Pilot Induced Oscillations (PIO) and Power Push-Over (PPO)
Part 5
By Greg Gremminger
The traditional safety issue with Bensen-derived gyros, throughout their history, has been the frequent fatal incidents of PIO and PPO. Pilot Induced Oscillations are simply when the pilot is unable to provide the proper and precise compensating "commanded" cyclic inputs for the reactions of an unstable airframe. The pilot's rotor control inputs are in the wrong direction and/or wrongly timed to "dampen" the natural oscillating tendencies of the unstable gyro. For such a gyro system, if the pilot's reactions are not precisely applied, the oscillations will continue or diverge to extreme amplitudes. And, to make it even more ominously difficult for the pilot of such an unstable gyro, the errant airframe motions are providing a feedback reference that is prompting these wrong pilot inputs. Typically, PIO oscillations are very quick and "diverge" to fatal extreme in just a few cycles. This is akin to aerodynamic "flutter" of an airplane's control surfaces. Some doubt that any pilot could even react properly once divergence initiates. It is this divergence that is actually initiated by the pilot's wrongly timed or applied inputs or reaction to a disturbance. The pilot's imprecise compensating reactions further cause a worse airframe reaction followed by a worse pilot reaction - thus Pilot Induced Oscillations. The pilot's initiating control actions are often in reaction to a wind gust disturbance, but can be initiated from over aggressive flight maneuvering beyond that pilot's skill in that unstable gyro.
Pilot over-reaction or over-control is one significant root of PIO. A truly stable gyroplane will "dampen" or diminish the effects of pilot over-control, and a truly stable airframe will minimize the over-reaction or ill-timed tendencies of even less proficient pilots by the comforting solid feel of the aircraft reactions. The Magni design applies all appropriate aerodynamic means to self-stabilize the airframe and aircraft, and provide proportionate and appropriate airframe attitude cues to the pilot. The attitude and g-load cues to the pilot preclude pilot over-reaction and wrong reaction on the controls. Magni also employs another pilot feedback mechanism that is rarely provided on other designs - cyclic stick feedback forces. In most other aircraft types, some degree of control force feedback is employed naturally - the force resistance of an aileron or elevator, for instance. Traditionally, gyroplanes rotors have been designed to the contrary, to offer very light or even force-free cyclic stick resistance forces. The lack of cyclic stick force feedback to the pilot however, is one element in possible pilot over-control. With no rotor/cyclic force feedback, the pilot cannot readily perceive the neutral positioning of the cyclic or how much actual cyclic control is applied. Neutral or constant cyclic control inputs cannot be immediately, if subconsciously, perceived through stick forces. Without this additional feedback mechanism (loop), the pilot must rely on the resultant action of his/her cyclic input - attitude or g load change - to reference the effect of that cyclic input. This means some inertial delay in the feedback (loop) which can lead to over or ill-timed control inputs. Indeed, with little or no cyclic force feedback, un-intentional or over-aggressive pilot reaction is not perceived by the pilot until airframe over-reaction might occur.
Magni gyroplanes employ significant cyclic force feedback. This is accomplished through a combination of the inertia of the rotor and the rotor airfoil and feathering axis arrangement. The result is that cyclic feedback forces constantly and immediately provide pilot cues of the aggressiveness or amplitude of his/her cyclic input. With such a system, un-intentional reactive cyclic inputs are tempered or prevented. The pilot is well aware of his/her cyclic input, must intend to be making that particular cyclic input, and is less likely to make inadvertent or over-reactive cyclic control inputs. This may be the very first Magni characteristic that unfamiliar gyro pilots recognize when first flying the Magni. Some may suggest that this significant force feedback limits the "nimbleness" of the Magni gyroplane. In actuality, the gyroplane is just as "nimble", the rotor can be maneuvered just as quickly and aggressively, but the pilot must certainly apply intentional cyclic stick forces to do so. This means that the pilot is less likely to apply aggressive cyclic control inputs inadvertently. The cyclic force feedback is immediate, with no delay in perception of the actual cyclic input. This additional and immediate feedback "loop" allows the pilot to "fine tune" his/her control inputs, before the other feedback cues arrive. The result is that the pilot is now acting in harmony with the other harmonious reactions of the stable gyroplane. This harmony provides the exceptional comfort and reassuring feel of the Magni, even in very turbulent air. You feel truly "in harmony" with the air and the aircraft. The result, in the already truly stable and PIO resistant gyroplane, is additional PIO resistance for even the most novice of gyroplane pilots. It has been demonstrated that flight control of the Magni, even in turbulent air, can be mastered by complete novices in just a few minutes of instructive demonstration.
