Why does taxiing produce large lateral loads on the 747's engine pylons? While reading through a bunch of old AARs recently, I found (in the context of a 1993 747 in-flight engine separation) this interesting tidbit:

  [...]  Boeing engineers report that the modifications will increase the pylon's vertical and longitudinal strength. However, the modification will provide a slight, if any, increase in the structure's lateral load-carrying strength. Additionally, it was provided by Boeing engineers that the greatest lateral loads on the pylons normally occur during taxiing.  [Page 40 of the PDF of the aforelinked AAR, numbered as page 33; emphasis added.]

I'm having some trouble seeing how taxiing would be capable of generating any significant lateral loads on a 747's engine pylons; assuming that one is not attempting to drift one's 747, nor taking it excessively close behind another jet running its engines up for takeoff, taxiing it at a reasonable taxi speed should not produce any large lateral accelerations that would make the engines want to swing on their pylons, nor any significant lateral airloads that would tend to push the engines sideways relative to the wing.

Intuitively, one would expect that the situations placing the highest lateral loads on the engine pylons of a 747 (or, for that matter, any jet with engines mounted on vertical pylons1) to be those involving one or both of the following:

Large, abrupt accelerations from side to side (such as during severe turbulence, as experienced by [for instance] the aforementioned 747 whose engine separated in midair).
Flight at large sideslip angles (in which case the direction of the airflow experienced by the engines and pylons would be out of line with the longitudinal axis of said engines and pylons, generating considerable, though steady, lateral airloads on the engine-pylon assemblies).

So why would normal taxiing produce such large lateral loads on the 747's engine pylons?

1: For a jet with horizontal engine pylons (such as a DC-9 or CRJ-1000), one would instead expect the highest shear loads2 placed on the pylons to be experienced during abrupt pitch changes, high-angle-of-attack flight, large vertical gusts, or (in the mostly-hypothetical case of a jet with wingtip-mounted engines) high-roll-rate manoeuvring.

2: The loads that try to snap the pylon in two, rather than move the engine longitudinally or stretch/compress it out of/into the pylon.

The slip-skid ball indicates the ratio of "sideways" apparent acceleration to "upwards" apparent acceleration, in the aircraft's own reference frame.  In normal constant-airspeed wings-level flight (i.e. not looping or pulling out of a dive), or while taxiing, the "upwards" apparent acceleration is 1 G, so the sideways deflection of the slip-skid ball is a direct indication of the total lateral force acting on the aircraft and the total lateral acceleration experienced by the aircraft.

Turning at a high rate while taxiing will displace the slip-skid ball far to one side.  During high-speed taxi, and the portion of the takeoff and landing where the wheels are on the ground, steering corrections will typically involve low turn rates but may still cause a large displacement of the slip-skid ball due to the high speed involved.  In either case, the large displacement of the skip-skid ball indicates that a strong lateral force is being transmitted to the aircraft, including to the engines through the engine pylons. The fundamental origin of this lateral force is the sideload generated by the tires.

But we can't assume that in actual flight, a similar displacement of the slip-skid ball indicates that a similar sideload is being exerted on the engine pylons.

In actual flight, during a sideslip, the tires are no longer involved.  Instead, the sideways airflow interacts with the various surfaces of the aircraft to generate the aerodynamic sideforce that displaces the slip-slid ball.

Therefore, in flight, the lateral inertial loading created by the mass of the engine whenever the slip-skid ball is displaced, is at least partly offset by the aerodynamic sideforce generated by the engine nacelle.

Consider an aircraft banked to the left, with the slip-skid ball displaced to the left, showing that the airflow is blowing from left to right across the aircraft.  (A "yaw string" would blow toward the right.)  Which is generating more aerodynamic sideforce per unit mass-- the engine nacelles (including the engines contained within), or the rest of the aircraft?

If the former, the nacelles will be forced toward the right relative to the rest of the aircraft.  If the latter, the nacelles will be forced toward the left relative to the rest of the aircraft.  If the nacelles are generating exactly the same aerodynamic sideforce per unit mass as the rest of the aircraft, then the nacelles will not be forced either to the left or the right relative to the rest of the aircraft, so the pylons will not experience any lateral force at all.

Regardless of which of these three scenarios actually apply, we can be sure that unless the total G-load is quite high, for a given displacement of the slip-slid ball the lateral load on the engine pylon is higher if the aircraft is taxiing than in actual flight.


Would there be any advantages to a wing with a concave upper surface? A wing with a concave upper surface could have the same area as one with the normal convex shape of an aerofoil.

I can think of many potential disadvantages: additional complexity of design and construction, reduced volume for fuel tanks, having rain fill up rather than run off the the wing.

Could there be any advantages to such a design? Has it ever been tried, or studied?

For clarity, this is the kind of cross-section I have in mind. Yes, it's ugly, and I don't think that better draughtsmanship would improve it.

The idea behind this design seems to be that the principal cause of lift
of a wing is that the top has a larger area than the bottom, so if we
substitute a top surface with a different shape but the same area, we will
still get similar lift.

This is the idea behind the equal-transit-time theory of lift,
which is incorrect, disagreeing in several ways with
with actual observations of air flowing past airfoils and the
lift forces that result.
NASA has produced a basic but thorough refutation of this theory;
among the points they make are

  The lift predicted by the "Equal Transit" theory is much less than the observed lift, because the velocity is too low. The actual velocity over the top of an airfoil is much faster than that predicted by the "Longer Path" theory and particles moving over the top arrive at the trailing edge before particles moving under the airfoil.

and

  There are modern, low-drag airfoils which produce lift on which the bottom surface is actually longer than the top.

What you generally want in an airfoil is for it to establish streamlines
that carry the air at very high speed over the top and shoot it downward
past the trailing edge. The speed is not determined by the surface area
or length along the top surface; the speed is determined by other things,
and two air molecules that start near each other but go on different sides
of the wing (one on top, one on the bottom) get to the trailing edge when their respective speeds carry them there, generally not at the same time as each other.

Putting a big hollow space on top of the wing
where the air can pile up and then forcing it over a second hump to get
to the trailing edge seems counterproductive.
It might just lead to the wing stalling at relatively low angles of attack,
which would limit the amount of lift you can get from it.
But I am not an airfoil designer, and I don't know exactly what the result would be.


The Mig 21 and the Mirage III have been criticized for lack of range, is this a limitation of the wing configuration? Mig 21
Range: 1,470 km (910 mi; 790 nmi) at 10,000 m (32,800 ft) with 2 × K-13A missiles and 800 l (180 imp gal; 210 US gal)
(Wikipeda)

J-7
Combat radius: 850 km (459 nmi, 528 mi) (air superiority, two AAMs and three drop tanks)

JF-17
Combat radius   1 350 km

The J-7 and JF-17 are developments of the Mig 21, why is their range greater?

No. Their range limitation stems from their original concept: relatively cheap simple lightweight agile fighter (MiG-21) or interceptor (Mirage III).

There are plenty of long(er)-range aircraft with delta wing (if that's what you mean). Concorde, for one. B-58. Eurofighter.

For a given class of aircraft, range is primarily a function of fuel quantity and therefore (almost directly) of the maximum takeoff weight. JF-17 is 35% heavier and so has significantly greater range. (It also helps that it and its engine are two generations newer). J-7 is practically the same as MiG-21 and has very similar performance.


Can I land a helicopter anywhere? If I own a helicopter and am appropriately rated, can I land the helicopter anywhere that isn't private property, and is a safe place to land (clear of obstructions, appropriate surface, etc.)?

My question is specifically about the regulatory requirements to land a helicopter off-airport in the United States.

Per §91.119 of the FARs, yes. That said, you can't just find an open field-there may be local/state regulations regarding helicopter operations.

14 CFR 91.119:

  (1) A helicopter may be operated at less than the minimums
  prescribed in paragraph (b) or (c) of this section, provided each
  person operating the helicopter complies with any routes or altitudes
  specifically prescribed for helicopters by the FAA


What was the takeoff run distance of a Fw 200 Condor bomber/transport? The Fw 200 Condor was a large plane of Germany during WW2. It was used in recon and bomber roles against convoys. Wikipedia lists several specs including max takeoff weight of 24.5 tons and 4 engines producing 895 kW each.

But I could not find takeoff run distance, neither there nor in google. I would like to know how long of a runway was needed for it.

Unfortunately i could not find this information for the basic variants, but for the Fw200F an Focke Wulf document from 1943 states following

on a paved runway, 4x 323R engines on standard start power

********** (deleted / see graph)

on max power

   ********** (deleted / see graph)

EDIT:


What is the difference between VFR Flyways, Corridors and Transition Routes? What is the difference between:

VFR Flyways
VFR Corridors
VFR Transition Routes

I can find a Flyway chart on the reverse side of the TAC chart and also Transition Routes in the TAC chart. But where can I find a VFR Corridor chart? I Googled it, but there’s nothing.

VFR Flyways

These are the most loosely defined, and can be found on a TAC (Terminal Area Chart) as shown below. They suggest general routes and altitudes near busy airspace to keep VFR traffic out of restrictive airspace. This is one answer to the question of how pilots know if they are inside controlled airspace or not.

VFR Corridors

A corridor is a section of airspace that has been cut out of more restrictive airspace, like Class B, to allow VFR traffic to transit without explicit clearance. There will often be suggested altitudes and frequencies to prevent conflicts. Below you can see that there is a hole in the KSAN class B from 3300 to 4700 ft AMSL.

VFR Transition Routes

These are also found on the TAC as shown below and are charted routes for VFR aircraft to fly through more restrictive airspace. They will require contacting ATC, but provide a route for pilots to quickly request and then fly to avoid having to take a longer route around the area.

See also: Special Flight Rules Areas

Sometimes the basic provisions above aren't enough and the FAA defines a Special Flight Rules Area (SFRA). This defines special procedures and requirements for operating in the area. An example is the Los Angeles area where pilots are allowed to fly through the Class B without contacting ATC as long as they follow certain requirements.


