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Old Jan 12, 2011, 07:49 AM
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United States, OH, Bradford
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We have another case of "the tail wagging the dog" going on here.

Incidence is something unique to each aircraft design. Yes, you can get away with about the same numbers on a general class of aircraft (and aft tails are somewhat more forgiving in this respect than canards), but the forces involved vary from airplane to airplane. Just because certain numbers worked on one of Chuck's airplanes does not mean those numbers are appropriate for yours.

C/G sets static stability. Once that is determined, then the balance between things like the pitching moments due to C/G, lift moments and aerodynamic pitching moments for the wing and canard, moments due to the fuselage and prop, and the desired trimmed airspeed all add up to determine what the final lift of the canard needs to be at zero elevator deflection, and therefore its required incidence.

The same is true of thrust line. Up or down thrust depends on the vertical and longitudinal location of the C/G, and the thrust angle you need to make the thrust line pass through it, assuming you don't have airframe interactions to deal with in addition to that. Right thrust depends on the inflow angle in the prop that results from the downthrust plus the plane's pitch attitude, and the resulting P-factor you have to counteract.

The incidence of the canard changes when you make an elevator input, regardless of whether it's an all-flying tail, or a canard+elevator. The basic incidence with zero elevator deflection that you build it at strongly influences the pitch trim in normal flight, but has very little to do with what happens at stall, with a large elevator deflection.

The best thing to do is make sure your canard incidence and thrust line are easy to adjust. The wing is less of an issue. If the plane is going to be large, complex and expensive, or there are other issues that make it impractical to experiment with these settings, consider building a smaller, cheaper, "breadboard" test model, perhaps a profile model just to keep things simpler.

Your Vvt is about double the usual number. However, the vertical tail moment arm is relatively short, which hurts dynamic stability in yaw, so having some extra might be good. Also, the long nose with significant side area, and the destabilizing effects of the tractor prop, will offset some of that Vvt of the fins alone. Overall your effective Vvt will be somewhat lower. OTOH, if you still have too much Vvt after adding the effects of those other factors, you can expect some spiral instability, particularly in cruise where the dihedral effect of the swept wing will be minimal.

The tractor prop right in front of the canard means you will have dramatic changes in elevator authority when you change power settings, and this will also alter your pitch trim and static pitch stability. It's likely that at high power settings you will be able to stall the wing long before the canard. You might be able to do some unconventional maneuvers, such as back flips (not the same as loops!). However, you might also find it difficult to avoid doing them unintentionally! You had better build your control surfaces, linkages and servos strong enough to withstand flying backwards.

OTOH, if you cut the power completely on final approach, it's quite likely you will not have enough elevator authority to flare for touchdown. You will want to keep enough power on to get at least to a "zero thrust" condition with the prop. The tricky part will be getting the power low enough for a decent glide slope on approach, while still getting enough airflow over the elevator to flare for touchdown.

The other option that would help some of these issues would of course be a pusher prop (I can't believe I just said that!!). However, going that route will cost you quite a bit of prop efficiency, which means more power and bigger batteries to get the same flight time and in-flight performance. That means more wing area to keep your stall speed down, which means a bigger airplane, which means more power, which means even bigger motor and batteries, and so on. It's easy for that sort of thing to snowball on you.

Other possible issues to discuss, but those are good for a start.
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Last edited by Don Stackhouse; Jan 12, 2011 at 08:08 AM.
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