Re:Angles and dangles

Bill: The "Cierva Rule," relating HS volume to rotor volume, is so rough an estimate that it's not much use in designing gyros in general. Cierva himself alluded to the possibility of changing the HS size with different control mechanisms. There are half a dozen other factors that go into sizing and angling the HS. Some of these can make the difference between a craft that's stable and one that's crashy (a cabin that produces a powerful nose-down "wedge" effect at high speeds can be your worst nightmare, for example).

Certainly, as a starting point, if the thrustline is at all above the CG, the HS must be set to counter that effect at all throttle settings and airspeeds. The PPO moment created by a high thrustline is measured in foot-lbs. The moment at wide-open throttle will equal the maximum engine thrust times the height of the thrustline above the CG, in feet. E.g., 300 lb. of thrust and 3" above the CG yields 75 ft.-lb. of PPO moment (300 x 1/4 ft.).

The HS must generate a counter-moment to this PPO moment. The HS uses a lever arm equal to the distance from the CG to a point 1/4 of the way back from the HS's leading edge. It comes out to 4-4.5 ft. on most Bensen-style gyros if the HS is mounted as far aft as possible. To counter the PPO moment mentioned above with a 4-foot HS lever arm, we'd need the HS to create a down-load of 75/4, or 18.75 lb.

To know how big the HS must be and what incidence to give it is a bit more complicated. Standard airfoils have known "lift curves" -- graphs that relate various angles of attack to pounds of lift and airspeed. Once you know the airspeed the HS "sees," you can pick a HS size and incidence. The HS is likely to have to be immersed in the prop blast to work at low airspeeds.

I don't know of a standard formula for HS size or down-load in the case of perfect centerline thrust. Something on the low end of the Cierva formula is probably enough for an open-frame gyro. In the case of centerline thrust, I'd give it a token negative incidence, no more than -1 degree. The aerodynamic effects of a cabin can throw off this number, however, requiring a much more powerful HS.

The process is to use these methods to get your HS in the right neighborhood. Specific flight tests for pitch stability are needed to see if your estimates are correct. Greg Gremminger has outlined a good, objective test series in the PRA mag.