Thrust line

[Dean]

Hello, I'm new to the building aspect of gyros. Would someone, please, tell me what would be the problem if the prop was below the centerline? I know what would happen if it were too high, but is there a problem with being too low?
Dean

[Doug Riley]

Yes. If the centerline of the prop is appreciably above or below the CG, then the prop is applying a torque to the airframe any time it is generating thrust. If the thrust offset is prop-below-CG, then the torque is nose-up. While that's probably better than nose-down, it still represents a potential imbalance of moments (torques) on the airframe. Consequently, for the gyro to fly level, some other force must be applied to create a nose-DOWN force that exactly counters the prop's nose-UP force.

The usual sources of this nose-down balancing force are rotor thrust or a lifting HS. A rotor thrustline that falls behind the CG will create the needed balancing force in steady flight. Using rotor thrust to counter imbalanced prop thrust (whether high or low) has its shortcomings, though. Rotor thrust varies with G-load, while the nose-up prop force varies with throttle setting. They don't necessarily vary together in a way that consistently preserves the balance.

In practice, with a substantially low thrustline, you add additional nose-up moment when you add power. The rotor thrust won't vary to cancel this effect; instead the gyro actually will nose up, often slowing down as you power up. This will require the addition of substantial forward stick pressure just to maintain airspeed; the pilot's failure to add such pressure can result in the gyro mushing out of a climb. This obviously isn't as catastrophic as a PPO, but it's at best an unnecessary complication. With heavier gyros, the stick force you need to add can be rather high, almost demanding that you have in-flight adjustable trim.

If the throttle is chopped from a high setting on such a machine, you get the opposite effect: the rotor thrust won't decrease in tandem, so the nose will drop in response to the now-unbalanced rotor thrust. A small dose of this behavior is good; you WANT the nose to drop a little as power is cut, so the machine goes into a glide and doesn't lose airspeed. A large, abrupt nose drop isn't a good thing, though, as it tends to unload the rotor, costing you rotor RPM. A severe loss of RRPM can cause destructive in-flight flapping that could finish you off. This may have been what happened to the late Steve Adler in his very-low-thrustline machine a couple years ago -- although no one knows for sure.

A lifting HS is probably a better device to balance a low thrustline than relying purely on rotor thrust. The best setup, however, is one in which the prop applies very little pitching moment to the frame:neither nose-up nor nose-down. If he designer wants a nose-up or nose-down bias (there are reasons to have a small nose-up bias), he's much better off creating it by varying the HS incidence thna by slinging the prop thrust line way above or below the CG.

[Dean]

Doug, Huge thanks...I just wasn't sure what the implications were. Like I said I know about the thrust line being too high, but I didn't even think about power-up and power-down ramifications. I guess that's why this forum is so great. Shame on me for trying to re-invent the wheel. I sincerely appreciate you taking the time to explain it fully. Thanks again! Dean