How much altitude can a glider gain only from losing speed? Say we have a glider flying near its top speed. If pilot decides to "trade speed for altitude" how much can it climb before hitting stall speed? Assuming pilot will use most effective climb rate for the task.
I know it depends on many factors, but what kind of figures are realistic?
Reverse situation - how much altitude glider needs to gain speed (say its top speed)?

You can calculate a theoretical maximum (which will be well above the actual practical height gain) by considering the conversion of kinetic to potential energy. You can also make estimates based on observing a glider performing a loop.
Calculate the kinetic energy of the glider at maximum speed as mv2/2, where m is the glider mass and v is the maximum level velocity. Then do the same for the stall speed mu2/2. Take the difference. This is the kinetic energy lost in the maneuver.
Assume this is all converted to potential energy, which is given by mgh, where m is the mass, g is the gravitational constant and h is the height gained. Rearranging this (and cancelling m) gives us

h = (v2 - u2) / 2g

Again, the real number will be much less than this, because of drag during the maneuver, but it will give you a ballpark.
According to Wikipedia, a Schleicher ASK 21 has a stall of 40 mph (18 m/s) and a top speed of 170 mph (76 m/s), which seems to be one of the larger ranges among common (and non-performance?) gliders. With g=9.8m/s^2, and calculating to two digits of precision, that's a theoretical max of h=280m or 920 feet. (Thanks K A Buhr)
When a glider performs a loop maneuver, it does so by diving to achieve a high speed, pulling up to a low speed (upside down) and then diving again. This is somewhat analogous to what you are describing, although gliders don't reach their theoretical maximum speed in the maneuver, and they may in fact go under their stall speed at the top. An examination of performance gliders looping show that they can gain several hundred feet during a maneuver.


What is the difference between AHRS and ADIRS? Whats is the difference between Attitude and Heading Reference System and Air Data Inertial Reference System ?

The most important difference is that an ADIRS contains an inertial system, which gives a position estimate and can be used for navigation. This is done by aligning to a known position on the ground and then integrating all changes in orientation (measured by ring laser gyroscopes) and velocity (measured by quartz accelerometers) to update the position. As a by-product you also get the orientation (attitude and heading).
The AHRS only gives this orientation information, but no position. It therefore replaces conventional gyroscopic instruments like a directional gyro and an artificial horizon. An ADIRS can usually be switched to ATT mode (instead of full NAV mode) in flight and then function as an AHRS without navigation capabilities:

(source: 737 NG FCOMv2 (11.10.21 - Flight Management, Navigation - Controls and Indicators))
The ADIRS also performs all computations done by an Air Data Computer (ADC), which usually comes installed separately next to an AHRS for glass cockpits, and therefore replaces the conventional pitot-static instruments.


Can multiple helicopters be used to lift a heavy load? If the load is heavier than a single helicopter could lift, how possible is to use multiple helicopters for this task?

I have only seen this in some movies. The biggest doubts are

If we attach two ropes to the single point of the load in a V shaped way, a side force will be created pulling the helicopters together into collision.
Even without this, the flight of the two machines probably needs to be very coordinated.

In movies like "Pacific Rim" a whole swarm of helicopters lift a heavy robot. How realistic are these visions?

This video on YouTube apparently shows that it has been done at least once. A rigid beam was used to counter the forces that would draw the two helicopters together.

Screen grab:

I also found this fascinating report (.PDF link) from Piasecki (now part of Boeing Defense, Space & Security) that studied the feasibility of attaching two CH-53D helicopters to each other, including interconnected powerplants and flight controls!


Could the An-225 use Trent 500 engines? Currently, the Antonov An-225 Mriya is being produced with Progress D-18T engines, rated at 229 kN each. However, I've looked at the A340 and I love its Trent 500 engines. But the Trent 556, rated at 249-270 kN, is only used on the A340, and the A340 is not being made anymore. Would it be possible to re-engine a future An-225 to use six Trent 500 engines?

There have been various proposals to use Western engines on new versions of the An-124:

An-124-200: Proposed version with General Electric CF6-80C2 engines, each rated at 59,200 lbf (263 kN)
An-124-210: Joint proposal with Air Foyle to meet UK's Short Term Strategic Airlifter (STSA) requirement, with Rolls-Royce RB211-524H-T engines, each rated 60,600 lbf (264 kN) and Honeywell avionics—STSA competition abandoned in August 1999, reinstated, and won by the Boeing C-17A.

Antonov fell victim to the fall of the Soviet Union: its HQ is in independent Ukraine which doesn't have the best relations with Moscow, so Antonov's sales cratered and left them with no development funds for new versions of the An-124. Western-engined versions were proposed to generate some sales in the West, but potential buyers were scared off by having to buy a paper project (and having to pay for development of the new version).

Along with the modest improvements to the An-124 being considered for new production of the aircraft, the Antonov organization has also investigating a more ambitious cargolifter, an improved hybrid of the An-124 and An-225 called the "An-124-300". It would feature the fuselage of the An-124 and the extended wing of the An-225, kitted up with modern avionics and engines. Since more powerful engines are now available, only four would be needed instead of six. The cargo floor would be extended, and a palletized loading system would be installed. It would have a maximum range of 11,500 kilometers (7,145 miles) with a 100-tonne (110-ton) cargo. Development costs were cited as less than a billion USD.
Given that the relationship between Ukraine and Russia is antagonistic, the Antonov organization is very interested in "Westernizing" the An-124 and any follow-ons -- using Western avionics and systems at the very least, and possibly re-engining the aircraft with Western turbofans.


Why is the nacelle outlet of the tail engine of the L-1011 Tristar lower than the engine itself? I've noticed that the nacelle outlet of the Lockheed Tristar is situated below the engine itself. It is at the end of the fuselage. The nacelle outlet of the tail engine of the DC-10 on the other hand, is at the same height as the engine. Why did Lockheed choose to lower the nacelle outlet? Perhaps this picture will help show you guys what I mean.

As Federico points out, the engine is located directly in front of the exhaust portion. This is true for almost all engines with ducted arrangements. It's much easier to duct the cool, slow intake air to the engine than duct the hot, fast exhaust air away from it, for both temperature and drag reasons.

In this cutaway you can see the intake duct leading down to where the engine is:

Source

Putting the exhaust nozzle lower allows more room for rudder above it. Putting the nozzle higher, like on the DC-10 series, means the rudder is pushed higher up the tail.

This arrangement keeps the rudder as close to the vertical center of gravity as possible, which reduces the stress on the tail and limits the rolling motion induced by the rudder. It also keeps the engine forces close to the vertical center of gravity. This configuration also allows the supports to tie in to the existing structure at the rear of the fuselage. Placing the engine further up the tail would add surface area (and therefore drag), and require additional structure to support the engine above the fuselage.


Why are the toggle switches in cockpits of new aircraft painted white? The toggle switches in the cockpits of new aircraft are painted white. e.g. 737 MAX:

but a 727

Why?

The reason is threefold:

White knobs offer better contrast, and are thus easier to spot regardless of lighting conditions

The material used has improved and consistent friction and "feel" to it. Bare metal switches, especially when worn out by use may become slippery, and since some of the toggle switces have detents, operating them might be compromized.

The plastic coating (on at least most) of the switches can be renewed without replacing the whole switch assembly. For example on the larger switches white knobs are screw on -type


Would discharging a firearm in-flight cause an explosive decompression? I was reading a BBC FAQ about Federal Air Marshals, which has this chilling quote in it:

  A gunfight on a passenger airliner is seen as an option only of last resort.

Presuming an air marshal has to discharge his weapon on-board, and it goes through the fuselage, would this be likely to cause an explosive decompression?

No, there was actually an episode of Myth Busters about this exact subject:

http://www.youtube.com/watch?v=Fi1_1l7M8FA

They had to try really, really hard to cause an explosive decompression, resorting to actual hollywood style explosives to make it happen.

Most airliners can actually even maintain normal pressurization even with a coffee can sized hole in the fuselage.

As far as firing their weapon on board the aircraft, they are trained to not shoot towards areas like the cockpit or the floor where most of the "important" stuff is.  Avionics, hydraulic lines, electrical wiring, etc. don't take bullets well.  The rounds that they use would put holes in the aircraft, but as the above episode shows it won't create a pressurization problem.


What could cause GE and Rolls-Royce turbofan engines to sound different? I've often as a passenger noticed the difference of the sound coming from different airliners. I believe the two manufacturers I have compared are Rolls-Royce and GE, where the latter's engines sound more "electric" (funny enough) and the former's more distorted and noisy.
Do these observations make sense? And what could possibly create such a difference i sound? I was in the belief that engine design of turbofan engines is somewhat established and detailed.

The GE90 engines have a higher bypass ratio than the RR Trent.

The majority of engine noise, especially the "ripping, howling, typical jet noise" comes from the engine core.

The higher the bypass ratio, the less air passing through the core for any given thrust and the more cold air meets the core exhaust at the back reducing the shear

Jet engines with no bypass ratio and deep intakes produce a wonderful sound


Are there any aircraft with retractable vortex generators? From this answer, “Of course, these also add drag, even when the wing is not stalling, so they are only added where needed”.  Are there any aircraft that have vortex generators that are retractable so that if the flight computers determine that the wing is stalling, the vortex generators are automatically deployed without them causing drag all the time?

Bell Textron filed a patent for retractable vortex generators in 2006. The patent is here.

Boeing also filed a patent for vortex generators that extend simultaneously with a leading edge slat here.


How many wing attachment nuts are there on a DC-3/C-47? How many wing attachment nuts are there on a DC-3/C-47? I once participated in putting one together and is now writing about it, but cannot remember how many nuts there are, only that it took forever to line them all up.

Information on PPRUNE (without a source) says:

  328 per side (102 x AN4-7A & 226 x AN410A bolts to be exact).

I was able to verify the 328 figure from various books.

  Each wing of a DC-3 transport plane is fastened on with 328 nuts and bolts. Unless the stress and strain are distributed equally, some of the bolts shear off. The answer was found in Elastic Stop Nuts. These nuts can be given precisely the right tension—then lock fast. This is one of the important structural fastening jobs which Elastic Stop Nuts have solved.

