Where does upward momentum come from in leading edge vortex? I believe I am beginning to understand how a leading edge vortex creates lift. Faster air creates a lower pressure above the wing making lift.

But I am hoping to get another view of this in terms of momentum.

Does this vortex draw in air from above, pulling it downward and as a result lifting the plane up? Or is the vortex shed downward off the tail of the wing? How does this work?

Does this vortex draw in air from above, pulling it downward and as a
  result lifting the plane up?

Yes, the bound vortex in combination with translational flow accelerates air downwards.

  Or is the vortex shed downward off the tail of the wing? How does this work?

A vortex has no beginning and no end. When the wing begins to move, the vortex starts out of nowhere and has four parts: The bound vortex which travels with the wing, one parallel starting vortex which stays in place and two free vortices which connect both, running from the wingtips to the point where the wingtips were when the wing started moving. To be precise, an infinite number of vortices of infinitely small strength is created, and some cover only the center of the wing while others run over most of the wingspan.

Idealized vortex structure (own work). The wing moves to the left with the speed v. The vortices are shown as discrete lines - to be precise, the bound vortices should be a sheet of infinitesimally small vortices which extend both over the chord and the span of the wing. Chordwise, they bunch together close to the leading edge, so their average streamwise position is at one quarter of the wing's chord.

And now let me give you my advice: If you do not want to operate or to write a potential flow code, do yourself a favor and forget all that. Read here why you must endure the vortex explanation of lift, and read here for a much more intuitive explanation of it all.

If by leading edge vortex you mean the swirling air coming off the leading edge of a highly swept wing, this is a result of flow separation. The inertia of the air molecules makes it impossible for them to flow around the leading edge, and they jump off the contour, leaving a volume of lower pressure behind. This pressure bends their flow path towards the wing and sucks the aircraft upwards. Since more air follows the same pattern downstream, the whole air above the wing starts to rotate while it flows downstream. This low pressure area sucks more air in from above, just like the regular low pressure region over a straight wing does.

Along and past the inner part of the wing this downward motion is prevalent and keeps the air spinning like the regular downwash from a straight wing.


How long have commercial aircraft been continuously in the sky? In 2018, it's obvious that at any moment of the day, there is at least one (and in fact many) commercial aircraft in the sky. In 1908, this was not true.

Which was the last year there might have been a moment with no commercial aircraft in the sky? Please argue for your estimated year.

This is a great question and I will try to answer as I think it was intended. Based on reasonable assumptions, I can estimate that at least one commercial passenger flight has been in the air somewhere in the world continuously since the early 1930s.

A key help in answering the question is that passenger aviation grew very rapidly throughout the mid-20th century. Moreover, the chance probability of having zero flights in the air at some moment is mathematically very sensitive to the overall rate of flights. Thus, the historical window in which an "empty sky" is plausible would close quickly. Past some point, the overwhelming growth makes it virtually impossible that the skies would ever be devoid of commercial flights again, unless some event stopped them in sync almost everywhere in the world.

What about World War II?

In North America, despite the diversion of a significant fraction of people and resources to the war effort, many commercial passenger flights continued. Schedules in the early 1940s showed numerous routes operating at all hours of the day and night. So much growth had occurred in the 1930s that even the dent from the war left the industry well ahead. (This may well have also occurred in Australia, another developed region mostly spared the direct violence of the war domestically.)

What about 9/11?

Although US and Canadian airspace was closed after the attacks, flights continued within the rest of the world -- again making a dent but not undoing decades of growth.

So why the early 1930s?

Due to the aforementioned rapid growth, we will be pretty close if we merely get the orders of magnitude right. Assume a typical flight length of 2 hours or 200 miles in this era. Also assume that flights were uniformly distributed over a time period (say a year) -- equally likely to be scheduled at any time 24/7/365. Even within one region, flights operated day and night, and superimposing the various global time zones (even just North America and Europe) further homogenizes the flight schedules.

If we can estimate the average number M of commercial flights in the air at any moment, we can model the fluctuations statistically as a Poisson distribution. According to the Poisson distribution, the probability of finding zero flights at a given moment is e^(-M), where e ~ 2.7. This result is fairly robust; for example, if we take a different model where there are 2*M planes in the global airline fleet, each of which is flying half the time (thus M are in the air on average), then the chance that no plane is flying at a given moment is (1/2)^(2*M) = 4^(-M). The key result is that the probability decreases exponentially with M.

Roughly every 2 hours, the flights "turn over" (current flights have landed and a new set has taken off), giving us a new sample from the probability distribution. During a year, we get about 4,000 samples, and the probability that we see zero flights at least once is roughly 4,000 * e^(-M). Of course, M is increasing from one year to the next as the industry grows. We conclude that once M surpasses about 15, the probability of ever again having the sky globally devoid of commercial flights is negligible.

In the year 1930, there was already a noticeable passenger airline industry in the US (at least 6,000 passengers flown, which is likely very low because another source says over 300,000) and in Europe (10,000 in Italy, 18,000 in the Netherlands, not to mention the UK, France, and Germany).

If we estimate a worldwide total of >100,000 passengers for the year, and consider that a typical ticket was for several flight segments, say 1,000 miles, then we get >100 million passenger-miles in 1930. As further confirmation, the US alone is quoted at 84 million passenger-miles that year.

The planes were very small and load factors were low, so the typical number of passengers on board was likely around 3. Thus, passenger flights worldwide traveled at least tens of millions of miles in 1930 (compare 37 million revenue miles for US airlines, which may however include some cargo flights without passengers).

At an effective speed of 100 miles per hour, the total flight time was at least hundreds of thousands of hours. With the year containing about 9,000 hours, the sought average number M of planes in the air at once would be dozens. By the statistical reasoning given previously, this is the point at which the "empty sky" probability becomes negligible. And if we are off by say a factor of 2 in estimating M (e.g., due to seasonal variations), this corresponds to just a few years of growth.


How could a Focke-Wulf 190 pilot see anything at all from his cockpit? As one can see from the images below, the seat seems to be placed so low on the cabin, I cannot figure out how the pilot could see anything.

Can anyone explain how a Focke-Wulf 190 pilot would be able to see out of the cockpit?

Source

Source

It looks like the pilots are performing taxi-runs for show. For an actual flight, by sitting on the seat pack parachute (and/or a cushion) the eye level is raised as shown below:

(warbirdsnews.com)

(worldwarphotos.info)

For how to taxi while sitting so low, it's usually down by taxiing in an S-shape or following hand signals from a marshaller.

Related:

How are aircraft controlled that have no forward view out of the cockpit?
How do pilots manage to keep aircraft on centerline during taxi?
How does a taildragger steer (taxi) differently to tricycle gear?
Would the Gee Bee be allowed to fly today, with so little forward visibility?


Why do Dassault Falcons only have one thrust reverser? Using all 3 engines would slow it down much faster. No?

As always, it is a trade-off.
On the positive side, thrust reversers allow you to slow down faster and reduce brake wear.
On the negative side, there are costs to install and maintain them. They add weight, which increases the required take-off length reduces the maximum payload and increases fuel consumption.
Dassault found the optimum for their design in one thrust reverser, others go with TR on all engines, none or optional.


What makes a weight-controlled microlight tumble, and can it recover? As shown in this video, what causes a weight-controlled microlight to tumble, and is it possible to stop it from tumbling once it starts?

what causes a weight-controlled microlight to tumble

There seem to be several mechanisms. One is an excessively sharp nose-down control input after entering or approaching a stall.

  and is it possible to stop it from tumbling once it starts?

Not without an external safety device (presumably a throwable parachute or something similar)

  The tumble mode is a little known mode of departure from controlled flight experienced by weightshift controlled microlight aeroplanes. It has been a very significant factor in fatal accident records, being non-recoverable without the use of external safety devices. The mode consists of a nose-down autorotation at a rate of up to 400°/s. The tumble entry mechanism is explained, and advice to operators developed which should prevent tumble entry. Evidence is shown of the nature of the developed tumble – both modelled and through wind tunnel results, which explain how the autorotation occurs. It is also shown how this theory may be applied during testing of an aircraft to develop a tumble resistant aircraft.

See A theoretical and experimental appraisal of airworthiness evaluation techniques for small light aeroplanes

This seems to be an excellent paper 255-page dissertation which contains far far far more than the average reader of this website could ever want to know about this phenomenon.

You should note that this contains stills from the Youtube video in the question. It is therefore directly relevant. If you can find a better analysis I will print out my answer and eat it.


Are any flight schools or clubs in Canada using Pipistrel's Alpha Electro or Velis Electro? I'm interested in the business and environmental possibilities of using electric trainers for training.
Are there flight clubs or flight schools training with electric aircraft?
I thought I'd read that Canada provided some kind of type rating for the Alpha Electro, and I'm hearing that Pipistrel's newer Velis electro is certified (at least in Switzerland).
Is anyone doing this now in Canada? If not, why not? Are there regulatory obstacles?

The only Pipistrel Electro registered in Canada is an Alpha, C-IZAP, registered to a private owner in British Columbia.  So the answer is for now, no.
Pipistrels, including the Electros, are licenced in Canada as Advanced Ultralights.  I don't see any reason an Electro couldn't be used for training under the Advanced Ultralight regulations (not for Private Pilot however).  Probably cost is an obstacle.


What keeps an aircraft engine from flaming out at high altitude? High altitude atmosphere has lower oxygen level. How do aircraft, especially the military ones, prevent engine flameout?

The oxygen level at high altitude is the same as at sea level, namely 20.95%. What is lower is the oxygen pressure.

To avoid flameout, pilots must avoid to throttle the engine down too much, so a minimum pressure in the combustors can be maintained.

Designers can increase the altitude performance by building longer combustors. The speed of ignition is pressure dependent, and with lower pressure it takes longer for the fuel-air-mixture to ignite. More length allows more mixing and leaves the mixture more time to react. Also, they can add flame holders which leave pockets of burning fuel close to the injection nozzles, so the mixture is heated more rapidly and ignites more reliably. Normally, flame holders create drag and are reduced to the minimum necessary.


