In the upper air, do pilots use flaps to generate lift as well slow down an aircraft? I know, flaps are mostly used during take-off or landing to generate both lift and drag simultaneously.

I am wondering if the pilot uses flaps to generate more lift so that they can climb in less distance (due to drag of flap) and possible reasons for doing it. I have an idea that the flight control system may prohibit flap deployment above a certain altitude or at higher speeds due to flap structural limitations.

It's a little complex, but here is a shot. Take a look at the following graph:

graph http://www.navlog.net/wp-content/uploads/2012/10/PowerAvailable.png

What you can see is multiple airspeeds. For a plane, the rate of cruise is maximised around Airspeed B is reached- this is when the difference between power available and power required is the greatest and therefore the most excess energy is available for climb.

The slope here indicates how much power is necessary for that airspeed- the steeper the curve, the worse it is. The value is really high for airspeeds all the way to left.

If you were to deploy the flaps, you would sort-of shift the power required curve to left- adding drag, reducing the difference between the power available and power required, reducing the maximum rate and in the process increasing the power necessary.

(source: ppl-flight-training.com)

In the equation above, you want to increase lift. Yes, you can do this with flaps- it would increase CL. As you yourself state, it takes a lot of drag to use flaps and slats. Another option is to increase velocity. This is more efficient and uses less energy.

The reason we need is flaps is since as the velocity drops for landing, we need to increase the CL to maintain sufficient lift for landing. Airspeed for B for landing though is much too fast, so you want to slow down to the left side of the graph. See this post on a little more detail on this relationship.

As for flap limitations, they appear to be 20,000 ft for the Boeing 737- Mostly because boeing found them unnecessary to use at higher altitude.


Was this the intended flight path? I have verified numerous approaches from the NW to PVR airport and they all look like this, basically approaching on the left side of the mountain then taking a right towards the airport:

But then noticed 1 flight whose path is different:

Do you think that was the intended flight path or they suddenly realized they need to be further to the left ?

Puerto Vallarta (MMPR) has a VOR/DME on the field (PVR), which is used in seven different approaches (that I can find) depending on where you're coming from and which runway you're landing on:

VOR DME 1 Rwy 04 (Teardrop)
VOR DME 2 Rwy 04 (DME arc from NW)
VOR DME 3 Rwy 04 (DME arc from SE)
VOR DME 1 Rwy 22 (Teardrop)
VOR DME 2 Rwy 22 (DME arc from NW)
VOR DME 3 Rwy 22 (DME arc from SE)
VOR Rwy 04 (Procedure Turn)

Your abnormal flight seems to exactly match the VOR DME 2 Rwy 22 coming from BLASK on airway V1/J141. So yes, that was the intended flight path.

What's interesting is that your normal flights don't match any of the procedures. I suspect that ATC is vectoring those planes to final, which saves the time, fuel and effort of flying a DME arc, teardrop or procedure turn.

If ATC can do that, though, why did that one flight do the DME arc as charted? It's possible ATC was too busy to vector at that time, or their radar was down, or there was a radio problem, or the pilot requested the full procedure to maintain proficiency, or something else. Without a recording of the ATC frequency, it's hard to say.


Is it legal to fly an aircraft with no navigation beacons, and only solid lighting? Is it permissible for an aircraft to fly without red, green, and white strobe position lights?

Is it possible for any aircraft to legally fly with only a solid white or solid red light on the underbelly?

This answer assumes that the question is in the context of the FAA's jurisdiction.
The red, green and white lights are position lights. They are required from sunset to sunrise by 14 CFR 91.203.

(a) During the period from sunset to sunrise (or, in Alaska, during the period a prominent unlighted object cannot be seen from a distance of 3 statute miles or the sun is more than 6 degrees below the horizon)—
...
(1) Operate an aircraft unless it has lighted position lights;

A solid red or solid white light on the underbelly doesn't correspond to a configuration that I am familiar with and doesn't meet the legal requirements.
I have, however, seen some older varieties of rotating beacons that if broken may become a solid red light mounted on the underbelly. Some older models actually have a motor that actuates rotation. If one had a failed motor it could appear as a solid red light.

The strobes are anticollision lights. The answer is also in 14 CFR 91.203 . If anticollision lights are installed, they must be used unless the pilot turns them off in the interest of safety. The most common reason for this is if the aircraft is flying in the clouds and the diffuse strobing lights are a distraction or cause of disorientation such as flicker vertigo.

(a) During the period from sunset to sunrise (or, in Alaska, during the period a prominent unlighted object cannot be seen from a distance of 3 statute miles or the sun is more than 6 degrees below the horizon)—
...
(b) Operate an aircraft that is equipped with an anticollision light system, unless it has lighted anticollision lights. However, the anticollision lights need not be lighted when the pilot-in-command determines that, because of operating conditions, it would be in the interest of safety to turn the lights off.

14 CFR 91.205 is also relevant. Parts of it deal with the FAA requirements for both position and anticollision light systems. It states that aircraft may not be operated without position lights. It also states that aircraft may not be operated without anticollision lights installed with some exceptions.
There are some provisions by which a flight may be conducted legally with broken lights so that it may be repaired. One such provision is also found in 14 CFR 91.205.

(11) For small civil airplanes certificated after March 11, 1996, in accordance with part 23 of this chapter, an approved aviation red or aviation white anticollision light system. In the event of failure of any light of the anticollision light system, operation of the aircraft may continue to a location where repairs or replacement can be made.

If you're really interested, and I mean really really interested, the FAA released AC 20-74 which contains a lot of technical data on exactly how these lights are to be built in terms of intensity, color and position. Delving into this document should be reserved for the truly bored and those who manufacture aircraft lighting.


What are some advantages and disadvantages of have the wing on top of the airplane? Most commercial airplanes have the wings on the bottom of the aircraft to lift the body better, but what about the sugar volt concept that has the wing on the top

The SUGAR Volt and SUGAR Freeze concepts use a high wing purely to make effective bracing possible. It is a design for a world where oil costs $200 per barrel and above. Mainly, two things make its aerodynamics much more efficient than current designs:

A very high aspect ratio of its wing, enabled by the bracing, and
A lower flight Mach number, which in turn makes the wing more efficient by requiring less sweep. Boeing doesn't give a Mach number in their web page, but I would bet it is somewhere around 0.75. See here for a discussion of a very similar design.

Todays airliners use a low wing to enable them to stow away their long landing gears and to put the wing spar below the passenger deck. Long gears make it possible to stretch the fuselage and still be able to rotate during take-off. Large high-wing aircraft, with their low fuselage position, are easier to load and unload, at the price that the fuselage taper has to start shortly aft of the landing gear, so no stretching is possible.

Both concepts use rather speculative engine designs, in case of the SUGAR Volt even one with hybrid natural gas / electric propulsion with onboard batteries. The SUGAR Freeze uses a less speculative engine burning liquefied natural gas. In the SUGAR study, Boeing set the goal to reduce fuel consumption by 70%.


Electrically heated windshields are mostly transparent, but their colors vary from light blue, yellow or light pink. Please explain Found it on a quiz worded this way. No other details unfortunately. I can only assume it has to do with the heated element materials of tungsten or molybdenum reaction with film layer materials. Perhaps it is a coating to reflect nothing but infrared. Hope to explain to students taking aviation maintenance course on ice and rain systems.

The heating element in cockpit windows is typically a thin film comprised of a transparent conducting oxide - indium tin oxide is common. In thin layers, it is transparent to the visible light spectrum but acts as a mirror in the infrared. As with all transparent conductive films, there will be some  tradeoff between transparency and conductivity.
Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other. Since the wave cannot be reflected, it is completely transmitted instead. Thus when white light, which consists of a range of wavelengths, is incident on the film, certain colors are intensified while others are attenuated. Thin-film interference explains the multiple colors seen in light reflected from soap bubbles and oil films on water. It is also the mechanism behind the action of antireflection coatings used on glasses and camera lenses.
For any certain thickness, the color will shift from a shorter to a longer wavelength as the angle changes from normal to oblique. This interference produces narrow reflection/transmission bandwidths, so the observed colors are rarely separate wavelengths, such as produced by a diffraction grating or prism, but a mixture of various wavelengths absent of others in the spectrum. Therefore, the colors observed are rarely those of the rainbow, but browns, golds, turquoises, teals, bright blues, purples, and magentas.
Additionally, keep in mind that in its thicker, bulk state, indium tin oxide is naturally yellowish-grey. Therefore, the thicker the film, the more it's natural color will influence the light passing through it. I don't know the natural colors for every transparent conductive film material, but it stands to reason that there will be some variation in natural coloration there as well, owing to the different elements included in the composite.


How do pilots estimate glideslope visually without PAPIs? If my (enthusiast-level) understanding is correct, on instrument approach there are specific glideslope paths a plane is expected to follow. On ILS this is self-evident, but (again, if I read approach plates correctly), VOR DME approaches also entail a certain altitude that must be crossed at given DME distances. I assume, crews estimate their rate of descent based on their ground speed etc.

Now, when visually, apart from PAPI lights, is there any other way of estimating the correct glideslope? With C172s and 55 knots, I expect it's easier to simply crawl to the runway and adjust accordingly. But how does it work when you have a B737 or A320 landing on a 6000ft/1800mt runway without PAPI lights? I expect a certain kind of precision is required, but does it come merely with experience, or is there some other way?

There's no instrument you need for a good approach angle in good weather, it's all done by eye. When you train as a pilot you develop a sight picture of what a good approach looks like, and you develop a sense of what a runway looks like when you get too low or too high. It comes from experience and practice combining with the human brain's excellent spatial processing capabilities.


In Raymer diagram "Take off parameter- Take off distance", what is the difference among "balanced field length", "over 50 ft", "ground roll"? Raymer in his book suggests a method to find wing loading W/S (for example we consider jet with 2 engines) (starting from specific thrust $\frac TW$, $CL_{to}$, density/reference density etc.).