(American Aviation - Volume 7, 1943, p.82)

I tried to find more info, to no avail.


Why do these safety instructions have a ruler on them? I was on a Wizz Air flight the other day and the safety instructions leaflet had a CM ruler running down the side of it.

What is the purpose of this?

This looks like just a courtesy tool on the safety card, but doesn't actually have a requirement or purpose. Previous versions of their safety card did not have this courtesy tool.

(Image Source: aviationfreaks.freeforums.org)


What is the material composition of aircraft compressor blades? I read up that aircraft compressor blades are generally made out of titanium. I was curious as to why is titanium the choice of material used for aircraft compressor blades? What alloys are involved and are there alternative materials used in the designing of such blades?

Have found a reference to "high grade steel" and to titanium. According to the (paper format) book, the parameter that dimensions the compressor is the perimeter speed (tip speed) of the blades $u_2$.

For steel: maximum $u_2$ = 450 m/s. With a total inlet entry temperature of 300 K, pressure ratio is 4.2
For titanium which has a higher strength/mass ratio: $u_2$ can be up to 500 m/s, leading to a pressure ratio of 6.

The book was written at the time of the B757/767 engines so there may be more modern materials around.

Also refer to this answer.

Update

A more recent reference is provided by this presentation for the Trent 1000 engine:

Fan and LP compressor blades are made from titanium alloy, containing small amounts of Fe, V, O, and Al.
IP compressor: 8 stages of titanium blades.
HP compressor: 6 stages of titanium blades.

The reason titanium is selected is that up to a certain temperature, titanium has the highest specific strength (strength/weight ratio), and in the compressor the temperatures stay below the transition temperature for nickel alloys:


How can I calculate a flightpath given a set of waypoints? I have made a GUI that allows the user to create waypoints defined by latitude, longitude, altitude, and time-of-arrival. I'm looking for some software that can generate a flightpath between the waypoints.

For example, say that I have placed 5 waypoints. All that I know is that I want my aircraft to be at those five points in space at the times specified. To create my 'flightpath', I want to calculate the latitude, longitude, altitude, pitch, roll, yaw, and velocity of the aircraft as it moves from one waypoint to the next. I'd like to calculate those values for time intervals of, say, one second.

I've made a hackish solution, but it isn't very realistic. Someone must have tackled this problem before. Does anyone have any suggestions on software or algorithms I can use?

The Haversine equations will calculate the bearing and distance between any latitude/longitude pair, and can calculate the location and next bearing for points along a Great Circle.

They go back to the days of sail, shortly after the invention of the marine chronometer, which allowed calculation of the current longitude. As such they assume that the points are on the surface of an idea sphere; as Earth is slightly oblate, there is an error of upto 0.6% in calculations of locations. For any realistic altitude, the oblateness (23 km difference between polar and equatorial radius) is likely to be larger than the change in altitude.

A discussion of them is here http://www.movable-type.co.uk/scripts/latlong.html


What makes the ailerons on an aerobatic airplane different than the ailerons on a non-aerobatic airplane? I often hear a spec of the "roll-rate" when talking about aerobatic airplanes and the term "fast ailerons." What exactly does that mean, and what makes the ailerons on an aerobatic airplane different than the ailerons on a non-aerobatic airplane?

How does the design of the ailerons affect the roll-rate of the airplane?

The term "fast ailerons" is an informal term that generally refers to the response of the plane relative to the pilot's input to the aileron controls on the stick or yolk. The faster the plane responds, the faster the ailerons (or other controls).

The roll rate is the speed at which a plane can complete a roll. This depends on a number of factors, including aileron size, maximum aileron deflection, wing length, and speed.

Ailerons that can deflect a lot of air without disrupting the airflow over the wing too much are desirable in an aerobatic plane, so the roll rate can be increased and so the plane will have better roll control at lower speeds. While larger ailerons are desirable on an aerobatic plane, they are also more susceptible to flutter at high speeds, which is not desirable because it tends to rip them off the wing. There are several design techniques that can be used to mitigate this, such as gap seals, counterweights, countersurfaces, etc.


What is this military turboprop aircraft? I took a video of this military turboprop aircraft on August 6, 2020 at 7:03 PM in Long Beach, California. Here is a screenshot of it:

From that really grainy, blurry picture, it looks like a C130 Hercules.

bulbous fuselage
Straight, ironing board-like wings
High wing configuration
4 turboprop engines
Huge, high empennage
The rear is shaped like it could have a ramp.

If you are sure it was a turboprop, it looks like the C130 is a strong contender.


What construction standards are used to increase impact survivability of passengers in case of a crash landing? There are several equations on the site about effectivity of passenger seats and restraints, and how beneficial they would be in case of a crash landing:

This question about lack of shoulder restraints
This question about possible decrease of fatalities when using a shoulder harness.
This one about the best brace position.
This one on turning the seats backwards.

The latter one is the only one of the four that cites any research and quantifications on increasing survivability rates in a crash.

What are the requirements imposed on airliner seats and restraints in order to make sure that preventable deaths from impact are actually prevented? What research has been conducted to improve safety standards in the last few decades?

In the US, the FAA has no requirement for crash integrity or safety of the air-frame (and they have been heavily criticized for this). There is no requirement for crash analysis like is done for automobiles. The only requirement is the seat integrity. From 1988 to 2009 the FAA has phased in the requirement for the seat and restraint system to remain secure - from 9G to 16G. New York Times

This is similar to the question,
"Why are passengers not given the option of shoulder harnesses?"

[Safety Report]


How tight should my orbits be? I'm flying VFR circuits at a towered aerodrome, and the circuit is quite busy. With traffic ahead of me to land, the tower calls on the radio and asks me to orbit on downwind. I know it's good airmanship to keep a neat orbit, centred over one point on the ground, but how tight should I make it?

A tight orbit takes up less space (I've known there to be three aircraft all orbiting on downwind at once), but at the same time, it seems like a waste of effort and fuel to be making tight turns the whole time when the point of orbiting is to kill time. A lazy orbit (at say rate 1) would kill time just as effectively while flying at a much more comfortable attitude - and more stable in windy conditions.

It sounds like "orbit" is UK-speak for making a 360-degree turn for spacing in the pattern.

I'm not sure what the custom is in the UK but in the US I was taught that you make a Rate One turn (3 degrees per second) for this, giving the tower two minutes between you and the aircraft in front of you (or the one on the ground they're trying to get rid of before you arrive).

The logic is that the maneuver is supposed to create spacing - flying a tighter circle takes less time, and thus creates less space between you and the aircraft in front of you. It also obviously requires more aggressive banking at low altitude compared to the relatively gentle bank required for a standard-rate turn at traffic pattern airspeeds, which as you noted will be easier to control in windy/gusty conditions.


Why do the transponders introduce codes instead of using aircraft registration numbers? (A non-pilot here, just a Aviation.SE reader, recently into listening to ATC. I am wondering...)

Why do the transponders use their own codes? (squawk) It seems to me, it would be safer and less of a burden, if each aircraft programmed their own registration number into the transponder and everyone used that?

To expand upon voretaq7's answer, understanding the history and evolution of transponders can more deeply explain why this is the way it is (for now).

When Allied (US/UK/Others) WWII planes returned from combat missions over Germany and mainland Europe, the (brand new technology) radar controllers had no way of knowing "who" the aircraft were. To solve this problem, aircraft began carrying an electronic box that could transmit a radio signal automatically in order to identify themselves. This system was the original IFF (Identification: Friend or Foe).

This piece of electronic wizardry could be set on the ground by crew members, prior to departure, using an 8-position dial numbered from 0-7. The box would receive a transmission signal from the ground, and then transmit a reply of two bursts of energy per cycle: the first signal was the "I'm beginning my transmission" signal (a slightly louder burst of energy), and the next signal was delayed a specific amount of nano-seconds to match the frequency wavelength (and would be slightly lower in intensity).  If the transmitter was set to 0, the delay was nothing. That is, the second burst would be transmitted on the next "instance" following the begin-transmission burst. If set to 1, then there was a 1-step delay after the start-signal... all the way up to 7, which would induce a 7-step delay.

The rudimentary "computers" on the ground, co-located with the radar, would receive these transmissions. Based on the known radio frequency, the timing of these short bursts of energy onto the carrier wavelength could be interpreted back into a number. If the radio on the ground received something like:
START-nothing-nothing-SIGNAL-nothing-nothing-nothing-nothing-nothing-START-nothing-nothing-SIGNAL-nothing-nothing-nothing-nothing-nothing.... then it could be calculated that the box onboard the aircraft was transmitting the "number 2".  If the secret number for that day (briefed to pilots before they left) was "2", then the plane was identified as a "FRIEND" in the IFF system. This was the very first transponder prototype. (BTW: It is called a "transponder" because it does two things: it TRANSmits a resPONSE to a specific signal on a specific frequency.)

But the obvious first hurdle was the fact that there were only 8 numbers available, and so the transponder boxes were quickly upgraded to two settings (56 possible code combinations) and then 4 settings (4,096 possible "code" combinations).

But there was still a "problem": the codes were set on the ground and could not be changed. Any mid-mission compromise of the number being used that day prevented aircraft already airborne from changing their codes. The next technological leap was to develop a panel-mounted system where the pilot could change each of the four dials manually.

This technology eventually entered the civilian arena. To distinguish between different types of transponders and who was using them (mostly a frequency issues - military freq vs. civilian freq), different categories came about, called "modes".  A "Mode 2" transponder was the above described 4-digit, octal, set-on-the-ground-and-leave-it system. A "Mode 3" transponder was the newer, pilot-sets-it-in-the-air version, but still using military frequencies.

When the civilians began using the same technology on approved civilian frequencies, the designation was "Mode 3-A", often just called a "Mode A" transponder. Combined with the advent of civilian air traffic surveillance radar, controllers could now distinguish "who was who" on their screens, by assigning each aircraft its own unique number.  This was great for a while, but the only thing controllers could see was the "blip" and its assigned number. The radar computer could also calculate ground speed, but not altitude. So, the controllers relied on the pilot to report their altitude verbally.

