How to maximize your practical lift/weight ratio in a multirotor? My friend and I are currently working together in a multirotor competition.  He's an experienced flyer and I'm more of a numbers guy (though my background is in software engineering, not aviation).

Our latest challenge involves maximizing our Lift to Weight ratio.  Essentially, we're going to measure how much our craft can lift and divide that by our craft's weight.

My preliminary research seems to show that the most obvious way of doing this is by adding more motors+props to the craft.  So, we're considering building a bare-bones dodeca-copter.

This seems like a very simple approach... maybe too simple.  I'm worried that although it's very intuitive it may also be very naive.

So, I ask the community, is our approach sound?  Are we missing some common optimizations that could help us?

In other words:  How to maximize your practical lift/weight ratio in a multirotor?

Update

We've been able to find more powerful motors (E800 instead of E310).  We have fewer of them, but I think we're still better off with them.  Here's the the breakdown of our build:

Main components (Flight controller, receiver, nuts, screws) = 313g
We're using E800 motors

Unit weight: 106g
Unit lift: 2100g

ESC weight = 44g
Prop weight = 14g
Battery weights = {410g, 584g}

The only frame material we have access to right now is square aluminium tubing.  Each "bar" of tubing weighs about 88g.

I did some calculations for a few different setups and this is what I got:

4 motors, 4 arms

This configuration requires 1.5 "bars" of frame materials.  This gives me the following total weight:

(4 * (motorWeight + EscWeight + PropWeight)) + (frameWeight) + batteryWeight + MainComponentsWeight
(4 * (106g + 44g + 14g)) + (1.5 * 88g) + 410g + 313g
= 1511g

The total lift is 4 * 2100g = 8400g which gives us a L/W ratio of

8400g / 1511g = 5.56

6 motors, 6 arms

This configuration requires 4 "bars" of frame materials.  This gives me the following total weight:

(6 * (motorWeight + EscWeight + PropWeight)) + (frameWeight) + batteryWeight + MainComponentsWeight
(6 * (106g + 44g + 14g)) + (4 * 88g) + 410g + 313g
= 2059g

The total lift is 6 * 2100g = 12600g which gives us a L/W ratio of

12600g / 2059g = 6.12

8 motors, 4 arms

This configuration requires 4 "bars" of frame materials but has two motors mounted per arm.  This gives me the following total weight:

(8 * (motorWeight + EscWeight + PropWeight)) + (frameWeight) + batteryWeight + MainComponentsWeight
(8 * (106g + 44g + 14g)) + (4 * 88g) + 584g + 313g
= 2561g

The total lift is 8 * 2100g = 16800g which gives us a L/W ratio of

16800g / 2561g = 6.56

Conclusion

If I'm doing my calculations correctly then it seems that our best option, given our limited resources, is an octocopter on 4 arms.

I'm not all that surprised since this configuration has the highest %age of total weight associated with motors.

Is 6.56 a decent Lift to Weight ratio?  Is there anything else we can do to improve it?

Competition Results

Weigh-in: 2460
Lift (meters of rope): 34m @ 346g/m
Lift (grams): 11764g

Lift/Weight ratio (excluding craft): 4.78

Lift/Weight ratio (including craft): 5.78

So, all in all, not too far off from our "ideal" prediction.

To maximize lift, use the most powerful motors and let them turn the propellers at the highest possible speed. To minimize weight, make the structure as light as possible (use round carbon fiber/epoxy tubes instead of square aluminium ones, for example) and use only as much batteries as needed to achieve lift-off.

I would expect that there should be some more rules to ensure that the winning entry is actually useable. You did not mention any of this. Now efficiency comes into play, and the number of parameters to watch goes up. A lot.

Make sure that your propellers act on as much air volume as possible. Let them run at a reduced speed for better efficiency. You need to test your particular engine-propeller combination where the sweet point is - there is no general number for best efficiency.
Put as little drag in the way of the airstream as possible. This requires careful study of all shapes and cross sections. See on the right side of the picture below for a general overview of the drag coefficient of different 2D shapes.

Figure 33 from Sighard Hoerner's Fluid Dynamic Drag, Chapter 3.

As you can see, round is better than square. However, depending on the local Reynold number, it can help to add turbulators or local roughness to trip the boundary layer.

Also, adding motors to one arm will reduce the efficiency and thrust of the second propeller. Your 8 motors / 4 arm design will end in a disappointment - you cannot simply add the thrust of an individual motor-propeller combination once they interfere with another unit.

Without detailed knowledge of the rules of your competition, it is hard to give more detailed advice. I expect that a lot of educated trial and error will produce the best outcome.


What are these two tail elements of the Avro RJ100? What are these two tail-side elements of British Aerospace Avro RJ100 and what is their function:

Photo source: Eric Verplanken, jetphotos.net.

The one behind is an aerobrake. The tail opens up to increase drag. It is much better seen in this Wiki image:

The one in front looks instead like the APU exhaust.


Which current commercial flights have the highest ETOPS ranking? While the Airbus A350 is technically certified for ETOPS-370, what are currently the highest rated commercial ETOPS flights? Which airline actually operates flights under ETOPS-330 or ETOPS-370 rules today?

Air New Zealand's Auckland (AKL) to Buenos Aires (EZE) is the longest ETOPS flight I know of, under ETOPS 330. It started in 2015:

  Air New Zealand completed the first flight approved for 330-minute extended twin-engine operations (ETOPS), flying a 777-200ER powered by Rolls-Royce Trent 800 engines. The December 1 flight departed from Auckland, New Zealand, and landed in the Argentinian capital Buenos Aires about 12 hours later.

As for ETOPS 370, I'm not aware of eny regular commercial flights, but some candidates are Sydney-Johannesburg and Sydney-Santiago de Chile


How can the glide ratio in a balanced turn be estimated? I am trying to estimate the bank angle/turn radius required to lose a given amount of altitude during a full orbit. This has led me to trying to estimate glide ratio in proportion to $L/D_{MAX}$ of an aircraft in a balanced gliding turn, knowing only $V_{MD}$ and $L/D_{1g}$ for $V_{MD}$.

My approach is to use the following relationships:

$D = D_0 + D_I$

$D_{0} = C_{D_0} 0.5{\rho}v^2 S$

$D_I = C_{D_I} 0.5{\rho}v^2 S$

$C_{D_I} {\alpha} {C_L}^2 $

$C_L {\alpha} L $

$L {\alpha} n $

$n = \frac{1}{cos({\phi})}$ (${\phi}$ = bank angle)

Assuming $D_0 = D_I$ at $V_{MD}$ I think I should be able to calculate:

$L/D_{n_g} = \frac{L/D_{1g}}{0.5 + 0.5(1/cos^2({\phi})}$

Since altitude lost in a full orbit becomes $\frac{2{\pi}r}{L/D}$ where r = turn radius and

$r = \frac{v^2}{g tan(\phi)}$

Altitude lost in a full orbit should ${\alpha} \frac{1}{cos^2({\phi}) tan({\phi})}$ which looks like this. Does my method look alright, or have I made any incorrect assumptions or errors?

I get a variation of the glide ratio with $\frac{2}{\bigl(\cos\varphi + \frac{1}{\cos\varphi}\bigr)}$ for circles flown with a bank angle of $\varphi$ when compared to straight flight at the respective best speed for optimum L/D.

Your initial assumptions are right:

Lift: $L = c_L\,\rho\,\frac{v^2}{2}\, S\, n_z$

Load factor: $n_z = \frac{1}{\cos\varphi}$ with the bank angle $\varphi$

Drag: $D = D_0 + D_i = \Bigl(c_{D0} + \frac{c_L^2}{\pi\, AR\,\varepsilon}\Bigr)\,\rho\,\frac{v^2}{2}\, S$ (zero-lift drag + induced drag), and both are equal at optimum $L/D$:

$$\Bigl(\frac{L}{D}\Bigr)_{\rm opt} = \Bigl(\frac{c_L}{c_D}\Bigr)_{\rm opt} = \frac{\pi\, AR\,\epsilon}{2\, c_L}\text{ for straight flight at }1\, g \text{ ($n_z = 1$)}.$$

Now with load factor $n_z = n$ and unchanged speed:

$$\frac{L_{ng}}{D_{ng}} = \frac{\frac{c_{L_{1g}}}{\cos\varphi}}{\frac{c_{L_{1g}}^2\bigl(1 + \frac{1}{\cos^2\varphi}\bigr)}{\pi\, AR\,\epsilon}} = \frac{\pi\, AR\,\varepsilon}{c_{L_{1g}} \bigl(\cos\varphi + \frac{1}{\cos\varphi}\bigr)}.$$

The ratio between both glide ratios is $\frac{2}{\bigl(\cos\varphi + \frac{1}{\cos\varphi}\bigr)}$, which makes sense because it will become 1 for $\varphi = 0$ and become less than 1 for nonzero values of $\varphi$. Note that the glide ratio at n g doesn't carry an "opt" subscript anymore, because by increasing $c_L$ we move away from the optimum. If the best L/D would be maintained for circling flight, speed would need to increase, not lift coefficient.

The altitude loss in straight flight of a distance equal to the diameter of a circle with the radius $R = \frac{v^2}{g\tan\varphi}$ is $2\,\pi\, R \, \frac{D}{L}$. When flown at the correct bank angle, this altitude loss per circle $\Delta h$ will become
$$\Delta h =  \frac{\pi\, v^2}{g \tan\varphi} \Bigl(\frac{D}{L}\Bigr)_{1g}\Bigl(\cos\varphi + \frac{1}{\cos\varphi}\Bigr).$$

Plot of the glide ratio with bank angle divided by the glide ratio for straight flight (blue line). I also added the altitude loss for a full circle (red line, Basis: $L/D = 18$ at 100$\,$m/s in straight flight), both for flight at constant speed and constant lift coefficient:

Use the results from this plot with caution at higher bank angles: Lift would need to double at a bank angle of 60°, doubling $c_L$, so we fly much more slowly than any pilot would. The red line gives more realistic values, since here the aircraft speeds up for flying higher bank angles. Flying at constant speed gives less altitude loss simply because the circle becomes smaller than when flying at constant $c_L$. Note that for every meter flown the altitude lost is greater when flying at constant speed, indicated by the blue glide ratio degradation curve.

Two factors conspire to increase aerodynamic losses when flying at constant speed with higher bank angles:

You need to create more lift, which causes higher induced drag, and
Your optimum $L/D$ is at a higher speed when the bank angle increases, which again increases aerodynamic losses because you move away from the optimum.

