71
votes
Accepted
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 ...
- 230k
40
votes
Accepted
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 ...
- 230k
37
votes
Accepted
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 ...
- 1,837
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 ...
- 8,531
16
votes
Accepted
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 ...
- 230k
15
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 ...
- 126k
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 ...
- 4,454
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 ...
- 8,436
14
votes
Accepted
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 ...
- 230k
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 ...
- 230k
14
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 ...
- 251
13
votes
Accepted
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 ...
- 230k
13
votes
Accepted
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-...
- 70.9k
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 ...
- 1,092
13
votes
Accepted
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 ...
- 61.3k
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 ...
- 230k
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 ...
- 61.3k
12
votes
Accepted
What is a laminar airfoil and what are their pros and cons?
The fist decades of aviation used empirically determined airfoil shapes which usually had most camber near the nose. Such airfoils tend to have pressure peaks near the nose, followed by a long and ...
- 230k
12
votes
Why does a flat plate create less lift than an airfoil at the same AoA?
It shows that at the same positive AoA, a flat plate generates less lift compared to the NACA airfoils.
From an arbitrary guess, the lift force generated should at least be the same for both
Your ...
- 9,589
11
votes
Accepted
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 ...
- 768
11
votes
Do airliners like Boeing and Airbus use airfoil variation along the span for their wings?
Every good design varies at least the thickness: While the root needs to be thicker to enclose an efficient spar, the tip can be optimised for aerodynamics, which usually results in a root thickness ...
- 230k
11
votes
Accepted
Should the pitching moment be up or down?
All points you mention are correct (for the nitpickers: Point 5 is only true below transsonic speeds). That the pitching moment of the wing is drawn nose-up is probably because it is positive this way ...
- 230k
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 ...
- 1,722
11
votes
Accepted
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 ...
- 9,013
11
votes
Accepted
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 ...
- 230k
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 ...
- 11k
11
votes
What are the effects of having the aileron hinge line on the pressure side vs the suction side of a wing?
Here's one thing large endurance aircraft can do: they use spoilerons instead of ailerons and rudders.
You simply leave off the downward pointing off-side aileron, which creates adverse yaw.
The ...
10
votes
Accepted
Do negative angles-of-attack create lift?
A symmetric airfoil will generate no lift at no angle of attack, and negative lift at a negative angle of attack. However, cambered airfoils are curved such that they will generate lift at small ...
- 340
10
votes
Are airfoil profiles patented? If not, why?
Patenting an airfoil would be difficult- the owner of the patent would need to show that they had invented something new that wasn’t obvious, and to enforce the patent they would have to prove that ...
- 1,637
9
votes
Accepted
Why does linear airfoil theory break down at hypersonic speeds?
The linear theory makes certain assumptions in its formulation:
The flow is irrotational and isentropic.
The perturbations are small; i.e. the bodies are small at small angles of attack.
However, ...
- 100k
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