Why is the force that moves an airplane forward called THRUST and not PULL? As we know, there are four forces acting upon an airplane. Thrust ($T$), Drag ($D$), Lift ($L$), and weight ($W$).

Since most propeller-driven airplanes over history have had the propeller mounted in the front, the power or forces generated were to pull, not to push. So why is it called thrust (the meaning of which is equivalent to push), and not pull??

Thrust is the force reaction from accelerating a gas from here to over there.  If the element creating the thrust, a propeller, is in front of the engine, it's called a "tractor" configuration because it's pulling the engine, like a tractor (any normal front engine airplane).  If the prop is at the back, or more correctly, behind the engine, it's a pusher configuration (A Lake Buccaneer amphibian is a pusher even though the engine is amidships, not at the tail, because the prop is on the back of the engine itself).  Lots of other designs have put the prop all the way at the back.

In a jet engine, you don't really have an element pushing or pulling the engine itself because the thrust reaction is actually happening inside the engine, mostly in the area ahead the convergent nozzle of the tailpipe where the flow is forced to accelerate. So for jet engines it's just thrust.


Is there a site that tracks all flights? Some of my flights are tracked on sites like Flightradar24 and others are not, regardless of flight plans being opened and closed. It would be nice to go to a website that tracks all flights, so I can review the flight dynamics after I land.

I am interested in a website that provides this service, not an app (I have had limited success with these).

No, there will always be flights that are private, or for any other reason not traceable.


What is profile drag? I know about induced drag, parasite drag, form drag, skin drag, wave drag, and interference drag. But I don't encounter the term profile drag often, and I can't find a good definition of it.

Is profile drag the same thing as form drag? If not what is it?

The composition of total Drag as it is shown below:

Thanks to DRAGBUSTERS... This document can solve this question, and analyses the drag in detail.

  Profile Drag Definition:
  Profile Drag is the drag incurred from frictional resistance of the blades passing through the air. It does not change significantly with angle of attack of the airfoil section, but increases moderately as airspeed increases.
   Taken from here.

Definitions of profile drag as they are in dictionaries:

  The portion of the wing drag that is due to friction and turbulence in
  the fluid and that would be absent if it were nonviscous

  The part of the drag on an aerofoil or aircraft which arises directly
  from its profile and from skin friction (i.e. the part not
  attributable to lift).

By reading this...(in fifth par.)

  Form Drag Definition: Drag which depends on the shape of the aircraft, is called form drag.

The following definition has been taken from here.

  Profile Drag or, sometimes called form drag, is the drag
  caused by the separation of the boundary layer from a surface and the
  wake created by that separation. It is primarily dependent upon the
  shape of the object. Form or pressure drag is caused by the air that is flowing over the aircraft or airfoil.

And finally, the most commonly types of drag encountered are,

  Parasitic Drag, composed of

    Form Drag, which is the result of the aerodynamic resistance to motion due to the shape of the aircraft,

    Skin Friction Drag, which is due to the smoothness or roughness of the
    surfaces of the aircraft, and

    Interference Drag, which may occur where surfaces with different characteristics meet (e.g. wing and fuselage)

  Induced Drag, which is a secondary effect of the production of lift,and

  Wave Drag which comes into play when shock waves are developed close to the surface of the aircraft in transonic and supersonic flight.

  Taken by this Skybrary Article which describes very nicely the types of drag.


“, remain VFR” — may I enter Class C airspace? During a flight yesterday, the approach frequency for a local Class C was unusually busy. I was heading back to an underlying Class D and the frequency finally quieted down enough for me to poke in a call of Huntsville Approach, Cessna 123AB, VFR with request.

ATC responded with <identifier>, remain VFR. I am not positive whether he gave the full callsign or just Tree Alpha Bravo — either way, definitely not Aircraft calling. He did not instruct me to remain clear.

I understand that radar advisories are workload permitting for VFR traffic, and clearly ATC had all the workload they could handle. Does that reply establish two-way radio communication and thus permit entry into their Class C airspace, just without flight following? What if the reply had been Tree Alpha Bravo, remain VFR? Assuming it does establish two-way communication, is it good Aeronautical Decision Making to fly into known busy airspace without advisories?

Rather than requesting confirmation on a busy frequency, I descended below the shelf feeling like a scud runner. Approach and landing at the Class D were otherwise uneventful.

Yes, you were allowed to enter the class C in that case, but you weren't cleared to do it because no clearance is needed to operate in class C, just communication with ATC. The AIM 3-2-4 has the best explanation of this (see also the basic regulation in 14 CFR 91.130):

  If the controller responds to a radio call with, “(air craft callsign) standby,” radio communications have been established and the
  pilot can enter the Class C airspace.
  If workload or traffic  conditions prevent immediate provision of  Class C services, the controller will inform the pilot to remain
  outside the Class C airspace until conditions permit the services to
  be provided.
  It is important to understand that if the controller responds to  the initial radio call without  using the  aircraft identification,
  radio communications have not been established and the pilot may not
  enter the Class C airspace.

So if the controller used your callsign then you've established radio communications and you can enter. This also applies to class D but not to class B, where you need an actual clearance to enter. (This is a classic exam question/scenario, by the way.) As @rbp suggested, the instruction to "maintain VFR" was probably the controller's way of letting you know up front that he was too busy to give you a pop-up IFR clearance or whatever else you might have wanted. (Maintain is an official ATC term, remain is not, although the controller may still have used it accidentally.)

As for flying into busy airspace, it's up to you as PIC to determine what's safe and what isn't. If you had the HSV ATIS (so you can anticipate traffic patterns), were squawking 1200, were in communication with ATC and were keeping your eyes outside the cockpit then there's not too much more you can do (you could turn on all your lights if you think you need to make yourself more visible). But, if you ever feel unsafe or uncomfortable in any flight situation then you shouldn't hesitate to take action - like staying clear of the class C for a few minutes - if you do feel it's necessary, that's simply part of gaining ADM experience.


What is this plane with its thick cockpit? Today I saw this unusual plane while going to work. I'm in France, near Paris. It flew at a rather low altitude, even if I can't tell precisely - I noticed the low altitude because of the loud sound.

We have an airport at 20km, the Charles de Gaulle Airport (CDG).
The plane was going towards South-West, probably directly departing from it. It came from this direction.
I was in the place named "La Défense", a district in the west of Paris.

I did make an outline sketch but it's not very precise :

It looked like a very small airliner; what sparked my attention was the front of the plane (the cockpit, very probably), which was thicker than the fuselage, making it look like some sort of hammerhead shark.
The wings were posistioned more towards the back of the fuselage than usual, but I can't remember if they were straight or oriented like usual.
As well it had a T-shaped tail.

I'm not very knowledgeable about planes but I have a some interest and I have never seen one like this.

I'm curious because it really looked unusual to me and that cockpit was funny.

Potentially a Piaggio Avanti, based on OPs description of an unusual nose.


If ATC doesn't assign an obstacle departure procedure in an IFR clearance, what should you do? If ATC doesn't assign an obstacle departure procedure (ODP) as part of an IFR clearance can you use one anyway?
And if there is no ODP, do you then follow the 35ft at runway threshold / 400ft at first turn / 200ft/nm rule, then turn on course on your own?

To answer your question directly, yes you can use the ODP to depart an airfield regardless of if ATC has specifically given it to you. You may use it until you reach the prescribed altitude in your IFR clearance or the beginning of the cleared routing. However, if ATC has given you specific instructions for your departure, you have to either accept those instructions and follow them. Or, decline the clearance and ask for what you need. After all, you are the Pilot in Command solely responsible for the safety of your own flight.
That being said, I have never heard an IFR clearance where the departure procedure is not explicitly spelled out. It does not have to be in the form of an official charted Obstacle Departure Procedure or complex instructions. As a matter of fact, it is usually a fairly simple instruction in order to put you in a position for ATC Departure to pick you up on Radar. It should sound like this:

C - Cleared to [your destination]
R - Fly runway heading. Radar vectors to the [blank] departure. Then as filed.
A - Altitude 3000 [or so]. Expect 10,000 in 10 minutes.
F - [Frequency you will use for departure].
T - Squawk [transponder code].

The “fly runway heading” statement means that you will fly the course of the extended center line of the runway of departure. And, continue to do so until told otherwise by ATC.
They may replace that statement with “Upon departure, turn right [blank] heading. This generally means you have the leeway to begin your turn once you are beyond the traffic pattern and/or above 1000 feet AGL.
If they stipulate for you to make an immediate turn to a specific heading as practical, do so as soon as you are above a safe altitude to bank the aircraft. I would generally make the turn when I have reached the Departure End of the Runway and above 400 feet AGL. ATC may give you an altitude at which to make that turn. In which case, make the turn at that altitude regardless of if you have reached the DER or not. Typically, ATC will not have you make the turn lower than 400 feet AGL. They would deem this as the practicable altitude for a turn. In an emergency or urgent situation, ATC may request a turn as low as 200 feet AGL. They would not want you banking low and slow in IMC.
If ATC does not give you a specific set of departure procedure instructions, the FAA recommends in the AIM that Part 91 flights should follow the charted ODP if one is available. This is only a recommendation and not a requirement. I would highly recommend informing ATC if you plan to amend their clearance instructions by departing on the ODP.
Something you may want to consider is that the numbers you alluded to in your questions are bare minimum performance values for certification of a Departure Procedure. You, as the PIC, should not be following these numbers. If your aircraft can not achieve 35 feet AGL way before the DER, and a climb rate of 200 feet per Nautical Mile, you probably should not be flying that aircraft, at that airfield, on that day, in those conditions (weight, weather, etc).
Below are the FAA recommendations for this found in the Aeronautical Information Manual Chapter 5-2-9:

a. Instrument departure procedures are preplanned instrument flight rule (IFR) procedures which provide obstruction clearance from the terminal area to the appropriate en route structure. There are two types of DPs, Obstacle Departure Procedures (ODP), printed either textually or graphically, and Standard Instrument Departures (SID), always printed graphically. All DPs, either textual or graphic may be designed using either conventional or RNAV criteria. RNAV procedures will have RNAV printed in the title; for example, SHEAD TWO DEPARTURE (RNAV). ODPs provide obstruction clearance via the least onerous route from the terminal area to the appropriate en route structure. ODPs are recommended for obstruction clearance and may be flown without ATC clearance unless an alternate departure procedure (SID or radar vector) has been specifically assigned by ATC. Graphic ODPs will have (OBSTACLE) printed in the procedure title; for example, GEYSR THREE DEPARTURE (OBSTACLE), or, CROWN ONE DEPARTURE (RNAV) (OBSTACLE). Standard Instrument Departures are air traffic control (ATC) procedures printed for pilot/controller use in graphic form to provide obstruction clearance and a transition from the terminal area to the appropriate en route structure. SIDs are primarily designed for system enhancement and to reduce pilot/controller workload. ATC clearance must be received prior to flying a SID. All DPs provide the pilot with a way to depart the airport and transition to the en route structure safely.
e. What criteria is used to provide obstruction clearance during departure?