This led to the next technological leap forward in transponders. If that same radio could somehow be connected to the aircraft's pressure system (the same one that drives altimeters and vertical speed indicators), then it could probably somehow be configured to transmit that altitude data as well. That technology was thus invented, and the designation "Mode 3-A/C" was applied to the civilian versions... commonly (and still popular today) called a "Mode C Transponder".

This (finally) gets us to the original question: Why don't aircraft program their own registration numbers into the transponder?  Since there are only four octal digits available, there are three immediate problems: first, the numbers 8 and 9 are not usable; second, no letters are available; and third, with only 4096 possible combinations, and way more aircraft than that in the world, it is very highly possible that two aircraft in the same vicinity could possibly have the same number.

The advent of "Mode S" transponders, with "current-day" technology, can now transmit (as well as receive) all kinds of information, including the aircraft's specific identity like the question-asker thought should be the way it is. But, backwards compatibility for all of those aircraft still flying around out there with "Mode C" transponders still requires ATC to assign a unique code to an aircraft and the pilot to plug that number in.

Additional trivia, not directly related to the question: Since Mode C transponders are limited to the numbers 0-7, how can they transmit altitude data that might contain the numbers 8 and 9?

The answer is in an aspect of binary coding called a "Gray Code" (or sometimes a "Gillham Code"). The world's avionics people standardized on a set of codes to represent a set of altitudes in 100-foot increments. As a couple of examples, an aircraft at a pressure altitude of 8,500 feet would send the 4-digit code of 6220.   Sitting at a pressure altitude of 0 would result in the code 0620. The receiving ground station would evaluate both the pressure altitude received by the aircraft, as well as the current station pressure setting (commonly called the "altimeter setting"), and convert these into the aircraft's actual MSL altitude for display on the radar screen.

I was able to find this list of altitude-code conversions:

https://web.archive.org/web/20120726003224/http://www.airsport-corp.com/modecascii.txt


Why does the Rutan Voyager use canards? Can anyone point me to why Burt Rutan's Voyager is a canard?

In other words, what reasoning could be behind it. What problems Burt Rutan might have tried to solve by using Canard configuration in this specific aircraft
and why those problems may not be solvable or less feasible in a conventional configuration?

For  comparison I would prefer to use predator or global hawk.

I believe this is because noticeable CG shift during flight and possibly has some advantage in relation to dynamic responses.

But I would like to get a second thought from you guys.

PS: English is not my mother language so apologies in advance if Im not clear enough. Unfortunately the answers I've got so far are either not very well thought or incorrect.

The canard is a design which stabilizes in front but also adds lift, whereas elevators at the back actually subtract lift.  As the Voyager needed to take energy efficiency to the extreme, this would be a prime design choice.  His motivation may be in the book written about the Voyager, but a significant number of his designs were canards.


Do manufactures list the maximum speed of an aircraft based on its theoretical maximum speed or dive speed? I know that the absolute maximum velocity of an aircraft is based on the thrust available. However the aircraft is usually limited beforehand by other speed limits such as dive speed and Mach Never Exceed velocity.
So when researching the maximum velocity of an aircraft, is the max speed listed the absolute max speed or the dive speed as that would make the most sense since its the structural limit. The example I am specifically thinking of is the C-130J Hercules which according to this brochure (linked below) from Lockheed, is around 365 kts.
https://www.lockheedmartin.com/content/dam/lockheed-martin/aero/documents/C-130J/C-130Brochure_NewPurchase_May2020_Web.pdf

The 365 knots listed in that brochure is (as noted in the text) the maximum cruise speed. That is the maximum speed at which the aircraft can be expected to operate under normal circumstances in level flight. It is not related to the Never Exceed speed, which depends on the structural limits of the aircraft (as well as air pressure and atmospheric conditions etc.). The maximum cruise speed is interesting for planning purposes, as it gives you an indication of how far you can expect the aircraft to go in a certain time window. It is not, however, very interesting to pilots, who will operate the aircraft with regards to the actual structural limits.


Are the aircraft used for the A380 display ready to be delivered to airlines? For displays in international airshow, Airbus uses A380 whose livery is the one of customers. Here are some examples:

Le Bourget 2011: Korean Air
Farnborough 2012: Malaysian Airline
Le Bourget 2013: British Airways

Are those aircraft regular newly built aircraft ready to be delivered to airlines (with all the seats and passenger equipment inside)? Are those aircraft specially prepared?

In all cases the answer is yes, those aircraft were all outfitted with seats, and at least very close to being ready for delivery to the customer. Installing interiors and seats is a regular part of the production process, and it's unlikely that the plane would be pulled from the line before seats are installed, and worked back in later. From looking at rollout and delivery dates, these aircraft all had about 7 months between rollout and delivery, with the air show appearance happening within a month of delivery. Also, the aircraft are used as static displays while not flying, allowing people to walk through and see the interior.

In 2011, the Korean Air A380 indeed had seats in it. It was delivered in the middle of July.

In 2012, the Malaysian A380 also had seats installed. It was delivered at the end of July.

In 2013, the British Airways A380 probably had seats installed. I can't find any pictures of the inside, but it was delivered shortly after in early July.

Airbus also tends to bring a flight test A380 like MSN001 (first to fly, registered F-WWOW) for display, and has a more unusual interior. The Korean Air A380 was used for the display in 2011 after the flight test plane MSN004 had a bit of a mishap.


What range or variation should we expect from gyroscope in a helicopter? I need to measure the angular variation of a helicopter. Hence, I am looking for a 3-axis gyroscope? However, what kind of specification the gyroscope should have, like sample rate etc? Therefore, I want to know how the angular velocity changes (in any axis) in helicopter. I need to know the range only.

Any reference would be highly appreciated.

3-DoF magnetic gyroscopes are a dime a dozen nowadays, together with 3-DoF accelerometers, magnetic field sensors, and GPS position transducers. These are in mobile phones now, tiny and frugal with electricity. A google search for "9-dof position acceleration gyroscope" results in a lot of options.

For the sampling rate: this needs to be at least twice the frequency of the fastest vibration - more is better, since a frequency at half the sampling rate shows up as a triangular wave.

For the frequency range of angular displacement, this article might be of interest. It shows vibration data of a car driving over the road, using an iPhone to measure the vibrations. The article mentions a sprung frequency (inside the vehicle) of 1 - 5 Hz. A helicopter usually has higher inertia than a car, so sprung frequency should be lower. If you take 0 - 10 Hz as a baseline for the range, you should be safe. The ground reaction angular frequency range is always higher than the airborne one: responses in the hover are measured in seconds, not in Hz.

Above graph shows pitch response of an example helicopter, from Prouty, Helicopter Performance, Stability and Control. A helicopter with a rotor mounted on top is unstable in the hover, but in order to still be controllable for human pilots the response frequency may not be too high - it turns out that humans can comfortably control instabilities with a time period in the order of around 10 sec. The military have requirements for the stability response in MIL-H-8501A


What triggers the PTU (Power Transfer Unit)? As highlighted by this answer and by this page the PTU (power transfer unit) is an hydraulic driven pump. It gets energy from one hydraulic system and uses it to pressurize another hydraulic system.

What triggers the functioning of this pump? Is it automatic or manual? Is this pump available even if a leak is detected in the hydraulic system it tries to restore pressurisation to?

(A320 PTU (Vickers), source)

The Power Transfer Unit is well known on Airbus due to its barking-like sound (refresher: Video of a PTU in action). The sound comes from a pump which creates a 3,000 psi pressure being quickly switched ON and OFF.

(Source: Youtube)

The PTU is a safety system to ensure hydraulic pressure is available, specially at takeoff, even if one or two hydraulic circuits have failed.

The PTU can restore pressure in a deficient circuit without actually transferring fluid from the working circuit to the deficient one. This prevents throwing hydraulic fluid from the working circuit overboard if the deficient circuit is leaking.

The PTU is always activated, but is actually triggered based on a pressure differential.

When is it activated?

On Airbus A320 series, the PTU is actually the second source of pressure for the yellow and green hydraulic systems, after the EDP. It's always ON, ready for the pump activation if there is a pressure drop on the yellow or green hydraulic system and the differential pressure is more than 500 psi.

  Airbus A318, A319, A320 and A321 aeroplanes are equipped with three
  hydraulic systems and they can be safely controlled with only one
  system operative. A bi-directional PTU enables the yellow system to
  pressurize the green system, and vice versa, without the transfer of
  fluid from one system to the other.

  In flight, the PTU operates
  automatically if a pressure drop in one of the systems is detected.
  The PTU ensures optimal hydraulic system availability for take-off.
  (Source: EASA safety bulletin)

Automatic inhibition below 1,500 ft in case of leakage

The PTU cannot restore the correct pressure in a circuit with a permanent leak. In this case, the PTU, and possibly the working hydraulic system, eventually overheat. The crew receives a failure amber caution on the ECAM display and, up to recently, had to switch OFF (inhibit) the PTU manually to prevent damage.

  In case of loss of hydraulic fluid of the yellow or green system, the PTU
  cannot pressurize the failed system and must be switched OFF as
  required by ECAM procedure to avoid a PTU overheat which may occur two
  minutes later.
  (Source: EASA safety bulletin)

A320 PTU switch (source: Baltic Aviation Academy which, by the way, has plenty of quality training videos)

To avoid distraction from non-essential warnings during takeoff and landing, the ECAM message is not displayed when airborne below 1,500 ft height. There were several incidents (e.g. G-EZDM) where overheating became harmful before the amber ECAM message appeared at 1,500 ft, causing the temporary loss of a second system. Airbus added logic for automatic PTU inhibition when the aircraft is airborne, but still below 1,500 ft.