PIO is not itself the real "killer". However, PIO can quickly progress to such pitch attitude oscillating extremes that reduced g loads or even negative g loads (airflow on top of the rotor) can cause the rotor to slow below its ability to autorotate upon the next high g cycle. This is typically considered to result in Power Push-Over when the gyro either tumbles forward from loss of rotor thrust or the rotor slows dramatically and folds when centrifugal forces cannot support the gyro load.
Power Push-Over is just one form of what is also called a "bunt-over". These typically refer to the gyro pitching rapidly forward to inverted. This typically results from an unbalanced propeller thrustline that tends to rapidly rotate the gyro forward once the rotor loses significant lift and drag (thrust).
PPO is typically considered to occur from any or all of several factors:
• Pilot rapid forward cyclic stick motion causes reduced rotor lift and drag, allowing the rotor to slow, perhaps too much!
• A strong wind down-gust disturbance which also "unloads" the rotor - reduces rotor lift and drag.
• Upon severe and/or extended loss of lift, the rotor slows below its ability to autorotate and support the gyro's weight.
• An unbalanced propeller thrustline offset pitches the airframe rapidly nose-down upon loss of rotor lift and drag.
• The unstable airframe, pitching rapidly nose-down results in moving the rotor thrustline forward, relative to the CG, causing further nose-down pitching in reaction to reducing or negative rotor thrust.
For a properly aerodynamically balanced and damped airframe, with complementing rotor and airframe inertial responses, the above factors can be avoided. If the propeller thrustline is properly balanced (by the Horizontal Stabilizer and other airframe surfaces), the CG / rotor lift vector stable relationship will be maintained and disturbances will be reduced rather than amplified. The inertial reactions of both the airframe and rotor can be "harmonized" so as to prevent disastrous slowing of the rotor for both natural and pilot disturbances.
There are other "pitch related" incidents that, while they cannot be accurately described as PPO, or even a "bunt-over", still result in the same thing. These may be incidents where the airframe reactions allowed the "rotor to be pulled back into the tail" or propeller by an over-reacting pilot, or the rotor incurs a "precession stall" from improperly balanced loading and inertia reactions. In the end, all of these fatal incidents are the result of some aerodynamic pitch stability flaws in a design, which allow certain extreme conditions to progress into these critical issues.
The Magni gyroplane, while it is not claimed to be totally impervious to extreme pilot abuse of the flight parameters, is designed with all these factors addressed and compensated. Some of the issues are avoided by good aerodynamic airframe reactions. Some issues are addressed by the balance of the moments acting on the airframe to properly align the CG and rotor lift vector. Some issues are addressed through "harmony" of the various inertias. Some issues are addressed by harmonizing the gyroplane and controls reactions to the human capabilities of the pilot - including the accurate pilot reference of a stable airframe. And, last but not least, all of these issues are addressed through good pilot flight and knowledge training. For any gyroplane, whether as stable as a Magni or not, the best avoidance of PIO or PPO or any other related situations, is the full understanding and appreciation and avoidance of the factors involved.
Summary:
Personal preferences and perceptions on gyroplane stability will continue. The challenge and satisfaction of mastering control of a wild beast has attraction and alure for some. However, to many, the difficulties with mastering sometimes-dangerous quirks can make the risks not worth the allure. Even the pilot, who has attained a safe proficiency in less stable gyros, in windy conditions will likely tire from pilot workload - however automatic and unconscious that effort may be. In my personal opinion, having flown many breeds of this beast, I find it more comforting, enjoyable and reassuring, knowing that my "beast" is tamed - knowing that an unexpected villain won't suddenly appear. The precision and solidity of flying a truly stable aircraft, to me, far outweigh the thrill of unnecessary risk. The Magni gyroplane abundantly provides the many extremely enjoyable unique thrills of gyroplane flight without the worry and concerns that something as insidious as PIO or PPO might bite. And, the pure solid feel of the superbly stable Magni gyroplane, in even the gustiest of turbulent days, makes that enjoyment no longer restricted by concerns with the wind.
Greg Gremminger - Magni USA, L.L.C.
