How does the F-35's roll posts work, and how does its engine turn down 90 degrees? I've heard about the F-35's roll posts but I don't get how they work and how its engine turns downwards 90 degrees, any ideas?

The purpose of the roll posts is to let the pilot have some control authority in hover and stabilization, along with controlling aircraft attitude. Basically, the Rolls Royce Lift System used in the VTOL F-35 has three components connected by drive shaft (along with the engine):

The LiftFan at the front
Three-bearing swivel module (3BSM), which turns the nozzle through 95$^\circ$, and
Two Roll posts on either side of the fuselage.

F-35 Liftfan system; image from nationstates.net

Basically, some bypass air is directed towards the (individually adjustable) roll post ducts to ensure stability during hover, which is done by the onboard computer. In this image, you can see the roll post doors opening on outboard sides of the MLG doors.

F-35 roll post doors opening; image from nationstates.net

The 3-Bearing Swivel Module (3BSM) has three segments driven by two actuators (the first one drives the first segment, while the second one drives the second segment directly and the third one through an angle gearbox). The three segments are cut at an angle, so that when transferring from forward flight to hover, the central segment ends up with its longer edge at the top (so that their sections with maximum length are on the same side), thus 'summing' the bearing offset angles to give a 95$^\circ$ deflection of the jetpipe nozzle in the downward direction.

From codeonemagazone:

  The three-bearing swivel nozzle, 3BSN, design uses three segments of the engine exhaust duct cut on an angle and joined by two airtight circular bearings. External motors drive geared teeth in these segments to rotate them to create the deflected thrust. The forward and aft segments always stay aligned with one another in the rotational axis. The center segment rotates through 180 degrees relative to them. The third bearing is on the back of the engine aft of the turbine stage and provides the ability to swivel the nozzle thrust axis in yaw at any pitch angle.

Three bearing swivel module, from codeonemagazine


With flight dynamic equations, how do I return to positive alpha if alpha is always negative with negative w? I'm coding a flight dynamics simulation, but my simulation runs into a problem with negative angles of attack. Once alpha is negative, it is impossible to get back to a state where the plane is generating positive vertical velocity.
At the start of each frame of the simulation we have some forward velocity ($u$) and vertical velocity ($w$), and at the end of each frame we calculate new values for $u$ and $w$. To keep things simple, assume the plane generates 0 lift at 0 alpha, we're always in level flight with no change to pitch or roll, and the pilot only makes changes to $u$ with the throttle.
If $u$ decreases enough that we generate less lift force than gravity, $w$ becomes negative and the plane begins to descend. The usual formula for angle of attack ($$) is a function of $u$ and $w$:
$ = arctan(w / u)$
This means a negative $w$ and positive $u$ will always give a negative $$.
I calculate lift with this equation (simplified. $q$ is dynamic pressure):
$L = 2π *  * q$
This means a negative $$ will always give negative lift.
And finally, the next frame's $w$ is just a function of lift and gravity (in level flight):
$w_{next} = w + L / mass - g$
So it is possible to have positive $$ and lift but end up with a negative $w$ due to gravity.
If a frame of my simulation ends with negative $w$, the simulation is put in a state where it is impossible to ever return to positive $$, because negative $w$ always gives negative $$. Negative $$ always gives negative $L$. And negative $L$ always gives negative $w_{next}$ Negative $w_{next}$ always gives negative $$ on the next frame.
Increasing $u$ eventually should return me to positive $L$ and $w$ as $q$ increases, but doesn't - it only generates more negative lift. What factors am I missing to be able to get back to positive $w$ with just an increase in $u$?

I think your problem is in part, that you assume that $w$ is positive if your aircraft is ascending. The opposite is the case. The aircraft coordinate system is defined as a right-handed system with the z-axis pointing downward.

This means a positive $w$ indicates that your aircraft is descending.
Therefore, if your plane keeps a constant velocity $u$, the body-fixed downward velocity $w$ will get positive because of the gravity pulling you down. This in turn results in a positive $\alpha$ providing lift. Eventually if you have implemented your simulation correctly, the $\alpha$-angle (and $w$) will oscillate for a bit at the beginning of your simulation, but after finding the steady-state, it will steadily descend with a positive angle-of-attack and a positive value of $w$.
P.S.: For the calculation of $\alpha$ it is very helpful to use the atan2() function, as it correctly discerns the various sign cases.


For training with a grace month, is it legal to fly in the month after training expires before going to school? Most 135 training/testing says something like this:

  §135.343   Crewmember initial and recurrent training requirements. No
  certificate holder may use a person, nor may any person serve, as a
  crewmember in operations under this part unless that crewmember has
  completed the appropriate initial or recurrent training phase of the
  training program appropriate to the type of operation in which the
  crewmember is to serve since the beginning of the 12th calendar month
  before that service. This section does not apply to a certificate
  holder that uses only one pilot in the certificate holder's
  operations.

Before that though, 14 CFR 135.323 states:

  §135.323 - Training program: General.

  ...

  (b) Whenever a crewmember who is required to take recurrent training
  under this subpart completes the training in the calendar month
  before, or the calendar month after, the month in which that training
  is required, the crewmember is considered to have completed it in the
  calendar month in which it was required.

  ...

So let's say that I completed initial training in March.  This says that if I complete recurrent training in February or April that they consider the training to have been completed in March.

So what happens if a year passes and recurrent training is due.  I don't make it in February or March, but the company schedules me for recurrent training towards the end of April.  Is it legal for me to fly in April before I go to recurrent training?

At this point, I don't meet 135.343 (because I am no longer within the required 12 calendar months) but I haven't been to training yet, so 135.323 doesn't really apply.

From my interpretation of the regulations I would say no, however every 135 company that I have ever flown for continues using pilots until they go to school, or stop flying at the end of the late grace month if they still haven't gone.  The POI's have never complained about it.  I feel like I must be missing something.

Is it legal for me to fly 135 trips during the late-grace month??  Note that the same the same scenario applies to 61.58 PIC checks as well, which also have a provision to train during the eligibility period (the month before or the month after it is due).

Yes, the FAA issued a Letter of Interpretation from their Chief Counsel in May 2015 which covers Section 61.58 proficiency checks and specifically addresses this issue.

It says that a pilot may continue to act as PIC during the grace month under Parts 91, 121, and 135.  It says (in part, and emphasis added by me):

  Section 61.58(i) states that, if a pilot completes the proficiency
  check in the calendar month before or the calendar month after the
  month it is due, "the pilot is considered to have taken it in the
  month in which it was due for the purpose of computing when the next
  pilot-incommand proficiency check is due."

  You have asked whether the language in paragraph (i) creates a "grace
  month" during which a pilot whose currency has lapsed under paragraph
  (a) may continue to act as pilot in command. The FAA has previously
  stated in a legal interpretation that § 61.58 includes a "grace month"
  for purposes of complying with the 12- and 24-month proficiency check
  requirements. We did not explicitly state, however, that a pilot may
  continue to act as PIC during this grace month. Legal Interpretation
  to Sean Conlin (Feb. 24, 2000). We have indicated, however, that
  similar provisions in parts 121 and 135 create a "grace month" during
  which a pilot may to continue to act as PIC during the month after the
  pilot's currency has lapsed? Legal Interpretation to Gregory S. Winton
  (2006), Legal Interpretation to J. Dennis Vanatta (June 25, 1996).

  The FAA intended the grace month to limit disruptions in operations
  conducted under parts 121 and 135 by accommodating pilots who are
  unable to complete recurrent training requirements in the month they
  are due. At the same time, the FAA did not want operators to use the
  grace month as a means of regularly extending a 12-month proficiency
  check to a 13-month proficiency check. For that reason, a proficiency
  check under part 121 or part 135 that is completed in the grace month
  is considered to be completed in the month it is due. The FAA believes
  that the same approach is appropriate for§ 61.58 proficiency checks.
  As such, a pilot may continue to act as pilot in command during the
  month after a§ 61.58 proficiency check is due. However, when a pilot
  completes a §61.58 proficiency check during this grace month, it is
  considered to have been completed during the month it was due for the
  purpose of calculating the next proficiency check.


What is the rate of acceleration based on downward pitch? In other words, I want to understand how gravity converts into thrust in respect to pitch. What is the general mathematical relationship? (For visuals, imagine a bird diving to gain speed.)

Assuming this is the standard inclined plane task from physics textbook, as here for instance, the acceleration is

$a = g \cdot \sin(\theta)$

where $a$ is acceleration, $g$ is the free fall acceleration (9.8 m/s^2) and $\theta$ is the pitch angle down (0 assuming horizontal).

This equation applies does not take the air resistance into consideration so would only apply to relatively slow speeds. The equation also assumes that the plane orientation matches the flight direction, this is obviously simplification.

Interestingly, under these conditions the acceleration does not depend on the mass of the plane. The equation should also apply for the positive pitch angles, but the acceleration would be negative in this case (slowing down).


Why were german pilots more successful on the eastern front than the western front? I was wondering why big german aces shot down the most enemy aircraft while fighting on the eastern front. Is this because the soviet pilots were undertrained, or their flying machines were inferior? Is it something else? You see all these German aces that have shot down 100+ aircraft and most of them fought on the eastern front. Why is that?

Several factors.  The ones you mentioned, plus the different purpose of most of the Soviet aircraft and the longer flight career of German pilots on the Eastern Front.  Beyond that was the fact that, early in the war, the Germans enjoyed near-complete air superiority (reinforced by their superiority in other factors).

As you note, at the beginning, most of the Soviet aircraft were inferior to the German ones; some of them were ten or even fifteen year old designs (or even older -- one squadron, known as the "Night Witches", flew biplanes into 1943), while the Germans flew new designs, either late pre-War or fresh off the drawing board.  The newer aircraft were faster, more robust against damage, carried heavier loads and more/larger guns.  This aircraft design deficit was reduced and possibly even reversed later in the war, but by then the Germans were on the defensive, being pushed back week by week.

The German pilots, at least early in the war, were far better trained -- theirs was the best air force on Earth in 1940 (though they started to run out of good pilots later in the war, because they had a far smaller population to draw replacements from and were resource limited in building aircraft, both for combat and for training).