Unless specified otherwise, required obstacle clearance for all departures, including diverse, is based on the pilot crossing the departure end of the runway at least 35 feet above the departure end of runway elevation, climbing to 400 feet above the departure end of runway elevation before making the initial turn, and maintaining a minimum climb gradient of 200 feet per nautical mile (FPNM), unless required to level off by a crossing restriction, until the minimum IFR altitude. A greater climb gradient may be specified in the DP to clear obstacles or to achieve an ATC crossing restriction. If an initial turn higher than 400 feet above the departure end of runway elevation is specified in the DP, the turn should be commenced at the higher altitude. If a turn is specified at a fix, the turn must be made at that fix. Fixes may have minimum and/or maximum crossing altitudes that must be adhered to prior to passing the fix. In rare instances, obstacles that exist on the extended runway centerline may make an “early turn” more desirable than proceeding straight ahead. In these cases, the published departure instructions will include the language “turn left(right) as soon as practicable.” These departures will also include a ceiling and visibility minimum of at least 300 and 1. Pilots encountering one of these DPs should preplan the climb out to gain altitude and begin the turn as quickly as possible within the bounds of safe operating practices and operating limitations. This type of departure procedure is being phased out.

Note: “Practical” or “feasible” may exist in some existing departure text instead of “practicable.”


Why does a hovercraft lift blade look one way, but a propeller another? The lift blade of a hovercraft looks like this:

Note the ten rectangular blades.

The blade of a propeller-craft looks this this:

Note the three, traditional airfoil-shaped blades.

Why are the two different?

Edit: My question is different from the other question cited because it asks about both the number of blades and their shape, as well as other differences I may as of yet be unaware of.

Because lift propellors need to provide static pressure to inflate the airbag and hover. This means more fan blades to stop backflow.

The thrust propellors need to provide force which is a combination of static pressure and airflow but relies mostly on airflow. This means less fan blades to facilitate large airflow.


What are the effects of shockwaves on normal and supersonic aircraft? What are the effects on the speed of the plane, does it effects the body of the plane, and do shockwaves cause major damage to the plane.

I know that flying at Mach 1 is really risky, but what's the actual effect of the shockwave?

Not much, really, the shock wave just streams off of the nose of the plane, and an anti-shock from the tail. They stream away from the plane and don't hit it.

Image source

The shock itself is the compression that the flying aeroplane imparts upon the unsuspecting air in front of it: disturbances travel with the speed of sound, if the aeroplane travels faster there is no warning and air gets compressed suddenly when the aeroplane arrives. This is the nature of the shock.

The amount of compression is spread out It's not a shock like a bomb explodes, only a loud noise. Flying over Mach 1 is not inherently more risky than flying subsonic. When first attempted 70 years ago there was some unexpected behaviour to iron out, but that's aviation in a nutshell for you. Once understood, a problem is only an invitation to find a solution, and that is what the aero guys did.


Is this route of a airforce jet on flightradar faked? While looking at the flight routes of Indian airforce jets on flightradar, I have noticed that the trainee jets show a very weird flight pattern. Here's an example,

The width of this screenshot is around 20KM. I don't think a jet can take such sharp turns. If so, why are these turns so sharp? Is this data somehow been faked, or is this issue with the radar?

No, that has not been faked.

Flightradar is updated with a low frequency (from that screenshot I'd estimate once per 10 seconds, maybe more). This is done to reduce the computational demands on both the service that sends the radar info to Flightradar, and on the Flightradar servers.

Each dot in the track is one position update, and the website software just draws straight lines between them, which results in sharp corners. The real flight path will have a radius.


What surface materials do airplanes use to reduce friction? Full swimming body suits were banned from Olympic Games a few years ago because some materials have less friction than skin and contestants that could not afford gear to increase their performance were disadvantaged.
However, low friction materials could benefit many industries including aviation. Are there any special surface coatings either experimental or used by manufacturers to improve the aircraft performance and decrease drag?

In a way, yes. By using smooth metal surfaces with countersunk rivets, modern aircraft use a surface with low friction already. In the past, wooden structures were varnished to give them a smoother surface. In the second World War, German crews would polish the normally matte paint of some of their airplanes to eke out the maximum in top speed.
There have been several attempts in the past to improve the smooth metal surface. The most attention was heaped on riblet surfaces, a grooved plastic foil the surface of which was inspired by shark skin. Since the 1980s several experiments were made with partially riblet-covered airplanes, boats or wind turbine blades. The gains of up to 8% in drag reduction are real, but the application and maintenance of riblet-foil covered surfaces made the technique impractical.

Shark skin surfaces (picture source). The common understanding is that the streamwise grooves will suppress oscillations in the flow and reduce friction drag. The overall effort to maintain this surface has so far been considered too high in order to achieve the small reduction in drag that is possible with a grooved surface.
And now for something completely different …
Monocrystalline solar panels would result in a polygonic surface and need to be embedded in clear resin to produce the desired curved outer surface. Solar foils are much easier to apply and weigh much less, but also have a lower efficiency. So far, only experimental airplanes have used solar panels on their skin and the integration of those panels was a major part in the overall cost of those airplanes.

The Gossamer Penguin used a rather clumsy way to add solar panels in order to avoid the major work of integrating them into the surface (picture source). It should be obvious that this way of integration adds both drag and weight.

Solar panels of the Solar Impulse II (picture source). It used a rectangular wing planform to make the integration of the solar panels easier since the same mold could be used for all wing panels. Now the solar panels do not add viscous drag, but the weight of the panels, their wiring and the clear resin needed for the smooth, curved surface adds weight which causes an increase in induced drag.


what are the Common roll angle and roll rate limits for civil flights? I'm currently doing simulations for eVTOL type of vehicles and for the purpose of controlling the attitude, I'd like to know the common limits for roll and roll rates (more important) that are in place for civil flights. I'm interested both for airplanes and helicopters even though I do prefer information for rotary vehicles.

I've spent a few hours already searching through the web but for some reason, I'm not able to find literal values in documents such as ICAO annexes etc.
So it would be really helpful for me to know at least the approximate values. I appreciate any answer in advance. Thanks

UPDATE with respect to comments:
I am interested in upper boundary, i.e. what rate should not the vehicle exceed. I imagine that there is some at least recommendation that aircraft of a particular type should not exceed this rate, otherwise the passengers can spill their coffee, get sick etc. So it could be also partially dependent on the current wind situation, gusts and so on.

In terms of particular regulatory office, it doesn't really matter as this information should just give some overview of possible values because right now I really dont have any clue.

The minimum roll rate is documented in 14 CFR 23.157 (pre Amdt 64), which is used as an accepted means of compliance for the current 14 CFR 23.2135.

As far as I know, there is no regulatory requirement on maximum roll rate. For large transport aircraft, the design target at maximum roll authority is typically around 20 deg/s to 30 deg/s. For example, A320 has a roll rate limit of 15 deg/s at maximum lateral input in Normal Law, while A220 has a roll rate limit of 20 deg/s in Normal Mode.

There is no regulatory requirement on maximum roll angle. For reference, the A320 has a maximum commandable roll angle of 67 deg in Normal Law, while the A220 has a limit of 80 deg in Normal Mode. The autopilot disconnect bank angles would be much lower than these.


F.X Wortmann airfoil naming convention Does anyone happen to have a reference or such that explains the naming convention used by Prof. Wortmann for his airfoils?

Usually it follows that for symmetrical ones, the first 2 digits are to indicate the planned transition location at zero alfa and the last 3 for for the thickness.

ex:
fx76-120 -> fx (prefix for FX.Wortmann) 76 (transition) 120 (12%)

would like to double confirm this with a reference if available and also the meaning of the middle letters used in some foils.

Ex: FX76-MP-150 (MP for man-powered in this case but there are many more)
2 is known thanks to Peter Kampf. Any idea on remaining?

FX stands for Franz Xaver, Professor Wortmann's given names.

The next two digits stand for the year this airfoil has been designed. Add '19' in front and you have the full year.

Then may follow one or two optional letters which specify a dedicated use, but they can be missing for general purpose airfoils. For example, the 'L' in FX 71-L-150/30 stands for "Leitwerk" which in German is the name for the combination of the horizontal and vertical tail surfaces.

The next three digits denote the relative thickness times one thousand. So the "150" in FX 71-L-150/30 tells us that this airfoil is 15% thick.

The last digits are again optional and denote the relative flap chord in cases of control surfaces, this times in percent. The FX 71-L-150/30 is optimized for a 30% chord control surface.

Reference: Stuttgarter Profilkatalog ISBN 3-528-08464-2, 1981.


How to determine angle of bank caused by shift in centre of gravity in an aircraft? I’m trying to figure out the angle of a bank caused by a shift in the centre of gravity of an aircraft. How should I do this?

What avenues should I go down to calculate this? I know that the plane will bank until c.g. is below centre of lift.

You can't. Aircraft are not stable in roll!

Once there is an offset between the centre of gravity and the lift action line, there will be a rolling moment until the aircraft is banked at 90°, at which points the forces become perpendicular, so they can't generate any moment.

The moment will cause roll acceleration, but as the wing going down will have higher angle of attack, its lift will increase, causing opposing moment that will stabilize the aircraft at some roll rate. Which will slowly decrease as the moment between lift and gravity decreases with the increasing bank. But it will not vanish until 90° bank, so the aircraft keep rolling and end in a graveyard spiral.

