What experimental techniques should one employ during cruise flight to determine correct airspeed to fly for max range in a headwind or tailwind? Theory demonstrates that one ought to increase speed in an headwind, and decrease speed in a tailwind to achieve maximum range.  A lengthy discussion on the theoretical solution was answered, though not clearly, here.  It is unlikely that a pilot will have the Thrust Required chart on which to draw a tangent line through the x-axis intercept at the given wind speed.

What simple techniques should the pilot employ to determine the best speed to fly given specific environmental conditions (fixed headwind or tailwind)?

An old technique is "Half the Headwind, all the Tailwind".

So if you're flying into a headwind of 20kts, increase your cruise speed by 10kts. If it's a 20kt tailwind, decrease your Cruise Speed by 20kts (but only as far as BestEnduranceSpeed).


Did Air Europe fly to Toronto from Lamezia Terme? I am looking for more information regarding a flight from Lamezia Terme (SUF) to Toronto Pearson (YYZ) that according to some of the people I spoke to used to be operated by Air Europe around 1993.

I have researched wikipedia's Air Europe page and it looks like they have never even had an aircraft capable of servicing the 4,087 nm route.

Has any of you any idea of how I could retrieve more detailed information? For example, is there any way of knowing which airlines operated the route back in 1993?

The Wikipedia page you checked is for the British Air Europe. The Italian airline of the same name operated 12 Boeing 767's in the 90's.

The image below shows the Italian airline at Toronto in the mid-90's.

(airliners.net)

The 767-300ER shown above is capable of reaching the West Coast of the US from Italy.

(gcmap.com) The Boeing 767-300ER has a range of 5,980 NM.


How possible is a successful landing just with 1 wing? Having in mind this famous F-15 incident :

Would this be possible with any other fighter-jet? Or was there something really specific to F-15's design that allowed this ?
Would this be possible at all with any commercial aircraft ? Specifically for those that have their engine(s) mounted on the rear ? or what about for those that have engines mounted on the wings, but only with a partial wing loss (eg having both engines intact) ?

This would be possible with other fighter jets that are of similar design to the F-15, i.e. have a low aspect ratio, much of their lift produced close to the centerline, and a lot of control authority. They would also need the other two key ingredients for a landing of this kind: a strong aviator at the controls and a bit of luck.

Specific contributing features of the F-15 were its fuselage's lifting design, side-of-fuselage engine placement, blended wing with strakes, and large control surfaces, common to fighters.

If you look at the planform, it wasn't missing a full 50% of its wing, but only about 20% of its lift-contributing planform area (lift distribution isn't equal across it of course), with the body, the intakes and wing root strakes continuing to provide lift. As a result, the center of lift didn't move as far away from center as it would have for a tube-with-wings design.

In principle this is a more violent version of a runaway aileron scenario, and one has to compensate for roll from the lost wing area. The F-15, being a fighter, has large control surfaces for combat maneuvering, and they proved sufficient for stopping the roll in this case. The loss of lift was not so much of an issue.

The red box is the lost area, the green box the remaining planform, and the dashed line its very approximate center.

It's possible that other fighter aircraft could do something similar. Commercial aircraft, however, almost universally feature high aspect ratios, and with such a wing, the center of lift will move too far away from the design centerline in a similar scenario. Partial wing loss, however, can be compensated for, and there's a lot of incidents where planes landed having lost some part of their wing.


What is the standard for tachometer calibration / Hobb's equivalency? I've heard that 1 tach hour = 1 Hobb's hour at maximum RPM.  I've also heard that tach equivalency is at "cruise" RPM, although that seems vague.

I believe I've also heard that the accuracy of tachometers is regulated (I think to within 50 RPM).

Because engine time, and often TTSN, is measured by tach hour, there surely must be a strict standard for each aircraft for what RPM the tach counter should be set to.

What is the standard for tachometer calibration? Max RPM? Cruise RPM? Something else?

Hobbs time is most useful for tracking log time (and billable time in rentals.)

Tach time is simply a counter of revolutions of the engine, displayed as "hours."  Calibration is basically at normal cruise RPM, which can vary for different engine/prop/desired speed.  The tach counter is approximate at best, but it is what we have.


Categories of aircraft (A,B,C,D): Is it possible for an airplane to move into a different category? Categories are defined for the Vat (Speed at threshold) which is Vso multiplied by 1.3, or Vsldg multiplied by 1.23 .
Since these speeds are related to the weight of the aircraft is it possible for an aircraft to start the flight in one category but then move into another one?

The FAA's Instrument Procedures Handbook, Chapter 4 has a good explanation of this (emphasis mine):

  An airplane is certified in only one approach category, and although a
  faster approach may require higher category minimums to be used, an
  airplane cannot be flown to the minimums of a slower approach
  category. The certified approach category is permanent and independent
  of the changing conditions of day-to-day operations.

And:

  A pilot must use the minima corresponding to the category determined
  during certification or higher. Helicopters may use Category A minima.
  If it is necessary to operate at a speed in excess of the upper limit
  of the speed range for an aircraft’s category, the minimums for the
  higher category must be used. For example, an airplane that fits into
  Category B, but is circling to land at a speed of 145 knots, must use
  the approach Category D minimums. As an additional example, a Category
  A aircraft that is operating at 130 knots on a straight-in approach
  must use the approach Category C minimums.

In other words, the aircraft's "certified approach category" never changes. However, for operational reasons a pilot may use a higher category's minimums for an approach, but never a lower category's. One possible scenario would be if the flaps are inoperative and the pilot can't slow the aircraft to its normal approach speed.


Can jet engine ingest (suck in) concrete from runway? In the question Is there any emergency situation in which jet engines would not be shut down before passenger evacuation of a transport aircraft comment says:

  At full engine power, those vortices are quite capable of lifting something the size of a 1-foot cube of concrete from the ground into the engine!

Incredible! Did it really happened? I guess it did not ended up well for the engine. Any links/images?

I found this:

Can a human get sucked into a jet engine? - but concrete is denser than water (human body), so it would be harder to suck in
Can jet airplanes operate on 'rough' airfields? - mentions gravel, not concrete tiles.

Small, loose pieces, it sure could.  But those are small flakes which would fit in your hands.  Large chunks, it’s doubtful that the suction could lift them to ingest them off the runway.  Now there’s a chance such a piece could get kicked up by the nosewheel of an airplane during a ground roll and thrown into the intake of an engine.  Airports generally have surface crews which periodically do FOD sweeps of the runways and taxiways looking for debris like that.


How do airlines train their pilots to deal with the stress of delays? There are some good answers here dealing with boredom during long flights and with the physical stress of piloting, and with what pilots to do during a delay. I'm interested in how pilots cope with the mental stress of delays.

Consider - you're flying home, you've been sitting on the tarmac for three hours with no idea of when you're going to be cleared to take off, and your child has a birthday party you really want to make. Another five minutes and you're going to have head back to the terminal because of the mandatory rest period; the tower starts to give clearance to the planes in front of you but they seem to be taking an interminable time to complete their checklists.

These are the sort of things you face as a passenger; luckily for most of us they're pretty infrequent.

For a pilot, however, they must be quite regular, especially on certain routes. And the added complication is that the temptation to rush or cut corners could lead to disaster.

Do airlines have specific training on how to cope with the stress caused by  situations where they will find themselves under time-sensitive / delay pressures?

Robert Hays under pressure in Airplane!

(My answer is limited to carriers based in the USA, things can be very different in other regions, many countries pay monthly salary and culturally have different view of company expectation verses safety expectation)
Pilots are paid per hour of block time, they are getting paid for all that time waiting on the taxiway after leaving the gate and starting an engine. The flight plan will usually include an extra hour or two for flying to an alternate airport if the weather turns bad at the destination airport, this is not commonly used with modern forecasting but it is part of the plan. So generally a pilot should not have a strong financial motivation causing delay stress. If a poorly timed childrens party is causing distraction then my opinion is that the company should consider strongly encouraging this pilot find a less safety critical career.
Pilots in the USA in recent years do tend to get a lot of general stress-management guidance during training, even a fair bit at the private pilot level.(depending on the school) Often this training is lumped under labels like "Human Factors" or "crew resource management", and FAA materials even include specific advice for avoiding situations just as described in your question they call it "get-there-itis". The free FAA materials tend to be a bit superficial but better than nothing in calling attention to a possible problem, industry training courses on the other hand have some good depth. This type of training has been growing since the late 1960s, but it got a large boost in the early 1980s after the 1977 Tenerife disaster where a KLM captain apperently hot to make up for delays started takeoff without full clearance and slammed his 747 into a Pan-Am 747 taxiing in the fog, just at rotation speed, killing over 500 people.


How do large drones communicate with ATC? For large drones like the Reaper, X-47, and aircraft like that, how do the "pilots" communicate with ATC when they need to?

They have an onboard VHF radio that can be used to receive and transmit on local frequencies.

For example, on the MQ-1B Predator you will see a small fin on the back of the aircraft behind the GPS dome. That is an omni-directional antenna which can transmit VHF. The Global Hawk has a similar capability.


How can I calculate the drag coefficient for a given wing design? I am building an RC plane (for the first time. I am doing it for a competition) with a twin boom configuration like this:

My wing shape is like this:

I decided that the lift should be 0.8g and I need to use this equation to find the wing area (as per the instructions)

$$F_l=\frac{1}{2}\rho v^2 S C_d$$

$F_l=0.8$

$\rho=1.225$

$v=12m/s$

Can you help me in figuring out the value of $C_d$ considering that I am using Clark Y aerofoil design?

After that, I will measure $b$ using the aspect ratio.

$$\frac{b^2}{S}=\text{Aspect Ratio}$$

My instructor asked me to obtain a wing span of 80-100cm. So I will have to choose Aspect ratio as well.

You may visit http://airfoiltools.com/airfoil/details?airfoil=clarky-il
You can estimate the value of Cd from the plots. For different angles of attack, the Cd and Cl varies. So, you will have to select one based on your requirement.


Can a mid wing airplane have no wing spar running through the fuselage? Can a mid wing be attached to a fuselage such that no features of the wing run through the fuselage? (so that the pilot can seat comfortably and close to the center of gravity) Airplanes such as the mini max has its wings bolted to the sides. Does that mean there is no main spar running through the fuselage and connecting both wings?