Further, the German aircraft that the aces flew were dedicated fighters, designed from the ground up to destroy other aircraft in the air.  The BF-109 and Focke-Wulfe 190 were superb airplanes for the day.  The Soviets, on the other hand, especially early in the war, had a heavy preponderance of ground attack aircraft, and their primary mission was to destroy German armor, infantry, and supplies.  This made them sitting ducks for the German fighters and fighter pilots.  Further, there were lots and lots of those differently tasked and outdated Soviet airplanes -- the Soviet Union had more than twice the population of Germany and tens of times the landmass and resources; that gave them the ability to do the air equivalent of "human wave" attacks, which would still succeed even after a "turkey shoot" by the Germans.

Because of the disparity in pilots and aircraft, the German pilots lasted a great deal longer on the Eastern Front, where they weren't flying against the elite of nations larger than their, who hadn't had a decade or more with no powered aircraft and who had P-38s, P-47s, and P-51s, Hurricanes and Spitfires to pit against their aircraft.  The Western Front pilots had a strong tendency not to come home, even in the early years of the War, and much more so after the Normandy invasion.

Additionally, especially after the first winter of the Russian invasion, German pilots stayed on the line until they were physically incapable of flying -- either dead, or seriously injured (loss of eyes or limbs, for instance), or shot down behind lines and captured.  As noted in comments, also, there was a tactical difference, with German squadron leaders often flying without any thought for defense, trusting their mates to protect them while they simply destroyed as many enemy aircraft as possible, and this worked better against the relatively less experienced pilots in the Soviet forces.


What is airflow direction in turn? Let imagine that we put wind indicator on rotating merry-go-around.
Will wind indicator stay prepedicular to the radius of circle,showing tangetinal  airflow velocity?
(Wind indicator has equal mass and distance from pivot point so moment about pivot point is zero.It desing like this to stop centrifugal force to move one end out of turn..)
I think wind indicator will be prependicular to radius of rotation..
(I ask this because of glider yaw string...)

Let imagine that we put wind indicator on rotating merry-go-around.
Will wind indicator stay perpendicular to the radius of circle,showing
tangential airflow velocity?

Assuming that the indicator has a mass distribution such that the apparent "centrifugal force" from the rotation has no effect on it, and also assuming that the physical length of the wind indicator is trivially small compared to the radius of the circle, then yes, the wind indicator will remain perpendicular to the radius of the circle at the point where it is located, showing  that the "relative wind" is tangent to the circle.
A very lightweight piece of yarn would do essentially the same thing, though there would be a very slight tendency for the apparent "centrifugal force" to displace the free end of the yarn toward the outside of the circle.
However the problem gets more tricky if the length of the wind indicator is not trivially small compared to the radius of the circle.  If the length of the wind indicator is not trivial, this means that the "vane" or fin is located well aft of the weathervane's pivot point.  The relative wind at every point along the circumference of the circle is aligned with the line that is tangent to the circle at that point.  So when the vane streamlines itself to to the relative wind which is tangent to the circle at a point well behind the pivot point of the weathervane, this means that the weathervane is not parallel to a line drawn tangent to the circle at the pivot point of the weathervane.  Rather, the weathervane is skewed with its nose outboard, and its tail inboard, of the line drawn tangent to the circle at the pivot point of the weathervane.
Your intuition is correct that this is why tend to see some sideslip in circling flight, especially in slow-flying aircraft whose linear dimensions are not trivial compared to the radius of turn.  The vertical fin tends to streamline itself to the relative wind, which means that the fuselage tends to be parallel to a line drawn tangent to the circular flight path at a point well aft of the CG.
This effect is illustrated in section 8.10 of the "See How It Flies" website.
In short, in turning flight, the "relative wind" is curved, rather than linear.  It curves to follow the curvature of the flight path.


Are jet simulators a commercial business or are they owned by airlines? Are simulators owned by airline operators or are they property of private companies who are specialised in this business and offer the services on an outsourcing basis to airlines that require it?

I'm talking about proper simulators like the Airbus or Boeing ones and not PC based ones.

Both!

Some airlines operate simulator facilities. For example, United has a training facility in Denver with a number of simulators. In turn, they rent out training services to others who will pay to use the facilities (more info on their offerings here). They can also provide trainers, cabin crew training, ground school training, and other such services. Many airlines have similar programs. They've even auctioned off simulator time to customers willing to bid frequent flyer miles for the privilege.

When an airline doesn't own and operate the necessary equipment itself (even an airline that runs its own simulators may not have a suitable quantity for every aircraft in its fleet), it can contract for such services from someone else. That someone else might be another airline, a company that provides simulators (example) and may offer other training services like instructors, or a provider affiliated with the manufacturer (example).


Why is an aircraft carrier's arresting cable made from steel? Steel seems to be what modern aircraft carrier arresting cables are made from.

It's strong, provides some elasticity, and serves its purpose well. However, it does need to be changed after every so many traps due to metal fatigue (about every 125 landings from what I've heard).

Why not use a more modern material such as Kevlar? Wouldn't a material such as Kevlar reduce the frequency which the cable must be changed? It's lighter, more flexible, more durable, can tolerate more temperature extremes, is not susceptible to corrosion, and it might even be cheaper - since it's made from nylon threads. It also has the added advantage of being less dangerous in the rare event of a cable break under load.

This BBC Documentary (no longer available) posed the same question, but didn't really provide any answer.

Why are modern aircraft carrier arresting cables still made from steel?

The single most important thing for an arrestor cable is that it works reliably, compared to this incremental differences in weight or durability are fairly minor considerations.

One of the major advantages of kevlar is that it has a very high strength to weight ratio, however all this really achieves in this context is that the cable is thinner and lighter and I would guess that the weight of spare arrestor cables is vanishingly trivial in the context of the total payload of an aircraft carrier.

Another consideration is that Kevlar has much lower stiffness than steel, which will be further exacerbated by the differences between a fibre rope and a braided steel cable. This means that it will extend further under the same load. Not only does this mean that it allows the aircraft to travel further before it stops but you also need to fundamentally redesign the whole arrestor system to account for the very different material properties of the wire and then the new design would need to be tested, evaluated and certified.

Also, while kevlar is considered abrasion resistant by the standards of textiles it is nowhere near as resistant to abrasion as high strength steel cable. Consider that industrial steel cables may have a design life in the order of decades (think of suspension bridges) whereas high strength polymer climbing ropes are typically retired after a few years, mostly because of the cumulative effects  of abrasion, dirt and UV exposure. Also steel is almost entirely unaffected by contamination by spilled fuel, solvents, lubricants, hydraulic fluid etc.

Similarly the arrestor hooks on aircraft are made form high strength steel, which is much harder than Kevlar so even a small burr or sharp edge (as you miht get form landing or combat damage) could cut through a kevler rope like knife whereas a steel cable is much more resistant to sharp edged shearing.

125 loading cycles is nowhere near where metal fatigue would be an issue, especially in steel, so it seem more likely that the life of the cable is based on direct wear and local overloading from the arrestor hook. Steel being relatively hard while having reasonable ductility  is much better at tolerating this sort of wear and tear than most other materials.

It's also worth noting that overloading beyond the yield stress (but before total failure)  will often produce  a noticeable kink in a steel cable whcih is a warning sign that it shod be replaced. This gradual failure mode (ie damage is measurable before it becomes critical( is one of the major reasons why steel is preferred for safety critical structures ie you get some waring of impending failure.

However the most compelling answer is that steel cables are proven to work well for the job and so there is no compelling reason to redesign the whole system to use a new material for the sake of fairly marginal (if any) benefits in performance.

It's also worth noting that textiles are use for the arrestor nets for emergency landings where it is known or suspected that the landing gear on a particular aircraft is compromised. Here the light weight and greater stretch is a distinct advantage as it mitigates potential damage to the air-frame.


Has anyone successfully solo'd a helicopter with no actual flight experience? Especially at low speed, such as during takeoff, while hovering, and while landing, helicopters are famously difficult (impossible) to control for someone with no experience. Is there a documented instance of any person ever successfully taking off, hovering, and landing a helicopter with no actual flight experience? It must all be in one flight, first time--someone teaching themselves over multiple short flights / hours does not count.  Simulator use is OK and does not count as actual flight experience for the purposes of this question.

Yes, Eric "Winkle" Brown took delivery of a Sikorsky R-4 and flew it after reading the flight manual and getting a briefing from a USAF Sergeant. He had no experience or training in helicopters. He wrote about it in "Wings of the Weird and Wonderful".


Why does ATC ask a crew who has declared an emergency if their aircraft will be overweight when landing? I have heard ATC ask an emergency aircraft if they will be overweight when landing.
Why do they ask?
I am not asking about the consequences of an overweight landing or what procedures are required following one, but rather why ATC would ask for this in advance of the landing.

E.G.

Does ATC take it into account when offering a runway?
Does passing it on to emergency services somehow allow them to be better prepared than if they were informed after the fact?

If an aircraft encounters a serious problem quite soon after departure that forces it to land immediately, the aircraft may be above its certified maximum landing weight. This is because there is still a lot of fuel in the tanks, which adds a lot of weight.

As @RonBeyer mentioned in a comment, landing overweight can have a number of serious consequences. When landing a heavy aircraft, stopping it on the runway already requires a lot of energy (or rather, a lot of energy needs to be converted into something else). If the aircraft is heavier than it is designed to be on landing, this may cause excessive stress on the brakes. In extreme cases, this can make the brakes catch fire. Now, open flames under an aircraft that is almost fully loaded with fuel is the recipe for disaster.

In some cases, even though the crew brakes hard on landing, the aircraft may be so heavy that stopping on the runway is impossible. A runway excursion will then follow (a situation where the aircraft leaves the runway onto the grass or similar). This is not neccesarily dangerous, but it certainly can be, depending on the surrounding terrain:

(image from http://avherald.com/h?article=46f3d531. This aircraft was not overweight, but I still think the situation shown in the picture is relevant to this question.)