If you try to express the altitude loss per circle $\Delta h$ as a function of $\varphi$, you will get a quadratic equation and two solutions, one high and one low bank angle:
$$\frac{\cos^2\varphi + 1}{\sin\varphi} = \frac{g\,\Delta h}{\pi\, v^2} \Bigl(\frac{L}{D}\Bigr)_{1g}.$$
Maybe MathSE can solve this algebraically; I would resort to iteration or tabulation here.


How to record flights and cockpit radio com using an Android phone? I am a student pilot, and would like to record my training flights (in a C152) to look back on. I do not have a Go-Pro, and do not want to fork over the money for one. I do however, own a Samsung Galaxy S5, that has a pretty good camera.

I was wondering if anyone had some ideas as to how I could record my flights using my phone? I have bought a mount, a Y-cable connection for the 1/4"" phone jack, an TRRS cable and a Headphone/Microphone splitter cable for recording the headset audio.

I have, however, noticed that the avionics interferes with the recording and causes the phone to stop recording. Does anyone happen to know why this is?

In addition, this question is not limited to my specific cables/connectors.

Please let me know if anyone has found a working solution with their Android phone in the past. A similar question was answered for IPhone users. Does anyone have experience with Android phones?

Android by default has Google Now enables, which include voice search. Voice search listens for the words "Ok Google" when you speak, but phones also detect a long press in a headphone button (on headphones with controls). It can also be fooled into activating by odd signals like the pop when you turn on the avionics master switch or loud droning noises like motorcycle and airplane engines.

First thing I would try is to turn off the voice detection, see this page for instructions. Now google voice should stop popping up if the phone is being fooled by noise.

However, that may not be the end of it if the phone is detecting static or other signals as a long headphone control press, in which case it may still interrupt your setup. In that case you will need to use an app to disable the headphone controls, the one I've heard of is SoundAbout, or your phone's distribution may have settings to let you do it.


Why is water-contaminated fuel bad, but water-injection is not? A "wet" takeoff of a KC-135 with J57 engines—By USAF Photographer (USAF photo) [Public domain], via Wikimedia Commons

Why is water-contaminated fuel bad, but water-injection is not?

The way I understand it, water-injection lowers the combustion temperature, which is good and bad, good for cool yet increased power-output / mass flow, bad for wasting [unburnt] fuel.

Why is then water dangerous when a trace amount of it is found in fuel? What's the worst that can happen apart from black exhaust and higher power?

Water-injection in jet engines and piston engines is mixed with fuel before it enters the combustion chamber, so I'm thinking fuel already mixed with water should not be any different. Of course I'm wrong. But why?

Because of the quantity of water in the fuel, as opposed to a careful introduction of water into the combustion process.  Typical water contamination is bad in fuel tanks as water is denser than aviation fuels and settles at the bottom of the tanks.  Aviation fuels are also hydrophobic (non-polar) and so do not readily mix with water.  Therefore water will settle unmixed at the bottom of the tanks, in the fuel sumps or feeder hoppers and when ingested into the engine will fuel starve it and cause it to shut down.  Water in fuel lines can also freeze there during cold weather, causing fuel starvation as well.

Fuel samples should always be taken of the gas when it is both in storage as well as prior to flying the aircraft, especially after rainstorms or the aircraft has sat on the ground for long periods with low fuel in the tanks in a humid environment.  Fuel samples should also be taken approximately 30 minutes after refueling and the gas allowed to settle in the tanks.  A preflight sample of water contamination taken from the fuel tank sumps of a GA aircraft can be seen here.

Water injection, however carefully controls the amount of water going into the engine along with fuel at the point of combustion, allowing for continued engine operation.


How are further structural failures prevented if an aft pressure bulkhead fails? In a modern airliner, if the aft pressure bulkhead experiences a structural failure, what design features are incorporated into the empennage to prevent the empennage from being overpressurized (i.e. at or near cabin pressure vs the normal case, which is at or near outside pressure) to the point of structural failure?  Or, in other words, what keeps the next aft pressure bulkhead failure from blowing the vertical tail, or some other important part, off the airplane a la JAL123?

If what's escaping a compromised bulkhead is cabin pressure, not an explosion; the airframe structure aft of the pressure vessel is wide open for air to escape.  The structure is hollow from one component to the next with "lightening holes" wherever possible designed to minimize weight (nothing to do with thunderstorm lightning).  Additionally, the lightening holes provide for plumbing, wiring, maintenance, inspection access, and ventilation.

Unless there's an explosion or some catastrophic failure of the pressure vessel, escaping cabin pressure by itself cannot pressurize the open empennage.  Pressurized aircraft also have a safety valve to protect the pressure vessel from structural damage in the event of outflow valve failure.  The prospect of aft pressure bulkhead failure before other skin fracture is unlikely.

If you are able to peer in between the rudder and vertical stabilizer of an aircraft, you will likely see the openings.


Are there countries where not all plane crashes are investigated (even if deadly)? background information
I was struck by the news of this deadly plane crash on 20 Dec 2018 which claimed 8 lives and apparently is not investigated as the plane and its potential survivors were not even searched for:

A Gomair Antonov AN-26 on behalf Central Electoral National
Independent Commission (CENI), registration 9S-AGB performing a cargo
flight from Tshikapa to Kinshasa Ndjili (DR Congo) with 7 or 8 crew
and election materials, was cleared to descend to 5000 feet on
approach to Ndjili's runway 06 when the aircraft went missing about
20nm short of Kinshasa's runway 06 at about 10:00L (09:00Z). The
aircraft was found about 24 hours later in hilly terrain (peaks rising
up to 700 meters/2300 feet MSL) about 19nm before runway 06. Local
Congolese media report Authorities did not launch a search for the
aircraft. The aircraft was found by random chance by a local, who
reported 8 bodies in the wreckage. (source)

question
Aren't countries bound at least (if nothing else) by international treaties and ICAO regulations to

perform SAR operations for missing aircraft lost in their sovereign airspace
and investigate the cause of the accident ?

In which countries can deadly plane crashes lack either one or both ?

scope
Q (from a comment below): "There are countries which may not have the resources to conduct their own investigation of a crash and may request assistance from another country."
A: Good point. Countries unequipped to handle S&R or crash investigations can voluntarily outsource either one or both to other countries. For the pupose of this question, a crash in country X investigated by country X or Y does equally count as being investigated, so this is NOT what my question is about. I'm asking about dead passengers searched for by NOBODY and crashes investigated by NOBODY.

As of 2017 191 of the 193 countries are signed to the ICAO anex and there are 122 national aviation authorities so at least some countries lack serious formal aviation authority and often look to other nations for help. Smaller, war torn nations or nations that do not have the facilities of first world countries may simply not have any ability to provide SAR operations. Its not just about having a helicopter or two hanging around but having a trained crew, a flight doctor and a hospital to bring the potential survivors back to.

Countries are under no obligation to provide SAR operations and generally speaking international treaties can't really force any country to offer something like that. In some cases the crash sites can be difficult to access or outside the range of practical rescue at the time of the incident. In these cases there may be incidents where a country simply cant investigate as it may risk the lives of those investigating.

There are some notable exceptions, the FAA will provide aid to any investigation involving an American built aircraft or having Americans on board anywhere on the globe. This is of course assuming they are permitted to provide such aid (which may not always be the case). The FAA also has a lot of tooling for recovering black box data and in many cases foreign investigations will send the black box to the FAA facility if they lack the data reading tools.

Its not always known when or where planes crash. This generally applies to smaller aircraft that fly VFR from uncontrolled fields. Broadly speaking its possibly to not tell anyone you are going flying, depart at night, and crash in a remote area unknown to anyone that you were ever up or what your flight path was. In this case its possible no searching is done if no one even knows the aircraft needs to be located.


Is there a difference in stability between a tail downforce (cg ahead of ac) and a tail upforce (cg behind)? So from what I understand, having a -

1.cg ahead of the neutral point(ac) with a tail downforce

and

2.cg behind ac with a tail upforce

are both stable configurations.(Please correct me if I'm wrong.)

Will the first configuration be more stable due to less lift at the tail?
What are the advantages and disadvantages of both these configuration?

Also if I have a configuration with cg between cp and neutral point, neutral point being the most aft point, will it be stable?

No, cg ahead of neutral point is stable, cg behind it is unstable.

Neutral point is the centre of pressure when both main wing and horizontal stabilizer fly at the same $C_L$. If the cg is just a bit ahead of neutral point, the tail still flies with positive lift. If it is ahead of the centre of pressure of the main wing, tail flies with negative lift.

The further forward the centre of gravity is, the bigger the difference between the coefficients of lift and therefore the stronger restoring force. But note that since it is second order negative feedback, the restoring force creates phugoid oscillation and if that is too fast, it will be difficult for the pilot to stop. So the stability has to be just right.

This is still true in canard configuration, but there the forward surface that flies at higher $C_L$ in stable configuration is the canard.


What features enable the Su-25 Frogfoot to operate with such a wide variety of fuels? It is widely attested* that the Su-25 Frogfoot's engines (the Turmansky R-95Sh and later the R-195) were designed to function using a wide variety of fuels, including diesel, gasoline, petrol, kerosene and avgas.

For a rough-field front-line attack aircraft, the ability to load it with whatever was at hand, even tank fuel, would be very useful.

It's not a feature I've read about anywhere else. The question is, what specific engine modifications allow this level of fuel-tolerance in comparison with, for example, the T-34 which powers the A-10, an aircraft designed for a similar role.

 * for example miltaryfactory.com lowflying.net

 Su-25 Grach  (NATO reporting name: Frogfoot)

(Previous questions asked whether diesel engine aircraft can run on car diesel (yes) and if you could safely run a 777 on diesel (no).)

Being a ground attack aircraft, the Su-25 has an operational ceiling of 23,000 ft and will not encounter the low temperatures to which a Boeing 777 is regularly exposed. Therefore, the risk of diesel fuel gelling and clogging the fuel lines is much lower.

On the other side of the temperature scale, gasoline could vaporize in hot environments and equally endanger the fuel supply of the engines. Those extremes are rarely found in the countries of the former Eastern Bloc, and the Su-25 was primarily designed for operations in moderate climates.