ETOPS between Johannesburg, South Africa, and Perth, Australia South African Airways flies over 6 000km across the Indian Ocean from Johannesburg, South Africa, to Perth in Australia.

I don't see any airports along the way in the event they need to divert.

Does anyone know what would happen in an emergency?

PS Yes, yes, I know that ETOPS does NOT mean "Engines Turn Or Passengers Swim"!

ETOPS originally was an acronym for Extended Twin Operations. While it has been changed to just Extended Operations, the focus is still on twin engine aircraft. South African Airways currently operates the route with an A340-300, which has four engines. If it has an engine failure, it is not as much of an emergency as in a twin-engine aircraft, so it does not necessarily have to adhere to the same ETOPS rules for alternates.

Johannesburg to Perth is certainly a fairly remote route. But at least eastbound it's less than 10 hours, and, for example, the A350 is certified for 370 minute ETOPS. This is 6 hours, and while that is at a one-engine cruise speed, it's still fairly close to covering the entire route.

Looking at the Great Circle Mapper, a 240 minute ETOPS would be enough to fly the route with only a slight diversion from being direct. This is with 300kt diversion groundspeed. From the map, it appears that Mauritius and Cocos Island could serve as alternate towards the middle.


How can I use XFLR5 on a shape with no maximum thickness at one point? I want to run an analysis with XFLR5 on a shape that is not really airfoil shape since it has no maximum thickness at one point only, it is basically like the shape below:

But when I run an analysis, the program keeps showing that at all angles it does not converge. Could the problem has to do with the XFLR not knowing the maximum thickness and at what position? or is there another possible reason?
Here are the coordinates of the shape:
http://www.filedropper.com/elipse

XFLR5 uses XFOIL to perform airfoil analysis. From the User Guide:

  XFOIL is a (...) program for the design and analysis of subsonic isolated airfoils.

  The XFOIL code is not foolproof, and requires some level of aerodynamic expertise and common sense on the part of the user.

The most general equations to describe the motion of fluids are the Navier-Stokes equations. XFOIL doesn't solve the NS equations directly but uses panel methods instead. This makes it much faster than other CFD simulation tools at the cost of restricting the geometries it can analyze.

Normally an airfoil has the following properties:

Simple, smooth geometry
Smaller thickness than length
Sharp trailing edge

The problem with your shape is not the maximum thickness (a relative thickness of 20% is still OK) or the constant distance between the upper and lower side but the blunt trailing edge. To some extent the really large thickness at over 75% of the chord affects too.

Whitcomb's supercritical airfoil for example has a nearly constant thickness from 20 to 50% chord but can be analyzed without errors.

The panel method splits the airfoil in many small panels and tries to find a circulation for each one of them so that the air follows the airfoil geometry (normal speed to each panel equals zero). To get physically more realistic results other equations like the Kutta-condition have to be used. One of the assumptions for this equation is that only shapes with a sharp trailing edge can produce lift.

Trying to solve equations when the basic assumptions are not met normally doesn't work and that's why your calculations do not converge.

Note: I'm not an XFOIL power user and perhaps someone can tweak it to get good results, but I doubt it. While it is possible to analyze airfoils with a blunt trailing edge the shape above is very far away from being an airfoil.

TL;DR: XFLR5 can only analyze airfoils. Your shape has not a sharp trailing edge and it is therefore not an airfoil.


Why A320/330 use C* FBW control without speed stability? The A320 and A330 family are FBW aircraft that use C* longitudinal control laws for longitudinal sidestick command tracking. In essence, stick neutral commands 1G flight. When stick is deflected longitudinally, a blend of pitch rate and normal load factor is commanded to provide a consistent G-feel to the pilot; once the stick is released, 1G flight resumes, but the airplane is steady-state at the new pitch angle (or flight path angle to be more exact).

Unlike B777, B787 and A220, however, the A320/330 family do not have artificial speed stability. Once the aircraft is steady-state at the new pitch attitude, and with the autothrottle disengaged, the airspeed will settle to a new trim point. In another word, unlike a conventional aircraft, the A320/330 only has neutral speed stability.

This is why the envelope protections of the Airbus aircraft are not nice-to-have features, but essential features to ensure the airspeed does not rapidly bleed off to stall or increase to overspeed, all while the stick is neutral.

My question is, why is a C* control law without speed stability desirable for a transport category aircraft? The A320 does not need to perform precision target tracking as a fighter aircraft would, and it changes the flying characteristics drastically from those of a conventional aircraft.

Many accidents have shown over and over that once the handling becomes muscle memory, the cue that having to pull harder means you are losing speed is easily missed anyway, so it's not as important as it seems.

So the artificial neutral stability instead decouples the controls: stick for flight path angle, thrust levers or A/T for speed. Nice and easy and you don't have to fiddle with the trim button that most pilots end up pressing rather mindlessly.

Large aircraft tend to be flown with auto-thrust all the time anyway, including e.g. a 777 which is stable in pitch and has trim. And when they are, the logic is rather like Airbus—control column for flight path angle, maintain flight path angle when released, just after changing speed you have to fiddle with the trim to get it stable again, which Airbus Normal Law does for you.

Note that aerodynamically all Airbus aircraft are still longitudinally stable, so if the sensors fail and the system degrades to Direct Law, the trim behaves like in any other aircraft.


What is the most accurate geometric plane for referencing terrestrial flight attitude? I have found various definitions for attitude.

Merriam Webster: inclination of the three principal axes of an airplane in flight to the relative wind.
Wikipedia - Attitude Indicator: the orientation of the aircraft relative to Earth's horizon.
In my words, based on my current understanding: the angular orientation of the longitudinal and lateral axes relative to a 2D geometric plane, which is orthogonal to the Earth's average curvature at the point on the surface below the aircraft.

What is the correct reference plane for flight attitude in regards to terrestrial aerodynamic flight?

There are a few definitions because there are a few ways to look at it.

The most general thing to say would be something along the lines of,

  An object's angles in pitch, roll and yaw from a set of chosen zero planes

It boils down to what you want to use as your point(s) of reference. Attitude indicators use the horizon and if you really want to talk about it even space ships have gimbals that measure attitude relative to a plane. Your definition assumes only attitudes of aircraft flying over the earth (using the surface as a reference plane) which is valid for most cases but not all.

Furthermore, attitude indicators are referenced to the horizon because it is an easy to find and generally a universal level plane with respect to the Earth's surface. In aircraft flight we care about the Earth's surface because that is where we want to get back to and subsequently not smack into in the process, so its convenient to reference to its plane. Furthermore at high altitudes the horizon is still the horizon; it simply becomes a space/Earth horizon as opposed to a sky/ground horizon.

Here is the view out of the SR-71's cockpit (@73,000ft, this is also the highest-flying non-experimental plane ever made) to illustrate the point.

(source)

To extend the frame of reference explanation, we can look at the Apollo rockets FDAI, which is a nice example of a 3-axis attitude indicator that was referenced to multiple planes instead of a single horizon. In this case the flight attitude was simply the angle from a chosen set of zero planes.

  The first one is called "Inertial Stellar". That means that the reference is a 3D system of coordinates which is fix relative to the stars. That does not mean that there's only one system of reference. In fact, there is a system of reference for almost every phase of the mission. Some are reused during the flight and. in order to be accessed multiple times, they are stored in the computer as "REFSMMAT", which stands for REference For Stable Member Matrix.

-- EDIT --

Since the root question has changed somewhat ill add to my answer. Generally speaking airplanes are oriented to the gravitational force below which is taken to be the "vertical". The vertical is often taken to be the tangent of the ground beneath it but since the ground may also not be level this is a generalization. There is a bit of real world discussion involved here since attitude indicators, the point of reference for an aircrafts attitude may not always be perfectly level to the ground. A pilot is mainly concerned with setting his 0 degree point with a pitch that will yield flight that does not increase or decrease in altitude. This is called "straight and level flight". In reality, due to the design of aircrafts this may in fact be a few degrees of pitch away from true 0 on the reference plane. Power settings will also effect this angle.


Do pilots who have color deficient eyesight see light gun signals differently? Can pilots with color deficient eyesight expect to see the red, green, and white light-gun signals as such? In other words, if the controller is giving them a green light, will they see green? If they are being given a white light, will they see white?

It depends on the type and severity of the color deficiency and on the individual.

When I first applied for a 2nd class medical, I couldn't pass the color test. I took a practical test with an FAA inspector to get a waiver. We walked outside the FSDO office, and he had me identify the color of the lights being directed at me by the tower. Fortunately, I had been warned to have the tower do this for me on a previous day so I wouldn't be surprised. In my case I had to remember that if I saw yellow, that was white. If I saw white, that was green. Red was red.

When I first applied for a 1st class medical, I again had to take a practical test, but this time from an airplane. I noticed that from the airplane, the white of the green signal was more white, and the yellow of the white signal was more yellow.

Whether the difference from the ground test was due to the increased distance or to my individual daily variation I don't know. I have noticed during physicals through the years that my performance on the color test varies greatly.

In my opinion every pilot should have the tower shoot light-gun signals at him during training so that she/he can see how they look.


Do I need to call Clearance Delivery for IFR Practices when taking off from a D airport? I have the following question:
If I am going to practice IFR approaches under VFR, do I need to contact clearance delivery for that or it's enough to contact ground and state my intentions?
I would deffinetly contact clearance delivery on a C airport, but I am not so sure about a D airport. For sure I will contact clearance delivery if the practices were under IFR with a filled IFR plan.

When flying VFR from a Class D airport, there is no need to contact clearance delivery to practice IFR approaches. You may, on the other hand, have to call clearance delivery if that is the local procedure for all departures at that airport. This is not very common at a Class D airport. If a clearance is necessary for VFR flights at a Class D airport, most often it is given through ground control. The US Chart Supplement and/or the ATIS will indicate if it is otherwise.
Normally, a simple indication to ground control of your intentions will suffice. This lets them know how to prepare for your return with the rest of their traffic flow and provide separation. This would be a demonstration of common and professional courtesy. It is not mandatory.
You can request a practice approach once airborne and inbound back to the airport without notifying ATC before your departure. As a matter of fact, it is very common to request a practice approach at any class of airport at a distant destination when making your initial call and position report prior to entering the airspace. Be prepared to be denied the approach if the airport is extremely busy during VMC.


is boundary-layer jet-turbine a scam? While exploring open jet turbine designs, I've stumbled upon one CAD drawing which has made me wonder.