The previous quotes are from an EASA bulletin related to this modification (also detailed in Safety First, the Airbus safety magazine), with a failure scenario that had to be prevented:

Principle

A PTU is connected to two independent hydraulic systems: The first to drive an hydraulic motor (or turbine) mechanically connected to a pump. The second hydraulic system is pressurized by the pump.

PTU principle, source

PTU can be reversible (each hydraulic system can drive the motor or be pressurized by the pump) or not (the same system always drive the motor).

(Actual PTU schematics, source)


Are there any ILS approaches in Class E airspace? When descending on an ILS you can descend to 100' above the TDZE if you have the approach lighting system in sight. Cody Johnson wrote an article explaining how one pilot mis-interpreted what the rules allowed him to do, descended below 100' when there were obstructions in the way and crashed into a hill. (https://iflyamerica.org/misunderstanging_part91.asp )

Section 91.129(e)(3), Operations in Class D Airspace, is also extremely applicable in this accident. It says, 'An airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide slope until a lower altitude is necessary for safe landing.'

Class C and B incorporate Section 91.129(e)(3) by reference, so the rule applies in that airspace as well. It doesn’t specifically apply to operations at airports in Class E airspace.

My question is are there any ILS approaches in Class E airspace? Is there a a rule that prohibits them, or conversely, only allows them when there is a tower?

There are plenty of ILS approaches in Class E surface area (e.g. KCLM, KHQM, KPWT). In fact the presence of an ILS will usually require that some kind of controlled airspace extends to the surface. For an untowered airport this means a Class E surface area.


What is this hole on the F-22 Raptor? I noticed this little hole that is right in front of the cockpit window of the F-22 Raptor. In some photos, it seems to be closed, while in other photos, it's open.

Does anyone know (or think they know) what the purpose of this hole is? Is it a ram air intake for air conditioning? Or is this classified?

Here are examples:

Closed hole:

Open hole:

The appearance of being closed/open is a light trick; light reflecting off the tiny transparent window of the missile launch detector.

This detector typically sees in IR and would detect the plume of a heat-seeking missile. According to Wikipedia, the model used on the F-22 is AN/AAR-56 (IR based). (There are videos available on YouTube for its output; example.)

In the below cutaway, it's label number 7. Note it says windows, so I suppose there's more than one for the different angles (after some eye strain I couldn't find another 7).

Source: pinterest.com

The F-35 uses the AN/AAQ-37, below are the various locations:

Source: daumcdn.net (Korean)


What is the power curve Someone help me understand this thing in a simple way.
I understand it helps determine Vx.

It's an expression of power required to maintain level flight throughout the speed range of the airplane and forms a U shape.  The lowest point of the curve, bottom of the U, is the point of minimum power required for level flight and is normally just below maximum L/D speed (flattest glide speed).  To go faster moves up the U to the right and requires more power.  To go slower moves up the U to the left and also requires more power.

The main bogey man that pilots have to be aware of is the issue of being on the "back side". If you slow to below min power airspeed, slowing any more will require more power to keep from descending; that's the back side.

This becomes a crisis situation in marginal conditions, especially when flying at very high altitudes, and is one of the things that makes flight at the upper flight levels riskier than at lower altitudes; if you are flying at Min Drag  speed or close to it at service ceiling, say FL410, and fly though some down flowing mountain wave air that is sinking at 100 fpm (nice smooth air that you likely won't notice), the autopilot in ALT hold mode will just pitch you up to keep from descending and the jet will slow down.

If you aren't paying attention and let the speed bleed off too much, you will find yourself in a pickle entering the back side of the power curve, with little reserve thrust to speak of, and will have no option but to tell ATC you have to descend NOW and start down until you can recover your speed.


Why do propeller blades not have winglets? On the first look this question might sound ridiculous and maybe it is.
But as propeller blades act by the same physical laws as wings, and winglets reduce the induced drag by quite a bit, then why are they not put on propeller blades?

I could imagine that it might be a material problem. But are there any more reasons? And why exactly might it be a material problem?

Winglets on wings help because they increase the volume of air on which the wing can act. Extending the wing span would be much more efficient, but when span is restricted or the maximum wing bending moment is limited, winglets bring a small improvement in efficiency at high lift coefficients.
On propellers, however, the winglets would run through air which is already affected by the tips of the propeller. No additional air will be involved, so no efficiency increase will be possible. Please note that propulsive efficiency is increased by accelerating more air by a smaller amount. The formula for the propulsive efficiency $\eta_p$ of an air breathing engine is
$$\eta_p = \frac{v_{\infty}}{v_{\infty} + \frac{\Delta v}{2}}$$
where $v_{\infty}$ is the speed of the inflowing air and $\Delta v$ the speed increase of the air affected by the propeller disc. A smaller $\Delta v$ acting on a higher mass flow makes the engine more efficient. This effect is most pronounced when $v_{\infty}$ is low.
The prop tip winglets would operate in a region of high dynamic pressure and generate more friction drag without contributing to the prop's efficiency.
Things become different when the propeller blades have a very low aspect ratio and there is a hard restriction on propeller diameter: This is true for ships, where draught puts a hard limit on propeller size. Here a sort of winglet does indeed help: The Kappel propeller has its tip bent forward and increases efficiency by 3 - 6 percent. Given that marine propellers even for large ships have efficiencies between 50% and 60%, this is a noteworthy increase in efficiency.
By the way: Whoever tells you that winglets reduce induced drag quite a bit has something to sell to you, but I digress.


Are ICAO and FAA airport signs and markings the same? Are ICAO and FAA airport signs and markings the same or are there differences? If so what are the differences?

Principle

ICAO Airport markings are defined by ICAO in Annex 14 of Chicago Convention, Aerodromes.

ICAO also requires at section 5.8 Notification of differences in the AIP of Doc 8126 Aeronautical Information Services Manual, that differences are explained in the country AIP, Section GEN 1.7 Differences from ICAO Standards, Recommended Practices and Procedures.

US regulations are not entirely compliant with ICAO SARP, and differences exist for airport marking.

Example of ICAO SARP

For the touchdown zone lights, ICAO states in Annex 14, at paragraph 5.3.13.2:

  The lateral  spacing between the innermost lights of a pair of
  barrettes shall be equal to the lateral spacing selected for the
  touchdown zone  marking. The longitudinal spacing between pairs of
  barrettes shall be either 30 m or 60 m.

US example

As US regulations are not fully compliant, the differences are explained in GEN 1.7 of the US AIP, page 129:

  U.S. does not require the lateral spacing of touchdown zone lights
  to be equal to that of touchdown zone marking when runways are less
  that 45 meters wide.

In general

All differences can be found at this location, sorted by ICAO reference and chapter. Differences related to Annex 14 start on page 124 (actually labeled GEN 1.7 81). Listing all of them (15 pages in the AIP) would be too broad for this answer.


How heavy is too heavy to land on a grass strip? This is actually a tricky question, because it's not just about gross weight, but about how much weight each individual tire imparts into the ground.  This, of course, depends on the design of the suspension and the number of wheels.  Thus a really big craft, with a well designed undercarriage, can land on grass just fine (like a C-130 for example).

The point being, "how heavy is too heavy" has more to do with specific weight supported by each wheel during landing, and less to do with total landing weight.

So, kind of a two parter.  Firstly, how much weight can a grass field generally support when an aircraft is landing?  Secondly, how do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?

Your second question is really easy to answer:
Q: How do aircraft designers make landing gear that can land on a grass field without plowing into the ground and ripping off?
A: Big Fat Low-Pressure Tires.

A wide, low-pressure tire has a larger contact patch with the ground, spreading the aircraft's weight over that larger area helps prevent the tire from digging into the ground, and lets the aircraft roll more easily on rough or soft surfaces.

Your first question - How much weight can a grass strip support? is a much tougher one.  The bearing strength of an unimproved surface depends on a number of factors (some big ones: The composition of the runway bed, its drainage, and what kind of surface cover - turf, rocks, etc. - it has).

If you look at aerial photos of Edwards Air Force Base you'll notice part of the runway complex includes a dry lake bed:

Some pretty heavy stuff has taken off from and landed on those runways - space shuttles have landed there.
Similarly TACA Airlines Flight 110, a Boeing 737, landed on a levee at NASA's Michoud Assembly Facility (and was subsequently repaired and took off again, though they used an access road for the takeoff rather than the grass levee).

Conversely if you tried to land a Cub on a grass strip that's saturated with water after a rainy month there's a chance your tires might sink into the mud.

CBR Technologies has a nice presentation on unpaved runways, which includes some discussion of how their bearing strength (and thus maximum landing weights, tire pressure limits, etc.) can be calculated, but each unimproved runway will be different, and the values will likely change over time.


What is motive flow and why is it used in some aircraft hydraulic systems? What is motive flow and why is it used in some aircraft hydraulic systems? Why is motive flow used in favor of alternative means of causing fluid flow?

By hydraulic systems, I mean any system that moves a fluid whether it be fuel, oil or some other fluid.

Motive flow is required in design of ejector pumps in fuel system. Ejector pumps (also known as jet pumps or eductor pumps) have no moving parts and therefore represent a highly reliable, low cost means to move fuel in an aircraft. All jet pumps contain the physical characteristics illustrated in below figure.

In this pump, the motive flow comes from a high pressure source such as a primary boost pump, or the returned fuel from the engine. When the high pressure motive flow passes through the throat it creates a suction which sucks fuel from its neighborhood and the total fuel flow are sent to another location. The overall efficiency of an ejector pump is low when compared to electric motor powered pumps but they are frequently used due to their extremely long life, high reliability and low cost.

The ejector pumps generally are used for two functions:

Ejector pumps are often used to provide the primary fuel boost function in smaller transport aircraft applications such as business jets and regional aircraft.
In order to minimize the unusable fuel, scavenge pumps are often employed to suck up fuel from the remote corners of fuel tanks and to discharge this fuel at the inlet to the main feed pump(s). Ejector pumps are used for this purpose and the motive flow for these scavenge devices may be taken from the feed pump outlet if other motive sources are not available.

Note: Most of above material are from: Langton, R. (2009). Aircraft fuel systems. Chichester, U.K Wiley.


