Canard Incidence vs. Pitch Stability

Copied verbatim from Cozy Newsletter #72. Make sure this is done early in flight testing:

The Cozy Mark IV is designed to have positive pitch stability and to be resistant to a main wing stall throughout its approved c.g. range. But for this to be true, the canard must be set at the correct angle of incidence. During your initial flight tests, you should determine whether your canard is set at the correct angle of incidence, and there is an easy way to do this safely in flight. If you notice on large drawing M- 18, there is a protractor which shows the full travel of the elevator from minus 15 degrees at full forward stick to plus 30 degrees at full aft stick, and there is a notation that at cruise, the elevator should be in trail, i.e. at zero degrees. In newsletter #56, page 5, there is a plot of elevator position as a function of c. g. and speed for our plans-built model, and on this plot, the zero elevator position occurs at a c.g. of 101 and 150 knots IAS. For several years now, we have been including a copy of this page with each Owner’s Manual. It also lists several changes for you to make to the Owner’s Manual. So, before you expand your flight envelope, and before you fly at slow speed and aft c.g., check your elevator position. It is perfectly safe to do this at a c.g. of 101 and 150 kts IAS, or even better, at a c.g. of 100 and 150 kts IAS, your elevator position should be at less than minus 1 degree, i.e. about 1/16th inch trailing edge down. If your canard is set at too low an angle of incidence, it is a dangerous condition. The indication will be that the trailing edge of the elevator will be too low in cruise. The symptoms will be difficulty in rotating, pitch instability, and a tendency to react too fast to back stick. And the result could well be a main wing stall.

Very recently two builders complained of pitch instability, and difficulty rotating. I went for a demo ride with one of the builders in his Mark IV. With both of us in the front seat (and estimated c.g. of 100), the elevator trailing edge was down almost 3/8 inch or 5 degrees. For comparison, we both went up in our plans-built Cozy and the trailing edge of the elevator was zero to 1/16th trailing edge down. We concluded that his canard was set at too low an angle of incidence. After a telephone discussion with the other builder, he sent me this e-mail: “Hi Nat, Thanks for your input the other day. I adjusted the canard incidence by 2 degrees. Did so by relocating the guide bushings that engage the alignment tabs, as we discussed. Had to slightly modify the canard cover, but it was really much less of a task than I’d envisioned. Bottom line is that the aircraft now flies lovely all the way out to the forward c.g. limit. The c.g. and weight envelope is fully expanded as well as all flutter testing and low speed/min speed tests. Cooling is pretty good though I’m still working on it. Will provide more soon. Regards, Rob”

This is an important matter. Make sure you check elevator position early in your flight test program.

Last updated: 2021-08-02 18:00:57 -0700