If it is possible, can it be done on the cri cri by Michael Colomban? And what would be the necessary adjustments to the fuselage?

Yes it can, albeit at the cost of some added weight.

By using reinforced bulkheads, the bending loads can be transferred into the bulkheads which leaves the center section free of structure. That is how large engines and their intake ducts can be placed where normally the spar would go. A classical example is the Lockheed F-104 Starfighter.

Lockheed F-104 Starfighter cut-away drawing (picture source).

Using bracing like on the Mini-MAX reduces stresses in the carry-through, but does not remove the need for something to keep the wings apart. In other words, the compression member of the wing spar is still required to transverse the fuselage where the wings are attached. Also, at high angle of attack the wing produces substantial nose thrust which could fold the wings forward were it not for a second compression strut at the wing nose.

Mini-MAX 1100 (picture source)

In case of the Cri-Cri, which is a low wing design, the pilot sits on top of the spar. Moving the wing up to a mid position would create a much heavier aircraft and would require a wider fuselage, too.

 Colomban Cri-Cri (picture source)


How do the Airbus Beluga's wing/control surfaces deal with its high center of gravity? This is the Beluga:

It's used for carrying pieces of a fuselage (of much larger planes) from one place to another so they can be fitted together to make a plane.

As I recall, it's based on an Airbus A300, and then they added on the huge bulging fuselage piece so it could carry more stuff and moved the cockpit down a bit to make way for the huge door at the front of the bulge.

My question is what did they do in terms of wing/control surface design to keep the plane from becoming unstable.  Clearly when this plane is fully loaded the CG is much much higher than a regular A300.  Which seems like it would make it more likely to want to tip over in some fashion. What did they do to counter-act this?

As long as the aircraft flies without much sideslip, the vertical position of the center of gravity is not important.

It is a common misconception that the aircraft could "tip over" when banking due to a high location of the center of gravity, or even that dihedral would right the aircraft up because the lower wing will create more lift. This is all wrong!

To understand why, look where gravity is pointing in banking flight. By gravity I mean the total acceleration felt by the pilots and passengers, the apparent vertical direction.

A320 front view straight and banked with force vectors in the aircraft's frame of reference.

The force vectors in banked flight don't differ from those in level flight, only their magnitude grows. In both cases the apparent vertical is parallel to the vertical plane of symmetry and will go straight through the center of gravity, regardless of its vertical location. Therefore, it has no lever arm and produces no momentum around the center of gravity.

To put it another way, the aircraft banks in a coordinated turn to keep the apparent vertical in its vertical plane of symmetry, now that a horizontal force needs to be added to change the aircraft's angular momentum.

Therefore, neither the wing nor the control surfaces need any special adaptation to the huge fuselage. Only the vertical tail needs to be increased to compensate for the increased side area ahead of the center of gravity. In case of the Beluga, two vertical fins were added and the strake enlarged. This is a very common procedure for modifications of existing aircraft, because it is easier than designing a new vertical and improves handling compared to a new, scaled-up fin.

Side view of the Airbus Beluga (Picture source)

The fuselage has its aerodynamic center ahead of the center of gravity, so increasing its size will also increase its destabilizing yawing moment in a sideslip. This requires more stabilizing area at the rear, which can be kept relatively small since its aspect ratio is much higher than that of the fuselage, making it more responsive to sideslip angle changes. Both together will create a side force in sideslip which will be substantially bigger than that of an unmodified A300 in the same conditions. This sideslip-induced side force and the drag which comes with it will make the flying characteristics of the Beluga different to that of an A300. Since it acts quite a bit above the center of gravity, it will add a rolling moment, increasing the dihedral effect. I expect that it will be harder to build up a sideslip angle in the Beluga, and if the pilot crosses controls, the aircraft will slow down more quickly than a regular A300.

When designing the Beluga, it was clear from earlier planes (Super Guppy, 3M-T or BM-T Atlant) that the oversized cargo bay would not make flying impossible.

The 3M-T/BM-T modification for rocket stage airlifting. Picture source.


Why wasn't Solar Impulse 2 cleared for takeoff? I've recently been watching videos of the Solar Impulse 2 (HB-SIB) flights, and I noticed that on several occasions, instead of giving clearance for takeoff, the tower told the pilot that "departure is at your own risk".

In particular, this happened at Kalaeloa (PHJR) in Hawaii, at Dayton (KDAY) and at Lehigh Valley (KABE).

But, at Moffett Field (KNUQ), Tulsa (KTUL), and out of KJFK, the flight was "cleared for takeoff".

I couldn't spot anything obvious in the tower conversations to suggest why departure was at the pilot's own risk. Out of KDAY, the tower said that the runway the plane was using was closed (of course it's closed, there's an experimental solar airplane and ground crew on it), and at KABE the tower mentioned a jet that landed on the other runway a few minutes prior.

What happened here? Why was departure at the pilot's own risk?

Interesting question. The ATC orders mention only two reasons that controllers have to say "at your own risk":

The pilot "insists" on using a closed or unsafe runway (3-3-2)
A helicopter takeoff or landing uses a non-movement area (3-11-2, 3-11-6)

Obviously Solar Impulse 2 isn't a helicopter, so that leaves the closed or unsafe runway reason. I don't know for sure why the tower used the "own risk" phraseology - it would be great to hear from an actual controller - but some possibilities are:

The runway was marked as closed (for Solar Impulse 2) on the tower controller's displays, and he said "at own risk" more or less out of habit
There were vehicles on the runway, therefore it was "unsafe"
Solar Impulse 2 is such an unusual aircraft that the controller felt he couldn't contribute anything to the safety of the takeoff, so he was simply saying "go for it, I can't do much to help you"

Of course, the problem with all those suggestions is that they should apply at all the airports, not just half of them. But I did find this quote from a controller on the AOPA forums (members only):

  Although it's normal for helicopters, I issued this "departure will be
  at your own risk" to an aircraft once. The runway was open, he wasn't
  departing from a taxiway or anything like that. The problem was fog,
  where I couldn't see the runway from the tower, and therefore I
  couldn't determine whether the runway was actually "clear" for his
  takeoff.

  I don't know if this is the right response in this situation or not,
  but I was a pretty new controller at the time, hadn't faced this
  situation before, and figured the "at your own risk" was appropriate.

That could have applied to the Solar Impulse situation: with other vehicles and personnel on the runway, the controller can't say it's "clear". And just like the controller on the forum, perhaps some of the controllers handling Solar Impulse 2 felt that the non-standard wording was appropriate for a very unusual situation. The other controllers who did issue a takeoff clearance presumably felt differently.


How far does an A320 travel horizontally while climbing to / descending from cruise altitude? I am trying to determine how far an A319, A320, and A321 travel horizontally while climbing to / descending from cruise altitude? Would anybody be able to help me calculate this?

Of the main variables are the weight at brake release, and how high the cruise FL is. Luckily the manufacturers publish this data for flight planning (actual mileage will vary).

The climb distance you are looking for is the lower-left value in each box in the example below, which is applicable to the 320-family:

You can find it in the FCOM under Performance > Climb. A similar table exists for descents. Of course the values do not include the wind correction. For climbing other tables exist for the various deviations from ISA (lapse rate).

If you want to calculate it from scratch, then you need to have more data before you begin, I recommend these posts:

How to find total climbing time based on Rate of Climb?
Is there a formula to calculate ground distance traveled given rate of climb and true airspeed?
Is it possible to determine the approximate climb rate of an aircraft knowing its top speed, engine power and weight?
How can I calculate maximum rate of climb?


Can you re-register a de-registered aircraft with the same N-number? If I'm selling a plane that has been de-registered, can the new owner re-register the aircraft with the same number (or do they have to get a new one)? Does the airworthiness certificate transfer as well? How long does this process typically take?

The FAA has a page that sort-of takes you through this process.  Basically it boils down to this:

If your registration has expired you still have ~90 days to register the aircraft

  If a renewal application was first received by the Registry after the expiration date, the renewal process no longer applies. An aircraft owner may apply for registration of an unregistered aircraft under 14 C.F.R. §4731(a), by filing an Aircraft Registration Application, AC Form 8050-1, the $5.00 registration fee, and evidence of ownership (if it is not already on file at the Registry).

So it's a "new registration", but you can keep your N number. (Yes, you have to use an official, FAA-Provided 8050-1 form - you can't download them anymore. You're also going to want to talk to someone at the Aircraft Registration Branch or your local FSDO before you start filling out forms because you don't want your form to show up after the 90 day window or you wind up in the other category...)

After about 90 days your N number is Cancelled & unavailable for 5 years

  When an aircraft registration is not renewed, the cancellation of the N-number assigned to that aircraft will take place approximately 90 days after the expiration of an aircraft's registration. Upon cancellation, an N-number will be unavailable for re-assignment or reservation for a period of five years.

At this point you have to talk the aircraft registration branch, but my understanding is you are out of luck -- That N number is void and a new number will be issued when you register the aircraft (at which point you must apply the new N number to the aircraft in accordance with FAR 45, which typically means repainting at least part of the tail).
You must get a new airworthiness certificate bearing the new registration number from your local FSDO.

In both of these cases your airworthiness certificate is invalid* (because without a valid registration you have no authority to operate the aircraft).

In the first case you will not need a replacement airworthiness certificate (because your N number hasn't changed), but if your N number has been cancelled you will  need a new airworthiness certificate issued with the new N number on it.

Total time for all the paperwork to be processed depends on which bucket you fall into -- the aircraft registration branch usually has about a 30-day backlog (check here), plus time for the post office to move the paper from point A to point B, so a lag of a little over 30 days is what you should expect.
If a replacement airworthiness certificate is required that will probably add a few days to the process as well.


How to find the remaining fuel without electrical power on the MD-80? After a flight, how to find the remaining fuel without electrical power (even the APU) on the MD-80?

The normal Digital Fuel Quantity Display in the flight deck does not work without AC power, meaning you need to have either the APU running or external AC power connected:

  Fuel Tank Quantity Indicating

  A standardized modular fuel gauging system is used which requires no
  adjustment or calibration after system installation or component replacement.

  The Digital Fuel Quantity Display is located on the center instrument panel
  to indicate the quantity of the three tanks and gross weight. Operation of this
  display requires normal AC power.

