Is an ekranoplan less efficient than a regular plane? This are my conclusions upon researching the matter:

Ground effect is the cushion of air that can not escape down because of the ground. The hovercraft creates that cushion using a skirt which prevents as mush as possible a lateral escape of the air from underneath the ship. In effect a pressure differential between underneath and over the ship is created, enough to counter the ship weight. The value of this overpressure is within millibars.

Now returning to ekranoplan. While in the water, the ship has to fight the water drag. If the wings provided just enough lift to defeat the water drag, and the air cushion former under is rendered slow enough at escaping, then the ship becomes airborne. Will ride on a dynamically forming air cushion, better known as ground effect. However, the air drag becomes now the main opposing force. Increasing the angle of attack will increase the air drag and thicken the air cushion to the point where the ground effect is lost and the aircraft will loose altitude down to ground effect. Since the power output is limited, the machine is restrained at flying in ground effect only.

Now in terms of efficiency, the answer is no. The ekranoplan uses more thrust to fly in ground effect because it has to overcome air drag which is significantly stronger due to high angle of attack required to maintain the ground effect. The main reason the airplanes are efficient is due to low drag cruising. The wing develops enough lift to maintain the aircraft in the air while the angle of attack makes the wing a blade cutting through the air. Since the ekranoplan rides at way lower speeds than a regular airliner, it is required a high angle of attack which in turn determines a high drag.

There are a couple of inaccurate trains of thought in the question.

  The ekranoplan uses more thrust to fly in ground effect because it has to overcome air drag which is significantly stronger due to high angle of attack required to maintain the ground effect.

All aeroplanes need to overcome air drag, which grows quadratically with airspeed. Ground effect creates more lift at a given speed, therefore requires less thrust at that speed. Or: ground effect creates more lift at a given wing area, requiring less thrust than the larger wing required for the conventional aeroplane.

  ...thicken the air cushion to the point where the ground effect is lost and the aircraft will loose altitude down to ground effect.

Thickening an air cushion cannot result in losing ground effect. The ekranoplane cannot fly out of ground effect - increasing Angle of Attack does not make much sense and altitude will auto stabilise to within ground effect, indeed at higher thrust level at higher AoA for a given speed. I believe this is what OP means, but it does not equate to ekranoplanes being less efficient than conventional aeroplanes. Ground effect creates extra lift without a drag penalty.

Whether ekranoplanes are more or less fuel efficient than standard aeroplanes is not due to the ground effect, but to the interference with the water surface, sea state, having to lift the engine out of the spray etc. Depending on the speed, they may be less efficient than ships, but the payload arrives a lot quicker.


What's the meaning of "the break is a level" in a recovery operation of an aircraft carrier? The break is a level, 180° turn made at 800 feet (240 m), descending to 600 feet (180 m) when established downwind. Landing gear/flaps are lowered, and landing checks are completed.

(source: wikimili.com)

"Level" isn't a noun here--it's not that the break is a level.  Level is an adjective describing the kind of a turn.  A break is a turn; that turn is a level turn (neither descending nor ascending), and a 180 degree turn, and it is made at 800 feet.


What is the theoretical maximum speed of a rocket-powered aircraft in the atmosphere? I read that the scram jet has a theoretical maximum speed of Mach 24. What is the theoretical maximum speed of a rocket-powered aircraft designed to operate in the atmosphere?

The upper limit is given by the L/D of the rocket-powered vehicle and its thrust.
In order to follow the curvature of the Earth so it will stay within the atmosphere, the vehicle must create enough downforce by flying inverted so it can bend its flight path enough to not escape to outer space.
A practical altitude for flying this way would be dozens of kilometers up in the higher atmosphere, so drag and heat loads stay manageable. For the L/D I use the approximation given by Dietrich Küchemann (replace with a more recent one if handy):
$$\left(\frac{L}{D}\right)_{max}=\frac{4\cdot(Ma+3)}{Ma}$$
which would result in an L/D of 4 for Mach growing very large. Add to this the effect of gravity and your maximum downforce is the weight of the vehicle plus four times its rocket thrust. This force now has to balance the centrifugal force which results from flying at constant altitude:
$$m\cdot g + 4\cdot T = \frac{m\cdot v^2}{R_{Earth}+h}$$
where $R_{Earth}$ is the earth radius of approximately 6367 km and $h$ the flight altitude above ground. Now solve for flight speed v:
$$v=\sqrt{\left(g + 4\cdot\frac{T}{m}\right)\cdot\left(R_{Earth}+h\right)}$$
By flying in dense air close to the ground the lift force will be immense, but so will be the required thrust. Since flight altitude is small compared to the radius of the Earth, the equation can be simplified with a small loss in accuracy:
$$v=\sqrt{\left(g + 4\cdot\frac{T}{m}\right)\cdot6.400.000 m}$$
A more precise result would now add the effect of Earth's rotation, so flying eastwards along the equator will reduce flight speed by 464 m/s and add the same amount to the speed calculated above when flying westwards.


Have there been any incidents caused by an inability to shut down an engine? I read many items on the avherald.com site where it says "engine shut down during flight" or words to similar effect.

I'm wondering if there are any instances where an engine could not be shut down or the pilots had trouble shutting down the engine when commanded. What sort of backup systems are in place to avoid this kind of scenario?

Qantas Flight 32
Enroute to Sydney, Engine Number 2 of the A380 disintegrated explosively. The shrapnel of the explosion destroyed many systems, including a hydraulic system, the anti-lock braking system, flaps and electronic controls.
After emergency landing, the pilots were unable to stop engine number 1, because the controls had been destroyed by shrapnel. Emergency services had to douse the engine until flameout was achieved.

Source


Are AWOS data open for commercial use? I 've found out this great website where you can get historical AWOS weather data. I would like to use it for a commercial application but I don't find any data license. Do you know if the AWOS data are free to use for commercial purposes? Can you point me to a relevant link?

Your website source from Iowa State University may or may not be open for commercial use. As Freeman has said in his comment, it would be difficult for anyone here on the internet not affiliated with the university to know. Your best opportunity to use their data is to contact the university directly. Their contact information is at the bottom of the link you provided.
Otherwise, your best option is to contact NOAA, the NCEP or the National Weather Service (basically, one and the same organization). They collect, consolidate, and archive the data at their source. That is where many commercial applications get their data.


Does skin friction drag decrease with velocity? The wings of subsonic planes have usually a very low surface area and aspect ratio as big as possible. However, in supersonic aircraft, wings with more wetted area (e.g delta wings) are used. Is this because skin friction decreases with speed or because in supersonic flight this type of drag is no longer the main concern?

Your question is about friction drag while the text is mainly about aspect ratio. You seem to explain the choice of aspect ratio to be based on friction drag. This is not correct.

Here and here are some good answers to explain the choice of aspect ratio. You will see that it has mostly to do with structural strength and wing volume to pack enough fuel. For supersonic aircraft the delta wing provides a highly swept leading edge, a stiff, light wing and good low-speed characteristics even with the thin airfoils which are helpful in supersonic flight.

Generally, aspect ratio becomes less important the faster the aircraft flies, because it has more air flowing past per unit of time  for creating lift. At supersonic speed some of the drag is proportional to the local inclination of the surface, so the designers try to make the aircraft as sleek as possible. To put it bluntly, a subsonic aircraft likes to be as wide as possible to reduce induced drag, while a supersonic aircraft likes being longer to reduce wave drag.

Friction drag will go up with the square of velocity if all other parameters are constant. Since a higher velocity increases the Reynolds number (the ratio of inertial to viscous forces), in the real world the increase is slightly less than with the square of velocity. Since lift also goes up with velocity squared, the airplane can fly at higher altitude where the density is lower, which reduces both lift and drag.

Now you need to consider the change in temperature over altitude: Air gets cooler when it expands, and cooler air causes higher viscous forces. By flying higher, the Reynolds number goes down again and the viscous drag goes up. In total, flying faster and higher will result in a slight increase of the absolute friction drag at the same lift.

If you plot the drag coefficient over flow path length you will find that lengthening the flow path will generally increase drag less than proportionally. Only when the boundary layer is laminar and lengthening the flow path adds a turbulent transition will you find that friction drag briefly goes up more than proportionally. Sometimes it is helpful to force that transition, but mostly it comes sooner than the engineers like.

Please follow the links and read the linked answers, only then is this answer complete.


How much distance do I need between quadcopter propellers to avoid issues? For a given propeller size (say 5"), what is the minimum clearance needed between propellers to ensure that not only do they not hit each other (easy to calculate), they also don't interfere too much with each other (i.e. tip vortices, airflow, etc.)

For example consider the following design: an X-shaped quad-copter where the propellers are at the end of each tip of the X - and the X across the diagonal (max distance from the center of the propellers) is 180mm - then 5" propellers will only have .279mm of clearance between them - now assuming that mechanically there are no vibrations/imperfections/etc. that cause them to run into each other, what else is there to avoid in terms of airflow? if they were raised such that two diagonal propellers are lower/higher than the others, would there be any issues? does it depend on the propeller pitch, or if it's a 2-blade or 3-blade propeller?

P.S. - I am unsure of the tags, please adjust as necessary

Well, you gotta keep in mind that the airflow is mainly down, but there will always be a little flow side to side, (thus frames, also used for safer collisions,) so you should put them roughly 1/3 of the diameter of the rotors themselves away from each other. This provides enough space to keep your interference down, as the interference spreads  out to about 1/4 of the radius of the rotor. I have not seen that the number of blades matters much, as long as you follow the 1/3 diameter rule. You want to avoid the counter- airflow that can make your craft less efficient or jack the blades. Note also that raising two of the blades would cause serious problems with your efficiency, because two rotors blowing downwards in a cone, unless very effectively controlled, will kill the stability, creating unnecessary strain, or in bad cases, frame torque. The two above would create interference for the lower. This could, however, be avoidable if the top two were about 3/4  rotor diameter away, and no higher than 1/4 a rotor diameter. But then you'd be flying a very odd and somewhat unbalanced copter.