Obviously, in an emergency ATC, and in turn the fire and rescue crew at the airport, need to be prepared for such events. If ATC expects an aircraft will be landing overweight, getting the crew to confirm this will allow ATC to inform the fire crew that they can expect brake fires or a potential runway excursion. It can also enable the rescue leader to position the fire vehicles at the best possible position, which would be further down the runway than normal, since the landing roll will be longer. So basically, it's a way to get information to the rescue crew so that they can prepare for what might be coming.

Depending on the type of emergency, ATC will also ask the crew about other details, in some cases also with the purpose of passing this information on to the rescue crew, since the rescue crew will normally not be in direct contact with the cockpit as long as the plane is in the air. See also this related question: Why does ATC ask emergency aircraft about fuel on board?


What is the ceiling altitude for the Airbus A320? About a week ago, I was flying from Boston to San Juan in an A320 and noticed that the flight was staying at about 42000 feet for the major part of the trip, according to the flight map screen on the plane's entertainment system. That struck me as an unusually high altitude and I am even more surprised now that I have read a little on the subject. I am wondering if that was even possible, or the altitude readout to the entertainment system was simply wrong. The flight proceeded uneventfully in other ways, and was on time.

What is the ceiling altitude for the Airbus A320?

This question is a bit more generic. Ceiling altitude depends on many things such as (environmental conditions, engines operative, a/c aerodynamic characteristics, e.t.c), each factor by itself and in combination, can affect the ceiling altitude. You can get an idea and some definition from here: https://en.wikipedia.org/wiki/Ceiling_(aeronautics) .

  For most of the A319/A320/A321 aircrafts Maximum Certified
  Altitude according to FCOM is FL391 considering (2 engines operative
  and environmental allowing.)

Now according to FCOM DSC-22_20-50-10-28, (controls and indications) you can find the MCDU page,

REC MAX:
This field displays the recommended maximum altitude (in magenta), that is computed based on the current gross weight and temperature, and assuming that the anti-ice is off (Refer to QRH graphs, if icing conditions are expected). It provides the aircraft with a 0.3 g buffet margin, a minimum rate of climb at MAX CL thrust, and level flight at MAX CRZ thrust. This field is limited to FL 398.
If one engine is out, this field displays the recommended maximum engine-out altitude, that is computed based on the long-range cruise speed and assuming that anti-ice is off.

Note: The OPT or REC MAX field may display a value up to
FL 398. However, the current aircraft’s maximum certified altitude is 39 100 ft. Disregard the OPT or REC MAX value, if it exceeds FL 391.

Now for entertainment system, (again according to the books), reads and display data taken from the FMGC.

One scenario is the input from the pilots, in MCDU was the displayed LVL, and subsequently in the entertainment system, the displayed LVL
was that (after the CRUISE phase), even though the actual altitude was
different that the displayed.But again it's impossible to insert an altitude above the value of "FL398" MCDU says "ENTRY OUT OF RANGE".

  So the conclusion As kevin said already, possibly wrong data were
  displayed.


FAA Turbine engine FCU regulations Does the FAA have any regulations governing the design of a turbine engine FCU? I'm being questioned about an older Beechcraft PT6A FCU - in particular, how can I guarantee that when the power condition lever is at the shutoff/cutoff position, no fuel is moving to the combustor...? Any help would be appreciated... Thanks!

Edit - for those needing clarification:

Does the FAA have any regulations governing the design of a turbine engine FCU?

Simply as stated, does the FAA regulate how a turbine engine FCU can be designed - pressure allowances, flow volumes, etc.

I'm being questioned about an older Beechcraft PT6A FCU - in particular, how can I guarantee that when the power condition lever is at the shutoff/cutoff position, no fuel is moving to the combustor...?

As stated, I'm being asked to prove that when the power condition lever in the cockpit is moved to the shutoff/cutoff (pick one term) position, fuel will not flow into the combustor. Tying into my first question, does the FAA regulate that a turbine FCU MUST absolutely cut off all fuel moving to the combustor, or is that just left to the designer to ensure...?

Hope that clarifies everything. Not sure how much more detailed I can get without specific and possibly pointed questions.

Subpart E (Powerplant) of Part 23 (AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES) has quite a few regulations regarding fuel system operation. In particular, section 23.995, "Fuel Flow".

However, I could not see in there a regulation requiring no fuel to flow in the off position, but given the safety implications of that not occurring, I would fully expect that is a requirement that needs to be demonstrated.

It appears to exist in Part 33 (AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES), subpart E (Design and Construction; Turbine Aircraft Engines) section 33.75 "Safety Analysis". This states:

  The applicant must analyze the engine, including the control system, to assess the likely consequences of all failures that can reasonably be expected to occur.... The safety analysis must also include, but not be limited to, investigation of the following: (vii) complete inability to shutdown the engine.

I don't expect there are any regulations defining how it needs to be designed to do this - but rather regulations saying what it needs to do, and the way of achieving it is left up to the designer.


Do fuselage rip/crack/damage sensors exist? Are there sensors that can detect a rip, crack or damage in the fuselage skin by not using air pressure differential on aircraft?

Such sensors certainly exist. The sensor is as simple as can be: usually a piece of wire. The bonding of the wire to the skin or structural elements dictates what kinds of damage it's sensitive to.

The "wire" can be in the form of a thin metallization on a thin non-conductive substrate that's intimately glued to the skin - such gives the most sensitivity to cracks, and is a crack detector.

If you wish to detect whether the rocket is in the process of breaking up, you simply need to run a thin, insulated wire along a longitudinal structural member, such as a tank, and attach it in a few places, but certainly rigidly at the ends. The input is used sometimes for automatic flight termination.

The major problem with pressure differential sensors is that you really need a big hole to overwhelm the pressurization systems on turbine-powered transports. A pencil-sized or even coin-sized hole is noisy but essentially irrelevant, and no, you won't get sucked out through it.

Why is it so? For cabin ventilation to work to begin with, there must be an outflow valve. Such valves, in steady state, have openings way larger than a coin. The more unplanned-for holes you add to the pressurized section, the smaller the outflow valve's opening will be. Only when the outflow valve completely closes and the pressure is still insufficient, you've got a problem...

So yes, the outflow valve's position in steady state can be used as a measure of air leakage elsewhere.


Why don't commercial airliners use variable sweep wings? I know they're not used in military aircraft because it decreases the stealth factor, but why don't commercial plane manufacturers make variable sweep planes?

Variable geometry is not used in commercial aircraft design for the same reason that they initially became popular in the 1960s but have fallen out of favor in more recent times: any kind of aerodynamic performance gains associated with the design is not worth the additional weight increase, and subsequent decrease in payload, associated with variable sweep wings.

There have been attempts to do so in the past. The ill-fated Boeing 2707 SST was supposed to make use of swing wings but designers could not achieve their payload goals with the design.   Usually more judicious application of aerodynamic principles during initial design is far more advantageous then beginning to design out right with a swing wing.


Why is the most common hydraulic fluid so poisonous? The usual hydraulic fluid, Skydrol, is poisonous. If it makes it to the air conditioning system, it causes fume events that may have adverse health effects up to and including at least one fatality. And it is not easy to completely prevent the possibility of it creeping into the air ducts as bleed air is also used for pressurizing the hydraulic reservoirs (at least on some aircraft).

So why was this organophosphate-based fluid selected over anything safer? Are there no safer alternatives?

Update: I should have linked the Qantas A332 at Sydney on Dec 15th 2019, hydraulic leak prompts evacuation accident, which actually inspired the question. That one is clearly hydraulic-related (while in most other fume events we never know for sure), and there is a comment down the page:

  By geezer on Monday, Dec 16th 2019 14:48Z:

  Nothing to do with the apu other than the duct by the packs. Hydraulic reservoirs are (gas) pressurised by either an engine bleed or from the pneumatic duct in the pack bay. Hence a tee piece. If one system loses pressurisation, say overnight, then with a couple of leaky check valves one reservoir can slowly fill another back through the hydraulic service manifold, as all three systems are serviced from one inlet using a three-way valve. Eventually one tank is overfull to the point where the fluid now travels back down the air pressurisation pipe (a small bore pipe). Another check valve stops it flowing back to the engine so it ends up dribbling into the pneumatic duct in the pack bay. We know this because we have found skydrol in these small bore pressurisation lines. I hope you get all that. I think I need a little lie down now.

It should be noted that this is not verified and may not be the cause, but A330 has APU intake on the bottom, so the APU could have also ingested the hydraulic fluid dripping from the bottom of the fuselage into the inlet.

Fume events are from two sources in the vast majority of cases:  Deicing fluid, and engine oil.
The Aviation Herald event linked was poisoning from (really severe) engine oil contamination (mainly the toxic TCP additive; it's the same stuff used as a lead scavenging agent for running 100LL fuel on low octane engines, and it's also the oil additive that Lycoming specifies for the H2AD piston engines for the cam spalling problem). Phosphate Ester hydraulic fluid had nothing to do with it.
The oil contamination comes from 3 sources:

A failed compressor section seal on the main engine at some forward bearing that is able to feed oil into the compressor upstream of the bleed port used for air conditioning.
A blown seal on an air cycle machine (the turbocharger-like cooling unit in an air conditioning pack) allowing ACM lube oil into the conditioned bleed flow.
An APU compressor seal failure (more or less the same as an engine oil seal failure) getting into the APU bleed and into the A/C packs.

Probably the majority of fume events are from oil traces, only enough to make smells, but which is really hard to remove, and deicing fluid (when airplanes get deiced and too much was sprayed into the engine and the engine didn't get enough of a purge run to clean the compressor out before turning on the bleeds).  Deicing fluid fume events happen a lot and are probably the most common cause of fume turn-backs.
Skydrol/Hyjet is mostly a hazard to the techs that work on the planes.  Anyone here who turns wrenches on transport airplanes will tell you that Phosphate Ester fluid is nasty, nasty stuff to get on your skin and probably the single most unpleasant aspect of the job.


