What is the difference between types of propeller? I have been noticing almost every Multirotor vehicle uses a single thin double-bladed propellor on each axis. And yet clearly, there are some that do not fit the norm.

Can someone explain what the effect of each of these changes would do to a normal quadrocopter?

1) Adding more Propellors.

2) Adding more Blades on each Propellor.

3) Replacing all Blades with Wider Blades.

4) Making the angle of each blade steeper.

5) Replacing propellors with Contra-rotating propellers

6) Spinning Propellors faster

There should be two parameters you should know about:

Blade loading: The amount of force per unit of area a blade is creating. Like with a wing, you can formulate a lift coefficient, a dimensionless number which removes the influence of size, air density and speed. Ideally, this number should be between 0.5 and 0.7, so the propeller makes best use of its blade area.
Advance ratio, which is the ratio between the circumference of the propeller disc and the height it would screw itself up into the air in one rotation. Increasing the blade angle would directly increase this advance ratio, for example.

Now you should be able to guess the answers to your questions:

Adding more propellers: Will distribute the load over more propellers and helps if blade loading was too high, but makes matters worse if blade loading was low already.
Adding more blades: See 1.
Replacing blades with wider blades: See 1.
Making the angle of all blades steeper: If you run the prop at a lower speed, your blade loading will go up and thrust will be the same (if you don't mess up the flow over the blades by changing the angle). If you run the prop at a higher speed, blade loading will go up; again until the flow cannot cope with the angle anymore. This is similar to stalling a wing. Changing the blade angle is the preferred means of adjusting the prop to different flight speeds in airplanes.
Adding a second, contra-rotating prop is like adding another sets of props (see 1), but helps to reduce swirl losses. If blade loading was at a proper value before, halving blade loading will create more losses than reducing swirl losses. You add contra-rotating props only if your blade loading was high already.
Spinning propellers faster will create more thrust but also more losses. If your thrust is not sufficient, it is better to increase prop diameter (a parameter you forgot to include).


When may pilots leave the flight deck during flight? Are there regulations as to when commercial pilots may leave the flight deck during flight, i.e. due to physiological needs? Anytime when it is practical (obviously it would not be practical during take-off, approach and landing), or only during the cruise period (i.e. controlled resting only allowed in cruise)?

14 CFR 121.543

  (a) Except as provided in paragraph (b) of this section, each required flight crewmember on flight deck duty must remain at the assigned duty station with seat belt fastened while the aircraft is taking off or landing, and while it is en route.

  (b) A required flight crewmember may leave the assigned duty station -

    (1) If the crewmember's absence is necessary for the performance of duties in connection with the operation of the aircraft;

    (2) If the crewmember's absence is in connection with physiological needs; or

    (3) If the crewmember is taking a rest period, and relief is provided -

      (i) In the case of the assigned pilot in command during the en route cruise portion of the flight, by a pilot who holds an airline transport pilot certificate not subject to the limitations in § 61.167 of this chapter and an appropriate type rating, is currently qualified as pilot in command or second in command, and is qualified as pilot in command of that aircraft during the en route cruise portion of the flight. A second in command qualified to act as a pilot in command en route need not have completed the following pilot in command requirements: The 6-month recurrent flight training required by § 121.433(c)(1)(iii); the operating experience required by § 121.434; the takeoffs and landings required by § 121.439; the line check required by § 121.440; and the 6-month proficiency check or simulator training required by § 121.441(a)(1); and

      (ii) In the case of the assigned second in command, by a pilot qualified to act as second in command of that aircraft during en route operations. However, the relief pilot need not meet the recent experience requirements of § 121.439(b).

tl;dr:
You need at least one pilot at the controls at all times, unless completely necessary to operate the aircraft, or take care of physiological needs.


How are gyros used on a turn indicator? So I understand how precession works, and that gimbals are used to isolate the gyro from outside rotation, however when a yawing motion is applied to the gyro, it'll essentially translate to a rolling (horizontal) motion on the gimbal to turn the indicator.
Since the gyro maintains its plane of rotation, would it be acceptable to say that if I applied force to a gyro, the precession from the gyro would be displayed on the gimbal not the gyro, since it has to maintain its plane of rotation?
To summarize, I'm just wondering how a gyro's precession affects the gimbals or how it affects the instrument if it doesn't affect the gimbals.
Thank you!

The picture given is not complete, it is missing several components. Unlike the other two gyros, the turn coordinator is not allowed to remain rigid in space while the gimbal moves freely around it. The plane of rotation is forced to change with changes in the heading, because of this it does not use a full gimbal. It has one axis other than the gyro core. The housing applying force to the gyro causes precession. In other words, the gyro would like to remain fixed in space but the housing applies force as the aircraft changes heading and bank angle. The resulting precession is restricted by springs.
The higher the rate of change in heading the more torque is generated by the gyro attempting to precess. This force can be measured by the change in spring length, indicated as movement on the gauge. By tilting the free axis up from zero pitch, changes in bank angle will also cause precession. With 90 degrees or a vertical axis the instrument would only precess with changes in bank, angles between 0 and 90 provide a varying mix of the two signals.(Angling the axis down will cause bank to react in the opposite direction, usually this is not desired.)


Why do modern fighter aircraft still use transparent canopies instead of cameras feeding imagery to screens? The F-35 loves to boast about the HUD letting pilots see 'through' the aircraft, but I was wondering why they even have a transparent canopy instead of embedding the pilot deep in the fuselage like the BAe P.125 (https://www.reddit.com/r/WeirdWings/comments/b240pq/bae_p125_the_cancelled_british_equivalent_to_the/)?

There is no major problem to be solved by moving the cockpit inside.

The canopy adds minimal drag, considering how much drag the whole of a fighter's airframe produces. It also offers the highest quality of vision.

To expand on the latter, let's define a few items that go into visual quality:

Field of view
Acuity, including in motion
Noise
Dynamic range
Reaction speed

The canopy's field of view is about half the sphere, while cameras can cover most of the sphere. A win for synthetic vision.

Good human vision (healthy young pilot) offers enough acuity to distinguish high-contrast objects down to about 0.1 milliradian in size. The best commercial video camera heads offer 4.5K*2K to 8K*4K resolution. To cover 12.6 steradian with 6 cameras, restricting each camera to a ~2:1 rectangle, somewhat correctable with an anamorphic lens... It's a textbook math problem, but we have the advantage of knowing it's not solved in a mathematically optimal way in real fighters using it.

For a "cube with overlaps" solution, which is viable for a fighter, the FOV of each camera needs to be about 1.6 radian vertically. This results in a best-achievable instantaneous resolution of 0.5-0.7 milliradian for Arri sensors or 0.4 for the RED (which has other drawbacks). These are commercial cameras that are miles ahead of the F-35's DAS, but indicative of what could be used in a fighter designed today and meant to fly in the 2040s.

This can be improved, 6 cameras are not some hard limit, but you're looking at 100+ to fully match the acuity of the human eye (a more accurate term than resolution, since the eye doesn't perceive the whole FOV simultaneously). At that point the system won't be reasonably compact and easy to add anymore.

At their maximum resolution, all cameras produce significant amounts of noise.  So does the human vision, but its noise is filtered out very well by the brain and almost eliminated in the macula. For small megapixel-chasing cameras, the noise is so heavy that their effective resolution is reduced several times. Above we assume professional cameras with full-format sensors. At any rate, camera+eye give more noise than just the eye.

The dynamic range of human vision, at 20 stops, is better than the best cameras at 15-16 stops, and much better than LCD at 10-12 stops (20 for OLED). But this is acceptable in daytime, and will mostly matter at night, where a fighter can rely on IR systems instead. The night-time performance of IR cameras, even though they lose a lot in resolution, gives synthetic vision a win on this.

For reaction speed, at their open gate resolution, cameras offer a best-case framerate of just 60. I won't copy the full math, but combined with a 60 fps display, this results in a minimum lag of ~40 milliseconds. This sounds acceptable at first. The real pain comes when you consider the motion resolution resulting from this low framerate.

To explain motion resolution, see how a moving picture looks on a display. This can be better demonstrated with a few illusions. Unless the motion is very slow, your motion resolution can drop from the camera's 8Kx4K to only 100-600 lines depending on the rate. In practical terms, you're going from discerning objects at 0.4 milliradian all the way to 2-10.

This won't matter much in level flight, but the moment a pilot with synthetic vision begins to maneuver, they will suffer major degradation of their vision. Video gamers learn to compensate by turning instantly and returning to a slow-moving image that can be seen at sub-1000 fps, but a plane doesn't do things instantly. Whether the ability to see while spinning will make a difference, losing it can affect pilot behavior just by trying to avoid that motion-induced blindness.

All in all, you've reduced the pilot's visual acuity, added noise, and greatly compromised their motion resolution - for what? We're not chasing Mach 3 anymore; it won't help against Mach 8 SAM. We've learned to make canopies stealthy through shaping and RF-reflective metal coating, so there's no need to shy away from one. We've learned to project images onto a transparent visor, so we can get the benefits of night vision and extra FOV without losing natural vision.

Most importantly, in the last 10 years we've also learned to protect data with authenticated encryption and send it fast over vast distances. So if things like best possible acuity, motion resolution, reaction speed, or tolerance to avionics damage don't matter much, there's no reason for the pilot to be inside the aircraft at all.


Why isn't transition altitude standardized in Europe? In the US, the transition altitude is standardized at 18,000ft, however in Europe it is different for every airport.

Why is it not standardized like it is in the US? I could see situations where someone has their altimeter calibrated differently, and thus giving different altimeter readings, which would seem dangerous, especially for smaller GA aircraft that might not have TCAS?

You're right that having a common transition altitude is better- efforts are underway in Europe to set a common transition altitude. Multiple studies have been carried out by Eurocontrol and it has been noted that it is better to have a single transition altitude. For example, from an aircrew prespective:

  The multiplicity of transition altitudes and the national rules and procedures make the European environment  very  complex.  This  can  lead  to  a  lack  of  altitude  awareness  and  altimeter  mis-settings  and  is  operationally  unsatisfactory.

and

  Establishment of a common transition altitude has a clear safety benefit.

