Canard characteristics and CG

[av8or91]

How does CG affect canard airplanes? Forward and aft CG?

[tgrayson]

Originally Posted by av8or91

How does CG affect canard airplanes? Forward and aft CG?

Canards aren't fundamentally different from conventional aircraft; the same equations apply, but there's merely a sign change by having the "tail" up front. In either case, having the CG in front of the aerodynamic center (AC) of the aircraft is essential for stability; having it too far forward of the AC will result in an excessively stable airplane, and having it behind the AC at all will result in an unstable airplane. (BTW, the AC for the airplane is usually referred to as the "Neutral Point", NP.)

Since the canard and the main wing are both lifting surfaces, the NP will be somewhere in between, obviously closer to the main wing, since it's larger. This tends to put the NP much further ahead in a canard airplane than it would be in a conventional one, which makes it more difficult to ensure that it remains behind the CG. The end result is that there is normally a more limited CG range in a canard.

[sharkey]

Since both the wing and the canard make upward lift and share the load of the airplane is there any performance difference with a shift in CG?

[tgrayson]

Originally Posted by sharkey

Since both the wing and the canard make upward lift and share the load of the airplane is there any performance difference with a shift in CG?

Probably. Even in an airplane with a tailplane, there is a point of inflection as as you move the CG rearward. As less and less download is required, performance increases, but once the tailplane moves into positive lift, further shifts rearward require an increase in AoA on the tail, resulting in worse performance. The smaller tailplane requires a higher AoA to support a given amount of weight than does the main wing, and induced drag rises rapidly with increasing AoA. I would imagine the same thing happens as the CG moves forward in a canard.

[ProudPilot]

I'm a bit confused, I would assume that pound for pound, and any airplane where you simply move the wing and change from a tail downforce to a canard you would gain additional CG range. The assumption going that for the same weight in payload the wing would have to produce less lift than before since instead of compensating for tail down force, there is a net gain in lift due to the canard.

Example, 2100 lb aircraft, requires 200lbs tail down force, so wing produces 2300lbs of lift for level flight. If changed to a canard, and the canard produces 200lbs up force, total lift produced by the wing would be 1900lbs, hence less weight. Less weight equates to a lower moment allowing for a greater arm of cg before max tail up force is reached, giving a net gain in CG range.

Am I missing something basic here? Are we simply talking about a normal aircraft that has had a canard added in the front to create a smaller elevator/stabilator?

[tgrayson]

Originally Posted by ProudPilot

I would assume that pound for pound, and any airplane where you simply move the wing and change from a tail downforce to a canard you would gain additional CG range...

You lost me in your chain of reasoning. For one, it looks like you may be equating trim with stability. They're not the same.

Regardless, stability is determined by the position of CG with regard to the aerodynamic center of the aircraft. Note that it's the positions of these things, not their magnitude that controls stability. That's because with stability, we're concerned about how the aircraft behaves with changes in lift, not how it handles the lift it already has.

The problem with a canard is that it's inherently destabilizing. If you get an updraft, which will increase the AoA on both wings, what you want to happen is for the airplane to lower its AoA; however, the increase in lift on a canard wants to increase the AoA, not lower it. To compensate for this, you have to increase the stabilizing tendency of the main wing. This means ensuring that the CG is further ahead of the aerodynamic center than it would be on a conventional airplane.