If there is some roll-yaw and yaw-roll coupling, and the imbalance is small enough, the side-slip created by the incipient turn will generate enough opposing roll moment to return the plane to a dutch roll instead. But that is an oscillating motion, so you still won't have stable bank angle.


What is the parasite drag for Airbus A320? What is the parasite drag or CD,0 for the Airbus A320?
How do we calculate CD? Is it correct to use CD = CD,0 + ( CL2)/$\pi$AR

For numerical usage (in some question) you can use the above formula, but for exact drag (and lift) calculation, the industry standard is to use Computer Solvers to compute this drag because of the following reasons:

The formula used above is correct but the value of Cd varies for every point on the wing, as things like wing twist, sweep back, wing tip sharklets contribute to a different Cd throughout the wing.
Other effects like drag due to fuselage etc can not be calculated by using a generalized equation for the whole plane, as usually local Cd for every point on the airplane surface are known.
Application of force on the surface of an airplane (Lift/Drag force) causes deformation in the geometry of the surface and this leads to recursive change in the value of Cd/Cl, which hence needs further set of calculation to again arrive at a value of Cd from the Solver.

For approximate usage though an averaged out Cd, Cd0 and AR if known, then the above formula can be applied to arrive at the results.


What are some good resources on Jet Design? I'm a student in high school, so please imagine you're talking to a toddler first, and second where would you advise I look for good documentation and guidance in jet engine design and tools to familiarize with.

That one depends on a lot of things.  Real jet engine design is going to require an understanding of advanced physics, computational fluid dynamics, thermodynamics, heat transfer, and metallurgy.  It will also require college level engineering coursework as well as at least calculus and in some cases partial differential equations on order to properly design such an artifact.

Not to be deterred, amateur jet engine builders have constructed self sustaining gas turbines using old turbochargers and a custom built isobaric combustion chamber as this YouTube video shows.  Both the reactive thrust and/or mechanical power one can get out of such a device is limited.

There are some existing plans available on the market for building small jet engines as this Instructables shows.

RC Aircraft modellers can also buy small scale turbine engines for around \$3,000-$5,000, depending on manufacturer, type, thrust output, etc.

Designing a proper jet engine from scratch is going to be much more difficult, though it is possible. It will be expensive and time consuming, but if you really want to learn the art of building turbine engines, it's possible.  I would advise waiting until college and pursuing a degree in aerospace engineering. The major aero schools (e.g. Michigan, Purdue, MIT, CalTech, etc. all have extra curricular clubs as well as design and testing facilities and equipment for this sort of stuff and as I said before you will need higher level engineering and mathematics education in order to do it right.  That's not to say this is impossible; the Rocket Boys, a pack of coal miner's sons whom the film October Sky was based upon, were designing Di Laval nozzles using differential equations in high school, so if you really want to learn, it's always possible, though not always easy.

If you do want to start down that path and want a challenge, I'd recommend The Design of High Efficiency Turbomachinery and Gas Turbines by Wilson and Korakianitis.  I'd also recommend primers on thermodynamics and heat transfer as well.

You might attempt the earlier turbocharger projects first or disassemble an RC jet engine to get a much better familiarization first, then attempt the engineering of an engine from scratch.

In addition, seek out other jet engine builders and talk with them.  They will be a wealth of information for you at the beginning of the project.

You might also seek out professors at a good aerospace engineering program somewhere and ask them what direction they would recommend you to begin such a project.  Remember: Old people find nothing more flattering than a youngster who will sit patiently and listen to what they do for a living; employ that to your advantage.

A long shot but it might work as well - get involved with Linkedin and look for profiles of engineers who are employed at places like UTC/Pratt & Whitney, GE Aircraft Engines, Rolls Royce, Eurojet, etc.  You might just find someone there who designs jet engine parts for a living and can be a good resource.

Also, as with any kind of engineering or programming, start simple and work up to complex.  Your not just building an artifact, you're building a database of knowledge of how to build such an artifact as well, which can be built upon in increasingly more complex designs.

Oh, and one final caution: gas turbines belch large, noisy quantities of high temperature, high pressure gas and operate at very high temps, creating the possibility of serious injury, explosions and/or fire hazards.  Approach the design and testing process with caution and take appropriate safety measures i.e. don't test this thing in your parent's basement, etc.


Is GLS (GBAS landing system) an RNAV approach? PBN which is contrast to conventional navigation consists of RNAV and RNP. And I know that general RNAV approach of IAP has up to 4 minima such as LPV, LNAV, LNAV/VNAV, and LP.

And I know that GLS approach is precision approach that use GBAS as well as GPS, but GLS approach procedure chart is separated with RNAV approach chart. I guess that GLS approach is RNAV approach because it use GPS system not conventional aids. Is it correct? is there any official document?

Is GLS an RNAV approach? Short answer is no. It is also not in the framework of PBN.
RNAV approach
Out of confusion the naming was not standardized, which led to more confusion. This will be fixed, from 1 Dec 2022 only the term RNP will be permitted for the minima you mentioned. (ICAO Circular 336, starts at page 19)
RNAV?
It stands for area navigation, which is the ability to fly any course in an area, not bound by VOR radials for example.
GLS
GLS uses a ground augmented signal, anchor point, course, and slope, so it does just the one thing (like an ILS).
PA, APV, and NPA
Another reclassification will start to impact the older terms precision approach, approach with vertical guidance, and non-precision approach. There will be only two main types, A and B. Here's a Eurocontrol slide that breaks them down, and also shows that GLS is not in the PBN scope:

(eurocontrol.int)
Last but not least in case any of the above is confusing, this big Airbus presentation (icao.int, Sep 2017) covers the history from conventional navigation to PBN and beyond, and also talks about the GLS.

Addendum based on comments:

A 'conventional procedure' in the slide above includes NDB, LOC, LDA, VOR, ILS, with or without DME, and MLS approaches. ILS and MLS would fall under PA / Type B.
For how the GLS plots the approach, see: How do GLS avionics plot an entire accurate approach path?
One of the things that sets the GLS apart from RNAV is the need for a ground component at the airport, the VHF Data Broadcast (VDB) transmitter.


What's the difference between aquaplane and hydroplane? If one reads through 10 articles describing a plane which overran a contaminated runway, the term aquaplane will be used 9 times and the term hydroplane once (and on that one time there will be a snarky comment at the end of the article.)

Articles describing automobile crashes, on the other hand tend, to use the term hydroplane much more frequently. A google N-gram search turns up far more instances of hydroplane but this is perhaps simply because more people write and think about cars losing control.

My question is this: Is the use of Aquaplane in aviation the result of some recognizable difference in the loss in braking/traction by an aircraft wheel? If so what is the difference between the definition of hydroplaning and aquaplaning? I've been unable to find an answer because most web searches just turn up information about the automotive phenomenon.

I'm beginning to think this is just a result of aviation preferring to use more technically correct terms (like saying a runway is contaminated rather than wet) rather than a scientific or etymological difference between the two terms (like how physicists use the term centripetal while the rest of us prefer centrifugal)

Can anyone cite an aviation-specific definition of aquaplaning that distinguishes if from hydroplaning?

It is the same thing, even wikipedia cross lists them, however in the case of aviation it is both a noun and a verb

Aquaplane

(source)

Aquaplaning

(source)

Non-Aquaplane-AquaAquaplaning

(source)


Does an airplane need a key to start? Does an airplane require a key to unlock the doors and start the aircraft engines?

Is it somewhat like how a car does?

My 1973 Cessna Cardinal has a key to unlock the doors and another to start the engine.  Just like a car.

I could use a horn to scare deer & geese off the runway.


Is it right to extend flaps only in the white arc? If the cruise speed is >80kts on the downwind leg but I can only extend flaps in the white arc, am I right in saying I need to get to the white arc, even if I'm on - for example - 100kts?

For reference, note the 'white arc' on this airspeed indicator.

Yes you must slow down to the white arc, or whatever your flap extension speed is for a given condition, regardless.  If you are 10kts above the white arc and drop flaps anyway, it's not going to make the airplane come apart, and if you did it once, slap yourself on the wrist and don't do it again.  It's putting stress on the flap attachments beyond what they are certified for, and eventually if it's done repetitively over a long period you might start seeing cracks in hinge arms, brackets etc.

On transport airplanes if you do that you get an overspeed warning and you are expected to declare the incident which will trigger a fairly straightforward visual inspection of the flaps.   On a small a/c, if nothing is bent, don't worry about it if it was just a few knots over, but don't keep doing it.  If it was WAY over, like 20 or 30 kt above the white arc, you should tell whomever owns or maintains the airplane about it so someone who knows what to look for can look it over.

In fact, as a matter of good airmanship, you should get into the habit of being well below the top of the white arc, within reason, before extending flaps (I slow down to not much above the flaps up approach speed before dropping flaps - it's a matter of being on top of how your aircraft flies, and staying ahead of it, mentally).  This is simply a matter of taking care of the machinery and not beating the crap out of it; it'll mean the difference between having to replace hinge bushings at 5000 hrs for the pilot who takes care of the equipment vs 2000 hrs for the pilot who treats his/her plane like a mule.


Why is full carb heat recommended when reducing power below normal setting? In my first flight lesson today, my instructor asked me to apply full carburetor heat when we had to reduce power to 1900 RPM in our Cessna 152.

I was surprised by this as I didn't remember learning this in ground school, and I later checked the Jeppesen textbook I am using, and referring to the section on carburetor icing, I saw that when power is reduced below normal operating limits, full carb heat is recommended (must have missed it when studying the ground material).

My understanding is that the cause of carb icing is the sudden drop in temperature in the venturi due to drop in pressure as the fuel speeds up. If I am reducing power, I am reducing fuel flow, so shouldn't this reduce the chances of icing? So my question is why is full carb heat recommended when decreasing power below normal operation?

When you close the throttle for descent, the airflow is constricted.  This constriction is going to cool the air more than it is during cruise configuration, and may increase icing.

From This AOPA document:

Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the. throttle valve.


"right traffic pattern" vs. "right-hand traffic pattern" Is it officially called "right traffic pattern" or "right hand traffic pattern"? I'm mostly interested in EASA terminology, but also FAA / other areas of the world.