Similarly, from the ATC perspective:

  The establishment of a common TA for ECAC States and the EUR Region is a  fundamental element in achieving the goal of a unified sky and the safety policy of reducing risks to the greatest degree practicable.

The reason such an unsatisfactory state of affairs have persisted is that people have become used to that- the second document makes that point:

  ATC providers, as is human nature, grow comfortable with what they are most familiar.

The main reason is that there is no requirement for any common transition altitude. The relevant ICAO document simply states:

  2.1.2.1    A transition altitude shall normally be specified for each aerodrome by the State in which the aerodrome is located

and

  2.1.2.3    As far as possible, a common transition altitude should be established: ...

As a common transition altitude is not exactly a requirement (in-spite of its advantages) different European states have historically established their own TAs, to be decided by the aerodromes or the regulatory authorities.

As they have become comfortable using it (and no major accidents have happened because of this), there is no serious move towards the establishment of a common transition altitude (through arguably, this is the case with much of the world- common transition altitude is not that common). In the UK, CAA has launched a consultation process to raise the transition altitude to 18,000 ft, which notes that,

  ... it has been agreed that the TA in the UK will be raised to 18,000ft.


How secure is an airplane on the ground whilst passengers are boarding the aircraft? On my recent flight to Ibiza, as I and the other passengers were boarding, there were many occasions where I could see right into the cockpit as the pilots and cabin crew were communicating back and forth for whatever pre-flight checks they were conducting. Being near the front of the queue at the gate, I was one of the first seated, the 10-20 minutes it took for the rest of the passengers to board was the time I refer to when I say I was able to see into the cockpit.

With it being my first flight in quite a few years, understandably I was extremely nervous. From the open cockpit to the smell of burning as the engines spooled up...

If we ignore terrorism, what is stopping a drunken passenger from, instead of turning right when they board at the front loading door, turn left and take a few steps into the cockpit, and if they were able to get into the cockpit, assuming they know absolutely nothing about the operation of a plane besides that pushing that pushing the throttles forward = more thrust = plane moves (faster), what damage could they do with a plane parked at a jetway or a plane parked out on the tarmac that requires a shuttle to drop passengers to the plane in order to board it.

If we introduce terrorism into the mix, what is stopping a group of strong men from forcing their way into the open cockpit past 1 or 2 cabin crew?

Those guys would gain no more than they'd gain if they hijacked the bus taking you to the airport.

If fact they'd gain less as they'd have nowhere to go and there's a lot more (armed) police at the airport to take them down than there is in some remote place they could drive that bus to.

The greatest risk to airliners isn't people forcing their way into the cockpit on the ground who are among the passengers waiting to board.
It's in the ground crew who can (at least in theory) bring aboard almost anything they wish (including bombs) with very little oversight (in fact this has happened in the past).
And there security has been seriously tightened (and it needed to be) over the decades, making that scenario far harder.
I won't say impossible, because nothing is impossible as long as there is a human factor involved anywhere in the chain, but bloody hard.


Can a holder of a G4 Visa legally receive flight training and/or acquire a Private Pilot Licence? I am planning to attend a flight school and acquire a PPL, but I am currently staying in the US with a G4 Visa. Am I allowed to receive flight training legally?

You would need an M-1 (for training at a flight school) or F-1 (for training at a college & some flight training academies) Visa to receive flight training.

More information here.


What sort of issues related to extreme heat (115°F, 46°C plus) would prevent takeoffs or landings? I recently flew into and out of McCarran Airport Las Vegas (LAS) and a local told me that they never report the air temperature at the airport above 115°F (46°C) because the FAA does not allow landings above that temperature.

What sort of issues do large passenger jets (or small for that matter) have to deal with in high temperature environments either taking off or landing?

Short Answer

Very high temperatures limit how much payload and/or fuel you can load on a plane.

Engines

To help you visualize it, in hot air the air molecules are more energetic, which means the engine will have hard time compressing hot air vs. cool air. Losing thrust.

Wings

Hot air—with the molecules spaced farther apart—has less density, which reduces the lift capability of wings.

Hot and High

Vegas is not only hot, but also high, which means the air pressure is already less than at sea-level.

Each plane has its own performance charts, and on each flight, the flight dispatcher and crew make sure the weather, elevation, etc., would allow a safe take-off and climb.

A plane is configurable for such changing elements. For example using higher thrust setting, or more flaps to augment the lift.

If you have a very heavy plane, hot and high situation, and a short runway, then things become very marginal—

Engine Out

—not just for the take-off run, but also in case of an engine out during take-off, i.e., losing an engine. The plane still has to be able to climb.

That's why in places like Ethiopia, South Africa, Australia, etc., airlines preferred1 quad-jet planes over twin-jet planes. To have bigger margin of safety in hot and/or high operations, i.e., to be allowed to carry heavier loads.

1 When tri/quad-jets were commonly offered alongside ETOPS twin-jets of same class.

Example of Impact

(Map generated using Great Circle Mapper)

A cargo MD-11 plane taking off from Ethiopia en-route to Europe can't take off fully fueled, and has to stop in Cairo for a re-fuel. Note that if Ethiopia were at sea-level or cooler, a direct flight would have been possible.

Landing

For landing in a hot and/or high airport, the landing speed will be higher, so what matters is the runway length available for landing. Again there are performance tables for that.

Rest assured, those things are checked and double checked. And no one can tamper with reported weather, because it is very critical to flight safety.

Also, planes do have their own temperature sensors. Newer planes can even alert the crew if the gross-weight, configuration, runway length, temperature, and elevation combo isn't sufficient.


Is an approval required to fly from the right seat? I asked this as a comment in this question, but I thought it was worth asking properly to preserve it for future Aviation.SE generations.

Is there any sort of permission required to fly from the right seat of an aircraft? If I've done all of my training on the left side of a C172 is there anything stopping me from deciding one day to stick a passenger in there and fly with my right hand?

I acknowledge that for experienced pilots there shouldn't really be a safety risk in doing this, but many light aircraft displays/instrumentation are designed for the pilot being in the left seat.

As Ron mentions in the comments here in the US it is not illegal by any specific regulations however...

Most GA aircraft (the smaller ones at least) only have a single set of instruments that reside in front of the left seat as you mention in the question. Lets say hypothetically you were flying right seat and something went wrong. Due to the angle of view you were not able to read an instrument and this lead to an accident. Lets say you lose your primary CDI/Glide Slope while flying an ILS approach and you need to use one on the far side of the cockpit. For some reason you are alone and in the right seat. If an accident did occur as a result of this you may be able to be held to

  §91.13   Careless or reckless operation.

  (a) Aircraft operations for
  the purpose of air navigation. No person may operate an aircraft in a
  careless or reckless manner so as to endanger the life or property of
  another.

Since the left seat was open and available for you. But other than that there is no regulation preventing such operation.

Ron  posted an excellent article in the comments below that has at least one quote worth mentioning

  Aircraft insurance policies and flying club rules often specify that
  all flying is to be done from the left seat unless the pilot holds a
  current flight instructor certificate or has specific authorization.

While this is not an FAA regulation insurance companies or the rental/flight club you get your plane from may stipulate left seat only flying.


Regarding 49 CFR 830, what is considered an in-flight fire? 49 CFR 830.5 requires immediate notification for in-flight fires.

Does that mean there must have been visible plasma?  Would the presence of smoke alone be enough to report that there was a fire, or would something that simply smolders not meet the standard of an in-flight fire?

No, there does not need to be visible plasma, and smoke alone is enough to constitute a fire onboard an aircraft.

FAA Advisory Circular 120-80A contains the following definitions that indicate that a visible flame is not the only definition meaning "fire" pertaining to aircraft.  In my experience, it's reasonable to expect that the FAA's definitions in this case would fall in line with the NTSB's definitions.

Hidden Fires:

  Fires that are “hidden” are not readily accessible, may be difficult
  to locate, and are more challenging to extinguish. Some examples of
  hidden fires would be fires behind sidewall paneling or in overhead
  areas.

Smoldering Fire:

  Combustion without a visible flame and a slow combustion rate are
  characteristics of a smoldering fire. A smoldering fire left
  unattended or an incompletely extinguished fire can ignite and grow
  into a larger, uncontrollable fire in a short time.

Suppressed Fire:

  A fire that has been partially extinguished which may or may not have
  visible flames. A suppressed fire, if not extinguished, may
  reestablish itself and grow into a larger, uncontrollable fire in a
  short period of time.


How do preceding aircraft affect the ILS signals? (airliners.net) Planes on approach in sequence to London Heathrow.

When LVP is in effect, the ILS area is protected.

I'm wondering, when there is a queue to a runway, how does the long queue affect the signals for those behind?

I'm guessing it does somehow, and the answer may shed some light on the new Enhanced ILS.

I've never noticed any perturbation of the ILS signal from aircraft ahead in flight. Aircraft on the ground crossing the runway, absolutely yes. Something big enough (747 or A-380) exiting at the far end, perhaps briefly, but not generally something you notice.

I'll let somebody else explain the theory, but that's my observed results.


Is there any requirement that airports have distinct names? Is there any requirement that airports have reasonably distinct names from each other?

I've known about SYD versus YQY for a while, but I've just read that there's two airports mentioning Genghis Khan: ULN in Mongolia and NZL in Inner Mongolia in the PRC.

There's no requirement for names. Location codes can't be the same though.


What should I do to resolve an incorrect birth date on my FAA medical certificate? While taking my pre-solo exam, I noticed my medical has my birth date one year earlier than it actually is. I am expecting to solo tomorrow.

Would this error legally prohibit me from soloing until it is corrected?