Nevertheless, some modifications are advised in order to avoid trouble when exotic fuels are used:

Regular fuel pumps use kerosene for lubrication. Kerosene is a good lubricant and allows for a simpler pump design. By using better bearings, the fuel pump can also be used with gasoline or petrol without suffering long-term damage.
Running the fuel lines away from hot surfaces reduces the risk of vapour bubbles forming inside. I don't know the details of the Su-25 design, but I would be mightily surprised if any fuel line runs right below the upper skin of the aircraft where the sun can heat the surface to temperatures in excess of 70°C even in moderate latitudes. This is supported by a light grey paint scheme as shown by the Ukrainian Su-25 in your question.
Don't do fancy things with the fuel, like use it as a hydraulic fluid, but restrict its use to being burned in the engine, and maybe cool some engine oil on its way into the combustor. Then any liquid hydrocarbon can be used.

That turbofans can run on a wide variety of fuels has been established in the answers to this question.


How safe are extreme aerobatic flights? Statistically, how safe are extreme aerobatics flights like this one?

The maneuvers listed are not really extreme, and it is certain that, catering for the general public, they will be accomplished at a relatively high altitude. The airplane is widely used in the aerobatic field and is generally regarded as reliable. Therefore I believe that there is no more danger than there would be if you were doing a regular flying lesson.
Edit: I have dug up some more numbers for the aircraft type. There has been one non-fatal accident listed involving an Extra 300 in the Aviation-Safety database for the year 2020. From 1992 until present there have been 82 accidents listed of which 42 were fatal, resulting in the deaths of 67 people, including spectators listed for this aircraft type at Aviation-Safety.
As far as aerobatic flying is concerned, you can refer to this article for an overview of aerobatic safety. Most accidents took place during airshows rather than during training.
Below is the NTSB's summary of aerobatic mishaps from 2008 - 2015.


Can a lifting body work at supersonic speeds? Can lifting bodies be made for supersonic flight, say up to Mach 3? What would they look like? How would they differ from lifting bodies for normal subsonic flight?

Lifting bodies were extensively used for reentry vehicles. In the Fifties and early Sixties there were two camps with different ideas how to go to space:

The Air Force wanted to increase vehicle speed and altitude incrementally such that finally a horizontally-launched, winged vehicle would reach space. Since the materials and propulsion were still lacking, they started at both ends of the flight envelope: One culminated in the X-15 research program, and the other researched how best to land a vehicle suitable for re-entry. This produced craft like the Northrop HL-10, which is shown below.

Northrop HL-10 with B-52 launcher overhead (picture source)

The US Army, on the other hand, tried to increase the size of rockets, traditionally used as an alternative to artillery, and got a helping hand from some other ex-Army guys who had lived through exactly the same split of technologies, where the Wehrmacht preferred rockets and the Luftwaffe preferred flying vehicles.

So not only can lifting bodies be made for Mach 3 and higher, they pretty much originated from the work done to fly at hypersonic speed, at least for a short while.


Why is the ratio maximum range speed/maximum endurance speed the same for any airplane? It seems counter-intuitive that the ratio of speeds for maximum range and maximum endurance is always the same, $3^{0.25} = 1.316...$ for any heavier-than-air aircraft, regardless of its mass, size or form. It's hard to believe that such ratio may be identical for an ultralight autogyro and for the F-35 fighter, but so it seems to be...

Why is it so?

The ratio is the same for all aeroplanes if you accept a number of assumptions:

The propulsion efficiency is constant, regardless of speed or power setting
Aerodynamic drag is the sum of parasite drag and induced drag
Parasite drag is proportional to the square of airspeed:$ D_p = k_p \cdot V^2$
Induced drag is inversely proportional to the square of airspeed: $D_i = \frac{k_i }{V^2}$
There is no wind

Since we assume the propulsion efficiency is constant, the fuel consumption rate is directly proportional to the power required. The power required is drag times airspeed:
$P = D\cdot V = D_p\cdot V + D_i\cdot V = k_p\cdot V^3 + \frac{k_i}{V}$
For the maximum endurance we need to minimise the fuel consumption and thus we need to find the speed that minimises the power.
$\frac{dP}{dV} = 3 k_p V^2 - \frac{k_i}{V^2} = 0$
Solving for $V$ results in
$V_{endurance} =  \sqrt[\uproot{1}4]{   \frac{k_i}{3k_p}} $
For the maximum range we need to find the speed that minimises the fuel consumption per distance travelled, which is found when the ratio of power to  speed over ground is minimal. As we assume there is no wind, the ground speed and the airspeed are equal.  Since the ratio of power to airspeed is drag, we have to find the speed for minimum drag:
$\frac{dD}{dV} = 2 k_p V - 2\frac{k_i}{V^3} = 0$
Solving for $V$ results in
$V_{range} =   \sqrt[\uproot{1}4]{   \frac{k_i}{k_p}} $
We can now show that the ratio of maximum range speed to maximum endurance speed is:
$\frac{V_{range}}{V_{endurance}} = \left. \sqrt[\uproot{1}4]{   \frac{k_i}{k_p}} \middle/  \sqrt[\uproot{1}4]{\frac{k_i}{3k_p}} \right. = \left. 1 \middle/  \sqrt[\uproot{1}4]{\frac{1}{3}} \right. =  \sqrt[\uproot{1}4]{ 3} = 1.316... $

Now as long as the assumptions hold, for any aircraft the ratio of the speed will be approximately 1.3.
For the F-35 the ratio may be approximately right in the subsonic domain. Effects of compressibility will cause the drag to be higher when the aircraft gets into the transonic domain so depending on the max-range speed the last two assumptions may not hold.
For an autogyro at higher speeds, the relation of induced drag to airspeed is inversely proportional to the square of velocity, just like conventional fixed wing aircraft. This is because the fundamental way of creating lift by deflecting the incoming airflow downwards is the same for autogyros and aeroplanes. Therefore autogyros have also a factor pf approximately 1.3 between their maximum endurance and maximum range speeds.


Was the adoption of conventional tailplanes as 'everyday aircraft' inevitable, or are there other designs that can be just as good? I originally asked a variant of this on Worldbuilding.SE, but they didn't seem to like it. So I apologize if it's out of place even here.
I ask, because I've been forming in my head an alternate history or two that would logically involve a completely fresh start to aviation; no Wright Brothers, no Otto Lilienthal -- the entire world looks slightly different on a political map.
Because the world is different, it would be lazy of me to just import elements of our timeline without understanding why things developed the way they did. Our world, in general, appears to have rolled with canard planes and later adopted the tailplane for just about every conceivable function for fixed-wing aircraft.
That alone seems to imply that the tailplane is the superior choice to any other design, but is it really? Or was it just a case of technological and economic inertia?
Since 'superior' can be subject to opinion, I'll try to narrow down some criteria:

The design has comparable advantages in stability, lift production and maneuvering.
The design has comparable economic advantages; it doesn't necessarily require more time and resources to build and maintain.
The design does not (necessarily) incorporate concepts that require an advanced and well-developed understanding of fixed-wing aerodynamics. These would be among the early aircraft designs, or at least a short time into successful flight, so things like vortilons, fly-by-wire, etc. would not be present.

There are definitely plenty of wacky creations like Burt Rutan's Quickie, the V-173 and Miles M.39, but I'm sure there are good reasons none of them ever became popular...right?

If you write an optimizer which modifies the wing spans of a virtual aircraft, you will end up with a big wing in the middle and a smaller one at the tail if your goal was to achieve good performance, a wide cg range and docile behavior. So yes, a concurrent evolution would also result in a conventional layout.

Why no canard?

The forward wing is more highly loaded for stability. Adding control surfaces there would reduce the possible control effectiveness. Putting them at the large rear wing will result in higher stick forces and higher lift changes for the same pitch moment changes.
For the same reason, the wing cannot utilize its full lift potential because the canard has to stall first.
Since the wingspan of the canard is smaller than that of the wing, the wake of the canard will hit the wing and disturb the lift distribution over span there.
For directional stability a tail is still needed, even though it is only used for vertical surfaces. Alternatively, the canard wing can be swept and winglets be used for directional stability, but adding sweep will reduce the efficiency of the wing.

This all puts the canard at a distinct disadvantage. It works, but comes at a price.

Why no flying wing?

Flying wings have low pitch damping and a very limited center of gravity range.
They often cannot have trailing edge flaps for extra lift during slow flight, so their take-off and landing speeds are higher than those of a comparable tailed aircraft. Or the wing loading has to be made lower, which hurts at high speed.
Even though a good flying wing design has sweptback wings, they are poorly suited to high transonic speeds. Their wing airfoils cannot have rear loading, so the critical Mach number at the same lift coefficient is much lower than for conventional configurations.


What is the regulation related to weather radar use? I think most commercial aircraft have a weather radar located in the nose cone, like this:

(Source: PhotoBucket, by intelmani5)

It's well known that radar emissions can be harmful, e.g. one could get their eye burnt like an egg in a microwave oven.

However I don't know:

About the typical frequency, power and duty cycle (time in transmission mode vs time in reception mode).
Whether there is a risk for a harmful radiations exposure.

My question is: Based on current radar specifications and regulation, is there an overall ICAO (or local, e.g. US or EU) regulation forbidding the use of the radar on the ground to prevent radiation exposure for people close to the aircraft nose (or conversely to be ON at certain phases of the flight for cumulonimbus detection)?

The FAA has AC 20-68B, which limits ground operations to qualified personnel only.


Is "General Aviation" a well-defined term in the US, in the EU and/or elsewhere? In this earlier question I tentatively assume that GA means civil aircraft operations other than scheduled passenger transport - what I think of as "airliner" operation though it could include quite small aircraft operating a scheduled fare-paying passenger service.

It has been suggested to me that GA can sometimes or often be used to mean all aviation in general - perhaps in some contexts more than others.

I had thought that most people here used GA to mean aircraft operations other than scheduled service, but now I have my doubts.

So, is there an authoritative definition of GA in the USA (and elsewhere, or worldwide) or is it a vague and varyingly interpreted general phrase that I should avoid to prevent misunderstanding?

The FAA does not include a definition of General Aviation in CFR 14 (aka Federal Aviation Regulations) Part 1 (General Definitions and Abbreviations), but, from a functional perspective, in the US, GA is generally used to refer to any civilian operations falling outside of Part 121 and some Part 135 operations (Commuter and Charter Operations - regional feeders have and may still operate under this section if they can meet the requirements, as it is more relaxed and easy to comply with than part 121).

With respect to the FARs, it's important to remember that Part 91 covers ALL types of flight operations UNLESS its provisions are EXPLICITLY overriden by another paragraph (as would be the case with an air carrier's Operating Specifications, part 121, 135, and other sections dealing with more specialized aerial applications). Part 119 also deals, in a very general sense with conducting flight operations for compensation.

