PPRuNe Forums - View Single Post - Which aircraft have pos/neg elevator lift?
Old 29th Mar 2009, 16:34
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bjornhall
 
Join Date: Mar 2007
Location: Right here
Age: 46
Posts: 419
I would submit that a canard does not have to always produce lift. The same argument that a tail "could" produce lift applies to a canard producing down force.
I don't see that, at least not by the same argument. For a positive stability contribution from decalage, the rear surface must have a lower angle of attack than the front surface, as explained in Denkerīs book referenced earlier. (Not suggesting his is the only good description, but it is one readily available online).

The wing always has to have positive angle of attack in steady flight, obviously. This means that in a conventional aircraft, the tailplane can therefore have a negative or zero angle of attack, or a slight positive angle of attack (but never more positive than the wing). But in a canard aircraft, the canard must always have a more positive angle of attack than the wing does; otherwise the surface at the front would have a lower angle of attack than the one at the rear, and the decalage would give a negative contribution to stability.

To emphasize: A tailplane can provide positive decalage if lift is negative, zero or slightly positive, but a canard can only provide positive decalage by producing positive lift.

The only way an aircraft could be stable with a canard having zero lift, is if there are other positive contributions to stability, resulting in a net positive stability despite the negative decalage contribution. That might be the case if you take an already stable delta wing aircraft and add a canard for maneuverability. I wonder if there are such aircraft? Some Kfir, perhaps?

So yes, you could in principle have a stable aircraft with a canard producing no lift. But we can tell from the very high angle of incidence displayed by most canards that they are designed to always provide positive lift.
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