I read the FARs for this one, and there is a rule:
14 CFR §67.403 Applications, certificates, logbooks, reports, and records: Falsification, reproduction, or alteration; incorrect statements.
From what I gather, the rule is there to get liars in trouble.
If you didn't lie, which you clearly have no reason to because your age does not disqualify you, I don't think you have to worry about getting in trouble because it was the doctor's mistake.
However: (From 67.403)

(c) The following may serve as a basis for suspending or revoking a
medical certificate; withdrawing an Authorization or SODA; or denying
an application for a medical certificate or request for an
authorization or SODA:
(2) An incorrect entry, upon which the FAA relied, made in any
logbook, record, or report that is kept, made, or used to show
compliance with any requirement for a medical certificate or an
Authorization or SODA. [My emphasis]

Let's pull out the important words and we get this:
"An incorrect entry ... made in any ... record, or report that is kept ... or used to show compliance with any requirement for a medical certificate..."
A medical certificate is certainly kept or used to show compliance with a requirement to posses a medical certificate §61.3(c)(1)
You have discovered an incorrect entry.  Call the doctor that issued your medical as soon as you can in order to get it resolved, and do not accept a solo sign-off on this incorrectly filed document now that you have knowledge of the mistake.
To give you a straight answer based on my interpretation of the law, yes; this error would prohibit you from soloing until it can be corrected.


What is a "Lifeline Extension Belt"? I was recently flying on an United Airlines A320 and noticed a narrow section between the overhead storage bins with a placard labeled "Lifeline Extension Belt" on it. It was over the wing exit row. What exactly is a lifeline extension belt?

Unfortunately I didn't get a picture, but what I'm referencing can be seen in these photos (not copied here since they are copyrighted):

https://www.flickr.com/photos/oliverporter/7653848312 (on the top-left)
https://www.flickr.com/photos/49111993@N00/13890509784 (narrow section is visible rising from the forward exit to the overhead compartment, no placard)

I think what the placard actually says is.

Lifeline
Extension Belt

(ie, its 2 things, not one!)
An A320 equipment specification indicates the narrow OHB you reference as number 16. Which is titled "Lifeline". This line is used to aid evacuation from the overwing exits and more details can be found in this answer
Extension belts are used for infants on their carers laps, an extra belt loops round the child and attaches to the carers lap belt.

(source: cabincrewsafety.com)


Can anyone help identify this old 1930s plane? I would like to identify this plane, if anyone can help? All I can tell you is that it is from an old photo album and probably an English Airshow in the 1920's or 1930's.

That is the Boulton & Paul Overstrand. A twin-engine biplane medium bomber, built for the Royal Air Force of the United Kindom starting from 1933. The last one was already retired in 1941. Apparently this was the first bomber with a closed power-assisted gun turret.


What are the take-off parameters for the B-25 Mitchell? What is the take-off distance and speed in standard atmosphere (59°F, 29.92 inHg, 0% humidity) for the B-25 Mitchell with max, minimum, or average payload?

You can find the full POH here, no wind, standard atmosphere, at gross weight you are looking at 3800 ft. of ground roll and 4700 ft to clear a 50 foot obstacle. Take a look at the charts for the other weights as well as other runway surface types.

(source)


Why did the manifold pressure gauge on the Ki-61 have negative values? The manifold pressure gauge of the Japanese Ki-61 has values which go from +40 to -40. I know this has to be relative to something since negative pressure can't exist (you can't get more vacuum than a vacuum), but I'm not sure what it's relative to.

Why did the manifold pressure gauge on the Ki-61 have negative values?

Disclaimer: I do not know if my answer is correct, but I think it is plausible.

The Ki-61 engine was the Kawasaki Ha-40, a Japanese version of the Daimler-Benz DB 601 Aa turbocharged inverted-V inline engine, so I would assume that the engine instrumentation was inspired by what was used in Germany at that time. Manifold pressure was measured in ata (Atmosphäre absolut, close to 1 bar), which is multiples of the standard atmospheric pressure. The picture below shows the manifold pressure gauge of the Me-109 G, which used the DB 605 A. This was derived from the DB 601, and the manifold pressure gauge has a range from 0.6 ata to 1.8 ata.

The DB 601 A had a maximum manifold pressure at sea level of 1.35 ata. This lets me assume that the Ha-40 was also turbocharged to 1.35 ata, which is 1.368 bar. The critical altitude was probably only 3700 m. The regular DB 601 A had a critical altitude of 4500 m, which was reduced in the DB 601 Aa export version to 3700 m. I could not find a direct reference for the critical altitude of the Ha-40.

At the 11,600 m service ceiling of the Ki-61, the atmospheric pressure is 206.6 mbar. Now we need to factor in the ram pressure gain of approx. 20% at 300 kts TAS, and with the 2.14 compression ratio needed for a critical altitude of 3700 m, this would translate to a manifold pressure of 530 mbar or 0.522 ata. If the Kawasaki engineers raised that to the 2.37 compression ratio of the original DB 601 A, the manifold pressure at 11,600 m could even had been 578 mbar or 0.570 ata.

The most plausible explanation for the numbers on the manifold pressure gauge now would be percentage over rsp. below 1 ata, the static pressure at sea level. Since it is more important to not overstress the engine at low level, the +40 upper limit fits well to the maximum 1.35 ata setting. This fits also well with the static value of 0, meaning 1 atmosphere. At high altitude the compressor would run at maximum setting anyway, and the scale here is for informing the pilot about available power, and would not cover the lowest values close to maximum altitude.

I could not find a performance chart for the Ha-40 or the DB 601 A, and the nearest Google dug out for me is the performance chart of the DB 601 E below. Please note that this engine had more performance (2700 RPM and 1.42 ata manifold pressure for takeoff; the DB 601 A used 2400 RPM and 1.35 ata) and a higher critical altitude of 4800 m:


Can an aircraft operate IFR without ground-based navigation? If I were to redo my avionics to only include a WAAS GPS unit and two comm radios, would anything prevent me from operating IFR? 14 CFR 91.205(d) only states that my airplane must have:

  (2) Two-way radio communication and navigation equipment suitable for the route to be flown.

I'm aware that this is not the most bulletproof way to fly hard IFR. In this case, assume that the aircraft is primarily used for currency/proficiency and the occasional light IFR flight.

The AIM contains the information required to operate under IFR using GPS1.

To summarize the requirements,  WAAS (but not "traditional" GPS) may be used stand-alone without an alternate means of navigation provided:

Installation requirements:

The WAAS navigation equipment is approved in accordance with TSOC145 or TSOC146
The installation is done in accordance with AC 20-138

Operational Requirements:

The WAAS AFM procedures are followed
It is not be used by a commercial operator

Preflight Requirements:

The GPS database is current
The GPS database provider does not have any notices prohibiting the use of any fix or procedure that you are going to use
GPS NOTAMs/Aeronautical Information is reviewed prior to each flight
A RAIM prediction for the time and route is done that shows no loss of RAIM
The flight plan navigation equipment is listed as RNAV

Note that commercial operators are required to have redundant systems or ground based backups in order to use it, and while not required for you, it's a pretty good idea.

1 More specifically, see:

AIM Chapter 1, Section 1 - Navigation Aids

1-1-18. Global Positioning System (GPS)

d. General Requirements
e. Use of GPS for IFR Oceanic, Domestic En Route, Terminal Area, and Approach
Operations
f. General Database Requirements

1-1-19. Wide Area Augmentation System (WAAS)

c. General Requirements


What is defined as "day VFR" under EASA rules? I received a document where it it specified that the flight relevant for such document can only take place in "Day VFR" conditions undear EASA regulations.

Is this connected to VMC? What does EASA defines as "Day VFR"?

It's by opposition to Night VFR (NVFR) which allows to fly by night in visual meteorological conditions. So the answer will focus on NVFR, rather than on day VFR.

EASA harmonization over EU countries is done by enforcing Standardised European Rules of the Air (SERA). SERA states that a night flight must be conducted in IFR, and therefore the crew must hold an Instrument Rating.

Prior to SERA, some countries did not allow VFR at night for security reasons (aircraft not visually identifiable, and no transponder), but other did, that's the case for the UK that I'll use as an example here.

When implementing SERA, EU countries can use acceptable means of compliance, as described in ED Decision 2013/013/R and its annex.

NVFR is implemented (or denied) in EU country by country. So rules may differ between countries. UK example:

UK license can be extended for NVFR.
More for UK NVFR: Safety Notice: SN-2012/007.
In practice, VFR at night is permitted in accordance with SERA requirements. Aircraft leaving the vicinity of an aerodrome must maintain 2-way communication with ATC and file a flight plan. The process of 'booking out' is still accepted as an alternative method to filing a formal paper or electronic flight plan in certain circumstances. Similarly, abbreviated flight plans filed in flight are still permitted. There are also more restricting weather minima:

Minimum cloud ceiling of 1500 ft AMSL
Flight visibility of 5 km, or 3 km in the case of a helicopter flying outside controlled airspace
Maintain sight of the surface when flying at 3,000 ft AMSL or below
Minimum height of 1000 ft (or 2000 ft if over high terrain) above the highest fixed obstacle within 8 km of the aircraft except when taking off or landing

Night is defined for  this purpose:

e.g. for UK in Air Navigation Order 2009 Art. 255:

  'Night' means the time from half an hour after sunset until half an
  hour before sunrise [...] at surface level.

Aircraft must be compliant for NVFR, e.g. this equipment is required:

artificial horizon
Instrument Lighting
Landing Light
Position and Anti Collision Lights

In countries where NVFR is not allowed, IFR applies at night.


Why shouldn't you inflate your life vest until you're outside the airplane? On commercial flights, in part of the pre-flight safety demonstration, you're told in the event of a crash, not to inflate your life vest until you're outside the airplane — why is this?

Ethiopian Airlines Flight 961 was the flight that resulted in this addition to the safety briefing.

After being hijacked, the plane eventually crash-landed in the ocean. Many passengers died because they inflated their life jackets in the cabin, causing them to be trapped inside by the rising water.

Mayday/Air Crash Investigation S03E13 (Ocean Landing) covered the incident. The relevant clip is here (from 23 seconds in):


What is the aerodynamic reason that the An-225's vertical stabilizers are not simply vertical? I am trying to understand the aerodynamic consideration that the two vertical stabilizers of this gigantic Antonov An-225 are not simply made vertical and are slightly pointing in. Then my questions are:

What is the reason?
What is the degree they lean?