Background: After I always said "right-hand traffic pattern" on frequency I got feedback from other pilots that I should leave out the word "-hand" to keep communication on frequency on a minimum - so I was now wondering, if there is any official terminology for that to be used. When looking in the Eurocontrol Phraseology Database, I only found "right hand pattern" for holding patterns.

"Right traffic pattern" and "right hand traffic pattern" are one in the same.  Usually on the radio either a controller or a pilot will abbreviate it to simply "right traffic" to expedite radio communications.

For example, A C-152 student pilot is inbound for a controlled airport and says something like "Plainview Tower, Cessna one two five X-ray ten miles to the south at two thousand, inbound with ATIS Golf."  If a controller wants a student pilot to fly a right traffic pattern to land on runway 16 which is active, they might say something like "Cessna one two five X-ray enter right downwind runway 16.  Report abeam the tower."  The pilot then responds reading back the instructions to the controller and might say "right downwind one six.  Report abeam.  Two five X-ray."


What do these symbols mean? What do these symbols mean in an airport?

The answer is stated in ICAO Annex 12 (Search and Rescue)PDF. (See Appendix I relevant to Chapter 5, 5.8 of the Annex.)

Ground-air visual signal code for use by survivors:

Ground-air visual signal code for use by rescue units:

  Note 1.— Symbols may be formed by any means such as: strips of fabric, parachute material, pieces of wood, stones or such like material; marking the surface by tramping, or staining with oil.


What is the working principle of a Gurney Flap? Recently I came across a airfoil improvement called a 'Gurney flap', see image.

I don't really understand the working principle of a Gurney.
Wikipedia states:

  The Gurney flap increases lift by altering the Kutta condition at the trailing edge. The wake behind the flap is a pair of counter-rotating vortices that are alternately shed in a von Kármán vortex street.In addition to these spanwise vortices shed behind the flap, chordwise vortices shed from in front of the flap become important at high angles of attack.

I don't understand how the flap could generate vortices in front of the flap.
Wikipedia states the following effects:

  The Gurney flap increases the maximum lift coefficient ($C_{L_{max}}$), decreases the angle of attack for zero lift ($\alpha_0$), and increases the nosedown pitching moment ($C_M$), which is consistent with an increase in camber of the airfoil. It also typically increases the drag coefficient ($C_d$), especially at low angles of attack, although for thick airfoils, a reduction in drag has been reported.

Is it simply a ridge that stops the flow, and thereby increases the pressure at the top surface?

How come the drag is decreased for thick airfoils if it generates extra vortices?

Adding the flap to an existing airfoil increases the angle of attack of the airfoil - just draw a straight line from the leading edge to the trailing edge: after adding the flap, the trailing edge has moved down to the tip of flap. The line before and after resemble flat plates at different angles of attack.

The vortex in front of the flap is produced because the flow can't make the sharp right angle turn around the flap, it can't perfectly follow the geometry.

The increase in CLmax should come from an associated increase in effective camber.

Total drag consists of skin friction drag (mostly, for an airfoil that is just "thick") and pressure drag. Maybe the increase in effective camber changes the relative amounts of both, leading to a net decrease of drag.


What are pilot position reports? I do a lot of shortwave radio listening, and I often listen to MWARA air traffic radio transmissions based out of San Francisco for flights crossing the Pacific Ocean. When giving position reports the pilots always report fuel, position, and some strange names. Usually they give the strange name, and then report "NEXT --strange name here--". Some of the names I've heard are "Diallo", "Gallop", "Gitsol" (sp?). What are these?

Navigation Fixes

The words that you are hearing are navigation fixes that are published on the pacific crossing charts.  They are used so that aircraft can report their position to Air Traffic Control using a standard, easy to say name that is located at a specific geographic location.

You can think of them like street intersections.  Instead of saying 29 degrees 44 minutes 18.8 seconds North, 95 degrees 24 minutes 38.5 seconds West (GPS coordinates), you would say the intersection of South Shepherd Drive and West Alabama Street1.  In the case of aviation though, they take it one step further and give it a five letter (usually) pronounceable name like the ones that you are hearing on the radio.

The navigation fixes that you refer to are standard reporting points located on airways going between the SW coast of the US to/from Hawaii:

GALIP and DIALO are on R576 (outbound airway)
GITLE is on R585 (inbound airway)

To see an actual chart and to follow along, the FAA offers IFR Enroute Aeronautical Planning Charts on their website (these would be on the PORC chart).  Here is a low resolution excerpt from the area that we are talking about:

A typical flight will file an airway with only the start and end fixes along with the airway name instead of specifying every fix on the route.  For instance, it might look something like GALIP R576 OGG which tells ATC that they will fly directly between all of the fixes on the airway: GALIP DADIE DIALO DUSAK DRAYK DENNS OOG.  The filled in triangle at each of these fixes means that it is a mandatory reporting point, and they will be required to make a position report every time that they cross one.

Position Reports

Position reports are given in a standard format specified in the AIM 5-3-2:

  d. Position Report Items:

  1. Position reports should include the following items:

  (a) Identification;

  (b) Position;

  (c) Time;

  (d) Altitude or flight level (include actual altitude or flight level
  when operating on a clearance specifying VFR-on-top);

  (e) Type of flight plan (not required in IFR position reports made
  directly to ARTCCs or approach control);

  (f) ETA and name of next reporting point;

  (g) The name only of the next succeeding reporting point along the
  route of flight; and

  (h) Pertinent remarks.

Example (from 5-3-1):

  (Name) CENTER, (aircraft identification), (position), (time),
  (altitude), (type of flight plan), (ETA and name of next reporting
  point), (the name of the next succeeding reporting point), AND
  (remarks).

Or a better example:

Los Angeles Center, N1234, over GALIP at 1645, Flight Level 370, Estimating DADIE at 1730, DIALO next

Then, when they get to DADIE, they do it all over again:

Los Angeles Center, N1234, over DADIE at 1730, Flight Level 370, Estimating DIALO at 1818, DUSAK next

This continues until the aircraft is identified on radar again at which time they can discontinue reporting unless specifically requested by ATC.

1 Mapped here for the curious.


What categories/classes is a Ground Instructor able to instruct on? Under §61.215 Ground instructor privileges, it says

  (a) A person who holds a basic ground instructor rating is authorized
  to provide—

  (1) Ground training in the aeronautical knowledge areas required for
  the issuance of a sport pilot certificate, recreational pilot
  certificate, private pilot certificate, or associated ratings under
  this part;

  (2) Ground training required for a sport pilot, recreational pilot,
  and private pilot flight review; and

  (3) A recommendation for a knowledge test required for the issuance of
  a sport pilot certificate, recreational pilot certificate, or private
  pilot certificate under this part.

But it doesn't say anything about what kind of category and class the ground instructor may instruct on. Is a separate GI rating required for each category and class? or is there one GI rating that covers all categories and classes?

The basic ground instructor covers everything you quote.  The only additional privileges you can get are those granted by the advanced ground instructor rating.  My ground instructor certificate just says "Ground instructor" on the front and "advanced instrument" on the back.  No other ratings or limitations.

Things like reading charts, familiarity with regulations, weather information, fundamentals of flight and other required knowledge topics do not depend on category or class of aircraft.  You just need the BGI (and IGI for instrument) to teach ground and endorse written tests regardless of the aircraft the student is or will eventually train in.

The only other benefit to having a ground instructor certificate is that it is required to get a gold seal on your CFI certificate.


How does a fan differ from a propeller? Why is the fan in a turbofan different than a propeller in a small airplane? Why do propellers typically have 2 or 3 blades, but the fan has a lot more? And why is the fan shrouded but the propeller open?

I read that a ducted fan is more efficient at very slow speeds (it generates more static thrust than an open prop), but at high speeds the drag becomes too great and you're better off with an open propeller. Which is why they use a ducted fan in hovercrafts, for instance. But if that's true, shouldn't the fan in a turbofan be open (unshrouded), considering the high speeds (higher than a propeller)?

Also, how does the fan get away with supersonic tip speeds and the prop don't? Would the fan be more efficient if tip speeds were lower?

The simple answer that covers the majority of engines is that a fan has a shroud. The possible exception are unducted fans or Open Rotor Engines, which are a hybrid between a turboprop and a fan engine.
A more scientific answer is based on the difference in disk loading: How much power per disk area can be pumped into the fan for propulsion. For propellers, this is distinctively less than for fans. Some examples:

Cessna 172 with O-320 engine (150 hp) and McCauley 1C160 prop (1.9 m diameter): 39.45 kW/m².
P-51D with Packard-Merlin 1650 (1590 hp) and Hamilton-Standard prop (3.4 m diameter): 130.59 kW/m².
Lockheed C-130H Hercules with Allison T-56 engine (4590 hp) and Aeroproducts propeller (4.1 m diameter): 259.25 kW/m².

With jets, this comparison needs power when thrust is listed. Therefore, we make the heroic assumption that the speed at the fan is Mach 0.4 in static conditions and thrust is split according to bypass ratio (which is completely imprecise but will do for the purpose here):

BAe 146 with Lycoming ALF 502 (31 kN static thrust, 5.7:1 bypass ratio and 1.02 m fan diameter): 4254.35 kW/m².
Boeing 747-200 with Pratt & Whitney JT9D (213 kN static thrust, 5:1 bypass ratio and 2.34 m fan diameter): 5388.74 kW/m².
Boeing 777 with GE90 (388.8 kN thrust at take-off, 8.4:1 bypass ratio and 3.124 m fan diameter): 6077.23 kW/m².

I guess by now you see where this is heading: Turbofans are simply a different class when thrust per frontal area is concerned. And for this to be possible, three conditions must be met:

A well-designed intake which produces uniform flow over the engine face. The shroud is only the consequence of providing the fan with an intake.
A high solidity factor of the fan/propeller (the ratio of the total blade area of the propeller to the disk swept out when the prop turns)
High dynamic pressure at the engine face, so the engine power can be absorbed by the fan/propeller.

