Rutan Defiant canard question.

Anyone (Orion?) want to to take a crack at why the Defiant canard is such a larger percentage of total area than the other 4-place canard designs? Yes I know there is another engine up there. However couldn't you build, say a CozyIV with 2 engines and the standard canard as long as the cg was maintained the same as the single version? I'm wondering if it has to do with the effects of the prop slipstream acting on a smaller percentage of the total canard area. I'm guessing that the prop blowing on the canard might allow it to fly to a higher aoa and allow the rear wing to stall first. On the other hand might not the prop blowing on the canard give it more elevator effectivness to fly the plane out of a deep stall situation?

In case you haven't guessed by now I would like a smaller version of the Defiant (Baby Defiant ) like Dave Ganzers old Gemini, for two corvair engines @120hp each. I have searched high and low for canard aircraft design criteria and have found very little. Any suggestions?

John

Not much historical or technical data exists in connection with the Defiant. The answer to your question could be as simple as that which you guessed at (nose engine) or it could be more complex.

Given the size of the airplane and the capacity it was to have, the variable payload was a bit further ahead of the main wing than what you see in airplanes like the Cozy or the Ezes. To maintain balance and reasonable performance, the configuration would therefore need a larger canard in order to maintain stall speeds and control authority. (BTW canards don't rally have a "deep stall" region)

Simply said though, the sizing and proportion of the surfaces is most likely the result of a combination of factors that are similar to those used in sizing the wing and tail of a conventional aircraft.

In designing your own, using proportion as found on another, similar aircraft is OK only as long as you understand that that "guess" has to be just that, a guess. This is especially the case in an airplane where you're considering going to two engines. The sizing based on load distribution is only the first order consideration based on a static assumption. You must then also account for the second order issues, which may be a function more of the dynamic factors like maneuvering and momentum.

In short, in designing a new airplane it is a good idea to approach it as from scratch, even if it is based on an existing design.

Canards are unusual airplanes.

Let's cover static pitch stability for a moment. Several things have to simultaneously be true during unaccelerated flight:

Summation of vertical forces equals zero;

Summation of moments equals zero;

First Derivative of Pitching Moment with respect to angle of attack must be negative.

In canards, that means that the canard must be loaded more heavily than the main wing. The Wright Brothers figured this out when they were flying from Huffman Prairie, prior to their spectacular demonstrations in New York City and Le Mans.

For safety, the canard must also stall ahead of the main wing, that is, each stage of stall in the canard must precede the same stage in the main wing. This drives you to select stall behaviours in both foils to be similar. Once the loading is higher in the canard, the safety can then be covered.

And despite all of the knowledge gained in Vari-Viggen, Vari-Eze, Long Eze, and Cozy, the Velocity series ended up having to reduce the canard span to allow full range of CG to be used. Rutan Aircraft Factory also ended up reducing the spans of their canards...

I think that it is important to note that even Burt Rutan has abandoned canards. Before he was done with the Catbird (a significant three-surface airplane) he had sawn the starboard canard off entirely. Why not reduce both sides symmetrically? Well, there were antennas in the port side of the canard, but no one knew where, so they only removed canard from the starboard side. And symmetry is over rated anyway - look at the Boomerang, which is the best configuration twin yet.

Anyway, a canard with an engine at each end has its CG pretty far forward, and so, the canard that was about right would be pretty large...

Billski

Orion,

Thanks for the quick response. Yes, if I were to do this I would address wing and canard issues from scratch. The 747315 airfoil you refer to in your technical paper seems a good match for the main wing on a canard design. It has a very low Cm and a low unflapped Cl. I was wondering however about the cp movement when the flow trips due to contamination. I have heard that cp movement is why Burt choose a turbulant airfoil for the LongEZ and later designs.

I made the reference to the Cozy just as an example. What I really want is 2 place plus baggage, again similar to the Gemini built by Ganzer back in the eighties (the "heyday" of the canard).

Structural design I can work through. It's the aerodynamics that I can't find much info on, lot's of reading between the lines seem's to be necessary. Can you recommend referance sources that might address this issue in depth? If I do go ahead with this concept I would install a sliding weight system for flight test much as Nat Puffer did with the Cozy.

As far as the "deep stall" I was refering to the control authority of the elevator system to fly the nose down if the aircraft were to stabilize in the nose high,low airspeed attitude some canard designs have experienced. I don't know which would be dominant due to prop slipstream effects, increased canard stall aoa or increased elevator effectiveness.

Again, thanks for your input.

John

Billski,

Thank you for your response as well. It must have come in while I was typing my response to Orion.

John