How useful are simulators in pilot training? For commercial pilots' training, airlines rely heavily on the training at computer simulators (fixed base simulators), where one can learn the cockpit controls and how to use the cockpit displays to control the airplane. But this does not give them the physical feel of controlling the airplane (which can be felt while using a real physical simulator (Hydraulic Simulators), as used by NASA in training their astronauts). Do airlines use such physical simulators to train pilots or are computer simulators the only ones used? Moreover, what is the usefulness of simple computer simulators as such then?

As a follow-up, do military pilots undergo training in physical real life simulators or they too are trained first in the computer simulators and then directly in the training planes?

Flight training devices (FTD) are the non-moving, no-visuals cockpit replicas you seem to be reference as "computer simulators".  These provide a real cockpit layout (real avionics, switches, buttons, etc) that are used as procedure trainers and to learn cockpit flows.  The main purpose of these is so you are aren't wasting time in the expensive full flight simulator and can get right to work once you get in there.

Once FTD training is finished, you move into the full flight simulator (FFS, typically a level-D sim per FAA certification) which are simulators with exterior visuals and a 6-axis hydraulic motion platform.  These are the devices used for the actual flight training in airline training.  You are expected in your first sim session to already know your way around the cockpit and not be too distracted by where all the buttons you need are so you can focus on the flight training.  Being at that comfort level for the first sim session is the purpose of spending time in the FTD to start.


Can I fly in the USA with an FAA ATP, if I don't have a commercial certificate? I am Taiwanese and I have an FAA ATPL, but I don't have an FAA CPL.

Can I fly commercial operations in the US?

Maybe. Your ATP certificate does give you commercial privileges, as per 14 CFR 61.167:

  (a) Privileges. (1) A person who holds an airline transport pilot
  certificate is entitled to the same privileges as a person who holds a
  commercial pilot certificate with an instrument rating.

But, that doesn't authorize you to work in the USA or even to enter the USA, which is an entirely different question about immigration law. As a foreigner, you'll need an appropriate visa or a green card in order to be legally employed and paid. You can ask more about that on another site.


What kinds of planes can tow banners? Planes can sometimes be seen towing banners around large cities or events.

What kind of plane is good for this type of flight? With banners they appear to be flying slower than usual. Is this just an illusion?

What are the practical or regulatory requirements that the plane must meet to be capable? I am tagging with FAA but information about other jurisdictions is welcome.

What kind of plane is good for this type of flight?

The cheap kind. Joking aside, the exact airplane to use depends on the size of the banners or billboards you're going to be towing, but generally speaking you want something reasonably rugged, with good slow flight characteristics, sufficient engine power to be able to overcome the added drag from the banner, and, hopefully, some decent fuel consumption.

I'm not entirely kidding about the cheap part though; banner towing is hard on the airplanes and carries a higher-than-average degree of risk, so you do want to stay away from airplanes that will be expensive to fix, if you can help it.

As for specific types, I've seen banners towed with everything from -152s to Grumman Agcats; a large banner tow operation apparently had a Super Cub with an O-540 bolted to it, so you'll see a ton of different things, but tailwheels seem to be the norm, at least in the US, and many outfits will make additional modifications such as removing engine cowlings to help with cooling.

  With banners they appear to be flying slower than usual. Is this just an illusion?

No, they are indeed flying slower. First of all, the added drag from the banner slows the airplane down all by itself. Secondly, you're being paid to advertise, so you want to make sure that people can read whatever sign you're towing. You're not flying the ragged edge of the stall (it's dangerous, uneconomical, and unnecessarily hard on the engine), but you're probably going to be flying on the backside of the power curve.

  What are the practical or regulatory requirements that the plane must meet to be capable?

In addition to the practical considerations we've talked about above, banner towing operations require a waiver from the FAA. The company has to apply for it and every pilot must be trained in banner tow operations, pass a review from an approved FAA inspector (who will be watching you perform picks and drops from the ground), and be added to the waiver. The aircraft and tow hitch mechanisms also have to be inspected and approved for banner tow operations, and added to the waiver.

Here is the current FAA Banner Tow Information handbook, covering equipment, operations, waiver information, and other assorted miscellaneous if you're curious to learn more.


What is the comparative roughness of fabric surfaces? "The Design of the Aeroplane" by Darrol Stinton (1983) lists comparative roughness of aircraft skin material (pg 154). Unfortunately it does not list dope & fabric.

What is the comparative roughness of dope and fabric surfaces?

It varies with the level of finishing effort, and depends mostly on how much the weave of the fabric was filled in.  If the weave shows in the fabric, you can expect the surface roughness to be similar to lacquered wood.  If there are enough dope coats to fully fill the fabric, you'd have something similar to commercial metal painted finish.  But if you really go to town you can get to the filled and polished glass surface finish.  You'll see some award winning homebuilts at Oshkosh with a fabric finish that looks like shiny mylar plastic, but is actually regular fabric and dope.   Many many coats, and many many hours of sanding and rubbing.

Some of the latest finishing systems can produce that finish without zillions of coats and hours of rubbing.

This is all notwithstanding all the surface irregularities you have with fabric due to the use of reinforcing tapes etc., which may make the whole issue of surface smoothness moot anyway.


Is it correct to say "the up-elevator position decreases the camber of the elevator"? Aft movement of the control column deflects
  the trailing edge of the elevator surface up. This is usually
  referred to as the up-elevator position.
  The up-elevator position decreases the camber of the elevator (my emphasis)
  and creates a downward aerodynamic force, which is greater
  than the normal tail-down force that exists in straight-andlevel
  flight. The overall effect causes the tail of the aircraft
  to move down and the nose to pitch up. PHAK 6-5

This is an excerpt from PHAK on the elevator. Do you agree with the statement "the up-elevator position decreases the camber of the elevator"? I've always thought the camber of control surfaces indicates their curvature, which is designed by aircraft designers or controlled by pilots using high-lift devices, and it is the angle of attack that that changes when pilots give control inputs. So isn't it more correct to say "The up-elevator position decreases the angle of attack of the elevator"?

You're absolutely correct, camber equals curvature. The camber of the horizontail tail is changed by deflecting the elevator, not the camber of the elevator itself. And yes the local angle of attack changes when the elevator deflects, resulting in an aerodynamic moment.


Why would FlightAware show this meandering flight path? Today's BA219 from LHR to DEN (2018-03-20) appeared to fly 5 zigzags just before and over the Hudson Bay in Canada at around 18:15z. Prior to that, its flight path matched its planned route very closely.

I know that congestion avoidance can sometimes lead to flights being asked to fly doglegs (as in Why did this plane fly in a zigzag pattern?), but this flight was still 3 hours from its destination and still exceeding planned speed when it appeared to fly zigzags. I don't see any weather issues on radar at the time.

After the zigzags seen in the FlightAware capture, the flight track straightened up and flew parallel to its planned route. This appears to be a track data problem, not the actual plane's path.

But on FR24 that path is not there:

I've seen 'Why do aircraft on Flight Radar 24 jump around randomly sometimes?', since they are different services, what could be the reasons for FlightAware to behave like this? Also note the linked FR24 post is a different more jagged path than the one here.

FlightAware tracks from other flights show similar zigzags in the Hudson Bay area. Presumably this is just noisy tracking data. Any idea why that happens in this particular location?

It looks as if the GPS location was only updated at intervals, and the software extrapolated the missing path information using bezier curves.


Would a canard work better for emergency pitch down? In order for a conventional horizontal tail plane to generate a nose pitch down effect in a stall condition, it must increase its angle of attack even more (by use of elevator or pitching up more).  A front mounted canard would pitch DOWN into the airstream, increasing chances of remaining unstalled. A canard would also be in relatively undisturbed airflow.

The old fashioned solution is to mount a lower aspect, wedge , or delta shaped horizontal horizontal tail that clearly allows the main wing to stall first.  Newer designs, especially in airliners, feature reduced size Hstabs of similar planform to the wings, relying on computers to maintain pitch control.

Would a downward pitching canard be a better solution to avoid deep stall or stalls in general?

A couple of confusion here. For starters, unless we are dealing with T-Tails, there is no deep stall for a tailed airplane. On the other hand, a canard airplane has the potential to enter deep stall.

Next, for a transport category aircraft, tail stalling in the positive incidence is generally not a concern. Tail stalling in the down load, however, is; for example, zero-G pushover in icing condition is a real issue to consider during design. In a well designed aircraft, the tail would have a higher sweep and lower aspect ratio than the wing so that the wing stalls earlier than the tail.

(Zero-G pushover may stall the tail because of the pitch rate build-up. Nose-down pitch rate equals negative flow incidence on the tail. Once the tail stalls, you lose the ability to pull.)

It could be an issue, however, if you have a T-Tail. For a swept wing in cruise configuration, the pitching moment would likely reverse near stall. At that point, the elevator would still have good effectiveness, but depending on how aggressive the pitch up is, there may not be enough time to lower the AOA before the aircraft enters deep stall.

Once in the deep stall, the authority on the tail as a whole decreases dramatically. Even with full nose-down stabilizer and nose-down elevator, you may not have enough authority to break the stall. In that situation, without an anti-stall chute, it's game over. The point is, you don't want to ever get there. Preventative measures include stick pusher and envelope protection functions.

Would installing a canard help? Sure, more authority always helps. But does it warrant the complexity and weight increase when other solutions exist? Probably not.


Spanwise flow and wingtip vortices in forward-swept wings One of the benefits of forward-swept wing is that spanwise flow direction is from tip to root. However, in this answer an X-29 has quite significant sweep angle, at which drawbacks such as aeroelasticity effects dramatically increase.

What happens to the wingtip wake on a forward-swept wing?

Here are some points I'd like to touch:

Does sweeping wings forwarg completely eliminate it, and if it
depends on sweep angle?
How to estimate sweep angle, at which the
spanwise flow starts to flow to the root?
Considering a tampered wing with only trailing edges swept forward, is it enough to get wing characteristics (primarily spanwise flow) like (or close to) forward-swept one?

