2
$\begingroup$

Does using wings with different wing loadings work the same as a swing wing, without swinging the wing?

Was just wondering: we need the maximum co-efficient of lift at slower speeds at takeoff, but a much smaller co-efficient of lift at cruising speed.

For a triplane, wouldn't you want the top wing for flying and the other wings just for takeoff?

In other words:

you could make the top wing, a classic unsymmetrical wing like a NACA2415, set at around 5 deg angle of incidence, where I believe these airfoils are most efficient.

The other wings could be a wing with a high wing loading, like a high speed laminar foil, like the canard of a velocity, set at a incidence of 0 deg during straight and level flight.

In that way, the lower wings provide no lift, but also no induced drag ( drag due to lift) during straight and level flight, just like a classic tail airfoil.

During takeoff, you would have to rotate early prior to takeoff, ( like the Concorde) to get an angle of attack for all wings.

Say all 3 wings are the same size and you size your wings to get an overall wing loading of 7psf, which means you can land slow and short, but during cruise you would only be using 1 wing, so your effective wing loading is a very efficient 21psf. If it's gusty out, you could always land long and fast , using all 3 wings. So this would be like a swing wing, without the complexity of actually swinging the wing.

Is this correct, at least conceptually?

$\endgroup$
  • $\begingroup$ Efficiency does not depend on wing loading but rather on wing span. At slow speed due to interference the three wings have induced drag like an only slightly wider single wing, and at high speed you are still getting the form drag of the two wings not taking part of lift generation. So I'd say no. $\endgroup$ – Jan Hudec Aug 14 at 19:05
  • $\begingroup$ Actually I'm comparing a triplane with 3 similar wings to a triplane with one high lift wing and 2 low lift wings. Not comparing a triplane to a monoplane. $\endgroup$ – Fred Aug 14 at 19:20
  • $\begingroup$ Are you? I thought you were comparing it to swing-wing, which has only ever been realized as monoplane. $\endgroup$ – Jan Hudec Aug 14 at 19:22
  • $\begingroup$ As for triplane with different lift coefficients on each plane compared to triplane with the same lift coefficient on each plane, the later is always more efficient, because the vertical spread does give you some increase in the affected air mass. $\endgroup$ – Jan Hudec Aug 14 at 19:24
  • $\begingroup$ @ Jan: I guess I wasn't clear. What I'm asking is if the making the 2 lower wings high wing loading would improve the efficiency of a triplane, versus a triplane with all 3 wings the same. I think the effect would be the same as a swing wing monoplane in wrt it would have about 66% less induced drag. $\endgroup$ – Fred Aug 14 at 19:51
1
$\begingroup$

For a triplane, wouldn't you want the top wing for flying and the other wings just for takeoff?

No.

  1. Form drag is proportional to dynamic pressure and frontal area. By varying the lift coefficient between the planes you are not doing much to the frontal area, so the form drag is roughly the same.

  2. Induced drag is proportional to square of lift and inversely proportional to square of span and dynamic pressure. By generating three times more lift with one of the wings, you are making it produce more than tree times the induced drag.

    It is not all the nine times the basic equation suggests, because the efficiency is still significantly reduced by interference when all the wings are producing lift, but it is still worse, and it is worse whenever the lift is not distributed uniformly (maybe the interference makes a slight increase in the middle wing desired, I am not sure here, but it won't be big difference).

So the configuration is not better than evenly distributed lift in any flight regime.

$\endgroup$
  • $\begingroup$ Thanks for the answer: 1.FYI, not talking about form drag. That would be another question, reducing the span of a triplane, vs a monoplane. $\endgroup$ – Fred Aug 14 at 19:58
  • $\begingroup$ So, if I need 1.5cL at takeoff, and 0.5cL during cruise, it would be better to have 3 wings at .5/3, or .17cL, than 1 wing at 0.5cl. Is that correct? $\endgroup$ – Fred Aug 14 at 20:03
  • $\begingroup$ @Fred, it would be better to have 3 wings at .5/3, or .17cL, than 1 wing at 0.5cL given they are all the same span. Taking the three wings and putting them side-by-side to get a monoplane with 3 times the span is better still, because it removes the interference. $\endgroup$ – Jan Hudec Aug 14 at 20:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.