(MD-80 FCOM Sec. 13 - Fuel)

The Fuel Load Display Panel on the right wing however can be powered via the battery:

  Fueling POWER Switch

  OFF - Removes power from fuel load selector display panel and fuel fill valve switches.

  ON - Supplies power from battery or external power to fuel load selector display panel and fuel fill valve switches.

(MD-80 FCOM Sec. 13 - Fuel - Controls and Indicators)

You can read off the current fuel levels in the upper row of the display panel:

(image source: MD-80 FCOM Sec. 13 - Fuel - Controls and Indicators)

Without battery power, manual measurements of fuel tank quantities are still possible with a magnastick:

  Fueling and Defueling

  [...]

  All fueling operations can be accomplished with airplane or externally
  supplied electrical power. If electrical power is not available for fueling
  operations, all fuel tank valves can be manually operated and fuel quantity
  measured using magnasticks.

(MD-80 FCOM Sec. 13 - Fuel)

The following section from a Dash-8 manual describes how the magnastick (or magnetic dipstick) works:

(image source: Dash-8 Q400 FCOM)

  Magnetic Dipstick Magnet
  Float Magnet
  Fuel Level in Main Tank
  Calibrated Magnetic Dipstick

  the magnetic dipsticks give an alternate means to measure the fuel quantity when the aircraft is on the ground
  the magnetic dipstick is accessed at the bottom of the wing
  when the rod is released, it moves slowly down until the rod magnet is attracted by the float magnet

(Dash-8 Q400 FCOM - Fuel)

  Magnasticks

  Two magnetic dipsticks (magna sticks) in each tank provide an alternate means of measuring fuel quantity when on the ground. Each consists of a calibrated rod, which slides within a tube extending vertically from the bottom of the fuel tank. When the rod is released, allow it to fall slowly until resistance is felt as the rod magnet becomes attracted to the float magnet. Read the tank contents from the bottom of the rod in U.S. gallons or in litres.

(Dash-8-200-300 FCOM - Fuel)


Can a Skylane fly without electricity? Some Hobbs Meters uses master electricity switch to keep the track of time.

So what If an cheapskate Skylane pilot wants to fly without electricity to lessen rent?

Which main six instruments would keep working?
Would the engine fail? (There are both carburated engines
and fuel injected engines which complicates things a little bit)

The engine wouldn't fail. Avgas powered engines use 2 magneto systems which are independent of the battery and alternator, so if your alternator dies or you lose the electrical system it will run as if nothing happened. Starting it does require the battery and electrical system though to run the starter motor. Aviation engines with computerized ignition systems have backup batteries so they will also not fail due to an electrical system malfunction.

As for the "6 pack" cluster of instruments 3 are not powered at all: Airspeed Indicator, Vertical Speed Indicator, and Altimeter. 3 are powered, although there is no hard and fast rule the Attitude Indicator (AI) and the Heading Indicator (HI) are usually vacuum powered from the engine and the Turn Coordinator is usually electrically powered. There are many exceptions to this.

So keeping the electrical system off doesn't affect the primary instruments much, provided the airplane stays in VFR. What captain cheapskate won't have for sure is any radios, transponder, GPS, ADF, VOR, or any other communication or navigation aids.

All these things you could probably do without on a completely VFR flight, provided you don't need communications. However, the engine temperature and oil pressure gauges, EGT gauge, and fuel gauges all work using electricity as well, and I'd really hate to lose those. A typical steam gauge Cessna isn't going to have an electrical fuel pump, but many piston singles do. Running without the electrical system means you'd lose that to, and that's considered an important safety system to back up the mechanical one in case of failure.


How is the RAT on the Airbus A320 family manually deployed? I watched Captain Joe's video on this topic but he only said in case the RAT does not deploy that you can manually deploy it from the cockpit, but unfortunately he never stated how this worked? (I'm only new to the industry and just an av geek)

There is a pushbutton on the overhead panel in the hydraulics section to manually deploy the RAT:

5 RAT MAN ON pb
The flight crew may extend the RAT at any time by pressing the RAT MAN ON pushbutton.
Note: The RAT extends automatically if AC BUS 1 and AC BUS 2 are lost. (refer to 1.24.20).

(A320 FCOM - Hydraulics - Controls and Indicators)
There is another button on overhead panel in the EMER ELEC PWR section, which will also deploy the RAT and connect the emergency generator:

1 MAN ON pb (guarded)
AUTO: When the following conditions are met:

AC BUS 1 is not electrically supplied.
AC BUS 2 is not electrically supplied.
Aircraft speed is greater than 100 knots.

.

The RAT extends.
The blue hydraulics system drives the emergency generator.
As soon as the emergency generator electrical parameters are within tolerance and the landing gear is retracted, the emergency generator is connected to the aircraft network.

Pressed: This selects manual RAT extension. Emergency generator coupling occurs 3 seconds after the RAT supplies the emergency generator.

(A320 FCOM - Electrical - Controls and Indicators)
Here is the full RAT deployment and emergency generator logic from the FCOM:


Why Rotax 912 "burps the tank"? The Rotax 912 engine has dry sump forced lubrication system. They say rotating the prop in the correct way (counter-clockwise when looking from the prop side) will get the oil from the bottom of the crankcase to the oil tank.

Questions:

How does this "force back by blow-by gases" work? Is it air which gets through the carburetors and cylinders to do the crankcase what pushes the oil from the engine? If oil gets from the bottom of the crankcase to the top part of the oil tank, where the "burping" sound emerges then? In the oil line, or at the bottom of the crankcase?
If the oil gets pushed by air, why is it bad to rotate the prop in the opposite direction? Air would get to the bottom of the crankcase and air would get to the oil input line and "burp" at the bottom of the oil tank. But if engine then runs in correct direction again, won't the oil be sucked from the bottom of the oil tank again? My only guess is it won't due to the air in the oil pump which would just spin without sucking any oil...

The Rotax 912 engine uses a dry sump, and does not have a scavenger pump to return oil to the sump. Instead it uses pressure escaping past the piston rings to force the oil back into the sump.
The "burp" you hear coming from the (open) oil tank happens when most of the oil in the crankcase has been returned to the tank: the air escaping past the rings bubbles up through the oil tank.

The propeller is pulled through "forward" (in the normal direction of rotation) for a few reasons, mainly that it's the direction that the gearbox & accessories are all designed to turn. Turning in the normal direction of rotation also ensures that oil is pumped through the rest of the system, particularly the valve train.
Reverse rotation can pump air into the system which may cause problems if you try to start/run the engine immediately after. I believe the recommended procedure is to "burp" the engine a few times to re-prime the oil system if it's pulled through backwards.


What advantages and disadvantages do tri planes have? A triplane, mainly used in WW1 on the german side, was a biplane with 3 wings.

What were the advantages to having more wings?

And, why don't countries use them today?

The accepted answer is wrong.

Triplanes have all the disadvantages of biplanes, only more so. But they share their advantages as well, and the main reason to build them in WW I was roll maneuverability:

The reduced wingspan reduced roll inertia, which helps to accelerate quickly into a high roll rate. Note that roll authority changes linearly with span (if the ailerons are scaled with the span), but inertia will change with span squared.
The reduced wingspan reduced  roll damping, so the achievable maximum roll rate is higher. The maximum roll rate is reached when the aileron-commanded rolling moment equals roll damping, and roll damping is proportional to span squared.

When the Sopwith Triplane became operational early in 1917, German pilots felt at a disadvantage and Reinhold Platz at Fokker created the famous Dr.I. The next generation were biplanes again, however, and the latest Fokker creation of WW I was actually a monoplane. The high climb rate of the Triplane was caused by its low wing loading and high power-to-weight ratio and more due to the lightweight construction and the advances in engines than the triplane layout.

  What were the advantages to having more wings?

Adding a second wing creates a lightweight, stiff box girder. As for why people added even more wings, see this excellent answer.

  And, why don't countries use them today?

Because countries don't use airplanes, pilots do.


What is the Space Shuttle's command law? The Space Shuttle was fly-by-wire. In today's fly-by-wire systems, the command law commands either the load factor and the roll rate (e.g., A320's normal law), or the surface control deflection (e.g., A320's direct law).
What was the case for the Space Shuttle?

The Space Shuttle's manual stick mode is called Control Stick Steering (CSS).

It commands load factor in pitch (yaw is similar). In pitch it of course accounts for the existing load from vertical deceleration. And rate in roll. When released it holds the attitude.

A second submode is indicated airspeed hold (the stick controls speed via the speed brakes), else S/B operation is manual.

(NASA Space Shuttle Digital Flight Control System)


What is the "optimal" age of a fighter pilot? Fighter pilots are always portrayed as young, fit men (and woman), in their 20s and 30s - and most of the F16 pilots I know are that age.

However, today someone, who is over 50 years old, mentioned that he was asked to be an F22 pilot.  His background is astonishing, flying in multiple "wars," and he was even the commander of the Thunderbirds in F-16s for a while.

Even though this man clearly has the background of the stereotypical F22 pilot, would age be a problem?  Wouldn't someone whose body could take more abuse be better suited for that type of assignment?

I can't directly tell you what's "optimal" because I don't know what that means, but the US Air Force does provide some basic demographic information so we can indirectly see what they consider to be a good age mix for fighter pilots. I used their report tool to get data on fighter pilots1 and it shows this breakdown:

17-24 :    43
25-34 : 1,341
35-44 :   968
45+   :    83
Total : 2,435

This isn't very fine-grained, but on the other hand we can immediately see that your example of someone over 50 is really at the extreme end of the curve: only 3.4% of fighter pilots are over 45.

Of course, this only applies to the USAF and it doesn't tell us anything about other branches of the military or about what other countries do. But we can say that the USAF appears to consider 25-34 to be the 'optimal' age: 55% of their fighter pilots are in that bracket.

(I should add a small disclaimer that I know nothing about the military, so hopefully I didn't make any stupid mistakes in pulling or interpreting the data.)

1 To be specific, the search criteria I used were: Active Duty Air Force only (excludes Air National Guard, Air Force Reserve and civilians); officers only; AFSC 11FX specialty (fighter pilot) only. The data is from April 2015.