In step 1:  he suggests to use graphics "Fig. 5.4 Takeoff distance estimation": you enter takeoff distance (in ordinate), so .. in abscissa you can determine Takeoff Parameter ("TOP")

Step 2: From "TOP" you can find $\frac WS=TOP\cdot \sigma \cdot CL_{to} \cdot \frac tW$

The questions are: in figure 5.4 you can find many diagrams: takeoff distance for:

a) "Balanced field Length"
b) "over 50 ft"
c) "ground roll"

Which one I have to use?
What is the difference among them?
Which curve you have to use, in order to calculate "take off parameter"?

Source: Daniel P. Raymer, Aircraft design: a conceptual approach. Chapter 5 Thrust-to-weight ratio and wing loading, paragraph 5.3 Wing Loading, sub-paragraph Takeoff Distance, page 88-89 and "Fig. 5.4 Takeoff distance estimation".

Let's clarify:

Large commercial airplanes take off in a way which ensures there is sufficient runway length to stop in case of aborted takeoff. There is a speed threshold, V1 speed or decision speed, that determines whether the takeoff can still be aborted, or the airplane must continue and liftoff whatever happens. See balanced field takeoff on Wikipedia.

The balanced field length is the minimum runway length to execute a balanced field takeoff for a given aircraft. The V1 speed in this case is the lowest possible. Additional runway length allows greater V1 to be selected (greater V1 means a greater portion of the ground roll with a possibility to abort the takeoff).

V1 value and balanced field length (source)

50 ft distance: This is the distance from the start of the ground roll to the point where the aircraft is airborne and 50 ft over the runway, to ensure obstacle clearance. 35 ft is another typical obstacle clearance in the US, and indeed the distance to climb to 35 ft is shorter than the previous one (a 35 ft clearance is implicitly used to compute the balanced field distance)

Takeoff roll distance and Obstacle clearance distance (source)

Using the diagram

From this slideshow which reproduces the figure you refer to:

(Source)

This depicts the relationship between takeoff parameter and different distances, either the ground roll distance or the obstacle clearance distance (35 and 50 ft) in the three types of takeoff described above.

There are three groups from left to right:

Jet using balanced field T/O (the obstacle clearance is implicitly 35 ft).
Jet not using balanced field T/O.
Propeller (not using balanced field T/O).

In these groups, there are one or two data provided:

Jet using balanced field T/O. The only distance provided is the balanced field length (distance to accelerate, abort the T/O at V1 and decelerate). You must choose the right curve, depending on the number of engines (twin, tri or four-engine).
Jet / propeller. There are two distance provided and you may need both. Ground roll is the distance until the aircraft leaves the ground (liftoff); 50 ft is the ground roll + the distance to climb to 50 ft.

See also:

What would be the ground roll and total distance to clear a 50ft obstacle given these conditions?
How is minimum runway length related to V1?


At what altitude does a wing become useless? At what speed or altitude does an aircraft become more efficient as a rocket then a plane using thrust vectoring instead of the wings to steer? At what altitude did the flight control surfaces of the Space Shuttle become useful again or X-37B is still useful?

Source: http://www.xairforces.net/newsd.asp?newsid=817&newst=7#.XCbNY7hhrwc

Related: https://space.stackexchange.com/questions/33152/could-rockets-launched-from-the-ground-use-wings-in-the-stages

Is GOCE a satellite or aircraft?

Maybe wings never mattered...

Altitude:

The answer depends a bit on the design of the craft in terms of control surfaces/wings. Atmospheric reentry begins at the Karman line at an altitude of 100 km (62.14 mi / ~ 54 nautical mi) (327,360 feet) above the surface. Which is generally when air resistance starts to matter but that does not mean control surfaces will do anything at precisely that altitude. In terms of practical vehicles that have been flow:

The space shuttle starts maneuvering with its aero surfaces ~30 miles up

  At an altitude of approximately 30 miles [158,400 feet/50km], the orbiter makes a series
  of maneuvers and S-turns to slow its speed. At 9.5 miles in altitude
  and at a speed of Mach 1, the orbiter can be steered using its rudder.
  The on-board computers fly the orbiter until it goes subsonic (slower
  than the speed of sound: Mach 1). This happens about 4 minutes before
  landing.

Neil Armstrong famously flew the X-15 high enough to need the reaction control rockets to keep from skimming off the atmosphere, he was up around 207,500 feet.

So somewhere up around ~175,000 feet, practical control surfaces start to work. Its possible aircraft could be built to get above this region but they would need the ability to either fly up there or survive the decent which is a whole different can of worms.

Speed:
Any speed so long as your thrust to weight ration is greater than 1. All rockets start from a speed relative to ground of zero. Rockets simply have engines capable of providing 100% of the thrust needed to move (generally directed in the up direction). Planes have enough thrust to overcome drag then use the wings to help them with the lift (up direction) portion of the problem. There are aircraft that have a thrust to weight ratio greater than 1 and are capable of "flying" in a way without wings.


What are the differences between single and double annular combustion chambers? The combustion chamber of a jet engine may be can type or annular type. Annular type combustion chamber can be SAC (single annular combustion chamber) or DAC (double annular combustion chamber). What are the major changes between these  two types of combustion chamber? Is the CFM56-7B26 a SAC or DAC engine?

See this answer for the difference between a SAC and DAC combustor. The EASA type certificate for the CFM56-7 also identifies which engines have the DAC and which have the SAC. The answer depends on exactly which -7B26 version you are referring to.


Why does V1 increase with weight? (B738 AFM, 0-2000 feet elevation, up to 27°C) Example of V1 increasing with weight.

This valid question came up the other day here in chat.

The heavier the aircraft, the harder it is to stop.

So why isn't V1 reduced to allow for better stopping?
How is the max weight / V1 determined for a specific runway?

V1 is the speed by which the decision is made to continue or reject the takeoff.

Other than being controlled by Take-Off Run/Distance Available (TORA/TODA) or Accelerate Stop Distance Available (ASDA), all field limits, maximum $V_1$ is also controlled by $V_R$ (Speed for rotation) and $V_{MBE}$ (Maximum brake energy speed, but let's ignore $V_{MBE}$ in this context). $V_1$ must not exceed either.

$V_R$ increases with increasing gross weight.

This means that at low gross weight, theoretically, if the aircraft had not yet been rotated past a $V_1$ limited by $V_R$, sufficient distance may still exist to stop the aircraft on the remaining ASDA.

At higher gross weights, the higher $V_R$ required might allow for a higher $V_1$. Therefore, in response to: "So why isn't V1 reduced to allow for better stopping?": The higher $V_1$ still provides adequate margin for stopping on the ASDA, but gives you the option to abort the take-off at a higher speed with the increased $V_1$.

Rejected take-off (RTO) is still initiated when the engine failure is recognized, so a failure before the "increased" $V_1$ will result in RTO at the time of recognition of the failure, and not at $V_1$.

If performance allows, I prefer $V_1$ = $V_R$, as opposed to say $V_1$ = $V_R$ - 20 kt, on a short, slippery runway, which would imply that given an engine failure past $V_1$ you'd have to keep accelerating the aircraft with asymmetric thrust, on said runway, close to $V_{MCG}$ (minimum control speed on ground, which can be the lower limit for $V_1$).


Why then don't aircraft fly even higher, for even greater efficiency? After reading various superb QA on here I now see that (basically) aircraft are more efficient per passenger-mile, at higher altitudes.

Why don't we go even higher, than current typical airliner cruising altitudes?

What's the deal?

If there's an efficiency transition, have we reached it?

Some do (or have in the past) but very high altitudes present their own issues. Historically the Concorde cruised anywhere from FL550 to FL600 and was actually allowed to climb and descend at its discretion up there since they were well clear of any traffic. However the increase in pressure differential on the airframe as well as supersonic flight meant the airframe saw much greater wear and tear per cycle than its lower altitude friends.

At some point you get near the coffin corner a point at which, even if you have enough thrust your stall speed exceeds your critical mach number (effectively your wing can't work right). The U2 spy plane is capable of flying right on this edge.

One of the big practical limiting factors is also the rapid descent requirement for airframe certification. The FAA requires that in the event of a depressurization the aircraft can get down to 10,000 ft. (no oxygen required altitude) in 10 minutes as discussed here. The higher you go the faster the emergency descent needs to be, eventually this becomes an engineering issue and the airframe becomes the limiting factor since you don't want to exceed Vne in the dive.


What is an airliner pilot's first go-to if he or she doesn’t have a positive climb rate immediately after take off? Assuming a typical transport-class aircraft, what would the pilot do if the aircraft does not have a positive rate of climb immediately after takeoff?

Would the pilot just lower the nose to pick up airspeed, then raise it back up once they’re satisfied? Or maybe retract the flaps a bit? Assuming the throttle is at 94% N1, would applying full throttle at that point even stop the decline in speed?

The first thing I would do is advance thrust to maximum available thrust, make sure both engines are indeed operating.
Next I'd check is if the speedbrake is stowed.

I would not consider retracting flaps because when you are too slow to climb you are definitely too slow to retract flaps... could be deadly.

Normally this kind of situation can't happen unless you have at least one engine failure and the remaining engine not working at full thrust or the wings are contaminated with ice or snow. The aircraft is perfectly capable of climbing on one engine, so if you have two engines on max takeoff power and are still not climbing then something is seriously wrong.
In case of ice/snow on the wings, well there is not much you can do about that in the air, put on wing anti ice and hope that it is quick enough to clear the wings of any contamination, this comes at a penalty of losing precious thrust because of the bleed air extraction. This situation is usually avoided by removing snow and ice from the wings whilst still on the ground - before taking off.


Can large airliners do aerobatics such as loops? This subject has been partially covered before here Is it possible to do a barrel roll in a large aircraft like a B737? and here Is it safe to roll an airplane that isn't approved for it?

If a airliner such as say a 777-300 had enough height, could it theoretically do a loop or similar aerobatics?

I am supposing that there is no passengers, luggage, hostesses and minimum fuel on board etc - so minimum weight.

Also, taking into account the loop - if it couldn't achieve the loop by pulling up - could it achieve it by going nose down instead?

IE: The aircraft goes nose down till it is upside down then pulls up - a mirror version of a normal loop basically.

The other questions go over the reasons why not to roll a large aircraft, I want to know if other aerobatics (specifically loops) are theoretically possible.