Why are private jets more expensive to travel in than commercial aircraft? I am not sure if this is the right site for a question that asks about the economics of aviation rather than the technical aspects of it, but I couldn't find any other stack exchange site so I am asking this here.
As far as I understand, the reason why commercial aircraft are cheap to travel in is because the operational costs of the aircraft are divided among the many passengers in the flight. A fair share of the maintenance costs come from depreciation of the aircraft and the engine overhaul and inspection. In a business jet, I would assume the cost of fuel and the landing fees are much smaller than that of commercial aircraft and, as a result, do not add much to the cost per seat (since the costs are proportionally smaller - approximately at least). So I suspect that costs such as engine overhaul and aircraft inspection do not scale with aircraft size and are approximately the same for both business jet and large commercial aircraft. As a result, the price that each passenger has to pay is much higher.
I tried looking for a few sources on the internet that confirm this but couldn't find any that show a detailed account of expenses. If this hypothesis is correct, I would appreciate it if you could direct me to a source that compares the operational expenses of any commercial aircraft with a business jet.
So is it safe to say that travelling/ transporting cargo is cheaper in a large aircraft than a smaller one if the fuel efficiency is comparable?

Aside from the economy of scale effect of a large aircraft vs a small one you can compare apples for apples by looking at the same type used on both roles.
The Canadair CRJ200 was sold as both a regional airliner and a corporate jet, called the Challenger 850, so it makes a good comparative example.
The Challenger 850, being, mechanically, a CRJ200 with a corporate interior, has pretty much the same numbers for fuel burn etc, as a 200.  But it's only carrying 1 to 12 passengers, vs up to 50.  So even "full", its fuel burn per occupied seat mile is always going to be several times higher than the airliner version operating with decent load factors (and they aren't much lighter; the corporate interior is very heavy and with 12 bodies aboard you are close to gross weight with a large fuel load - add in a long range fuel system option by STC for that airplane, and the payload with full fuel was 2 persons... yes, 2).
Then there's utilization.  The CRJ 200 airliner averages about 2500 hours a year.  A Challenger 850, like most corporate jets, averages 300-400 hours per year (corporate shuttle versions fly a lot more, maybe over 1000, but they have regular airline style interiors).  All the fixed costs of operation are amortized over a way smaller number of operating hours (most corporate airplanes would fall apart if you put airliner hours on them; they are not designed for that level of use - this made the Challenger 850, with an airframe meant for 2500 hours/year, absolutely bulletproof in its corporate role).
So you have a cost double whammy.  You're using several times more fuel to get around for each warm body on board (what I might call "warm-body-miles"), compared to an equivalent airliner, and you're doing it infrequently and so are paying through the nose for all the fixed costs like insurance and hangar space, etc.
The reason for the existence for most corporate airplanes is the time valuation of the "warm bodies" travelling on it.  A large company makes the calculation that the value of the CEO, and the CEO's lieutenants' time, justifies the astronomical operating cost of a corporate aircraft; in effect, the jet makes money for the company by making the senior management's job more efficient (in theory - no doubt a lot of it is just "because they can").
If you're the other type of corporate jet owner, the ultrawealthy who own the planes personally, economics doesn't even come into it (I knew a guy who looked after corporate jet purchasers pre-delivery, and he told entertaining stories of bizarre customer meetings, with oligarchs and their children arguing over 500,000 dollar interior options as if they were fighting over who got to lick the cake icing spoon).


Why does an airplane require 3 pilots on board? A car requires only 1 single driver. Why do airplanes require 3 pilots given that each pilot does not come cheap to the airline? Furthermore, a plane can go on auto-pilot while a car cannot. Could there be more room for efficiency by reducing the number of pilots needed?

Actually, for all but very long haul flights, airline aircraft only require two pilots, a Captain and a First Officer. For those long haul flights that exceed the duty time in a pilot seat, a relief pilot is also carried.

Older aircraft often also had a flight engineer. Technically speaking the f.e. wasn't usually required to be a pilot, but he was a very necessary crew member. You could say that airlines became more efficient crew-wise when they moved from 3-man cockpit crews to 2-man cockpit crews. For example, the 747-100/200 series uses 3-man crews, but the 747-400 uses 2-man crews. The change was made possible by using automation and instrumentation that wasn't available when the first 747 models came out. Indirectly the increase in engine reliability also helped. If you lost an engine in an 3-man crew 747, the typical manner of handling the loss was to have the first officer fly the airplane while the captain and the f.e. worked the problem.

To illustrate the increase in automation, the 747-100/200 series had, typically, seven or nine fuel tanks, and there was a fuel burn schedule that had to be followed to keep the structural stresses and balances between different levels of the tanks within limits. The f.e. took care of that, doing it manually by throwing switches on his panel. The 747-400 and follow on models do that automatically by computer.

Basically the number in the cockpit crew is determined by safety. There's the basic consideration that if you had only one pilot, and that pilot died or otherwise was disabled, that might well be catastrophic. Beyond that, though, there's the matter of what all has to be done and for how long. Pilots make mistakes, and the longer they've on duty, the more mistakes they make. And if an emergency occurs, the work load can get very high.

You can't reasonably compare an aircraft to a car for at least two reasons. First, a car is moving in only two dimensions, an aircraft in three. Second, a car can stop moving, an aircraft can't; it has to keep moving. Let's say your car loses power. Usually that won't result in anything more than inconvenience. If your aircraft loses power, you've got a much more serious problem. Most airline aircraft have at least two engines. Losing one isn't going cause a crash in most two engine aircraft, losing all power will.


What's the difference between fog, fog banks, and fog patches? My meteorology textbook (from Pooley's Air Pilot's Manual series) has a list of abbreviations used in met reports and forecasts. They include:

FG for "fog"
BCFG for "fog patches"
PRFG for "fog banks"

What's the difference between these three?

Thanks to mins's link, the Australian Government Bureau of Meteorology explains everything in detail. To summarise the particular part I was asking about:

FG on its own means fog (visibility < 1000 m) covers more than half of the horizon
PRFG and BCFG are fog covering less than half of the horizon
PRFG (fog bank) is more extensive than BCFG (fog patches)

Another way of looking at it is like cloud cover descriptors. FG is like BKN or OVC: more than half the horizon is covered. PRFG is SCT, and then BCFG is FEW.

All the above are used when the temperature is above freezing point and the fog reaches more than 2 metres above ground. If the fog is only below 2 metres agl, it's shallow fog MIFG instead. If the temperature is below freezing, so the fog consists of supercooled water droplets, it's freezing fog FZFG.


Understanding "non-WAAS G1000" in a 2006 Turbo 182 All,
I am trying to get my head around something my local avionics shop is telling me about a 2006 T182 that has a GTX345R fitted. Surely, the GTX345R adds WAAS capability but he referred to this configuration as a "non-WAAS G1000"

I feel like you'd need to be Stephen Hawking to understand this stuff!

Can anyone shed light on the way in which the addition of a GTX345R (replacing GTX-33) means it is not WAAS???

Thanks in advance,
Michael.

The GTR345R is a transponder that can be purchased with a built-in WAAS GPS (link)—which this one apparently has. So it provides the WAAS GPS positioning required for ADSB. The GPS in the transponder does not provide a GPS source to the G1000.

From what the shop said, it looks like the G1000 is an older version that is not WAAS capable. That’s probably why they chose to install the transponder with built-in WAAS since it was much cheaper than upgrading the G1000.


Boundary layer momentum thickness Could someone please explain the concept of the boundary layer momentum thickness and its difference to boundary layer thickness calculation? Some explanation about the application of the boundary layer momentum thickness is also much appreciated.

Thank you in advance.

Both are different ways to express the boundary layer thickness in a deterministic way.

The boundary layer according to wikipedia:

  a boundary layer is an important concept and refers to the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant.

Since there can be debate about what significant means, multiple formulations for the boundary layer thickness exist. And depending on the problem to solve, one of them can be more suitable than the other.

Therefore the boundary layer thickness is by typically specified as the distance where the velocity reaches 99% of the free stream velocity.

The boundary layer momentum thickness can be exactly specified as the distance a uniform flow field should be displaced by to equal the total momentum flux (m*v)*v of the real boundary layer (non-uniform).

There is also the boundary layer displacement thickness, comparing the mass flux deficit (m*v) rather than the momentum flux deficit (m*v)*v, as highlighted by the slides in Juan Jimenez's comment.

Take a look at this quora answer as it contains good explanatory figures.

Update:

I found a paper with comparative figures of the displacement and momentum thickness based on XFOIL results. It's worth to take a look at. Iliev, Sergiu. (2016). Aerofoil Analysis using XFOIL - Practical Implementation for Preliminary Wing Design. 10.13140/RG.2.2.30861.54243.

An example of where displacement and momentum thicknesses are useful can be found in the XFOIL documentation.
The displacement thickness can be used to couple a boundary layer solver with a potential flow solver (ISES code).
XFOIL uses the momentum thickness to calculate the viscous drag.

  Viscous Formulation

  The boundary layers and wake are described with a two-equation lagged
  dissipation integral BL formulation and an envelope e^n transition
  criterion, both taken from the transonic analysis/design ISES code.
  The entire viscous solution (boundary layers and wake) is strongly
  interacted with the incompressible potential flow via the surface
  transpiration model (the alternative displacement body model is used
  in ISES).  This permits proper calculation of limited separation regions.
  The drag is determined from the wake momentum thickness far downstream.
  A special treatment is used for a blunt trailing edge which fairly
  accurately accounts for base drag.


If the EPU (Emergency Power Unit) on a modern fighter jet fails, will the aircraft drop out of the sky? If the EPU (Emergency Power Unit) and batteries plus engines fail on a modern fighter aircraft, basically they will fall out of the sky, there is no dead stick landing - is this correct?

How Are Modern Fighter Aircraft Controlled?

The F16 was the first US military aircraft to be designed around the revolutionary fly-by-wire concept.  This meant there were no direct mechanical linkages between the cockpit and major control surfaces, and that all pilot input was electronically carried to the hydraulic system via wire.  However, since the F16 burst into the scene, there have been further major advancements in aircraft systems design.  Late Gen 4 aircraft, such as the F/A 18 Super Hornet, are designed around a concept now called control-by-wire.  Control-by-wire aircraft not only have removed mechanical linkages to the hydraulic system, but now the pilot is only a voting member.  This means that all input is received by a computer bank that then computes what it thinks the pilot is intending to do, and then autonomously operates the control surfaces to achieve the effect.  This means that Gen 4 aircraft rely heavily on both computer integrity (and there are plenty of self-healing backups), and the power required to run those systems.