IMO, you're not likely to go wrong using the term to refer to everything civilian outside of scheduled carrier and medium/large cargo operations (e.g. Atlas, Evergreen, FedEx, UPS). The caveat is, of course, that GA operations are so diverse that this is more of an "everything else" category rather than the much clearer images conjured up by a flag/commutter/regional carrier or a large cargo operator. If you're unclear about whether a specific application or air operator falls under GA, go ahead and ask.

The FAA division of airports is more likely meant to reflect that certain airports have certain primary intended purposes (e.g. many of the nation's largest airports do not allow Part 91 operations, only air carriers) and may require certain types of infrastructure in order to allow them to perform those roles.

And yes, using a 737 as a personal plane under Part 91 would fall under GA - Boeing even makes a specialized version just for that, see the BBJ.


What was this mystery flash on an airport runway? Recently I was onboard a commercial airliner and we had just landed and were headed for the gate. I cannot remember exactly which airport but it was either Schiphol(Amsterdam Airport) or Bristol. As we were taxiing I was looking out of the window and saw an object(?) fall vertically into my eyeline from about 10 m up. This object was maybe 50 m away and was at the edge of an intersecting adjacent runway/road and the grass. I noticed it because it had a faint trail of smoke emanating from it. At around 3 m above the ground it ignited, there was a short white flash and it made a small white cloud of smoke that lingered for a short while.

Does anyone know what this might have been? It was almost like a flare/firework (which makes no sense at an airport) so is there another explanation?

It's likely to be a flare / flash bang for bird control. Schiphol uses these (amongst a wide variety of methods).

  Bird dispersal equipment

  There are various resources a bird controller
  can use to keep birds away from the runways. Standard equipment
  includes a flare gun with noise blanks, a bird alarm call system and a
  green laser.

  The flares and noise blanks come in various types,
  including a number of ‘whizzers’. The flare used depends on the bird
  species and the number of birds the controller wants to scare away.
  Some species respond better to a loud bang and others to a screeching
  noise. Bird controllers also have all the most common bird alarm calls
  at their disposal, which act as a warning for the other members of the
  species to leave the area as soon as possible. All bird controller
  vehicles are equipped with a mobile green laser. This is a type of
  torch that emits a green laser beam to chase water birds away from the
  surrounding ditches.

Source: Schiphol Bird Control information brochure

At Schiphol the Bird Control vehicles are equipped with ADS-B. This makes them visible on flightradar24. If you remember the day of your flight, you maybe able to see if they were operating in the area near your taxiing aircraft. The cars have callsign Kievit (peewit / lapwing) and use abbreviation "KV#" as flight ID on ADS-B.


Can gliding your plane save fuel? Aimed towards lighter piston planes including a powered glider, if a pilot wants to extend a plane's range unconventionally, could flying like a glider by climbing and then gliding down or would flying straight, lower or slower save fuel; even if it means extending flight time?

This approach of using an engine is called pulse and glide. It generally works because each engine has an optimal power setting at which it converts fuel into power most efficiently.
If the most efficient power is higher than is required, something should be done to accumulate and later use the excess energy. Raising the vehicle up looks like a solution, better than accelerating.
This method can only work if the power to efficiency curve of the machine is known. Otherwise, it is easy to do worse.


What are the other clearance and instructions that must be read back? The following items must always be read back:

ATC route clearances;
clearances and instructions to enter, land on, take off from, hold short of, cross or backtrack on any runway; and
runway-in-use, altimeter settings, SSR codes, level instructions, heading and speed instructions, and, whether issued by the controller or contained in Automatic Terminal Information Service (ATIS) broadcasts, transition levels.

In the definition it goes on like this: Other clearances or instructions, including conditional clearances, must be read back. What do I have to obtain from these "other clearances or instructions" I think it is the really broad term?

Generally speaking, you should read back all clearances, instructions and warnings. You may in some cases not need to read back information, unless it is safety critical (for example QNH).

One of the only instructions I can think of that you do not need to read back is an instruction to report something (report passing 5000 feet, report established, report overhead XYZ). A proper reply to an instruction to report something would be "WILCO". But in all other cases, if ATC directly instructs you to do something, it should be read back.


How to become pilot in India with lack of money Hello I am from India currently I am in class 12 pursuing PCM and want to become a commercial pilot but belong to a middle class family how can I pursue my dream. I can't afford course of 60 - 80lacs.

Airlines in most countries (outside of US and Canada) usually offer ab initio pilot programs, meaning they will train a pilot with no experience. Training will be paid for in the form of a loan which will be deducted from your future salary.

However there are some caveats. First, if you fail, you will be responsible for paying off the loan. There is no guarantee you will pass or be offered a job.

Second, these programs are highly competitive. Most programs require a college degree simply due to the number of applicants. Even then, you will have to be high in your class and be the best in all of their tests. I understand typical numbers at European airlines are 1 accepted out of 1,000 applicants.

Some people will suggest the military. Keep in mind that military pilot positions are often equally competitive. In the US, you must have a four year college degree to become a military pilot as well. I believe India is similar to the US as somebody after secondary education may apply to obtain a degree in a military academy, though that is highly competitive in itself.

Therefore, the best way may be to save money and/or get loans and gain your licenses yourself.


What is the smallest manned aircraft with four engines? What is the smallest (by operating empty mass) quad-engine aircraft that satisfies these conditions?

Current or past airplane
aircraft with fixed wings, not a rotorcraft, not a lighter than air, etc
4 engines, pistons or turbines
fuel as a main source for propulsion
actual aircraft, not scaled model
piloted by a pilot or a crew
used for an actual military or scientific goal or business (not for design tests only).

Added to prevent flying suits to be proposed:

Need to takeoff and land under its own power.
Not a worn wing (flying suit), the aircraft must have a flight deck where the pilot can be.

If this one had been piston or turbine, it would be a good candidate:

... but it's an electric-powered aircraft.

I present you Yves Rossy's Jet wingpack, which satisfies your conditions (I know I'm pushing it, but couldn't resist).

Yves Rossy's Jet wingpack, By Rama - Own work, CC BY-SA 2.0 fr, Link

It is certified by FAA as an aircraft, powered by four Jetcat P400 turbojets, piloted by an individual, not a rotorcraft and used in show business.

According to the official site, it weighs 150 kg.

@ymb1's logic is correct. To get to lightweight four engined aircraft with, you have to go back when the engines were not powerful enough for large aircraft. So, I present you the Sikorsky Ilya Muromets, the World's first four engined bomber.

Sikorsky Ilya Muromets, By www.cultinfo.ru, Public Domain, Link

The Ilya Muromets Type S-23 V is reported to have an OEW of around 3700 kg, powered by four Sunbeam Crusader engines. Its predecessor, the Sikorsky Russky Vityaz, was still lighter, though it never saw service in numbers, sadly.

Of course they are not autonomous as per your requirement. Why would you want a manned autonomous aircraft anyway?


What is the difference between regulatory and nonregulatory airspace? The first paragraph of the FAA PHAK Chapter 14 - Airspace says:

  The two categories of airspace are: regulatory and
  nonregulatory. Within these two categories there are four
  types: controlled, uncontrolled, special use, and other
  airspace.

It never says anything further about regulatory and nonregulatory. So what do these terms mean and what are their differences?

Non-regulatory airspace is MOAs, Warning Areas, Alert Areas, and Controlled Firing Areas (source 1), (source 2).

Other information should be located in the AIM Ch 3-4, Special Use Airspace. Specifically 3-4-1(c)

Basically non-regulatory areas mean that the FAA does not make regulatory enforcement in those areas due to special (military) operations. Usually the non-enforcement is only for participating aircraft, and generally recreational/commercial aircraft would not fall under any non-regulatory rules.

For example, in an active MOA with aircraft practicing low-altitude speed runs, the FAA would not enforce the 250 knot speed limit on participating aircraft.

This is in the AIM, Ch 3 Section 4, Part 5(b)

  Military pilots flying in an active MOA are exampted from the provisions of 14 CFR Section 91.303(c) and (d) which prohibits aerobatic flight within Class D and E surface areas and within Federal Airways. Additionally the Department of Defense has been issued an authorization to operate aircraft at indicated airspeeds in excess of 250 knots below 10,000 feet MSL within active MOAs..

For another example, consider warning areas, which are areas extending 3 miles from the coast. Since this area may include international waters (think of the area between Alaska and Russia), the FAA can't really regulate operations in all these areas. The US Navy also controls some of these areas and "shares" jurisdiction with the FAA.


Why all the taxiway construction at PHL? Perusing Google Earth around Philadelphia, I noticed some new taxiway construction at PHL/KPHL.  The "time machine" feature of GE shows this to be new (latest imagery data is May 2016)--the paved areas in the red circle below don't appear in past aerial imagery.

What are all these paved areas for?  Are they extending 10R/27L?

From Google Earth

Note:  The Google Maps image for this area shows an older aerial image (without all that extra concrete).

From the KPHL website

  Realign Taxiway H and Establish Taxiway EE (North)

  This is the first phase of a multi-phased project to provide a 1,494
  linear-foot extension of Runway 9R-27L, construction of associated
  eastside taxiways and hold pads.  In this phase a new Taxiway EE will
  establish a connection between Taxiways H and K and Taxiway H will be
  realigned between Taxiway EE and the existing Taxiway D.

  Projected completion date: August 2016

As well as,

  9R-27L Extension - Site Prep, Utility, & Fuel Line Relocation

  This is the first phase of a multi-phased project to provide a 1,494
  linear-foot extension of Runway 9R-27L, construction of associated
  eastside taxiways and hold pads.  In this phase a new Taxiway EE will
  establish a connection between Taxiways H and K and Taxiway H will be
  realigned between Taxiway EE and the existing Taxiway D.

  Projected completion date: October 2016

The imagery predates the projected completion dates so it makes sense.


How much are noise-related landing costs for a long-range aircraft at Heathrow? I am trying to estimate the noise-related costs for a long-range platform aircraft (for example, the Boeing 787) at Heathrow Airport.

I have found the following table (PDF, page:24) outlining some costs for night/day landings, but can anyone tell me which 'chapter' a B787 would considered? I have looked around extensively, but cannot find any clear explanation on this.

As DeltaLima states, the 787 is a Chapter 4 aircraft.

Directly under the table in your question is another table listing the requirements for each category.

So the categories are based on the certified Chapter, and the margin to Chapter 3 standards. Based on these values, the 787 would be categorized as Chapter 4 Low.

