Gyroplane Thrustlines vs. Center of Gravity


bee beleiver
Dec 1, 2011
Middleville MI
Total Flight Time
Kitplanes has been running a series on Gyroplane stability. They are relating speed to stability and suggesting using an elevator for control at high speeds and rotor tilt for low speeds. Comments?

Doug Riley

Platinum Member
Jan 11, 2004
There's an erroneous impression out there that gyros become unduly sensitive at high airspeeds. That simply isn't true for gyros in general. It IS true for gyros that have an unstable airframe layout (misplaced prop thrustline, misplaced center of fuselage drag) -- but it makes more sense to fix the underlying design problem that to introduce a second system of control.

A gyro that's properly designed to begin with develops a firmer and firmer "feel" as it speeds up. It's the opposite of twitchy.


Gold Supporter
Jul 2, 2007
London/ Kilifi Kenya
Gyrs, RAF 2000/Mgni/Bnsn/Hrnet/Mrlin/Crckt/MT-03/Lyzlle AV18-A/Prdtor. Pax ArrowCopter
Total Flight Time
100+ gyro, 16,000+ other
Great point Doug.


Unobtainium Member
May 21, 2006
Columbia, SC
Air Command Tandem w/ Arrow 100hp; GyroBee w/ Hirth 65hp; Air Command Tandem w/ Yamaha 150hp
Total Flight Time
Having a prop thrust line (PTL) close to or through COG is not really all that much of a concern when flying at constant speed and rotor lift.

The problem on a gyro arises when lift and the force due to acceleration of gravity changes dramatically and quickly. Even though a good gyro pilot may be able to compensate and prevent anything bad resulting from a zero G moment, there comes a point where a poor gyro design with excessive HTL will result in zero G recovery failure, the rotor will flap and strike the prop/tailfeathers, and the gyroplane will fall out of the sky.

The general consensus gyroplane community figured this out decades ago. Or so one might think.

For if this is true then why am I now reading about and seeing advertising for - and gyro builders endorsing! - a new, three-gear, extra tall PSRU, designed and intended for use on tractor fixed wing aircraft and trikes, being promoted as gyroplane equipment?

When you lower the engine 3 inches on a single place gyro while leaving the PTL alone you lower the COG by 1-1/2" and thus raise the HTL by that much.

And if, for example, the gyro already has a 1-1/2" HTL then you have doubled the HTL and the risk of a rotor blade tail strike during a zero G event - a catastrophic disaster. By doubling PTL torque about the COG it may become impossible to recover from a power push over (PPO) and prevent a rotor flap strike.

Maybe it wasn't all that big a deal with a 100 HP 160# 912, a 110 HP 195# Viking Honda, or a 150 HP 310# Lyco. But IMO when you are talking about 150-165 HP 165# Yamaha with 650# thrust or more, the physics of acceleration about COG in zero G is radically different.

Why would anyone risk using an unnecessarily tall PSRU which increases HTL on a high-powered, light-weight, hot rod gyro when other, much safer options are available? See ya on the other side.
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