Note that those conditions depend on each other: Without the solidity, much less power could be absorbed. Without the intake, the supersonic flow at the fan tips would produce horrible losses and noise.
Now for the crazy ones. The misfits. The rebels. The troublemakers. The round pegs in the square holes:

Antonov 70 with Progress D-27 (14,000 hp and 4.5 m prop diameter): 656.4 kW/m².
MD-81 with GE36 UDF (6661 kN thrust at max climb and 3.25 m prop diameter): 803 kW/m².

Here it is hard to make a clear call whether these are fans or propellers. Their proper name "propfans" hints at that already.
A last word on terminology
Efficiency is a measure how much effort is needed for a certain result. An efficient prop needs less power per unit of thrust produced. It has few blades, turns slowly and has a big diameter. A subsonic fan would be more efficient, but would create much less thrust for a given size.
If you want to express the capacity for thrust creation at a specific diameter, use disk loading, not efficiency.


When is my biennial flight review due? I logged into my flying club's online scheduling website today (May 19, 2015) and it was warning me that my Flight Review was about to expire (on May 31, 2015). My last flight review was on June 7, 2013.

According to 14 CFR 61.56 (c), the look-back period runs from (my emphasis added):

  ... the beginning of the 24th calendar month before the month in
  which that pilot acts as pilot in command ...

So am I right in calculating that my last Flight Review is actually valid through June 30, 2015?

Yes. Your interpretation is the correct one. Several FAA legal interpretations cover the "calendar months" phrasing; the best I could find was the Harvey interpretation from 2012:

  Based on the facts that you have presented, a biennial flight review accomplished on April 2, 2002, would not lapse until May 1, 2004.

Basically, your flight review is legal until the end of the month (ie, June, or August) it was completed in two years before. The same calendar logic applies to maintenance like the annual inspection.


Why is tangential velocity a loss of kinetic energy in a turbine section? While researching turbine engines I came across a statement

The mass of airflow naturally does not change in the transfer of energy to the rotor system, but the velocity of mass flow being slowed axially is said to account for the energy of flow as the energy of shaft power is increased. In other words, tangential velocity is essentially a loss of kinetic energy to the engine.

Why and how does this the loss occur, and how can this loss be reduced?
I'm not trying to get an in-depth mathematical aspect on that topic as I'm still a beginner with the study of gas turbines, a simpler theoretical non-math explanation would very much be appreciated, thanks!

We take axial flow turbines as our example here.
Tangental flow of air (flow that circulates around the engine's central axis) is imparted to the air by viscous drag between the air and the rotating fan and compressor blades. Between those fan discs are stator blades that act to straighten out the tangental flow and help convert the kinetic energy of the axial flow into pressure rise in the compressor stage. Viscous drag losses occur in the stator stages too as they perform this function. These losses dissipate some of the compression work being performed on the air by the compressor as a whole, and that amount of work then cannot contribute to thrust generation in the engine.


Will 3D wing has smaller drag than 2D at same AoA and airflow speed? We meassure lift and drag for 2D and 3D wing case,we use same airfoil at same AoA and same airflow speed for both case.
Wind tunnel can change test section walls width.

2D wing test:

So first we put airfoil in wind tunnel so walls touch arifoil tips(2D wing),at AoA=15degrees and airflow speed=20m/s  and meassure lift and drag..

3D wing test:

Now we increase walls width so airflow "act" like 3D wing in test section.We use same AoA=15 and same airflow speed (20m/s) over airfoil like in 2D wing- test.
note: AoA=15 is angle between chord line and freestream airflow not angle between chord line and effective airflow at 3D wing

Question is,what will be drag of 3D wing-test compare to 2D wing-test,smaller ,equal or larger?

My logic is that 3D airflow leakage on tips  will decrease overall pressure difference at wing ,so lift will be smaller than in 2D wing-test but drag will be also smaller than in 2D-test,because when you integrate smaller pressure difference over airfoil surface you will get smaller drag as well.

(BUT if we must have SAME LIFT IN BOTH TEST,than 3D wing test will have larger drag,because then we must increase AoA at 3D wing to compensate lift reduction caused by airflow leakage,so now  when we have increased AoA we also have larger drag..)

3D wing will have less drag,drag decrease when AoA decrease,3D wing "feel" effective AoA,so it fly at smaller "AoA" than 2D wing.

how blue-lift can be larger than red-lift,wing at AoA=10 can not produce larger lift than wing at AoA=20 ??

Summary

At relatively high Reynolds number (>1 million) and in attached flow, where the boundary layer is thin, 3D wing should always have larger overall drag than a 2D airfoil (infinite span), at the same free-stream incidence angle.

Why?

The downwash generated from trailing vortices is actually very small compared to the free-stream airspeed; therefore, the reduction in effective AOA is also very small.

This results in three effects:

Reduction in lift due to tilting. This effect is exceedingly small, since cosine of a small angle is essentially unity.
Reduction in lift and pressure drag due to lower effective AOA. The reduction in lift is small but definitely non-negligible. For unseparated flow around an airfoil, the overall pressure drag is very small to begin with, so the reduction is even smaller.
Creation of induced drag due to tilting of the lift vector. This is the main one, since sine of a small angle produces non-negligible linear factor. For example, if the local lift coefficient, $C_l$, is 0.4, and we have an effective decrease in AOA of 1.0deg, then $C_l\cos{\frac{1.0\pi}{180}}\approx 0.9998C_l\approx0.4$, while the local induced drag coefficient, $C_{d_i}$, is $C_l\sin{\frac{1.0\pi}{180}}\approx0.0175C_l=0.007$.

By the way, the drag polar you cited doesn't help your argument since it's for the whole wing (at aspect ratio of 6). What you really want to cite is the airfoil drag (from Airfoil Tools):

As you can see, the form drag (includes pressure drag) is a small portion of the overall 3D drag at aspect ratio of 6, in the linear lift range. The pressure drag increase is even smaller.

Addendum:

From your comments, it seems you have totally missed the point about induced drag. Induced drag is completely absent in 2D. In fact, if you assume that the flow is inviscid and without local supersonic flow, you have zero drag on any 2D airfoil. This is called d'Alembert's paradox.

As mentioned above, induced drag appears in 3D inviscid flow due to finite span and induced downwash.


What was the decrease in approach speed of the KFIR over the Mirage 5 due to the canards? The Mirage III approached at 170 knots according to the Dassault. The Mirage NG and the KFIR C2 as well as the Swiss Mirage 5s added canards to the design, what was the decrease in approach speed due to these changes? I see improvements in low speed handling stated in some references, but does anyone have any references to the approach speeds of the modified aircraft?

The KFIR has similar airspeed specs as the Mirage 5. However, I'm not clear on what you mean by "Approach Speed".

Approach and Landing Phase:

Vectoring Speed: 220-250 kts
Max Gear Speed: 270kts
Approach Speed: 190-200 kts
Touchdown Speed: 170-180kts

Source


Why do modern fighter jets use 20mm guns? Why do modern fighter jets use 20mm guns? Is the armor of a modern jet that thick? Does, for example, .50 (12.7mm) ammunition lack velocity to be effective?
I'm just curious on why is that 20mm is so widely used in air-to-air guns.

20mm is the smallest caliber that is practical to load with a bursting charge. Some fighter jets use 23mm, 27mm, or 30mm guns.

The reason 20mm has become one of the most common aircraft gun calibers is that it produces the lightest gun that meets the minimum requirements to be useful in air combat: enough rate of fire to hit a maneuvering jet, good enough ballistics for meaningful range, enough bursting charge capacity for meaningful damage.

The majority of modern combat jets do not carry armor, but they are built with considerable redundancy and internal subdivision. A bursting charge (explosive payload) is needed to do meaningful damage to a modern combat aircraft.

Smaller calibers can cause some damage that will require repairs, but not enough to rely on them to bring down a jet in combat. With missiles prioritized, most air forces want the lightest gun that's still effective.

Larger calibers have a cost in design. A large caliber rapid-firing gun will be much heavier, its propellant gases (muzzle exhaust) can cause flameouts in the engine, and firing will throw the aircraft's aim off center. A slower-firing one can be suitably light, but still has the same recoil force. Another issue with large calibers is collateral damage - 30mm+ rounds send fragments a couple hundred meters, far enough to damage friendly aircraft in air combat maneuvering.

The 20mm rounds used by combat aircraft have limited armor-piercing capability anyway (comparable to .50 BMG). Armored aircraft are rare, and some exceptions include CAS aircraft such as the A10 and to a lesser extent the Su-25 and the Su-34. This armor is mostly designed to protect them against ground fire, not enemy fighters.


Why do airplanes have to retract landing gear? Why do all the airplanes have to retract their landing gear, once they reach some specific height? Why can't they simply leave their landing gear deployed all through the flight?

In the early days of aviation it was simply easier to have the gear fixed. The landing gear adds drag, but so does a second wing and wire braces. Airplanes generally did not fly very fast or far, so the drag from the landing gear (and that other stuff) was not a big issue. The first design with retractable gear dates to 1911. Since then, retractable gear has been a design decision each airplane must make.

Retracting the gear into the airplane allows for a cleaner form, which reduces drag. However, this is at the expense of added weight. The retract system, which is usually hydraulic, must be added, and the plane must be designed to make room for the gear somewhere at least mostly inside the aircraft. The plane must also be designed to handle situations where some or all of the gear does not extend properly. For planes that need to be fast and/or efficient, retractable gear is worth the reduction in drag.

The additional drag has to be supported by the structure as well. Aircraft with retractable gear will have a maximum speed at which the gear can be extended. Even gear with proper fairings would add significant drag at the speeds airliners fly at. This would be much more critical at supersonic speeds. In these cases the weight of reinforcing the gear could be even higher than the weight of the retracting system.

Particularly smaller general aviation planes tend to not have retractable gear. The retract system adds extra weight and complexity (and therefore cost), which will be fairly significant in a small plane. Fixed gear is simple and can be designed to minimize drag as much as possible. These planes do not typically need to fly fast or have large range, so the added drag is less of an issue. Other small planes do have retractable gear, which will allow for greater speed and range. The PA-32 type shown in Dave's answer was later produced in a retractable gear version as well.