I'm fairly sure (but have no sources to confirm) that the reasons are purely structural. The vertical stabilisers are simply set at a right angle to the horizontal one. This is just the easiest thing to do, and it also provides better clearance with deflected lower rudders. The aerodynamic consequences of this (for such small angles) are pretty minor.

Now there are good aerodynamic reasons to have dihedral on the horizontal stabiliser (which causes the whole tilt), but this is a different question, and it applies to many airplanes.


How do general aviation airports inspect their runways? How do general aviation airports inspect their runways for cracks, ruts, holes or anything that would impair their integrity?  How often do airport operators conduct standard inspections of their runways as well as taxiways and ramp areas?

Runway inspection is usually performed visually. In this answer there is a list with several items which have to be checked in EGNX, but the frequency and exact list check items will be different at each airport. I'm not aware of any binding standard.

In every GA airport I know there is at least one daily inspection performed shortly after sunrise (or before the airport becomes active). This is especially important in grass airfields, where wild boars could have dug holes at night. If something happens during the day there is normally someone who sees it and can either inform the operator or fix it himself. Larger airports operating at night will probably inspect the runway more often.


What is that black pod on the left wing of a TBM? What is the black pod on the left wing of a TBM?

(Source)

That is the front of the housing which encloses the 10" GWX70 Color Weather Radar antenna.

Here are some close up shots of a TBM depicting the weather radome.

Source: Own work

Note the lightning diverter strips:

Source: Own work

Apart from the simple fact that airborne weather radar is useful, weather radar is a common option due to the equipment requirements imposed on air carriers operating such aircraft. Specifically, in 14 CFR 135.175, the FAA generally prohibits operation without airborne weather radar equipment along routes where thunderstorm activity is forecast.

Many such single engine aircraft include the option for wing-mounted weather radar, as seen on the TBM 900 in the original question. Since weather radar requires a relatively unimpeded area for signal transmission and receipt, the antenna assembly is typically located at the nose of the aircraft, such as on most multi-engine aircraft. In aircraft with nose mounted engines, a mid or outboard wing mounted position makes sense. See the following photos depicting examples of wing mounted weather radar on a Piper PA-46-500TP Malibu Meridian and a Cessna 208B Super Cargomaster, and a nose mounted weather radar on a Beechcraft King Air B200, respectively:

Source: Own work

Source: Own work

Source: Own work

According to the TBM 900 POH, installation of the optional radar system results in a 5 KIAS decrease in maximum cruise performance and a 3 KIAS decrease in long range cruise performance. This change is due to the slight increase in drag from the radar antenna housing.


How to find if an aircraft is inside an airspace using latitude and longitude? I have the data of latitude and longitude of $n$ points on the boundary of the airspace of a country. I also have the coordinates of the current location of an aircraft. Using this data, how do I find out of the aircraft is inside the airspace or outside.

For the sake of simplicity we can assume the boundaries of the airspace form a convex polygon.

It is just a well-known computational geometry problem called PIP (point-in-polygon). I assume that the airspace is pretty small, and we can consider its boundaries as lines (not curves) in a plane.

Firstly convert latitude and longitude minutes and seconds to fractions. Then use any of PIP algorithms (or libraries, if you do it in any programming language).

For more information check: wiki or this link.

You can also use WolframAlpha for that, or any programming language with a specific library (for example, OpenCV does it very well in C++ or python).


Is there any difference between "EMERGENCY LANDING" and "FORCED LANDING"? Emergency landings and/or forced landings are stressful situations, but talking about meanings, are they the same, please?

Not all forced landings are emergencies, and not all emergencies are forced landings! (See below)
In general, the term "Emergency Landing" is a superset which covers a bunch of different scenarios where a landing is not quite to plan. Such as

Forced landing due to mechanical failure. Note that a forced landing can absolutely be at an airfield. It can also be in a literal field.
Precautionary landing, such as where continued flight is possible but inadvisable
Ditching in a body of water
Belly landing due to inoperable landing gear

A forced landing can also be one where the pilot is forced to land due to military interception, which is not always an emergency.
This is all covered in great detail here

The FAA also publish some useful information on this subject which repeats much of the above information, reproduced below for clarity

Types of Emergency Landings
The different types of emergency landings are defined as
follows:
• Forced landing—an immediate landing, on or off an
airport, necessitated by the inability to continue further
flight. A typical example of which is an airplane forced
down by engine failure.
• Precautionary landing—a premeditated landing, on
or off an airport, when further flight is possible but
inadvisable. Examples of conditions that may call for
a precautionary landing include deteriorating weather,
being lost, fuel shortage, and gradually developing
engine trouble.
• Ditching—a forced or precautionary landing on water


Why do large aircraft have their engines mostly on the wings, while smaller ones tend to have them in the tail or the tip? Large passenger planes almost always have their engines hanging from the wings, Fighters usually have them in the rear, and small Cessnas have them on the nose.

What is the reason for these variations in design?

Small aircraft need very low mounted wings to allow for a decent cabin height. The last commercial airliner which required the passengers to climb over the wing spar on their way to their seat was the Boeing P-247. Low wings do not allow the necessary space for engines, so the next best location is on the rear fuselage. Also, small aircraft have a relatively short fuselage, and the gear can be made short (and light) without risking a tail strike on rotation.

Fighters need to have small moments of inertia, especially in roll, so they cannot afford to put engines on the wings. Even the Me-262 was initially planned with fuselage-mounted engines, that's why it had this peculiar triangular fuselage cross section. Only when the engines grew in diameter during development they had to be placed on the wing.

Pros of rear-mounted / fuselage-mounted engines:

The wing is clean, having low drag and optimum conditions for lift creation.
The engines are easy to access (if mounted at the rear fuselage. Different story with buried engines!).
Smaller vertical tail needed for asymmetric thrust conditions.
Shorter, lighter landing gear possible (if rotation capability is not compromised).
Low roll inertia.

Cons of rear-mounted / fuselage-mounted engines:

No bending relief for the wing; heavier wing spar needed.
No help in flutter supression (a mass ahead of the elastic line helps).
Possible interference with wing wake at high angle of attack.
Forces the tail to be a T-tail configuration.
More structure needed to carry the thrust forces forward.
Intake noise is more audible, especially for passengers in the rear seats.

This picture of a Cessna Citation X should illustrate the point nicely.


What if pilot feels too tired before his flight? Today is a day when I feel terribly tired. I'm not ill, but I could sleep all the day, though I had enough sleep last night.

This is not a problem if you have an office job where you can probably be not as productive than on other days. But what about pilots? They are expected to have a high level of concentration and vigilance during the whole flight?

This question, What should a pilot do if they feel sleepy? concentrates more on upcoming sleepiness during the flight. The accepted answer says:

  If a pilot is not rested enough to safely operate the aircraft, they should not fly. [...]

Well, "should not" sounds more like  advice than a rule.

You may say that there are always two pilots on (not so long) commercial flights, but then you can also start with one faulty engine, because the second engine still could bring you to your destination. For me, this breaks the safety concept.

So, can a pilot go to a doctor and tell, "I'm tired today so I can't work"?

So if you are well rested and not ill, but still feeling tired, there has to be something else wrong. From your description, it appears to be fatigue. The common signs for fatigue are:

Falling asleep
Yawning
Poor visual acuity
Feeling "sluggish" or "drowsy"
Decreased reaction time
Decreased concentration

Although, the most commonly known reason for fatigue is lack of sleep, but any or all of the following can cause fatigue:

Lack of quality sleep
Sleep disturbances
Interruption of circadian rhythm
Mental or emotional stress
Physical exertion, such as heavy exercise
Poor health, including dehydration or poor diet

Pilots can assess their own fitness to fly by the IMSAFE mnemonic. FAA has a small handout about fatigue.

Now what pilots (and generalizing, people) can do and actually do are two very different aspects. A survey about European pilots mention:

  ... many are afraid their fatigue reports could have negative
  consequences for their professional future (i.e. reprisals by
  management) ...

FAA has a new rule which states:

  The FAA expects pilots and airlines to take joint responsibility when
  considering if a pilot is fit for duty, including fatigue resulting
  from pre-duty activities such as commuting.  At the beginning of each
  flight segment, a pilot is required to affirmatively state his or her
  fitness for duty.  If a pilot reports he or she is fatigued and unfit
  for duty, the airline must remove that pilot from duty immediately.

The answer to your question is: Yes, they can!


Did airlines fly their aircraft slower in response to oil prices in the 1970s? Modern airliners generally have lower cruise speeds than those of comparable aircraft from 40 years ago (the venerable Boeing 737 seems to be something of an exception; it has always had a more leisurely cruise speed).

Airlines had to respond much faster to the oil crisis than manufacturers could - to what extent were airlines able to economise on fuel consumption by flying more slowly, and what sort of margin was available to them at cruise (and by extension, what sort of margin is available in modern aircraft, should economic factors drive operators to seek lower fuel consumption by flying more slowly)?

As a French who grew up in the Caribbean, I can tell that flights are getting longer. A flight between France and my island that took 7.5 hours 15 years ago is almost 9 hours long today.

This post published by the MIT’s School of Engineering says (emphasis mine):

  Specified cruising speeds for commercial airliners today range between about 480 and 510 knots, compared to 525 knots for the Boeing 707, a mainstay of 1960s jet travel. Why? “The main issue is fuel economy,” says Aeronautics and Astronautics professor Mark Drela. “Going faster eats more fuel per passenger-mile. This is especially true with the newer ‘high-bypass’ jet engines with their large-diameter front fans.”

Today's high bypass engines are more efficient at low speed:

  Observant fliers can easily spot these engines, with air intakes nearly 10 feet across, especially on newer long-range two-engine jetliners. Older engines had intakes that were less than half as wide and moved less air at higher speeds; high-bypass engines achieve the same thrust with more air at lower speed by routing most of the air (up to 93 percent in the newest designs) around the engine’s turbine instead of through it. “Their efficiency peaks are at lower speeds, which causes airplane builders to favor a somewhat slower aircraft”

This article also states that slower aircraft's structures are cheaper to build:

  “A slower airplane can also have less wing sweep, which makes it smaller, lighter and hence less expensive.” The 707’s wing sweep was 35 degrees, while the current 777’s is 31.6 degrees.


