raquo wrote: ↑
Tue Mar 06, 2018 3:38 pm
I saw this mentioned quite a bit, but I don't think I've seen a good explanation of why canards are bad in thermals. Could you elaborate on that please?
It should be kept in mind that a properly designed and built canard aircraft flies with the canard having a higher wing loading than the main wing. Since it is more heavily loaded than the wing it flies with a slightly higher AoA than the wing does, and so the stall occurs first on the canard. In a 'normal' slow deceleration stall, the canard stalls and the nose drops.
This is considered a safer platform than the 'conventional' design that has the little wing in the back. They never gained much popularity outside of the homebuilt airplane arena. A design I always admired is the Burt Rutan designed "Defiant", a twin-engined canard having centerline thrust. Twins with their engines mounted out on the wings are horribly inefficient with one engine inoperative, what with the ailerons and rudder having to be deflected just to keep flying in a straight line. And if the pilot isn't right on top of things when the engine quits, especially when flying with full throttle and a low airspeed, well, the result is often ugly.
Rutan's Defiant is a push-pull design, if an engine conks out the pilot notices a drop off in performance, but no control problems. And on top of that, you can't stall it, and if you can't stall you can't spin.
I don't understand how a manufacturer can get product liability insurance for conventional twins. I imagine a courtroom scene where the lawyer for the plaintiffs (a woman and two kids crying) asks the designer "If you knew that designing your airplane like the Defiant would have saved those children's father, why didn't you do it?"
I've heard that the canard sailplane that Rutan designed was not all that efficient. It's difficult to get the main wing up to it's highest lift AoA because the canard stalls before it gets there.
I have built and played with some model gliders that use a "free floating" canard wing. It is in one way the opposite of a normal canard design, in that the canard wing NEVER STALLS. The canard has a reflexed pitch-stable airfoil, and is mounted in such a manner that it can rotate in pitch all the way around, 360 degrees. If I adjust the glider's Cg too far aft, or tail-heavy, the glider will nose up and slow down, then the MAIN WING stalls and the glider descends vertically, and I can easily see the canard pointing straight down, and it is still "flying" at it's trimmed AoA.
The canard is now "lifting" forward, and it pulls the glider forward and the main wing un-stalls, starts flying, and then the whole scene repeats.
It' looks funny when I toss the glider straight up, it stops and tail-slides, and I can watch the canard pivot nose down and point back at the wing. You should make one, they're fun to play with. They seem to "co-ordinate" the glider in pitch when circular airflow, or airflow curvature if you prefer, exists, such as in circling flight, because the free-floating canard always seeks it's trim AoA. When the Cg is corrected it flies like a regular glider.
I keep hoping that someday I'll find a practical use for the design, to justify it's existence. I came up with it one day when I was thinking those "what if" thoughts that seem to plague me most of the time. Maybe with pitch controlled by weight shift it would prove to have desired characteristics. If not, then I guess it's just a novelty.
Hey, I just realized that if the glider flew into a thermal the canard would adjust itself to the new relative airflow direction and....... hmmm, gonna have to Rc one and see what happens...Maybe the problem of getting the main wing up to max Cl would be solved by using weight-shift to "unload" the canard....Gonna have to experiment some more.