Misconceptions 6-7
Gyroplane Stability Misconceptions
By Greg Gremminger
While presenting a number of downsides of popular misconceptions, be assured that a proper design can actually provide gyroplanes that exhibit few or none of these downside issues. This article is just to make you aware and avoid falling into “over-confidence traps”. When designed properly, as verified by actual flight testing, gyros can present stability and transient safety margins well beyond those capable on even fixed-wing aircraft.
Misconception #6: “Air on top of the rotor makes the rotor slow down!”
Air on top of the rotor would actually still provide autorotation in the same direction – maybe not as effectively, so that a 1 G negative load on that rotor might provide an even higher autorotation rotor RPM!! Rotor RPM would still support that negative G load! Blaspheme you say! If you could get air suddenly on top of the rotor continuously, and the control, buntover and coning angle issues weren’t a concern, the rotor would continue to autorotate. (Try turning the airfoil blades on a gyrokite upside down and prove it to yourself.)
The real issue is that reduced G-load on the rotor, between plus and minus 1 G, does slow the rotor down and would probably never allow you to get “air on top” before the gyro starts flapping and hitting things! I’m not saying it is OK to go negative Gs. In fact I’m saying it may be dangerous to incur even somewhat reduced Gs! This may be just academic, the results are still the same, but I think it is important for pilots to appreciate the limits of their machine! The issue is not “air on top of the rotor”; the issue is reduced G loads on the rotor (less than 1 G). Some gyro/rotor combinations, due to inertias and degree of G-Load static stability, may tolerate lower and longer reduced rotor G loads than other gyros, but everyone should recognize that they should not present either rapid or prolonged forward stick – purposefully rapidly reduced G load to the rotor. A G-Load unstable gyro might very easily initiate a rapidly diverging forward bunt upon just a sudden reduced G-load – it does not necessarily need “air on top” or “zero Gs” to be deadly! Most rotors can handle some quick and short rotor downloading, such as a wind gust. But, the real issue here is whether the G-Load pitch stability is adequate to prevent an immediate precession stall, buntover, or rotor strike. An adequately statically stable gyro, with complimentary dynamic stability characteristics and rotor inertias, can be VERY resistant to low G buntovers – the airframe and rotor reactions resist commanded rotor unloading and rapidly self-restore rotor loading before the rotor has a chance to slow down too much. But, you can’t rely on a “cook book” remedy alone to assure that your gyro has such adequate static and dynamic stability characteristics for the wind gusts you might be tempted to fly in.
Misconception #7: “Pilots can stop PIO if they chop power and reduce speed!”
Partially true, but only for specific gyro configurations – for other gyro configurations, this can be a dangerous reaction. Reducing power rapidly has been a traditional remedy that really only applies to traditional high, unbalanced thrustline gyros. For these gyros, airframe nose-up pitch reaction to suddenly reduced power (Power instability), does cause the RTV to reposition rapidly aft of the CG. This immediately restores static G-load stability, and reverses the required control inputs to a more intuitive direction so that the pilot might more readily dampen dynamic oscillations or stop long-period PIO. For such unbalanced high prop thrustline gyros, chopping power is THE correct thing to do!
But, for a low prop thrustline gyro, the rapid nose-down pitch reaction of a rapid reduction of power could actually initiate an immediate precession stall or buntover, or make the aircraft immediately G-load unstable by rotating the RTV rapidly forward of the CG. In fact, pulling aft on the cyclic at the same time (reducing speed!) also moves the RTV further forward of the CG to further aggravate this transient situation with worse static G-load instability! This rapid nose pitching is also a sharp dynamic transient that could possibly trigger a true short-period PIO. In effect, chopping power on a low prop thrustline gyro can worsen the stability and control issues by changing static stability and initiating a dynamic response or subsequent over-reaction from the pilot.
This popular remedy or misconception may not even be possible for a true and fatal PIO event. PIO can come in different rates. Very quick, or short-period natural oscillations would be difficult or impossible for a human pilot to dampen or stop. These short-period natural and undamped oscillations (probably a 5 second period or less), are probably the precursor of a true fatal PIO. Slower, long-period oscillations are more readily dampened by the pilot, and probably do not evolve directly to a short-period PIO event. Long-period oscillations or PIO are likely the commonly observed pitch nose “bobbing” of unstable gyros. But, these long-period oscillations would not be expected to suddenly pitch violently and more quickly into a true PIO event. An experienced pilot could learn to control those slow oscillations – thus the old remedy – to stop recognizable slow pitch oscillations.
But, quick, short-period oscillations of a fatal PIO are probably not recognizable or correctable. They happen at such a rate and such severe pitch excursions, that it is probably not possible for a human pilot to stop the short-period, rapid oscillations of a true, short-period PIO event. The point is that quick and fatal, short-period PIO occurs so rapidly that the old remedy probably does not really apply because the pilot may have no time to react at all!
It is my arguable point that the pitch oscillations you can recognize and stop by proper pilot control are not those rapid pitch oscillations of a real, short-period and fatal PIO. I am suggesting that it is not even possible to recognize and stop true fatal, short-period PIO, once it has started. The only true defense to fatal PIO is to provide adequate “passive” damping (an adequate HS) that avoids sustained short-period natural oscillations. The long-period, slow rate pitch “bobbing” often observed is probably not the precursor to a fatal PIO. But a tendency for long-period oscillations (“bobbing”) probably does indicate inadequate pitch damping of the HS and that that gyro may be susceptible to the higher rate fatal PIO pitch oscillations.
