Question about how airplanes fly.

Factual Questions
41
      • Ummm, have you ever tried to throw a regular Frisbee upside down? As I recall they pretty much seem to pull downwards after release…
        ~
42

Yup - they don’t fly near as well, but they will fly. You need to start the thing at a steeper angle above horizontal. It helps to use a lot of spin.

I knew a guy who was an ace at this - he threw it upside-down, backhanded, very fast and very far. (Of course, he could throw it a lot farther rightside-up.)

43

I’ve mentioned this before in another thread. The high speed jets all have what amounts to two different wings for high and low speed. The leading and trailing edge slats and flaps not only increase the camber, they increase the chord and provided airways to prevent, or at least reduce, turbulence at the higher angles of attach at low airspeeds.

44

There seems to still be some questions as to what really makes lift, the curved surface, the deflection angle, or the downwash (momentum of air going down). What happens is that the curved surface of the airfoil combined with the angle of attack forces the air to move in a certain direction over the airfoil. This results in different speeds over different parts of the airfoil. This results in both a pressure difference on the top and bottom and the resulting downwash. You can say that the downwash can be used to explain lift, but only as far as it is a direct measure of the pressure difference.

You see, the pressure on the wing is way the energy is transferred between the wing and the air. In a car it is the combustion fuel that causes the engine to run, but the only way that combustion can be utilized is by the high pressure it exerts on the piston, that is how the energy is transferred.

So, if you ask what is causing the plane to fly, it is the way you have caused the air to move, an effect of the shape of the airfoil and the angle of incidence. Even if you have a symmetric airfoil, the amount of lift you get for a certain change in angle of incidence is still depedent upon that shape even though the airfoil would be producing zero lift at zero angle of attack. This relationship is defined by the Coefficient of lift, or Cl, of the airfoil, and is only valid over certain speed ranges. If you have a non-symmetric airfoil upside down it will produce negative lift. By increasing the angle of incidence you can counteract this, and in some cases be able to still produce enough lift to fly (this in no way disproves the Bernoulli theory on airfoils) but it will just be very inefficient.

This gets us to that sailplane airfoil. Sailplanes are hyper-efficient (so much so that sticking your hand out the window, which I have done before, makes a noticeable drop in performance) but only for very low speed ranges, which is why they are not used on other aircraft.

David SimmonsIn fact, they have many different wings for different flight conditions. Slats and flaps can be partially extended for different speed ranges. Modern passenger aircraft will often have several trailing edge flaps that are extended together or seperately to create a continuum of different airfoils depending on the flight condition. This allows the aircraft to be efficient at high speeds and still be able to produce enough lift to stay in the air at low speeds.

45
      • It seems that for some reason all aircraft wings seem to have upper surfaces that are either longer or the same length as the lower surfaces. …So what I might be willing to consider as possible evidence of the “deflection” theory is–an example of any aircraft that has been built with the top surfaces of its wings flat or shorter than the lower surfaces. The problem with asserting symmetrical airfoils as an example of deflection is that if the aircraft flies inclined to its direction of travel, then due to the typically-rounded leading edges, the path over the wing effectively becomes longer than the path underneath.
  • So essentially, what would qualify would be a wing with a sharp leading edge and that has more “bulge” of the lower surface. Are there any?
    ~
46

Probably not, DougC. The airfoils with the stereotypical airfoil shape, and symmetrical ones, are pretty efficient for their purposes, so existing designs will use one of those. Could one be built? You could probably build a flying model plane with sharp edges for the leading and trailing edges, with more curvature on the bottom side, and get it to fly. You can make pretty much anything fly with enough of an engine.

But when you said “deflection” theory - with the quotes and all, it sounds like you doubt what’s being said, so let me clarify. The way I learned how a wing flies, both in freshman physics and in flight school in the late 1970s, was that the top surface is longer, and since the air going over the top must meet up with the air going under, with equal transit times, the air on top must be going faster, which according to conservation of energy (the Bernoulli equation), must have lower pressure, so that holds the wing up. Further, wings do not fly by deflecting air downwards, and this was accompanied by airflow diagrams which indicated no downwards movement after the air passed the wing.

The problem with this explanation is that the equal transit times assumption is wrong - wind tunnel photos clearly show that the air that goes over the top actually arrives at the trailing edge in less time, so there’s something else going on. And wings do deflect air downwards, necessarily. A wing that doesn’t deflect air downwards produces zero lift. Think about it - the helicopter rotor as a wing is a good example - a helicopter can’t stay aloft if it’s not blowing air down.