Tuesday, August 16, 2011

John Roncz / OSH11: aspect ratio has nothing to do with induced drag

I was listening to John Roncz's presentation in Oshkosh 2011. We had just talked with few aerodynamics people about the induced drag and that it is actually tangent of downwash angle (except for the tip vortex which forms from leakage of pressure over the tip and due to the movement of the plane, a swirling motion is created).

John Roncz was talking in his presentation about wing span, and finally noted about wing area, that he does not care about wing area, because induced drag has nothing to do with aspect ratio. That's why gliders have not only skinny wings, but also very long wings. That's why Rutan's aircraft have long wings, not only skinny wings. Lots of span is needed for low induced drag.

However, there is a geometrical relation about AR to the drag: The lower the AR, the higher is the wetted area for the given span. Wetted area is bad, it causes drag, you don't want extra wetted area. So the wing becomes skinny by definition. But now the wing is skinny (high AR) but also very long, and not only skinny.

There is another problem: I would think then that I want 20 meters long wing span, but very very narrow chord. The chord can not be infinitely narrow in order it to be structurally any sound, especially in speed. Therefore the higher the AR gets, and the lower the wetted area gets, the heavier the wing becomes. And the heavier it becomes, the worse gets the span loading if this is added to the weight of the plane. Then there is the another consideration, where I could taxi such plane which would have 20 meter span? On our airport even the Diamond's comparatively modest wing span is in some places a bit tricky.

Interesting dilemma. This also answers why there are multiple pods on some Rutan's aircrafts, along the span. The reason is to reduce induced drag, by moving the weight from the center more along the span. Then the lift required on the center where the lift given by the wing is worst does not give that unfavorable dip to lift distribution. And it reduces induced drag. On planes, like Globalflyer, induced drag plays major role in how much range the Brequet's equation gives.

But there is even more to this: the higher the AR gets, the lower the Re gets. The higher the altitude, also the lower the Re is again. The lower the Re, the higher is the profile drag. To get high L/D and thus efficiency one has to get also the profile drag down. And airplane efficiency is all about L/D (lift/drag), no less.

So my basic concept remains and does not need to be revised for another configuration alternatives:
- conventional (to be able to use efficient flaps)
- large span, low span loading (to reduce induced drag)
- high aspect ratio, relatively high wing loading (to avoid extra wetted area and that way to reduce drag and AR also to have steep lift curve slope (in other words, closer to the 2D airfoil simulations of infinite wings)

- larger than minimum size elevator for larger CG allowance - this is for practicality rather than minimum trim drag
- The large AR is also needed for this: cruising with high wing loading causes need for high Cl for cruise, which in turn causes high alpha. To reduce alpha, the steepness of the lift curve slope is your friend. The lift curve slope steepness will make the plane to cruise fairly low angle of attack despite of flying at high Cl at high altitude with high wing loading.

1 comment:

Claus Maximilian Zeumer said...

I am glad you point this out - it's a widespread misconception.