How frequently are avionics on large commercial aircraft updated? How frequently are avionics on large commercial aircraft updated? And how are the systems updated?

Infrequently. Large fleets, lots of down time, lot of equipment to buy.

The fleets are cycled thru at a refittng center where the panels are disassembled and re-assembled. Here is one example


What is the typcial descent rate of a helicopter? I'm not a pilot, just asking out of curiosity. What is the typical descent rate for a helicopter when landing? I found the answer of 300 feet per minute on the web, but I want to confirm this. If this is true, than a helicopter descending from 2400' to land would take 8 minutes. Is this correct?

That is of course dependent on airspeed at approach, and the approach angle. I have a flight manual of an older type helicopter, I cannot reproduce the contents, but it shows True Airspeed on the horizontal axis, Rate of Descent on the vertical, and a set of inclined lines for the angle of Descent.

In the Emergency Procedures - Normal Approach, an Angle of Descent of 8 - 10º is specified, and a typical TAS of 65 knots. This amounts to a Rate of Descent between 800 and 1100 ft/min. Maximum allowable ROD at that speed is 1600 ft/min which is marked as Approximate Autorotation.


Are glider winch launches rarer in the USA than in the rest of the world? Why? I had heard before that winch-launching gliders was a relatively rare practice in the USA, while quite common in the rest of the world.

My data is mostly anecdotal, but I do know gliding clubs in Europe that exclusively use winches.

A recent post in The Hangar reminded me of this: Tanner claims their club owns the first winch launcher sold by Tost to a USA customer, and that they have not used it in years.

As far as I know general aviation scene in the Asia-Pacific region takes after that in the US, but I have no concrete knowledge.

Is winch launching relatively rarer in the USA than in the rest of the world? If so, why? How does it compare to other regions?

To expand on Quiet Flyer's answer, it's technical and economic at the same time.

Technical: Winch is limiting because you are deposited in the same spot over the field, whereas a tow, if the pilot knows what he's doing, can take you to a thermal up to a few miles from the field.  Plus you can get towed higher.  If you had to choose the launch method that gives the highest probability for a successful flight, aerotow's a no-brainer.

Economic: Because of the higher average individual purchasing power in the US (and Canada to more or less the same degree) of disposable income (for example, power flying is a middle class, even lower middle class, activity in the US and Canada, and an upper middle class activity in EU), glider pilots can afford the higher cost of a tow, but which is still cheaper than a tow in Europe because overall operating costs for power planes are less.

So given the choice between paying 15-20 bucks for a winch launch to hopefully 2000 ft (if you have a runway long enough to fit the cable) and 30-40 bucks to be hauled to 3000 feet in a Pawnee, whose pilot, if reasonably skilled, will find and drop you off right at a thermal, most North American glider pilots will pay the 30-40 for the tow.

So the result is there is very little market demand for winch operations in North America, except with some small clubs in out of the way places without the money and resources to run a tow plane.


How is the camber calculated? In this picture, how do you determine the amount of the camber? Is it the average difference in the distances between the mean camber line and the chord line from the leading edge all the way down to the trailing edge?

The mean camber line is the locus of points halfway between the top surface and the bottom surface (which are sometimes referred as upper and lower cambers). For a symmetrical airfoil, it is merged with the chord line.

This curve is described by a polynomial function at each point along the chord axis. An airfoil may have a camber line which changes direction at one or more point, and which possibly crosses the chord.

All measures are taken perpendicular to the chord which is considered having a length of 1 or 100%.

Adapted from Wikipedia.

What you ask for is the maximum camber, which is determined by the value of the maximum difference between chord and mean camber line, and the distance from the leading edge.

Adapted from Wikipedia.


Can TAS at different altitudes be calculated if only one performance point is given? If the POH only gives TAS at only one altitude, can we calculate the TAS at other altitudes?

Here is a snippet from the POH of Remos GX. If we set the RPM to 5,200 then at 3,000 feet MSL ISA we get 107 knots. Can we calculate the TAS at 6,000 feet?

If you keep 5,200 rpm at 6,000’, the engine will be producing slightly less power due to density altitude. There will also be slightly less drag so the speed will only drop about 1-2 knots. Because the engine is producing less power, fuel consumption will be slightly less and range will also increase.

The performance figures for a Cessna 150 can give you an approximation of how the numbers change with altitude.


Where does Air Force One refuel? Obama came to Greece and then to Germany. That's a long trip, even for the majestic Jumbo. There was a lot of discussion about where the aircraft was refuelled during that trip. Do we know, in general, where Air Force One is refueled?

There were claims that it was refuelled in the air by American aircraft, but isn't that more costly than refuelling on the ground?

According to the Washington Post, Air Force One never refueled in mid-air with the president on board.
If it needs to refuel, it usually does so at one of the U.S. military bases across the world.

The retrofitted Boeing 747 that usually serves as the presidential jet can indeed refuel in flight — but it has never done so with the president on board, military officials say.
It has enough range to fly from Washington to Iraq without needing more fuel. On longer trips to Asia, it typically stops at U.S. military bases in Alaska or Germany.
— Five myths about presidential travel


Why is the mammatus cloud formation so dangerous? I have read and heard that the mammatus cloud formation presents a danger to aircraft and is often associated with funnel clouds and tornadoes. How is it formed and why is it dangerous?

Source: Wikipedia

Nobody knows exactly how mammatus clouds are formed, it's been studied and there are several theories about it but none are accepted. They are most often seen below anvil clouds, ie cumulonimbus clouds with extremely powerful updrafts. One thing that the theories all agree on is that where they are formed on the underside of cumulonimbus clouds there are huge shifts in temperature, moisture and pressure within a short space of time. Although these clouds can form under other types of cloud they are mostly seen under the most powerful of storms.

So why are they dangerous?

They are usually made of ice: ice accretion is dangerous, especially for light aircraft without de-icing systems
They are almost always associated with severe low level wind shear. Wind shear is sudden changes in wind direction which is dangerous for aircraft. You can suddenly find that a 30kt headwind is a 30kt tailwind so you've lost 60kt of airspeed, this is especially dangerous during takeoff and landing
They are almost always associated with severe convective activity, ie very strong updrafts and downdrafts which are dangerous for aircraft. You could get sucked up thousands of feet or pushed down as well, and there's be nothing you could do about it
They are almost always associated with severe turbulence which is also dangerous for aircraft. Severe turbulence is at best extremely uncomfortable, at worst can cause a loss of structural integrity

So mammatus clouds are usually an excellent sign of an area to be avoided.


Can the clearway be located on a lake? From the FAA AIM:

  All 14 CFR Part 139 airports report declared distances for each runway. Other airports may also report declared distances for a runway if necessary to meet runway design standards or to indicate the presence of a clearway or stopway. Where reported, declared distances for each runway end are published in the Chart Supplement U.S. For runways without published declared distances, the declared distances may be assumed to be equal to the physical length of the runway unless there is a displaced landing threshold, in which case the Landing Distance Available (LDA) is shortened by the amount of the threshold displacement.

Can the clearway be located on a lake?

From USA AIP (emphasis added):

  The U.S. does not require or provide criteria for clearways in its design standards. It does encourage ownership and clearing of the land underlying the innermost portion of the approach out to where the approach surface is 10.5 meters above the level of the take-off surface.

You may also want to check AC 150/5300-13A - Airport Design (no mention of water).

FWIW, ICAO includes water in the definition (Annex 14 to SARPs):

  Clearway. A defined rectangular area on the ground or water under the control of the appropriate authority, selected or prepared as a suitable area over which an aeroplane may make a portion of its initial climb to a specified height.


What are these devices on or adjacent to an airport taxi-way? Can someone identify and provide some description of the objects marked in this photograph?

Especially those marked with yellow and red seem to be placed in the middle of a road and that seems to be making the road unusable. How can a plane or airport car go that way, if there are some devices placed in the middle? What am I missing?

It is not a road that you are looking at, it is the edge of the runway / taxiway.

The double yellow lines are used to define the taxiway
edge from the shoulder or some other abutting paved
surface not intended for use by aircraft.

The blue lights in the yellow boxes are taxiway edge lights.

The poles next to the lights are blue reflective markers that will mark the taxiway in case the light fails. Systems for light control often work for groups of lights (e.g. sections of taxiways) or, if individually controlled, through a control signal on the power line powering the light. In the latter case, there is a small piece of electronics in each light housing controlling the intensity of that individual light and reporting back if the light fails.

The yellow objects in the blue boxes are signs. They indicate names of taxiways, runways, holding points etc. You see the backside of light casings, not the signs themselves. A bit further to the right you can see similar signs.

I am not sure what the item in the red box is.


Why do pilots disable the air-conditioning 'packs' during takeoff? I've realized that some pilots turn the air conditioning units (also called 'packs') OFF during takeoff, like in this video:

LH456 Takeoff Video

Is there any operational reason for that?

The "packs" (or A/C on the aircraft) are powered by engine bleed air. When bleed air is extracted from the engines, less thrust can be produced. This may be a problem if the airplane is taking off at a high elevation airport or the departure path requires a certain climb gradient to clear obstacles. This action does create some discomfort in the cabin so it's usually not preferred unless required for safety.

Some newer airplanes are manufactured with an "APU to pack" configuration - i.e. the pilots leave the APU running during takeoff, which provides bleed air to power the packs. The workload on the engines are now reduced.


Where can I download the flight plan data around the world? Where can I download the flight plan data around the world, so that I have analyze their route?

Are they provided freely?

For historical FAA IFR flight plan data, you can get it from www.flightaware.com.  They show five minute delayed, and with a free registered account you can get access to the last 90 days worth of data.  You can pay for data older than that and I believe in different data formats.

I'm not aware of a similar source for other parts of the world.


What is dual mode adaptive landing gear system? Referring to the NASA document Design and Development of the Blackbird:
Challenges and Lessons Learned.
On page 31, there's a mention of dual-mode adaptive landing gear, which is quoted below.