What is the maximum altitude a skydiving plane can fly? Is there a standard maximum altitude which skydiving planes can fly?

I am mainly asking about the sports activity, not military parachuting.

Qualifier: I am a master parachute rigger and taught sport skydiving for 10 years.

Standard jump altitude is 12,000 feet AGL - this is one turn of the standard parachutist's altimeter. This height provides 1 minute of freefall, is within reach of a Cessna with a decent engine and does not require supplementary oxygen. For most places, this means an operating altitude between 12,000 and 15,000 feet MSL.

Dropzones in high areas like Denver will reduce their jump altitude to 9,500 feet AGL mainly because the piston-powered planes take too long to climb higher and/or the upper part of the climb runs into the oxygen zone.


Are the military tilted-thrust aircraft in the film Avatar feasible? As full-scale aircraft or as models / UAVs / drones?  Is anyone working on making these real or are they fundamentally flawed designs, or needing new materials to be invented?

I presume you're talking about the fictional Aerospatiale SA-2 Samson. From that page:

  Ducted fan VTOL aircraft have been flown since at least the 1960s. A notable example was the Bell X-22, a quad-ducted fan experimental aircraft built for the US military. It has no relation to the later V-22 Osprey tiltrotor. Though successful, no military aircraft since have used that technology.

The Bell X-22 looked like this:

which has similar overall characteristics to the SA-2 depictions.


Do pilots on approach receive automated warnings of sudden windshear or seismic motion? I've read about Windshear Awareness and there is mention of a Predictive Windshear System (PWS) which I believe is a function of the radar system of the aircraft itself.

To complement the reactive windshear system and provide an early warning of potential windshear activity, some weather radars feature the capability to detect windshear areas ahead of the aircraft.
This equipment is referred to as a Predictive Windshear System (PWS).

Are there any gound-based systems that detect possible windshear situations that generate automated warnings directly to planes on approach? Or is the information only distributed verbally by controlers  (e.g. LLWAS)?
Albeit exceedingly rare, are there any automated warning systems to pilots indicating other sudden dangers, for example large seismic motion? For example, the automated stopping of high-speed trains - sometimes before the sesimic wave even reaches a particular train - is being explored (in places where earthquakes and high speed trains are both frequent).

Windshear, yes. Seismic events, no.

Most major U.S. airports have a LLWAS -- Low-level Windshear Alert System, which is essentially an array of anemometers around the airport which detect wind speed and direction.  The values from each station are processed by a computer that uses algorithms to detect a windshear or microburst event, and it notifies controllers in the tower.  As a simple example, if the anemometers around the perimeter of the airport are all pointing toward the center of the airport, that's a strong indication that there is a microburst over the center of the airport, with air descending into the ground and then spreading out in all directions.  Obviously, the algorithms can detect cases beyond just that.

And, many airports supplement that system with Doppler radar as well.

Regardless of how they're generated (LLWAS with anemometers, Doppler, pilot reports, visual observations of blowing dust, etc), the warnings are broadcast by the tower controllers, rather than by any automatic system.  In the aircraft, aural warnings from the GPWS (reactive windshear -- noting changes in groundspeed and sink) and the EGPWS (predictive windshear -- returns from the aircraft weather radar) are automatic, but they don't block a radio frequency.  Automated alerts over the tower frequency might block an even more important transmission, so it's left to the controller's judgement when to broadcast the alerts.  (They're usually pretty prompt!)

No airport I'm aware of has a system similar to what the OP described for seismic events.  I would assume that the risks to a high-speed train from an imminent earthquake -- i.e. the train may derail at high speed with catastrophic results -- are far greater than the risks to aircraft.  I've never read anything about what happens when you're landing or taking off in an earthquake, but I'd doubt that anything other than the most severe would have much effect.  And if the earth is shifting enough that you can't keep the aircraft on the runway, well, it's a really bad day for pretty much everybody in that state that day!


Does hypoxia affect pilots differently from people on the ground? In my experience, many people don't have serious problems camping, skiing, hiking or doing other physical activities at 12,000 feet (3,600m).  We may get tired easily, and be out of breath, but we still function physically and mentally.

But for pilots (who aren't exerting themselves physically), hypoxia is a much more serious problem, e.g. see this article. Pilots aren't involved in heavy physical activity the same way mountaineers are, yet 91.211 requires O2 for periods as short as 30 minutes.

Is it accurate to say that hypoxia hits pilots harder than campers for some reason? Is there a clear reason why?

The analogy that if someone can become acclimated to working at high altitudes and therefore should not need supplemental O2 in an airplane is flawed. While flying - at least straight and level - does not require strenuous exertion, it is very cognitively taxing. It's a constant process if taking in a deluge of incoming data, be it instrument read outs, navigation information, external stimuli, radio chatter, the grunt work of flying the aircraft etc.

Unfortunately, the conscious and cognitive centers of the brain as well as the eyes require an oxygen rich blood supply in order to operate fully; if this is denied or reduced, serious deterioration of these abilities will occur and can happen rapidly within seconds.

Night vision is also severely impaired by hypoxia as the rods in your eyes require a lot of oxygen as well. I did an experiment one night in C172 where I took off at sea level and climbed to 12,500 feet. Looking outside the aircraft at those altitudes I could see a few bright stars but after I had landed back at close to sea level, I looked up at the night sky while tying down the airplane and could see a sky filled with stars as well as the bright gaseous strip of the Milky Way. The effect was dramatic; you really are affected by hypoxia at night.


Is a recoil start (hand propping) allowed if it isn't mentioned in the aircraft manual? Is a recoil start (hand propping) allowed without it being referenced in the aircraft manual? In a Cessna 150, for example.

You didn't say exactly what "allowed" means, e.g. do you mean approved by the manufacturer, or legal in a certain country? But if you're asking is it legal in the US (under FAA regulations), then the answer is yes.

AOPA has a nice article on exactly this question, called Hand propping: A legal primer. It says:

  There is no specific FAA regulation that applies to hand propping an
  airplane, either to prohibit it or to direct how it is to be done

But:

  The FAA contends that hand propping is a two-person operation and has
  expressed this view in the Airplane Flying Handbook (FAA-H-8083-3A)
  under the section titled “Hand propping.” Of course, this publication
  is not regulatory, but the NTSB was surely influenced by it in a 1983
  legal decision. In that case, the FAA sought to suspend a pilot’s
  certificate for being careless or reckless when, while attempting to
  start a VariEze experimental aircraft, it “got away” and ran into a
  parked aircraft.

The NTSB concluded that the pilot had violated 14 CFR 91.13 (careless and reckless operation):

  The board affirmed the administrative law judge’s finding that there
  had been a 91.10 (now 91.13) violation

That means that hand propping itself isn't illegal, but doing it wrongly and against the FAA's general procedures could get you in trouble:

  There have been at least two previously issued NTSB (full board)
  decisions and one subsequent decision that refer to these generally
  accepted procedures and precautions for hand propping. The precedent
  has been set. So, hand proppers beware; if you fail to follow proper
  precautions and the airplane gets away, the FAA might pursue action
  against you for being careless or reckless.


Can an APU be designed to also provide emergency thrust in the event of a double engine failure? Commercial airliners seem to be moving towards twin jets with incredibly powerful 100,000+ lb thrust engines becoming available for use.  Would it be possible to design the APU to provide enough thrust to push an aircraft to a nearby runway in the event they
both fail as a safety measure?

No, it would not be possible with current technology.

To scale the APU to an engine that provides thrust, you would basically have to turn it into another engine, with significant weight penalty. The position of the APU inlet and outlet would have to be re-designed, not to mention mechanisms that allow it to produce variable thrust. In short, it would be turning the plane into a tri-jet.

Furthermore, the chance of a double independent engine failure are slim. If a condition would lead to both engines fail, it would very likely lead to failure of any remaining engines. Converting the APU into a thrust producing device would be a complex engineering exercise with little to no benefit in risk reduction.

Perhaps decades later, we can produce a powerful jet engine using lightweight materials.


Is an alternate airport always required when flying internationally? When flying a domestic US flight, an alternate airport is not required unless the weather is below specific minimums at the destination.

What are the rules when on an international flight plan and leaving or returning to the US?

In larger airplanes, carrying the extra fuel required to fly to an alternate (which costs money) is a little silly when there isn't a cloud in the sky and there are multiple airports in the vicinity of your destination that can be easily reached with your already required 45 minute IFR reserve.  We should always have a plan for when things go wrong, but if it can be done safely, is it legal to file an international flight plan without an alternate?

As a simple anwer, no, it is not always required.
ICAO Annex 6 4.3.4.3 Destination alternate aerodromes

For a flight to be conducted in accordance with the instrument flight
rules, at least one destination alternate aerodrome shall be selected
and specified in the operational and ATS flight plans, unless:
a) the
duration of the flight and the meteorological conditions prevailing
are such that there is reasonable certainty that, at the estimated
time of arrival at the aerodrome of intended landing, and for a
reasonable period before and after such time, the approach and landing
may be made under visual meteorological conditions; or
b) the
aerodrome of intended landing is isolated and there is no suitable
destination alternate aerodrome.

Also, there are similar descriptions for the requirements for take-off and en-route alternates.
EASA adds aditional requirements in OPS 1.295 in the form of maximum flight time of 6 hours and at least two runways at the airport, but it is still allowed. I can only assume FAA does a similar thing


Is the "effective wing span" reduced in a turn? The Ace the technical pilot interview book says that in a bank the aircraft's lift is reduced due to an effective reduction in wing span. I know in a bank the vertical component of the lift vector is reduced due to the inclination of another component of lift towards the banked side. So the overall lift required to counter the weight is increased. That's why we need to increase the AOA or speed to maintain a level turn. But I am a bit confused about how the effective wing span is reduced, can anyone explain?