It is based on a famous Tesla turbine and its highly-efficient way of compressing the fluid.

At first glance the concept seems to make sense, but there should be a reason why it hasn't been explored by the industry? (the only alternative exploration I've found is a cold jet design, which is also a subject to a "scam" tag).

In short, they are both scams that fall heavily into the category of Not Even Wrong or cargo cult science.Tesla's work is sadly a great favourite of conspiracy theorists who do not have the technical background to adequately understand why these devices do not function as intended.
Your "Cold Jet" link should scream "I do not know what I am talking about" not only because of the broken, rambling sentences (although admittedly thats not exactly a rare quality amongst many actual professors) but also because of the over reliance of handwaving any problems away to just an "on board compressed air source"; without any mention of exactly how this on board compressed air source will itself be powered.
As to the tesla gas turbine itself as pictured, just because some guy can poorly model something in Solidworks and take a screenshot of it does not make it a viable design, nor does simply claiming "it has a high efficiency!" make it so.
The disc shaped turbine design apparently works on the basis of the boundary layer viscosity. This is a real effect and to say that boundary layers have been studied intensively is like saying that Everest is larger than a grain of sand.
In this sense the turbine works, very much like an automotive plate torque converter with air instead of hydraulic fluid. It may be an "efficient" design (although I have not been able to find any kind of source that gives any source of figures, accurate/reliable comparison or indeed methodology; and it has never been in any single one of my undergraduate or graduate textbooks) but the torque it is capable of producing is minimal due to fluid shear.
Any kind of small deviation in the surface will lead to turbulence which, on the scale of the disc separation will cause a complete destruction of the boundary layer on which this device depends; which makes this an incredibly temperamental device probably only useful for a single speed which is no good for any practical application (apart from any other flaws it has).
Note that I have concentrated solely on the turbine aspect of this - again, I have not been able to find anything remotely approaching a scientific analysis of this concept with applications as a tesla compressor. Simply having a bunch of discs revolving in close proximity does not a compressor make. You might manage to raise the static pressure of the fluid in close proximity to the rotating elements by a few Pascals solely because of the velocity effect, but you will not experience anything like the compression needed to sustain efficient combustion.
In both cases (compressor and turbine) the close spacing of the plates of the devices will paradoxically probably rob them of any claimed "efficiency" due to their very model of operation (fluid shear) and the boundary layer drag that results. This will also severely limit the mass flow rate through any such device; which in turn severely limits the practical application.
You are better off looking elsewhere. Even model radio control jet aircraft use a conventional miniature centrifugal compressor wheel taken out of a super/turbocharger; with an two-stage axial turbine to provide power.


How would flow angle change along the length of a propeller airfoil? Let's assume I have:

windspeed of 15 m/s
blade length of 10 m
rotational speed of one revolution per second

Can someone please help me calculating how the flow angle would change along the length of an airfoil? I have been trying to do the process for the last couple of days but I don't know how to do it.

In order to explain the situation, I have drawn a couple of pictures.
Consider a propeller rotating with the rotational velocity $\omega$ (in $\frac{rad}{s}$), which is flying at a velocity of $u$ (in $\frac{m}{s}$).

(Please excuse my crude drawing skills...). If we cut the propeller at four places along the radius $r$, we get the following velocity triangles (at the respective positions 1, 2, 3 and 4). Note that in the following we look at the propeller from the tip in direction of the hub:

Note that the tangential velocity at each crosssection rises, however the forward velocity stays the same!
The tangential velocity can be simply calculated by the factor $\omega \cdot r$.
Now everything which is left to do is to calculate the angle $\alpha$, which can be simply done by taking the $atan()$ function (btw, I believe that Fern got this point wrong)
$$\alpha = atan(\frac{u}{\omega \cdot r})$$
Therefore in your specific case with $n=1 \frac{rev}{s}$ we get:
$$\alpha = atan(\frac{15}{2\cdot \pi \cdot r})$$
with $r$ going from 0 to 10 meters (in order to remain practical from 0.1 to 10 meters)


Why are the boundary layer suction holes placed far forward from the engine? Edit: I know that the suction holes reduce the boundary layer; I am wondering if there is a reason why they are placed far ahead of the engine’s intake. My thinking is that the long duct past the holes will form its’ own boundary layer, hence reducing the effectiveness of the holes.

Looking at the picture of the Eurofighter inlet, the suction holes for the boundary layer are located far forward from where the engine actually starts.

Source

Another example is the SR-71 intake - the suction holes are located in the middle of the spike which is also located far ahead from the engine:

Source

Why are the suction holes not located closer to the engine?

Surely, the long duct aft of the suction holes will develop its' own boundary layer - which is what the suction holes were reducing in the first place?

Ideally, the engine inlet supplies the engine with appropriate air-flow across the operating range of the aircraft while incurring a minimal loss in total pressure. Under supersonic operation, shocks form near the inlet lip. The shock wave boundary layer interaction (SWBLI) thickens the boundary layer around the shock, develops an adverse pressure gradient, and increases total pressure loss. Boundary layer suction is used to prevent further pressure losses from flow separation due to the adverse pressure gradient. So, suction removes low-energy air to help control SWBLI near the lip and subsequently improve inlet pressure recovery.

From (Titchener 2013) specifically relevant to the SR-71:

  In inlets where there are multiple SWBLIs distributed suction is often employed. Fukuda et al. (1977) suggest that a major reason for this lies in the complexity of these inlets which leads to a heavily iterative bleed system design. In such a case, a model with a large number of distributed holes that can be easily switched on and off helps to lead the design to optimal solution more quickly. Consequently, the complex mixed compression inlet system on the SR-71 (see Smeltzer et al. (1975)) and the Boeing/NASA SST inlet (see Tjonneland (1971)) both utilize substantial areas of distributed suction. As do the F-4, F-14, and F-15 which have multiple compression ramps on which distributed suction was deemed necessary.

The following are good references for further reading (included the thesis since the first link requires access to AIAA papers):

R.K. Scharnhorst, An Overview of Military Aircraft Supersonic Inlet Aerodynamics, AIAA 2012-0013, 2012.
N. Titchener, An Experimental Investigation of Flow Control for Supersonic Inlets, Ph.D. thesis, University of Cambridge, 2013.


Is the seat-belt sign activation when a pilot goes to the lavatory standard procedure? Yesterday, I took an inter-European commercial flight with a popular budget airline. Before take-off, the captain announced that the First Officer would be taking the entire flight today.

During the middle of the approximately 2 hour cruise, the seat-belt sign came on. The flight attendant, who was next to me at the time, quickly turned to look at the front of the cabin.

The captain came out of the cockpit, a flight attendant took his place and locked the door behind him, he spoke over the PA system and asked everyone to fasten their seat-belts. A couple of people stayed up and he firmly told them, again over the PA, to sit down and fasten their seat belts.

Despite largely overcoming my fear of flying recently, I was slightly alarmed at this point, so I asked the flight attendant if we were expecting some turbulence. He told me that no, we weren't and that the seat-belt was only because the captain needed to use the restroom.

Sure enough the captain used the restroom, came out, re-entered the cockpit and a few minutes later turned the light off.

My question -  is this standard procedure or is it likely the flight attendant was giving me the 'passenger friendly' version of the story? I have since wondered if it was the FOs first 'solo' flight and if that might have been related.

In the US they do very similar theater when a pilot needs to relieve himself. There is an announcement that nobody is allowed to come forward, and grim looking flight attendants are blocking the aisles with coffee trolleys. The details surely depend on the airline.

Each of these procedures goes back to some bad incident. The cockpit doors are locked since 9/11. Since Germanwings 9525 (PIC on toilet, co-pilot had a death wish, cockpit door locked), a flight attendant has to replace the pilot in the cockpit. I'm not sure if there was an incident calling for body-guarding a pilot outside of the cockpit, but maybe someone can comment.

Even though it is much more likely to get killed by your angry spouse or just falling down the stairs at home, security procedure theater is in fashion, and it is just getting worse and worse. Yes, it is very plausible that the seat-belt sign activation in your flight was standard procedure to ensure that the PIC could pee in peace.

This photo of Thomas Cook Airlines (UK) is courtesy of TripAdvisor

Some similar reports found in the depth of the internet:

boards.ie in August 2013:

  Noticed this last night on a Ryanair flight.

  Internal phone call to stewardess was then followed by the fasten seat
  belt sign going on. We were forced to sit down with a by then crying
  baby (who had been just about dozing off being rocked whilst standing
  up) Once we were all sat down cockpit door opens, one of the pilots
  exits and was replaced by an air hostess in the cockpit. He visits the
  loo, comes back out, rings in, door opens, swaps with the air hostess
  and then the lights go back off. Not a hint of turbulence during the
  time period involved. [...]

flyertalk.com in August 2008:

  In the middle of a flight today from LAX-ORD, the seat belt sign was
  turned on and the pilot or co-pilot came out of the cockpit to use the
  bathroom. As soon as the pilot went back into the cockpit, the seat
  belt sign was turned off. It was perfectly smooth throughout this
  portion of the flight, so it was obvious that the seat belt sign was
  turned on just for the pilot to use the facilities. [...]


What, if any, equipment on a commercial airliner is (allowed to be) inoperative most of the time? Since airliners are bound by their minimum equipment list, I was wondering, which parts/systems that are not on the minimum equipment list are often inoperative on normal commercial flights.

If a system goes south while sitting at the gate or the ramp, that brings on a cockpit indication, the crew will break out the MEL/CDL using whatever protocol or procedure they follow and if it's not covered, they are grounded until the system is made serviceable again and will take a significant dispatch delay, or worse, a cancellation.