There is no magical switch that gets flipped once you cross a specific sweep angle. Things are gradually changing with the cosine of the local sweep angle. Now let's look at your questions one by one:

  What happens to the wingtip wake on a forward-swept wing?

Nothing special. It is part of the full wake system and behaves like in any other aircraft of comparable aspect ratio and lift distribution over span. For high sweep angles and high lift, the wing torsion will increase local loading towards the tips, making the tip vortex relatively stronger.

  Does sweeping wings forward completely eliminate it, and if it depends on sweep angle?

Why should it? No, the tip wake of the wing with more forward sweep will be stronger for comparable wings with different sweep, for aeroelastic reasons as explained above.

  How to estimate sweep angle, at which the spanwise flow starts to flow to the root?

See here for the reasons for spanwise flow. As soon as forward sweep results in a lateral local speed component, spanwise flow will happen.

  Considering a tapered wing with only trailing edges swept forward, is it enough to get wing characteristics (primarily spanwise flow) like (or close to) forward-swept one?

With a straight leading edge, flow will not exhibit any sweep effect. With a large taper ratio, forward sweep can still become significant towards the trailing edge, but most flow acceleration happens around the nose, so overall sweep effects will still be small.


How to control yaw in stick aircraft? How do you control yaw in aircraft with a stick control?

It seems like forward and back are used for pitch, and left/right are for roll, so where is the yaw?

There are a few ways that yaw control is achieved.

The easiest and the first to be successfully used would be to have cables from rudder pedals on each side of the rudder to deflect the control surface and control yaw.

Airplane Flying Handbook pg 2-1

The first application to control yaw unsuccessfully was the development of gears and linkages that will control both roll and yaw with the pilot stick.  It was quickly discovered that there are cases where you may need to use the controls separately.
Yaw damper is an electronic device that sense yaw rate and will provide inputs to the rudder to keep the airplane coordinated.

https://www.gulfcoastavionics.com/images/products/3552/detail/detail.jpg


What are the factors that determine the speed flutter appears on wings? I'm doing a study on the flutter phenomenon of airplane wings. I'm interested in factors that influence wing stability, and how one can increase the speed where flutter starts.

What are the parameters that determine this speed?

It's an aeroelastical phenomenon driven by pitching moment, torsion stiffness, and inertia of the wing. It's a classical spring-mass-damper problem: increased aerodynamic pitching moment causes torsion on the wing structure, which causes more pitching moment until the local airfoil AoA is such that the air flow separates. Torsion is counteracted by the wing torsional stiffness, thin wings have less torsional stiffness.

Parameters that determine flutter speed:

Wing profile: aerodynamic $C_m$ and wing thickness
Wing aspect ratio
Number of jet engines mounted forward of the aeroelastic axis
Wing sweep


I would like to insure my foreign registered aircraft in the USA I am looking to fly my UK registered aircraft in the USA this summer.  At present UK insurance companies will not insure aircraft flown in the USA and US insurance companies will not insure foreign aircraft.  I see 2 possible solutions:

Find a US company that will insure a foreign plane
Temporarily register my plane in the USA.

Does anybody know if either of these is possible?

Thanks

You could do either or neither.

I assume you are flying a small, piston-engined aircraft. Any insurance company that insures turbine powered aircraft would have provisions for this. I don’t specialize in aircraft insurance. Having been in insurance for 20 years, I know that anything and everything can be insured for the right price. If your insurance company won’t do it, there are others that will. You just might not be willing to pay the price the insurance company requires.

The question you might ask yourself is how do people who ferry aircraft across the pond insure the aircraft? If you are planning a cross-Atlantic flight, you might want to contact an experienced, professional company to help you plan it. You have to think of more than insurance. Communication, water safety/survival and egress, auxiliary fuel planning, weather, sovereign airspace entry, enroute landing airports, customs, and other items need to be considered.

An alternative is to rent an airplane or airplanes here in the US. I have run into a few of Brits, Germans, Aussies and New Zealanders who have done that. They made it seem surprisingly easy and something they did on a regular basis. Some annually. I have even checked into doing it in South Africa. It would have been simple with the renting organization even arranging for validation of my US pilot certificate for flight use in South Africa.


Why does the MiG-15 have a cruciform tail? The horizontal stabilizers of the MiG-15 are halfway up the tail. This is called a cruciform tail.

Why did they put them there? Why not a T-tail or the normal tail where the stabilizers are on the fuselage?

The tail position is a compromise which is influenced by these factors:

Interference with the wing's wake: The tail should be outside of the boundary layer flow coming off the wing root at all normal angles of attack. A high tail can cause a deep stall, something that was not known in 1947. Today all similar designs try to get the tail surface below the wing's wake.
Lever arm: By mounting the tail higher up on the swept vertical tail, its lever arm (in horizontal direction) can be increased without lengthening the tailpipe. A longer tailpipe means more mass and more internal drag and is best avoided.
Flutter: The higher the horizontal tail is along the vertical, the higher the danger of flutter. Here the longer lever arm (now in vertical direction) will lower the eigenfrequency of the bending mode of the vertical and the torsion mode of the rear fuselage. I suspect that for that reason the full T-tail was not chosen.

Mounting the horizontal smack in the middle of the tailpipe would require a heavy spar there to carry the bending moment across the fuselage. Mounting it on the vertical tail allows for a much lighter construction.


Why do turbofan engines not have a contrarotating second fan? Why aren't planetary gears used to have a fan in front of the fan in a turbo fan engine. Would this not lower energy consumption etc?

The power to drive a fan through a gearbox is not beyond current technical capability. Pratt & Whitney have developed the geared turbofan, the PW1000G that first ran in 2008 and entered commercial use in 2016, flying on 4 different aircraft, including the Airbus A320neo. This enables the fan to spin at a slower speed than the low pressure turbine, such that the turbine spins at a higher speed that is more efficient for power extraction from the relatively high velocity mass flow, while the fan spins at a lower speed that avoids the blade tips lossing efficiency or creating noise at take-off, due to near sonic flow.

The engine design involves trade-offs. The complexity and weight of a counter rotating two stage fan needs to be balanced against the achievable pressure ratio with a single stage fan, and the added benefit a two stage fan would provide. Some design work has considered counter rotating compressors for example, this thesis. or even earlier work in  1992. The gas turbine industry can be quite conservative - it costs huge amounts of money, and time, to develop a new engine design, and radical steps are much less common than evolution. Pratt took a gamble, that appears to have paid off, though they do still have some issues. However, it took them decades, and around $10 billion. Rolls Royce are considering a similar design (the UltraFan), but GE are more wary. They have chosen the use of new materials as they felt the addition of the extra component would add reliability, weight, and durability challenges that they could avoid by using material improvements instead. For example, the use of ceramic matrix composites in the hot section of the CFM56 LEAP. They claim this gives 15% fuel savings, but has taken 25 years to develop. In comparison, P&W claim the GTF is 16% more fuel efficent than current generation engines. So each manufacturer choses the route they are comfortable in, and may get to the same end point.

In general, the desire has been to increase by-pass ratio, as this lowers specific fuel consumption. However, increasing BPR results in larger diameter fans, and higher tip speeds, approaching sonic velocity if the turbine is run at the optimum rpm. Hence, the geared fan approach. Clearly, Pratt & Whitney did the trade-off analysis, and decided this was the best approach for a geared fan, balancing techncial risk, reliability, performance, in-service cost, and weight.


Do all B777 family aircraft have the same engines? I think there are multiple variants of engines for the different variants in B777 family but I am not sure but is it GE90-115B in one while Trent and PW in others?

TL;DR: No.

For the true answer refer to the FAA Type Certificate Data Sheet

777-200

2 Pratt and Whitney Turbofan Model:
PW4074, PW4074D, PW4077, PW4077D,PW4090,PW4084D, and PW4090-3
(Engine Type Certificate No. E46NE)

2 General Electric Turbofan Model: GE90-76B, GE90-85B, GE90-90B, GE90-94B
(Engine Type Certificate No. E00049EN)

2 Rolls-Royce Turbofan Model: RB211-Trent 875-17, RB211-Trent, 877-17,
RB211-Trent 884-17, RB211-Trent 892-17, or RB211 Trent 892B-17, RB211 Trent 895-17
(Engine Type Certificate E00050EN)

777-300

2 Rolls-Royce Turbofan Model: RB211-Trent 884-17, RB211-Trent 884B-17,
or RB211-Trent 892-17
(Engine Type Certificate E00050EN)

2 Pratt & Whitney Turbofan Model: PW4090, PW4098
(Engine Type Certificate E46N)

777-300ER

2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)

777-200LR

2 General Electric Turbofan Model: GE90-110B1

2 General Electric Turbofan Model: GE90-115B

(Engine Type Certificate No. E00049EN)

777F

2 General Electric Turbofan Model: GE90-110B1

2 General Electric Turbofan Model: GE90-115B
(Engine Type Certificate No. E00049EN)


Selecting radials for radial interception I’m a beginner in intrument flying so sorry for my ignorance. Few months ago I was doing BIR training and with my CFI we were intercepting radials, we were doing that on HSI (Garmin G500).
He told me that If I want to intercept radial inbound I should set the tail of the needle to the desired inbound radial and if I want to intercept radial outbound I should set the head of the needle to the the desired outbound radial.
My question - is this way of setting desired radials correct? Because I read somewhere that we always should set the head of the needle to the course that we mean to fly, no matter if it is outbound or inbound - to avoid reverse sensing.
So if I want to intercept and fly radial outbound 180 I should set head of the needle of my HSI on 180 and if I want to intercept and fly radial 240 inbound I should set the head of the needle of my HSI on 240. Now I'm confused, please help me out.

This is a terminology problem - and I think you're actually both right but you need to learn the terminology. Radials always radiate out from the VOR.
You should absolutely always set the needle to the heading you intend to fly in order to avoid reverse sensing.
However, if you're intending to "fly inbound on the 240 radial" then you're actually on the reciprocal heading of 060 - which is what should be set on your OBS. The flag should indicate "TO".
If you're flying outbound on 180 then you should set 180 on the OBS and the flag should indicate "FROM".