Design

The key point is that these vehicles are not designed to perform such maneuvers.

A coast-guard lifeboat is designed to self-recover from a capsize and can roll through 360 degrees without significant structural damage. A cruise-liner would not survive being rolled over completely.

If a vehicle is not designed to perform a high stress maneuver it is safe to assume an unmodified vehicle cannot do so without incurring severe damage.

Inside loop

Regarding an inside loop, the world's smartest man says

  Looping a 747 or a DC-10 would be trickier [than rolling one ...]  Boeing suspects its planes could make it, but since no one has ever been silly enough to try, there's no way of knowing for sure.

Outside loop

An outside loop is much much more stressful on airframe and pilots as it involves negative G. Airliners have flight envelopes that have much less capability under negative G. There must be a very high probability the aircraft would break up when attempting an outside loop.

Roll

A barrel roll is a less stressful maneuver.

60 years ago, in 1955, "Tex" Johnston, a Boeing test pilot performed a barrel roll in a Boeing 707. So far as I can find out, no one has since deliberately attempted to replicate this stunt in a commercial airliner. At the time of publishing his autobiography, the pilot claimed in a newspaper report that the stunt was planned by him but unknown to his boss.

in 1985 China Airlines 006, a 747, inadvertently executed a half roll but suffered structural damage. It rolled to 60 degrees before the pilot disengaged the autopilot, the pilots suffered spatial disorientation and the roll continued into a dive. The pilots obviously weren't flying the aircraft within its limits - meaning you can't read a lot into this, other than it might be easy to break up the aircraft while attempting this sort of maneuver.


What should I do if the FAA requests health records that no longer exist? I am working on obtaining my FAA Medical Certificate and I was deferred.  I received a request for more information and one of the items that was requested were medical records related to a surgery I had as a young child over 30 years ago and the records no longer exist?  How can I resolve this?

I had a very similar situation. Along with the documentation they requested that was available to me, I submitted a letter explaining my lack of the older records. This was apparently acceptable to the physician reviewing my case because I was issued a third-class medical cert. You can borrow from the verbiage I used (identifying information removed):

I am unable to provide any medical records or previous imaging related
to the injury in question. I contacted the hospital that treated me,
[hospital name] in [city, state], and was informed the records have been
destroyed. [State] law [citation of statute] requires retention of records
for “[quote document retention language from the statute].”
The injury and treatment occurred in [month] of [year].
[Hospital name] does not retain records past this requirement, and
the records you are requesting were destroyed more than 10 years ago.
Furthermore, the physician who treated me, [physician name], is no longer
practicing.

Good luck!


Is there a term for the general design used in most modern fighters? Is there a term that encompasses the overall design of modern fighters (e.g. F-15, F/A-18, F-14, Su-27, Su-30,33,35,37, Mig-29, J-11, etc.), with dual vertical stabilizers, leading edge extensions, dogtooth extensions, very similar nose and canopy shape/profile, air intakes, dual engines, and so on?

I know they each fit into one or more of the air superiority, strike fighter or interceptor categories, but I'm looking for a term (if it exists) which describes and/or encompasses this particular design style. Or is "modern air superiority / tactical / strike fighter" as close as I'm likely to find?

That particular empennage design is called a “four poster” amongst the OEMs.  It’s popular because it provides both excellent longitudinal and directional stability with a minimum side profile.  As to leading edge extensions, I’ve heard of them referred to by aviation scholars as Leading Edge Extensions (LEXs) or Leading Edge Root Extensions (LERXs) they’re popular because they create a turbulent boundary layer over the inboard sections of the wing and fuselage which provides better high alpha controllability.  Canopies, such as those found on the Eurofighter Typhoon or the F-16 are sometimes called ‘blister’ or ‘bubble’ canopies because of their outraised profiles against the OML of the fuselage, providing the pilot unobstructed 360° visibility with a minimum of blind spots, so critical to air to air combat.  The serrated  or ‘dog tooth’ edges of panels are done to minimize the radar cross section around panels or skin joints.

I don’t know if there’s a particular name for such a fighter configuration, save that for that kind of a performance requirements and flight envelope, it just seems to work very well.  Similar to Bill Sweetman’s analysis in his article How the 747 Got Its Hump, given certain criteria and other demands on designing and building an airplane, this is just the best solution to the problem of designing a large twin engine fighter airplane.  It’s an open ended design so there’s no ‘right’ or ‘wrong’ answer here; it’s just certain solutions work better than others.

They don’t always follow the mold though.  Take Lockheed’s F-22 in comparison to Northrop’s F-23 design.  The YF-22 was a stout, little chunky, conventionally styled twin fighter with stealth characteristics, while the Northrop YF-23 had a trapezoidal planform with smooth, voluptuous curved surfaces and a large pelican tail with twin all moving ruddervators.  Both aircraft excelled at what they did; a high speed low observable air superiority fighter but seemed to come to very different conclusions.  While the F-23 lost the contract, its design philoophy did not go unnoticed.  The Sukhoi design bureau seemed to draw very similar layout conclusions in the development of their Su-50 stealth fighter.  The same is true of the F-35 vs the F-32 aircraft.  LMACO’s entry was very conventional and the Boeing jet was an ungainly, unconventional delta wing design with a pelican tail, laid out largely for simplicity and ease of manufacture (which didn’t work out!).


Why don't all GA aircraft have yaw dampers? Why don't they have yaw dampers if they are always "suffering" from adverse yaw?

A yaw damper means added cost, weight, complexity, and one more system that has to be maintained. That's the downside. The upside, for most GA aircraft is that they relieve the pilot of having to make a "coordinated" turn. In other words, having to apply appropriate rudder input along with aileron input. Is that convenience worth the downside when the pilot has internalized doing that without having to think about it? That's a matter of opinion. For myself, a yaw damper in any of the, say, single engine Cessnas would be a waste.

There are some GA aircraft that could benefit from a yaw damper in other than banking into a turn or leveling from a turn. For example, in level flight the old V-tail Bonanza would yaw around the vertical axis in turbulence to an objectionable degree in my opinion. Very uncomfortable, especially to passengers in the back seat. It would have been great to have had a yaw damper.


Why does Lift and Drag calculation over an airfoil ignore shear stress? why in Lift and Drag calculation over an airfoil we ignore shear stress effect and focus on pressure distribution?

We "Do" take viscous shear stresses into account when calculating the drag of an airfoil.
This is ALWAYS true for subsonic flows. The underlying reason behind this is called D'Alembert's paradox. For incompressible and inviscid potential flow, the drag force is zero on a body moving with constant velocity relative to the fluid.

BOTH pressure drag and profile drag on an airfoil is viscous phenomena.

In order to provide a numerical example, please see the inviscid and viscous calculations of SD7037 airfoil. What you see in the inviscid case as drag, is merely a numerical error (- value).

PS: In order to really understand how drag is formed, best to start looking up the "Von Kármán Momentum Integral" equation. Also to convince on the analytical results one can use XFOIL's VPlo menu under OPER and observe the momentum defect via delta* distribution and kinetic energy defect via CD distribution


How do droops help to avoid stalls? Droops are deployed on the leading edge of a wing of an aircraft, and they work by changing the camber of the wing.

How do they help in delaying stall?

Similar to a leading edge flap, fixed in the extended position with no gaps: The nose is lowered, so at high angles of attack the flow still has its stagnation point at the round nose where on an un-drooped nose it would had slipped below the nose section.

This reduces the suction peak near the nose of the upper surface pressure distribution and consequently makes recompression towards the trailing edge much easier. This helps to delay flow separation and to achieve higher angles of attack and lift. If an aircraft had a roll instability at high angle of attack, adding a drooped nose on the outer wing will cure it of misbehaving.

The downside is that now at low angles of attack a suction peak will form on the bottom side and might cause local flow separation. This increases the viscous drag just when this drag component dominates overall drag. Consequently, aircraft which had been modified with a drooped nose do not reach their old top speed anymore.


Are aircraft engines classified as DAL A or DAL B? I am confused regarding the Design Assurance Level (DAL) of aircraft engines. I attended a seminar in which the speaker mentioned aircraft engines as being DAL level B; when we asked about it, he answered that it's not required by the FAA to have two engines running because an aircraft is capable of flying with only one engine. Hence, it's classified as DAL level B.

If I'm not wrong, the FAA states that an commercial aircraft should be able to fly with one engine with for at least 1.5 to 2 hours. So was the speaker right about the DAL classification?

DAL started in the software world with DO-178, but it has been extended to the entire development process.  This is documented in ARP4754A Guidelines For Development Of Civil Aircraft and Systems and ARP4761 Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment.

But to the direct question, 14 CFR 33.75 Safety Analysis, describes the safety levels required for the engine.  Matching the wording there

  (3) The applicant must show that hazardous engine effects are
  predicted to occur at a rate not in excess of that defined as
  extremely remote (probability range of 10−7 to 10−9 per engine flight
  hour).

with ARP4761, it is describing DAL B.  So I would have to agree with the speaker.

But I will offer one point of personal experience which is that one FAA certification officer insisted the DAL for the FADEC software should be DAL A.  The argument is that a common mode fault in the FADEC software could result in simultaneous loss of both engines.

One other fine point of certification is that complying with Part 33 allows you to get a Type Certificate for the engine.  That doesn't mean it meets all the requirements necessary to allow an aircraft with that engine installed receive an aircraft Type Certificate (under Parts 23 or 25).


What keeps Russian airliners from being as fuel efficient as their Western counterparts? Peter Kampf points out in his answer on IDing Tupolevs that the Russian transport aircraft designs tend to be quite aerodynamically refined, in some ways better than their Western counterparts on that front.  What kept them from matching the West when it came to fuel efficiency then?  Was it purely a matter of the Russian need to build sturdy aircraft to withstand their rough strips in the middle of Siberia?  Or were there other factors, such as engine technology, in play?  And have the Russians "caught up to us" regarding aircraft fuel economy, or are they still trying to get the hang of building fuel-sipping airplanes?

The engines.

Russian engines were for decades behind the state of the art. One reason was manufacturing tolerances. I remember that we studied the Progress D-27 three-spool turboprop and found that just the cast casing could be made 250 kg lighter with Western tolerances. The Russian designers kept their tolerances wider to make sure that the minimum wall thickness would not be undercut.

