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71 votes
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Why do solar planes have many small propellers instead of fewer large ones?

Because they are very lightweight and fragile. Therefore, thrust and propulsion mass must be distributed over span - a single, large propeller and motor would put too much force into the structure ...
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40 votes
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Why are birdlike airfoils not used?

Early biplanes did use similar airfoils. Not as extreme as the Eppler 376, but still very thin and highly cambered. When Otto Lilienthal started his glider experiments, he tried to copy storks. He ...
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36 votes
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Why does my wind tunnel experiment give two different stall angles of attack?

This is called stall hysteresis. You have two different situations and the flow reacts differently in each of them. When increasing the AOA The flow is attached to the wing and the boundary layer is ...
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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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Why do airfoil's trailing edges point downward?

You are right, the trailing edge does not need to point down. Take symmetric airfoils - here the trailing edge runs parallel to the airfoil chord. Or take reflex airfoils (like the HQ 34 of the SB-13 ...
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17 votes

Why isn't there a single best airfoil for subsonic flight?

For the same reason why we do not have one single type of aircraft flying all commercial and military missions worldwide: flight has many variables, and there is no single optimum solution. First ...
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16 votes
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What is the performance of a flat plate wing?

I cannot answer all your questions, but maybe point you to some facts to come closer to an answer. Most important is the thickness of your parallelepiped - more than a few percent will just increase ...
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15 votes

Why are birdlike airfoils not used?

This is more an addendum than an answer, regarding "birdlike" airfoils. Ignoring the fact that birds can modify geometry, chord and camber of their wing when required, what can at best ...
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14 votes

Can a steady flow have stagnation points?

The idea of a stagnation point is an idealization. This point is infinitesimally small, and air particles flowing along a streamline which leads into it will slow down on their way. The closer they ...
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14 votes

Why do solar planes have many small propellers instead of fewer large ones?

Conventional commercial designs try to maximize kg-kms per dollar fuel cost or kg payload per dollar investment. Or minimize operating & maintenance cost. All these goals need an economy of ...
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14 votes
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How would an airfoil react if it was flown backwards?

What would happen? Flow separation on the suction side, but it would still produce lift like a regular airfoil. The L/D ratio would be lousy, however. Only at a small angle of attack range will the ...
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14 votes

Why isn't there a single best airfoil for subsonic flight?

In aircraft, size does matter. Smaller aircraft flying at the same speed, air temperature and altitude than a larger aircraft have a smaller Reynolds number which characterises the boundary layer ...
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14 votes

Why are the wings of some modern gliders tadpole shaped?

The flaps and ailerons are "reflexed" on this glider. They have been raised to a setting above the normal zero position, above the airfoil's normal chord line. A number of flapped gliders have this ...
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13 votes
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Are NACA airfoils used in modern aircraft design?

Just three data points: The tail surfaces of the Pilatus PC-12 still use the venerable NACA 0012, even though a better alternative (from the Wortmann FX 71 L series) was proposed. It did not help that ...
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13 votes
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Does lift coefficient vary with the wind velocity for a given angle of attack?

Yes, it does vary slightly due to viscous effects. In inviscid flow, the flow speed would not affect the lift coefficient - angle of attack relation. However, increasing the flow speed will result in ...
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13 votes
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Which books should I study to understand more about Supercritical airfoils?

Introduction to Transonic Aerodynamics Roelof Vos, Saeed Farokhi (more) Written to teach students the nature of transonic flow and its mathematical foundation, this book offers a much-...
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13 votes

Why do solar planes have many small propellers instead of fewer large ones?

Don't forget the electrical advantages of multiple motors. No need to conduct all the current to a single place with long lengths of heavy wire, no need to control a large current. Also, many small ...
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13 votes
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Was the B727's wing supercritical (even if not so designed)?

I've got an aged book on pre-design of aircraft, which states that the B727 wing thickness was 13% at root and 9% at tip, average thickness 11% chord. $M_{M0}$ = 0.9, first flight of prototype is ...
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13 votes

Why are highly cambered airfoils rarely used in propellers or wings?

First propeller use: A highly cambered airfoil would cause high pitching moments and twist the propeller blade. Of course you can pre-twist the blade so it will assume the correct shape in the desired ...
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13 votes

Are airfoil profiles patented? If not, why?

There certainly are airfoils that are patented. Here is an example. Here is another airfoil by the famous aerodynamicist Richard Whitcomb. I think it is tricky in practice to obtain a patent in that ...
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  • 241
13 votes

Why are there two local maximums on some CL vs AOA curves?

The mini stall is the stall that we normally talk about when considering the flight of an aeroplane, and is therefore the main stall. Below the AoA where the first stall occurs, the wing profile is ...
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12 votes

Why doesn't an airfoil rotate?

It does rotate. In fact, in aerodynamics, this rotation force has a name: pitching moment. Pitching moment changes depending on the angle of attack. Airfoils can be designed with almost any pitching ...
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11 votes
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What are the advantages of the B747-8 airfoil over B737's airfoil?

The airfoil profile you've shown is called a supercritical airfoil. Typical airfoil sections are curved on the top and the bottom. The airflow over the top of the airfoil is accelerated i.e. the ...
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11 votes

What is the method to calculate a finite wing's lift from its sectional airfoil shape?

There are indeed several approximations, depending on the shape of the wing. Generally, the lift curve slope is $2\pi$ only for a flat plate in inviscid 2D flow (with Kutta condition fulfilled). With ...
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11 votes
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Why doesn't an airfoil rotate?

There are no two (aerodynamic) forces. Force is not caused by the up- or downwash itself, but by the change from up- to downwash. The air is only accelerated downward and this change in it's momentum ...
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11 votes
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Why should the leading edge be blunt on low-speed, subsonic airfoils?

When using an airfoil with a sharp leading edge in subsonic regime, you need to adapt the angle of attack so that the stagnation point occurs right on the sharp edge. At every flight condition there ...
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  • 758
11 votes

How do symmetrical airfoils generate lift?

By using a non-zero angle of attack. When the trailing edge is pointed downwards, and assuming the airstream leaves the trailing edge smoothly, the exiting airstream is deflected downwards. This ...
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11 votes
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Which are some planes shaped like an inverted wing?

Your intuitive feeling that the fuselage could be used to create additional lift is correct. Indeed, some aircraft are specifically shaped such that fuselage could provide substantial lift (and that's ...
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  • 8,565
11 votes
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Is drag coefficient lowest at zero angle of attack?

Obviously, drag should be smallest for symmetrical airfoils at zero angle of attack. However, most airfoils have camber, and then the lowest drag is at positive lift coefficients in case of positive ...
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11 votes

Why are highly cambered airfoils rarely used in propellers or wings?

You can't just cherry-pick aerodynamics and exclude everything else when it comes to aircraft design. But let's entertain flight dynamics alone for this instance, and use Selig S1210 or S1223 as ...
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