Why is the wing of large commercial jet aircraft usually located below the fuselage? When the wing of an aircraft is located above the fuselage, the aircraft is generally considered more stable.

When the wing of an aircraft is located below the majority of the fuselage, the aircraft is generally considered more acrobatic.

Why then, do large commercial jumbojets generally have their wings located towards the center (from top to bottom) or bottom of the aircraft?  On first glance, this design seems odd since one of their primary goals is stability in flight.

First of all, I do not agree with your assessment: The wing of large commercial aircraft is clearly below the fuselage, to allow the cabin not to become divided by the wing spar. The A380 would have better aerodynamics if the designers had interrupted the lower cabin by the wing spar, but the airlines did not want this at all. Please look at the picture below: I hope you will agree that the wing is clearly below the majority of the fuselage. Why this is done has been answered here and here.

Large commercial aircraft have low wings to stow away their long landing gears. Long gears make it possible to stretch the fuselage and still be able to rotate during take-off. Large high-wing aircraft with their low fuselage position are easier to load and unload, at the price that the fuselage taper has to start shortly aft of the landing gear, so no stretching is possible.

A mid-wing design is common to aerobatic aircraft. For ideal roll handling, all parts should be aligned along a single axis, and doing this complicates fuselage design. Only for aerobatic aircraft the benefits outweigh the disadvantages. See the picture of an Extra 300 below:

A high wing is selected when the gear height should be low for loading and unloading, when the downward field of view of the pilot should not be restricted (think parasol design in WW I), when the wing should have more ground clearance during landing (think gliders) or when the wing is meant to pivot horizontally (see picture below for an example). Note that the combination of a high wing and wing sweep will produce too much sideslip stability and needs to be corrected with anhedral. All three (sweep, wing position and anhedral) change with angle of attack, and the combination will still have too much or too little stability away from the design angle of attack.

As Jan Hudec pointed out, pitch and yaw stability depend mostly on tail size and location, and even roll stability is affected by more than the wing's location. The location is a compromise of many, sometimes diverging parameters, and stability is only a small part of it.


Is RTCA DO-260 Change 1 the same as DO-260A? I am a bit confused as to the history and the naming for 260 documents. This is the timeline I perceived.
From earliest to latest:

DO-260                  ( September 13, 2000 )
DO-260 Change 1         ( June      27, 2006 )
DO-260B                 ( December  2,  2009 )
DO-260B Corrigendum 1   ( December  13, 2011 )
DO-260B Corrigendum 1   ( December  13, 2011 ).
This one says it is supplemental to DO-260B for corrections in regards about typographical errors, etc.

Am I missing something? Please correct me if I'm wrong.

At the moment I assume DO-260A is the same thing as DO-260 Change 1 yes? Another is what are the major differences between DO-260B and DO-260B Corrigendum 1 ? Thank you in advance.

Sources:
RTCA store website,
Fourth Meeting of APIRG_ICAO

DO-260 change 1 is not DO-260A.

DO-260A released in April 2003
DO-260A change 1 in June 2006
DO-260A change 2 in December 2006

The releases marked as 'change' contain editorial changes, clarifications and corrections, but are effectively the same specification.
The alphabetical versioning is for subsequent releases of a new specification of the same system. In these releases the specification is changed, i.e. the behaviour of the system specified is different.


Why do we use dimensionless expressions in flight mechanics and aerodynamics? Why do we use non-dimensional expressions in flight mechanics and aerodynamics? We could as well directly calculate forces and moments; would that not be more relevant to the specific problem?

Here is a dimensionful equation: $$L = \frac{\rho}{2}\cdot v^2\cdot \frac{2\cdot\pi\cdot b^2}{1+\sqrt{1+\left(\frac{b^2}{2\cdot S}\right)^2}}\cdot\alpha$$
Note that all ingredients are physical, measurable values. Now here is the same thing again, now in dimensionless form:
$$c_L = \frac{2\cdot\pi\cdot AR}{1+\sqrt{1+\frac{AR^2}{4}}}\cdot\alpha$$
The dimensionless version is independent of speed $v$ and air density $\rho$ and also needs only the aspect ratio $AR$ and the angle of attack $\alpha$ to give a result, while the dimensionful version needs wingspan $b$, wing area $S$ and angle of attack. However, while the dimensionful version yields an actual force, the dimensionless version yields a coefficient which needs to be multiplied with speed, density and wing area again in order to result in a force.

By working in the dimensionless domain, engineers can work on a generalized form of the problem. This was especially helpful in the time before computers: The complex calculation to arrive at a coefficient was needed only once, and computing the dimensionful value from there was trivial.

Also, when comparing between different airplanes, or between wind tunnel models and the real thing, with dimensionless coefficients the numbers will be very similar, which allows much better to error-check results or to make first assumptions on a new design.

However, there are two drawbacks:

You need to know the reference values! It is easy to screw up, e.g. when combining values from the wing and the tail which are referenced to their individual area. While lift can be added right away, the lift coefficients must first be converted to a common reference value.
Sometimes thinking in coefficients can be misleading: Just consider induced drag. If written in dimensionless coefficients like this
$$c_{Di} = \frac{c_L^2}{\pi\cdot AR}$$
you can be forgiven for thinking that induced drag goes down with increasing aspect ratio. But this is wrong! Now the same again with dimensions:
$$D_i = \frac{L^2}{\frac{\rho}{2}\cdot v^2\cdot\pi\cdot b^2}$$
This shows that induced drag at the same speed is inversely related to span loading, the amount of lift created per unit of wing span.


Would a large airliner such as a 747 or an A380 survive an inversion? Could any large airliner or GA aircraft survive a horizontal roll? Or would a large aircraft break up from structural damage?

Could it? Yes.
Tex Johnson famously performed a roll in a Boeing 707. If performed properly it is a 1G maneuver (the airframe experiences +1G stresses through the maneuver - as far as it's concerned nothing unusual is happening).

Should you attempt such a maneuver without aerobatic training? No.
Should you attempt it in an aircraft not certificated for aerobatic flight? Absolutely Not.
This is one of those "If you do it wrong the wings fall off" maneuvers, and chances are you're not Tex Johnson.


What are the labels for this aircraft's JSON? Can someone please help me identify the labels associated with the below JSON?

The data comes from FlightRadar24

c5cded9":[
          "06A0F8",
          51.3056,
          6.4601,
          283,
          43000,
          459,
          "3245",
          "F-EDDK1",
          "A359",
          "A7-ALD",
          1486231556,
          "DOH",
          "LHR",
          "QR1",
          0,
          64,
          "QTR1",
          0
]

I get that 51.3056 & 6.4601 are the latitude and longitude. I also get that 43000 is the altitude and 1486231556 is a timestamp, but I do not know what the others should be...

For example, LHR I guess is London Heathrow, but is what is the official term?

I'd like official terms to match the JSON.

I want to do some data visualizations with the aircraft data.

Community wiki answer:

06A0F8,         Mode S ICAO 24-bit address
51.3056,        lat
6.4601,         long
283,            heading
43000,          altitude in feet
459,            ground speed in knots
3245,           squawk (transponder code)
F-EDDK1,        radar receiver identifier
A359,           ICAO aircraft type Airbus A350-900
A7-ALD,         aircraft tail number / registration
1486231556,     time-stamp
DOH,            departure airport Doha (IATA code)
LHR,            arrival airport Heathrow (IATA code)
QR1,            flight number (IATA)
0,              ground flag (1=on ground, 0=airborne)
64,             vertical speed (feet/min)
QTR1,           ATC call-sign
0


What is the purpose of proximity sensors on hydraulic systems? One of the planes I worked on had ground spoiler response checked with proximity sensors. The reason mentioned for that was - "The ground spoiler is hydraulic only - hence its response will be checked with proximity sensors"

Landing gear control unit reads the proximity sensors and flight control system receives the signals indicating deployment of ground spoilers by landing gear control unit.

What are proximity sensors?

Why are they used because the surface is hydraulic?

A proximity sensor is any sensor that detects whether there is something in its proximity (within specified distance).

There are many kinds of proximity sensors (optical, magnetic, capacitive etc.) that detect different kinds of target objects and to various distances.

In this case, proximity sensor is used to detect position of the spoiler. Proximity sensors are preferred over micro-switches because they are more reliable, since they have no moving parts and thus don't wear out.

For parts actuated with hydraulics they are the only way to detect the position, since hydraulic actuators don't provide any direct way for detecting their position (compared to electric actuators, where in some cases the response of the actuator can be used to detect the position; the principle is similar to a magnetic proximity sensor anyway though).


Do vapor clouds increase the radar scross section of an aircraft? The F-22 is known for its stealth capabilities, which is mostly achieved with an extremely low RCS. Due to its high maneuverability we also see this plane often creating extreme vapor clouds while maneuvering:

Source: Wikipedia

We know that weather radar can detect clouds, so do such vapor clouds increase the RCS of the airplane?

As with most things involving the military, the latest generation of radar detection is classified, and no one really knows all the tricks it uses to work.

However, ignoring the newest bells and whistles let's answer your question with simple radar implementations.

At it's core, radar is a broadcast of an electromagnetic pulse that bounces off anything which reflects it. This reflection is then picked up by a receiver, and after some computer math you get radar predictions.

So the important question to ask, is what band of electromagnetic radiation is being used, and will that particular band of radiation be deflected by cloud vapor?

Most aircraft detection radar uses radiation in the low frequency areas. As you can see on the chart below, long waves are also called "radio waves" is the normal frequency.

This is also what weather radar uses, so it's quite obvious that yes the vapor cloud of the plane will deflect the radio waves. In fact most ham radios operating off similar frequencies rely on the deflection of clouds/upper atmosphere imbalances to communicate with other radios without having a line of sight.

However this is the interesting part. The vapor cloud a jet fighter produces doesn't normally last that long, and when the receiver starts getting the strange data of a small cloud appearing and disappearing it's up to the radar operators/manufactures what to do with this data.

Do you write it off as noise? Or do you say "hey this cloud is moving at mach 1, and is appearing and disappearing in a predictable flight path, we should probably shoot a missile at it."

I'd imagine the military would chose the latter, and I would think the military of any powerful nation would learn to exploit these vapor clouds.

However again that's kind of classified information, so I'm really guessing at this point.