Also, since Russian designers had less powerful computers available to them, they could rely less on CFD, so the turbo machinery was less refined. Improved engines explain maybe two thirds of the increase in fuel efficiency over the last 70 years.

Another reason is system redundancy. Russian engineers made sure the aircraft would still be operable after being maintained by a substandard maintenance crew. Especially the military designs were built with the goal of maintenance-free operation for several weeks. This thinking was also applied on the civilian side, since all Russian aircraft were designed with dual-use in mind. In case of war, all Aeroflot planes would become military transporters, and Aeroflot crews received most of their training on military aircraft. Why else would designs like the Tupolev Tu-134 have a glazed nose?

Tu-134 (picture source)


What's the relation between TAS and Mach in tropopause? I understand that TAS decreases with altitude with constant Mach number in troposphere, however I'm not sure what happens in tropospause, as the temperature is constant, however the density is still decreasing. Does that mean the TAS will continue to decrease, just at a slower rate?

The speed of sound depends only on air temperature, not density. So, for a given temperature, a given Mach number will always equate to the same TAS.

In other words, for a given Mach number, TAS remains constant throughout the constant-temperature region above the Tropopause (ie. 37000ft to 65000ft in the International Standard Atmosphere, ISA).


What is this strange cutout in Class B of Kansas City International Airport? To the Northwest of the KMCI Class B airspace, there is a strange cut out in SFC/80 airspace, which I highlighted at this link (and shown in picture below):

highlighted in purple - Image Source: www.skyvector.com

There's no airport there, and no obvious reason for the cut out.

What possible reasons could there be for a strange shape like this?

There are in fact two airports in the cutout:

An unmarked field (A in top map) just east of Platte City (see below) that corresponds to the NW limit of the cutout. Note that this edge of the cutout appears to parallel the main runway at MCI.

The private Elton Field (B in top map). In the image below, field is boxed in red and runway in blue.

And there's also the VFR reporting point over north Platte City


Need help identifying planes, near Toronto Any help will be appreciated. Thanks

The last one is almost undoubtedly a Pilatus PC-12 operated by Ornge.


Are Cargo Holds Pressurized, Non-pressurized, or vacuumed? I watched an episode of Air Crash Investigation, which depicts a plane catching fire due to it shipping many expired oxygen generators for planes. The narrator says that a fire in the cargo hold is very unlikely, as the cargo hold is vacuumed, to prevent fire; but however in this situation, the oxygen generators were actually generating oxygen so the vacuum was futile. I also know they carry pets in the cargo hold. So are the cargo holds pressurized, non-pressurized, or vacuumed? Or does it depend on the carrier, or aircraft?

The cargo hold is indeed vacuumed, using a vacuum cleaner, regularly.
It is however NOT drawn vacuum (so having all air removed).
Some cargo holds may be pressurised, some are not. Depends on the aircraft and the requirements of the cargo (life animals want a pressurised cabin of course, just to give one example).


How do I explain what makes an airplane fly to a non-technical person? As an engineer I can explain in very technical terms exactly what makes an airplane fly, however, it isn't easily understood by non-technical people.  How can I explain it to a non-technical person, or a nine year old, in a way that isn't really incorrect but is far less technical than what we learn as pilots?

The simplest explanation is the "hand out of the window" one:

  Hold your hand out of the window while in a moving car and hold your hand at an angle. It gets pushed upwards. Your hand represents the wing of an airplane. The plane's engine simply makes sure the "hand" keeps moving forward against the force of the wind.

Even a 4-year old understands the concept using this explanation.


Why is the available power in propeller engine is almost constant for every velocity? I surfed on the internet for an answer but I didn't find any. I'm taking Aircraft Performance course. I saw that available power for propeller engine is almost constant for every velocity.  But I don't know why...

It is a gross simplification, assuming a constant speed prop.  If a fixed pitch prop, it is non-linear.  Even with a constant speed prop it is not exactly linear.

Lesser factors for power available are the angle of attack/angle of incidence, which will change with airspeed, assuming level flight.  Ram air is also a minor factor on some engines.

The diagram you show is a simplification meant to show you that the power is the same, regardless of airspeed, but in reality it varies.  How much? I can't say for certain in general, but on one aircraft I collected data on, I recall it varying about 6% in the 20 knot window we were looking at.  And to be clear, that was power converted to thrust, not simply power plant power.


What does the sign "Caution your blast" mean? At London City Airport (LCY) there exists a "CAUTION YOUR BLAST" sign. This isn't one I've seen before. What does it mean?

I'm fairly sure that sign is beside taxiway C*, and as you can see from the aerodrome chart it is right next to the main apron. Aircraft holding in this position have the main apron directly behind them.

The sign is instructing pilots to use the minimum thrust necessary to move off from that position, as there could be personnel, vehicles etc on the apron. As this is a bit of a mouthful "Caution your blast" is used.

This is further backed up by the aerodrome textual data which states:

  Pilots are requested to use minimum power when manoeuvring on and off parking stands and when entering the runway.
  The use of minimum power is particularly emphasised when holding at the the entry points to the runway and when
  entering the runway.

The other hold points around the apron seem to have a similar problem Taxiway B has the same sign. Perhaps the others do too.

(Image source: https://www.airlinequality.com/review-pages/gallery-airline-reviews/)

* This was based on personal memory of seeing this exact sign, and of being aware at which hold point we were at. I went and checked using google earth/street view. There is a good view from the terminal however the sign itself is obscured by an aircraft. However you can clearly see the building opposite from the original picture, and so the position for Taxiway C lines up.


In the Lockheed Martin Flight Services system, what are MIFR, MVFR, YFR and ZFR flight rules? When I file a flight plan I can file under either IFR or VFR flight rules. However, the Lockheed Martin Flight Services website has the following options in addition to IFR/VFR:

MVFR
MIFR
YFR
ZFR

1 and 2 are only available for US domestic flight plans; 3 and 4 are only available for ICAO plans. What are all these options and when should I use them?

According to the tooltips on the Lockheed Martin site, the flight rules are:

  MVFR - Military VFR; restricted to military flights
  MIFR - Military IFR; restricted to military flights
  YFR - Initially operated under IFR, followed by one or more changes of flight rules
  ZFR - Initially operated under VFR, followed by one or more changes of flight rules

There's very little information on MVFR and MIFR flight plans that I could find, but the FAA's FSS procedures do include specific actions that are required when receiving a military flight plan (section 6-4-7).

YFR and ZFR are ICAO rules; the FAA calls combined VFR/IFR flight plans composite flight plans (see section 5-1-7 in the AIM) and on a US domestic composite flight plan both VFR and IFR should be checked. Interestingly, the Lockheed Martin site doesn't allow that.


What does "RA" mean in the FAA Terminal Constraints? I'm trying to interpret the Terminal Constraints listed on the Operations Plan Advisory (https://www.fly.faa.gov/adv/adv_spt.jsp). RA seems to be used in the context of weather, but I haven't been able to find a definition for what "RA" means other than "Resolution Advisory" which I don't think is applicable in this context.

Example:

TERMINAL CONSTRAINTS:
NY METS/PHL/DC METS-RA/LOCIGS
CTL/ATL/MCO/TPA/FLL/MIA-RA/TSTMS

-RA is the METAR code for light rain.

LOCIGS is low ceilings, and TSTMS is thunderstorms.*

* Air Traffic Management Glossary of Terms (faa.gov).


Does a portable ILS ground station exist? Does anyone know of a good example of a portable ILS ground station?  I mean, it can be used for temporary airfields.

NASA did some research on it in 1986 and published this paper. Thales was contracted to build one that the US Army has ordered and it made it to prototype phase. Eventually it was tested for a while and appears to have made it to production and the military ordered more.

It goes under the name Deployable – Instrument Landing Systems  (D-ILS). here is a short two page brochure on the unit which has some good pictures of the whole thing.

It looks like the units cost around $4.68 million each.


Aborted take off. What happens next? A rare -- as I was told -- situation of aborted take off for a large passenger jet. What is next?

Everything goes as smudge as possible after getting jetliner to full stop. Plane clears the runway and goes back to the either parking position or designated place at taxi way.

What steps must be performed in order to make this aircraft back to take-off waiting queue?

Approximately how long will it take for this plane  to be taking-off again? I know that this vary on many circumstances that's why I am asking about approximate only. Is there any priority for the plane that should already went off? Will that take minutes, tens of minutes or hours?

Clarification: This question is for the sake of the story that I am writing. It (both story and the question) involves Boeing 777 taking off from LHR in August 2014.

Aborted take of is due to a misleading information coming from cargo doors' sensors that reveal a possible problem only during take-off, not before (i.e. during taxi). The doors are actually closed (no real danger posed after check). But there's a tiny piece of fabrics from cargo loader's uniform stick in between doors. This question is a follow-up or alternate version of this question. Here I'd like to know what happens, if such situation will be detected after V1 or shortly after take-off and thus after such aircraft returns safely to the airport and clears the runway. In linked question I am asking about the same situation detected before V1.

I am aware that most airline experts will say that such situation is not possible at all. I am, on the other hand, a system designer and I know that such "ghost" situations of "neither 0 nor 1" sometimes do happen. But, for the sake of this question, if you feel uncomfortable with such reason -- let's assume that take-off was aborted due to any other reason, i.e. engine failure. I belive that such reason has similar severity as the need of checking whether cargo doors are really closed due to fact that on-board computer provides misleading data.

I'd lilke to underline that this is purely hypothetical question. I am aware that while such questions are sometimes hard to answer, they're in general quite welcomed in this site and are not off-topic.

A lot depends on whether the reject happened during the high energy or low energy phase of the takeoff.  The boundary between the two is the reason for the speed call "xx knots", usually 80 knots or 90 knots, during the takeoff roll.