The OEM will try to get as many items covered under the MEL as possible and will continuously try to expand it, because it has a big effect on overall dispatch reliability.  An unreliable system system or subsystem that has a lot of failures will just be maintenance cost issue (and possibly minor delays) if it has MEL coverage, but if it doesn't, it becomes a cost plus dispatch delay/cancellation problem, much more expensive.


What would happen if someone covered a small hole in the fuselage of a plane with a soft body part? If I punched a hole of the size of a small coin into the fuselage of a plane, and then covered it with the palm of my hand, what would it happen to my hand?

I assume the hole has been properly deburred, otherwise you could suffer cuts on the sharp edges of the hole.

What happens else depends on altitude and flight speed.

At low level, the outside temperature should be tolerable and the pressure difference between inside and outside should be small, so your hand will be fine.

At higher altitude, outside air temperature can drop to -60°C rsp. -76°F. Since air density is lower, the aircraft will travel fast (approx. 250 m/s rsp. 560 Mph in case of an airliner), and convective cooling at the hole will soon cause hypothermia and frost bites. Also, inside pressure is 0.6 bar rsp 8.8 lb/sq-in higher than outside pressure, and the exposed skin on your palm will feel this as a sucking sensation.

I further assume that you need to remove all cabin insulation to put your hand over that hole, so your whole hand will be exposed to the airplane's metal skin which will have the same temperature as outside air. If the hand is dry and does not freeze to the metal skin, you might manage to keep it in place for a few seconds, but it will hurt. Keep it in contact with the metal skin longer and you will suffer progressive frostbite. The part of the skin which covers the hole should suffer less, since convective cooling should be less intense than the thermal conductivity of the metal skin, which will quickly pull all body heat from your hand where it touches the metal.

The pressure difference will not damage your hand, but the low temperature will.


Why are row numbers 10 and 11 skipped on Frontier's A-319? I'm sitting behind an exit row (the exit row is row 12), but am still not sure why rows 10-11 are missing from this plane. Note the number goes from 9 to 12.

Why are the rows numbered this way? It's the same on both sides.

Airline row numbering has more to do with fleet consistency than sequential numbering in one airplane type.

Head over to SeatGuru and look up Frontier. They fly 3 types of mid-size narrowbody - A319, A320, Embraer E190 - and not surprisingly row 12 is an exit row in each one. The E190 and A319 don't have row 11, only the A319 is missing row 10.

It's the same reason 2+2 seating skips seats B and E ( AC DF ), where 3+3 seating goes all the way across ( ABC DEF ). Widebody aircraft have a different sequence because it doesn't need to match - the same cabin crew will rarely work both major classes and they don't tend to run regional airlines on widebody (long-haul) routes.

Look at Air Canada's fleet - row 18 is the beginning of economy and an exit row in almost the entire widebody fleet (the exception is probably historical).


What does "S-ILS" signify in an FAA NOTAM? Consider this NOTAM for LAX from the current list:

  FDC 5/0341 LAX IAP LOS ANGELES INTL LOS ANGELES CA. ILS OR LOC RWY 25L AMDT 12C... S-ILS 25L DA 395/ HAT 291 ALL CATS VISIBILITY ALL CATS RVR 2200. ADD NOTE FOR INOPERATIVE ALSF-2 INCREASE S-ILS 25L VISIBILITY ALL CATS TO RVR 4500. EXCEPT WHEN ADVISED BY ATC THAT EQUIPMENT IS DOWN. CONSTRUCTION EQUIPMENT 178 MSL BEGINNING 1399 FEET NORTHEAST OF RWY 25L. 1502052152-1508042152EST

What does S-ILS25L signify, how is it different than just ILS25L, and where can I find the source document?

The S is for the straight-in portion of that approach (this is explained in the Abbreviations section of the Terminal Procedures Publication (See here - it's in the Abbreviations section of "Legends and General Information" at the bottom).  In this case the decision altitudes, height above thresholds, and weather minimums have changed.

Check out the approach plate and at the bottom you'll see S-ILS 15, S-LOC/DME 15, CIRCLING, etc.


Why maneuverability and top speed are still issues in 5th generation air superiority fighter jets In a previous question someone asked why do modern fighter jet speeds decrease compared to older fighter jets, and someone hinted that nowadays top speed is not a factor considering modern rockets which will anyway exceed the maximum G forces that a human body can withstand.

Now, in the F-22 vs T-50 comparisons many hint at superior maneuverability and top speeds of the Russian T-50 design.

Shouldn't modern fighter jets be all about BVR (beyond visual range) capabilities and thus speeds and maneuverability be "surpassed".

I imagine modern fighter jets to be basically bricks in the air with a lot of high tech long distance warfare technology.

Fighter jets are not bricks with powerful radar and long range missiles. BVR combat is an extremely complex affair conceptually, operationally and technologically.

For BVR combat to be initiated, the enemy aircraft should first be identified and sorted as a threat. However, assuming that both have similar radars, the inverse square law ($S \ \propto \ \frac{1}{R^{2}}$) means that you are exposing yourself to the enemy before identifying him. Combat between two stealth aircraft (F-22 vs T-50) is not unlike two men with a gun and flashlight in a dark room, as someone noted. The first one to switch on the radar is actually at a disadvantage and against a highly trained enemy, possibly suicidal. Also, let's not forget SAM ARM missiles that can home on the aircraft radars.
Though the number of aircraft shot down by radar guided missiles (as opposed to IR missiles, which are usually fired WVR) have risen significantly both in numbers and also as a percentage (of total aircraft shot down), the probability of achieving a kill by a BVR missile is still uncomfortably low. For example, the following table shows the ratio of 'kills' by radar guided missiles post WWII.

Table from Promise and Reality: Beyond Visual Range (BVR) Air-To-Air Combat by Lt Col Patrick Higby, USAF

The high percentage of BVR kills in Gulf war is basically due to two factors- AWACS and IFF(Non-Cooperative Target Recognition (NCTR) system), which gave sufficient confidence to permit BVR shots. It is highly unlikely that such a superiority level and confidence is available in case of war between two peers.
Another thing is that barely any of the BVR shots are fired head on due to the high closing speeds. So in all probability, you have to maneuver yourself in a position where a tail (or an above) shot is possible.
Also, if some missile is fired at you, speed and maneuverability are critical to carry out evasive maneuvers.
Most of the combat aircraft use in present day has nothing to do about air superiority. They are mostly used as precision guided bomb trucks. Optimizing fighters for BVR combat sacrificing other parameters is, at the least, foolhardy.

So far, the most important contributors to BVR victories (in Gulf War, for example) are not due to some inherently superior technology, but due to human factors- pilot skill and opponent's weakness. It is potentially catastrophic to generalize the doctrine against a near-peer opponent.


Why has the A340 not been fitted with only three engines? The A340 was created because the airlines wanted an aircraft they could cross the atlantic ocean with (They couldn't with their twins because of ETOPS so far). So Airbus upgraded the A330 and added two engines. Because of the four engines, the plane was now allowed to cross the ocean.

However, four engines instead of two increase fuel cost dramatically (I assume). And the A340 is the same size as the A330. The A330 could fly with 2 engines. So why hasn't Airbus built the A340 with three engines instead of four. The fuel cost could have been reduced significantly without loosing the permission to cross the atlantic ocean. For example the MD-11 had three engines and also had permission to cross the ocean.

Because the third engine would need to be near on centerline. That means in the tail.

Putting the engine in the tail significantly changes its design requirements.

Supporting an engine requires a lot of structural support. Then you need to run fuel to it. The effective height of the rudder is decreased. You probably need to switch over to a T-tail. The center of mass is also shifted which influences where the wings need to go.

In short, adding a single engine requires a redesign of the entire tail. While adding an engine to the wings is much simpler.


Why do super sonic fighter jets have narrow waist? Some of the supersonic fighter jets (such as F-4 Phantom and F-5) have narrow waist on their fuselage.

Why do these fighter jets have narrow waist?

It is likely due to application of the Area rule, which helps to reduce drag due to shock waves in transonic and supersonic flight.

It basically is an attempt to keep the cross-sectional area of the craft from varying rapidly as you move along the length.  So at a point where the wings area is increasing a lot, the area of the fuselage may be reduced to compensate.

This reduction of area is mainly intended when these airplanes fly at transonic speeds (0.8-1.2) but not when flying at supersonice speeds where a front shock wave is created.


What are the ACARS frequencies and what is their usage? I have been playing with a SDR dongle. I have been able to decode some ACARS signals using SDR# and acarsd. I have noticed in my area (35 miles west of DFW airport) there are a number of frequencies used. I have been able to decode transmissions on 130.450, 131.125, and 131.475. Are there other common frequencies? What determines the frequency used by a particular plane? I find it odd that I can pick up an AA flight 30 miles away and yet when a similar flight passes directly overhead, I am unable to find their ACARS on any of those frequencies.

According to the specs ACARS supports direct to satellite linking so planes flying over you may not be broadcasting on one of those frequencies. Similarly, if broadcasting over VHF you are line of sight limited. At high altitudes you may see ranges up to 200 Nautical Miles however if the planes are low overhead they may not be broadcasting as they are out of sight of the possible ground station. Your dongle may also filter/ignore low power signals or it simply cant pick up certain signals due to their strength.

Here is a list of common ACARS Frequencies.


What are the costs involved with getting a 2 seat ultralight? About 7 years ago I got my private pilots cert but I let my medical cert lapse.

Now that life is more settled I'd like to get back into flying but I want something that I can keep at home. I was thinking of a two seat ultralight with folding wings.

My questions are:

What do I need to do to get back into the flight seat
What are some good options for a 2 seat folding wing ultralight that has a few hour range
What are some hidden costs of owning a new or used ultralight

What do I need to do to get back into the flight seat

Legally, not much. With the recent third class medical reform you dont even need to get a medical. You will need to get a bi-annual flight review to get current as well as your takeoff/landing requirements for passenger carrying as per FAR 61.57. While your bi-annual only needs to be an hour of ground and an hour of flight, if its been 7 years it will be well worth your time (and possibly an insurance requirement) to spend some more time on both.

