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59 votes
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How to build a 2 mile long runway on a 1 mile square island?

Anyone who has ever flown at Zar (EPZR) in Poland knows how much a runway slope can reduce the needed field length. For aircraft which need a 2 miles long level runway, a 1 mile long one with a 15% ...
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44 votes
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Is gravity less on an airliner at cruise altitude?

There is less gravitational force, but by how much? An insignificant amount. The gravitational force of attraction between two objects is given by, $\displaystyle F_{\mathrm g} = \frac{G m_{1} m_{2}}{...
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43 votes
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Why/when is the blunt nose better?

In subsonic flow the air can react to the approaching vehicle since the pressure field around it extends forward, too. This means that a suction area on one side of the fuselage or wing will already ...
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40 votes
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Why can gliders fly for so long?

A glider and a paper airplane operate on the same principle: Exchanging whatever potential energy (altitude) they have for the kinetic energy (airspeed) required to keep air moving over the wings so ...
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  • 67.2k
40 votes

Could propeller wash provide sufficient lift to take off - even in theory?

Yes. The wing doesn't care what is causing air to flow past it. Headwind, propwash, 747 wake, gopher sneezes. If the airplane is restrained from forward motion, and the propwash over most of the ...
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39 votes

Why/when is the blunt nose better?

Aircraft design is full of compromises. In case of aircraft nose design, the main factor is to reduce drag. As you yourself noted, the main difference is in flow regime, i.e. subsonic and supersonic. ...
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38 votes
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Would an aircraft with contra-rotating propellers longer than the plane's wingspan be able to fly?

Short answer: This design will probably work, but it will not be very efficient. It can be tweaked into flying, but when you start tweaking, you would continue such that the outcome would look ...
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30 votes
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Can planes take off from a treadmill?

Everyone collectively went "Oh god, not this one" because this same question has sparked some intense debates in the past. Aircraft rely on airflow over the airfoil (wings/tail etc) to produce lift - ...
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27 votes
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Is there an aerofoil that gives reasonably good lift for both air flow directions: forward and backward?

There is no airfoil with good lift in both flow directions, but one with some lift is conceivable. However, the lift-to-drag ratio will be nothing to write home about. One reasonable candidate would ...
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22 votes
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When does the use of afterburners save fuel?

If such a pair of states does exist it ought to be straddling a Mach boundary (I think) e.g. A subsonic & B trans-sonic. Or A trans-sonic & B supersonic etc. Reasoning: To get from any ...
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  • 8,224
22 votes

Principle of aerodynamic lift: are misconceptions also taught in flight schools?

I am a CFI who teaches at a large (+200 students) flight school in the United States. You might be surprised to hear this, but... We really don't worry about how a wing works that much. As far as I'm ...
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  • 22.3k
20 votes

Is gravity less on an airliner at cruise altitude?

Gravity itself @aeronalias is absolutely right. Given the gravitational acceleration of $g=9.81m/s^2$ on the ground, a perfect spherical earth of radius $R_E=6370km$ with homogenous (at least: ...
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20 votes
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How does the bypass air provide thrust?

The bypass air is accelerated by the fan at the front of the turbofan engine. This changes its velocity and therefore its momentum, which is the definition of a force (in this case: thrust): $$ F = \...
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  • 44.5k
18 votes

Why can gliders fly for so long?

The primary difference between a glider and a paper airplane is that most folded paper airplanes have very bad aerodynamics. Most folded paper airplanes have short stubby wings. This in itself makes ...
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17 votes
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Is it possible for an airplane to enter another airplane?

Clearly the plane must enter and exit through the belly of the larger plane Why so? If you drop this condition, things become much easier, because you could attach the smaller planes externally and ...
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  • 8,605
16 votes
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Why are the power and drag curves called polars?

It is an historical name. The first polars were drawn by Otto Lilienthal in polar coordinates. Here (sorry, German link) we find an example:
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  • 31.5k
16 votes

Why does an aircraft pitch up when the speed increases?

What you ask is: If a model glider is thrown at a higher speed than its trim speed, why does it pitch up? Short answer: Because the rear horizontal surface produces less lift per area than the ...
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15 votes

Can planes take off from a treadmill?

Yes. Airplanes get their thrust by using the air. The wheels are not powered. The drag from the wheels will limit how fast the treadmill can go before the plane won't be able to take off anymore. ...
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13 votes
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How long would a cruising aircraft take to crash if everyone spontaneously vanished?

There is a far more important question (and the real deciding factor) How much fuel is on board? If left unattended a plane will fly until it runs out of gas quite literally, the more gas in the ...
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  • 96k
13 votes

Are aircraft capable of sustained inverted flight at constant altitude?

Yes they are! Aerobatic aircraft can have symmetrical wing to improve inverted performance. So with these aircraft there is no any problem at all. Other agile aircraft, gliders can fly inverted as ...
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  • 3,684
12 votes

Could propeller wash provide sufficient lift to take off - even in theory?

In principle yes, but why would you do it? For vertical take-off this would be grossly inefficient. Lift is produced by deflecting air downwards. This becomes easier as more air is available for ...
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11 votes

Would a 90 kg object jettisoned from an aircraft create a detectable movement?

Yes It's not just weight, it's also aerodynamics. First let's assume that the aircraft was trimmed slightly nose-heavy (no passengers), therefore it's trimmed slightly "tail down" anyway When the ...
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  • 10.3k
11 votes

Is there any engine that doesn't use a propellant to produce thrust?

Yes there are. Although they are not for commercial aviation. One example are solar powered engines: Source Wikipedia Alternatively, there are other means of storing energy - rubber bands, but this ...
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10 votes
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Principle of aerodynamic lift: are misconceptions also taught in flight schools?

I can only speak on behalf of the Australian Syllabus as put out by CASA (Civil Aviation Safety Authority - Australia), but we are expected to teach both Bernoulli's Theorem, and the Theorem of Air ...
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  • 2,011
10 votes
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Is the maximum lift-drag ratio found at minimum drag?

Well, for all L/D curves and D curves, the assumption is that the weight of the aircraft is constant and that there is no acceleration. Therefore the lift equals the weight (neglecting the small ...
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  • 74.8k
10 votes

How to build a 2 mile long runway on a 1 mile square island?

A seaplane would be a suitable solution. Then the only infrastructure required on the island is a jetty and fuel/servicing. This also enables a much larger aircraft and so easier cargo resupply and a ...
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  • 301
10 votes

Why can gliders fly for so long?

The glider and a paper aircraft are designed and operated in different ways which affects their stalling characteristics. Both aircraft trade one form of energy (potential energy or altitude) in ...
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  • 98.5k
10 votes
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Where does upward momentum come from in leading edge vortex?

Does this vortex draw in air from above, pulling it downward and as a result lifting the plane up? Yes, the bound vortex in combination with translational flow accelerates air downwards. Or is ...
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10 votes

Are aircraft capable of sustained inverted flight at constant altitude?

Most aircraft use a cambered airfoil. Such an airfoil only gives you a higher stall speed, otherwise it will just cope fine with prolonged inverted flight. The wing's twist will most likely increase ...
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10 votes

How does the bypass air provide thrust?

A bypass fan provides thrust in the same way a propeller provides thrust: by increasing the energy content of the gas mass passing through the disk. The added energy is most effectively converted ...
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