I could end the answer here, but I want to explore another aspect of next gen radar which is the use of infrared radiation. Also called "heat" radiation.

Infrared radar uses a receiver to detect objects in the sky that are emitting heat. Jet engines obviously leave quiet a bit of heat in their wake so this works pretty well.

Now can the vapor cloud diffuse this type of radiation? Maybe, but probably not.

I say maybe because I've never been able to measure the temperature of a jet's vapor cloud, but I'd imagine it's pretty hot. However if you want to make the argument that it's only slightly above ambient temperature then maybe this will temporarily help mask the aircraft's heat signature.

If though you think like me that the vapor is very hot considering the extreme pressure the air just went through to make the vapor cloud in the first place, then this will work against the aircraft to expose more heat to ir detection.

So to answer your question, yes this could cause a problem with radar detection, however I hope that anyone who puts billions of dollars into building these aircraft know about this limitation and inform pilots not to do a vapor causing maneuver within enemy radar when stealth is needed.


How much simulator training for general-aviation? How much training/refreshing is necessary/desirable for instructing general aviation pilots? All I see is commercial pilots been trained in simulators.

A real general aviation airplane might be not much more expensive than a realistic flight simulator (full-motion  simulators with hydraulics), but wouldn't training specific scenarios (like engines-off, bad weather) in a simulator be a significant enhancement in their education?

Under FAA rules, simulator training is not a requirement for any certificate/license. Only a certain amount of simulator training can be substituted for training in an actual aircraft for each certificate. Even then, only simulators certified by the FAA can be counted towards training time. The cost of the hardware and complexity of the software necessary to be good enough to receive FAA certification makes only high level FFS devices financially advantageous for airlines. These devices compared to the average home simulators is like the difference between 1970s TV night to holographic day.
Then, the question is if the allowed simulator time makes sense for the General Aviation pilot. Part 61.109 allows 2.5 hours of sim time to be substituted for actual flight time towards a private pilot certificate. That number doubles if the training is done in a Part 142 training center. By the time you get to the 1500 hours required for an ATP, ...

Not more than 100 hours of the total aeronautical experience requirements of paragraph (a) of this section or §61.160 may be obtained in a full flight simulator or flight training device provided the device represents an airplane and the aeronautical experience was accomplished as part of an approved training course in parts 121, 135, 141, or 142 of this chapter.

If the time requirements for aeronautical experience were arbitrary to the need for training, a flight simulator can be a viable option. A realistic AATD (Advanced Aviation Training Device) would be affordable for a General Aviation flight school.  Compared to the millions of dollars of a full-motion FFS (Full Flight Simulator), an AATD would be less than $100,000. Most flight schools in my area would charge a student roughly half the hourly rate for an AATD that they would for the typical 4-seat, 6-pack, single-engine, fixed-winged trainer aircraft.
Unfortunately, the lack of realism in an AATD makes it a valuable tool for procedures training only. An AATD lacks the feel and feedback of real flight. Its value is only as good as the student, instructor, or pilot’s diligence to following the procedures. To gain the most training benefit from an AATD carry out the flight using flight planning, flow checks and checklists to their fullest. Role-playing ATC will further add to this benefit. Treat it like a real flight and not like a game.
Anecdotally, I have trained on sims ranging from full-motion Redbird BATDs with custom G1000 bezels to stationary AATDs with fully functional instruments and controls, home sims on personal computers to multimillion dollar full-motion, gimbaled FFSs at the American Airlines Training Center. Except for the AATC FFS, none of them had the realistic force feedback of a real flight. None but the FFS had software sophisticated enough to mimic landings or wind interaction with enough realism. And, the FFSs are far too expensive for general aviation use. Actual flight time in a GA trainer aircraft would be cheaper.


What aircraft might have white wing tip lights? I live near O'Hare airport and see hundreds of planes every day.  This evening a plane flew past that did not have the standard red - green wing tip lights.  It was white only on both sides.  It caught my attention because of that, and because the plane was unusually quiet...not silent, but quieter than most everything accept the smallest private planes.  Any suggestions as to what I saw?

You don't tell us if you're viewing this aircraft from the front (approaching you) or the back (moving away from you), so let's look at all the options:

Aircraft have three colors of steady-burning navigation/position lights: Red on the left (port) side, Green on the right (starboard) side, and White at the aftmost position (tail). This diagram shows the required lighting coverage (as implemented on a typical small aircraft):

(With the exception of a "mast light" these are identical to naval vessel position lights - the function of a mast light is performed by the landing light, though it's not used as a "position light" the way the others are.)

On some aircraft there will also be aft-facing white position lights on the wingtips, combined with the navigation and strobe lights:

The aft-facing position lights on the wingtip improve visibility at night, particularly flying over brightly lit urban areas (a problem sailing vessels don't have). They also give a visual reference to determine the size (wingspan) of the aircraft.

Finally (just to throw another wrench in the works) some aircraft are equipped with wingtip "recognition lights" (forward-facing white lights on the wingtip):

These are typically used on the ground or in approach to landing, but when illuminated the brighter recognition lights can make the forward position lights difficult to see, particularly from the ground where your eyes probably aren't adapted for darkness.

So putting it all together:

If you're viewing the aircraft from the rear (it's flying away from you) you will only see white lights (either one or three white lights) and the aircraft's strobes or other intermittent anti-collision lights. You will thus know the aircraft is traveling away from you.

If you're viewing the aircraft from the front (it's flying directly toward you) you will see a red and green light (possibly with white taxi/landing/recognition lights as well, but the red and green at the wingtips are what you're looking for to identify the aircraft's direction of travel).
Again if the aircraft has wingtip "recognition lights" you may have trouble picking out the navigation lights.

If you're viewing the aircraft from the side (it's flying perpendicular to you) you will see either the Red or Green light until it passes in front of you, at which point one (or more) of the white lights will come into view.


On what principle(s) do aircraft smoke detectors work? I know there are smoke detectors in aircraft but on what principle(s) do they generally work?

Commercial smoke detectors can use several different techniques like optical sensing, heat sensing and particle sensing. Do aircraft smoke detectors also work using one of these technologies or something different?

The typical aircraft smoke detector works using optical sensing which exploits the Tyndall light scattering effect.

From Boeing:

(Source)

An LED illuminates a sample of air while a light sensor, which is not in line-of-sight with the LED, looks at the same sample of air. If enough light is scattered by smoke particles then the detector will signal an alarm.


What protects the location beacon (pinger) of a flight data recorder? The pictures of flight data recorders show that the location beacon (pinger) is outside the main protection encasement:

Is it really a good idea to keep such a vital instrument unprotected? Or is that round metallic thing strong enough to resist impact and fire?

Is it really a good idea to keep such a vital instrument unprotected? Or is that round metallic thing strong enough to resist impact and fire?

The casing of the ULB is itself designed to be strong enough to withstand the forces expected when an airliner crashes into an ocean, lake or large river (which are the only circumstances where a ULB is useful).

See teardown video starting at around 4:45

Copies of the relevant standards cost $70 each. However, a 1968 report from the early days of ULB development says

As voretaq7 notes in a comment: ULBs need to be external to the FDR/CVR canister because they need to have batteries changed regularly without affecting the integrity of the data recorder canister.


Helicopter Flight: What is the reason for decrease in Induced drag with increase in speed? As per the figure the induced drag will decrease only if the AoA of the blade is decreased — But in the forward flight, cyclic input increases and decreases the AoA throughout the blade rotation cycle.

Also, in forward flight, a component of the Lift force is used to compensate for the parasite drag, so the overall Lift vector will increase. This should increase the induced drag further.

Where I am getting wrong? What causes Induced drag to decrease with an increase in speed?

As you mentioned, the main rotor thrust must support the weight of the helicopter in hover, and must also provide a propulsive force to overcome drag in forward flight.  However, the induced drag is also a function of how the air is deflected downward by the rotor.  The velocity of this downward deflection of air at the rotor is often called induced velocity.  This induced velocity is a function of not just thrust, but also the helicopter forward speed, rate of climb, air density, height above ground (when near the ground it decreases, a phenomena known as ground effect), etc.

Relevant to your question is how the induced velocity decreases as the helicopter goes from hover to moderate forward speed (even though thrust increases).  At hover, the rotor is sitting in its wake with a large induced velocity.  With some speed, the helicopter is moving away from it's wake, resulting in a smaller induced velocity.  This smaller induced velocity increases the angle of the incoming air to the blade, and therefore decreases induced drag (and even overall power required).


Is it legal to fly without a radio in Germany? Most people in the aviation world know that it's legal to fly a Super Cub without a radio in the US. Is it legal in other parts of the world, specifically Germany?

I stumbled across this section in a copy of the German AIP VFR:

  GEN 1-12 Radio and Air Navigation Equipment of Aircraft (NfL II-11/05)

  ...

  Radio and air navigation equipment for VFR flights

  (1) For VFR flights, aeroplanes, rotorcraft, powered gliders, gliders,
  aerodynamically controlled ultralight aircraft, gyrocopters, airships,
  and free balloons shall be equipped with one VHF transceiver covering
  at least those frequencies in the range 118.000 to 136.975 MHz that
  are required for the planned flight; the transmitter output power and
  receiver sensitivity shall be at least such as to ensure clear
  radio-telephony communication with the ATC and AIS units, taking into
  account the operational characteristics of the aircraft and the route
  flown.

  ...

  (3) Para 1 does not apply to flights at aerodromes without an ATC unit
  which are conducted during the day and which do not leave the vicinity
  of the departure aerodrome (Art. 3a para 3 of the German Aviation
  Regulation [LuftVO]). Local regulations of the responsible authority
  of a Land (Art. 21a para 1 of the German Aviation Regulation [LuftVO])
  shall remain unaffected.

Does this mean cross country flight without a radio is illegal in Germany? Or is the part "shall be equipped with one VHF transceiver covering at least those frequencies ... that required for the planned flight" a loophole meaning you can go as long as you don't cross into any areas where radio is required? (thus effectively rendering 3.(3) above superfluous)

I understand that as long as you remain in class E and G airspace, I don't actually have to talk to anyone; would I still be legally required to carry a radio?