What is the maximum altitude ATC would deal with? What's the maximum altitude ATC would deal with?

Any example of jets service ceiling certified for above 45.000? Commercial & Business jets?

There are many examples of aircraft with high service ceilings. While most commercial aircraft have service ceilings of FL410 and rarely fly even that high, many business jets have a service ceiling of FL510, such as the Dassault Falcon 7X, Gulfstream G650, and Bombardier Global Express.

That being said, military jets can fly higher and will be talking to ATC while in their airspace. Fighter jets are capable of high altitude flight, and planes like the U-2 and Global Hawk still fly up there as well.

In the US, all aircraft from FL180 to FL600 must be in contact with ATC, but above this they are not required to. ATC will still provide services when able though.


ADS-B receiver, dongle vs SBS-3 or Garmin Is there any differences between FlightAware Pro Stick USB and Kinetic SBS-3 or Garmin for aircraft tracking purpose?
Something like: SBS-3 can receive twice more data.. is there any benefits?
Assume, that I have power PC.

Most ADS-B receivers in the market (including USB sticks such FlightAware Pro Stick USB) are receiving the same data using the standardized GDL-90 protocol. There is not a lot of data available on their amplifier characteristics.

Both in the air and on the ground, the main differenciator will be the quality of the antenna and its positioning. In that respect, avoid some USB sticks with a tiny embedded antenna without an antenna connector, as they will lead to lower performances than those of devices that are connected to much more efficient antenna.

Furthermore, with any device with an antenna connector, you will be able if needed to install the antenna at a more convenient location, as it is important the antenna has unobstructed view of the sky and when installed the ground be located as high as possible. According Garmin GLD-5x installation manual, ADS-B receiver antenna should be a standard 50 Ω vertically polarized antenna with a VSWR < 1.7:1 at 978 MHz and < 1.5:1 at 1090 MHz.

On that respect, the FlightAware Pro Stick USB, Kinetic SBS-3 and Garmin GDL-5x are all supplied with an external antenna connector, so I guess the cheapest one will do the job.


How does the weather radar work? I was reading this question about turbulence and how pilots foresee it, and in one of the answers, @Rhino Driver states:

  Weather Radar Just like using your eyes, except the radar can see further through haze and other clouds. Typically this is useful for finding embedded thunderstorms, but it can also be useful to find areas of potential turbulence

And that make me wonder...

How does the airliner's on-board weather radar work?
Is it connected to ground base stations to synchronize the weather info?
What kind of "weather types" can it detect? (i.e. thunderstorms, light rain, clear weather)

It works the same way any radar works.  An antenna broadcasts a pulse of energy and listens for its reflection and interprets the power of the reflection to calculate the reflectivity factor Z, expressed in decibels dBZ (helpful because return power varies across many orders of magnitude).  Weather radar can see water, hail, bugs, birds, bats and bigger things, and the ground but not clouds.  Higher return power can mean more objects or bigger objects.  A basic single-polarization radar cannot tell the difference between a tons of small rain drops and a fewer big raindrops.

The particulars of the radar on any aircraft are generally going to be determined by the antenna size, as the optimal frequency of emission varies for a given antenna.  The large S-band* dishes that make up the ground based WSR-88D network are quite big (around 10m).  S-band is a good choice because they have low attenuation and can penetrate through storms and see things behind them.  Airplanes can't use S-band though, because the antennas are too big to put on the airplane and require too much power to fully utilize them.

Airplane radars are generally in the X-band*.  X-band can see smaller particles than S-band but attenuates easier.  This means if a big storm is in front of you, the energy doesn't penetrate far into the storm and may not show you the whole storm.  These are great short range radars and work well in airborne applications as long as you are aware of the issue with attenuation.

Airplane radar displays typically only display information from the on-board radar and do not downlink any additional information from ground stations.  Onboard radar are usually adjustable in the vertical by the pilots, so they can point the radar up or down as needed.

Here is an example of a radar antenna and associated electronics on a Cessna Citation 501:

Image by Dtom, Wikimedia Commons.   https://en.wikipedia.org/wiki/File:Cessna501_radar.JPG

*S-band is 8-15 cm / 2-4 GHz
 X-band is 2.5-4 cm / 8-12 GHz


How does this vortex form inside a jet engine? I have seen this in a picture once online and thought it was very strange and wondered about the formation of this vortex and how could it happen.

Source

Not everyday do you see this happening. So how does this happen, and why does it not happen very often?

Source

Another picture of this strange occurrence. This generally happens with thrust reversal.

Whenever there is under-pressure in air, the air flows along the gradient of this under-pressure, i.e. into the direction of the source of the under-pressure. (In first order.)

If there's a wall (or the ground) in the way, air will flow along the surface of this obstacle and will change its direction at some point, as shown in this sketch:

Now, imagine the whole air around the engine is rotating slowly for some reason around the symmetry axis in the sketch.

From physics, you may know the conservation of momentum, or in this case, of angular momentum. It says that the product of distance to rotation axis times RPM of this rotation is constant for a certain particle. If a particle reduces its distance to the axis, its RPM increases.
This is like the spinning ice skater, who rotates faster when he pulls his arms to his body (to the axis).

This law also applies to packets of air, which are sucked to the axis, though internal friction brakes the air to some extent.

The fast rotating air near the axis encounters a centrifugal force, i.e. the air is pulled away from the axis, forming a region of strong under-pressure. In this region, the air expands and so, cools down. This can cause condensation of water, if the humidity of the air is high enough. And that's what you can see on your pictures.

There's still one question unanswered: I wrote, this happens if the surrounding air is rotating slowly. This usually happens, because the wind is deflected from the aircraft itself, which means it changes its direction, which is some kind of rotation.

And of course, a strong, non-uniform air flow from the rear, as generated by the reverse thrusters, can cause more / stronger rotating air, which results into stronger vertices. So, it's no wonder when they appear (or better: become more visible) in this case.

Here is a nice picture of a simulation, which also shows that the fuselage can be the wall where the vortex is created.

Source: http://gallery.ensight.com

It seems the wind comes from the camera position, or a bit more right of it. It gets deflected by the fuselage and flows along it, which explains the straight lines coming from the rear and the curved ones coming from the front left of the aircraft. If you imagine the air flow along / under the fuselage, you may also understand why the two vertices are rotating in different orientations.

As the strong under-pressure inside the vortex, it may also suck in dust or even larger objects, which can damage the engine. For aircraft landing on unpaved airfields, unpaved strip kits may be available, which can contain a vortex dissipator:

Source: http://www.b737.org.uk

This is a nozzle blowing bleed air into the region where vortices could apper, reducing the under-pressure and so the risk of ingesting objects from the ground.

Notes:

You can see something similar in the sink of your bathtub. As soon as there is a little rotation, it is amplified above the sink, the centrifugal force pushes the water aside, and forms this fast rotating vortex.

A tornado is a giant version of an inlet vortex. The air is moving along ground over far distances, until it moves upwards at some point, and a slight rotation leads to this fast rotating hose sticking to the ground and sucking in everything due to the strong under-pressure inside.

And a bit off-topic:

Sometimes, you can see vortices at the wing tips or at the edges of engaged flaps of aircraft flying through humid air. The reason is similar: The air is put into rotation due to the higher pressure below and the lower above the wings. (But that's not exactly the same as inlet vortices.) And sometimes, you also see fog forming above the wing just because of the under-pressure.
On this picture, you see both, and may imagine the rotation for the second vortex from the right.

Source: http://avioners.net


Are military aircraft required to have black boxes? In the United States, the FAA requires certain types of aircraft to have a Flight Data Recorder (FDR) and/or Cockpit Voice Recorder (CVR).  I understand these regulations to apply mostly to civil aviation.  Does it apply to military aircraft as well?

Another question researched whether fighters have them or not; my question is whether they are required to, and for military aircraft in general, not just fighters.

Military aircraft need not adhere to FAA regulations. Modern military aircraft with digital controls have black boxes, older ones may not.


What will be the immediate action by the pilot and ATC if any plane blocks the runway while landing? Suppose, if the Airport has only one runway and one plane is trying to take off at the end of the runway and other plane trying to land from the other end of the runway.

Due to any reason if the pilot decides to abort the take off.

In that situation that plane is blocking the runway and it is in the way where the other plane is trying to land at the same time.

What action will be taken by the pilot who is landing the plane and what action will be taken by ATC?

Is there any chance of collision?

ATC will tell the landing plane to go-around. The pilot will then execute the go-around by flying the missed approach procedure as published in the airport charts.
Go-arounds happen daily and are pretty much a routine action.

In any case, if the landing pilot feels that the runway is not safe for landing, he can decide to go-around by himself without ATC order (if the controller missed the issue or if the airport is uncontrolled for example).

For reference, the missed approach procedure can be as simple as:

Stopping descent
Climbing back to a published altitude
Flying a published pattern
Trying a second approach


Can lift be created without air downwash? I was looking at the great site code7700.com, but I don't really understand their explanation of lift and drag:

     The so-called infinite wing
  displaces air for a while, but the air particles return to their
  original positions along the relative wind. The relative wind
  experience up wash as it approaches the airfoil and down wash as it
  leaves the airfoil. But the up wash and down wash are equal so the net
  effect is cancelled. Because of this, the aerodynamic force occurs
  perpendicular to the relative wind and their is no induced drag. In
  other words, all of the aerodynamic force is producing lift, none of
  it is pulling the wing backwards.

  Source

It seems this is a big confusion. Could someone just confirm that lift is created solely by the downwash and Newton's 3rd law of motion?

(Why air is deflected is not part of the question.)

If no downward acceleration occurs, there can be no lift.

However, a wing of infinite span will use an infinite amount of air for lift production, so an infinitely small amount of downward acceleration already suffices to produce lift.

Clearly, this is a thought experiment that cannot realistically be reproduced in reality.


I don't quite understand the bypass air I am writing an essay on how a jet engine works, so I need specific answers on this.