Lockheed engineers designed a dual-mode adaptive landing gear system for the YF-12A. The configuration included a strut with an optimized air load-stroke curve during landing, and an automatic switch-over system to allow for a flatter air load-stroke curve during taxi. The study demonstrated the effectiveness of a dual-mode adaptive landing gear system in reducing the dynamic response of an airplane during taxi. It also provided a database to aid in determining the degree of correlation between analytically predicted responses and actual test results with a full-scale YF-12A.63

From what I could understand, it seems it's focus was to give the landing gear loading a smoother curve and reduce the dynamic loading during taxiing, in simpler terms, dampen the minor vibrations while taxiing.
How is this type of landing gear different from the basic oleo shock struts employed today, if at all there's any difference. I mean, it pretty much does the same thing. Is there any construction difference?
An image reference would very much be appreciated.

From Testing and analysis of dual-mode adaptive landing gear, taxi mode test system for YF-12A the purpose was to obtain a very progressive rate to keep the gear from bottoming out on landing, but then move to a flatter rate to minimize ground shock when taxiing.

Two air reservoirs of differing capacity and pressure were valved to the hydraulic struts. The smaller reservoir was used for landing as it reached higher pressure faster with a given amount of fluid displaced by the compressing strut. After landing the strut was switched to the larger reservoir.
My reference provides a description of the test configuration as being similar to the actual construction, which it says is in its Reference 1 which I was unable to locate.

In any case, it is simply an additional air (now nitrogen) cylinder connected to the service port of the strut, a somewhat higher pressure is used and there is no valve since they were only testing a single mode, taxiing. The smaller reservoir is likely just the self contained strut air reservoir as in the unmodified design.
Unmodified strut

Automotive shock with reservoir to show test configuration (this was not used, it is not even a strut)


Why is this airplane flying in circles randomly? This G650 has been flying around for over an hour now.  Is this somebody with a mechanical problem who is burning fuel before returning to their airport of origin?

What you observed is probably a Gulfstream production test flight, either an initial or a snag clearance flight, or possibly a Customer Acceptance from Savannah Air Center, which is a completion center across the ramp from Gulfstream. It could be possibly an experimental test flight if Gulfstream runs experimental flying from Savannah (I think they have a test operation in Wichita), but production flights are far more common.

The manufacturer will have designated test areas by agreement with the appropriate ATC units, with specific boundaries, and they will flight plan into the test area for production test flights (aircraft coming off the line) where a Functional Test Procedure, usually taking several hours to complete, is performed.  The ATC clearance will be to maneuver anywhere within the test borders, with a block altitude clearance they are free to operate within, until they need to go higher or lower for different tests and request a new block.

The pilot flying will be steering this way and that pretty much randomly, within the test area borders, while the other pilot performs systems tests, and the two of them will do a number of flight tests where they operate the airplane to, but not beyond, it's certification margins (things like stalls, engine shutdown/relights, yaw damper function, anti-ice tests, those sorts of things).

The airplane will return with or without snags, and the plane will get snags cleared, and go back out for another flight to check the snagged items.  On a corporate aircraft it'll then go on to a Completion Center (usually a separate company selected by the buyer for high dollar corporate a/c) for the interior.

Once the completion Center is done the interior, the customer or a customer's agent is taken on a Customer Acceptance flight before accepting the a/c.  They will go through the cabin to make sure everything is perfect in the cabin, both fit and system wise, getting it fully cold-soaked to make sure valves don't freeze, and subject to pressurization changes that can make interior panel joints open up (and I mean perfect - the slightest flaws in panel fits will be snagged).


Why don't we use hydrophobic (icephobic) coatings over traditional anti-ice methods? I am a design engineer on GA aircraft and I had a thought whilst flying in icing conditions the other day. I know there are lots of ways we deal with ice buildup (as per the chart below from this post, many of which are complex in nature and require more attention in design and operation as compared to passive coatings.

Why don't we use hydrophobic (or rather icephobic) coatings instead of using the more common methods like deice boots, TKS or bleed air? I would think a strip of hydrophobic material such as this 3M™ Microfluidic Diagnostic Film or a hydrophobic paint additive that covered the leading edge would work on most general aviation aircraft. I am sure there are downsides that I am not seeing otherwise it would be used much more.

I have no doubt that a lot of experimentation has been done using coatings on GA airplanes and to the extent that a coating would work, the main problem that I see is degradation of the coating from UV and erosion.  A coating would likely have to be restored regularly and would be likely too expensive for most tightwad GA pilots.

The other thing is that clear ice that forms quickly can tend to sit in place because it's "cupped" around the leading edge and a non-stick surface may not help since the ice shape can be more or less mechanically held in place even if it's unable to actually stick to the surface.  Rime ice that tends to build up out in front of the nose of the LE would probably shed from a non-stick coating a lot better.

On a side note, your chart doesn't include "evaporative" anti-ice, which is a sub-type of heated anti-ice.  On jets this is often used with hard (non-slatted) leading edges because of the need to avoid "runback ice".  A leading edge hot enough to melt ice, but below the boiling point, will allow the water to flow back and re-freeze farther back on the non-anti-iced part of the wing.  Anti-ice systems in that case will be evaporative, that is, so hot that water immediately vapourizes when it touches (like a steam iron).

Such anti-ice systems run at 105+ C temperatures.  A coating that could prevent adhesion of runback ice behind the LE might be quite useful and could allow those wings to run lower temperature systems (the heat plays havoc with the LE structure over the long run).

Runback ice is tolerable on slatted wings and those anti-ice systems will run a lot cooler, around 50 deg C.

On yet another tangent, I always used to wonder why some jets anti-iced horizontal tails and some didn't. Turns out you don't have to anti-ice the surface if you make it big enough.


In USAF are people WSO by choice or is it because they didn’t make the cut to be a pilot? Generally, is a US Air Force Weapon Systems Officer (WSO)

someone who wanted to be an WSO or is specifically assigned and trained to be an WSO from the start, or

someone who failed flight training in some way?

Short answer:  #1 is the most correct statement, but it doesn't tell the whole story.  #2 can happen, but it's very rare for the reasons described below.
TL:DR
To understand this you need some understanding of how the USAF personnel system operates. One key factor that constrains how the system operates is that it's designed for a constant flow of personnel through the system where everyone starts at the bottom (for officers, that's a Second Lieutenant). Over time, each year group (people commissioned in the same year) will eventually go before a promotion board. Failure to be promoted normally results in being forced out. Add in people who just quit (after they've met any service commitments), and the year group gets smaller as you move up the ranks. This makes sense as there are fewer people at the top of the chain of command.
The downside to this is that the USAF can't go hire a bunch of 10-year Captains or 15-year Majors if they find themselves short of personnel in that range. So everything in the system is based on long term projections - not just how many do we need today, but 5 or 10 years down the road.
This process is applied to each career field, of which there are dozens. In general, when you start in a career field you will be in that field for your entire career. It is possible to change career fields, but it's not all that common. The biggest factor will be staffing level in your current field and the field you're trying to move into. Moving from an overstaffed field to an understaffed field is possible. The reverse is virtually impossible.
The pilot and WSO careers are complicated by the fact that it has a very long training pipeline and there's an age limit to start the training. So the process of selecting pilot and WSO candidates starts before you ever get commissioned. For the AF Academy and AFROTC cadets, this starts when they enter program where they get to state their desired career field. The number of available slots in each career field are set based on projected needs 5 to 8 years in the future.
Realistically, almost everyone wants to be a pilot. But it can't happen. When I went in the AF limited the pool by setting very high medical standards. The need for uncorrected 20/20 vision cut the candidate pool down to a more manageable size. The standard for WSO/Nav, etc. at that time was 20/70. So WSO was a backup for those who couldn't meet the 20/20 eyesight requirement (or get a waiver). Grades in school as well as performance as a cadet determined whether you got a slot. Some who asked for pilot were offered WSO slots. (I was an engineer and couldn't pass the eyesight for WSO.) And if you screwed up in school, you could lose it the next year.
My senior year (AFROTC) there was a force cut which cut the number of slots. Pilot candidates that lost their slot were given options. Some were offered Nav/WSO or Missile slots, others were given the choice of taking a non-rated career, going in the reserves, or just dropping out of the program with no penalties. I know people that took each of these options.
But even with all this planning, things change. So if you did come in as a non-rated officer, there are some limited opportunities to make it into pilot training. It's a long shot, but if you do really well as a Lieutenant there's usually a few selected each year to become pilots. You don't have much time to do this due to the age limit to start pilot training.
As to the second option, if you were to wash out of pilot training you would not be automatically considered for WSO as that pipeline is likely full. You could apply for it, but again that would be long odds (and the reason for washing out of pilot training would definitely have a bearing on your probability of success.)


Is a magnetic assisted takeoff feasible? I have seen that there are many companies working toward hybrid electric aircraft. It is very exciting some of the ideas.
The primary reason seems to be for limiting CO2 emissions acceleration, especially during take off. In theme parks acceleration is often performed by powerful magnetic forces.
I was wondering if in the long run a powerful electro-magnetic system could pull planes, fitted into a track on runways for take off?

Catapults are used to launch airplanes on aircraft carriers, most are currently steam powered but the US Navy are developing an electronic catapult system called EMALS to replace steam ones in the future. Other countries are developing the same technology. The US Navy system has been shipboard tested, apparently there are still some problems but they will likely get worked out, so the technology is viable - it is possible.
Shipboard catapults are used to accelerate airplanes quickly in the very short distance you have on an aircraft carrier deck, not to save energy or fuel. It doesn't make practical or economic sense to use the technology to accelerate airplanes in a normal airfield for the following reasons:

It's very expensive technology to buy and maintain. It would cost enormous amounts of energy to deploy this technology
It will limit airplane operations to those airfields which have the technology. I can fly a Cessna 172 into and out of remote strips, but if I require a catapult of some kind my options become very limited
It doesn't actually help much. It's not the take-off roll which is energy intensive, it's the climb to altitude, and a catapult wouldn't help with that. When you think about it the take-off roll is a minute, climbing to altitude is 20 minutes in a commercial jet, you'd be reducing the energy needed by at most 5%, which isn't worth the investment


If you make up your own IFR waypoints, how does ATC know where they are? If you make your own IFR waypoints, how does ATC know where they are?