There will be specific procedures to follow following a reject, depending on the airplane and the airline's own procedures.  Normally the first thing is to bring the airplane to a complete stop on the runway, set the parking brake, and evaluate the situation (you might find something's on fire and you need to order an emergency evacuation right there).  You'll have called reject to ATC when the decision was made, so ATC will expect you to stop and will tell aircraft behind you to abandon the approach (and will likely have dispatched the fire trucks to your location just in case).  Now there are situations where judgement and common sense is applied and if you rejected at the beginning of the roll because a binder fell on the floor and made a big bang but you were only going 40kt, and you realized what it was right away, you might just tell ATC what you are doing and taxi clear.  However with rejects, it's usually assumed the runway is contaminated with FOD and ATC may close it anyway to have a runway inspection done.

As soon as you are satisfied that no evacuation is required and it's safe to taxi, you'll taxi to the ramp.  If it's a low energy reject, and you have a BTMS (Brake Temperature Monitoring System) you can expect to only have to wait for the BTMS indications to get back into the green before going out again.  If the BTMS is inoperative and was deferred under the Minimum Equipment List, you will likely have to consult some QRH  (Quick Reference Handbook) brake cooling time tables to determine how long you have to wait.

If the reject was high energy (above the 80/90 kt speed call), or if the BTMS indications are giving an extreme overheat indication (red) because you rejected after you'd already nearly overheated the brakes while taxiing from riding them, you can expect to be required to have a brake and tire inspection performed before you can go out again.  Rejects close to V1/decision speed are considered high risk maneuvers in the general scheme of things.  In any case, a call to Maintenance Control will be the next thing after getting clear of the runway to figure out what to do.

Brake use by crews during taxi has a big effect on things and prudent brake use is really important.  It's easy to get brake temperatures close to the yellow (or white) range just from riding them constantly to hold the airplane back if it has more thrust at idle than it needs to roll at taxi speed. You have to either let it speed up, apply brakes, then let them off over and over, taxi with one engine, or use idle reverse (which airlines don't like you using because it uses up reverser cycles with a maintenance impact).  The CRJ200 is really bad for this and can can get rolling to pretty high speeds with both engines at idle; it's like driving a car with the idle speed set way too high when you just want it to creep along.


What is the rationale behind placing arrestor gear on a long runway? I'm looking at the obstacle chart and the aerodrome chart (pdf; source: Hellenic Civil Aviation Authority) for LGLM (Lemnos, Greece), which serves both military and passenger aircraft.
The runway length is 3016m (9895ft), and, according to the charts, there are also "Arrestor Barriers (nets) 75M before THR RWY 22L", as well as "Arrestor Gears (hooks) 583M, 554M beyond THRs RWY 04R and 22L respectively".
Considering that arrestor gear can have operational consequences for civil aircraft (Skybrary link), apparently its presence is operationally justified for the Hellenic Air Force.
My question is
What could be the reason that arrestor gear is placed on a fairly long runway? The fighters operating from this base are, I believe, F16s and Mirage 2000s. Would a 3-km/10k-feet runway not be enough for such an aircraft, even if we assumed braking problems?
Note: I have been on the island many times as a civil aviation passenger, and the arrestor gear is always in place, so it seems to be a (semi-)permanent installation.

I can think of 4 reasons:

Pilot Training to use the cable arresting system
Rejected takeoff, fighters don't usually have reversers and may take long space to arrest
Under certain bad runway conditions, like ice, the required landing distance may be very high, and a military airport should work (ideally) under any weather condition
Runway partial unavailability, if part of the runway is unavailable, with an arresting cable system you can still use the other part. This is especially important during a war, when the enemy may damage half of the runway and you don't have time to fix it. Military airports are designed considering also these possibilities.

Source for 3 and 4: https://www.wbdg.org/FFC/AF/AFTTP/afttp_3_32.12.pdf


Why the "Terminal" in Terminal Aerodrome Forecast? Why isn't it just called Aerodrome Forecast? Where does the "terminal" part come from?

Terminal refers to the airspace near the airport, more or less similar to the ATC usage of terminal as in "terminal airspace" or area. TAFs cover the airspace in a 5 mi radius of an aerodrome, the immediate terminal area. The T in METAR also refers to terminal as in "Meteorological Terminal Aviation Routine Weather Report".


Why are the call signs of some airlines completely different than their name? The call sign of U.S. Airways is "Cactus", and that of British Airways is "Speedbird"; while for Malaysia Airlines is "Malaysia", Qantas is just simple "Qantas".

Why is it that the call signs of some airlines are so different from their names?

I found this on Wikipedia:

  Some call signs are less obviously associated with a particular airline than others. This might be for historic reasons (South African Airways uses the callsign "Springbok", hearkening back to the airline's old livery which featured a springbok), or possibly to avoid confusion with a call sign used by an established airline. Companies' assigned names may change as a result of mergers, acquisitions, or change in company name or status; British Airways uses BOAC's old callsign ("Speedbird"), as British Airways was formed by a merger of BOAC and British European Airways. Country names can also change over time and new call signs may be agreed in substitution for traditional ones.

Before America West bought US Airways, their callsigns were Cactus (associated with the deserts of the American West) and USAir. After the merger, the combined airline kept the better-known (airline) name, and the other airline's callsign. Now, after the acquisition/merger of American Airlines, they're using that airline's name and callsign.  Leading to more than a few radio calls recently (late spring / early summer 2015) along the lines of

  "Center, Cactuuu...merican 123, FL 350"


What accident or safety data is available for ultralight aircraft? Is there any source of reliable accident or safety data available on ultralight vehicles (FAA Part 103 or similar)?
I'm interested in comparing them to other aircraft types.

This could match your need albeit this is a study from the Finnish Accident Investigation Board, based mostly on Finland numbers over 20 years:

Ultralight Aviation Safety and its Improvement through Accident Investigation (151 pages).

Chapter 9, page 87, also introduces information about other countries related to what they do regarding investigation: Croatia, Cyprus, Denmark, Georgia, Germany, Hungary, Iceland, Ireland, the Netherlands, Norway, Portugal, Spain, Sweden and the United Kingdom.

There is a comparison between ultralights and other types likes gliders and GA aircraft:

Source: Accident Investigation Board

Interestingly, while accidents are decreasing in Finland, this is the opposite trend in France (French article from UL federation, based on insurance notifications, from 15 fatalities in 2006 to 30 in 2010). Maybe just a matter of pilots number increasing.

Related:

Microlight/Ultralight Accident Statistics - Adventure/sport/aircraft: France, Germany, UK, Australia, Canada. From this document: "There is no good accident data for ultralights in the US".
Aviation statistics, including ultralights (1998–2007) Canada, TSB.
British Microlight Accident Analysis, many criteria (MS Word).
Recreational aviation risk survey -  Finnish
Transport Safety Agency.


Can the 737 Max autopilot be engaged when some sensors have failed? If some instruments on the 737 MAX 8 fail on the Captain's side and autopilot stops, can the First Officer engage autopilot (as the instruments on his side are fine)?  In the ET 302 crash the Captain tried to engage the auto pilot but it kept disengaging because of bad AoA readings (which in turn also affected other readings).  Could the First Officer have engaged the autopilot as instruments on his side were working fine?

The following applies to the Boeing 737 NG series, but probably there are no large differences for the MAX series.

The FCOMv2 (4.20.2 Automatic Flight - System Description) mentions only the following systems in the Autopilot Engagement Criteria and the Autopilot Disengagement sections:

  The A/P automatically disengages when any of the following occurs:

  [...]
  either left or right IRS system failure or FAULT light illuminated
  loss of electrical power or a sensor input which prevents proper operation
  of the engaged A/P and mode
  loss of respective hydraulic system pressure.

  Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand
  on system A, may cause A/P A to disengage.

The sensor input is probably what you are asking about because this would affect the instruments as well. In general, if a system only affects one side, the other autopilot should still work. One notable exception would be the IRS, where any IRS failure causes the autopilot to disconnect.

Edit:

The two autopilot systems on the 737 are called A and B, and they are not necessarily linked to Captain and First Officer. Both the Captain and the First Officer can press both engagement buttons on the MCP (marked with 1 in the image from the FCOM below).


Do aircraft always have the right of way on the ground? Or must they yield to police/fire/ambulance? I know FAA said aircraft always have the right of way over ground vehicles. What if a police car, ambulance or fire truck needed to pass?

Just because one has the right of way, does not mean they must exercise that right.

If a pilot sees an emergency vehicle, and is able to, I expect he would stop, and either flash his lights or wave visibly to allow an emergency vehicle to proceed.

The aircraft has right of way, however, because the pilot may not see an emergency vehicle, or may not be able to stop quickly and safely.

So, yes, the aircraft always has right-of-way over ground traffic, according to the rules.  They are reasonably expected to yield in certain circumstances.


How much lift must a helicopter rotor produce? How much lift must a typical helicopter rotor produce? For example, consider the rotor of what seems to be a common helicopter: the Robinson R22. Is the lift produced equal to one of the following:

The weight of the helicopter;
Double the weight of the helicopter; or
Simply some value greater than the weight of the helicopter?

For example: for a 700kg helicopter, what maximum lift will the helicopter rotor need to produce?

In the hover, it must produce a lift force equal to the weight of the helicopter, in order for the helicopter to remain at constant altitude. A helicopter with a mass of 700 kg must produce a lift of 700*g = 6,867 N to hover at constant altitude.

In forward flight at constant altitude, the rotor must produce more thrust with the thrust vector tilted forward at angle α. Vertical thrust component = lift must be equal to weight for the helicopter to fly at constant altitude. As the helicopter picks up speed, less power is required to provide the same amount of thrust due to reduction of induced drag - if power is not reduced, the helicopter starts to climb when transferring from hover to forward speed.

If in the hover the rotor produces more lift than the weight, let's say 710*g,it accelerates upwards, resulting in a vertical velocity component that results in an additional aerdynamic damping force, proportional to vertical speed, caused by change of rotor blade Angle of Attack and fuselage vertical drag. So for a 700 kg helicopter with a rotor that exerts 710*g = 6,965 N, the extra 98 N is used for overcoming aerodynamic drag associated with vertical speed.

Assuming that by "lift" only the vertical component of thrust is meant: more lift than weight results in a constant upwards velocity. Less lift than weight results in a constant downwards velocity, even in a free fall (terminal velocity).