Ignore the other airspace classes for now, as C has mandatory radio communication and F has its own can-of-worms rules in Germany, and neither A or B are used at all (class F is about to be discontinued and replaced with class G and a Radio Mandatory Zone in accordance with EU regulation 923/2012 "SERA")

paragraph 3 clearly states that the only exception to the requirement to carry a radio set is when you're not leaving the immediate vicinity of the airfield, and then only when there's no ATC coverage.
That effectively means you can fly a few circuits at low altitude over a small field, but even then many of those would be close enough to a larger airport that has ATC coverage that they'd be covered by that airport's ATC system rather than having their own.

The only "loophole" would be if you could convince authorities that your flight wasn't planned, good luck with that.
Effectively that'd most likely mean you're having to prove to them that you went to the airfield without wanting to fly, just jumped into an aircraft, took off, and started zooming around the sky. Something that's so far out there nobody's going to buy it.


Why two passenger airplanes travel side by side? Recently (2018-08-19 17:40 UTC) two planes flew above my city (Poznań, Poland). They were heading in the same direction, kept really close distance (side by side) - both flights to Riga:

https://www.flightradar24.com/data/flights/bt634
https://www.flightradar24.com/data/flights/bt684

Is this common? Why would they do that - are there any benefits of it?

The top one is Baltic 634, the bottom one BT684.

As you can see they had 2000ft seperation. That's 600m for us europeans not accustomed to aviation.

Furthermore, at 17:40:19 BT634 reported in at 52.409599,16.981899. 17:40:25 BT684 reported in at 52.423222,17.019314. While those are 6 seconds apart, the planes were north-eastern course at the time, so I guesstimate that they had 1-2km horizontal seperation as well.

As to why - they're going the same place. The separation is big enough that there's almost certainly no benefit to it.


During an IPC, is there any alternative to doing a precision approach? My friend's plane is IFR certified, but does not have either an ILS receiver or a GPS receiver, either one of which is required to fly a precision approach.  As he can't fly a precision approach in his plane, and such an approach is required to be flown during an instrument proficiency check (IPC), does that require him to find another plane in which to take the test?

Alternatively, can he take the test in his plane with a limitation being placed on the signoff to the effect that he is not qualified to fly precision approaches? Or does it not matter that a precision approach cannot be flown?

As far as I can see, there's no way to avoid flying a precision approach during an IPC. 61.57(d) says:

  The instrument proficiency check must consist of the areas of
  operation and instrument tasks required in the instrument rating
  practical test standards.

The instrument ACS requires a precision approach, as you said, and it's also in the checklist in AC 61-98C, Currency Requirements and Guidance for the Flight Review and Instrument Proficiency Check, and the FAA's accompanying instructor guidance. So I think it's fairly clear that the FAA requires it as part of an IPC.

As for limitations, the FAA's AC 61-65 Certification: Pilots and Flight and Ground Instructors doesn't mention any that are specific to an instrument rating. I've never heard of a 'non-precision only' instrument rating.

I can only suggest that your friend finds another aircraft. The 'good' news is that once he's current again, he can maintain currency indefinitely with only non-precision approaches and avoid another IPC.


Do leading edge flaps increase the critical angle of attack? So from my understanding, trailing edge (TE) flaps increase wing area + camber, effectively increasing the coefficient of lift for a fixed AOA which leads to higher lift at a fixed airspeed and AOA. This allows an airplane to fly at lower speeds + lower AOA to maintain lift.

But does the same also apply to leading edge (LE) devices like slats or flaps? I know that a slat moves to allow underside air to flow over the top and prevent airflow separation, leading to a higher critical AOA. Do LE flaps also increase the critical AOA? Also, it seems that both slats and LE flaps affect the wing camber, increasing it. Does this mean that their deployments lead to higher coefficients of lift for a fixed AOA like with trailing flaps?

Yes leading edge devices indeed postpone stall to a higher Angle of Attack. They are particularly useful for:

Airfoils which are prone to leading edge stall. Deflection of the trailing edge flap creates upwash at the wing nose, which reduces stall AoA mainly on thinner wing profiles.
Aeroplanes which use lift dumpers as speed brakes. Deflection of the spoiler reduces the stall AoA, a leading edge device provides extra AoA margin.

Image source and source of info below

The four main types of leading edge devices are:

  LEADING-EDGE SLATS are small, highly cambered airfoils forward of the wing leading edge, which experience large suction forces per unit of area and reduce
  the suction forces on the basic airfoil.

Note that before deflection, the slat was part of the wing nose. Its area is mainly deflected to a new position, not extended as in the case of a trailing edge flap.

Slat deflection does indeed change the camber, by reducing the local AoA. This potential reduction in local lift coefficient is compensated for by the changed suction forces around the nose, with a net effect of around zero.

  KRUEGER FLAPS perform in the same way as slats, but they are thinner and more suitable for installation on thin wings. Krueger flaps are often used on the inboard part of wings, in combination with outboard slats, to obtain positive longitudinal stability in the stall.

  (PLAIN) LEADING-EDGE FLAPS (hinged noses) are less effective than slats. They are mechanically simple and rigid and partic- ularly suitable for thin airfoil sections.

  FIXED SLOTS are the simplest devices for postponing leading edge stall, but their profile drag penalty is generally prohibitive for effective cruising, except on some low-speed STOL aircraft.

The picture below plots what happens upon deflection of flaps (left) and slats (right). Flaps increase lift for a given AoA, but reduce critical AoA. Deflection of leading edge devices does not result in increase of lift at an AoA, but postpone the critical AoA - in order to obtain more lift, the AoA needs to be increased.


Does clearance delivery assign VOR or GPS-A approaches? I've listened to clearance delivery feeds on LiveATC. Is it possible for clearance delivery to assign VOR or GPS-A approaches instead of standard terminal arrivals (STARs)? Or would they just give you routing to a VOR or another waypoint near the arrival airport, and say "direct" afterwards to indicate that the TRACON for the arrival airport will assign the specific approach?

No.  Normally you don't know which approach you are going to be using at your destination because you can't be sure which runway will be in use.  The STARs at larger airports are to provide standard routings for all/most the of airport's approaches/runways and each STAR is based on a different arrival direction, so the STAR in your clearance will usually be based on where you're coming from and you usually include one in your filed flight plan, that you've picked because its initial waypoint links in nicely to your route.  The actual routing within the STAR you will be using, you find out when you listen in on the destination ATIS to find out the approach in use as you plan your arrival.

Once you get there, you may only fly one or two legs of the STAR before ATC gives you a heading and now you are on radar vectors to get lined up for the approach and you just do what you're told and the rest of the STAR routing goes out the window.

The key thing in IFR is accommodation of communication failures. Comm failure procedures are intended to allow you to complete your flight by following your most current clearance and observing SOPs that allow ATC to predict where you're going to go and when, with reasonable confidence, so that they can work traffic around you.  If you've declared a comm failure (7600) enroute, you would fly the STAR routing and altitudes as published and use as much common sense as possible.

For example, a lot of STAR routes kind of drop you off at a Downwind Termination Waypoint for a particular runway and indicate radar vectors beyond that.  In a comm failure situation you are expected to fly the published route to the DTW and from there you are expected to find your own way to the approach by the most logical, predictable routing that causes minimum disruption.

If the destination has no STAR, your clearance will be to the last waypoint in your route that gets you close to the airport, and that will be your clearance limit and from there you would get radar vectors to line up with whatever approach is in use.  In a case of a comm failure, you are supposed to proceed to your clearance limit, hold there if you're early, then proceed to fly the appropriate approach at the time specified in your flight plan by the most logical routing, so that ATC can predict where you're going and when.


What is the distance at which my VHF communication radio can reach ATC? I just installed 2 new TKM MX11 radios and a new Garmin 340 audio panel. I also put in new coax cable and 2 new antennas in my 1964 Skylane.  What kind of distance from ATC should I expect at 4 or 5 thousand feet?

The TKM MX11 has an 8 watt transmitter which makes it a Class 4 transmitter as defined in RTCA DO-186B MOPS for Airborne Radios Communications Equipment. That specifies a maximum range of 100 miles.

In an area of relatively level terrain, you should see a range of about that 100 miles if you're at 5000 AGL or above. Below that, range will be reduced due to the curvature of the earth and multipath interference. At 2000 feet above level terrain, 40 to 50 mile range is typical based on an analysis I did about 20 years ago for a former employer.

One factor you can't control is the height of the ATC antenna.  The higher it is, the greater the range. Even a 50 foot difference in the height can affect the range by several miles.


How feasible digital radio communications would be? Its been over 100 years since we first flew and we still use analog radio with parasites and "radio effect" which changes your sound.

Why don't we ditch the old system and make a mandate a new law where any aircraft that wishes to fly in a controlled airspace has to be equipped with two digital radio communication systems?

What would the coverage costs be like? Doesn't digital systems require less power and antenna size etc to work which would make it better?

Digital is not always better.  Digital modes have been created for amateur radio use; the amateur radio VHF 2m band (144–148 MHz) is very similar to the VHF aircraft band (108–137 MHz).  I have a radio that uses both FM and a digital mode on the amateur 2m band.  Here are some notable differences between FM and digital:

When the signals are strong, the sound quality of FM is slightly better because there are no digital artifacts
When the signals are weak, digital sounds better because there is no static
When the signals are very weak, FM is definitely better because digital either degenerates into unintelligible loud tones ("sounds like R2D2" is what hams say) or is not decoded at all, but analog signals can still be understood with difficulty
Only one digital signal should be transmitted at a time on a channel; if two signals are there at the same time, the stronger signal might get through, but more likely neither signal will get through

The reason that old-fashioned AM has been kept for all these years is that if two signals are there at the same time, both signals can be heard and understood (perhaps with difficulty).  This is a significant benefit; a quieter signal, perhaps from an aircraft further away from the airport, has a much better chance of breaking through to be heard on a busy frequency.  FM and digital modes can't offer that benefit.

Digital radios are not intrinsically more capable than analog radios given the same power levels.  Digital radios can use slightly less bandwidth for the same audio quality, packing more channels into a band, but that's not very relevant for aviation.

The antenna size varies by the frequency, regardless of whether the radio is analog or digital: the higher the frequency, the smaller the antenna.  Switching to digital wouldn't make the antennas any smaller, unless the frequency band changes also.  A higher frequency band would make the antennas smaller, but finding a new large frequency band for aviation use worldwide would be very difficult.