How should night time be calculated when it is not night during the entire flight? I am B777 rated pilot and software developer. Nowadays I am working on an application about flights and this app has to calculate night time. Still I am trying to figure out how to calculate night time correctly for logging purposes. As you know night definition is easy,

Night means the time between the end of evening civil twilight and the
beginning of morning civil twilight, as published in the Air Almanac,
converted to local time.

But calculating a flight's night time is a little bit confusing.

Is this just related about being day or night at takeoff and landing airports?
Or whole the route should be considered if it is day or night at that time while flying?

I searched a lot but couldn't find any formula or detailed explanation about this. Just found some sites and apps can calculate night time; but interestingly every app gives different result; and still it is unclear for me that how they calculate night time.
Example:
Departure : KMIA
Destination : LTBA
Block Off Time : 03:03Z
Block On Time : 13:36Z
Block Time : 10:33

If you calculate this flight's night time via http://www.crewlogbook.com/utilities/night_calculator.php you will get:
Night Time : 01:47

I wonder how this calculation was made?

By the time that you're operating a 777, the amount of "night" time in the logbook is probably of little to no concern, so getting things exactly right to the minute may not be a great worry.
I'd simply grab a location every 15 minutes, either by GPS if your app has that, or else as (time elapsed / total time) * total great circle distance, and then for that location determine if you're between EECT & BMCT at that time. If so, log those minutes as night.
You could get fancier by interpolating the BMCT (for instance) between the last point you were in "night" and the first point you weren't. Then you have "the" time that you passed from night into day.
To implement that, you'll need to calculate the EECT & BMCT for an given point, plus find points along the great circle route. I'd expect that algorithms are available for both of those functions, but it's not something I've ever implemented.


What are the guidelines to execute a missed approach during circling? Alice is approaching KTCY on RNAV (GPS) RWY 12 at KTCY circle NE to RWY 30. When she is turning right base to RWY 30 final, she loses visual contact with runway environment and decides to go missed. Spotted her GPS map, she notices she is right over the RWY 30 touchdown zone. How should she execute the missed approach?

a) Climb to 700 (600 to 700, HDG 300˚), Climb left turn (~left 260˚ over circling NA region), 3000 direct ECA hold.
b) Climb to 700 (600 to 700, HDG 300˚), Climb right turn (~right 100˚) 3000 direct ECA hold.
c) Climb right turn (~right 100˚) 3000 direct ECA. Note Alice is above circling minimums 600 and initial climb to 700 is based on a HDG of 120˚.
d) Continue fly over LNAV MAP, make a right 180˚, then climb 700, climb left turn 3000 direct ECA hold

In more general senses, does circling nullify the initial climb, initial turn portion for a missed approach procedure, so that we may choose to intercept the following course, navigational radial, or direct to the fix from the side we turn with less degrees?

Flying a missed approach while circling can require some judgement, as the procedures are designed to originate at the missed approach point. Going missed while circling can mean starting the procedure some distance away from the MAP. The goal of the initial procedure should be to intercept the published missed approach course, while maintaining obstacle and traffic clearance. Put another way, a pilot may deviate from the letter of the procedure, if doing so follows the spirit of it.

In this situation, I would recommend option B. The procedure will provide obstacle clearance when started from three points: 443 MSL and about 4000 feet from the RW12 threshold (LPV minima); 638 MSL and about 11,500 feet from RW12 (LNAV/VNAV); and 600 MSL over the RW12 threshold (LNAV and circling). The procedure assumes you can climb at least 200 ft/nm, so to climb 100 feet would require less than 3035 feet across the ground. This puts you within the area where the published missed approach starts. Once you reach 700 feet, you can turn directly to the VOR and continue the procedure as published. The initial turn would normally be towards the landing runway, but in this example the runway is straight ahead.

If you descended below circling minima and needed a significant climb to get back, consider circling over the airport until either reaching 700 feet before flying towards the VOR.

The FAA has recommends the thought process above in the AIM, section 5-4-21. Emphasis mine:

  5-4-21 (c): If visual reference is lost while circling−to−land
  from an instrument approach, the missed approach
  specified for that particular procedure must be
  followed (unless an alternate missed approach
  procedure is specified by ATC). To become
  established on the prescribed missed approach
  course, the pilot should make an initial climbing turn
  toward the landing runway and continue the turn until
  established on the missed approach course. Inasmuch
  as the circling maneuver may be accomplished in
  more than one direction, different patterns will be
  required to become established on the prescribed
  missed approach course, depending on the aircraft
  position at the time visual reference is lost.
  Adherence to the procedure will help assure that an
  aircraft will remain laterally within the circling and
  missed approach obstruction clearance areas. Refer
  to paragraph h concerning vertical obstruction
  clearance when starting a missed approach at other
  than the MAP.

[...]

  5-4-21 (h): ... In the event a balked (rejected) landing occurs at a position
  other than the published missed approach
  point, the pilot should contact ATC as soon as possible
  to obtain an amended clearance. If unable to
  contact ATC for any reason, the pilot should attempt
  to re−intercept a published segment of the missed approach
  and comply with route and altitude
  instructions. If unable to contact ATC, and in the pilot’s
  judgment it is no longer appropriate to fly the
  published missed approach procedure, then consider
  either maintaining visual conditions if practicable
  and reattempt a landing, or a circle−climb over the
  airport...

The AIM includes a diagram that demonstrates a pilot turning against the missed-approach instructions when that is the safer course. The missed approach requires a climbing right turn to the VOR; the pilot in the scenario on the right chooses to make a left turn as that allows the initial climb to take place over the airport.

Source: FAA AIM


Are turbine engine reconditioned after severe FOD damage caused by bird strikes? I've just been watching a video of a 757 suffering a bird strike on take off. The pilots continue with their take off, declare a MAYDAY, shut the engine down and proceed to land without any issues.

Link to the video: https://www.youtube.com/watch?v=9KhZwsYtNDE

Are these engine reused after an event like this? Are they sent away to Rolls Royce, Pratt & Whitney etc to be repaired? Or are they always deemed a complete write off and replaced with new?

Obviously turbine engines are highly balanced complex pieces of machinery so I can imagine once FOD has damaged this balancing the damage is pretty high.

The damage is assessed and then a determination is made whether or not its economical to repair the engine.  It will truly be taken on a case by case basis.  However, FOD damage to the engine doesn't automatically mean the engine will be scrapped, and I think you'd be surprised with the damage they can suffer and still remain in service after repair.


What is typical ATC procedure when a non-pilot calls from a commercial flight for help? "This is ShinyWings 123, I'm not a pilot, the pilot and copilot are unconscious. Help."
Probably a day air traffic controllers dream of with dread, when a member of cabin crew, or even worse, a passenger, calls for help with an incapacitated pilot(s) and a plane full of souls still unaware of the situation.
I'd imagine given the incredible amount of training and experience put into air traffic controllers, this is a situation they prepare for, and have a standard operating procedure prewritten and pre-taught specifically for. What is that procedure?
To keep the question from being overly broad, I'll restrict this to (1) ICAO standard procedures, if one exists for this situation and (2) procedures within the United States and European Union.

There is absolutely nothing "typical" about such a situation, which is also why - as is the case for most abnormal situations and emergencies - there are no detailed and specific standard procedures for dealing with it.
Many ATC units around the world have adopted the general "ASSIST" checklist to deal with abnormal situations. This would likely also be the starting point for handling a situation like the one you describe:

Acknowledge: Let the person making the call know that you understand the situation
Separate: Provide extra separation to other traffic around the aircraft in question
Silence: Stop all unnecessary talk on the radio, switch other aircraft to another frequency, to ensure a good line of communication
Inform: Alert colleagues, supervisors, rescue personnel, the airline/flying club, other aircraft in the area etc. as appropriate. Get more people working on coming up with a solution.
Support: Help the passenger/cabin crew in any way possible. Maybe the controller has flying experience, maybe someone who was contacted in the previous step does and can step in and help guide the passenger/cabin crew.
Time: Try to estimate the possible timeline. Is the aircraft literally falling out of the sky, or is it flying along steadily on autopilot? How much fuel is left? Don't force anyone to make rash decisions if not necessary. Time produces good decisions.

But, again, there is no standard procedure, since such a situation could present itself in countless different ways. There are simply too many variables to come up with a "one size fits all" solution beforehand. Controllers are trained to remain calm under pressure, make good choices and adapt to an ever changing environment. The day we have a set checklist for every single possible emergency situation is the day we can safely retire and let computers take over our jobs.
Further reading on SKYbrary:
Crew Incapacitation: Guidance for Controllers
Guidelines for Dealing with Unusual/Emergency Situations in ATC


Is it legal to use RC quadcopters for commercial purposes? Is it legal to fly a drone with a go pro or something and take photos and video for commercial purposes?

This is what I found on the FAA website.

  All UAS operations for commercial or business purposes are subject to FAA regulation. At a minimum, any such flights require a certified aircraft and a certificated pilot. UAS operations for commercial or business purposes cannot be operated under the special rule for model aircraft found in section 336 of Public Law 112-95.

http://www.faa.gov/about/initiatives/uas/uas_faq/

Many news articles say that the FAA will not really care.

Is it legal to fly a drone with a go pro or something and take photos and video for commercial purposes?

Yes, many news businesses think so

  the numbers of drones licensed for commercial flights by the nation's Civil Aviation Authority (CAA), which regulates the use of unmanned flying aircraft, shoots up -- from 30 in January 2013 to more than 300 today

  Many news articles say that the FAA will not really care.

Most drone operators will not care too much what the FAA thinks. This is because the FAA don't have authority over most drone operations. This is mainly because they only really have authority in one country in one part of one continent. The FAA won't be arresting Mr 王 in 中国.

You can read what the FAA says about Unmanned aircraft.

  When the FAA discovers apparent unauthorized UAS operations, the agency has a number of enforcement tools available to address these operations, including a verbal warning, a warning letter, and an order to stop the operation.