How to know which data came from Mode S and which came from ADS-B (FIS-B)? I'm dealing with recorded ADS-B data as a part of my thesis (programming-oriented).

I have a table of ADS-B communication of an aircraft (image below) - I know, that some of these recorded data in the table come from Mode S and some from ADS-B system itself. I need to know which of these data (GPS coordinates, heading, TAS, Mach and meteo data) come from the Mode S and which come from ADS-B (or FIS-B).

I'm unable to find any source, that would distinguish between data sources of these data.

All the recorded transmissions you have, seem to come from the same aircraft. They come all from the onboard (Mode-S) transponder.

All transmission with Downlink Format (DF) 17 are ADS-B. Those with DF18 are TIS-B (or ADS-R), you don't have any of these (they are rare in Europe, and more common in the US)

Mode-S uses the following DF's:

DF4 - altitude reply (short)
DF5 - identiy reply (short)
DF11 - all call reply / Mode S squitter
DF20 - Comm. B / altitude reply (long)
DF21 - Comm. B / identity reply (long)


Do autopilots force the aircraft to fly over certain coordinates? As part of a project I'm doing I plotted the flight paths of about 2000 flights around the globe, I noticed over both the North Atlantic and Pacific that there is a pattern to the flight paths:

You should be able to see Europe on the right and the USA on the left. Why is there such a common pattern in the flight paths? They all seem to cross every 10 degrees longitude and 1 degree latitude. Is there any explanation for this?

Over the North Atlantic, there are no predefined low-level or high-level airways as there are over the continents. In order for aircraft to get a tailwind for eastbound flights and get out of the wind for westbound flights, Canadian and UK air traffic control set tracks based on the expected wind conditions twice a day.

The lines you are seeing are called an 'Organized Track System'; is this part of the world, they're called 'NAT' (North Atlantic) Tracks. Five to six roughly-parallel tracks will be defined and move around every day, but they will generally go between whole degrees of latitude and 10° increments of longitude.

For example, tonight's (Dec 31/13) track BRAVO ('B') will go from a intersection in the UK called...

ERAKA (at 58°N/10°W) to
60°N/20°W to
61°N/30°W to
61°N/40°W to
60°N/50°W to
MOATT (at 58.025°N/59.928°W) to
LOMTA (at 57.203°N/62.619°W).


What does the German AZF radio license test comprise of in the oral examination? I understand you're required to have an AZF-license, on top of a BZF-license to do IFR radio transmissions in Germany; I also figure the test is similar to the BZF license with a written and an oral part.

What's a typical sequence of radio calls in the oral test? Are there any particular gotchas one should be aware of?

I've read through a list of phrases (out of something called NfL), so I'm vaguely familiar with the differences from US phraseology, but I got from the BZF test there are other differences, like you tend to get a question "Are you ready to copy", rather than the "advise ready to copy" I'm used to.

There are a lot of differences in the US and European communication.

I got my AZF about a year ago and basically you have to do simulated IFR departure and approach. In addition you will have to fill out an IFR flightplan and answer 30 IFR communication questions.

I would say it is a similar difficulty like the BZF, it might be even a little bit easier, as you just have to repeat what the controller tells you (but you still have to be aware where you are).

An example on how radio communication is done can be found at http://www.caa.co.uk/docs/33/CAP413Supplement.pdf or http://training.ivao.de/trainingsunterlagen.html?tx_drwiki_pi1%5Bkeyword%5D=IFR-Funkbeispiel

I would however recommend a training at a flight school (which costs around 150 Euros).


How can a pilot deal with a magneto failure? As I know, the engine is independent from the electrical system, and it is supplied from magnetos. However, how can a pilot deal with a magneto failure? And also, when one magneto fails what problems can occur?

One of the things the checklist will have you do prior to lining up on the runway in a piston engine aircraft is a magneto check. That serves to verify that both magnetos are in good working order and that the grounding of both magnetos is intact.

Typically, that will involve bringing the engine to some given power setting (most likely expressed in terms of RPM, but I suppose it could be measured in some other way for some engines; manifold pressure, maybe?) while holding position, then turn one magneto off, then back on, then turn the other off, then back on. While doing so, you verify that there is a drop in engine power, and that the drop in power is symmetric (about the same for both magnetos), and of course that the engine keeps running on either magneto alone.

This verifies that both ignition systems (magneto, spark plug, etc.) are working, and that the grounding of the magnetos works as expected. Absent the power drop, you could have a hot magneto, which isn't a huge problem in flight but could be a major headache on the ground.

If a magneto fails in flight, especially in a single you might not even notice initially; there will be a slight reduction of engine power, just like during the magneto check, but the engine will keep humming along on the remaining magneto. That's part of the purpose of having two independent ignition systems: redundancy.

If there is an engine failure in flight, the checklist will likely dictate something like "select best magneto" at one point or another during the restart sequence. Basically, you'd be doing a magneto check in the air, and choosing whichever setting gives you more power. If one magneto has failed outright, this will very likely cause the engine to stop when you select only the bad magneto; you'd then have to select the other (or both) and restart the engine again.

The checklist may or may not specify that you should land as soon as possible after a magneto failure, but if it were me, I'd want to land regardless and have the problem checked out properly. That would mean either just landing and taxiing back to the ramp (if, say, you're in the pattern doing circuits), or diverting and landing (if you're on your way to somewhere). There's always the possibility that whatever caused the first magneto to fail might cause the second to fail as well, and I'd rather be on the ground at the wrong airport than in the air with a non-functional engine.


Is there a mathematical formula to find the most efficient propeller? Is there a mathematical formula to find the most efficient propeller? Is a five-blade better than a four-blade? Does software exist to aid in the calculations there?

Yes. The first good ones were published by A. Betz and L. Prandtl in 1919, and the latest substantial improvements were added by Larabee. Those formulas are the basis of Mark Drela's XROTOR. It will give you the chord and twist distribution over span for a prescribed speed, torque or thrust. It is best used for lightly loaded propellers - if you need anything with activity factors (power absorbed by the propeller per blade area) above that of purely subsonic airplanes, better consult prop manufacturer tables.

Generally, the most efficient propeller will be large, have two blades and will spin slowly. Depending on flight speed and desired thrust, you will need to reduce diameter, increase speed and add blades as required, at the price of lower efficiency.


When using the Source Panel method and dealing only with sources and sinks, why do lift and drag equal zero? I am trying to deal with the airfoil with the "Source Panel method", and when I try to calculate the lift and drag, they always equal to zero. And I have found an explanation in some articles, that sources and sinks don’t create any drag/lift. But I don't understand the reason. Could someone give me some explanation?

A single source, or a source sheet comprised of infinitesimal sources (sinks are just sources with negative strength), produces a velocity field that contains no circulation. Without circulation, it can't possibly generate any lift. The Source Panel Method is just a discretized computational method for source sheet. It, therefore, only produces non-lifting flow.
Drag, in the context of incompressible potential flow, only arises through induced drag. Without lift, induced drag would also be nil.
To predict lifting flow, vortex sheet modeling is typically used that, by construction, produces non-zero circulation (unless the integral of sheet strength is zero by prescription). An additional prescription of the Kutta condition is required to guarantee a unique physically correct lifting flow.

Image collated from: http://web.mit.edu/16.unified/www/SPRING/fluids/Lecture_Notes/f18.pdf
A mathematically equivalent methodology is to use doublet sheet modeling, using the following tranform:
$$\vec{\gamma}=\hat{n} \times \tilde{\nabla}\mu$$
where $\vec{\gamma}$ is the vortex sheet strength, $\hat{n}$ is the sheet normal, $\mu$ is the doublet strength, and $\tilde{\nabla}$ is the surface-gradient operator.
Somewhat confusingly, a doublet is a combination of a source and sink that are separated by an infinitely small distance. A single doublet is non-lifting. However, a lifting flow can be generated if the doublet sheet varies in strength. This video based on Drela, Flight Vehicle Aerodynamics provides an extremely good intuitive explanation.


How often is the cabin air replaced in A320? One of the A320 pilots told me that there is an icao rule that air should be replaced every 90 seconds?
Is this true ? can someone give me a supporting article for the same

Let's do some simplified arithmetic:

The packs airflow in an A320 is 1.2 kg/s per pack in HIGH setting, but let's take the normal setting 1 kg/s, thus 2 kg/s considering both packs
The cabin of an A320 is around 190 m^3
Let's assume the cabin altitude in cruise 8000 ft, i.e. the air density at 20°C is 0.897 kg/m^3
The total airmass of the cabin is then 190×0.897 = 170.43 kg
Assuming that the cabin pressure is constant in cruise (so all the air coming from packs is going out via the outflow valve), in 90 seconds we change the following mass of air: 2 kg/s * 90 s = 180 kg

So 170.43 ~= 180, then 90 seconds is a correct estimation.


When was the last time the US Standard Atmosphere 1976 was updated? I imagine due to slight changes in Earth's atmosphere or more accurate instrumentation it's been updated at least once, but also because it's understood to be a less precise model we might not have even bothered. I haven't been able to find any evidence/papers so far detailing the changes, however.

No, this model was last updated in 1976, and only differs from the 1966 model above 51km in altitude, where aircraft rarely fly anyway. There are no newer versions of this atmospheric model.
There are other models though, some of which may be newer.


Why can't Norwegian's Boeing 787 operate from Vegas during hot weather? In a German news article it was stated that the additional weight (Norwegian puts 291 passengers on their 787-8) leads to operations restrictions in Las Vegas when temperature conditions exceed 40°C.

Is the 787-8 really limited at 40°C? What would happen at airports that are not only hot but also high?

What are the temperature/altitude limitations for the 787-8 at MTOW in this specific case?

  European low-cost carrier Norwegian Air will suspend its four Las Vegas routes in March, saying hot weather there has created performance issues for its heavily loaded Boeing 787 aircraft.