  What are some good options for a 2 seat folding wing ultralight that
  has a few hour range

Dont know much in this space so I wont comment

  What are some hidden costs of owning a new or used ultralight

There is one guy on the field I fly out of who has one of the moving wing, snowmobile looking ultra lights. He keeps it in the back of another guys hanger and takes the wing off every time he stores it. With that in mind last time I spoke to him he said it takes some time to get the wing on and off and he needs to plan for that when he flys. If I remember correctly he bought his new or close to new and it was in the space of 75K which frankly is quite a bit considering you can get a really nice airplane for way less than that. The wing is fabric/composite so outside storage is a no go. You can hop around on nice days with it but its VFR only which depending on your goals can be a limiting factor.


How could Airbus better tailor the A380 to the modern market? I understand any company do things that give net profit. But, had not manufacturing newer engines that increase its Endurance and Range saved it from its decline?

Airbus had designed the A380 to carry as many people as possible so that airlines would be able to fly fewer flights. Instead of flying multiple flights between two cities, airlines could save money by flying just one A380 flight and cramming as many passengers into one plane.
What that would've meant, however, was fewer flights and less flexibility for travellers who don't want to rely on just one flight per day. Very few airlines used the model as intended and the A380 came too late as Boeing was already knee-deep in the 787's development by the time the A380 took its first flight.

Source
Though, bigger aircraft need more ground handling times and management. But with the oncoming innovations, it would have been decreased in future.
For example,
The Wheeltug
Video

As the electric power is used, it also reduces required staffs, cuts down time and Taxi fuel too.
And that may help the airlines too increase the number of flights.

The FlexSys Morphing Wing
Video

As the weight of hydraulics are removed here. This can also lead to higher fuel economy.

Hydrogen Fuel Cell Concept
Video1  Video2

It is nice that the Airbus is itself working on this concept for commercial airliners.
And it would had been nice too if it was implemented to A380 later too.

Reducing the wing span by folding wings on ground for smaller airports. (Though mentioned earlier) Improving engines for shorter Takeoff and Noise Reduction, etc. could help the airliners to reach more destinations with A380.
I want to know the practical feasibilty of introducing an A380neo. And how my above mentioned innoventions fail to be implied in A380? Also, could idling the outer engines thrust help in reducing the wake while landing or gliding, that eventually reduce the safety regulations and requrements for a "Super" aircraft (by this, I meant, will it lead to decrease the seperation required specially for Bigger Aircrafts)?

Plain and simple, to meet current market requirements, Airbus can do pretty much nothing to make the 380 more "current".
The technical improvements suggested do nothing to change the fact that the 380 is too big, that is, it has too many seats, thus not allowing enough flexibility in this market situation.
In addition to not addressing the main problem, they would need major re-design, except for the wheeltug, which on the other hand is questionable in it's economics: save 20 minutes on ground, haul extra weight for thousands of miles...
There is no sense in technologically further developing a model that has a major "flaw" in it's economics. There are no customers, or not enough at least, for 380neo.
PS: markets are fluid. It may very well be that in furure super jumbos are coming back, but at the moment it does not look like that. If SJ make a comeback, it'll prolly be with a clean sheet design.
PPS: idling outer engines does pretty much nothing for wake turbulence. Wake is "caused" by wings, not engines.


Can you become a pilot with an Asperger syndrome diagnosis in Europe? I have the diagnosis results for Asperger syndrome (scored 177 out of 76 required to get the diagnosis). Are there any regulations in Europe that can stop me from achieving my life goal: To fly commercial jets?

My autism makes me very strongly interested in aviation, programming and audio design. But aviation is absolutely on top and has been since I was 6. The others have come as I age.

It might be a little off-track: So I'll add this little story just to add some "meat" to the question:

I just recently went on a trip with a few of my co-workers that included a trip on a B737-800. They thought it was special that I filmed the takeoff and landing, took pictures of it at the gate, and viewed out the window the entire two hours trip (my neck still hurts). They also had a funny reaction when I explained why the airflow to the cabin (packs) suddenly went silent as the engines start up. There is nothing more that I enjoy than flying, and I feel like I really belong up there (that poor kid in the seat in-front of me had a very different opinion).

I really wanted to see the cockpit during these flights. But asking is hard for me with Asperger. It kind of makes me feel like a 4-year-old.

It might not count for much in the real world, but I have logged over 16,000 landings on my little home-simulator. Three of them are intended crashes just to test the simulation.

All I ever wanted since I was a kid was flying commercial jets.

In order to fly commercial you will need a first class medical certificate, and getting that with aspergers is going to be tough. The first consideration is the safety or people in the air and on the ground, so the criteria is conservative. Getting a first class medical would mean lots of extra hoops, even if it is possible. If you did get a medical you would then be competing against candidates who are neuro-typical, and would have to convince the airlines that you would be able to perform your job as well as non-aspergers candidates. That's not to say it's impossible, you would want to talk to a few aviation medical examiners about it to see what they say. If you can get a medical then you've got a chance, and if you are motivated enough and willing to work for it then I believe where there's a will there's a way.

You need to consider your own temperament when going for a pilots license of any kind, whether aspergers or neuro-typical. Flying is not just about the technical details, you need to be able to work with people, first of all your instructor! In commercial flying you also need to cooperate with other pilots and work with passengers, ground crew, and cabin staff. The extreme focus on detail and depth of knowledge on aviation you might develop with aspergers can be a huge asset in some ways, however aspergers could make it very challenging to work cooperatively in the cockpit, deal with troublesome passengers, and make quick and effective decisions. Think about some scenarios:

You are FO on a flight, the captain makes a decision you do not agree with
A passenger is angry after the flight was delayed and is shouting abuse at you
You are the captain of a flight that is low on fuel and had developed an emergency. You need to very quickly opt for continuing to the destination airport or diverting, and there are problems with either choice

Do you think you would be able to successfully deal with these types of situations in the time frame you would have to work with? Not everyone is cut out for that sort of thing, whether aspergers or neuro-typical, these are scenarios all potential commercial pilots would need to be able to deal with.

I think first you should figure out whether you like flying airplanes or not as a love of aviation and airplanes does not automatically mean you will love being at the controls. Why not go for your private license and find out? It's the first step in getting an ATPL anyway, and you could probably get a third class medical. You can then build on that if you want a career in aviation, or keep flying privately if you don't.


Can a someone with just a PPL log PIC time on a non-transport commercial flight? I have a job that requires me to travel on a small company plane with a company "commercial"pilot. I only have my PPL. If I am certified in the type of aircraft may the pilot allow me to fly "solely manipulate the controls" of the aircraft, while he clearly remains the PIC and responsible for the flight? If I am rated in the aircraft could I log those hours?

Yes, you can log PIC in an aircraft category and class that you are certified to fly. You can log that time during which you are the sole manipulator of the controls. This is the case even if another crew member (the commercial pilot) is the acting PIC.

See 14 CFR 61.51 (e) for this and other situations allowing logging of PIC.

Following is the relevant section of 61.51 (e):

  (e) Logging pilot-in-command flight time. (1) A sport, recreational,
  private, commercial, or airline transport pilot may log pilot in
  command flight time for flights-

  (i) When the pilot is the sole manipulator of the controls of an
  aircraft for which the pilot is rated, or has sport pilot privileges
  for that category and class of aircraft, if the aircraft class rating
  is appropriate;


Flight plan departure procedures vs real life departure procedures In the flight plans there are included departure procedures. However, sometimes something can change eg. different runway is assigned by ATC and then departure procedures in the flight plan is invalid, because SIDs are assigned to the runways.

Are flight plan amendments send after such change?
Does it happen often?
How airline dispatcher knows which departure procedure to fill in the flight plan if he doesn't know which runway will be used in the future or what will be the wind direction etc.

1) Not to my knowledge, these are short-term deviations from plan that do not require an updated plan (similar to ATC offering direct routings - shortcuts).

2) Not sure. What qualifies as “often”?

3) Experience, local and airport procedure knowledge and a weather (wind!) forecast go great lengths in predicting departure runways and routings.


What do manufacturers do to reduce cabin noise? It is being claimed by the Airbus that their new planes offer more sound proofing than those of Boeing, and I myself observed the same. How do airplane manufacturers tackle with the problem of soundproofing the cabin, so as to reduce the noise reaching inside from outside the plane (the engine sounds and the free stream noise)? What proportion of the total aircraft development cost would this be (approximately, say for an Airbus 320 and for Boeing 777)?

it is being claimed by the Airbus that their new planes offer more sound proofing than those of Boeing

I guess you mean this sort of claim:

  No matter where they sit, all passengers benefit from the quiet A380 cabin with on average half the sound energy of other aircraft. Passengers' feedback confirms the extraordinary contribution to their general well-being due to the absence of background noise on the A380.

  In market research surveys the A320 is regularly rated higher than its rivals for the low level of noise in its cabin. Effective insulation in the cabin is one reason why passengers say they find the A320 a quiet aircraft in which to fly. The A320 cabin is the subject of continuous improvements and in its latest version, the noise has been reduced by a further 1dB.

  how do Airplane manufacturers tackle the problem of soundproofing the cabin

There are a variety of methods

soundproofing materials in the wall of the fuselage between inner and outer skins.
choice of interior materials and furnishings etc.
adjusting cabin interior contours to allow lower airflow velocity.
fuselage aerodynamics.
engine nacelle accoustic liners.
scalloped edges on engine exhaust nozzle.
larger slower engine fans.
sharklets.
etc

BBC report

  One of the main sources of noise is wind. So during the design phase of modern aircraft, computational tools model the aerodynamics of the aeroplane to highlight areas of high airflow that are likely to increase cabin noise. This allows engineers to reshape their designs, says Alan Pardoe, director of product marketing at Airbus. Such techniques, he says, helped the double-decker Airbus A380 plane to be recognised by the UK Noise Abatement Society as the quietest jet airliner on the market.

Airbus say

  When it comes to making the A380 one of the quietest aircraft in the sky, it's the small details that count... in fact, more than 3 million of them!