What percent of the air that is sucked into the engine is bypass air? I have heard that bypass air provides about 60% of the thrust.  How does cold air provide thrust?  I was under the impression that hot air makes much more thrust than cold.

Cold air accelerated backwards provides forward thrust by reaction. What really matters is the amount of mass accelerated backwards. Its temperature is not important...

Concerning the 'percent of air sucked', it depends on the design. A pure turbojet doesn't produce 'bypass air', and a turbofan may reach a ratio of 10:1, that ratio being the relation between the mass of 'cold' air pushed backwards by the fan in a given time and the mass of air that passes through the combustion chamber...


Do pilots really wear "pilot's watches"? High-end mechanical wristwatches from Swiss and German manufacturers (e.g. Breitling, Rolex etc.) are often marketed as "pilot's watches", and have chronometer features to supposedly help pilots perform flight-related computations.

While I know next to nothing about aviation, I find it difficult to imagine that such a mechanical watch (which, for example, simply stops working if it is not moved for a few days) would be preferable to a modern quartz and/or digital watch, which keeps running for years on a single battery, self-adjusts with millisecond precision based on radio signals and even without that signal maintains time an order of magnitude more accurately than any mechanical watch.

To summarize: What kind of watches do modern commercial pilots actually wear? Or do they nowadays just use their cellphones for timekeeping tasks like everyone else?

High end mechanical pilots watches are very little use in the cockpit whether you are in a commercial airplane or a cessna 152. I love watches and I've searched for years for a watch that was a) very cool looking and b) actually useful when flying and I haven't found one yet. They usually fail on b) because their analogue chronometer and/or timer functions are very hard to interpret, and cannot be read at night. The digital readouts on the analogue/digital types are often small and rarely have lights for night flying. Some have lights that will only work when in calendar mode, which is about as useful as a chocolate teapot. Often the functions are hard to use and require a fair amount of fiddling to get them to work, which is not good when you have a high workload.

Commercial airplanes are usually so full of technology that a watch is completely redundant anyway, so for commercial pilots it's a matter of style. I'm not a commercial pilot, I fly older light aircraft with less technology, so having a watch with timer and chrono functions is handy. For this I wear a Casio G-Shock as it has a large and easily readable face, a light activated by its own button, simple to use, and very rugged. Digital kitchen timers also work very well.


Can fighter jets be reloaded with ammunition mid-flight? I know that we can re-fuel fighter jets mid-flight, but do we have also a mechanism to reload them with ammunition during mid-flight so they don't need to go back to base to reload?

Aerial rearming has been a daydream of military tacticians for decades. However, the task is much more technically demanding than aerial refueling, which is in itself a fairly demanding task for pilot and boom operator just to get a 50-foot fuel hose hooked up.

The technical issues include:

Physically capturing and docking a smaller aircraft with a larger one. The USAF tried it back in the 50s with a B-36 carrying a specially-modified RF-84F. In those days there were no computerized flight controls, and docking the fighter back up with the mothership after it had done its thing proved to be more dangerous to the bomber than just running the mission without a fighter escort at all. It would not be inconceivable for modern avionics to improve on that, however the relative size of fighters compared to larger support craft has increased considerably. Here is a KC-135, built on the "Dash-80" airframe that also sired the 707, refueling an F-15C that's not really that much smaller when you look at it:

Placement of most ordinance under the aircraft. Most fighters and ground attack aircraft carry weapons on external hardpoints under the wings (the RAF-variant SEPECAT Jaguar being a notable example of both overwing pylons and the insanity of British aircraft designers):

When in refueling position under a tanker, the aircraft's most deadly weapons are on the wrong side of the jet to try to get to them to replace or reload them. You'd have to either bring the aircraft inside the mothership, or have it land on top (and provide a sheltered work area for a crew to get to the ordinance pylons).
Relative fuselage width of mothership versus wingspan of fighter. The largest fixed-wing craft in the U.S. arsenal is the C-5 Galaxy. It's a big plane (though not the biggest, that distinction belongs to the Ukrainians with the An-225). Here it is with a couple F-15s and a C-130 parked nearby for scale:

Even with a plane this size, able to transport other planes, you can see from the below photo that neither the F-14 being loaded nor the Galaxy is in a flight-ready condition at the time:

I'd be interested to see whether one of the light weight fighters (F-16, F/A-18) might fit with less disassembly (the F-18 might if you folded the wings), but it remains that you're just not going to be able to fly a modern fighter into another aircraft in midair, at least not with any currently-flying airframe combination.
Getting crew access to the plane. If you've ever seen a ground crew loading a fighter for a mission, you know it's a fairly complex process of not only putting the ordinance (which can weigh up to 2500 pounds for large laser-guided bombs) onto the hardpoints, but wiring up the arming and firing circuits for each one, and programming the plane's FCC with the loadout. This requires getting a lot of people underneath the aircraft (it can take two or three people to load one hardpoint especially if the ordinance has to be muscled into place; here's two people loading probably the lightest piece of external ordinance the USAF has onto an F-15C, and it's only two people because they're using a hydraulic loader):

As the aircraft can't be taken inside the mothership (see previous point), the only other possible place for this to be done is on top of the mothership. That means you'd have a crew out on top of the aircraft muscling ordinance into place. That first requires a sheltered area on top of the mothership that the fighter can be docked and tethered to, as there's no way a crew is getting anything done in a 150-knot headwind. The fuselage vs wingspan difference is important here too; you're not going to be loading AIM-9s onto the wingtips of an F-16 or F-18 when the F-16's wingspan is 33 feet while the cargo deck width (a stand-in for the amount of usable fuselage width) is only 19 feet.

In summary, loading/reloading an aircraft is a serious operation requiring a dozen people to do quickly and efficiently. Putting those dozen people, and their equipment, and all the ordinance into a mothership, getting the fighter to safely approach that mothership and touch down on top of it, then getting the crew into position to remove expended hardpoint pylons and reload new ones, then getting the plane to safely get back off the mothership... It's just not gonna happen with any current technology.

There was a proposed design for a twin-fuselage C-5 as a shuttle transport aircraft; the idea was that this aircraft could be used instead of the risky crane system to pick the shuttle off the ground at an alternate shuttle landing site like Edwards AFB:

An aircraft of this scale might, might, be able to support an aircraft coming in between the tails, docking, then using remote arms to load ordinance. This aircraft would, by wingspan, be the largest plane ever to fly, and would compete closely with the An-225 on every other size stat except total length.


Control free and fixed static margin For many aircraft root and tip chord is given but the exact location of aerodynamic center and center of gravity is not given. Considering this information;
How can I calculate control free and fixed static margin?

The stick-free static margin depends a lot on the elevator hinge moment coefficients of the aircraft. Depending on the sign of the hinge moment derivative with angle of attack ($c_{r_\alpha}$), the stick-free stability can be greater than the stick-fixed stability (greater for positive $c_{r_\alpha}$ because then the control surface will deflect into the changed airstream). By how much depends also on the derivative over deflection angle $c_{r_\eta}$, but this one should definitely stay in the negative range, or the stick will be hard to center.

The stick-fixed margin is much easier to calculate. If aspect ratio is reasonably high, the surface-weighted mean of the quarter points of all horizontal surfaces will give a good approximation of the neutral point $x_N$, and the center of gravity depends on the loading of the aircraft. Normally, you would calculate the margin for both the most forward and the most backward cg location $x_S$ for which the aircraft is certified.

With the given planform, the mean aerodynamic chord $l_\mu$ is also easy to calculate, and the stick-fixed margin is (x backwards positive): $$\frac{c_{M\alpha}}{c_{L\alpha}} = \frac{(x_S - x_N)}{l_\mu}$$


How are IAS to CAS conversions calculated? How do I calculate calibrated airspeed (CAS) from indicated airspeed (IAS)? For example, how is 600 knots indicated calculated to 595 knots calibrated?

There are instrument errors in the airspeed indicator system, which consists, but is not limited to the air speed indicator, a pitot port (which senses ram air pressure) and a static port (provides ambient air pressure for airspeed and for altimeter and VSI instruments).

Because there are errors in the indicated airspeed, each manufacturer is required to measure airspeed and then provides data of indicated vs calibrated airspeed data.  This used to be a much bigger deal when flights were conducted by dead reckoning, and reliable airspeeds were more directly beneficial to the flight planning and enroute process.

Today, other instruments provide more accurate effective ground speed data.  For example, a low cost GPS can have greater accuracy in measuring effective ground speed than radar, and rivals lidar systems costing much more money.

Nevertheless, manufacturers still provide CAS / IAS tables.

Oh the instruments used to determining CAS?  Often supplemental pitot probes, which are on extended booms in front of the aircraft.

To a lesser degree GPS/IMU, radar and lidar may play a role in some testing, but these instruments do not actually measure airspeed, where as an extended boom pitot will measure ram air pressure, ideally outside any airframe and power plant perturbations.

Finally, a pilot may translate, using manufacturer provided CAS / IAS conversion tables, IAS to CAS when CAS figures are needed.  But CAS figures are not calculated, they are empirical.


Why the inward engines-placement on the An-124? I am curious as to why the engines (specifically engines 1 & 4) are positioned far closer to the fuselage on the An-124 compared to a typical 4-engined jet.

I am curious as to whether this design was totally due to wing aerodynamics, or if it may also be related to the shoulder wing design & wing droop when not flying.

Overlay that matches the wingspan of a 747-100 (top) to an An-124, showing the more inward placement of the jet engines.

In general, designers would like the engines to be closer to the fuselage, more like the An-124. With the low wing on the 747, the engines have to be placed far enough out that they can still have enough clearance from the ground. This trades with the height of the landing gear and dihedral of the wing at the fuselage. With a high wing on the An-124 they have plenty of room and can place them closer.

Although the engines do provide some relief to the regular bending loads from lift on the wings, there are lots of reasons to keep the engines closer to the fuselage. Better handling with engines failed is one. But it also affects the amount of wiring and tubing you need to get to the engines, and there are other loads like hard landings that are easier to design for with the engines closer to the fuselage.