Just finished my IFR course in Canada.

Was surprised to learn that you can make up
your own IFR waypoints.

If all IFR flights must register a flightplan, how does ATC know where your waypoints are if you make them up yourself, or do waypoints need to be registered first?

My flying club has their own IFR waypoints and I heard that some of the fly in fishing camps have their own IFR waypoints.

Yes, you can define your own waypoints. When we use the waypoint identifiers that are published ATC knows where the waypoint is. That's because we're using the same database they are which contains the Lat/Lon location for each of the waypoints in the database.

When we create our own waypoints, ATC doesn't know anything about them. So we just have to give them the waypoint in a form they can interpret.

Almost all FMS provide the capability for pilot-defined waypoints. They can be entered in the form of Lat/Lon, Place/Bearing/Distance (PBD), or Place/Bearing-Place/Bearing (PBX). Once created, the system assigns a short ID to be used in the flight plan leg list. Usually something starting with LLxx, PBxx, PXxx where xx is a 2 digit number. Some GPS navigators just have one pilot-defined list of WPTxx.

While these identifiers work well in your FMS or GPS, they don't help ATC. You have to give ATC the actual position reference in the Route section of your flight plan. The most common formats would be:

Latitude/Longitude can be entered in either decimal deg or in deg min format. A decimal point is used for decimal degrees. An apostrophe is used for deg min. Use NSEW to identify directions.  They can be either prefix or suffix.  Be consistent.  For example:

N28.52W83.68  or  83.68W28.52N  or  N28'51'W83'40'

Place Bearing Distance and Place-Bearing/Place-Bearing waypoints start with a known database waypoint - typically a VOR - but any fix in the database will work. Examples:

ATL/162/18 to identify Atlanta VOR, 162 deg, 18 NM.

ORL70/MLB348 is the intersection of the Orlando VOR radial 070 and Melbourne VOR radial 348.


How does aircraft weight affect landing distance? I am a private pilot. I posed a question to instructors in the flight school and got differing answers. For a given aircraft, landing at the same speed, with everything the same except for landing weight, and using maximum braking without skidding the tires, which will stop in a shorter distance after the wheels touch down:  the plane at max gross weight, or the same plane lightly loaded?

I submit that because the heavy plane has more energy to dissipate, it would take longer to stop. Some instructors said that because there is more weight on the wheels, the heavier plane would stop in a shorter distance. I think it is a simple energy conversion physics issue.

If you discount aerodynamic effects, and assume the same landing speed, on a theoretical level mass makes no difference. The additional kinetic energy to be dissipated is exactly cancelled by the additional normal force. The kinetic energy is given by the formula:

E = 1/2 mv², where

E is the kinetic energy
m is the vehicle's mass
v is the speed at the start of braking

The work done by braking is given by:

W = μmgd, where

W is work done
μ is the coefficient of friction between the road surface and the tires
g is the gravity of Earth
d is the distance travelled

The braking distance given an initial speed v is then found by putting W = E, from which it follows that

d = v²/2μg

d = minimum stopping distance
v = velocity when brakes were applied
μ = coeff. of friction between wheels and road
g = gravitational constant

Mass is removed from the equation. The kinetic energy is tranferred into heat, so the larger the mass the more heat is generated. But theoretically the stopping distance is unaffected.

In reality, though, the coefficient of friction of tires does not remain constant with changing normal force. As normal force increases the coefficient of friction (μ) of tires decreases.

Since the μ does not remain linear with mass a heavier plane will have an increase in kinetic energy which is higher than the corresponding increase in friction of the tires, thus a longer stopping distance.

All of this assumes that “maximum braking” means you have as much brake power available as the maximum friction force of the tires. Above a certain weight/speed this will not be the case for an airliner. They are designed for a particular amount of deceleration force at MTOW and that will be less than the amount of friction tires can provide on a dry runway. (See this question for more details on that.) That will also cause a heavier plane to roll farther.


How can cargo be jettisoned from a C-124 in flight? I was reading about a C-124 that was carrying three nuclear weapons over the Atlantic, and developed engine trouble. The article said that two of the weapons were jettisoned into the ocean for safety reasons. I was wondering how you can drop something from a C-124 in flight? Unless they were small enough to push out the back side doors. This happened on July 28, 1957.

The C-124 had a opening in the rear, through which cargo (or paratroopers) could be dropped. From globalsecurity.org:

  The C-124 was also used for airdrops. ... In the back of the plane, the underneath of the airplane had elevator doors that opened up and the crew could drop cargo out, or paratroopers could jump out.

Here is an image of the elevator doors in open position

The rear elevator opening of the Douglas C-124 Globemaster II where an elevator platform could be lowered or raised for bulk cargo operations (often the platform was removed so an interior hoist could directly move cargo) — photo by Joseph May; image from travelforaircraft.wordpress.com

Here is a C-124 using that door for an airdrop, presumably in Antarctica (I've put the image as I came across; I'm pretty sure that it's rotated)

Image from antarctica.recollect.co.nz

The crew must've used the rear door rather than the front one for jettisoning the nuclear weapon. The text of the official report reads:

  The C-124 aircraft was en route from Dover AFB, Delaware, when
  a loss of power from the number one and number two engines was experienced.
  Maximum power was applied to the remaining engines, but level flight could not be maintained. At this point, the decision was made to jettison cargo in the interest of safety of the crew and aircraft. Three weapons and one nuclear capsule were aboard the aircraft, and nuclear components had not been installed in the weapons. The first weapon was jettisoned at 4,500 feet altitude. The second weapon was jettisoned at approximately 2,500 feet altitude.

With the difficulties the crew was experiencing in keeping the aircraft level, it would only be logical that the rear doors were used, not the front.


There are some types of landings, can you, please, clarify them? Or even add some other types I'd like to have explanations of these landing descriptors - what do they mean?

deep landing
bounced landing
hard landing
nose pointed down landing
technical landing
"smooth" landing
belly landing
landing gear up landing
partial landing gear up landing

From European Action Plan for the Prevention of Runway Excursions 1.0, Appendix E- Aircraft Operators:

Deep landing – a certain distance behind the glide slope touchdown point
Short landing – touching down before the glide slope touchdown point

Belly landing (or pancake landing or gear-up landing)-  The aircraft lands without its landing gear fully extended and uses its underside, or belly, as its primary landing device.

"Thunderbolt II 080325" by U.S. Air Force photo/Brad White - Official USAF Website 1. Licensed under Public Domain via Commons.
Partial gear up landing- One/or more of the landing gears fail to extend. Similar to belly landing, but in that case, all the gears fail usually.
Hard Landing- The aircraft impacts the ground with a greater vertical speed and force than in a normal landing. The actual speed varies from aircraft to aircraft. This kind of landing usually requires some inspection afterwards.
Nose pointed down landing- Aircraft usually land at a positive attitude (i.e. with nose pointed up). However, in some cases (usually due to some problem), the landing can be carried out with nose pointed down. This is normally in violation of standard procedures.
Arrested landing- Usually done in carriers; an arrester cable is used to bring the aircraft to stop.

"FA-18 Trap". Licensed under Public Domain via Commons.
Smooth landing- If the landing has no problems like given above, it can be called a smooth landing. It is subjective though.


How do I get a Special Issuance? On the FAA medical exam there is a conditions section and one of them is mental disorders. I have/had OCD so I have to check yes. I'm getting off my med soon and then I will have another exam, so that I don't have to check this element anymore.

I heard I could use a Special Issuance to get a medical certificate. How do I request this Special Issuance?

On the FAA website it says:

  The applicant must demonstrate to the satisfaction of the Federal Air
  Surgeon that the duties authorized by the class of medical certificate
  applied for can be performed without endangering public safety for the
  validity period of the Authorization. The Federal Air Surgeon may
  authorize a special medical flight test, practical test, or medical
  evaluation for this purpose.

My OCD is not and was never that severe, I never had depressive episodes. I have a flight simulator and I don't repeat actions or have to check over and over again, or make sure everything is perfect; My OCD wasn't like that.

Right now I'm in contact with leftseat.com. They say they can get me a medical certificate, but nothing in the document they gave me mentions meeting a Federal Air Surgeon. I'm just really confused, as I think I need to meet one for the Special Issuance.

Could someone clarify this?

As with most specific medical questions, the only real answer here is "talk to an experienced AME, preferably one who's handled similar cases before". Some standard advice is to schedule a consultation only first, to review your case. Once you have everything organized and documented properly and your AME is confident about your application, then you can do the actual medical exam.

Having said all that, as far as I know you don't request an SI directly. If the AME determines that he can't issue a medical certificate because of a condition you have, he should work with you and the FAA to put together whatever combination of test results, documentation and general information that the FAA wants in order to issue an SI. But all of that should happen before you do the actual medical exam, to avoid delays.

If you're an AOPA member they can help you with advice and information and they have more details about the SI process. (Their medical forum is very useful too).


Why does the spiraling slipstream from a propeller hit the left side of the fin? In the picture below, if the fuselage was shorter/longer, would the spiraling slipstream from a propeller hit the fin on the right side as opposed to the left?

Is it a deliberate positioning if yes, to hit the left (port) side?

I know the diagram is cartoonish but I couldn't find wind tunnel or CFD illustration.

It may seem hard to hit the right side, but imagine hitting the top of the elevator on the right side. Would this slipstream affect the pitch as well as the yaw?

(Source)

Is the following image related? I mean the path of the vortex, not the cause of the vortex.

(Source)

The air flow is not a spiraling tube like shown on the picture for simplification, or as suggested by the condensation on the second image. Actually all air behind the propeller is:

In rotation, the direction being impulsed by the propeller rotation. This rotation is maximal for air close to the propeller, and by effect of viscosity, a gradient exists in air until a certain distance where it is negligible.
In translation, due to the aircraft moving forward (or air moving backward when the aircraft is holding on the ground).

The resulting motion relative to the aircraft is a cylinder of air moving in an oblique direction, as represented in your image. Note this is not a spiral with non-rotating air in between, but really a continuous cylinder.