  This is the number of tiny perforations in acoustic linings on the large nacelles that surround and protect the A380’s jet engines.  The acoustic material helps dampen noise created by the airliner’s four turbofan powerplants, ensuring the A380 is a friendly neighbour wherever it operates in the world.
  The A380’s nacelles are supplied by Aircelle, which has worked closely with Airbus in minimising the jetliner’s noise output during takeoff and landing.  Acoustic panels are applied throughout the engine – from the air inlet to the exhaust nozzle.

  Each panel, which is produced from composite material or titanium, is designed to attenuate noise at different frequencies, further improving the soundproofing effect.  Noise-reducing innovations include Aircelle’s 2-degree-of-freedom acoustic panels, which use a stack of two honeycomb composite sandwich layers separated by a porous wall.

  These panels dampen high-frequency noise from the engine turbines.  With much of this noise source successfully dampened by the acoustic treatment, engineers and designers are now focusing their attention on treating other sources of aircraft noise, promising even quieter jetliners for the future.

and also this

  The Airbus-developed Zero-Splice engine nacelle liner was designed to reduce the noise generated by the jet engines’ large fan, which represents almost half the noise during both approach and take-off. Following extensive studies carried out by Airbus on how to reduce fan noise, it was found that splices used to join the engine nacelle inlet’s acoustic panels generated forward fan noise – which was counter-efficient to the liner’s acoustic treatment. Airbus therefore developed the Zero-Splice nacelle liner as a single 360 deg. piece of composite, which is comparable in size to the diameter of an A320 fuselage.

It is, I think, obvious that measures that reduce engine noise on the ground also reduce it in the cabin.

They also write about how wing sharklets reduce noise.

I'd guess there are many other measures and design choices that are introduced to reduce noise.

  What proportion of the total aircraft development cost ...

I'd be surprised if Airbus or Boeing make public any breakdown of their development costs.

You can find general breakdowns like the following but I havent found any that break out soundproofing development costs.


Why are aircraft registration details made public? Recently, a friend asked me why aircraft registration details are made public since related car license plate numbers details are more regulated in access by law. Me, being an avgeek and taking this for granted, do not know why this is. Why are aircraft registration details made public?

Aircraft registration are no more or less public than car registrations (although they are a bit easier to get access to). Generally speaking you can go down to the DMV and request data on a plate number, for example here is the form for doing it in PA (there is a fee involved). Since cars are registered by state its up to the states how this works, planes however are registered federally and thus governed a bit differently, the FAA provides a nice portal here for looking things up.

From the FAA...

  In accordance with the underlying statutory framework (49 USC Chapter
  441) and as described in the applicable SORN, the information
  maintained in the aircraft record is available to the public upon
  request. This is to facilitate aviation safety, security, and
  commerce, the main purpose for which the record system exists. The
  public nature of the aircraft record allows for title searches by
  prospective buyers and/or financiers of aircraft purchases as well as
  for other interested parties to gather information concerning the
  aircraft including its airworthiness. The existence of recorded liens
  against the aircraft as well as the priority of those liens can be
  determined by a careful review of the documents maintained in the
  aircraft record.

This is done for a few reasons, first off many things in aviation may be billed by tail number. For example if you fly into an airport, say KPNE which has a city imposed 5$ landing fee, and the FBO for some reason does not collect this. The airport needs a way to look you up to send a bill. The FAA may also do this to make it easier for non-federal employees (local police etc) to access information related to an aircraft.

For what its worth the FAA also lists pilot data pretty freely, however you can now elect to hide your personal data (but not your name)in the FAA portal.


Why are contra-rotating jet engines so rare? Why are contra-rotating jet engines so rare? Such a jet engine would have contra-rotating turbines, or contra-rotating compressors, or both.

In fact I cannot think of even one aircraft that has this. If some prop-engines have contra-rotators, I don't see why some jets don't have it either.

It seems to me the contra-rotating parts would be more efficient in extracting power or compressing air. There are already contra-rotating turboprobs (Tu-95), and turboprops are very similar to jet engines, so I think it is not a question of technical knowledge. Another benefit would be reduced or eliminated gyroscopic forces hampering yaw or pitch torque, which I will ask another question about.

BTW, I am considering axial-flow jet engines only. I totally forgot there used to be centrifugally compressed jet engines, which have all but disappeared.

EDIT: The question is about jets (either turbofans or turbojets), but I mentioned the Tu-95 and its turboprop engines because turboprop spools are very similar to jet engines. However, I thought those Tu-95 engines had contra-rotating everything, but it turned out only the propellers were contra-rotating, not the turbines. Nevertheless, if there was a turboprop with contra-rotating spools, I would very much like to hear about it here because I think it would have great bearing on the question of contra-rotating jet engines and their efficiency.

If the low-speed and high-speed spool rotate in the same direction, the relative speed between them is low. Contra-rotating means you need to double the individual speeds to arrive at the relative speed, and this will increase bearing losses greatly. Both friction and wear will go up with little benefit in regular operation. A notable exception is the Rolls-Royce Pegasus engine for the Harrier jump jet.

Stators between the rotating stages ensure that air flow inside the engine is mainly in axial direction, so aerodynamically a different rotation direction between two spools will make little difference. However, contra-rotation will indeed make the stator between the last stage of the high- and the first stage of the low-pressure turbine obsolete. The video linked by @Moo cites a 10% reduction in part count as the advantage. Additionally, the first stage of the low-pressure turbine should show improved efficiency (like the second propeller of a contra-rotating propeller).


Could a plane land inside or on top of another plane in flight? I'm sorry if this is a little fantastical, but I'm just wondering if this type of maneuver is possible in any way. I understand if this question gets pulled. I'm trying to make a little action-animation-cgi and just wondering if this was ever a last-resort option for maybe two planes, maybe one is a bit larger than the other but maybe similar speeds could be maintained for the one plane that is running out of fuel could latch-on somehow.

The airforce used to have Parasite Fighters that could be launched and then recovered by a larger aircraft.

Parasite Fighter

The Space Shuttle was routinely transported on the back of two specially equipped B747s.

Shuttle Carrier Aircraft


Is taxiing a small aircraft on a residential street legal? I live with the back of my house to a Runway that runs north to south almost perfectly.  I live on the west side of the runway, Across the street from me on the further west most side of the road lives my neighbor who owns runway rights,(his house is completely unconnected to the runway) which from my understanding grants him parking privileges at a designated area on the runway itself. does owning runway parking rights make it legal for him to taxi his plane all 200 yards down the residential road in between my house and his to park his plane in his yard?

It doesn't matter whether or not the road is residential, it depends more on public or private. If the road is a public road, taxiing an aircraft will usually not conform to state motor vehicle regulations without a pilot car and a permit. In rural areas where nobody cares, it doesn't matter. When there are enough people around somebody will object, and then it does matter.

An airpark preserves community access to a runway via residential roads by making them private, and not subject to state or local motor vehicle law. Rules are enforced by conditions on titles to property, or by reference on the title that the property is subject to rules made by an HOA or other organization of the sort. Look at your title.

I think it unlikely you bought into an airpark without knowing it, but if your street signs look like this, taxiing on the streets is probably legal.


Why are the tips of the flap track fairings painted red? The fairings are painted red at the tips, look:

Source: Airliners.net

I have seen that on many new airplanes, not only the A330. So why are they painted red?

Elaborating on Peter Kämpf's comment: The flashy tips are for preventing accidents during ground handling operations, e.g. by belt loaders. This can be a painting or an adhesive tape (possibly reflective, useful at night).

According the pages referenced below, this is done for the A320 family aircraft for which the flap track canoe fairings are not low enough to be obvious, not high enough to not cause a problem.

It seems this is a company decision not required by regulation. Several airlines do it.

Variations on the painting (taping):

Only the fairings close to the locations where ground handling personnel works.
Only the right side of the aircraft.
Also the winglet.
Color used: Red, bright pink, reflective green.

Photo Taha Ashoori on Airliners.net (source)

Photo Vin Lane-Kieltyka on Flickr (source)

(Source)

References (non authoritative):

Air Canada Flap Track Fairing Speed Tape? (Airliners.net).
Why Do Airliners Paint Trailing Edge Flap Fairing (Airliners.net).
Colour On Flap Track Fairings (Airliners.net).


Why do many commercial aircraft have a red circle on the wing? Many commercial aircraft operated by japanese airlines like JAL and ANA have a red circle on the wing. Look:

Source: Airliners.net
And on this ANA plane, also:

Source: Airliners.net
Why do they have that red circle there?

That would be the Japanese sun symbol which also appears on that nation's flag. You can also see that symbol on the tail of that JAL 777 on top.

Same as the Japanese Flag:


Approach when weather condition is below minimum If airport weather condition is below minimum, but pilot report that flight weather condition is above minimum, Does controller can issue approach clearance to pilot?
If weather condition get worse than published minimum, should pilot cancel his approach and execute missed approach even in final approach phase?

You haven't told us what country you are referring to.  I am going to talk about the United States regulations.

There are two separate regulations that come into play.  Part 91 and Part 121 or 135.

Under Part 91.  The pilots are allowed to commence any approach (irrespective of weather) and determine if they can continue to land based on the criteria listed in §91.175(c).

Under Part 121 / 135.  The pilots can only commence an approach (proceed past the final approach fix) if the latest weather received is above landing minimums.  Mainly, pilots are concerned with visibility.  They will not be able to use "flight visibility" as the weather visibility is most likely different as the airplane descends towards the ground.  The pilots must use the weather reported from the airport.

To answer the second question, if the pilots are past the final approach fix and new weather arrives indicating the airport is below minimums, the pilots are allowed to continue the approach to minimums to see if they can land per §91.175.


Why a parachute for aerobatics only when carrying a passenger? Per FAR 91.307:

  Unless each occupant of the aircraft is wearing an approved
  parachute, no pilot of a civil aircraft carrying any person (other
  than a crewmember) may execute any intentional maneuver that
  exceeds...

So if I'm flying aerobatics solo, I'm not required to have a parachute. But if I have a passenger, both are required to have one.