Power Failure

If there was a catastrophic total electrical failure and the only remaining power supply was the battery, the only option is for the pilot to restart a generator.  In the event that the engines were operating and all power generation was destroyed, the aircraft would only be flyable for the duration which the battery could provide power to the aircraft--that's a very limited duration.

Possible other scenarios could include a dual-engine failure. In this instance, the aircraft may be able to fly fast enough to generate enough airflow down the turbine to turn the generator and perform an airstart, but this would only be useful in a dual engine failure, and only if there was enough altitude and airspeed to trade to make it a realistic option.  Otherwise, the battery would probably be used to power the APU, and attempt a relight.  Again, if both engines relit and the generators (including the APU) failed to produce power, then the aircraft would not be flyable.

But what about the RAT?

Although currently in limited use on some trainer aircraft like the T-45, modern fighter aircraft do not use a RAT.

So what usually happens in a total electrical failure?

If the aircrew cannot get a generator back online then they eject.


Have there been any cases of collision/airprox of small drones with manned aircraft? Small civilian mini-drones such as the DJI Phantom or professional surveying drones are becoming increasingly widespread, raising concerns about the potential collision/airprox with manned aircraft (especially GA where flight altitude is typically relatively low).

Have there been actual cases of collision and/or dangerous airprox reported so far? What were the consequences?

CASA (Australia's aviation authorities) has a report on an airprox between a crop duster and a surveying mini-drone:

ATSB incident report


Why do we need to fly at lower altitudes at night to prevent hypoxia? FAA recommends general aviation aircraft to stay below 5,000 ft at night time.
I thought the reason is that oxygen usage with our eye's rods since the rods uses much oxygen and rods can get hypoxia which makes tunnel vision and eventually not able to see outside.
But one of my friends claimed the reason why we are supposed to fly at a lower altitude to prevent from hypoxia is that at night there is higher pressure near the ground which makes fewer air molecules at high altitude at night than daytime.

Is this true that we get hypoxia more easily at night time because of pressure and that's the reason why FAA recommends us to fly below 5000ft?
If the question number 1 is not true (from my understanding), is rods problem the only reason why FAA recommeds us to fly below 5000ft at night time?

The main reason stated by aviation authorities is due to the impairment of vision, your rods provide the majority of your sight in low light conditions and are affected by the lower oxygen levels. However, your colour perception (provided by your eye's cones) is also affected and this is already degraded at night due to the lower light levels.

The below quote is from an EASA General Aviation Safety Briefing on Preventing Hypoxia

  Hypoxia also impairs night vision. Because the rod cells in the eye, which give us night vision, require a lot of oxygen, a lack of oxygen causes visual impairment.

Regarding your friend's point on the diurnal atmospheric changes, this should not be a factor as the suggested altitudes by the aviation authorities on hypoxia prevention should be given as a pressure altitude (i.e. the altitude with the standard altimeter setting of 29.92 inHg or 1013Hpa)

Hope this helps


If a pilot waves back, does it violate the sterile cockpit rule? There was a video posted to Reddit of pilots waving back from their commercial jetliners. What followed was a discussion of whether or not this was a violation of the sterile cockpit rule. Given that waving back is non-essential communication, is this a violation of the FAA sterile cockpi rule?

From rgl.faa.gov

  Sec. 121.542

  Flight crewmember duties.

  (a) [I omitted this, it's about noncritical (company) procedures]

  (b)
  No flight crewmember may engage in, nor may any pilot in command
  permit, any activity during a critical phase of flight which could
  distract any flight crewmember from the performance of his or her
  duties or which could interfere in any way with the proper conduct of
  those duties [...] such as [...] engaging in
  nonessential conversations within the cockpit [...]

  (c) For the purposes
  of this section, critical phases of flight includes all ground
  operations involving taxi, takeoff and landing, and all other flight
  operations conducted below 10,000 feet, except cruise flight.

One could argue that waving could distract the pilot taxiing, so according to the letter of the law, yes, you would violate the rule.

However, I think in the spirit of the rule it's fine, since it just means that the pilot not taxiing is paying attention to the surroundings as well. Reading between the lines, it basically says

  (a) During critical phases of flight, all crew should focus on flying, not selling cooked cardboard

  (b) Also, don't read the paper with a croissant while asking help with a crossword during said phases.

  (c) Critical phases of flight are when you're likely to read the bumper sticker on another jet.

I don't think a quick wave is quite as severe a violation as e.g. reading the paper (or, as the FAA calls it, reading publications not related to the proper conduct of the flight) during take-off. Compare it with a bus driver waving to a fellow driver or one reading the paper while driving. But then, if you do park your plane in a ditch while trying to wave to your mom, don't be surprised to be held fully liable.


Why do Turkey and UAE limit the age of aircraft flying over their territory? Why do the civil aviation authorities of Turkey (15 years civil, 25 years cargo) and United Arab Emirates (20 years) limit the age of aircraft flying over their territory? I think if an old plane receives good maintenance and doesn't have too many cycles, it's still a safe plane.

For reference, see this news article of a leased Kabo 747-200 that is being refused clearance by the UAE because it is older than 20 years.(German/English)

Sounds like a market protectionist move by the nations. i.e. The airlines of UAE and Turkey are relatively new. At least their current expansion to a scale that is competitive with the major international airlines is relatively recent. Their airports have also undergone fairly recent expansions to capitalize on their geographical location as hubs between N. America / EU to Asia-Pacific nations.

I suppose they give their own airlines a competitive advantage by restricting older aircraft. The US / European carriers can only allocate the fraction of their fleet that is new to flights to Turkey / UAE which constrains their activities.

The average fleet age of some airlines in 2013:

Delta  16 years
United  13 years
American + US 13 years
Lufthansa 13 years

Now compare with

Turkish  8 years
Emirates 6 years

Basically, international trade treaties have made it difficult for nations to overtly engage in protectionist measures. So bureaucrats figure out indirect means under the guise of technology, quality etc. that are ostensibly  not favoring domestic industry but are effectively a discriminatory measure.

Take the numbers with a pinch of salt because airline fleets are continuously upgraded so the average age of a fleet can change pretty rapidly.

Caveat: The average fleet age may not be representative of the equipment flying international sectors.

PS. I don't think the regulation has much to do with real safety because safety is concerned with many other factors outside of aircraft age. The safety aspects are better regulated by adopting the FAA safety watch-list based on national assessments.

At best, this sounds safety theater. Not safety.


What difference is there between stabilizers and stabilators concerning longitudinal stability? It is known how stabilizers affect the longitudinal stability of an airplane.

What about longitudinal stability in airplanes using stabilators (all-flying tails) or trimmable stabilizers?

Is it to be expected that certain combinations of angles of attack of the main wing and stabilators or trimmable stabilizers should lead to longitudinal instabilities?

There is no fundamental difference between them in such context. If you don't go to the extremes like 90° deflection (and generally stay in the linear range), you can mentally replace an elevator with a stabilator at a different equivalent angle. All-moving surfaces are typically a design solution when additional efficiency is required for whatever reason.

There are finer effects, especially at supersonic, but they don't change the overall picture for 'normal' aircraft. You can always find border cases when, say, elevator will stall but an equivalent stabilator will not yet, but they will generally be beyond the operational envelope.

Stability is primarily determined by derivatives, i.e. the way moments change with changing angles (of attack in this case). Absolute angles (tail vs wing) are less important and are usually the consequence of the desired derivatives.


What are the advantages and disadvantages of autogyro aircraft? What are the advantages and disadvantages of autogyro category of aircraft?  Is there a maximum speed an autogyro can attain?

Image source

From the same website:

  Zooming through the sky in something that looks like a flying chair, isn't for the faint-hearted.

  These aircraft look like a cross between a small helicopter and an upside-down lawnmower.

Autogiro's are used by bush pilots for mustering in Australia, due to their versatility and relatively low cost. And are also used for police patrolling in Tomball Tx.

Image source

Advantages of the autogiro:

Versatile and small.
It can fly very slowly, safely. From this site:

  Gyros cannot stall because the rotational speed of their rotors, and hence their lift, is not dependent on forward speed. At zero forward airspeed a gyro will slowly descend, to a survivable landing.

It can land anywhere.
It can takeoff from almost anywhere if the jump start is used: the engine spools up the horizontal rotor, then after take-off drives the propeller only.
Less complicated than helicopters, resulting in lower acquisition cost - much less complicated rotor hub.
Lower running costs than helicopters.

The disadvantages:

They are classified as recreational aircraft, with implications on licensing and oversight.
Some statistics show them to be more dangerous than most recreational aircraft. From this website:

  Aviation statistics show gyrocopters to be more dangerous than most recreational aircraft. An Australian Transport Safety Bureau report on accidents between 2004 and 2013 found half of all gyrocopter accidents to be fatal, with the aircraft responsible for the worst fatality rate per hours flown.

This can also be because non-fatal accidents with autogiros are under-reported.
Licensing (in Australia) is done through the sports association, which implies no access to commercial use of gyrocopters.

The simple autogyros shown above are not built for speed with their un-aerodynamic shape. Power to overcome parasitic drag scales with (engine power)$^3$, and aircraft designed for high speed have a much more streamlined shape than the gyrocopter has. From this site:

  Gyroplanes have somewhat different flying and performance characteristics to fixed wing aircraft. They have high drag and are therefore relatively slow, with typical cruise speeds of between 50 and 60 kt. The gyro’s advantage is not top speed but its ability to fly slowly, which is why under the name autogyro they were developed in the 1930s as a precursor to the helicopter.

But the flying armchairs for sheep mustering are not the only types of autogiro's. The fastest helicopters have a vertical thrust propeller as well:

Image source

Technically not an autogiro because the main rotors are driven, but the thrust propeller helps relieving the main rotor forward thrust. The Fairey Rotodyne was a pure autogiro, aerodynamically shaped, and had a maximum speed of 166 kts. Further speed increase is only possible when applying coaxial rotors like the X-2, to remedy instability due to retreating blade stall.