Actually the speed/pressure is not uniform within the cylinder, there is indeed a sinusoidal undulation, but the intensity never decreases to zero.

  If the fuselage was shorter/longer, would the spiraling slipstream
  from a propeller hit the fin on the right side as opposed to the left?

No, the result is that whatever the propeller pitch or the fin distance to the propeller, air is continuously hitting the vertical surfaces in a circular motion from the left (when seen from a location behind the aircraft).

  It may seem hard to hit the right side, but imagine hitting the top of
  the elevator on the right side.

The right side of the fin (as well as all surfaces in the shadow of the rotating air) sees a pressure decrease.

The more the distance from the the aerodynamic center of the aircraft, the higher the torque created on the aircraft, and the higher the tendency to rotate.

  Does this slipstream affect the pitch as well as the yaw?

The tendency is:

To yaw for the vertical tail components and the fuselage.
To bank for the horizontal tail components. This is not a pitch moment, because each side of the tail receives an opposite force. This force is balanced by the torque created by the propeller rotation on the aircraft.
To bank for the wing. This force is also balanced by the propeller torque.

Note that because there is a pressure undulation, the surfaces are hit by a slipstream varying slightly at the frequency of the blades.

For more details and theory: Propeller-Rudder interaction (in water, also applicable to air).

  Is the following image related?

The condensation happens at the tip and trailing edge of the propeller blades, and is then carried aft by the movement of the air cylinder. So it is completely related. Note that:

Air between condensation trails moves at the same speed (except the undulation).
There are two interleaved helices for a two-blade propeller.

Related effects:

The P-factor produces an additional yaw moment.
An increase in the propeller power changes the downwash, hence the efficiency of the horizontal stabilizer, and a pitch tendency.


Why do some approaches have suffix letters such as Y and Z? There are cases with instrument approaches where there will be multiple approaches to the same runway, in many cases uses some of the same nav aids and positions, but differentiated using alpha characters.

One example are the ILS Y or LOC Y RWY3 and ILS Z or LOC Z RWY3 approaches at Raleigh Exec Jetport at Sanford-Lee County in Sanford, North Carolina. These approaches both use the LOC/DME I-TTA there, and some of the same positions.

It seems that the differences are subtle in the overall procedure on things like the missed approach procedure and how positions are identified, but I'd love a textbook answer that explains it more thoroughly.

ILS Y and Z (click either image for higher resolution).

The naming convention is explained in Chapter 4 of the FAA's Instrument Procedures Handbook:

  When two or more straight-in approaches with the same  type of
  guidance exist for a runway, a letter suffix is added  to the title of
  the approach so that it can be more easily  identified. These approach
  charts start with the letter Z  and continue in reverse alphabetical
  order.

The TERPS explanation (section 161) is a little more specific:

  d. Duplicate identification suffix. When more than one procedure to the same runway uses the same type of navigation system for lateral
  guidance within the final approach segment, differentiate each
  procedure by adding a non-repeating alphabetical suffix using the
  letters “S” through “Z.”

The Handbook gives the example of two approaches at Chicago Midway (KMDW), which has the RNAV (GPS) Z RWY 13C and RNAV (RNP) Y RWY 13C approaches. They're both straight-in GPS approaches to the same runway, but one requires RNP and the other doesn't.

As the question itself and this comment note, there are plenty of other possible differences between approaches to the same runway; RNP is just one example. Having multiple approaches allows an airport to accommodate aircraft with different equipment and capabilities or, as section 140 of the TERPS puts it, to "gain an operational advantage":

  To permit use by aircraft with limited navigational equipment, an IFP [Instrument Flight Procedure]
  should be formulated using a single navigation source whenever
  possible. The use of multiple navigation sources of the same or
  different types may be permitted to gain an operational advantage

I have no idea how to find out why a specific runway has multiple approaches or how the approaches were designed; presumably it's something that the FAA and the airport operator (and maybe even the airlines) work out based on local runway use and traffic requirements.

In the example in the question, one plate requires GPS, the other DME or radar. So a non-GPS equipped aircraft can still use the ILS, but with different ways of getting to it, different minima, etc.

Note that there's a similar naming convention for circling approaches:

  Approaches that do not have straight-in landing minimums  are
  identified by the type of approach followed by a letter.  [...] As can
  be seen from the example, the first approach of  this type created at
  the airport is labeled with the letter A,  and the lettering continues
  in alphabetical order.

Their example is Rogue Valley International - Medford (KMFR), which has the following circling approaches:

VOR-A
LOC/DME BC-B
VOR/DME-C
RNAV (GPS)-D

There's no explanation that I could find in the Handbook or TERPS to explain why circling approaches must have a letter even if the navigation systems are different. But as @RalphJ mentioned in a comment, this provides a clear way to identify multiple circling approaches using the same guidance, e.g. a VOR-A approach from the north and a VOR-B one from the south.


Does rolling affect the thrust of a propeller? If a plane with a propeller, e.g., a Spitfire, does a constant barrel roll in the same direction as the propeller, will the plane travel faster as impractical?

for a propeller rotating at 2400RPM = 40 rev/second and a roll rate of 0.3 rev/second, the roll rate will not measurably affect the prop rotation speed and the plane will neither travel faster (if the rates are additive) nor slower (if the rates are opposite).


My driver's license was suspended for not having insurance. Should I even bother to apply for a medical? My insurance lapsed for a few days, and during that few days that I got caught.

Furthermore, it's the second suspension in my life because I got a bunch of tickets when I was younger.

I hold a sport-pilot license which would be invalidated if I was rejected for the medical.

A sport pilot license does indeed require a valid driver's license OR a minimum third-class medical certificate. If you don't have a medical certificate, you cannot exercise sport pilot privileges until your driver's license is reinstated.


What's the advantage of the F4U Corsair's gull wing design? What is the advantage of the F4U Corsair's gull wing design?

Is there also a disadvantage to this? The wing design just seems odd, and as far as I know it is one of the only planes to use this design.

Is this because the technology advanced so fast, or because the wing design was flawed?

The wing shape in the F4U Corsair is called the inverted gull wing. The main reasons for use of this is the large propeller used in the aircraft.

The Corsair design was in response to the US Navy RFP (Request for Proposal) in 1938, which mandated the following things:

It should have maximum attainable speed
The stall speed should be 70 mph
Minimum range of 1000 miles
Bombs should be carried under the wing (this was supposed to be dropped on aircraft formations from above, which didn't work out)

In order to achieve this, the Vought engineers used the largest available engine, the Prat and Whitney R2800 Double Wasp radial engine (this was done as the Navy, in a change of heart, vowed to consider designs with liquid-cooled engines, in contradiction with a policy settled in 1927 that required air-cooled engines for shipboard aircraft). Also, the wing area of F4U was the largest naval aircraft of its time.

During design, the oil cooler and supercharger inlets were moved to wing leading edges. In order to extract power from the engine, a large (~4m dia) propeller was chosen. This large propeller caused problems with ground clearance, which necessitated an increase in the landing gear length.

The Vought team overcame this by choosing an inverted gull wing, which starts out with a strong anhedral, and then curved upwards to strong dihedral, with landing gear at the lowest point in the bend.

"F4U-Corsair OE-EAS OTT 2013 04 main landing gear" by Julian Herzog. Licensed under CC BY 4.0 via Commons.

The main advantages of this configuration are,

Shortened landing gear
Elimination of the need for wing fillets, and reducing drag as the wing and fuselage were perpendicular.
Simplified wing folding (the wings were folded at the lowest point), with the folded wings nearly at the height of the propellers. This enabled simple automatic mechanism for folding/unfolding, unlike the F6F Sto-wing which was manual, as the hydraulic mechanism added too much weight.

Source: www.airport-data.com

The main disadvantages were:

The design added weight compared to straight wing.
(To some people) It was ugly

This design has been used in a number of other aircraft too, mostly to reduce the length of the landing gear and carry large external stores, the most famous being the Junkers Ju-87 Stuka.

"Bundesarchiv Bild 101I-646-5184-26, Russland, Flugzeug Junkers Ju 87 edit 1". Licensed under CC BY-SA 3.0 de via Commons.

The inverted gull wings were also used in Fairey Gannet, an ASW aircraft of Royal Navy.

Source: www.maritimequest.com

This was also used in some twin engined aircraft like the Caproni Ca.331.

Source: flickr.com

Note that in some books, this design is called the gull wing instead of inverted gull wing.


Do airlines require aptitude tests or other similar tests for new pilots? Do airlines require pilots to take an aptitude test? Are there any other similar kinds of tests that airlines perform?

Yes they do, and so do some independent training organisations. For instance EPST markets an aptitude test called Compass, developed to test the suitability of candidates before they engage in pilot training.

Some airlines only hire self funded students (who have paid for their own CPL, type rating etc) and they can be flight tested in the simulator.


The principle in 'Trompe' is applied to airplanes? 'Trompe' (see Wikipedia) is a water-powered air compressor, commonly used before the advent of the electric-powered compressor. A trompe is somewhat like an airlift pump working in reverse, used to provide compressed air for bloomery furnaces in Catalonia and USA. By sending a fast water flow through a duct, a pull effect on air is obtained.
I wonder if a pure Turborreactor working inside a larger Duct, with no Low Pressure Fan, would have same efficiency, with less parts, as a high Bypass Ratio Turbofan. Operation inside duct, or even inside fuselage, would result in reduction of sound and thermal prints. Salut +

You are basically describing an ejector pump. This would not be very efficient, I'm afraid (I'll try to dig up some comparative figures).

The arrangement in this case would be a jet engine used to accelerate an amount of air inside a duct. With a long enough duct, the end result would be a single airmass exiting at (somewhat) uniform speed. Unfortunately, because energy must be conserved, the kinetic energy of the air exiting the apparatus would not even match the kinetic energy of the air exiting the jet engine, because of turbulence, skin friction of the duct etc.


Why flaps can deflect up to 25°+ without stalling the wings? According to this graph,plain flaps have their highest Cl at about 2 and stalls at about 15°,my question is how then do airplane flaps reach 25°+ of delflection when this gragh clearly states that stall is at 15°?