Which suggests the FAA cares, at least about commercial UAS operations in the northern parts of the continent of America which aren't Canada or Mexico.

They also have a FAQ

  All UAS operations for commercial or business purposes are subject to FAA regulation. At a minimum, any such flights require a certified aircraft and a certificated pilot. UAS operations for commercial or business purposes cannot be operated under the special rule for model aircraft found in section 336 of Public Law 112-95.


How does an inoperative Anti-Skid affect V speeds? I am trying to figure out why is it that when you have an inoperative anti-skid system, the calculated V Speeds are lower?
I'm trying to see this from a performance perspective, could this be due to the brake energy limitation of the aircraft?

When you say it affects V speeds, I assume you mean V1 (and possibly VR), but not V2. V1 is usually calculated from the balanced field length, which is where accelerate-stop-distance and accelerate-go-distance are the equal:

(image source: boldmethod > Engine Failure On Takeoff: Do You Stop Or Go?)
While the accelerate-go-distance will be unaffected, the accelerate-stop-distance is affected by anti-skid because you will not be able to brake as efficiently as with anti-skid operating normally:

the antiskid system stops the airplane for all runway conditions in a
shorter distance than is possible with either antiskid off or brake pedal
modulation.
[...]
Braking with Antiskid Inoperative
When the antiskid system is inoperative, the following techniques apply:

ensure that the nose wheels are on the ground and the speedbrakes are
extended before applying the brakes
initiate wheel braking using very light pedal pressure and increase
pressure as ground speed decreases
apply steady pressure and DO NOT PUMP the pedals.

(Boeing 737 NG FCTM 6.45 - Landing, emphasis mine)
As you can see from the above quote, you will increase the stopping distance compared to a normal braking with anti-skid, which results in a larger accelerate-stop-distance, which then reduces the balanced field length V1.
It has nothing to do with the brake energy limitation because the maximum energy that can be absorbed by the brakes is the kinetic energy of the aircraft at the moment you start braking. It does not really depend on how you brake, but with anti-skid inoperative there is a chance that the tires will lock up while decelerating which actually reduces the amount of energy going into the brakes and increases the energy that goes into the tires (in the worst case, this energy will then cause a tire blowout).


What is the meaning of 'stall' in this sentence? I have no knowledge about aviation. Can somebody tell me what is the meaning of "he ran through a series of stalls" in:

  He ran through a series of stalls, killed the engine, and brought the
  plane down to a safe and quiet landing.

Source: Blended Coaching: Skills and Strategies to Support Principal Development

The sentence means the airplane was slowed down (generally) and the nose pulled up until the wing stopped generating lift (stalled) and finally "killed the engine" (typically by reducing the mixture until the engine stops running) and then landed by just gliding down to the airport.  Stalls are not a good thing when low to the ground and maneuvering for a landing, as there is no altitude left to recover, so we practice them to recognize when one is about to occur to avoid them, and to recover quickly at the onset of one.


Do airline transport pilot trainees ever fly a passenger-less aircraft? Do airline transport pilot (ATP) trainees ever fly an empty aircraft? Consider the example where a pilot is under training to operate a 777. Would the pilot ever fly the real aircraft without any passengers? Or is training only limited to simulation?

When pilots are trained for flying in an aircraft type they have not flown in before, they go through type rating, then line training. During the type rating, they learn where all the instruments and buttons are, what starts which system, how the plane behaves, how to land and auto-land etc.

During line training, they spend time in the cockpit together with an instructor pilot, usually a senior captain. Most airlines have a 3-person crew in the cockpit when a new F/O is receiving line training. This training is about the airline specific bit of flying, company procedures etc.

Note that before going to type rating, the pilot already has at least a commercial flying license: they can fly already. Depending on the country, the type rating can take place in a Level D simulator for 100%. Some countries still require a couple of actual flights in an aircraft, but there is an increasing trend to use Level D simulators only, for two reasons:

Back when the airlines paid and arranged for type ratings, it was relatively simple for them to arrange for an aircraft for a training flight, empty or not. But now a majority of type ratings is self funded, how does a student find the finance to charter an empty aircraft?
The realism of Level D simulators is incredible. My office used to be not far from the simulators - every once in a while I would hear the siren for the sim coming down at an unscheduled time, for an unscheduled break: the stress level inside is real, you reckon that you are really 30,000 feet up with an engine on fire, alarms sounding, actual (non-toxic) smoke entering the cockpit, etc. After clearing the emergency, some students needed time to reflect.

Boeing predicts that we need 30,000 new pilots every year for the next 20 years. How do we streamline it such that all competent persons can become pilots, not only people with enough money to charter a plane?

As @user71659 points out, at airlines that only fly widebodies, standard entry position is a second officer/relief first officer/cruise pilot on a long haul plane like the 777. (Cathay Pacific, Singapore Airlines, Emirates, for example).


How are duplicate call signs dealt with? Many airlines (at least here in the U.S.) have flight numbers that use different aircraft for different legs of the flight. For example, Delta 158 uses a 747-400 from Incheon International in Seoul to Detroit, then an A320 from Detroit to Boston a couple of hours later. This got me to wondering what happens if an earlier leg is delayed such that it's arriving around the time that the next leg is departing the same airport? In particular:

Will both flights use the same radio call sign? That is, will they both be "Delta 158" (ignoring the "Heavy" difference in this particular case?)

If they do use the same call sign, how do controllers know which one is talking, especially if they end up on the same frequency at some point?

It seems like this sort of situation would happen somewhat frequently, at least here in the U.S. where it's common for a given flight number to include multiple legs and for those legs to operate on different aircraft.

This question is related, but doesn't actually address this particular question: Why are some ATC call signs different than the flight number?

The scenario you're asking about is common.  Let's say that your Delta 158 from South Korea to Detroit is running several hours late, and the decision is made to operate the A320 DTW-BOS on time.  ATC doesn't accept having two aircraft flying with the same callsign at once due to exactly the sort of confusion you suggest, so something has to change.

What will happen is that the second flight -- i.e. the one that isn't already airborne, will get a new ATC callsign.  Sometimes you'll hear something like "Delta 158A" as the callsign of the flight, or they may use something entirely different like "Delta 9158" as the ATC callsign for that flight.  Each company will have their own method of resolving the conflict; some use letters, others don't.  Some can go to "9000 + original flight number," and others will simply use something like 9001 for the first "stubbed out" flight that day, 9002 for the next, and so on.

But, the basic answer is that the ATC callsign of the flight changes.

Which does NOT mean, incidentally, that the flight number as far as the passengers are concerned has changed.  It's possible to be "Delta 9158" or "Delta 9001" or "Delta 158A" or whatever for ATC and still be "Flight 158" when talking on the PA to the passengers.  The fact that the ATC callsign changed is transparent to the rest of the operation, since reservations, ticket counter, baggage handling, and so forth isn't upset when 158 ICN-DTW happens to overlap (in time) 158 DTW-BOS.  Only ATC worries about that, so the fix is only visible to them (and the pilots & dispatchers).


Can a student pilot start flight training before they get their student pilot certificate and medical? Let's say someone wants to start doing some flight training in the United States with the eventual goal of receiving a private pilot certificate. They have yet to get a medical certificate or student pilot certificate. Can they go ahead and start getting some flight training logging dual received?

Yes.

14 CFR 61.87(n)

  (n) Limitations on student pilots operating an aircraft in solo flight. A student pilot may not operate an aircraft in solo flight unless that student pilot has received:

    (1) An endorsement from an authorized instructor on his or her student pilot certificate for the specific make and model aircraft to be flown; and

    (2) An endorsement in the student's logbook for the specific make and model aircraft to be flown by an authorized instructor, who gave the training within the 90 days preceding the date of the flight.

14 CFR 61.23(a)(3)

  (a) Operations requiring a medical certificate. Except as provided in paragraphs (b) and (c) of this section, a person— ...

    (3) Must hold at least a third-class medical certificate—

      (i) When exercising the privileges of a private pilot certificate;

      (ii) When exercising the privileges of a recreational pilot certificate;

      (iii) When exercising the privileges of a student pilot certificate;

The student pilot only needs a student pilot certificate and medical when operating in solo flight.

With that said, if there is any doubt that a medical certificate won't be granted it is a good idea to get that out of the way before spending money on flight training you will never be able to use.


What are benefits of unloaded wingtips? What are aerodynamics benefits of using unloaded wingtips (zero lift)? does unloaded wingtip make inner parts of wing more efficient?
(I know that unloaded wingtip reduces bending moment,so structural weight can be  lower, thus reduction in L/D and higher net wing lift (lift minus wing weight), but you can connect wingtip with wire and use elliptical distribution..so..)

Twofold (at least): firstly it results in less energy going into the tip vortices, secondly, is the reduction in lift is gradual towards the wingtip then it tends to reduce the spin tendency - the angle of attack of the tip is typically lower than at the root and so it will stall later than the root.  In a slow-flying aircraft such as a paraglider the airspeed may be significantly lower at the tip when turning, and so the angle of attack for a given sink rate will be higher, increasing the risk of a spin.


During a touch and go (C150) should I first raise flaps or set full throttle? I am now finishing my PPL (on a C150 in Czech Republic) and today had quite a close call when doing a solo touch-and-go on a short runway. We train on grass (and therefore rarely completely level) runways and take off with flaps 10 (landing with 20 or 30). I stayed in ground effect a couple of seconds before touchdown and then, according to my instructor's (hmm) instructions, first raised the flaps to 10 and then applied full throttle (and carb heat full cold).

I saw trees approaching and cleared them by an uncomfortably small margin (which made my further way back on which I managed to lose track a bit seem like a joke I wouldn't care about).

The explanation I heard from other flying guys is that touch-and-go should be an imitation of landing followed by a take-off, and one should therefore retract flaps (to 10) and then apply full power. And if you are not sure of runway distance left, you should make a full stop and take off anew.

Does this actually hold true? Imitation and practicing is all good and well, but assessing the runway distance left (and more so at an unfamiliar unlevel grass field I was landing at) in one second you might have may prove too risky and may lead to quite different consequences I was lucky to have avoided.