What is the rationale for that?

I suppose that in something like a Super Decathlon, where the pilot must exit the plane before the passenger, it would be senseless to have one for the passenger but not the pilot. But is there anything more to this?

Well, as with many issues involving the FAA, the "why" is a bit of a mystery.  Perhaps they thought that the pilot can "lead by example" if a bail-out is needed.  I'd imagine that a lot of passengers would be a little reluctant to jump out of an airplane, but once they see the pilot go I can see them becoming highly motivated!

As far as not needing the parachute if only crew members are on board, the FAA has long held the view that they aren't protecting pilots from doing stupid things to themselves since they are knowledgeable and informed, but their regulations are to protect the "general public", or the passengers in this case, who don't know any better.


What is the effect of Mach number on efficiency, thrust and specific fuel consumption of a ramjet engine? My graphs show that overall efficiency and thrust first increase, reach a peak and then fall in the supersonic range (M=1-5) whereas SFC first drops and then increases rapidly near M 5.
Why does this happen?

The thrust of a ramjet depends on the dynamic pressure of the airframe since this determines how high the pressure inside the ramjet can become. Thrust grows therefore initially with the square of speed.
The efficiency of heat engines depends on the temperature and pressure ratio of their cycle, and again speed helps to improve that. The specific thermodynamic process of ramjets is the Joule cycle (in the US called Brayton cycle).
Why this doesn't grow indefinitely has to do with nonlinearities at high temperature. Once gas temperatures exceed 2000 to 4000K, Oxygen starts to dissociate, depending on pressure. Nitrogen follows beyond 8000K. Now the combustion heat will not be converted into a further temperature increase but will be wasted in the dissociation of air. As this limit is approached, less margin for heating is left and the SFC drops. The optimum with current technology is at approximately Mach 2.4.
A possible remedy is to slow down the air less, so the maximum heat inside the combustion chamber can be restricted. However, the flame propagation speed of conventional hydrocarbons puts a firm and low limit to that. The higher speed simply means that mixing, evaporation and burning will happen mostly once the air-fuel mixture has left the engine. Here you need to switch to hydrogen and a supersonic ram jet, but that will bring a host of new problems.


How similar/different are the roles performed by Military and Civil pilots? Broadly, Military and Civil are the only two categories in which the Pilots can be classified on the basis of the roles performed by them. What are the similarities and differences in tasks performed by them?

The biggest difference is that most everything in commercial aviation is considered administrative in the military.  The tactical employment of our aircraft, the non-administrative portions of flight, is what separates military sorties from commercial flights.  This emphasis on tactical flying has also led to significantly different cultures between commercial and military aviation.  There isn't a commercial pilot I would stand a chance against in a game of FAR/AIM trivia, but I also doubt they'd fare much better if the tables were turned and the questions were related to carrier operations.  We are two separate organizations with two unique mindsets to flying.


Why do hydraulic jack stalls result in pitch up and right roll? Quoting from page 162 of the book "Fatal traps for helicopter pilots" by Greg Whyte:

Hydraulic jack stall (servo transparency) in (AS-350) helicopter is an aerodynamic phenomenon can occur when the helicopter is flown outside it is normal flight envelope and subjected to positive maneuvering (g-loading).
It results in uncommanded aft and right cyclic and down-collective motion accompanied by pitch up and right roll of the helicopter.

My question:
What is reason for uncommanded aft and right cyclic collective down pitch up and right roll?

A moderately quick search reveals a few pprune threads and an accident report. From these I understood that:

The hydraulic jack stall occurs when the aerodynamic force on the rotor blades exceeds the force the hydraulic actuator can apply.

The actuators are controlling pitch of rotor blades and are applying force to keep the angle of attack high. When the angle of attack gets too high the blades stall which is associated with change in centre of lift creating a moment trying to rotate the blade to lower angle of attack, strong enough to overpower the actuator.

The right pitch moment is because the retreating blades have higher angle of attack and approach stall earlier.

The forward pitch moment is when the stall happens in higher G manoeuvre where the helicopter is pitching up and thus the blades in front are running at higher angle of attack and approach stall earlier. If the stall happens due to overspeed, the pitch moment will be back instead. In either case it will be opposite to the selected cyclic.

The down collective moment is because the angle of attack decreases on the forward (or backward) and right blades, but does not increase on the other ones, so the total lift is reduced.

The behaviour can be considered a good thing as it prevents aerodynamic stall from fully developing. The effect is also similar to what would happen if the actuators were more powerful and aerodynamic stall did fully develop. It can however be more surprising for the pilot since it may happen before vibrations that normally indicate incipient aerodynamic stall. The condition is self-correcting in that the reduction in pitch causes reduction in G loading and thus reduction of the aerodynamic forces, but the drop in performance can obviously be a problem if it happens too low.


Why do some U.S. airlines require passenger headrests to be lowered during takeoff? I've experienced a few times recently where cabin crew have asked passengers to lower their adjustable headrests during takeoff and landing and, in at least one case, it was even part of the pre-recorded safety briefing.

I'm wondering what on Earth would be the reason for this?

I completely understand needing to keep seatbacks and tray tables upright and aisle armrests lowered, but having headrests lowered makes no sense to me. As a tall person, having the headrest lowered is uncomfortable. For extended durations (e.g. a long wait to takeoff, etc.,) it becomes extremely uncomfortable.

Even more concerningly, though, it seems like this would significantly reduce safety in the event of a crash, due to forcing a tall passenger's spine into an unnaturally-forward position and having neither most of the spine nor the head supported. It seems like this would be much more likely to cause head/neck/spine injuries to the passenger during a crash than would allowing passengers to adjust their headrests to properly support their heads and necks (and not push their shoulders forward.)

I see that 14 CFR 121.311(e) requires seatbacks to be in the upright position during takeoff and landing, but I see nothing in the FARs about requiring adjustable headrests to be lowered for takeoff and landing during Part 121 operations, though I may have missed something.

So, my question is what is the reason for this requirement for headrests to be lowered? Is there some regulatory requirement for this or is it an airline-specific thing?

The answer appears to be one of clearing the view of the flight attendants. This FAA circular mentions the requirements that a FA be able to see clearly.

  (3)  Direct View.  As used in § 25.785(h)(2) [§ 25.785(h)(1)], "direct view" means direct (line of sight) visual contact with cabin area/main aisle(s), which enables the flight attendant to be made aware of passenger needs relative to safety when the flight attendant is seated with torso restraint (safety belt and shoulder harness) fastened.  Mirrors or other such devices are not acceptable equivalents to direct view, except in those cases where flight attendant proximity to the floor level emergency exit takes precedence over direct view.  Video systems may be an acceptable means of direct view, if the level of conspicuity is equivalent to that provided by line of sight visibility.

This forum thread talks about it with regards to FA regulations

  I have been told by many FA's that the reason for the headrests to be down is for FA visibility during takeoff and landing and taxi when they are in their seats. With them up they don't have a clear view of the cabin.


Is there a way to see the radar info ATC has? I want to track GA helicopter traffic coming and going from NYC. I know that most routes the helicopters fly require them to pick up a transponder code from ATC, which means they are in the system.

Is there a public feed of that ATC data?

EDIT:

Just to clarify, As far as I understand, sites like flightradar24.com and flightaware.com only track aircraft transmitting ADS-B out or with Mode-S transponders. I'm interested in tracking aircraft with Mode-C transponders that have picked up a unique transponder code in order to enter the class B.

If you are in or around NYC, flightradar24 will give you most, but not all, traffic including helos and ATC radar traffic.

flightradar24 most likely has several ADS-B receivers in that area that not only catch aircraft transponder/UAT output, but they also catch FAA ADS-B ground station broadcasts. The ground station will broadcast any known traffic (mode C radar traffic included) that ATC is aware of to any client aircraft that are in the area that have their ADS-B output configured to inform the ground station that they are capable of displaying traffic data. Most private GA aircraft with ADS-B are configured this way because there are iPad apps that will display it while airborne. The ground station will broadcast that traffic and a local flightradar24 receiver gets it too and up it goes on the website. The data may be limited to just aircraft in the vicinity of the client aircraft but in an area like NYC, there are typically many GA aircraft (clients) out there at any time that are configured as ADS-B in capable.


Sideslip vs Freestream Azimuth Angle Why is the sideslip angle, $\beta$, defined as

$$\beta=sin^{-1}\left(\frac{v}{|{\bf v}|} \right)$$

and not the same as freestream azimuth angle $\psi$

$$\psi=tan^{-1}\left(\frac{v}{u} \right)$$

where ${\bf v}=u \hat {\bf b}_x+v \hat {\bf b}_y+w \hat {\bf b}_z$ is the airspeed of the aircraft in body coordinates, $\hat {\bf b}$, and $|\cdot|$ is euclidean norm of $\bf v$. $\psi$ can be interpreted as the angle between $\hat {\bf b}_x$ and the projection of $\bf v$ onto the bodies $x$-$y$ plane. What is the physical interpretation of $\beta$?

Thanks in advance!

Dispensing with the math, because it makes my head hurt...

Azimuth refers to geographic direction relative to a compass reference point, so azimuth angle refers to the deviation between the body's longitudinal axis and its horizontal path parallel to the earth's surface (heading vs track along the ground).

Sideslip angle refers to the deviation, referenced along the lateral axis of the body (parallel to the wings in other words), between the body's longitudinal axis and the airflow, independently of direction relative to the earth.


Do fighter jets use elevators as ailerons? I saw jets in game using elevators to roll right and left, but I never saw them doing in real life. I was wondering if fighter jets actually use elevator to roll like they do in game.

Yes.  The airplane’s flight control computer can command a roll using all primary and secondary control surfaces, including differential actuation of the tailplane as Blue Angels #6 demonstrates during a preflight check.


What makes a bomber able to deliver nuclear bombs? Currently the German Luftwaffe has Panavia Tornados to deliver American nuclear bombs in case of war. The bombers are getting older and are currently modernized. The German Luftwaffe also has Eurofighter Typhoons which are said to not be able to deliver nuclear weapons. Why is that so? (Same goes for the F-35 and the F-22)

AFAIK all that's needed for an aircraft to deliver nuclear bombs is

The ability to actually carry the bombs, which probably conform to some standard "form factor".
Some special electronics for arming the bombs, which must be installed into the bomber - this probably also follows some standards.