The X-axis this is the angle of attack of the wing and not the angle of deflection of the flap. When the angle of deflection of the flap is 25%, the angle of attack of the wing will be much less.


Is landing without engine power routinely feasible? Considering that airplanes are very noisy, could one routinely land them with the engines turned off, gliding onto the runway? Could the engines be started fast enough for an emergency situation?

Not feasible. A start of each engine can take 30-60 seconds or longer and would require the APU (noisy). In some airplanes you lose the packs during start so you would impact pressurization. Additionally many engines have a required time at idle after start before significant thrust is applied.  Lastly, with all engines offline, the airplane will be in some sort of essential power mode and many systems will be offline or unavailable.

You might then ask that we land at idle but that can present issues with spool up time before thrust is available.

One of the primary reason we land with a high flap setting is so that we have the drag to maintain approach speeds with the engines spooled up. This gives us near immediate power if we need it to go around.

I'm sure someone else can provide historical data from when idle approaches in jets were normal and specifics into why we don't do this today.


Aerodynamics and fuel consumption of a powered glider I want to optimize the flight distance of a powered glider model airplane, without the use of soaring. I've studied some aerodynamics books, but still feel unsure about my conclusions. Please tell me if I'm wrong in the following:

1) The best fuel economy (liters per km) is for planes with the highest lift/drag ratio, typically so called gliders. The reason a 747 doesn't look like a glider to save fuel, is that it would be too difficult to build strong enough wings of that dimension.

2) Gliders are typically designed so that the best lift/drag ratio is when the airplane is in cruising mode with zero attack angle (horizontal). This is typically also when all control surfaces are neutral.

3) Optimum long distance cruising speed is at max lift/drag ratio, and when the lift is equal to the aircraft weight, given that the engine and propeller are also optimized for it. In other words, the speed where it sits stable in the air without any control. I can find this speed for my model glider by flying it at zero attack angle and with neutral control surfaces, and then increase the throttle gradually until the plane stops sinking.

4) Although the lift/drag ratio varies with speed, it varies very little around the speed that is optimum for long distance cruising. This is because lift-induced drag is a very small component of form drag (overall drag) at that speed. In other words, form drag is very close to proportional to the square of the speed.

5) If I want to increase the weight of the aircraft (for example add more batteries), all I have to do is fly proportionally faster to stay in the same zone of optimization for distance. Lets say the normal take-off weight of the glider is 1.4 kg. I quadruple the take-off weight to 5.6 kg. The lift equation says that my speed must now be doubled to make the airfoil provide four times the lift. This new speed will be very close to optimum cruise speed for distance, because of 4) above.

7) The speed has doubled, which means the drag has approximately quadrupled, so a new engine and propeller with four times the power and optimized for twice the speed must be installed.

I can talk about a couple of these...

  3) Optimum long distance cruising speed is at max lift/drag ratio, and when the lift is equal to the aircraft weight, given that the engine and propeller are also optimized for it. In other words, the speed where it sits stable in the air without any control.

Well, there are two speeds you're talking about:

The most efficient cruising speed, which is also the speed that maximizes L/D, which is also the best glide speed.
The "trim speed", which is the speed where the aircraft tends to maintain a constant pitch ("sit stable in the air") when the pilot is not applying any forces to the control.

For full-scale aircraft, the trim speed can generally be altered by the pilot in flight, using the trim wheel (or whatever trim control that aircraft has). So, the trim speed is usually not equal to the best glide speed.

That said, you certainly can (and it's probably a good idea to) design your powered glider so that its trim speed is constant, and equal to the best glide speed.

  I can find this speed for my model glider by flying it at zero attack angle and with neutral control surfaces, and then increase the throttle gradually until the plane stops sinking.

Yes, you can find the trim speed (but not the best glide speed) of your model glider by controlling the throttle so that the height remains constant. If the controls are neutral and your aircraft is not pitching, then it is flying at trim speed.

(I'm not sure what you mean by "flying it at zero attack angle", since you can't explicitly control the angle of attack without moving the control surfaces. For steady flight, the angle of attack is determined by the speed and the bank angle.)

  7) The speed has doubled, which means the drag has approximately quadrupled, so a new engine and propeller with four times the power and optimized for twice the speed must be installed.

That's almost right. If the speed doubles, then the drag quadruples. But power equals force times speed, so if the speed doubles, and the drag quadruples, then the needed power increases eightfold, not fourfold.

Responses to your comments:

  However, as I understand it the "trim" is just a minor adjustment to the elevator, correct? So in effect we are nudging at the angle of attack so the lift perfectly matches the weight and the plane can sit stable, but we aren't really flying at the optimum L/D?

I wouldn't call it "just a minor adjustment". In a Cessna 172, trim is what determines whether the aircraft flies at 55 knots or 155 knots. Other airplanes and gliders work the same way: there is one particular speed at which the aircraft will fly steadily without any force on the control stick, and trim is used to determine what that speed is.

Trim isn't used for making the lift match the weight. If the lift is too high, then the aircraft will begin climbing (or stop descending), and if the lift is too low, then the aircraft will begin descending (or stop climbing). You control this by using the stick and the throttle.

The aircraft can "sit stable" with the trim in any position, but for each trim position, there is only one corresponding speed at which the aircraft will fly steadily without force on the stick.

(There's an exception to the above: with some aircraft in some circumstances, if the trim is too far back, then constant forward pressure on the stick or yoke is required in order to prevent the aircraft from stalling.)

If you're sitting in an aircraft and you want to fly at optimum L/D, you'll usually do that by knowing what airspeed corresponds to optimum L/D, and setting the trim for this airspeed. (The process for setting the trim appropriately is: use the stick to fly at that airspeed, hold the stick in place, and then use the trim control to relieve the force the stick is exerting on your hand.)

  I'm just assuming that airfoils are designed to be at their best L/D at zero angle of attack, is that really the case?

I doubt it. In particular, some aircraft have symmetric airfoils, which have an L/D of zero at zero angle of attack.

  When I said "flying at zero attack angle", I mean that the plane sits completely horizontal in the air, and it does so without any compensation from control surfaces.

If the plane is completely horizontal, that's zero pitch, not zero angle of attack. Zero angle of attack is when the nose is pointed exactly into the oncoming air, rather than higher or lower.

The pitch that an airplane settles at depends on both the trim and the power setting.

  Last part doesn't seem right to me. If you look at it from the perspective of the engine, it creates a forward force. This force is constant, no matter the aircraft speed (taking aside the fact that a propeller cant be equally effective at all speeds). However the drag is related to the speed, and the drag is four times stronger. So it feels to me like the needed power is only four times more, not eight.

Well, the thing there is, an engine doesn't really create a constant force. In particular, you need to produce enough force to counteract drag. If your aircraft experiences four times as much drag, then your engine also needs to produce four times as much thrust in order to fly the same way.

So in order to fly twice as fast, you need to make the engine produce four times as much force (either by opening the throttle more, or by using a more powerful engine). The engine also needs to operate at twice the airspeed. Power is force times speed, so together, these two facts mean that the engine needs to produce eight times as much power.

Further responses to your comments:

  Re angle of attack and pitch, I might have just confused the terms. As I understand the terms, "angle of attack" refers to the angle of incoming air relative to the chord of the airfoil, so it relates to the wings only. Pitch is the angle of incoming air relative to the aircraft itself. So if the wing chord is mounted exactly horizontal, angle of attack and pitch are identical.

Pitch doesn't have anything to do with the angle of the oncoming air; it's exclusively defined by the way that the aircraft is oriented in space, relative to the horizon. Specifically, pitch is the angle by which the nose is pointing above or below the horizon. If it's pointing 10 degrees above the horizon, the pitch is 10 degrees; if it's pointing 20 degrees below the horizon, the pitch is -20 degrees. This is regardless of how the aircraft and the air are moving.

  It makes more sense to me when I imagine putting a piston engine with a propeller in a wind tunnel. Would the thrust force of that engine/propeller vary greatly with incoming airspeed when running the engine at constant power? I can't imagine why it would. As the air speed increases, the engine would increase its RPM to exert the same power on the air, and the propeller efficiency would vary slightly, but the engine power would still be the same. And overall the thrust force would be approx the same. Or am I thinking the wrong way here?

Well, the best way I know how to put it is "that's physics". If you want to push something in the same direction that it's already going (as an airplane propeller does—it pushes the air aft, even though the air is already moving aft), and you want to push it with a constant force, then the faster that thing is going, the more power is required in order to push it. I don't think it's easy to see why this is the case, but this is, in fact, the case.

Maybe it would be helpful to imagine an airplane flying straight up. Imagine that this airplane is heavy but sleek, so that the drag is very small compared to the weight. Then, in order for this airplane to fly straight up at any speed, its engine will simply have to generate thrust equal to the airplane's weight. (The only time the engine would need to generate more thrust than the airplane's weight, is when it's making the airplane accelerate.) And yet, if this airplane were to fly twice as fast, surely the engine would need to generate more power, right?

The moral of the story is that power depends on both force and speed. That's physics; there's no way around it. If you had an engine that generated a constant amount of thrust, using a constant amount of power, regardless of speed, then you'd be able to make a perpetual motion machine out of it.


Selling the emergency exit row seats The seats leading to an emergency exit normally offer more legroom and, therefore, the airline can cash a fee for them.

But, shouldn't they pick passengers that would have the higher chance of being able to open the door in case of an emergency?

They do.

If you are clearly not physically able, they will change your seat during check-in.

Then, as part of the safety briefing, you will be explicitly asked if your are willing and able to carry out the relevant instructions.  If you are not, the crew has the authority to re-seat you.

If all passengers are seated, and no-one is in the exit row, the crew will re-seat a capable person next to the door.

Here's but one reference to the requirements from the UK CAA.


Can a helicopter have a fully mechanic "elevator"? After a research, I've seen prototypes of planes with elevators. What if it could be used as part of the "Air Ambulance" technology on helicopters? Because helicopters have to land first to pick up patients, would it be better to use an "elevator" to lift them into the helicopter when it is hovering?