How are the "medical communication" headset sockets on some airliners used? I was recently on a flight during which a passenger had a medical emergency.  The passenger vomited and was reportedly unconscious.

The aircraft was either a Boeing 737-700 or 737-800.  I'm pretty sure it was the 800.  It had pretty blue lighting on the interior ceiling, including oval illuminated cutaways.

Located above one row of seats were two ports marked "Medical Communications" (or something like that).

During the emergency, one of the flight attendants connected a wired headset to these ports in order to communicate with others.

With whom was he likely speaking?

As an aside, the system was poorly designed.  One ports was black and the other gray; both ports otherwise looked identical and were the same size; the colors of the headphone connectors did not match the ports.  Both ports were required to make it functional.  The flight attendant had to try multiple times to get the correct wires into the correct ports.  I had to help the flight attendant get it connected properly.  Such a simple and important issue to fix.

Bonus question: Given that the multiple ports did not appear to be for redundancy, why was such a time-critical system not designed with a single port?

I'm going to assume you're talking about something like this:

Photo liberally stolen from here.

As other have mentioned, these are generally connections for a standard aviation headset (headphone and microphone plugs). When plugged in you're typically connected to the cockpit (so the pilots know what's going on), and on commercial airlines you may also be patched in to a medical consulting service via radio.

The principle behind this is similar to the early paramedic programs in the United States: Personnel with some basic training (like flight attendants) are connected to a doctor. They describe the patient's condition and symptoms, and the doctor advises on a course of treatment, which can begin immediately and hopefully give the patient a better chance of survival.

The radio patch can also be used by a doctor onboard the aircraft to consult with other physicians and to help the flight crew arrange for appropriate transport services to meet the plane when it lands.


How are thermals mapped in real-time? Related to "How thermals are found" this question about "Gliding your plane to save fuel" details how to use thermals to save fuel. To use thermals to maximize range, can thermals be mapped in real-time using Doppler or sensors or a data base of some type to fly by?

No.  Thermal activity in an area can be predicted in a general way based on atmospheric conditions, but not beyond that. The only way to detect thermals in real time is to stumble into them, and/or locate them by the clouds that form their tops.
In any case, the only conditions for which that sort of thing would work would be on one of those lovely soaring days where steady thermal sources and a moderate wind create "cloud streets" where the cumulus clouds line up for you in rows (the cloud being moisture in the air of thermal condensing at the dew point, creating a handy marker for glider pilots - if the air is dry, the thermals are still there but you have no clouds and you can only find them by trolling).
So it could theoretically work -
If: you had that one day of many where the conditions cooperate and you get decent cloud streets, and
If: the thermals were broad in diameter and,
If: your track just happened to match the direction of the cloud streets, and
If: you cruised just below the cloud bases to make it easy to locate them, and
If: you didn't mind the bumps and surges up and down and didn't have passengers that were prone to airsickness.
In those conditions you could push over and reduce power to maintain speed in the thermals when you feel the upward surge entering it, and then do the opposite when out of the thermal when out of it.  You do this automatically anyway when flying on bumpy days, to hold an altitude, although normally it's random as you stumble in and out of lift.
One problem is, between the clouds on the street, the air is sinking, so the period you are in sink cancels out the period you were in lift, to some degree, limiting the benefit pr maybe negating it.
Another problem is you need to slow the airplane down to do this effectively.
Another problem is you have to get good at identifying when you are in lift and in sink and if you have no soaring experience this can be tough.
Now, I do like to take my own homebuilt PL-2 power plane and soar it on good lift days from time to time, by turning it in a "virtual sailplane" by setting power to give a 200 fpm or so sink rate and fly around just above the stall working the lift (the engine is running at about 35-30% power to do this). I call it "power soaring". But I don't do that when going somewhere although I could try to do it cross country if I wanted and was willing to take an hour to go 40 miles.
If you are really serious about saving fuel, forget about trying to exploit thermal lift except as something to do for fun, and where the right opportunity might happen once in a blue moon, and focus on not being in a hurry to get from A to B.
If you aren't in a hurry, you can cruise your airplane at best range speed and power for the maximum MPG (maybe 40-50%), and you can go places in the early morning when the winds are light and the air is smooth, with your engine barely working, and enjoy the scenery sliding by a little bit slower than normal.  Best time to be in the air.
If you're renting however, and paying on a Hobbes meter, the benefit of the fuel saving is going to the airplane's owner, not in your pocket.


How do I enter the traffic pattern if I am coming from a nonstandard direction? I have some questions on entering the pattern, VFR at an uncontrolled airport:

Scenario: let's say I am going to land on a one-runway airport (36-18). Also,
let's say I will be landing on 36 (left pattern) and I would like to enter left downwind.

I would like to know the proper pattern entry for left downwind for 36 for these different routes:

What is the proper entry if I am coming from 090°?
What is the proper entry if I am coming from 020°?
What is the proper entry if I am coming from 180°?
What is the proper entry if I am coming from 200°?

I am answering this question based on U.S. operations and a plan to land on runway 36 using left traffic:

My opinion -

Option 1: If you are arriving from east of the airport (from 090 or 020) you should fly over the airport at an altitude well above pattern altitude, observing the traffic (if any) then descend to pattern altitude west of the airport well beyond/outside the traffic pattern.  After that enter left downwind at midfield from a 45 degree angle at pattern altitude.

Option 2:  If you are arriving from east of the airport (from 090 or 020) and have determined (visually and monitoring the CTAF) that traffic is extremely light or non-existent, flying over the airport and entering directly to a left downwind at pattern altitude (not descending) is an option.

Entering the traffic pattern after approaching the airport from the south or west (from 180 or 200) should be done by maneuvering west of the airport, well beyond/outside the traffic pattern, until you are able to enter left downwind at midfield from a 45 degree angle at pattern altitude.

See the examples from the illustrations below and familiarize yourself with AC 90-66B, which is an excellent guide.(illustrations below are from AC 90-66B)


Why is runway friction measured at 65 km/h and 95 km/h? Why do we measure at 65km/h and 95km/h when we do a Runway Friction Measurement? Are there differences?

A range of speeds were tested at NASA's Wallops Right Facility: 20, 40, 60, and 80 MPH.

20 was deemed too slow to cover a long runway in an acceptable time, and 80 was deemed too fast to achieve (accelerate to) in a short distance -- the test vehicle needs to reach the test speed by the touchdown zone (arriving from the other end also leaves no room for stopping).

So 40 and 60 MPH were chosen, which are 65 and 95 km/h. The two speeds measure different properties. The slow one is for the macrotexture of the runway, and the fast one for the microtexture.

  A complete survey should include tests at both speeds.

  Microtexture refers to the fine scale roughness contributed by small individual aggregate particles on pavement surfaces which are not readily discernible to the eye but are apparent to the touch, i.e., the feel of fine sandpaper. Macrotexture refers to visible roughness of the pavement surface as a whole. Microtexture provides frictional properties for aircraft operating at low speeds and macrotexture provides frictional properties for aircraft operating at high speeds. Together they provide adequate frictional properties for aircraft throughout their landing/takeoff speed range.

A good macrotexture result means the tires are able to drain the water and remain in contact and not aquaplane. A good microtexture is desired for airplane speeds below 100 MPH.

I haven't mixed macro/slow and micro/fast. Testing those properties has nothing to do with the actual airplane speeds. For example, if 20 MPH wasn't deemed too slow, it would have been the better speed for the macro testing for the aquaplaning at fast speeds.

  The lower speed determines the overall macrotexture/contaminant/drainage condition of the pavement surface. The higher speed provides an indication of the condition of the surface's microtexture.

  At speeds over 100 mph, the hysteresis component of friction governs. This component is the effect of damping or reacting elastic pressure of rubber when deformed around aggregate particles. The deformation is produced best by good macrotextured surfaces.

Summary:

References:

ICAO Doc 9137 Part 2 Pavement Surface Conditions
FAA Advisory Circular 150/5320-12C


Where is the APU fuel pump on the A320? I'm looking for APU fuel pump location for A320 aircraft series. I'll be much appreciated if you provide a link to some images, schemes.

According to the image below it seems that the APU fuel pump at the aft side of the center fuel tank.

And this is the view from inside the center fuel tank looking aft:

(instagram.com)

Note the pump still draws fuel from the left line, and not the center tank, it activates automatically if the left side engine fuel pumps don't provide the required pressure.


Is a magnetic compass required for an IFR flight? Is a stand-alone magnetic compass required for an IFR fight when the aircraft has a glass panel with a Horizontal Situation Indicator?

A magnetic heading indicator is required, but not necessarily a wet compass, like this one:

Image Source: Wikipedia - Author: Chopper

Digital devices or combined devices are also possible:

Image Source: WikiPedia - Author: Mysid

The required equipment for VFR flights states, as already posted in this question:

  §91.205   Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.

  (a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

  (b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:

  ...

  (3) Magnetic direction indicator.

  ...

The text further states for IFR:

  (d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:

  (1) Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.

  ...


What time zone is used for operating an airplane? This might be a silly question but my thinking was the following:

There has to be a definition of time in an airplane for the logging of data and so on, but what definition would that be assuming a flight goes across timezones?

Would it be the local time at the airport of departure? Or at that of arrival? or maybe even a third, commonly agreed upon time (e.g., GMT)?

The idea came from a story of a friend that was traveling with the ship from Italy (GMT+1) to Greece (GMT+2). The cafeteria on board had a sign saying that it opens at 14:00, but on 14:20 local Italy time, while the ship was anchored in Italy, he was denied service by the crew saying that the sign was referring to Greece local time; where the ship "belongs".

The pilot (and the ATC) use the UTC (Coordinated Universal Time). The flight's departure and arrival are in terms of the local times at the respective airports.

As for data logging, the FDR/CVR usually records in UTC (preferred) or the relative time count (usually count increments each 4 seconds of system operation). See Appendix M to Part 121 - Airplane Flight Recorder Specifications, for example.


Why did the speed of this plane drop so much? (Disclaimer: I know nothing about aviation. It is not even a hobby to me. I'm sorry if this question sounds obvious or silly to experts.)