Is that all? I seem to remember some special aerobatic maneuver the aircraft should be able to perform to allow the survival of the crew. Does that exist and is it a problem for modern fighter/bomber aircraft?

You are right in saying that's all that's needed, with the addition of a strategic or tactical need.

The need for a "special evasion" technique is only needed for aircraft dropping megaton range weapons in an air burst.  Kiloton range weapons could be dropped from altitude with no special evasion required except a lot of speed.  Sub-sonic aircraft would need to be "frisky" in their escape. Read about Tsar Bomba for a good example which demonstrates that even with the biggest bomb ever dropped, a relatively slow, lumbering aircraft can escape.

Even helicopters can drop nukes.

Most aircraft are only tasked to drop tactical, low yield weapons.  The real madness is assigned to subs and missiles.

As the comment above says, I suspect that most crews dropping strategic weapons will not care much whether they survive or not.


Airbus is to fly-by-wire as Boeing is to...? If the control system in an Airbus is fly-by-wire, then what is the control system in Boeings? How does it work? What are its general differences from the fly-by-wire?

Boeing uses a more direct approach to control (but both systems have warnings). In short it boils down to who has final authority of control surface actuation. Fly by wire does not just mean that control surfaces are controlled electronically (even the Cessna 172 has electric flaps) but that a computer in some way takes the pilot (or pilots) inputs and generates a control surface output. In the airbus case the computer has the final say, so if the pilot commands the plane to do something it should not, the computer will not move the control surfaces and allow it to enter such an attitude or speed. Boeing takes the reverse view and gives the pilot final authority so what ever control inputs are made translate to control surface movements.

With that in mind Boeing does have things like a yoke shaker in the event of approaching a stall and a yaw damper etc. Airbus will allow full control surface actuation with Direct Law however the plane is not normally in this mode. Boeings Standard protections and augmentations is similar but will allow the pilot to enter the conditions

  "...to inform the pilot that the command being given would put the
  aircraft outside of its normal operating envelope, but the ability to
  do so is not precluded."
  (source)

On the contrary vertical stick motion in an airbus in flight actually controls load factor and not necessarily pitch directly. So its always some form of proportion to the aircrafts speed.

If memory serves Boeing control surfaces are set up such that movement of one yoke will move the other yoke in tandem, likewise they can not be simultaneously moved in opposite directions while airbus controls can be moved in opposite direction and the computer will summate the inputs (although an override button is present on each stick to disconnect the others should something be improperly acting on it).


How high can I fly a model aircraft under 14 CFR 101 rules? For a UAS operated recreationally, may altitude exceed 400 AGL?  AMA rules state in several places that flight above 400 AGL should be avoided. And close to airports there is a more firmly worded restriction. But away from an airport, and presumably in Class G, what are the altitude restrictions?

This question presumes 14 CFR 101 applies.

Addendum to question: FAA appears to be attempting to impose a 400AGL restriction on 101 operations, without regulation. See the amended AC: https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-57A_Ch_1.pdf, stating: Model aircraft operators should follow best practices including limiting operations to 400 feet above ground level (AGL).

There's no specific altitude restriction for model aircraft anywhere in part 101 (that I can see). Even within 5 miles of an airport you only have to inform the tower, per 101.41(e):

  When flown within 5 miles of an airport, the operator of the aircraft
  provides the airport operator and the airport air traffic control
  tower (when an air traffic facility is located at the airport) with
  prior notice of the operation.

However, practically speaking there is an indirect altitude restriction in 101.1:

  For purposes of this part, a model aircraft is an unmanned aircraft
  that is:

  [...]

  (ii) Flown within visual line of sight of the person operating the
  aircraft

In other words, you have to be able to see the model aircraft and that limits the altitude and distance you can fly it at. It's also worth noting that 101.43 is very broadly written:

  §101.43   Endangering the safety of the National Airspace System.

  No person may operate model aircraft so as to endanger the safety of
  the national airspace system.

Arguably, that also implies you shouldn't fly any higher than you really need to.


What are some alternative benefits to electric propulsion? The context I am asking is regarding if electric motors could ease the fuel cost, take off weight, or runway length if they were set up in 'hybrid' configuration with regular hydrocarbon engines.

If so, could potentially a military aircraft (IE, F35) have any weight savings or feature benefit of having an electrically boosted STOVL/VTOL system? Any fuel savings over regular flight? And of course, would this make sense at all from an engineering standpoint, today or ten years from now?

I am not asking the ability of electrical propulsion systems to replace the current main prop or jet systems, but rather to the alternative capacity that they might offer in other auxiliary or backup uses.

For aircraft, none.

Hybrid drive trains are used because:

They offer, rather limited, efficiency improvement by being able to operate the engine closer to its optimal conditions.

Because spark-ignition engines are more efficient at higher power settings, the hybrid can get advantage by having smaller engine, running it at (relatively) high power or not at all and cover the variations in required power using the electric engine.

Cars accelerate and stop a lot and often cruise at small fraction of power, so they can make good use of this.

The advantage only applies to spark-ignition, i.e. gasoline, engines. Compression-ignition (diesel) engines have more uniform efficiency over their operating range, so there is no or only negligible advantage. Which is part of the reason you don't see hybrid diesel cars.
Batteries can be pre-charged with cheaper mains power. This is used in the hybrid cars, but it obviously requires lots of heavy batteries.
Electric motors can provide high torque from standstill. Piston engines can only provide limited torque from standstill and it stresses the clutch heavily.

This is particularly important for train engines. Train wheels have bearings lubricated by oil pumps attached to the axles that only operate when the train is moving, so when the train is stopped, the bearings are not lubricated and the force required to get it moving is much higher then when moving.
At high power outputs, electric generator and motor are simpler and more reliable than a gearbox and clutch. For big train engines and ships, mechanical clutch is out of question. Hydraulic one, which is a turbopump and turbine in oil, can be used, but that still needs variable gearbox. The electric drive-train acts as almost ideal continuously variable transmission, being able to deliver almost constant power over wide range or rotational speeds.

In either case the hybrid drive-train is heavier than mechanical one.

Now for aircraft:

The engine is run at constant speed and power setting, which is relatively high (usually cruise setting is 75% for piston and even higher for turbine planes). So there is no room to design a hybrid drive train to run the engine closer to optimal.
Weight is critical for aircraft, so the weight of electric generator and motor is a show-stopper there. For cars adding weight causes smaller increase in drag, so its less of a concern and trains even need to be heavy to have enough adhesion for their traction.
Since weight is critical, aircraft can't afford to take much batteries that could be pre-charged with cheaper mains electricity, the other benefit hybrid cars have.
Propellers need almost no force to start spinning and are operated at narrow range of RPM, so the ability of electric motor to provide torque from standstill and across wide range of RPM is of no use to aircraft.

So none of the benefits for which hybrid drive trains are used in other vehicles is applicable to aircraft, so there is no point in building one.


What is the difference between active and passive sidestick? What is difference between "active" and "passive" sidestick?

Which aircraft have active sidestick?

For references, I've used United States Patent Application 20080156939; BAE Systems Develops Civil Aircraft Active Control Stick; Active and passive side stick controllers in manual aircraft control; & the Side-stick wiki entry. The wiki is a general side-stick discussion listing (some) aircraft that have side-stick fly-by-wire (FBW) technology, but does not go into active vs. passive.

Generally speaking, if you have used a joystick on your PC to fly Microsoft's Flight Simulator, such as Microsoft's Force-Feedback Sidewinder Pro, you've used an active side-stick.

According to the Patent Application cited above, the patent is for an Active Side-Stick technology described as "pilot flight control stick haptic feedback mechanism provides variable force feedback to the pilot flight control stick." One of the 'issues' with the current passive side-stick technology (approved for use) used on aircraft such as Airbus' A380 is that there is no tactile feedback when using the side-sticks, such as a pilot would feel using an old-style column or yoke. In a Cessna 172, for instance, when you pull/push/turn the yoke, there is actual tactile feel, if you pull back strongly in-flight, you'll 'feel' the forces exerted against the airflow. Passive side-sticks have none of this 'feel'.

Active SS technology attempts to replicate the forces on the joystick.

Active SS technology is currently used on most miltary aircraft using side-sticks, but does not appear to have yet made its way into civilian airliners.

(If I've gotten stuff wrong, I'll remove/fix this answer, as I am not an airline pilot and have never had the opportunity to fly military hardware.)


How does a pilot aim for an aircraft carrier? This answer suggests the FPV is aimed ahead of the ship. But it doesn't make sense.

(Picture from linked answer.)

A runway running away at 25 knots, is the same as a stationary runway plus 25 knots headwind.

The relative ground speed is the same, i.e., the rate of closure, and the glide-slope is consistent. Shouldn't the FPV be targeted at the touchdown zone?

Perhaps a bit to starboard, toward the island bridge, because that runway is crabbed. What am I missing?

Hopefully this is a better view of what one should see on approach to a carrier.

Above is a typical textbook case approach to a carrier in an F/A-18E strike fighter. I chose a picture at dusk as it is a little easier to see all the symbology and optical lights more clearly.

Note the jet is aligned with the centerline of the landing area. The optical landing sight or 'ball' can be seen to the left of the landing area with the center amber light aligned with the green row of lights, indicating the jet is on a 3° glide-path and should allow it to touch down between the #2 and #3 cross deck pendants for a good 3-wire approach.

The flight path marker is centered in the 'E' bracket indicating an ideal Angle of Attack for the approach; this is also verified by the amber AoA indexer light to the left of the HUD.

The pilot will do a cyclical scan of the line up, ball and E-bracket, making corrections to the control inputs on each pass. In close, over the ramp, and in the wires this cycle shifts over to a focus on the ball, tracking it all the way to the deck such that touchdown should come as a surprise.