Image source


Is a firm landing an indication of the pilot inexperience? In a commercial airplane, when a landing has been a bit firm, a passenger may think it was not perfect, or maybe performed by an inexperienced pilot.

(Source)

A perfect landing is seen by passengers as one where the contact with the runway is almost imperceptible. Is that true? are there other technical or regulatory explanations?

First of all, the landing should be in the touchdown zone. Often I see pilots try to achieve a very smooth landing but floating far out of the touchdown zone before touching terra firma. Then they have to brake hard and taxi of at the far end. There are no points for landing outside the touchdown zone, even if it is the smoothest landing imaginable. In many overruns, landing late is a contributing factor.

A good landing is in the touchdown zone, firm enough to quickly spin up the wheels (and prevent wear by smearing rubber over hundreds of feet), and not so hard that it bounces up again. In calm winds, that can be very smooth, cross wind and wet / contaminated runways require a bit more of firm landing.

Hard landings1 can occur due to turbulence (wind shifts), upslope of the runway or when the air density above the runway is lower due to heating (typically in calm winds, with sun heating the runway surface)

So a hard landing is not necessarily an indication of pilot inexperience. However, some of the hardest landing I have ever experienced were when I joined in a training flight where new airline pilots were practicing landings in a Boeing 737 for the first time after their simulator training. I wouldn't be surprised if the runway was dented :-)

1Hard landing is used as a colloquial term here, not in the context of EASA CS 25 §473. See this answer for that context.


How long before departure can an IFR clearance be copied? When flying IFR under the FAA the departure procedure starts with copying an IFR clearance from ATC.

My question is, how long before departure is it okay to go ahead and copy down the clearance?

Sometimes clearances include void times. The context of my question is for clearances delivered, for example, be clearance delivery at a towered airport. These clearances don't include void times. What't the time limit on these clearances?

You can generally get your clearance 30 minutes prior to the departure time you listed on your flight plan.  If you call clearance delivery prior to that they likely won't have it and will tell you to call back 30 minutes prior to departure.


How do pilots fly an MLS approach? There is not much information on the cockpit aspect of the Microwave Landing System. Apart from Wikipedia, there's this excellent 15-minute FAA video from 1974 on TRSB (time reference scanning beam) MLS.

  Similar to other precision landing systems, lateral and vertical guidance may be displayed on conventional course deviation indicators or incorporated into multipurpose cockpit displays. Range information can also be displayed by conventional DME indicators and also incorporated into multipurpose displays.

How do pilots fly a non-straight-in (curved path) MLS approach on a Course Deviation Indicator?

(Source) CDI instrument.

I'll offer an analogy for the simple way I understand MLS: If VOR/DME were to be magically very accurate and to have slant information, an airplane would have been able to perform accurate area navigation using a single VOR/DME station. Instead, this station is the MLS system which covers the approach segment.

(Source–PDF) MLS depiction from 1978.

With the ever changing track and glide-path, I understand even in the 70's an FMS-like system was installed for the preset approach waypoints—in the lateral and vertical.

Even when coupled to the Flight Director, is there a requirement for a multi-crew, i.e., the Pilot Not Flying would be calling out the upcoming changes? Is there a requirement for an operative (must be on) autopilot? From the FAA video starting at 11:55, I get the feeling that for anything but a straight-in approach an autopilot is a requirement.

It is still being used in Heathrow,* perhaps there's information based on an MLS-certified civilian plane?

Apologies for the mess above, it all boils down to:

From the aspect of instruments and procedures, how do pilots fly an MLS approach?

* The link is now dead, it seems Heathrow no longer offers MLS approaches, but I can't confirm it.

Despite all the 'grand scheme' described in the documents you found and the fact that it is all possible, MLS development ran into a few development issues along the way.  The biggest was that it was extremely expensive to build for both the ground systems and the airborne receivers.  That slowed its development as its benefits over ILS weren't justified by the costs.

One big benefit that your reference describes is that it actually allowed for area navigation.  Where ILS just gives you a deviation from a path, MLS gives you a 3-D position (Azimuth-Elevation-Range) that can allow for the RNAV paths.  But this was complicated and then the second big shift arrived -- GPS.  Since GPS gives you RNAV capability globally, why spend a fortune on something that would work only locally with very high infrastructure costs?  Based on this, the FAA pretty much abandoned MLS in the late 80's in favor of expanding GPS/GBAS/SBAS systems.

Europe kept on with MLS work because they had a different issue with multipath interference to the ILS systems.  The effort though just focused on the final approach segment.  GPS/RNAV systems can handle the inital portions of the approach just fine.  To keep things simple, and to make them retrofittable into existing aircraft at a reasonable cost, the MLS receivers were designed to provide an "ILS Look-alike" output.  And to avoid dealing with new procedures and training, they kept the ground systems and approach design limited to ILS-Like functionality -- just with higher precision and less interference issues.

The US does have some MLS approaches, but they're mostly 'private' approaches paid for by the airlines that want them.  The FAA won't fund 'Standard MLS approaches'.

As a result, current MLS equipped aircraft can fly MLS approaches that are operationally identical to an ILS CAT III approach.  Most aircraft use a multi-mode receiver (MMR) that allows for both ILS and MLS approaches (and some now also support GLS).  The difference is that instead of an ILS frequency, the pilot selects an MLS channel.  Approach procedures (and the approach plates) are basically the same as an ILS, so crew training is minimal.

So the answer is that today, you fly an MLS approach just like an ILS approach.


What's the process for an uncontrolled airfield to become controlled/towered? I know some uncontrolled airports can have more operations per day than many controlled airports. That got me wondering what the process is for the change to a towered, controlled airport. I know a control tower was recently built (2005) at Provo, UT (KPVU) which is the second busiest airport in Utah.

Is there simply a threshold of average operations/day? Do the number of accidents and incidents come into play?
Does the FAA start the process or does the community have to lobby for the tower?
Who pays for the tower, is it all federal FAA funds or does the state and city where the airport is located pay?
Are there any US airports where towers are being built or considered? Given the recent sequestration and how close we came to having over 100 towers shut down, I'm guessing any current plans are on hold.

I finally found an AOPA article that discusses requirements for a control tower.

They say:

  The FAA has the authority to establish control towers or discontinue
  control tower services through the National Airspace System when
  activity levels and safety considerations merit such action.

In order to open a new tower, there must be a positive Benefit-Cost ratio (which is a complicated thing to calculate).  The FAA typically pays for the cost, and here is the criteria that they list:

  Criteria

  According to FAR Part 170.13, the following criteria, along with
  general facility establishment standards, must be met before an
  airport can qualify for a control tower:

  The airport, whether publicly or privately owned, must be open to and available for use by the public as defined in the Airport and
  Airway Improvement Act of 1982;
  The airport must be part of the National Plan of Integrated Airport Systems;
  The airport owners/authorities must have entered into appropriate assurances and covenants to guarantee that the airport will continue
  in operation for a long enough period to permit the amortization of
  the control tower investment;
  The FAA must be furnished appropriate land without cost for construction of the control tower; and
  The airport must meet the benefit-cost ratio criteria specified herein utilizing three consecutive FAA annual counts and projections
  of future traffic during the expected life of the tower facility. (An
  FAA annual count is a fiscal year or a calendar year activity summary.
  Where actual traffic counts are unavailable or not recorded,
  adequately documented FAA estimates of the scheduled and nonscheduled
  activity may be used.)

  The FARs specifically state that an airport is not guaranteed to
  receive a control tower, even if the airport meets all the criteria
  listed above. This is where the contract tower program comes in. The
  FAA, responding to an airport sponsor's request for an air traffic
  control tower, can elect to establish a contract tower. The FAA can
  either elect to pay for the service in its entirety, or enter into a
  cost-sharing agreement with the sponsor, depending on the results of
  the benefit-cost analysis. Typically, the airport sponsor is
  responsible for 10 percent of the cost of construction and operations.

For the Benefit-Cost calculation, they use the following criteria:

  Site-specific activity forecasts are used to estimate three categories
  of tower benefits:

  Benefits from prevented collisions between aircraft.
  Benefits from other prevented accidents.
  Benefits from reduced flying time.

  Explicit dollar values are assigned to the prevention of fatalities
  and injuries and time saved.

  Tower establishment costs include:

  Annual operating costs: staffing, maintenance, equipment, supplies, and leased services.
  Investment costs: facilities, equipment, and operational start-up.

The article contains a lot more information, along with links to the relevant regulations, etc.


How many types of tape do airplane mechanics use? I was reading this blog linked from this answer, and it mentions

  “We never use, and don’t even stock, duct tape,” explains a veteran
  airline mechanic. “Some of the tapes we use cost hundreds, even
  thousands, of dollars per roll.

Which brings up the question, how many types of tape are used by airplane mechanics and what are they all used for? Obviously speed tape is one type and duct tape is not, but what else is there?

I can't speak for General Aviation, but based on the number of tapes I've seen, it wouldn't be outside the scope of reality to think the aircraft manufacturers have a partnership with 3M to come up with as many different types of tape as possible.

For wiring repairs alone, I know our shop keeps at least 20 various types of tape on hand in our bench stock area to grab in a hurry. There are probably another 20-30 other types of tape we may use in rarer situations, but don't keep on hand. We'll usually order that on an as needed basis.

There are a number of different types of "Speed Tape" as well. These are commonly used as very temporary structural fixes for small holes, dents, etc... on certain parts of the aircraft. It looks a lot like a sheet of aluminum foil, but it's some of the stickiest stuff I've ever dealt with.

I know we also have different Mylar tapes that are used for a number of different applications. One of the applications, in particular, is when floor panels are sealed. The sealant can take 24-30 hours, in some cases, for a tack free cure. Since we want to use the airplane sooner, we can cover it with Mylar tape. It'll allow it to cure while under the tape. After the cure, we'll simply pull the tape up and continue business as usual.