You can look up aircraft in the NoisedB here. Each aircraft is tested for its noise margin. I looked at ID:BOEING_14816, which is a 787-8 powered by a GEnx-1B. It has a noise margin of 30.9, which is compared to the Chapter 4 standard. From looking up a Chapter 3 aircraft like a 747-400, it can be seen that the cumulative reduction between Chapter 3 and 4 is 7.3 EPNdB. I did not see any aircraft that would be above the Chapter 4 Low levels, and most are in the "30 or more" category.


If a shoulder harness becomes INOP in an aircraft certified before 1978, is it still required equipment? So we fly a 1973 Piper Archer, with shoulder harnesses installed from the factory. The question is:

Would the shoulder harness becoming INOP (jamming & not coming loose) ground this airplane or could you fly without it, since the FAA required shoulder harnesses as of 1978 (or some other year...?)?

Shoulder harnesses have been required for all aircraft manufactured since December 12, 1986 for all seats, July 19, 1978 for front seats.
14 CFR 91.107 (2) and (3) is pretty clear:

(2) No pilot may cause to be moved on the surface, take off, or land a U.S.-registered civil aircraft (except a free balloon that incorporates a basket or gondola, or an airship type certificated before November 2, 1987) unless the pilot in command of that aircraft ensures that each person on board has been notified to fasten his or her safety belt and, if installed, his or her shoulder harness.
(3) Except as provided in this paragraph, each person on board a U.S.-registered civil aircraft (except a free balloon that incorporates a basket or gondola or an airship type certificated before November 2, 1987) must occupy an approved seat or berth with a safety belt and, if installed, shoulder harness, properly secured about him or her during movement on the surface, takeoff, and landing.

So what I read above can be interpreted as "if it's installed, it's required".
Some aircraft required an STC to retrofit the belts, you'll need to find out if that is the case (if it is, you can't remove it, but if there is no STC it could possibly be removed, a call to your local FSDO can clear that up). However I've been told that if the aircraft is equipped with shoulder harnesses, then they are required to be used.


What are the main factors that make an aircraft have good STOL performance? What are some of the main characteristics and design principles utilized to make an aircraft have good STOL performance?

As a bonus, what are some disadvantages to some of those characteristics and thus reasons why they aren't utilized on all aircraft design (cost aside).

Good STOL performance is directly linked to how slowly an aircraft can fly and how quickly it can accelerate and decelerate when on the ground.

Low wing loading (a lot of wing for a given aircraft weight) with a lot of camber is the primary means for attaining slow flight. It is not used for every aircraft because a big, cambered wing creates more drag than a small wing, which limits top speed. Flaps and slats can help by modifying the shape of the wing to provide more lift when flying slowly while keeping drag low when retracted for top speed. These add complexity and weight so they are used by aircraft needing speed.

Rapid acceleration requires power, which costs money and is heavy. You pick how much you want. Rapid deceleration usually depends on brakes. It is limited by friction between tires and ground, and sometimes by aircraft configuration. Most every aircraft has brakes strong enough to make a tire skid.

Google "valdez stol competition" to see the world's shortest takeoffs and landings. These guys practice a lot, and you can see some of them nearly flip onto their noses by how hard they brake.


How can LMT be converted to UTC time? I am trying to solve some problems on converting from LMT (local mean time) to UTC (coordinated universal time) for flight planning purposes and I have some doubts. The problem is the following:

If the LMT in Delhi (LON=85ºE) is 17:45 (5:45 PM), what is the UTC?

First of all, what I've done is converting longitude into time as follows:
$$t=85\cdot4=340\hspace{1mm} min=5h\hspace{1mm}40m$$

But then, in order to compute the UTC time, what do I have to do, either adding up this time to the current LMT or subtract it (and what is the reason)?

$$UTC=Delhi(LMT)\pm t$$

I have checked a website that allows you to convert from LMT to UTC (https://savvytime.com/converter/lmt-to-utc-india-delhi) and I've seen the solution is 11:15 PM.

However, if I add or subtract my result, it is not the same as the given in the website. I suspect it is because in India they are using UTC +5:30 and probably these equations do not contemplate that, but I want to know the opinion of the experts anyway.

You have almost solved the problem yourself. The website you linked to does not help you because, for some reason, it assumes Local Mean Time (LMT) to be equal to UTC. For your problem, LMT needs to be for 85° of East longitude. So, ignore that website and let's move on.

For every degree of longitude, you calculate 4 minutes of difference from UTC. You have already correctly calculated the LMT difference from UTC for 85°E:

  First of all, what I've done is converting longitude into time as
  follows: $$t=85\cdot4=340\hspace{1mm} min=5h\hspace{1mm}40m$$

This leaves you with the question of whether to add or subtract this difference, as you stated:

  But then, in order to compute the UTC time, what do I have to do, either adding up this time to the current LMT or subtract it (and what is the reason)?

Let me start with the reason—the explanation of why we want to either add or subtract the difference between LMT and UTC—and then give you the solution.

LMT can be thought of as roughly equivalent to time based on the Sun's position. It is not equivalent to that, which is Solar Time, but it is related. Keeping that approximation in mind, consider the difference in time between when the sun reaches solar noon over Delhi, and when the sun reaches solar noon over London. As the earth rotates, the sun reaches solar noon first over Delhi, and then, a few hours later, over London. Thus you know that UTC will be a few hours behind Delhi LMT. This means that you need to subtract the calculated difference between LMT and UTC.

Another way to put this is: LMT for East longitude is ahead of UTC (subtract the difference to find UTC), and LMT for West longitude is behind UTC (add the difference to find UTC). Thus a more proper way to perform your calculation mathematically would be to properly incorporate the negative sign:

$$t=-85\cdot4=-340\hspace{1mm} min=-5h\hspace{1mm}40m$$

Therefore, if the LMT in Delhi (LON=85°E) is 17:45, then:

$$UTC=12h05m$$


How do planes know what altitude they're cruising at? I know that when planes enter the aerodome containing the airfield of destination, the ATIS will tell them an altimeter setting so the system knows how to calculate their altitude above their field; what about in cruise? Do they listen to the ATIS of every sector while cruising as well?

Above Transition Altitude (e.g. this is 5000ft in Germany) the altitude is measured in flight levels (FL) - each FL equals 100ft and is measured above an artificial QNH of 1013,25 hPa.
If you cruise for example at FL380 that will be 38000ft above the pressure level of 1013,25 hPa. This system assures that 2 aircraft which are 10 FL apart (e.g. FL370 and FL380) always have the required separation of 1000ft.
During descent when passing Transition Level you will change your altimeter setting to local QNH which ATC will tell you.


What is suction peak? Can anybody explain to me what suction peak is and does it have anything to do with lift and drag? I've read that suction peak is the point where the coefficient of pressure is the least and is located just behind the leading edge - is this statement true?

The suction peak is the point of lowest pressure on an airfoil, in other words the highest point on a typical pressure or Cp graph of an airfoil. On an airfoil it's typically located just after the leading edge on the upper surface, take a look at this image for instance. In that image you can also see that as you increase the angle of attack of the symmetrical airfoil (i.e. as you increase your lift), the suction peak becomes higher. At your stall angle of attack, the "adverse pressure gradient" becomes too high and the flow separates. An adverse pressure gradient means that the pressure increases.

Note that if you would put a symmetrical airfoil at a negative angle of attack, the pressure peak would appear on the lower side, just behind the leading edge.

Also, the suction peak doesn't have to be just behind the leading edge. It is for many airfoils because the point of highest thickness is quite close to the leading edge, but take a look at a natural laminar flow airfoil for instance. The suction peak is around 65%. Here the goal is to keep the flow laminar as far as possible. An adverse pressure gradient will also favour transition of the flow.


How far can airplanes glide? On an another question, an answer said:

  You don't need an engine to fly as airplanes are designed to glide without it.

I suspect this heavily depends on the type of the aircraft, so lets assume we are considering a small airplane.

How far could an airplane glide?
What governs the glideability of an airplane?
Is it possible, for an airplane with an engine/engines, to leverage this to save fuel while flying, or are they too heavy/otherwise unable to do this?

I suspect this heavy depends on the type of the aircraft, so lets assume we are considering a small airplane.

Absolutely right, it does vary widely. The parameter you're asking about is called glide ratio and it is directly related to another parameter called lift-to-drag ratio or L/D ratio. This is a fundamental characteristic of the aerodynamics of a particular aircraft. L/D ratio varies with airspeed; for determining best engine-out glide performance, the L/D ratio at "best glide airspeed" is used. "Best glide airspeed" is the speed that maximizes the L/D ratio, and this maximum value is known as L/Dmax.

The maximum L/D ratio (L/Dmax) of a Cessna 172 is about 9, so its glide ratio is about 9:1 - for every 9 units traveled forward it will lose 1 unit of altitude. So, it will glide about 9,000 feet for every 1,000 feet of altitude available. This is a fairly typical value for small planes.

To show you how widely variable this is, a modern glider can achieve ratios above 60:1, while the Space Shuttle ranged from about 1:1 at high speed, early in reentry, to 4.5:1 on final approach.

Notably, large transport aircraft tend to have significantly higher L/D ratios than small aircraft: a 747 can achieve an L/Dmax of about 17:1. With an altitude of 33,000 feet (~10,000 meters) that would mean a gliding distance of 100 miles (~170 Km).

  What governs the 'glideability' of the plane?

As above, its lift-to-drag ratio. Very casually speaking this is just a measure of how "aerodynamic" the airplane is, comparing its capability to generate lift with the drag it creates in the process. The better its lifting capabilities, or the less drag it generates, the higher the ratio.

For more information, I highly recommend a free online book on aerodynamics called See How It Flies by John Denker. It is written for pilots, not mathematicians or physicists, so it explains the concepts very intuitively without a lot of equations. It talks about L/D ratio and explains some of the factors that affect it. (I would recommend this book to any pilot anyway.)

  Is it possible, for an airplane with an engine/engines, to leverage this to save fuel while flying, or are they too heavy/otherwise unable to do this?

Since glide ratio is directly related to (indeed it's the same thing as) L/D ratio, you could say that airplanes already do take advantage of it. The higher their L/D at cruise airspeed, the more fuel-efficient they will be (because less thrust will be required to counteract drag in steady-state flight).

If you're asking about shutting the engines off during approach/landing to save fuel, there is another question here on ASE which specifically addresses this. (The answer, in short, is no, it's not practical.)


During WW1 Zeppelins began their missions with Hydrogen cells at 33% full. How did this provide enough lift to get off the ground? During WW1 U Class Zeppelins had hydrogen cells that contained 55,795 cubic meters of gas.