If this have been made before, can you point to a prototype?
And if not, why it haven't been made? Is it non-doable in terms of physics and engineering?

Using a building elevator would cut the payload of the helicopter in an unacceptable way. Also, none of the safety features of a building elevator could be implemented, and in the end you will have a cage lowered on a rope. Which is quite similar to existing winch systems.

Elevator components (source)

In addition to the motor and the pulleys, a building elevator has a car which runs along guide rails, and a counterweight to compensate for the heavy car. Adding all this to a helicopter would require a much bigger machine just for carrying the elevator components around. Note that using guide rails would dictate a specific hover height - no helicopter pilot would agree to have his options restricted that severely.

Lowering anything in the downwash of a helicopter rotor will cause it to swing and rotate, and putting the occupants into a cage will cut their ability to stabilize themselves or the cage. Once the cage is raised all the way up, it needs to be oriented correctly for easy egress. A rescuer on a rope can easily stabilize himself by grabbing handles on the outside of the helicopter. Sitting in a cage, he can only watch helplessly when the cage arrives in the wrong orientation.

There are many scenarios where rescue helicopters cannot land, like over water or over a forest. In those situations a winch is used to hoist one rescuer down to the ground, where he/she can pick up the injured and secure them to their harness. Both of them are then raised up and taken on board. Only when the patient must not move (think fractured leg), a platform is used to secure the patient. See below for a simulator installation, and note that still a rescuer is needed to stabilize the platform.

Helicopter rescue hoist trainer (source)

If the ground allows, landing the helicopter is simpler, makes much less noise and makes it immune to changing winds. Also, lowering the rescuer first and landing the helicopter will take a similar amount of time, so no substantial time savings can be expected.

AgustaWestland AW139 helicopter on SAR exercise (source)


What happens if fuel tanks are hit by gunfire? In June 2014, a PIA airplane was targeted by machine gun fire while landing.

I wonder what would had happened if the fuel tanks were hit? An explosion, perhaps?

But in an episode of Myth Busters, they did shot at a gasoline tank and found out it didn't explode. No ignition temperature was reached they said.

So, what would be the worst case scenario if a bullet hits the fuel tank on an airplane?

OK, let me expand my comments about inert gas to Peter Kämpf's answer to an own answer.

Argon or Nitrogen ?

It was asked if Argon is a suitable inert gas. While in principle it is, it is also a rare element and so expensive. A better choice is nitrogen. It makes up 79% of the air and is much cheaper.

For example, I found ~50€ for a 50l bottle of nitrogen and ~100€ for argon.
But one also has to keep in mind that the production capacity for argon is not very high, so if aviation starts to buy large amounts to use it as inert gas, the price would increase soon.

Argon and nitrogen are gases which can not be liquefied at normal temperatures, so they are stored under pressure in bottles like this:

They are made of steel to  withstand the pressure of up to 200-300 bar (atmospheres). So, a big 50l bottle has a weight of 77kg without filling (sorry, german).

Nowadays, also civilian aircraft are often equipped with an inert gas system, since the central wing tank of TWA800 exploded most likely due to electrical short circuits

But when it comes to larger aircrafts with larger tanks, gas bottles become a problem: The A380 has a fuel capacity of 325,000 liters. Imagine you want to fill its tanks from the 50l bottles mentioned above. The content of one bottle expands to 10,000-15,000 liters at sea level pressure, so you need 20-32 bottles for the A380.

This bottles have a weight of almost 2.5 tons and need some space. The high pressure is also an issue, and of course, you have to refill the bottles after each flight.

So, it's not a good idea to use this system on larger aircrafts.

I was quite sure that I once read that at least small aircrafts like fighters use bottles, but I don't find any evidence. On the other side, a F35 can carry up to 10,000l in its internal tank, so it would also need a 50l bottle. So, I think I misunderstood something.

CO2 ?

The benefit of CO2 is that it becomes liquid under pressure, You won't need so many bottles and the bottles themselves would be lighter. But CO2 may act corrosive if it finds some drops of water, as it turns to carbonic acid.

Exhaust gas?

A gas that is the product of a fire doesn't burn any more.
This is indeed used to fill the tank of large ships, but you can not take it from the main engine: Those are diesel engines always working with an excess of air, so the exhaust gas contains oxygen. The ships burn some fuel with a balanced amount of air in a inert gas generator.

As fans are similar to diesel engines in this context, exhaust gas is no solution. The gas also contains CO2 and water, so may be no good idea in general for aviation.

Halon

Many older combat aircrafts like the F16 use halon to extinguish fire, but also to fill the tanks.
As halon is one of the worst greenhouse gases, its production has been abandoned 1994 and this document describes the search for alternatives working largely the same way:

To explain it in short: Inside a fire, molecules of the burning material break up and connect to oxygen atoms. This connection is what gives the energy we want in our engines and we want not in the tank.
Halon jumps into the reaction and connects itself to the open ends. The reaction between oxygen and the material, which gives the energy, can not take place any more.

It is said that about 6% of halon in air is enough to extinguish fire, 10% and more can be survived, if exposure time is not too long.

Halon was not used as inert gas, but it was injected into the tanks shortly before a fight was expected

On-Board Inert Gas Generation System(OBIGGS)

Today's solution is to use a On-Board Inert Gas Generation System(OBIGGS) developed by NASA/Boeing

It uses some kind of membrane which can be passed by nitrogen more easily than by oxygen. Bleed air is lead to this membrane with some pressure. The oxygen is kept back and you get air with a significantly smaller fraction of oxygen behind the membrane. There are devices on the marked which produce 2lb/min with up to 1% remaining oxygen

The goal is to have not more than 9-12% in the tank, see the link to NASA above.

By the way: In WW I, Germany sent hydrogen filled airships to England. One could guess just shooting at them would take them down, but the holes were too small to cause a reasonable loss of hydrogen. And unless the bullets made sparks when hitting some steel in the ships, they also did not set the ships on fire. It turned out that firing standard munition at airships was quite useless.

The solution was to use phosphor based tracker munition which set the hydrogen streaming out of the leaks from the other bullets on fire.


In the US, in actual practice, workload permitting, will ARTC facilities grant SVFR clearance for surface-level Class E "extensions" (E4 airspace)? In the US, in actual practice, workload permitting, will ARTC Centers and Approach Control facilities grant clearance for Special VFR operations in surface-level Class E "extensions" (E4 airspace)?
Does the answer vary between different ATC facilities?

In the US, in actual practice, will ARTC Centers and Approach Control
facilities grant clearance for Special VFR operations in E4
surface-level Class E "extensions"?

Some will, and some won't.

Does the answer vary between different ARTC facilities?

Yes.
Note the range of comments below obtained from questions posed by telephone in May 2019 to various ARTC Center and Approach Control staffers-- in most cases, the "Watch Officer" on duty, or else an "Airspace Specialist"--
(The summaries below are based on written notes and are not verbatim transcriptions.  The actual airspace descriptions are not from the staffers but rather are from the FAA's "Airspace Descriptions And Reporting Points" document, JO 7400.11E.)
Note also that in no case did the staffer indicate that the specific reason that SVFR would not be approved for an surface-level Class E "extension" was because it was designated as E4 airspace rather than E2 airspace.  Airspace definitions for some airports are actually written so that the surface-level Class E (E4) "extensions" convert to E2 surface-level Class E airspace when the control tower is closed, but no staffer ever indicated that that change in status would have any effect on whether a Special VFR clearance might be granted for that airspace.  Several staffers were not even aware that surface-level Class E airspace was broken down into several different subcategories (e.g. E2 versus E3/E4) in the FAA's "Airspace Designations and Reporting Points" document.
Facility-- Harrisburg Approach (PA)
Airport: KLNS-- Lancaster-- PA
Airspace description-- Part-time Class D, turns to surface-level Class E (E2) when tower closes.  Surface-level Class E (E4) "extension" to SE, converts to surface-level Class E (E2) when tower closes.
Staffer stated that when tower is closed, Harrisburg Approach may issue SVFR clearance for the entire "surface area", which does include the airspace in the "extension", because it  goes down to the surface, whether the tower is open or closed.  In response to my questioning, said it has nothing to do with whether the airspace is E2 or E4-- it's all "Echo" to them.  Said tower is only authorized to issue SVFR clearance for the Class D airspace, but if tower contacted Harrisburg Approach with a request, odds are good a SVFR clearance could be granted for the E4 extension as well.  Stated tower has to call Harrisburg Approach anyway when granting a SVFR clearance so it would not be a burden to ask.  Noted that S37 is within the E4 extension so a pilot flying from KLNS to S37 might have good reason to want a SVFR clearance including the extension.

Facility-- Seattle Center ARTCC
Airport-- KACV-- Arcata CA
Airspace Description-- surface-level Class E (E2) circle around airport effective 24/7, surface-level Class E (E4) "extension" to SE effective 24/7, depicted as separate areas on the sectional chart--
Staffer said a SVFR clearance could indeed include the entire Class-E-to-surface area.  In response to my questioning, stated made no difference whether the airspace was E2 or E4.

Facility-- Salt Lake City ARTCC
Airport-- KSHR --Sheridan County-- WY
Airspace description-- surface-level Class E (E2) airspace, effective 24/7, extends far to NW and SE of airport
Staffer's initial response was that a SVFR clearance would be limited to a certain radius from the airport such as 5 miles, but after taking a closer look at the sectional chart said a SVFR clearance could indeed include the entire Class-E-to-surface area.  Stated that since the area is quite large, let ARTCC know if you only need the northern part or the southern part.
The staffer also offered opinions on the airports below, while noting that they are not actually in the Salt Lake City ARTCC area of responsibility--
Airport-- KBIH -- Bishop-- CA
Airspace Description-- surface-level Class E (E2) circle around airport effective 24/7, surface-level Class E (E4) "extensions" to N effective 24/7, all depicted by a single dashed magenta line on the sectional chart--
Staffer said a SVFR clearance could indeed include the entire Class-E-to-surface area. In response to my questioning, stated made no difference that extensions were E4 not E2.
Airport-- KTVL-- South Lake Tahoe-- CA
Airspace Description-- surface-level Class E (E2) circle around airport effective 24/7, surface-level Class E (E4) "extension" to N effective 24/7, depicted as separate areas on the sectional chart--
Staffer said a SVFR clearance could indeed include the entire Class-E-to-surface area.  In response to my questioning, stated made no difference that extensions were E4 not E2.