There are a number of other specialized tapes I've seen for various other applications. I do know that I have seen duct tape on bleed duct insulation. This is rare, but sometimes the insulation gets torn and people use duct tape to repair it. Quite frankly, I've never stopped to look and see what the approved tape is for that, but if there was one and only one application for duct tape, that might be it.

I can't offer you an exact number, but they most certainly number in the hundreds if not thousands! Oh, and one other thing I've noticed is that Boeing tapes are usually different than the tapes specified by Airbus! So, each manufacturer has chosen certain tapes for certain applications and purposes, although some are interchangeable.

If you really wanted to get crazy and technical, each tape comes in different widths too. All of these come in standard widths of 1/4", 1/2", 1", 2", and 3". I'm sure there are other width's too, but I know I've seen all of those sizes.

Quite frankly, it's dizzying and unbelievable how much tape is on an airplane!


drag force relationship with cube of speed? So I saw this physics website that said drag grows quadratically with speed. Meaning the drag force at 200 km/h is 4 times what it is at 100 km/h. Ok I'm fine with that. But then it said the work required to accelerate to that speed grows cubically. Meaning I need 8 times the power to accelerate from 100 km/h to 200km/h? this part I dont get.
So if my imaginary engine was producing x kilowatts or kilonewtons, and I pressed a button and its now doing 4x:

Isnt it logical that I should accelerate if I am going only 100 km/h, since drag isnt 4x yet??
Shouldnt I keep accelerating until equilibrium out at 200 km/h?

Update: the power vs force distinction solves it. I was mislead by the use of the word "accelerate" in that page. I thought it was discussion about drag and acceleration. There is no accelerate. The power required to go from A to B in half the time is 2x larger. And drag grows quadratically.

You seem to be confusing force (thrust or drag) with energy (under which concept work and power comes). Don't. You'll be spared much grief.
Work done is force times distance.
$$ W = Fs $$
Power is work done divided by time
$$ P = W/t $$
When you double velocity, force quadruples. As does work.
$$ W_\text{new} = 4Fs = 4W $$
Time halves as well, with the following effect on power.
$$ P_\text{new} = 4W/0.5t = 8P $$


What's an estimate for the cost of the cockpit technology on a 747 or 777? Can anyone share what the percentage cost or dollar costs of cockpit instruments and related software is within a 747 or 777?

PS-  I'm a physician working in medical safety.  Physicians can no longer memorize it all (haven't been able to for decades- but now admitting it with more frequency).  They need instruments as in a medical cockpit.  Currently hospitals spend hundreds of millions on electronic medical records, but do not heavily invest in software to aide diagnostic and therapeutic decisions.  I am trying to develop a comparison between professions that deal with complex systems and the type of investments in software and guidance systems to handle complexity.  Thanks for any help!

Upwards of 30%.

The avionics industry is now a major multi-billion dollar industry world-wide and the avionics equipment on a modern military or civil aircraft can account for around 30% of the total cost of the aircraft. This figure for the avionics content is more like 40% in the case of a maritime patrol/anti-submarine aircraft (or helicopter) and can be over 75% of the total cost in the case of an airborne early warning aircraft such as an AWACS.
— Introduction to Avionics Systems, 2002, p. 1

Boeing promises up-to-date prices (including profit) on their website. The cheapest 777 is listed at USD 283.4 million. For comparison, the engines on the 777 account for 20%.
Moving on to the military side to confirm that 30-40%—as of 2009 the [electronics of the] weapons systems of a single Boeing F/A-18E/F Super Hornet cost some USD 35 million, 44% of the flyaway cost (cost of the aircraft production). Source: navy.mil, PDF page 32.

While expensive, on the long run they're cheaper. Kind of like replacing a good digital watch, compared to repairing an expensive mechanical watch.

The airlines want the advanced technology, despite its higher initial price, because of the lower life-cycle and replacement costs it offers. The cost of maintaining conventional cockpits has soared. Dial-type gauges require watchmaker skills to repair, and skilled people are getting fewer. At the same time digital devices are getting cheaper.
— FLIGHT INTERNATIONAL, 26 April 1986


Is there a way to prevent (not relieve) the hotspot caused by wearing a headset for long periods of time? About a year ago, I bought myself a Bose A20.  Within two weeks, I had returned it.

I will be the first to say that the noise reduction for the A20 was fantastic.  The headset was amazingly light, and clarity was superb.  The reason I returned it was because after wearing it for an hour or two, I would start feeling a sharp pain on the top of my head, where the headset rested.  I'm not the only one to have this happen:

  Unfortunately, I have decided to return these headsets in the 30 day trial period. After the first week, these headsets started to "hotspot" the top center of my head and became uncomfortable after several straight hours of continuous flying. The sound quality was excellent but it came down to a comfort issue. There were a few annoying things.

Here's another...

  On one trip the airplane was equipped with a Bose headset and I thought I would let my wife use it. After 1 hour her head had a bad hotspot and she made me switch. After 30 minutes wearing the Bose I too had a hotspot.

And a quote from the A20 owners manual:

  During a long flight, you may feel a slight pressure point, which can be relieved by adjusting the headband position. Your experience with using the headset will help establish a sense of how it should sound and feel.

I wanted so badly to like that headset (in fact, I still do).  My faithful David Clark will be heading back to the factory for repair soon, and I'm willing to give the A20 another chance, if there's a way to keep that top-of-head hotspot from forming.

Is there a way to prevent (not relieve) the hotspot caused by wearing a Bose A20 for long periods of time?

The best way to prevent the "hot spot" pressure point from the headset band is to wear a headset that doesn't have a top band (like this or this). Of course this requires that you be able to wear an in-ear headset, and some people are more bothered by that than by the headband.

Short of wearing in-ear headsets there are numerous different headset band pads you can try - I know at least one person flying with this sheepskin pad.
Some combination of the right headband position and the right pad should make your headset more comfortable for long flights.


How do pilots clear an aircraft to fly through a hurricane? This video shows a pilot (Tim Gallagher/NOAA) flying what looks like a small jet into the eye of hurricane Matthew by Miami. It's clear that it is quite an interesting ride. The (small) caption states that the purpose of the trip is for "scientific research". I naïvely imagine that the research in question is the gathering of an idea of the velocity field, pressure, atmospheric aerosols, humidity, and spectral density/radiative transfer rates between different "parcels" of air within the large and complex structure in three dimensions. I'm a physicist, but not an atmospheric physicist, so I'm not entirely sure why you would undertake such an expensive and potentially dangerous trip like this.

As a result, I've got two questions:

How exactly would you pick an aircraft to survive a trip through the eye of the storm. Is it a big risk? I imagine that $V_{ne}$ of a fighter jet is an order of magnitude bigger than the bulk flow of any storm on earth, but the aircraft that he is in is not a fighter jet, and I could readily believe that there could be turbulent patches of air moving very quickly. Likewise, what is the risk of stalling the entire wing, or unexpectedly entering chaotic regions of airflow that could cause problems to control surfaces? How would you perform a "risk assessment" on what I presume is -- by the fact that it is a scientific flight -- the unknown?
What exactly are aircraft storm chasers after that cannot be measured from elsewhere? Satellites would provide potentially quite a lot of information about the highest few layers of the structure, and one could imagine that overflying it and dropping probes would be much safer, and more readily parallelised to getting some sort of information from more than exactly one path in $(\mathbf{r},\,t)$.

Here's a screenshot from the video, showing the sort of large wind/water flow the aircraft is experiencing; it's quite dramatic!

The aircraft they're flying is an Lockheed WP-3D Orion, not a jet- the NOAA doesn't fly its Gulfstream IVs into the storm, as I've noted elsewhere.

There is always a risk associated with piloting an aircraft through weather that pilots are usually told to avoid. But they have been doing it for decades, so, with proper training and planning, it appears that it's not that risky. The top speed of the WP-3D is listed as over 750 kmph. The $V_{ne}$ would definitely be over that. The top wind speeds in Category 5 hurricanes (the highest in the Saffir-Simpson scale, of which Matthew is one) are over 250 kph, though most hurricanes aren't quite that strong.

The NOAA seems to be aware of this issue; in this reddit AMA, they agree that wind shear could damage the aircraft and lead to loss of control:

  Planes are generally not destroyed by strong winds while in flight. Airliners routinely fly in jet streams with winds exceeding 150 mph over the U.S. during the winter. It's the shear, or sudden change in horizontal or vertical winds, that can destroy an aircraft, or cause its loss of control.

However, with proper training and by avoiding the most dangerous areas using real-time data, they can fly the aircraft within the safe limits.

  NOAA pilots and crew routinely fly in the high-wind environment of the hurricane and don't fear it tearing the plane apart. However, they are always monitoring for "hot spots" of severe weather and shear that they can often identify on radar and avoid if it's too severe.

The aircrew has things to worry about other than $V_{ne}$- the high velocity winds in the up and downdrafts can result in high 'g's, while the wing is susceptible to flutter in these conditions. This blog chronicles cases where the mission was abandoned due to aerodynamic flutter and a case where nearly 5.7 'g's were experienced. Another blog describes the penetration of Hurricane Hugo, that resulted in a near fatal disaster and grounding of the aircraft. From hurricanescience.org

  As they flew through the storm’s ey(e)wall they encountered sustained winds of 297.7 km/h (185 mph) (gusting to 315.4 km/h [196 mph]) and extreme updrafts and downdrafts (32-72 km/h [20-45 mph]). The massive jolts rocked the aircraft, causing gear such as tools, computers, all items in the galley, and even the 200 lb life raft to be dangerously thrown about the aircraft’s interior. The cockpit gravitational force(G)-meter, became pegged at 5.6 G's and -3.9 G's . Upon reaching Hugo’s eye, one of the engines exploded and had to be shut down. In the calm of the hurricane’s eye, the plane made repeated circles, a risky maneuver with the loss of one engine. NOAA 42 was able to successfully exit the hurricane ...