The instructor actually confirmed they were planning to change their route for the long nav flight because this field proved too challenging.

The question is, however, generic: first raise flaps or set full throttle?

Flaps in a C150 (and pretty much any type) take a few seconds to move. Throttle change is more-or-less instantaneous. If you were to throttle up first, and then change the flap position there is a good chance that you are lifting off while the position of your flaps are changing. This is not a good idea, changing the amount of lift generated by the wings in this crucial stage of flight can lead to a take off stall. You could (and probably will) be too low to recover.

To minimise the chance of this happening, I would say that on touchdown, one of the first things I would do is to set flaps for takeoff ( giving the actuators a second or two to change flap position) before immediately setting take off throttle.

So, yes, your instructor was right. They usually are. In your specific case, I suspect that as soon as you floated too far down the runway, you should have initiated a go-around before touching down. You did not leave yourself enough runway to safely perform a touch-and-go. You dont assess take off distance available once you touch down, you asses it while still flying - you had ~10 seconds to make that call (approach, float, oh shit, go around!)


What are the spiky things on the back of the Antonov-225 vertical stabilizers? On the Antonov-225's split vertical stabilizers, there is a 'spiked' structure pointing directly rearwards (as you can see below). What is the purpose of these two structures?

source

It appears to be the extension of the aerodynamic fairing between the vertical and horizontal tails (note that the horizontal tail extends outside the vertical one). It can be seen more clearly in the following image.

Image from diecastaircraftforum.com. Original image appears to be from airliners.net

This type of fairings (usually called bullet fairings) are found in 'T' tails and other types of tails (like cruciform tail) where two surface intersect each other at right angles and are used to reduce interference drag (and flow seperation). For example, the following image shows the fairing in C-141 Starlifter, an aircraft with 'T' tail.

airplanesofthepast.com


When temperature increases what happens to the air molecules and aircraft while landing and takeoff during hot weather? Just out of curiosity on a hot weather with  temperature increases what happens to the air molecules and aircraft during landing and takeoff?

If all else is equal, an increase in temperature will result in a decrease in density of air molecules. To fly, a wing leverages these molecules, so the aircraft will need to fly faster to generate the same lift. Therefore the takeoff and landing distances are increased.

Engine performance is also impacted. Most engines outside of NASA breathe air and so perform better when that air is dense. Jet engines also have a maximum exhaust gas temperature they can withstand, and in very hot temperatures the amount of thrust they produce needs to be lowered so as to not exceed that value. This is the basic premise of how Airbus ‘assumed temperature’ works, where the pilots tell the engine that the temperature is hotter than it actually is, so the FADEC automatically caps the thrust.


quadcopters — should the propellers/motors all be at the same height In quadcopters (or in any other drones), must the propellers/motors all be at the same height, or can there be symmetric height differences (like for eg, two of the motors at the "back" are elevated just a little like by say 5cm or even less... Is that going to make a significant trouble in the drone performance?

Tandem rotor helicopters have the aft rotor elevated to keep it clear of the front rotor's downwash and improve performance in forward flight (at the cost of reduced performance in rearward flight).

Most small quadcopters achieve substantial pitch angles in forward flight, often in excess of 30 degrees, which keeps the aft rotors out of the downwash anyway. Therefore, your proposal changes nothing in hover and fast forward flight, improves performance somewhat in slow forward flight and reduces performance in rearward flight.


How is the Cessna 172S G1000 Electrical System powered? Why, on a Cessna 172S, there is a positive voltage reading on the Main bus when only the standby battery switch is on? In the checklist it says

  verify M BUS Volts are 1.5volts or less.

I thought that the standby battery was supposed to power only the essential bus.

It would appear that there is a standby battery controller PCB.  It's important to note that M BUS VOLTS are read off the crossfeed bus as there is no main bus.  The label is a bit misleading in my opinion.  It's been awhile since I have flown a G1000 C172 but if I remember correctly you should not see 1.5volts on the M BUS VOLTS but zero.

The checklist is verifying the PCB box doesn't have a fault that will allow the standby battery to power more than just the essential bus.  Unfortunately, the POH is silent as to why this check is done.


Can a simulator (FFS) be used in place of a Technically Advanced Aircraft (TAA) to meet the commercial experience requirements? 14 CFR 61.129(a)(3)(ii) lists the aeronautical experience requirements for a commercial certificate. It says:

  (ii) 10 hours of training in a complex airplane, a turbine-powered
  airplane, or a technically advanced airplane (TAA) that meets the
  requirements of paragraph (j) of this section, or any combination
  thereof. The airplane must be appropriate to land or sea for the
  rating sought;

With the changes to this regulation to allow TAAs, does the FAA allow simulator (FFS) hours in place of the TAA hours?

You can't use a simulator for this time because the regulation specifically requires training time in an airplane; there's no mention of using a simulator. If you look at some of the wording in the currency regulations in 61.57 (for example) you can see that when the FAA allows you to use  a simulator they state it explicitly:

  The takeoffs and landings required by paragraph (a)(1) of this section
  may be accomplished in a full flight simulator or flight training
  device that is [...]

  A pilot may accomplish the requirements in paragraph (c)(1) of this
  section in a full flight simulator, flight training device, or
  aviation training device provided [...]


Why isn't there some optional setting that can precisely move thrust levers jointly? In the screenshots beneath (top, bottom), Capt Juergen Raps is fidgeting the A380's 4 thrust levers with his fingers to align them to the same thrust position.

Isn't it unproductive for pilots to tinker with thrust levels to align them manually?
Why can't pilots choose to bind and link all thrust levers, for moving together concurrently? If the pilot needs to move (at least) one thrust lever differently from the rest (e.g. taxiing with one option), this setting can be turned off. I'm not referring to the autothrottle.

When the levers are set manually to the same position, the engines depending on their aging will not develop the same nominal thrust, some differences will appear on the indicated EPRs or on the indicated N1s, so no point to have them linked together.

With modern aircrafts, for instance the B777, even when flying manual thrust, in flight, when the levers are  set just close to each other, an automatic « trimming » function is automatically activated, it is transparent to the pilots and it corrects the engines thrusts as similar as possible by sending corrective orders directly to the FADECs, but however also preventing the beat noise that might occurs with fans having exactly the same speed; this occurs without any motion to the levers.

Taxiing with a single engine out of two is frequent for fuel consumption consideration and for noise abatement in many airports.

While forced to wait in a long chain for your turn to take off, and advancing step by step, pilots do use frequently one engine out of two.

On modern aircraft, you do not fly manual thrust but with autothrust or with autothrottles

On AIRBUS, the throttles positions have notches, so that when you select takeoff, the notch will set the throttles at similar position, nevertheless the commanded thrust do not correspond to the EXACT throttles positions but to a computed FADEC order.

On AIRBUS, at thrust reduction altitude the throttles are set to position CLIMB, for all the remaining flight, the levers remain there, and the thrust is managed by the autothrust


How does a turboprop engine keep running during thrust reversal? For thrust reversal after landing, a turboprop engine reverses blade pitch and blows air forward. But the engine itself requires air flow for the turbine.

How can a turboprop engine keep running when the prop blows air away from it?

The prop is not blowing air away from the intake, atleast in that region. In modern propellers, the twist of the blades is such that the blade angle is smaller at the tip compared to the root.

Airfoil section in propeller blade; image from FAA Pilot's Handbook

Now, if reverse thrust is applied, the outer regions of the propeller is in the negative angle of attack, producing reverse thrust. However, the case is different in the root region, which won't have a negative angle, thus not blowing air 'away'. The net result is that the compressor can quite easily handle the reverse thrust condition.

Propeller angles in various flight regimes; image from pilotsofamerica.com

Another thing is that the reverse thrust is quite small compared to that of the forward flight and the speed is also quite small. modern compressors don't have much problem in this regard.

For comparison, consider the turboshaft engines in the helicopters in rearward flight- they can handle a much greater range of speeds in that condition. As an extreme case, consider the Russian aircraft doing tailslide- even the turbofan engines cope up with that, albeit in a much limited manner.


What are the advantages/disadvantages of in-fuselage engines (eg most fighter jets) vs wing/fuselage mounted engines? Most modern jet fighter designs incorporate their engine(s) within the fuselage, while most other medium/large aircraft mount them either under the wings or on the rear fuselage.

What are the advantages and disadvantages of in-fuselage jet engine placement, compared to those mounted on the fuselage or wings of an aircraft?

The answer is roll inertia.

Fighter aircraft need to maneuver quickly, and one of the most important parameters is roll speed. In order to achieve a high roll rate quickly, low moment of inertia in roll is helpful. Placing the engines on the wing would increase roll inertia without much benefit in air combat.

Wing-mounted engines can only be found in designs which were optimized for raw speed, such as the P-38 or the YF-12. Even the Me-262 was initially designed with fuselage-mounted engines, hence its triangular fuselage cross section. Only when the engines grew in diameter during development had they to be relocated to the wing.

Disadvantages of fuselage-mounted engines:

Worse accessibility for maintenance.
Variants using a different engine cost much more to develop.
Engine fires or loss of a turbomachinery blade pose a much bigger threat to the structure.
No bending moment relief or flutter damping for the wing.
Complex intake geometry with boundary layer splitter plate, unless the intake is in the fuselage nose. The air requirements of high bypass ratio engines cause high losses with side intakes.

Note that at supersonic speed the engine can actually benefit from the proximity of a fuselage: The precompression of the forward fuselage of the F-16 or the Dassault Rafale help to increase thrust, especially when it is most needed at high angles of attack (well, those relatively high angles in supersonic turns, that is).

Advantages of fuselage-mounted engines:

Low moment of inertia in roll.
Lower overall surface area, thus lower friction drag, when compared to podded engines.
Precompression of the forward fuselage can be used to increase supersonic thrust.


Where can a realtime map showing all flights and all dangerous areas be found? Further to my other question, how can I find a realtime map of all flights that's combined with the foregoing map of all perilous zones? My goal is to see which flights may be overflying them, and in particular, if there are any commercial flights doing so.