Facility-- Minneapolis ARTCC
Airport-- KSTC-- St. Cloud MN
Airspace description-- Part-time Class D, turns to surface-level Class E (E2) when tower closes.  Surface-level Class E (E4) "extension" to SE, converts to surface-level Class E (E2) when tower closes.
Staffer stated that tower could only issue a SVFR clearance for the Class D airspace, but if tower were closed and pilot contacted Center, a SVFR clearance might be able to include the extension to SE.  Wasn't sure about this, said no one had ever asked.  Noted that there is small airport just inside the far end of the extension so there might be reason to desire such a clearance.  Was unfamiliar with designations E2 and E4 and planned to do more research on the topic.

Facility-- Oakland Center
Airport-- KBIH-- Bishop CA
Airspace Description-- surface-level Class E (E2) circle 5 miles in diameter around airport effective 24/7, surface-level Class E (E4) "extensions" to north effective 24/7, all depicted by a single dashed magenta line on the sectional chart--
(Staffer indicated that the above description was only valid for the winter season.  Stated that the summer configuration was class G up to 1200 feet.  That has apparently now changed, as the 2-25-21 Chart Supplement simply says "AIRSPACE: CLASS E")
Regarding the winter configuration, staffer said a SVFR clearance would not be granted beyond 5 miles of the airport because the purpose of a SVFR clearance is just to get pilots up away from the airport or down into the airport, and the terrain to the north is mountainous so they don't want pilots flying SVFR there.  Also stated that the purpose of the extensions is to facilitate instrument arrivals and departures so they wouldn't want pilots flying SVFR there.  Said even in a case where terrain were flat they likely would not grant SVFR beyond a certain radius from the airport conforming to the basic inner Class D or Class-E-to-surface circle, because the extensions are to facilitate instrument arrivals and departures.  In response to my questioning, said it had nothing to with whether the airspace in the extensions was E2 or E4.

Facility-- Kansas City ARTCC
Airport-- KTOP-- Phillip Billard near Topeka KS
Airspace description-- Part-time Class D, turns to surface-level Class E (E2) when tower closes.  Surface-level (E4) "extension" to SE effective 24/7.
Staffer stated that a SVFR clearance could not include the extension because the purpose of a SVFR clearance is just get to pilots up away from the airport or down into the airport and if you can't do it within the D circle you should file IFR or not fly.  Stated that the purpose of the extension is to protect IFR traffic, so they wouldn't want to issue a SVFR clearance there.  In response to my questioning, stated made no difference that the airspace in the extension was E4 rather than E2 -- they just call it "Echo" and don't make that distinction.  Comments were not specific only to KTOP but also stating the general principle involved.

In essence, whether or not SVFR clearance will ever be granted by ARTC for a specific chunk of surface-level Class-E airspace that projects some distance from the basic circle of surface-level controlled airspace around an airport, seems very much to depend on the "culture" of the particular ARTC Center involved, as well as factors such as terrain hazards specific to the particular geographic location involved.

Note that the related answer Does an SVFR clearance extend to Echo surface extensions? quotes a 2010 letter from the FAA ATO Western Service Center stating that their opinion is that the FARs do not permit SVFR to be authorized in E4 surface-level Class E "extensions".  The original query that provoked the answer was apparently specific to KSTC, an airport where all the surface-level controlled airspace reverts to Class G airspace when the tower is closed, so all requests for SVFR would always be directed to the tower, and ARTC would not be used to receiving any requests for SVFR for any of the airspace associated with that airport during the hours that the tower was closed.  Yet the letter is phrased more broadly and indicates as a general principle that SVFR should not be permitted in any E4 surface-level Class E "extensions".
Clearly, not all ARTC facilities follow this guidance.

See these other related ASE questions and answers--
Does FAR 91.155c apply to class E surface extensions?
What indication has the FAA given that phrases like "surface area of Class E airspace designated for an airport" do or don't include E4 "extensions"?
What are the historical precedents of today's E2 and E4 airspace? (US)
Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization?


What are the requirements to convert an FAA certificate to EASA/JAA? If a pilot holding a US certificate wanted to convert it to an equivalent EASA (JAA?) certificate, what training, flight experience, and exams are necessary to obtain equivalency?

Does the process differ for private, instrument, commercial, ATP, and instructor ratings?

As @Pondlife states, this will depend on each member state's implementation of the EASA rules but I would expect that the implementations would be similar -- otherwise what is the point of EASA?  (That would be a good question except it would no doubt be closed as being too "opinion based".)

I've added a link to the relevant UK documentation and marked this as a Community Wiki answer so that hopefully others can expand on this for other states and perhaps add personal experiences.

UK

CAP 804 Section 4, Part Q deals with "Validation and Conversion of Non-EASA licences issued by States other than the UK".


How do pilots rest on long-haul flights? When flying on a long-haul airliner flight in economy, often I find it very hard to fall asleep. However, when off cockpit duties, they still have to get rest so that they are able to control the aircraft without being exhausted and collapsing on the controls when they are on duty. Are there are any specific methods of helping pilots to gain the rest required on board an aircraft? Have there been any studies about the effectiveness of these?

Since a picture speaks for a thousand words...

These are often called "coffins" or "sarcophagus" around here... (Portugal)

Source


Why are the flaperons on the 787 located so far in-board? Why are the flaperons positioned in between the outboard and inboard flaps (see 2nd picture)? Why not have them located as far outboard as possible to achieve maximum roll authority? I would think the order would be more efficient if the main flaps were both inboard, followed by the flaperons, followed by the ailerons. Thoughts?

In a previous post "How does the particular piece of flap behind an engine on a B777 work?", Peter Kampf explains the flaps would have a gap b/w them when extended because they extend perpendicular to the  hinge line and the trailing edge is non co-linear. However, the 737 tackles this problem just fine (see 1st picture) so I don't think it fully captures why the flaperon is placed where it is on the 787 and 777.

Quoting the answer @fooot linked to:

  At high speed, the outboard ailerons are locked and all roll control is achieved by deflecting the inboard ailerons (and spoilers, if needed). This helps to reduce wing torsion and twist.

Yes, roll authority will be larger once the force is applied further away from the fuselage (/ roll axis), but the cost at which this comes (wing twist) is larger than that benefit.


What cargo airplanes can transport fully assembled large turbofan engines? Is there a big demand for transporting overseas for turbofan engines with nacelles like TrentXWB or GEnx by cargo airplanes ? If yes, what planes are used?

Engine nacelles are part of the aircraft and not the engine. I think you're asking about fully assembled engines.

Oversized jet engines are usually transported by road when possible; in case air transport is required, the Antonov An-124 'Ruslan' is your best bet. It is the only aircraft capable of transporting engines like the Engine Alliance GP7000 and RR Trent XWB.

Image from antonov.com

For smaller engines (or partially disassembled ones), other aircraft like the Boeing 747F and A330-200F are used.

Interestingly, this filing with US DoT shows the An-124 being used to transport GE GEnx engines from Rickenbacker airport to Seattle to ensure smooth production in Boeing factories.


Are an aircraft's nuts and bolts generally either all Imperial or all Metric, or can they be mixed? This is a low-tech question, but I was wondering about the distribution of imperial vs metric hardware on aircraft.

When current commercial aircraft land at major airports, a mechanic will know which set of wrenches to grab. Still, I wonder is everything metric, or are some commercial aircraft still imperial? Could an engine or some subsystem use one kind of hardware on a plane that mostly uses the other?

What about at a more remote airport visited by a wider variety and vintage of aircraft? Would a mechanic need to go up and check before deciding which toolbox to use?

Found in this comment under this answer

Why not mixed? If the airframe is manufactured in a "metric" country but the engine is from the US, you will find metric nuts and bolts behind the firewall (and on the engine mount) but Imperial on the engine.

If I remember correctly, the Robin DR-300 and DR-400 that towed me many times had just that mixture. Lycoming engine and French airframe.


Is the mach number of an aircraft measured with respect to stagnation speed of sound? I was reading chapter 8 of JD Anderson Aerodynamics when I came across something I find weird. I found out that there are different types of Mach number, based on what the speed of sound was taken as. Now my question is this, when we say "That plane is flying at Mach 1", is it with respect to the stagnation speed of sound, ie, (approximately 330 m/s), or is it with respect to the actual speed of sound?

The Mach number is defined with respect to the static speed of sound.
Note however that the Mach number is not measured by somehow measuring the TAS (True AirSpeed) and dividing it by the known speed of sound because this would require knowing the static speed of sound, which is temperature dependent. Instead, we derive the Mach number from the ram air pressure $p_0$ measured by the pitot tube and the static air pressure $p$ measured by the static port:

$$ M^2 = \frac{2}{\gamma - 1} \left[ \left( \frac{p_0}{p} \right)^{(\gamma - 1) / \gamma} - 1 \right] $$
[...]
The velocity can be determined from
$$ V^2 = a^2 M^2 = \frac{a_0^2 M^2}{1 + \frac{\gamma - 1}{2} M^2} $$
but this requires knowing either the static speed of sound $a$, or the stagnation speed of sound $a_o$.

(MIT Lecture Notes on Subsonic Pitot Tubes)
You can see in the velocity equation that the Mach number $ M = V / a $ is defined via the static speed of sound $a$ instead of the stagnation speed of sound $a_0$.
The above equations are only valid for subsonic flow. If you want to determine the Mach number in supersonic flow, you have to solve the following equation iteratively:
$$ M \approx 0.88128485 \sqrt{\left( \frac{p_0}{p} + 1 \right) \left( 1 - \frac{1}{7 M^2} \right)^{2.5}} $$
(Wikipedia - Mach Number)