The crews also take some safety measures, such as flying at higher altitudes (UAVs are used at lower altitudes) to reduce the associated risk. As every hurricane is different, risk assessment is difficult and the crew has to fly sometimes based on imperfect information. From hurricanescience.org:

  Flying into the eye of a mature hurricane presents significant hazards as strong updrafts in the hurricane’s eyewall can give way to equally strong downdrafts on the measure of a few hundred feet. Horizontal wind speeds can also change by more than 161 km/h (100 mph) in a few seconds of flight time as the plane moves through the eyewall, putting strains on the airplane and its crew. Because of these hazards, Hurricane Hunter flights are typically done at 10,000 ft (3,048m).

There are two types of flights these aircraft are used for- reconnaissance and research. While satellites may give quite a bit of data, there are some things they can't provide and can be obtained only by flying into the storm. The main purpose of reconnaissance flights is to track the center of circulation and gather data, which will help determine the threat posed by the hurricane and the path it will take. NOAA notes:

  The primary purpose of reconnaissance is to track the center of circulation, ... , and to measure the maximum winds. But the crews are also evaluating the storm's size, structure, and development ... . Most of this data, which is critical in determining the hurricane's threat, cannot be obtained from satellite.

The research flights are for varied purposes such as to gather data about the structure of the storm and its interaction with environment, which will help them predict future hurricanes better. In fact, in the above mentioned flight into hurricane Hugo, the crew observed a long suspected phenomenon for the first time, which would not have been possible using satellites:

  Later analysis of the data taken during our amazing flight into Hugo revealed that we hit a tornado-like vortex embedded in the eyewall when the hurricane was at its peak intensity. These eyewall vortices had been suspected but never before observed, and ongoing research suggests that similar vortices may be responsible for some of the incredible damage hurricanes can inflict when they strike land.

While it is true that it is possible to drop probes while overflying hurricanes, it is not always possible- some of the larger ones have cloud tops reaching over 50,000 ft, higher than the ceiling of aircraft available (for example, the cloud tops of Hurricane Matthew reach over 16km), making it quite difficult to drop dropsondes and the data is relayed to the aircraft which then transmits to the center using satellite.

In recent times, UAVs are being used increasingly for this purpose (NASA uses a Global Hawk for this purpose) and, maybe, in the future, they'll replace the manned aircraft altogether.


How does jet engine water injection work? How did the "wet takeoff" of old days work? Was the water used ordinary distilled water, or did it have additives? Where is it carried in the aircraft?

On many early jet airliners water was sprayed it into the engine air intake or compressor where evaporation cooled the air and allowed cooler, denser air to be ingested by the engine. The result was increased thrust, especially needed for operation from hot and high airfields, or a cooler temperature in the hot section of the engine at the same thrust. From SAE Paper Number 2004-01-3108:

  Water injection was first used over 45 years ago on commercial
  transport aircraft to increase thrust. Boeing 707-120 aircraft with
  Pratt & Whitney JT3C-6 engines and later, the Boeing 747-100 & 200
  aircraft with Pratt & Whitney JT9D-3AW and -7AW engines, all used
  water injection for thrust augmentation.

This report discusses various techniques for water injection:

ahead of the low pressure compressor. This is the common method for thrust increase in hot and high conditions.
ahead of the high pressure turbine. This is used in cold weather, when injection ahead of the low pressure compressor would run the risk of ice accretion.
together with fuel into the combustor. This technique is commonly used with stationary gas turbines in order to reduce nitrogen oxide emissions.

And NASA-TM-2010-213179 is more specific on the system:

  Four water-injected Pratt & Whitney JT3C–6 engines were used on early
  Boeing 707–120 Stratoliner aircraft to augment takeoff thrust on days
  above 20 °F. This system used a belly tank to store demineralized
  water and an electrically driven boost pump to deliver water to the
  four engines. At that point, an engine-driven mechanical pump
  increased the pressure to about 400 psi for injection before the
  low-pressure compressor (LPC) on days as cold as or colder than 40 °F
  and for injection into the high-pressure compressor (HPC) only on days
  between 40 and 20 °F.

What would happen if kerosene instead of water was filled into the water tanks was tried out by a BAC-111 on September 6, 1971:

  The aircraft collided with a bridge, shearing off both wings, after a
  double engine failure occurred during takeoff. The water-injection
  system to cool the engines during takeoff was inadvertently filled
  with kerosene instead of water.

More recent work on water injection focused on the reduction of nitrogen oxides which form in lean combustion and high temperature. By lowering the temperature level of the combustion process, the nitrogen oxide pollution can be greatly reduced.


Are there aircraft with multiple inlets for a turbojet engine? Are there aerial vehicles which have multiple inlets with air ducts (shaped in S-forn) , which all lead the air to one single turbojet engine?
I have not enough space to put just one inlet area on my vehicle, so I thought about using inlets, located at the sides.
I think it is really difficult to combine the air of all inlets so that there is a uniform air flow in front of the turbojet -  because I don't have much space inside my vehicle.
Besides, I think is is really challenging when it comes to some maneuvers, where the flow sometimes will not flow through one of these inlets.
What do you think and do you know any vehicle from history?

It’s common on military fighters. For instance most single engine fighters eg F-35, JAS-39, J-18, etc. will have a pair of inlets on either side of the fuselage feeding these single engine.  Some aircraft will have auxiliary air intakes eg B-2, F-117, F-22, MiG-29/35, SU-27/30/33, etc. to provide additional air on takeoff or other high power settings or to provide protection against FOD ingestion.


Does the ARCA hoverboard violate known limits on small-diameter electric fans? The ARCA board is being hawked as achieving 430 pounds of lift from 36 electric fan motors for six minutes of 'flight.'

c.f. : http://www.theverge.com/2015/12/24/10663228/arcaspace-arcaboard-hoverboard-skateboard

To my uneducated eye, getting 12 pounds of static thrust from each of the small fan motors (430 pounds / 36 motors) seems unreasonable and perhaps impossible. The engineer says that it has 272 horsepower, so that would be 7.56 horsepower or 5.6kW per motor, which seems on the high side.

What are the a) theoretical and b) realistic amounts of thrust that one can assume from what looks like a 10-15cm diameter fan?

The key is that the Arca board only works in ground effect. Therefore, the pressure difference over the area of the board is the key for calculating its power needs.

From the video I would estimate the size to be 0.6 x 1.2 m thanks to @mins more thorough research we know that the board measures 1.45 x 0.76 m for an area of 1.1 m². To lift both the occupant and the board itself it claims to create 430 pounds of force, equivalent to 195 kg of mass in Earth's gravity, so the pressure increase between top and bottom is 1740 N/m² or just 1.7% of atmospheric pressure. This is not much when you consider that a turbofan compressor stage can create a pressure differential of 1.7, albeit at much higher speeds.

The concept is basically the same as the Hiller flying platforms of the 1950s, the first of which was powered by two 44 HP engines. Since the intake geometry of the Arca board is - how should I say - less than optimum, it can not fly as high as the Hiller vehicles and I find the claimed 272 HP hard to believe and the high power draw is due to the low efficiency of 36 closely coupled blowers.

Hiller VZ-1 Pawnee (picture source)

To calculate the exact amount of power needed I would need to calculate the mass flow from below the board at the given pressure, which depends nonlinearly on altitude. Then I would need to calculate the power to produce this mass flow at the known pressure differential with 36 small blowers. Sorry, but even my Hoerner books do not cover this. What is certain is that the thrust from the 36 blowers is not nearly sufficient to lift the 195 kg when out of ground effect. The board only works at very little altitude when the outflow cross section around the board is restricted. This gives it an automatic altitude limit of a few centimeters and makes it self-stabilizing in pitch and yaw.

It takes some thick skin to watch the full length of this platitude-filled piece of marketing for an obvious gimmick. The camera angles were chosen to make it look fly higher than it actually can, and in the one shot where it does fly higher you cannot see who or what is standing on top. Anybody spending 20 grand on this is going to regret it quite soon.

EDIT:

With the answer provided by Bruce Abbott to @mins' question on Electrical Engineering SE the question can be solved. Bruce approaches the problem from the electrical side and links to a 120 mm blower which creates 74 N thrust and consumes 7 kW of power. He calculates 65 kg for the batteries and 36 kg for the blowers, for an estimated mass of the board of 110 kg. However, in combination the thrust from closely coupled blowers will certainly be lower than that of an isolated blower with ideal flow conditions.

Conclusion: Looks possible, no limits are busted, but there is some potential for further optimization and badly needed mass savings on the board.


What type of altimeter is this? What type Altimeter is an SA-91? It has three pointers 100, 1000, 10000.
Don't know if it is Kollsman. I can't find any info as far as repairing unit.

It seems to be a C-12 or C-13 sensitive altimeter (SA) from Kollsman Instrument Division / Square D Company, manufactured circa 1940:

(Source)

The only difference seems to be the marking "ALTITUDE" replaced by "SA-91". As this is the 39th in the series, this could be the initial marking, later replaced by the more standard one. I don't see what could be "91" for.

The Kollsman window is black (as commented) because the setting dial has been turned past maximum pressure reference, as indicated by the bug between 7 and 8. The window value and this bug are linked.

The brown color seem to be related to phosphorescent markings and needles.

The C-12 has apparently many variants, likely designed on demand, especially for the USAF, like this one with alternate colors for the marks:

C-12 manufactured by C.G. Conn Ltd., (source)

C.G. Conn Ltd. is a music instruments company (saxophones), but at war time they provided aircraft instruments to the USAF (flying on instruments finally explained!).

Kollsman published handbooks about their instruments. Here is one from 1938. It doesn't cover this model of altimeter, but can be helpful though:

(Source)


What does the ATC instruction "Make left traffic, runway 19L" mean? I'm playing Prepar3D and when coming in for a landing I'm usually told something like Make left traffic, runway 19L. I'm not sure how to interpret this because I'm not a pilot.
Should I go to the left of 19L for my downwind leg so the runway is to my right, or should I go to the right, with the runway on my left?
Also with parallel runways should I go to the left of 19L exactly or is it the left of 1R? I'm very confused!

'Left' or 'right' traffic means that when you are flying the traffic pattern, all turns are to that direction.

The wikipedia article on traffic patterns isn't bad, and has some decent graphics, including this one from the FAA's Handbook of Aeronautical Knowledge:

The top pattern is left traffic, while the bottom pattern is right traffic.