I was casually checking the arrival time of a flight on FightAware when I came across this strange speed profile, apparently for a routine flight between Korea and Japan:

(Reference: https://ja.flightaware.com/live/flight/AAR152/history/20171009/0035Z/RKSI/RJSS)

Indeed, it seems that the speed of the plane went slowly down, in a controlled way, and reached 380 km/h for about 5 minutes. Then it went up again, much faster, and came back to its normal value. The event had no influence on altitude.

As I said, I am no aviation expert, but... 380 km/h at 10,000 m seems SO SLOW to me! The speed was more than divided by 2 and actually came close to the speed of a train. To my knowledge, I’ve never been on a flight that slowed down so much in mid-air (and it would freak me out).

I then checked many other speed profiles to see if it was a common pattern, but I couldn’t find anything close to this. How could one explain such a steep decrease in speed? Is it a common occurrence or a rare safety issue? What usually causes this kind of event?

At that point, altitude shows about 11 km, or FL330. Up there, 380 km/h, or 205 knots true corresponds to something like 124 knots indicated, well below clean stall speed.

This is ground speed though, so with very strong head-wind it could be possible. But the only sufficiently strong wind is the Jet stream and flying eastward they would have it in their backs.

Also for westbound flight, the dispatcher would have to make rather big mistake to send them across the jet stream—they do take forecast winds aloft into account when preparing the flight plan.

Data error is the only plausible explanation here.


Why did the MSA for Taipei (RTCP) change? I got a question about the Minimum Sector Altitude (MSA) on Jeppesen arrival plate (RCTP 20-2). The MSA circle  was divided into three sectors with three altitude before (2008, top image), but I found that it is only one altitude (not divided anymore) on the new version.

I've been looking for the reason for a long time but still can't figure it out. Does anyone know why they did such the change?

In 2012 South Korea presented to ICAO design issues with regard to PBN, of those design issues was the reference point for the MSA:

(...) in terms of PBN procedures, every component which is used for RNP approaches or RNP AR approaches, such as waypoint, variation, track and buffer value etc. is based on the airport reference point (ARP). Therefore, it is more reasonable that MSA for PBN Procedures should be centered on the airport ARP, instead of the navigational aids.

Since the old sectors were based on the VOR radials, which is now no longer the reference point after the introduction of the PBN elements, they simply take the highest sector (8,800 ft) and round it up (9,000 ft) to be the MSA for the whole 25 NM.
For uniformity, even the radio based approach (see below) now uses the same reference point as the RNP (RNAV) one. (The one on the left is an old MSA from 2016.)

ICAO's PBN mission is to have uniform specifications and cost-controlled implementation of procedures. So this 'common area' as opposed to the different sectors makes certifying and approving a procedure much easier and cheaper, and puts less strain on the minimum capabilities required from the airborne navigation equipment.
A basic GNSS unit doesn't say which radial/track to a point it is on if it is following a different point, whereas for the VOR reference point, looking at the RMI or spinning the OBS knob for the tuned VOR is pretty straight forward.
This is echoed by the FAA in the TERPS manual, page 2-17. However, note the different points required by the FAA:

RNAV procedures. For RNAV straight-in approach procedures, establish a common safe altitude within the specified radius of the runway threshold (preferred) or the MAP waypoint (WP); for RNAV circling procedures use the airport waypoint (APT WP).

The charts are from the official Taiwanese AIP: http://eaip.caa.gov.tw/eaip


How to pick a good airfoil based on Cl and Cd Let's assume I have 10 different airfoils graphs of Cl x Aoa, Cd x Aoa and Cl/Cd  x Aoa. How do I pick the best one that will give me the most efficiency (the longest flight time)?

I understand the information that those graphs provide and IMHO I think I should always pick an airfoil that has the best Cl/Cd ratio at Aoa (alpha) of 0 since at 0 the drag is usually the least. This is my "common sense" but nobody says this on the internet so I am pretty sure I am wrong!

So based on the three graphs I have of each airfoil, what math should I do in order to get the most efficient (more flight time) airfoil?

PS: the plane will be a floater/slow flier/glider/sailplane. Of course it will be a slow plane cause my focus is on efficiency and fast planes usually are not efficient.

The $\alpha$=0 point on the polar would be a poor metric - there is nothing special about it, and your aircraft will not fly at this polar point when you strive for longest endurance. But your common sense is correct in predicting that this will be a slow airplane.

The precise answer normally depends on your means of propulsion. Since this is for gliders, the answer will be quite simple.

Your definition of efficiency is minimum energy loss. Energy means potential energy in this case $E_{pot} = m\cdot g\cdot h$, and the loss of potential $\frac{dh}{dt}$ energy over time is expressed as the sink speed $v_s$. So we need to find the polar point at which the glider will have the lowest possible sink speed.

Let's start with the parabolic drag equation which splits the total drag coefficient $c_D$ in one component which is constant over the lift coefficient range and one which changes with the square of the lift coefficient.
$$c_D = c_{D0}+\frac{c_L^2}{\pi\cdot AR\cdot\epsilon}$$
 The symbols are:
$\kern{5mm} m\:\:\:\:$ aircraft mass
$\kern{5mm} g\:\:\:\:\;$ gravitational acceleration
$\kern{5mm} \rho\:\:\:\:\:$ air density
$\kern{5mm} v\:\:\:\:\:$ velocity
$\kern{5mm} v_z\:\:\;$ sink speed
$\kern{5mm} c_{D0} \:$  zero-lift drag coefficient
$\kern{5mm} c_L \:\:\:$  lift coefficient
$\kern{5mm} \pi \:\:\:\:\:$  3.14159$\dots$
$\kern{5mm} AR \:\:$  aspect ratio of the wing
$\kern{5mm} \epsilon \:\:\:\:\:\:$  the wing's Oswald factor
$\kern{5mm} S \:\:\:\:\:$  wing area

Next, we need an expression for the sink speed. This should be easy: It is the forward speed times the tangent of the glide path angle $\gamma$. If you allow to approximate the tangent function for small angles by the radian of the angle, you can write:
$$v_z = v\cdot tan\gamma ≈ v\cdot\gamma = v\cdot\frac{c_D}{c_L}$$

Now insert the drag coefficient
$$v_z = v\cdot\left(\frac{c_{D0}}{c_L} + \frac{c_L}{\pi\cdot AR\cdot\epsilon}\right)$$

and make the speed dependency of the lift coefficient obvious:
$$v_z = \frac{c_{D0}\cdot\frac{\rho\cdot v^3}{2}\cdot S}{m\cdot g} + \frac{m\cdot g}{\pi\cdot AR\cdot\epsilon\cdot\frac{\rho\cdot v}{2}\cdot S} = v^3\cdot\frac{c_{D0}\cdot\rho\cdot S}{2\cdot m\cdot g} + \frac{1}{v}\cdot\frac{2\cdot m\cdot g}{\pi\cdot AR\cdot\epsilon\cdot\rho\cdot S}$$

Now we are ready to differentiate with respect to $v$ and find the condition when the derivation will become zero.
$$\frac{dv_z}{dv} = 3\cdot v^2\cdot\frac{c_{D0}\cdot\rho\cdot S}{2\cdot m\cdot g} - \frac{1}{2\cdot v^2}\cdot\frac{2\cdot m\cdot g}{\pi\cdot AR\cdot\epsilon\cdot\rho\cdot S}$$

Now re-insert the lift coefficient, which simplifies the equation mightily:
$$\frac{dv_z}{dv} = 3\cdot\frac{c_{D0}}{c_L} - \frac{c_L}{\pi\cdot AR\cdot\epsilon}\,\overset{!}{=}\, 0$$
$$\Rightarrow 3\cdot c_{D0} = \frac{c^2_L}{\pi\cdot AR\cdot\epsilon}$$

So your energy loss will be minimized when the induced drag is three times bigger than the zero-lift drag. The polar point is then:
$$c_L = \sqrt{3\cdot c_{D0}\cdot \pi\cdot AR\cdot\epsilon}\;\;\text{and}\;\;c_D = 4\cdot c_{D0}$$
With wings of a high aspect ratio this gives a rather high lift coefficient, so you might need to pick a high-lift airfoil in order to actually trim this polar point. The figure of merit when selecting the airfoil should be the ratio $\frac{c^{³⁄₂}_L}{c_D}$; this should reach its maximum at the lift coefficient of minimum energy loss. Enter the lift and drag coefficients into a spreadsheet and create a new column for $\frac{c^{³⁄₂}_L}{c_D}$ (or $\frac{c^3_L}{c_D^2}$; doesn't matter). Select an airfoil that maximises this value at the calculated optimum lift coefficient.


What is this Perspex bubble? Found this during a garden clearance. Have included a photo of serial number. Could you identify where this came from?

dimensions: width 115cm length 140cm.

Few thoughts:

Label font does look British/Commonwealth in some way.
Flat bottom- unlikely to be cockpit canopy that curve down on the sides.
Not compatible with weapons- Search and Rescue aircraft?
Large curvature: Slow aircraft?

At some point I was considering the rear window of the Consolidated Catalina but it doesn't quite seem to fit the bill.

Tony Hisgett on Wikicommons/Source


How can I deal with insurance requirements for flight hours and training in order to carry passengers? I just completed a helicopter rating and was excited to take friends for a ride in the R22 and R44. Then I learned that my flight school's insurance requires a total of 100 hours in helicopters and attendance of a 4-day course at Robinson in California to allow for taking guests. Are there alternative ways to make it legal to take friends for a flight without having to meet these requirements? E.g. can I buy insurance elsewhere?
Thanks!

The flight school might issue a waiver of that policy but it’s going to be based on a number of factors, including instructor opinion of your pilot proficiency on rotorcraft, time in the particular type you want to rent, and whether the insurance company can do this for you.  I’ve had to do that during my flight training when renting high performance and complex airplanes which had certain minimums set for their solo operation under the terms of the school’s insurance policy.  Ask to speak with the chief flight instructor for the school and see if they are flexible with a similar arrangement.  If they feel comfortable with your skill and proficiency in operating the R22 and R22 and you have at least a few hours in each type, they might just approve this.  Again, this is largely their insurance company’s call and not the flight school’s.  I’d guess the school gets discounted rates for insuring their rental aircraft by requiring pilots to undergo factory authorized training programs.