According to Peter Kämpf's excellent, detailed explanation of Zeppelin mechanics they began missions with their hydrogen cells filled with roughly 33% hydrogen.  The rest of the volume contained air.  Because hydrogen expands as air pressure falls starting at 33% allowed Zeppelins to reach high altitudes without rupturing their gas cells.
However given the constants below I'm not sure how 33% hydrogen produced enough lift to get a Zeppelin off the ground.
Sea Level, 0 Meters:
Temp Celsius: 15°
Temp Kelvin: 288.15°
Pressure Pascals: 101,325
Gas Moles per cubic meter: 42.29
Air mass per cubic meter: 1.221 Kg
Hydrogen mass per cubic meter: .085 Kg
55,795 cubic meters of gas cells * 33% full = 18,412 cubic meters hydrogen
18,412 cubic meters of displaced air mass = 22,481 Kg
18,412 cubic meters of hydrogen = 1,566 Kg
Useful lift = 22,481 kg - 1,566 kg = 20,915 Kg
The empty weight of a U class Zeppelin was 25,750 Kg.  That's without fuel, ballast or bombs.  If useful lift was less than empty weight how did it take off?
At 5,000 meters altitude useful lift stays the same because although the hydrogen cells expand and displace more air the atmosphere is less dense and has less mass per cubic meter.  Lift at 0 meters altitude is the same as lift at 5,000 meters altitude.
Another detail I found interesting is that according to Wiki the U Class Zeppelin produced 64,750 Kg of useful lift.
When I plug in numbers at sea level that suggests that the gas cells were filled to the top with 100% hydrogen.
55,795 cubic meters of gas cells
55,795 cubic meters of displaced air mass = 68,126 Kg
55,795 cubic meters of hydrogen = 4,743 Kg
Useful lift = 68,126 Kg - 4,743 Kg = 63,383 Kg
However if the hydrogen cells were filled to 100% it couldn't climb without bleeding hydrogen all the way up.  It should be noted that at 5,000 meters the air is thin and useful lift drops by almost half.
Altitude, 5,000 Meters:
Temp Celsius: -17.5°
Temp Kelvin: 255.65°
Pressure Pascals: 49,586
Gas Moles per cubic meter: 23.33
Air mass per cubic meter: .674 Kg
Hydrogen mass per cubic meter: .047 Kg
55,795 cubic meters of gas cells at 100% full
55,795 cubic meters of displaced air mass = 37,605 Kg
55,795 cubic meters of hydrogen = 2,622 Kg
Useful lift = 37,605 Kg - 2,622 Kg = 34,983 Kg
34,983 Kg of lift is maybe barely enough to stay afloat with fuel, ballast and bombs.
But even if they did it this way when the Zeppelin descended the Hydrogen cells would be mostly empty and would fill with air causing the ship to gain weight and crash.
At 0 meters
55,795 cubic meters of gas cells filled with 66% air = 36,825 cubic meters
36,825 cubic meters of air mass = 44,963 Kg!

The gas bags always contained close to 100% hydrogen. No air was supposed to be in them. On the ground, the bags were only partially filled because of the high air density and pressure. The rest of the internal volume of the Zeppelin was filled with air which could enter and escape the hull through the many gaps and venting openings.
Compare that to a weather ballon. On the ground, the big envelope is nearly empty and only a small bubble of lifting gas (today mostly helium, but hydrogen would work even better) sits at the top. Only that the nearly empty ballon is in clear sight, while in a rigid airship the half full gas bag is hidden behind the rigid hull covering.
One cubic meter of hydrogen weighs only 90 grams compared to one cubic meter of air at 1.225 kg under standard atmospheric conditions. With the same amount of hydrogen a balloon will have the same lift, regardless of altitude, because its lifting gas will displace a bigger volume but the same mass of air as it expands with altitude. With Zeppelins, fine tuning of lift by adding dynamic lift is possible but limited to maybe 5%-20% of the ship's mass (more for smaller and faster ships and at lower altitude). By growing larger, the mass to volume ratio of Zeppelins decreased, so the latest WW I ships could be built light enough to fly with their gas bags only one third full.
Civilian airships rarely flew higher than 2000 m high and had to circumvent higher mountain ranges because they filled their gas bags to maybe 80% on the ground. Flying higher was not considered worth the reduction in payload and fuel that would result from less lifting gas.
Compare that to the wartime Zeppelins. They were in a continuous arms race for higher altitude with the British aeroplanes which were sent out to chase them. And until 1918 the most modern Zeppelin designs  were ahead. The limited altitude of older ships also meant that many were scrapped after one or two years of service. The table below shows the maximum altitude at which the gas bags were filling out the available volume of the ships. The list below gives the maximum altitude of the best ship of each series and the density ratio shows the degree to which the gas bags could be filled on the ground to achieve that altitude. Note that they still carried tons of water ballast and ordnance when flying that high.

LZ 112 tried with speed what could not any longer be achieved with altitude: It had 7 more powerful engines where older ships had only 5 and could cruise at 131 km/h as opposed to the 100 km/h of the older ships. The diminishing returns of flying higher and the stress of spending close to a day at -40°C and without a pressure cabin or a pressure suit made higher flight altitude impossible. What also held progress back was the size of airship hangars: Since LZ 62 the maximum diameter could not grow beyond the 23.9 m of the last wartime ships, so their volumetric efficiency went down when they grew larger by adding length.


Why does the Piper Malibu require a type rating? The Wikipedia page on type ratings says that most single-engine piston aircraft are covered under a single class rating but under JAA regulations the Piper Malibu requires its own type rating. (I'm unsure as to whether these regulations still apply or if the JAA even still exists.)

What is different about the Malibu that would require its own type rating? And do other regulatory agencies (FAA, EASA) require this?

Here in the US the FAA can impose a type rating on any aircraft it likes (see part 3 below). Generally speaking type ratings are either for big planes or planes that may require additional training for safety reasons.
You can find the full regulations relating to US type ratings in the FAR under
§61.31   Type rating requirements, additional training, and authorization requirements.
but basically,

(a) Type ratings required. A person who acts as a pilot in command of any of the following aircraft must hold a type rating for that aircraft:
(1) Large aircraft (except lighter-than-air).
(2) Turbojet-powered airplanes.
(3) Other aircraft specified by the Administrator through aircraft type certificate procedures.

In specific relation to the Malibu the US FAA does not require a type rating, you can find a full list of aircraft that require a type rating here. However it should be noted that to fly Pipers M600 and M500 Malibu variants, both of which have a service ceiling over 25,000Ft you would need a High Altitude Endorsement (along with all the other endorsements for a high performance plane like that).
The JAA may require such a rating due to the pressurization system in the plane.


What happens after a rudder hardover? I'm sure many of you are familiar with "rudder hardover", where an aeroplane's rudder deflects to an extreme position - full left or full right. Many of you will be aware that such an event, caused by a faulty Power Control Unit, was implicated in the loss of United Airlines Flight 585 and USAir Flight 427. A rudder hardover was also involved in an incident on Eastwind Airlines Flight 517 and the investigation into that incident unravelled the mysteries around the loss of the two aforementioned flights.

My question is about the effects of a rudder hardover on flight - and how it led to the two crashes. I'm interested in what happens, aerodynamically, after the rudder has moved to its extreme position and the sequence of events which follows. It's obvious to me that a rudder hardover must be an extremely destabilising event. But I'd like a more detailed explanation of precisely what happens. What effects does it cause? What happens to the affected aeroplane and why might it crash?

With a fully deflected rudder the airplane will fly with substantial sideslip, and by dropping the windward wing it can be held on a straight course. This is usually done intentionally with gliders and light aircraft when they want to lose altitude quickly, because the high sideslip angle will increase drag substantially. If $\zeta$ is your rudder deflection angle, the slip will normally have a sideslip angle $\beta$ of between -$0.5\cdot\zeta$ and -$\zeta$, depending on the details of the aircraft.

If the hardover happens at high speed, the resulting aerodynamic forces will certainly overstress the fuselage and tail structure, and the aircraft will break apart and crash. No amount of pilot skill can prevent this!

If the aircraft survives the hardover incident, the best the pilot can hope for is a controlled crash: The sideslip angle will cause the landing gear to be torn off, and the dropped windward wing will provoke a ground loop - if the pilot tries to level the wings during the flare, the resulting yawing motion will ground loop the plane in the other direction.

I once had the chance to fly the prototype of the Swift S-1, an excellent Polish aerobatic glider. During one maneuver my left pedal got stuck, and I saw myself already sideslipping the plane into the ground. In the remaining minutes I tried to get the pedals unstuck and got lucky, ending the flight with an uneventful landing. Afterwards, I found out that the pedal mechanism had caught the wire from the battery, and my maneuvering had freed it up again. This was the closest I ever came to land with a rudder hardover, and I am not eager to ever try this out for real.


How is the due date for landing gear replacement determined? I want to know how calculate the due date of Boeing MD-83 Landing gear replacement (it is every 10 year or 20000 FC).

Is it calculated from the installed date or shop visit date?

To my knowledge, all lifed components are tracked from installed date. That is from personal experience, but I can certainly have a look through the regs to find an official source, if you require.


Is the distance to PNR (point of no return) affected by wind? Is the distance to PNR (point of no return) affected by wind?

I did research it and found some links, but none discussed about the wind.

Well, short answer: yes. Intuitively how could the wind not affect the PNR?

But, intuition, especially mine, is sometimes wrong, so let's see. I seem to remember this one from an algebra class, so I took a moment to figure it out. However, my aging brain was obviously going to take a more than a moment (my last algebra class was more than 50 years ago) thus, lazy soul that I am, I went to skybrary.aero and found:

(skybrary.aero)

Let's say we've got 2400 nm to fly in a 500 kt airplane facing a 100 kt headwind. Thus

D = 2400, H = 600, O = 400 and the PNR (ETP) = 1440 nm.

In a no wind situation the ETP (Equal Time Point) would obviously be 1200 nm, so wind does effect the the ETP.

In real life things are not so simple, and flight planning software will take care of calculating such things using winds aloft forecasts along the route of flight as well as other things.


How can I calculate QNH from QFE? Can someone please explain in simple terms how to work out your QNH from QFE?

For example, you take off at 0' but leave the circuit, what do you set the altimeter to? How do you know the QNH?

In metric:

1 milibar is approximately equal to 30 feet.  This calculation is therefore also approximate but is good for airfield elevations to several hundred feet since we round to the nearest millibar.

Divide the airfield altitude in feet by 30 to get the number of millibars above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.

For example, the airfield elevation is 200 feet.  Dividing by 30 gives us 6.66r.  The QFE is 1023. Add 6.66 to get 1029.66 and round up to 1030 millibars, which is the QNH.