For example, I only learnt of this via CNN, but want to predetermine this by myself: http://edition.cnn.com/2014/07/22/world/asia/malaysia-airlines-plane-over-syria/

Actually what constitutes a "perilous zone" is arbitrarily decided by countries or airlines . Ergo, there are no zones that have been defined by the International Civil Aviation Organization as perilous .

http://www.icao.int/Pages/default.aspx

For example as on this date British Airways and Qantas  have demarcated Ukrainian airspace as dangerous and is therefore avoiding it .

As far as a realtime map of flights are concerned who may take a look at Flightradar24

http://www.flightradar24.com/20,77/7


Since shock waves have non-negligible viscosity, why do the normal shock relations ignore friction/viscosity? The normal shock relations (seen below) are derived by using steady, 1D, neglect potential, no shaft work, adiabatic and zero viscosity assumption. However, since shock waves are "thin regions of high velocity and temperature gradients, where, friction and thermal conduction play an important role," (Anderson, Fundamentals of Aerodynamics) why is the assumption of zero viscosity correct?

While I know that the normal shock relations give the correct answer, I am unsure of why neglecting friction is justified and does not lead to inaccuracies.

Neglecting friction does indeed lead to inaccuracies. Those, however, are small for weak shocks, so using the equations for inviscid shocks gives a good approximation. Especially in the times before electronic computers, this was a major simplification while the error was tolerably small.

Stronger shocks should be avoided and replaced by a cascade of weaker, oblique shocks in order to minimize losses, so for practical work there is little need for equations which include those losses. Only in academic work and the design of reentry vehicles with their blunt geometries would the precise calculation of stronger shocks be of any interest.

But how large are those losses? Wikipedia has a page on normal shock tables and the last column gives the ratio of the stagnation pressures ahead and past the shock. Since both should be the same in case of a lossless shock, the real number gives a good approximation of the energy losses incurred over the shock. The more the result deviates from 1, the bigger the losses. 1% of stagnation pressure is lost at a Mach number ahead of the shock of approximately 1.225. 10% is lost at Mach 1.587. It goes down from there: 50% is lost at Mach 2.5 and 90% at Mach 4.4. All values use a ratio of specific heats of 1.4, so they are valid for a two-atomic gas (or a mixture of predominantly two-atomic gasses like air).


How can I find out if there's an EASA STC for something that has an FAA STC? I want to purchase something for my Cessna from a US web shop.
The part has a Supplemental Type Certificate (STC) from the FAA, but I don't find any information about the EASA equivalent.

Is there any database to check this?

Unless I am reading the document incorrectly, EASA and the FAA have a reciprocity agreement since 2011 recognizing each other's findings and approvals. That means an FAA STC approval is good in EASA countries (except the ones that are not EU members) and vice-versa.


Why do wall-to-wall airfoils in wind tunnels produce no (or infinitesimal) downwash? There are several induced drag explanations out there saying that tip/wake vortices “come from” downwash and downwash “comes from” lift. I’m using quotes because the concept is that there are no cause and effect events but simultaneous events that occur in a flow.

Essentially, these explanations describe how the flow turns downwards immediately after leaving the trailing edge of a finite wing; examples in stackexchange can be found here and  here.

Apologies if I describe the above incorrectly, feel free to correct me.

What I want to ask is if someone can “adjust/adapt” these explanations to describe what happens on an airfoil placed wall-to-wall in a wind tunnel that has no (or extremely small) induced drag. I’m not talking about theoretical infinite wings and mathematical explanations, I’m looking for a physical description similar to the ones given above for finite wings.

The only difference between wind tunnels and free flight (in THIS case) is that in the wind tunnel the upper and lower surface flows can interact only at the TE while in free flight they can interact (touch, blend, etc.) at the TE and at the wing tip.

In this NASA page, it is said that “The wing tip vortices produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root.” Although this is not technically correct (see wake vortex), the importance of exposing the wing tip to the flow seems to be underestimated in the explanations above so I’m looking for an explanation of the wall-to-wall airfoil behaviour through the above explanations.

About Peter Kampf’s reply:

Now I think we’re getting somewhere…

Wake vortex is certainly more important than tip vortices, this is why I used the word wake with tip when referring to vortices. Perhaps I wasn’t very consistent/clear with that (like the NASA page) so apologies for any confusion.

You correctly say that the effect of the wake vortex is reduced by the tunnel walls and there are no tip vortices, which means induced drag is greatly reduced for a wall-to-wall airfoil (essentially a side effect taken care of by grating). If it was possible to add infinite size end plates with zero drag on real aircraft wings (acting as wind tunnel walls), the induced drag would be greatly reduced.

This is all I’ve been trying to say, that the only explanation for the induced drag increasing drastically when we move from a wall-to-wall model (2D) to a finite wing (3D), is that the wing tip is exposed to the flow.

Yes, a vortex sheet leaves the wing.
Yes, there's no spanwise spillage of lower flow towards the upper flow.
Yes, tip vortices do not create induced drag.

However, the free stream air “sliding” next to the wing tip (where the tunnel wall would be) definitely interacts (pushes, gets pushed, blends, etc.) with:

The upper and lower flows (vortices) as it travels the chord of the wing.
The “combined” upper/lower flow (vortex sheet) at/after the trailing edge.

so the more the wing tip is “walled”, the effects of this interaction get weaker and consequently the induced drag gets weaker....

The explanations correctly say that wingtip vortices do not create induced drag but, at least that’s how I understand it, neglect the importance of the physical exposure of the wing tips to the air flow that causes the “interaction” described above.

Thanks to everyone for bearing with me, I’ve never written about aerodynamics in a forum…

Who says that the flows only interact at the trailing edge?

The flow on each side of a wing creates (and is the creation of) a pressure field which influences what goes on on the opposite side in subsonic flow. Only in supersonic flow is that kind of interaction impossible because small pressure changes move only at the speed of sound.

First to the topic of flow turning. Half of that has already happened at the quarter point of the wing chord, so it is spread out over the whole wing. At the trailing edge the local flow direction is parallel to the trailing edge inclination if flow is not separated already. Past the trailing edge the flow readjusts to the downwash angle which is normally largest behind the center wing. In the case of an elliptic lift distribution, the downwash angle is constant over span, but it is almost never strongest near the wingtips. The experts at NASA certainly know that, too, but I wonder how often they are consulted by the marketing people who write those web pages.

Next to the tip vortex: It does not cause induced drag and is only a side effect of downwash. Please read this answer and try to follow all the links. There is not a single vortex but a vortex sheet leaving the wing, and to better understand what is happening it is best to forget all that.

Now to the conditions in a wind tunnel. The flow there is practically like one in ground effect, so no full downwash like in free flow is produced. In order to arrive at realistic results, wind tunnels use correction factors in order to make the results of their measurements close to those in free flow. If there is a gap between the trailing edge and the wind tunnel wall (and there certainly is), there is also some downwash, but less than in free flow. As explained in this answer, the part of the wake which flows down past the wing is smaller than the wingspan, so a crippled version of the wake behind a wing will also form in a wind tunnel, with air near the side walls moving up and air in the center moving down. Normally, this is all equalled out when the air flows through the next grating which is installed to remove turbulence or help the tunnel flow around the next corner.

  the explanations above describe downwash as the main (almost only) reason for induced drag and tip vortices as just a result of downwash.

Yes, tip vortices are indeed a result of downwash. You link already to two good explanations for that, so I need to go beyond those answers. The two vortex cores resulting from the rollup of the vortex sheet trailing behind the wing are not really the tip vortices you might see in humid air, like in the picture of an A340 below (picture source).

The condensation at the wingtip is from the high suction caused by flow locally negotiating the small radius of the winglet tip. This is the tip vortex. The much stronger vortex pair forming behind the wing is what the NASA page refers to, but that only happens when the air is already long past the wing. That is the wake vortex. In the cores of the wake vortex pair the pressure is lowest because the radius of the helical motion is smallest - vortex strength is the same for every point in a plane perpendicular to the direction of motion of the wing. Therefore, the NASA page is at least misleading, if not outright wrong.

In the wind tunnel the same happens (minus the tip vortex), but boxed in by the wind tunnel walls. They slow the wake vortex by wall friction and stop the wake from continuing downwards, just like the ground does in ground effect. But there is still a pair of vortices close to the bottom wind tunnel wall (or the top, if the wing section is mounted upside down).


Why do swept wings decrease the drag increase that occurs in transonic flows? Swept wings are used in aircraft that fly in the transonic regime because they 1) delay the critical Mach number 2) reduce the drag increase that occurs (smaller drag divergence).

I am trying to understand physically why a lower drag is observed. A common explanation is that only a component of the free stream is perturbed by the wing (see: Why are swept wings better for breaking the sound barrier?), but this does not explain why there is a lower drag when shocks do occur over the wing.

More specifically, I understand why the curve below shifts to the right as sweep increases, but want to know why the drag is not as high.

Source: https://history.nasa.gov/SP-468/ch10-4.htm

Shock waves form where air can no longer "get out of the way" and begins to compress.

Sweeping the wing effectively increases the chord and reduces the rate of wing thickness increases as airflow moves across it, essentially making the body longer and more "streamlined". Instead of running into a steep hill and compressing, it can flow over a more gently sloping barrier with greater ease.

What is most interesting is the decreasing drag of the 47 degree sweep at high subsonic.  As sweep increases, airflow over the wing may transition from the classical front to back flow seen in straight wings to the deflected "spanwise" airflow of swept wings to the rolling vortices seen in highly swept "slender delta" wings.

At some combination of speed and sweep, airflow will roll up into a vortex rather than washing off the back or end of the wing.  It may be that, at higher subsonic speeds,  with greater sweep, the vortices roll into the back of the wing and push the aircraft forwards.

It has been thought the same thing occurs underneath thin undercambered wings, which may help account for some of their "magical" slow flight properties.  Definitely worth studying.

As far as 11, and 35 degrees of sweep, they follow predictable patterns based on compression effects at transsonic speeds, as does the Area Rule.

Are swept wings better for breaking the sound barrier?  Not really, as the X-1 proved.  But they do significantly delay transsonic drag at high subsonic speeds.