Your understanding is correct.
"reduced effective wing span" is just an analogy to help some people understand the concept of lift in a turn. The Total Lift is not reduced, it is just applied in a different vector in relation to gravity. The vertical lift component (in relation to gravity) is reduced and you can then imagine that the effective wingspan is also reduced by the same amount.
Banked turn (from Wikipedia)


Why don't airports use arresting gears to recover energy from landing passenger planes? An average plane has 50t of mass and a landing speed of approx. 250 (lets be conservative) 200 km/h. Thus its kinetic energy at landing is about 75 MJ or 21 kWh.
The plane catches the wire attached to a pneumatic or hydraulic system, which will decelerate the plane and harvest its kinetic energy. The built up pressure in the system can be released via some kind of turbine to convert the pressure (potential energy) to electricity. Lets say the system has 50% efficiency. Leading to 10kWh harvested energy, per landing.

The airport in Hamburg, Germany is just average. Not large and not small. They had 150 000 flight movements in 2017. Lets say half of them are big planes and 1/2 were landings. That is 37,5k landings per year. Lets say 2/3 could use the arresting gear system:
This would provide a cumulative 250 MWh of harvested electricity per year. This is equivalent to the electric energy consumption of 100 two person households in Germany.
In Germany 1kWh electricity costs about 30 euro cents (including tax etc). An airport could save 75000 euro per year when harvesting this energy and use it, instead of buying it from the net.
I can't imagine, that such a harvesting system would not pay down after a couple of years.

Why there are no systems like this in the aviation industry?

Con:

You need to refit thousands of planes with a wire catching System
System has to be safe in case of a go-around
???

The energy unit costs for companies in Germany are much lower than for households. Where as a citizen you pay approximately 30 ct / kWh, the airport will pay much less, probably around 15 ct / kWh.

Adjusting your calculation for this price, we have 37.5 k€ per year in terms of energy cost saving to offset the depreciation and maintenance of the installation.

Let's assume our bean-counters want to see their investment money back in 10 years (10% depreciation per year), and the yearly maintenance costs are 10% of the initial system price. That means the cost of such an installation at Hamburg airport can be at most 187500.

But mind you, Hamburg has two runways, and each can be used in two directions. To catch 2/3 of the total traffic, we would need to install at least two systems. Runway 23 and runway 15 combined get approximately 75% of the landing traffic, they would be the preferred choice. (traffic percentages per runway are here)

This brings the maximal cost of a single installation to 95 k€; including project management costs, costs of the equipment, cost of the installation works, cost of closing the runway etc etc.

That is absolutely nothing! You can't develop and install a system that needs to recouperate 21 kWh in thirty seconds (the duration of the deceleration phase) for such a price.

And then there is the airborne side of the story. Who is going to pay for the modifications needed on the aircraft, such as an arrestor hook? And for the fuel that it costs to carry this hook around during flight (a hint, it will cost more energy to carry this extra weight around all the time, than that it would harvest at the airport)? And who is paying for the certification of the system?

There is no business case for such a system, not even close.


How do electric anti-icing systems work? How do they work for GA propeller planes? More specifically, how is it implemented on the leading edge of the wing and propeller blades? I'm more interested in the mechanics of electric heating, not the theory or where it gets power from (alternator or batteries).

What are the pros and cons of electric anti-icing compared to other systems?

How do they work?

An electric current is passed through a heating element built into the device needed to be heated. For example, 115 VAC may be applied to a heating element of anywhere between 5-300 ohms (resistance values vary widely depending on the heating application). The voltage applied may be dependent on a controller that will dictate how much voltage is applied.

Using basic Ohm's Law, we can deduce that the current draw of 115 VAC across an example 60 ohms coil, would be approximately 1.9 A, and the power 220W. Keep in mind, this isn't a strict calculation as phase angle and other variables might be involved in an AC circuit - but close enough... That 220W is dissipated as heat which thus heats the device.

This is virtually the same in most GA and commercial applications.

Are they viable?

If by viable you mean "They work, but do have occasional failures that falls within a reasonable rate of expectation," then yes, they are viable.


How much did Avgas cost in 1940? I am interested in historic fuel prices;

How much did Avgas cost in 1940?

100 octane fuel was the best possible aviation fuel at that time,introduced in April 1940 .By December 1942, production of 100 octane gasoline had risen to 5.5 million gallons/day; by June 1943, it was approaching 10 million gallons/day; and, by VE-Day, May 7,1945, production exceeded 20 million gallons/day. Avgas was introduced near 1970s. And, then it is very difficult to find exact price for that . But, the at that time the price of car fuel and aviation fuel were not very different. So, you can estimate it to be at 10-12 cents/gallon range. One other way to estimate the cost is by using the inflation adjusters, the current price for 100LL is about $5.5 . So, by using this calculator, it would have cost nearly 10 cents/gallon. (In 1940, Dollar and Pound were nearly same in value.) So, the final answer should be about 10 cents/gallon.


Pointers to Aviation Scholarships I want to become a pilot, but as you know, I should take some training course on aviation. Currently I am searching for Aviation Scholarships because I can not sponsor myself. Can you give me some pointers please?

Scholarships dot com has a list of a few scholarship programs:  https://www.scholarships.com/financial-aid/college-scholarships/scholarships-by-type/aviation-scholarships/

However I am not sure if you are looking to get aviation training for a career, or for personal fulfilment.  There are also some schools that offer various aviation related programs (Embry-Riddle being the best known in the US).  Some of them may have separate scholarship programs that are not specifically listed on the Scholarships.com site.

There are also some places throughout the world that offer "learn to fly" programs.  I have a friend that works for a company that even offers their employees all the coursework and flying required for a private pilot license as a training subsidy (Kollsman, in New Hampshire).


What is the direction of the sideslip induced by roll on a conventional aircraft? For the sake of this question let's assume that the considered aircraft is an A320. The question is what is the sign of the aerodynamic $C_{n_q}$ coefficient? In another word, when the aircraft starts rolling, will the aircraft experience sideslip and if yes in which direction (and most of all why) ?

Instinctively, I would say that if the right wing (seen from behind) goes down (positive roll rate), it will be delayed compared to the left one, however I already read somewhere that the coefficient tends to be zero (no sideslip induced). I even heard some aeronautical engineers say that it is the opposite...

If needed, we can set that the x-axis is pointing backward and z down.

To answer what you wrote: $c_{n_q}$ is the yawing moment induced by a pitching motion. In a conventional, symmetric configuration this is zero for the airframe and has a small value for propeller aircraft, depending on the rotation direction of the propeller. If the tractor propeller is spinning clockwise (when seen by the pilot), a positive pitch motion $q$ (nose up) will cause a small negative sideslip angle $\beta$. A positive pitch acceleration will add a precession moment which similarly pulls the aircraft's nose to the right. But I think you want to know something else.

To answer what you meant: $c_{n_p}$ is the yawing moment induced by a rolling motion. The axis system you propose (with z pointing up) is used to measure relative positions within the airframe, but for flight mechanics we prefer to put the x-axis in flight direction, y to the right and z down. A positive rolling motion $p = \omega_x\cdot\frac{b}{2\cdot v_{\infty}}$ (or $p = \omega_x\cdot\frac{b}{v_{\infty}}$, both definitions are used) is right-wing-down, and a positive yawing moment means the aircraft's nose moves to the right, causing negative side slip angles.

When a positive rolling motion happens in still air, the resulting yawing moment is negative, pulling the aircraft's nose left, as Federico said. Thus, $c_{n_p}$ is negative.

But the reason is different: When the pilot deflects the ailerons and causes a difference in lift on the left and right wings, and consequently a difference in induced drag, the yawing moment caused by that difference in induced drag is called $c_{n_\xi}$, the yawing moment due to aileron deflection. $c_{n_\xi}$ is positive.

Once the aircraft is in a steady roll, the roll damping $c_{l_p}$ will fully compensate the differential lift caused by aileron deflection. Now the rolling motion will increase the local angle of attack on the downward moving wing the farther you are away from the center, and decrease it on the upward moving wing. Since lift is perpendicular to local airflow, the air force vector on the downward moving wing is pointing slightly forward, and that on the upward moving wing slightly backwards. Please note that the total lift produced by each wing is roughly the same! The rolling moment of both is exactly zero once the roll rate is steady. However, the aileron deflection will decrease lift on the downward moving wing such that it just compensates for the increased angle of attack due to the rolling motion, and the same holds for the upward moving wing with its trailing-edge-down aileron deflection.

What is different is not the magnitude, but the direction of lift on both wings. This pulls the downward-moving wing forward and the upward moving wing back, causing a negative yawing moment in a positive rolling motion. That is why your conventional $c_{n_p}$ is negative.


Are pilots expected to keep an ear out for conflicting authorizations? I just watched the Mayday episode Cleared for Disaster, a dramatization of USAir Flight 1493. The episode covered how the controller lost situational awareness and gave a landing clearance to 1493 while SkyWest 5569 was still on the runway, as well as how the runway lights obscured 5569's lights. They did not, however, talk much about 5569. This was at least partly because 5569 was not carrying a CVR.

The tower would have given 1493 the clearance to land on runway 24L on the same frequency that 5569 should have been on. In other words, 5569 would have been sitting on 24L and hearing tower give landing clearance for 24L, which would be a big red flag. Are they expected to be listening and thinking about landing clearances given to other aircraft? If they noticed, would they be expected to speak up, or to shut up and trust the controller?

The short answer is yes, they are definitely expected to listen and act accordingly. This is one example of situational awareness in aviation, and pilots are expected to use all useful information sources to maintain awareness of what's going on around them.

A nice example is this one where the crew of a US Airways aircraft avoided an accident. There was heavy fog at the airport, and while the US Airways flight was waiting for takeoff clearance a United aircraft ended up on a runway instead of a taxiway. The controller couldn't see the runways because of the fog and she believed the United aircraft was on a taxiway, so she gave the US Airways flight clearance to take off even though the United pilot said several times that they were on a runway.

The US Airways pilot had heard the discussion (actually, almost a debate) between the controller and the United pilot, and he refused the takeoff clearance because he wasn't confident that the situation was under control. It's well worth watching the whole thing, but the key exchange is this:

  ATC: [...] USAir 2998, runway 5R, fly runway heading, cleared for
  takeoff

  US2998: Uh, tower, USAir 2998, 'til we figure out what's going on down
  there, we're just gonna stay clear of all runways

This level of awareness is expected of all pilots, including light aircraft pilots at uncontrolled fields. Although in that situation there's the additional complication that there may be aircraft around that don't even have radios!