  In imperial:

1 inch mercury is approximately equal to 900 feet.  This calculation is therefore also approximate but is good for airfield elevations to several hundred feet since we round to the nearest hundredth inches.

Divide the airfield altitude in feet by 900 to get the number of inches above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.

For example, the airfield elevation is 300 feet.  Diving by 900 gives us 0.33r.  The QFE is 30.12. Add 0.33 to get 30.45 which is the QNH.

If the airfield elevation is below sea level, subtract rather than add and vice versa.


What aircraft fly faster than 250 knots indicated airspeed under 10,000 feet for safety reasons? 14 CFR 91.117 limits aircraft to 250 knots indicated airspeed below 10,000 feet, but 91.117(d) allows aircraft to fly a higher speed if the minimum safe speed is higher. What aircraft and aircraft conditions might require an indicated airspeed higher than 250 knots?

A 747 taking off at or near max gross weight will have a flaps up, minimum safe airspeed and climbout airspeed speed greater than 250 knots. In such cases you didn't notify ATC as they expected it.

I tried to find what the exact numbers would be at 840,000 lbs, but I seem to have thrown out that manual. I did, however, find a reference for a 747-200 with JT9D-7Q engines that gives the flaps up holding speed at 800,000 lbs as 259 kts at 5,000 feet.

Also, I do remember being on an arrival once, with the weight low enough that we could safely maintain well below 250, but that when we saw we were icing badly, we speeded up to somewhere around 300 to get the ram air temp above freezing. In that case we notified ATC that we were doing so.

Also, ATC occasionally requests max speed. For example, on Hajj flights into and out of Jakarta Halim airport in the late 1990s it was common practice for controllers to put you down low to get you under the traffic into Cengkareng airport. They would then often request max speed.


Are there any remaining flying boat or seaplane airliners in operation? Is there anywhere in the world that still offers scheduled air carrier service using flying boat / seaplane types of aircraft - something akin to a proper airliner? Or are they all limited to charters and shuttle-type flights on de Havilland Otters and the like?

As far as I know, there is no actual flying boat in use at an airline. The flying boats that are still in commercial operation are typically used for fighting forest fires or maritime patrol. This may change in the near future since there are currently at least two flying boats in development that might be used by airlines:

Dornier Seastar CD2: This is a newer model of the previous Seastar, which had its first flight earlier this year:

Dornier Seawings flew its new-generation Seastar amphibian for the first time on 28 March from its base in Oberpfaffenhofen, Germany.
[...]
Dornier says the aircraft will be offered in various configurations including cargo, passenger, special missions and VIP transport.

(flightglobal.com, emphasis mine)

AVIC AG600: This is new aircraft developed in China and one of the biggest flying boats ever. It had its first flight in 2017 and according to Wikipedia is expected to be delivered by 2022, possibly also in a passenger variant:

Further variants may be developed for maritime surveillance, resource detection, passenger and cargo transport.

If you are not just interested in pure flying boats, but also floatplanes, then there are many airlines world wide that operate these on regularly scheduled flights. One of the biggest is Trans Maldivian Airways operating various variants of the DHC-6 Twin Otter. They operate 55 seaplanes, which according to Wikipedia makes them the biggest seaplane airline:

TMA currently operates the world's largest seaplane fleet.

They offer flights to over 80 destinations:

Trans Maldivian Airways currently offers transfer services to more than 80 Maldives resorts, flying over 1 million passengers per year to their holiday hideaways.

(transmaldivian.com)
This is their main terminal at Velena International Airport:

(image source: Wikimedia)


Aviation Embedded Systems I would like to understand the design philosophies of aviation embedded systems that provide real time control and instrumentation like EICAS/FMS/GPWS & TCAS.

What kind of embedded OS platforms do they use? VxWorks Linux, RT Linux
etc..?
Is there a specific design philosophy in Computer Science that is used for ensuring real time behavior? (Just like fault tolerant hardware for fail proofing?)

There are various standards out there and ways of doing things but DO-178C is the latest iteration of the document that outlines how real time avionics are to be programed. Certifying agencies like the FAA may have their own supplements/orders that specify specifics for a given jurisdiction, for example here is the FAA's order.

Avionics themselves often connect over the ARNIC data bus.

Many avionics and aircraft companies design their own OS's when they needed them for real time applications

  Honeywell, for example, created the digital "engine" operating system
  (DEOS) and Rockwell Collins, the virtual machine operating system
  (VMOS), which it engineered from an earlier LynuxWorks product in the
  late 1990s. Collins licensed the changes back to LynuxWorks in the
  2002-2003 time frame, and LynuxWorks now offers the new version of the
  software as LynxOS-178.

WindRiver did the OS for the dreamliner

  Smiths Aerospace chose Wind River Systems' VxWorks 653 RTOS for the
  B787's common core system (CCS), a cabinet that will host 80 to 100
  applications, including Honeywell's FMS and health management software
  and Collins' crew alerting and display management software.

There is a great podcast on designing spacecraft software (basically the same use case) that you can find here.

Its worth noting that the cost to certify avionics tends to be quite high (under the FAA) so code and methodology dont change all to much. Once avionics get certified companies have a way of continuing to make them for a very, very, very long time.


Are sequenced waypoints still displayed on the Navigation Display / FMS legs page? This question relates to the Flight Management System of a large airliner, e.g. Boeing 787:

During a flight, when the aircraft passes a waypoint, is this just passed waypoint (and the previous ones) still shown on the legs / FPLN page of the Flight Management System? Asked differently, can I scroll through all the past waypoints or are they automatically deleted once I have passed them?

Does the same apply to the Navigation Display (if I'd zoom out enough), would I still see all already sequenced waypoints and the past route, or do I only get to see the upcoming route and waypoints?

If they are not visible any more, why so?

Previous waypoints are not shown on the 787's CDU, and only the last waypoint or PPOS (present position at which a direct-to was initiated) shows on the ND.

The 787 EFB does not show the plane position. Only very recently the FAA allowed own-ship display on moving maps.

As to why, there is no use for it. The POS REPORT page on the 787 would show the three waypoints needed for position reports (from, to, and next).

Some airways are one-way, so it's not like a U-turn will be needed (there is also no reverse sequence function). Any emergency gets priority and a direct-to the nearest suitable airport (weather and terrain permitting).

If the whole plan is needed (on display) for some reason, it can be reloaded as a secondary inactive route (if saved, or if can be requested via uplink).

On the MD-11, the progress page shows the last waypoint (singular), but not the 787, so minor differences can be expected. Above applies to the 787's route legs and progress pages.

Source: 787 FCOM.


Are CTAF and UNICOM frequencies routinely recorded? ATC communications are recorded for later examination by accident investigators or regulatory agencies. Are CTAF frequencies at untowered fields usually recorded? How about manned UNICOM frequencies?

From the AIM:

  4−1−4. Recording and Monitoring

    a. Calls to air traffic control (ATC) facilities (ARTCCs, Towers, FSSs, Central Flow, and Operations Centers) over radio and ATC operational telephone lines (lines used for operational purposes such as controller instructions, briefings, opening and closing flight plans, issuance of IFR clearances and amendments, counter hijacking activities, etc.) may be monitored and recorded for operational uses such as accident investigations, accident prevention, search and rescue purposes, specialist training and evaluation, and technical evaluation and repair of control and communications systems.

    b. Where the public access telephone is recorded, a beeper tone is not required. In place of the “beep” tone the FCC has substituted a mandatory requirement that persons to be recorded be given notice they are to be recorded and give consent. Notice is given by this entry, consent to record is assumed by the individual placing a call to the operational facility.

The use of "may be" for the manned stations would hint at a "no" for the CTAF, UNICOM, etc. (Applies to official recording, liveatc not included.)


Why does the DC-9's antiskid system have to be manually disarmed during landing rollout following a ground-shift-mechanism malfunction on takeoff? An aircraft's ground-shift mechanism (usually a sensor wired to the nosegear strut that looks to see whether the strut is compressed or not) tells the aircraft whether it's on the ground or in the air.  This determines how the aircraft responds to certain situations; for instance:

Many aircraft's spoilers1 can only extend to the full-up ground-spoiler position if the aircraft is in ground mode.
Applying TOGA thrust with the flaps up triggers the takeoff-warning horn in ground mode but not in air mode, whereas the stall warning, in contrast, can only sound in air mode.
Reverse thrust, on most aircraft so equipped, is generally locked out in air mode, and the minimum thrust available changes from the ground-idle setting to the considerably-higher flight-idle setting when the aircraft lifts off.
The outflow valve on most pressurised aircraft automatically goes to the full-open position when the nosegear strut compresses (this depressurises the cabin, allowing the doors to be opened when the aircraft reaches the gate).

The ground-shift mechanism can occasionally malfunction, in which case fun things can happen, like the engines refusing to reverse upon landing and the stickshakers going off during taxi (if it fails in the "air" position) or the takeoff-warning horn going off when you raise the flaps during climbout, the landing gear staying locked down, and the aircraft refusing to pressurise (if it fails in the "ground" position); the usual workaround is to force the aircraft into one or the other mode using circuit-breaker fu.

One specific type of aircraft (the DC-9) has the following interesting entry in the abnormal-procedures section of the AOM, regarding what to do during landing if the ground-shift mechanism failed upon takeoff:

  Approach and landing:

  If landing gear was not retracted prior to landing, ground spoilers must be operated manually.

  AIRPLANE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. REPRESSURIZE (PNF)

  Ensure airplane is repressurized prior to landing.

  ANTI-SKID SWITCH (before 30kits ) . . . . . . . . . . . . . OFF (PNF)

  During landing rollout and prior to 30 kts, momentarily release brakes and place Anti-skid switch to OFF.

  GROUND CONTROL RELAY C/Bs (if pulled) (H20 and J20) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RESET (C or FO)

  Reset Ground Control Relay circuit breakers
  during taxi and verify that circuits are in the
  ground mode.

  [NTSB AAR-96/07, page 134 (page 144 of the PDF file of the report); my emphasis.]

Why does a ground-shift-mechanism failure require the DC-9's antiskid system to be manually deactivated during rollout?

1: (Though not all.)

Historical yet pertinent note: Concorde was the first plane to have electronic anti-skid, which means the DC-9 (also electronic) had the system installed after entry into service.

The relevant patent for this answer is: Anti-skid control system for aircraft.

The nose gear ground shift is part of the anti-skid system (see below; click to view).