  — usatoday.com

  — aerotelegraph.com

Norwegian’s heat issues stem from an unusually high passenger capacity aboard the Boeing 787-8 jetliners that the carrier uses on its Las Vegas routes. Most airlines operate the plane with an average of 200 seats, but Norwegian puts 291 seats for sale on the 787-8 aircraft to keep costs down as part of its low-cost model.
Given the weight of those additional Norwegian passengers and all that accompanies them, the airline this summer found close to 300 people on board, including crew, and its planes’ ability to take-off during high-temperature periods was sometimes limited.
— reviewjournal.com

You see that Norwegian carries more people compared to other 787 operators. And they do indeed cite the passenger count as the reason.
Hot and high limitations are explained here, the biggest limitation for twin-jet planes such as the 787 is the ability to climb if an engine fails during takeoff.
The longest runway at Vegas' McCarran International Airport is 14,512 ft; the elevation is 2,181 ft.

From the 787 performance dispatch manual at 2000 ft pressure altitude and 42°C outside air temperature, we find the limiting factor is the climb limit weight, which is 195 tonnes (429,000 lbs).
The 787-8's maximum takeoff weight is 228 tonnes (502,500 lbs), that's a structural limit.
The 33-tonne difference is equivalent to 300 passengers (80 kg adult + 30 kg luggage and carry-on). Of course the plane won't be fully fueled, despite the huge distance, but you see how big the limitation is.
It gets even worse above 42°C. 30°C seems to be ideal. Vegas record high is 47°C, and the mean maximum in July is 44.7°C.
Why not depart from Vegas at night? Most probably due to slot restrictions. Waiting for the sun to go down means financial penalties, which is confirmed by the German news article.

Several times the airline had to move the flights to the cooler hours. It costs.— Google Translate

Vegas to Europe is a huge distance, and a direct-flight without a technical stop to refuel is Norwegian's only choice—evidently and understandably. Their low-cost model that favors filling all the seats over fuel is the limiting factor.

(gcmap.com) The four Norwegian routes, they range from 4,462 NM to 4,701 NM, London being ~4,600 NM.

(Source—PDF)
Fully loaded with payload, the range is limited to 2,700 NM for a takeoff from a 42°C Vegas.
18 tonnes are to be shed to make it to London, i.e., 163 seats will be empty, or 90 empty seats plus leaving behind 8 tonnes of freight—airlines do depend on freight for profit. So, the typical quoted 200 passenger capacity would work. But if Norwegian doesn't do freight, then they're not to blame for cancelling the four European routes during the summer heat.

Boeing PDF on how to read the payload/range chart.


What is the fluid released from the tail of this clipped Turkish airliner? A Turkish A321 airliner was hit by an Asiana A330 on 13 May 2018 at Istanbul Airport. It seems a large amount of fluid was released from the damaged section. What is the fluid? There is no APU up there so only hydraulic fluid, am I right?

Some news: https://onemileatatime.boardingarea.com/2018/05/13/turkish-plane-tail-istanbul/

It is hydraulic fluid. This works on 3000psi so that would explain the geyser.

I am not familiar with the A320 family but on most aircraft hydraulic system there are usually isolation valves to ensure that part of the system remains serviceable if there is a sudden loss of pressure/fluid in one part of the system.


Why are high-wing aircraft more stable? A high-wing aircraft is considered to be more stable in a side-slip because of the pendulum effect. How does the pendulum effect increase stability in high wing aircraft?

This question is about high-wing stability, not about comparisons between low and high-wing design also in the question that talks about pros and cons of high and low-wing design, it doesn't really explain the stability from an aerodynamic point of view.

Are they really?

In a high wing aircraft the center of lift is above the center of gravity. This will not increase stability when the aircraft banks, however, as you rightly say, will help once the aircraft is in a sideslip. In order to keep the fuselage at an angle to the airflow, the wing has now to create a sideways lift component, which it does by flying at an angle to the horizontal. This roll angle of the aircraft will shift the center of gravity sideways, so it is not exactly below the center of lift, but offset laterally. This lateral offset seems to produce a rolling moment which works against the roll angle.

Below I sketched the major forces in a sideslip. Note that the aircraft has a sideways speed component which creates a side force on fuselage and tail (green) which needs to be counteracted by the sideways component of lift (blue).

Short Sandringham in sideslip (based on this source)

But still the weight (black) is attacking at the center of gravity, so it will not roll the aircraft, and the lift is in the vertical plane of symmetry, so it, too, will not create a rolling moment. Ergo, we have no stabilizing effect due to the high wing location!

In a sideslip of a high-wing aircraft the windward wing root will see a slightly increased angle of attack while the leeward wing root will experience a reduced angle of attack due to the crossflow around the fuselage. This would indeed create a rolling moment because it will shift the center of lift sideways, out of the plane of symmetry. In a static sideslip the pilot will deflect the ailerons to maintain the roll angle as well as the rudder to maintain the sideslip, shifting the lift vector back into the vertical plane of symmetry. Now any stabilizing effect of the lift asymmetry is on purpose cancelled by the pilot!

However, this is achieved in a low wing airplane by adding dihedral. Therefore, a low wing aircraft can easily be made as stable as a high wing aircraft in a sideslip.

You will notice that the lateral aerodynamic force (green) acts above the center of gravity and also produces a correcting rolling moment. This is dictated by the location of the vertical tail, which contributes little to the location of the center of gravity, but produces a considerable share of the lateral force (reduced in the sideslip case by rudder deflection, however). This rolling moment, though, is almost independent of wing location. In a low wing, the center of gravity will be lower in total, especially when the engines are mounted on the wing, so the rolling moment contribution of the vertical tail is somewhat higher. The effect is small in a sideslip since the rudder deflection means that the fuselage contributes the majority of the side force. And this is not the pendulum effect which you asked about and which does not exist.

In airships, the pendulum effect is real, however: Since buoyancy always acts against gravity, a lateral offset of the heavy gondola will create an uprighting moment, just as it does in a pendulum. When turning, the heavy gondola will be pulled sideways by centrifugal forces, and the airship will roll. Since the turn is commanded with rudder, the rotation will give the rudder a small nose-down moment, which must be compensated by a nose-up elevator command. The pendulum effect will ensure that the gondola is at the lowest point in straight flight.

P.S.: Thanks to the hint of @kepler22b I have now discovered the keel effect page on Wikipedia. It also mentions the pendulum effect and calls both the fuselage contribution to the dihedral effect. Man, if there is ever a competition for the most misleading name of an effect, this would be the winning entry.

A pendulum is a mass mounted below the hinge point, so it will stabilize in the down position. A flying airplane is not hinged, so all motion is around the center of gravity. There simply is no pendulum effect in airplanes.


Do missiles need anti-icing? Are anti-icing systems needed for any missile carried by a small combat jet? And if so, how is this achieved?
I realize of course that missiles typically fly at Mach 2.5+, so the compressive effects will heat the air around it. However, a jet is typically carrying the missile under its wing, and can be fighting at subsonic speeds. Flying through clouds or rain, or high altitude subsonic dogfight, are very ice-prone situations. If the missile has iced up and then is fired, it could fail.
Moreover, even at supersonic realms, there are other parts of the missile besides the nose and leading edges.

Top wing surface, if it gets iced, will affect the camber and lift.

Trailing edge of a control surface.

Main body could theoretically get too heavy with ice.

Seeker head could ice up at any time before firing, and if not ready, will prevent lock on.

I highly doubt the inside of the missile will be hot enough to cover all those areas, especially the nose and wings. So do missiles need anti-icing, and if so, how is it achieved? If not, why not?

In a history of NASA's Icing Research Tunnel, "We Freeze to Please," NASA SP-2002-4226, Fig. 5-2 shows Hellfire missiles being tested.  But it conspicuously mentions nothing about de-icing systems for them, unlike the many pages that it devotes to de-icing for other aircraft and aircraft parts.  So it's likely that such de-icers are either still classified, or were deemed unnecessary.
Dohn Joe's remark, that Hellfires this icy could not be laser-guided, suggests that classified is more likely.


What are the advantages/disadvantages of a forward-folding propeller? The AMS Carat A is a motorglider with a forward folding propeller.

To my knowledge, this is the only production plane with this characteristic. The most popular self-launching gliders with front motors use props which fold back.

A naive guess is that aerodynamics come hugely into play here, although I don't have a notion if they're better or worse than a rearward folding prop.

What are the advantages to this folding style, beyond the fact that a rearward folding prop would simply not be practical on an engine-cowled nose?

The problem has traditionally been that if you wanted to minimize propeller drag on a cowled tractor motorglider engine you had to go with a feathering prop.  Expensive, and a bit heavy because you have to have a hub with blades that can rotate on their axes to change pitch, with a passive or active pitch change system, so the hub needs a blade root retention structure with bearings that allow the blade to rotate on its axis and a control mechanism of some kind.

With an aft folding prop it's a simple job to hinge each blade on a bolt to passively trail, and when it's spinning the blades naturally move out and rest against structural stops or just sit in the proper position from centrifugal force.  You don't even need an actuating system because the wind blows them back for you when you're done.

This design tries to achieve a similar benefit but needs an active method to drive the blades forward against the air flow, so it adds an internal gas spring mechanism in the hub to push the blades into the forward position against the air flow when not spinning, and when spinning centrifugal force is enough to keep the blades from coning too much from thrust loads plus the gas spring (the VW based engine spins pretty fast, 3000 rpm, so there is plenty of centrifugal force).

So the main advantage is you get most or all of the benefit of a feathering prop (the only other option), without the elaborate hub/bearings required for feathering, just a simple pivoting bolt or pin and a spring system to push the blades forward.  The spring system is the only add-on relative to a super simple aft-folding prop.  Pretty elegant.

As a main disadvantage, it's easy to imagine that the blades when folded will be a little bit destabilizing either in yaw or pitch or both by adding surface area forward, depending on where the flat surface is oriented when it stops (like the way floats are mildly destabilizing in yaw and usually require extra fin area).  No doubt that is taken into account when sizing the tail surfaces and perhaps the blade surface area is too small to be noticeable.

The other problem is the engine can't be made to windmill, but that's also a problem with an aft folding prop as well. and in any case the VW based engine this airplane uses won't windmill even with a fixed prop (if the engine quits in your VW powered homebuilt, it'll stop at just about any speed you can fly at and you need a starter to get it going again).


How does a PESA radar work? I have been trying to understand how a PESA radar works. Essentially, it uses one signal that is electronically delayed. How is the reception coordinated and known by the receiver?

I am interested in understanding the logic behind in delay of signal and reception/identification.

Your question: What is the logic behind the delay of the signal and how is the echo signal later bound with the signal sent?

Short answer

A PESA is an array of individual antennas. Each antenna radiates the same signal, except the phase. The phase adjustment for each antenna allows for the non-moving array to behave like if it could be rotated, i.e. the direction the energy is sent to or receive from can be selected by adjusting the phase for the individual antennas.

Varying the signal phase for an individual antenna of the array can be done by delaying the signal for this antenna, which in turn is as easy as
making the electrical conductor (or waves guide) between the oscillator and the antenna longer.

PESA determines the transit time exactly like another more traditional one with a single rotating antenna. A signal sent by a PESA, after it has been sent, is indistinguishable from a signal sent by a rotating antenna radar.

Details

The principle of a primary radar is to send a radio signal in a given direction and wait for a possible echo. The direction of the radiation pattern is used in determining the direction (azimuth and elevation) of the reflecting object. The antenna must be very directional (small solid angle of the radiation pattern), else the angle determination cannot be precise.

Radiation pattern of an antenna with a parabolic reflector (source)

That means the antenna must be rotated to scan a useful portion of the sky (or ground).

To clarify the wording used, an antenna is usually composed of:

A radiator which receives the signal to be transmitted (it also collects the waves in reception). The radiator is the interface between electrons and waves and vice-versa.
A reflector used to reflect the waves from the radiator (it also focuses the waves on the radiator in reception).
Sometimes directors to narrow the radiation pattern of the system.

The radiation pattern of an antenna is mostly determined by the shape of the reflector (e.g. the parabolic reflector) which only illuminates a given fraction of the space and, due to interferences, also varies the energy based on the direction the energy is sent to or received from. Due to the reflector, a signal from the radiator will usually travels using different paths created by the multiple reflections.

Interferences are the result of combining the electromagnetic signals at a given point. For signals having the same frequency, the phase determines if signals are added (same phase) or canceled (opposite phases).

Phase determines how signals interfere (source)

We can apply this phenomenon to the multiple signals created in an antenna composed of a radiator and a cylindrical reflector.

Signals summation on a cylindrical reflector antenna creates a privileged direction of radiation with some secondary (unwanted) lobes (source)

Array of radiators (electronically scanned array, ESA)

Another way of varying the privileged direction of a radiation pattern is to create interferences on a voluntary basis, so that they create a stronger signal in the desired direction, and a weaker signal elsewhere. The advantage is that this electronic direction change can be much quicker than a mechanical rotation.

One way to create interferences is to send the radar signal from an array of small radiators instead of a single big one.

Close-up of the front of the old Cobra Judy radar (source)

This type of array is quite familiar under the name interferometer in the field of astrophysics:

Siberian Solar Radio Telescope (SSRT) interferometer observing the Sun at 5.7 GHz (source)

If all radiators are fed with the same signal, then interferences will occur the same way on the whole surface and the result will be that the direction perpendicular to the array is privileged (image on the left side).

Electronic beam-deflection (left: bore sight, right: deflected (source)

The right side of the image shows what happens if the signal sent to the individual radiators is delayed in a coordinated way: Delay increases from right to left on the array. It must be remembered that delay means phase here. The direction of the signal is now oblique due to the the location of constructive and destructive interferences. The red circles indicates signal with the same phase.

See this animation too.

Delaying an electric signal is quite easy: Just increase the length of the conductor (see analog delay line). A length variation between 0 and the wave length (e.g. 10 cm for a wavelength of 10 cm -- 3 GHz) allows to sweep the whole range of possible phase values from 0 to 360°.

Indeed there are other ways to delay a signal without adjusting the length, but they are electrically equivalent. When signals are digitized, it's really easy to process them, there is less electro-mechanical constraints.

By varying the phase on individual radiators we can steer the signal as if the antenna had been rotated, but we can also focus the energy on a target.

(source)

PESA vs AESA

There are two approaches for feeding individual radiators with phase-shifted signals: active and passive:

Active means the radiator has its own oscillator to create its own signal. The active element must receive the value of the phase shift to use from a central element which coordinates the whole array.
Passive means the radiator receives the signal to be sent and just radiates it. The signal must already have the required phase. This is usually done with a single oscillator which feeds an array of phase shifters which in turn feed the array of radiators.

(source)

Transmission vs reception

The beam formed by the array concentrate the energy during transmission. The signal sent is not different than one sent with a single large radiator (rotating radar antenna). This is also a general result of adding and canceling energy by interference.

An array conforms to the general rule: An antenna is reversible, the characteristics of the radiation pattern are also valid in reception. That means:

Receive the same signal on all radiators
Add individual delays (phase shifts) to obtain a privileged direction of energy origin. The phase values are inverted compared to the ones used for sending in this direction.
Mix all signals into a single one to have them interfer and obtain a single mixed signal, equivalent to the one that would have been received with a rotating antenna.

The effect is that energy originating from directions other than the privileged one will be weakened, and the energy in the privileged direction will be emphasized.

Note that the phase is used to change the radiation pattern (process known as beam forming), but the rest of the radar is unchanged compared to a rotating single antenna one.

(Tell me if there is something that is not clear, I'll improve the post as necessary.)

Additional details

Phase shifter

As mentioned a phase shifter is used on each individual antenna of the array. It should prevent signal loss, distortion and noise addition, and the phase must be controlled accurately and quickly, by steps or continuously.

For step by step (digital), delay lines are used, with a possibility to connect a variable number of them using switching diodes (e.g. pin diodes). See more on RadarTutorial.eu with an example of 4-bit shifter (16 phase-shift values). More details on this datasheet for a 6-bit shifter on S-band.

Continuous (analog) control can rely on a resonant circuit (LC) to delay the signal. The capacity value can be mechanically changed in MEMS.

(source)

This extends past what I know in the domain, but other technologies exist, and are detailed in Array Phase Shifters: Theory and Technology (Nasa, 2007), including:

  Another style of phase shifter uses coupled microstriplines as dc
  electrodes to polarize a thin [...] ferro-electric film [...]. These
  planar phase shifters are fairly compact, low loss, easy to fabricate,
  and can provide 360° of phase shift with bias voltages under 350 V.

Phase shifters design depends on the operating wavelength of the radar.

Radiators used in a phased array antenna

Again the operating frequency impacts the radiators design.

Low frequencies may use:

A pair of crossed dipoles to allow polarization changes during the pulse travel and the reflection on the target.
A Yagi antenna.

Higher frequencies use other designs:

Notch antenna (slot). Used on Russian N011M (also note the dipole array for lower frequency scan of this hybrid radar).
Patch antenna (microstrip).
Vivaldi antenna (tapered slot) for broadband spectrum.

More on RadarTutorial.eu.


Low speed, low wing loading? Just looked at the Lamac ( light aircraft manufacturers association of Canada) specs.  If I remember correctly, They want to see a maximum of 3 psf wing loading for Vc of 60mph, rising to a maximum of 12psf for Vc of 120mph and above.

Why only 3 psf?  Looks to me they just want standard wings with a cl max of 1.5.

The Zenith 701 stol has a wing loading of about 10 psf, yet has a Vc of about 60mph, with a cl of about 3.

It's a design standard to set minimum wing area for the airplane's intended speed range using "unassisted" Clmax as the basic parameter. And most low speed wings have a Clmax without enhancement of somewhere around 1.5/1.6.

The 701 gets its low speed range with small wings using both slats and slotted full span flaps to get its 3+ Clmax, so it doesn't really fall within the definition.


What area is used to determine the horizontal and vertical tail volumes of a vtail? What area is used to determine the tail volume of a 45 deg vtail?

Is the same entire area of the vtail used to calculate vertical and horizontal tail volume?

or do you use the projected areas in the x and y directions for horizontal and vertical tail volumes, respectively?

or other?

Depends on what the tail volume is used for.

For control authority and trim: Use the projected area.

For flight dynamics and damping: Reduce that by the cosine (horizontal) rsp. sine (vertical) of the V-angle, in your case multiply the projected areas by 0.707.

The control effectiveness of a control surface is proportional to the cosine of the V-angle, but the damping characteristics go down with the square of the cosine.


Is Doppler navigation still in use for civilian aircraft? Is it possible to find an airliner nowadays with Doppler radar used for navigation purposes?

Only in military applications nowadays, primarily helicopters as the system doesn't require the alignment process of the INS system, is not subject to GPS jamming, is very accurate when flying low (as opposed to high and fast over oceans), and most helicopters don't do the occasional barrel roll (the system is belly mounted to track the ground below).

(Calm seas messed with the Doppler early-on due to the double reflection – surface and sub-surface.)

However for the airliners, as you ask, it was short-lived, but it helped usher in a new era.

TWA in c. 1962 fitted its Boeing 707 fleet with dual redundant Bendix DRA-12 Doppler systems and CNA-24A computers (pictured above). After demonstration flights to the FAA for technology proving, TWA was the first to let go of the navigator (the fourth crew member back then). It was the first time long-haul over-water flights were operated without a dedicated navigator.

The inertial navigation system in that decade was still a top military secret. But the technology and computing power were making their way into the civilian world.

First was the sub-inertial navigation system (part INS; part Doppler), such as the Sperry SGP 500 gyro platform coupled with a Decca Type 72 Doppler. It was cheaper than a pure-INS system and had improved accuracy over a pure-Doppler system.

Initially operators favored the sub-INS system, because of the much quicker alignment time. In that era, any disturbance to the aircraft – while being refueled or boarded – would add to the alignment time. That was c. 1969.

1971 – just under a decade from TWA's initiative – was when the INS first made it onto an aircraft as default equipment straight from the factory. The plane was the Boeing 747, the INS system was the Carousel IV, and the rest is history.

(Doppler navigation was also used on general aviation aircraft, and much like the VOR-based RNAV equipment, such as the King KNS 80, both yielded to the GPS.)

References:

TWA Relies on Doppler, Flight International, 11 October 1962
New sub-inertial navigation system, Flight International, 6 March 1969
Carousel IV in the 747, Flight International, 1 July 1971