  Source: DC-9 FCOM

Here's how the circuitry works:

In flight (mode), the ground shift fakes a fast wheel spin, but with no actual wheel spin, the circuitry then thinks there is a lock, and sends a brake release signal. This ensures the plane doesn't touch down with the brakes on.
After touch down and wheel spin, speed transducers send AC signals, which are converted to DC for the circuits.
Once the wheels are above 10 MPH, the brake release signal is removed, i.e. braking is now allowed.
Once firmly down with the gear compressed (nose gear in the DC-9), the squat switch switches off the fake signal. However, that takes full effect only when the circuit is de-energized, which happens when the wheels slow down below 23 MPH (20 knots).

In the case here – with the plane still in flight mode due to the pulling of circuit breakers after the malfunction – any time the plane is stopped (or circuitry is de-energized), the circuitry would release the brakes (thinking there's a lock).

Note: The newer MD-80 has the same procedure minus the manual disarming of the anti-skid, which hints at newer circuitry.


Can biplane or triplane designs be revived with modern materials? With the new-age technologies and cutting-edge composite materials etc available currently, is it possible that biplane and triplane designs will make a comeback in the near future?

Biplanes (and triplanes) became (nearly) extinct not because high strength materials (like composites) were not available, but because they became available.

One of the major reasons for use of biplanes in the early days of aviation was that the materials available were of insufficient strength for the (wing) designs used.

The major disadvantage of biplanes (or triplanes) is aerodynamic- it produces a lot of drag compared to the monoplane and the wings interfere with each other.

So, the biplane was an (aerodynamically) inefficient solution to a structural problem. As higher strength materials like Aluminum became widespread, the biplanes fell out of favor.

While they can be revived, I don't see why someone would do that, except for nostalgia (or some special uses, like aerobatics).

There are some companies who manufacture or restore biplanes, but they usually don't use (modern materials like) composites, though modern avionics are used.


How does 'standalone' propeller efficiency change with altitude? I've seen questions regarding propeller aircraft and altitude and their interaction. But I'm wondering what the relationship between just the propeller and altitude (changing air density) is. How does the efficiency and ability of a propeller scale with altitude?
Real-life scenario which inspired this question: a two-bladed two-engine propeller system is being used to propel a payload on a balloon platform (i.e lift is handled by helium buoyancy, no minimum speed required to stay afloat). The propeller is only used to move horizontally slowly (< 5m/s). This occurs at 65,000 feet and the propellers were analyzed at these conditions. Now, the new altitude is going to be 85,000 feet. How does that affect the scaling and re-design of the propulsion system? The Reynolds number will of course be different for the same desired airspeed. Additionally, the system drag will be lower so less thrust will be required at the same airspeed. But if the required thrust needed was the same, how does that and other factors affect the efficiency? And knowing that the required thrust will be lower scaled to air density, what is the expected change in propeller requirements?
Addendum: can the same 65,000 feet propellers be used at 85,000 feet, or is a new design required?

Yes, the same propeller can be used.
Propeller thrust is proportional to air density. The same is true for aerodynamic drag, so your propeller will work at all altitudes equally well. Since the speed of sound drops with temperature, you need to pick your propeller tip speed for the coldest layer, which normally is the Tropopause. There, the speed of sound is 295 m/s and if the propeller tips don't exceed 85% of that, efficiency should be roughly constant over altitude. A small loss in efficiency with altitude must be accepted, however, since the propeller's friction drag increases with altitude.
This would not be true for airplane propellers where the drag of the airplane they are propelling is constant (actually, rises a bit) with altitude, but since your thrust demand drops in line with density, no new propeller is needed at 85,000 ft.


How many flying mileage a pilot should have per year in order to keep his licence valid? Please let me know if possible
Thank you very much

There’s no minimum flight time required to retain a pilot certificate, though there are minimum currency standards in order to act as PIC in certain flight situations, or to carry passengers.  See §61.57.

Pilots are also required to complete a Bienneal Flight Review ever 24 calendar months in order to act as PIC aboard an aircraft.  Any successful practical exam for an airman certificate additional category, class or type rating can be substituted for a BFR.

Typ ratings listed on airman certificates expire after 12 calendar months unless the applicant undergoes a recurrent training for the aircraft type and pass a practical exam administered be an aviation examiner or check airman.

Flight instructor certificates do expire after 24 calendar months, unless the CFI:

Passes a flight instructor refresher course containing both ground and flight training within the preceding 3 calendar months on the expiration date.
Pass a practice exam for one of the ratings listed on their instructor certificate
Recommends at least 5 students to take a practical examination in which 80% of whom pass, or
Work as a chief pilot or check airman for a Part 121 or 135 operation.

See §61.197


What is the constant in this formula for additional lift? Let’s consider the case of a plane flying s/l in a headwind with a positive gradient. A wing produces lift by inducing a bound vortex superimposed on the relative wind., and in the particular case of flying in a positive gradient, one can imagine an extra pseudo-vortex associated with the gradient itself, that re-inforces the bound vortex producing additional lift.

The question is now how to quantify that additional lift. From the variables that enter in the circulation theory of lift, one can guess that the variables involved are the density of the air rho, the airspeed v, the wing area A, the chord c and of course the wind gradient ß.

From dimensional analysis, it results that the extra lift L is:

L = k · rho · v · A · c · ß

where k is a constant to be determined. It can be probably done with a ‘material experiment’, but my question here is whether it can be derived from already known data or from a ‘mental experiment’.

The formula you have given looks strange to me, I have never seen it before. But possibly I can help you with the following brief explanation about kinematic relationships when flying under wind conditions.

Flight path velocity (VK) is the vector sum of aerodynamic velocity (VA) plus wind velocity (VW), see sketch. - When flying in a headwind condition aerodynamic velocity will increase and a pilot will need to decrease Alpha to keep the lift constant. It is the opposite way around in a tail wind condition. Now a pilot will have to increase Alpha in order to keep the lift constant and additionally will have to pay attention not to come too close to Alpha max. Or increase thrust.

If one uses the sketched kinematic relationships properly one does not need an additional formula for analysis.


What kind of malfunction is an "artificial horizon"? I was once (like ten years ago) on a transpacific flight with Northwest from KIX/RJBB to KDTW on a Boeing 747 and our flight as delayed at the gate for about an hour because of some issue with a "floating horizon" (I remembered incorrectly, the issue was a "artificial horizon").

Is that a malfunction with the attitude indicator? What exactly went wrong? It sounded like they fixed it by swapping out a component, maybe the instrument itself?

Best bet is that there was an issue with the artificial horizon. If it was on an older aircraft without a glass cockpit, the gyro or its gimbal was probably broken causing the face to 'float' around or just stick at an odd angle. Then it was then communicated poorly to you by someone. Other than that I have never heard of a "floating horizon" and a google search doesn't yield any results either.


Why don't stealth aircraft designs use podded engines? Normally, it seems all the stealth designs dont use podded engines like others. Is this just to avoid the radar cross-section?

Yes. From the PoV of stealth aircraft, the podded engine has significant disadvantages:

The engine pod has a high Radar Cross Section; In fact, stealth aircraft bury the engine inside their fuselages to minimize radar cross section. For example, the B2 Spirit engines were located inside the fuselage, as shown in the schematic below.

The exposed compressor blades in the podded engine reflects radar waves, which is not good. For example, in Boeing X-32, the location of the engine meant that the compressor blades will be exposed to the Radar and baffles were (proposed to be) added to reduce the Radar cross section. On the other hand, it can be seen that it is practically impossible to see the F-22 compressor blades from any angle of the inlet.

Source: f-16.net

In case of podded engine, it is difficult to shield the exhaust thermal signature. Compare the infrared signature of the typical airliner in the following picture with that of the B-2 Spirit in the next picture.

Source: laserfocusworld.com

Source: bestfighter4canada.blogspot.in

Of course, contemporary combat aircraft don't use podded engines at all (with the exception of the A-10, which is a ground attack aircraft).


Is it possible to revive a pilot who has had a medical condition in a seated position in a small aircraft? I recently learned of a business executive who sold his company jet (single pilot operation) after his corporate pilot (in his 50ies) fell to the ground during the pre-flight inspection of the aircraft and died. He and his family were about to go on a trip. Very lucky guy. Obviously, he could have just paid for a qualified 2nd pilot to always fly in the right seat, but thats not the point of this question.

It got me to thinking. Is it possible to revive a pilot who has stopped breathing and/or who's heart has stopped beating in a small aircraft in a seated position? Assuming of course aircraft is in cruise with plenty of altitude.

I'm going to preface this by saying I'm not a doctor, however I am trained in first aid and CPR.

If you are asking whether it would be possible to revive someone who is in respiratory or cardiac arrest to the point where they could fly again then the answer is absolutely not.

The movies make it seem like you can give CPR and 2 minutes later the person is running around shooting guns, however that's very far from the truth. CPR and rescue breathing are techniques which can, if done properly, keep someone's blood moving enough that radical emergency medicine may save their life. CPR cracks and/or breaks ribs and can cause internal injuries even when it's done right, rescue breathing doesn't generally cause injury, however anyone who has needed either will require days, weeks or even months of intensive care and recovery.

As to whether it would be possible to administer CPR or rescue breathing to someone in a light airplane then I would say yes, with extreme difficulty and a low chance of success. CPR requires a lot of force, you use the full weight of your upper body concentrated on one single point on the person's chest, unless you could lay the person flat on a hard surface you'd probably be wasting your time, so if the seats reclined a person in the back seat may be able to lean over and do it, although that's extremely chancy of working due to the angle and the cushioning of the seat. Rescue breathing is possible with a seated person, provided you can open the airway, getting the angle on that would be very tricky in the constrained area of a small cockpit. Again, if the seat could recline a person in the back would have a better chance of doing this.

Most importantly to remember is that if the sole pilot dies or becomes incapacitated someone else has to fly the airplane. Struggling to get into position to perform CPR or mouth to mouth on a seated person would be a great way to snag the stick and send the airplane out of control. If there's a back seater by all means let them give it a shot, but your best bet at survival is getting control of the airplane and calling ATC for someone to talk you through things, it's happened before and has a remarkably good survival rate.


Is using a VOR considered IFR or VFR? As the title mentioned.
And also, is using it in zero visibility (or near) a real life scenario?

You can navigate using VORs under either IFR or VFR. If visibility is zero, hopefully you are flying IFR.
These days, pilots are going to use GPS if at all possible, but if the GPS fails or if the plane doesn’t have GPS, we can fall back to using VORs. Most planes still have one or two VOR receivers for that purpose, even if they’re rarely used.