Maneuverability vs. Stability

Part 1

By Greg Gremminger

There are many viewpoints in the gyroplane world as to what goes into making a gyroplane stable. There are many viewpoints as to whether this is even an important parameter. The viewpoint I am presenting here is my view and just one viewpoint. Gyroplane stability refers to a number of issues. How easy is it to fly or to learn to fly? How well does it handle wind turbulence? Of particular importance is how resistant is it to Pilot Induced Oscillations (PIO) and/or a "bunt-over" - often called a Power Push-Over (PPO). All aircraft have varying degrees of stability as a result of their aerodynamic configuration. But, gyroplanes have particular issues because the "wing" of the gyroplane - the rotor - is not "fixed" to the airframe. In an airplane, the wing is fixed to the airframe, thus they are referred to as "fixed wing". The fact that the rotor in a gyroplane is not fixed to the airframe directly, but is controlled through cyclic action of the control system, makes gyroplane stability issues much more difficult and important. And, this fact makes the erroneous application of fixed-wing analogies particularly mis-leading.

Maneuverability vs. Stability:

One gyroplane property that people often associate with stability is maneuverability. A traditional fixed-wing analogy, inaccurately applied to gyroplanes, is a concern that if a gyroplane is very stable it will not be maneuverable - AKA, fun to fly! The flight path, or change in flight path, speed and direction, of any aircraft is the result of a change of attitude of the lifting element - wing or rotor -- not of the airframe. Because gyroplanes are not "fixed wing" aircraft, the lifting element - the rotor - is controlled independently by pilot input and does not depend on control surfaces to move the airframe to cause a change in the lifting element. And, because the rotor disk is moved or changed by a very powerful and responsive cyclic action, the rotor can be made to change attitude much more quickly and with much more authority than can a wing, which is attached to a high inertia airframe.

The gyroplane actually has the best of both worlds of this stability / maneuverability relationship. A stable gyroplane is one that actually has a stable airframe. An unstable gyroplane is one that has an unstable airframe. A stable gyroplane airframe is one that follows or accurately tracks the flight path - in other words, the rotor determines the flight path and the airframe follows or tracks like an arrow on that flight path.

Having said that the gyroplane rotor is not "fixed" to the airframe and that the airframe does not have to change attitude in order for the rotor to change attitude, the airframe attitude does have a part to play with the rotor attitude. However, the pilot is the interface between the airframe and the rotor. That means, without pilot input, through either control system friction or trim input, or pilot holding the control stick steady ("fixed"), the rotor WILL follow the airframe attitude - the same as a "fixed wing". This is actually what causes the "stability" of a gyroplane with a stable airframe - without pilot input, the stable gyroplane airframe will force the rotor to follow the airframe attitude. However, with pilot commanded cyclic input added to the airframe uncommanded cyclic input, the pilot can cause the rotor attitude to change much faster than the airframe would otherwise allow it, and therefore very powerfully adjust the flight path of the whole aircraft. In this case, the stable gyroplane airframe will be aerodynamically forced to follow the flight path dictated by the lifting element - the rotor.

Another quality of maneuverability is how precisely the pilot is able to control the aircraft in the desired maneuver - change in flight path or airspeed. The pilot controls that maneuver through commanded cyclic rotor control inputs. The pilot requires a reference from which to sense the progression of any maneuver and precisely provide further commanded maneuvering inputs - airframe attitude feedback. In a stable gyroplane, one where the pilot's reference - the pilot's seat and the airframe visual attitude reference relative to the horizon - accurately represents the actual maneuver of the aircraft. This is because the airframe attitude accurately tracks the flight path. In this case, the pilot has an accurate reference from which to make compensating or appropriate cyclic inputs. In an unstable gyroplane airframe, the airframe attitude does not track the flight path and requires extensive familiarity and experience with that gyroplane in order that the pilot may interpret the attitude responses of the airframe and provide accurate commanded cyclic rotor inputs to precisely control the maneuver. The definition of true maneuverability is both the ability of the aircraft to change flight path and airspeed, and the precision with which the pilot is able to control that maneuver. What is often interpreted on unstable gyroplanes as "maneuverability" is the perceived erratic response of the airframe when it fails to track either pitch or roll attitude relative to the flight path.

The bottom line on the maneuverability issue is, a stable gyroplane, no matter how stable the airframe, is equally as maneuverable as an equivalently sized unstable gyroplane. One thing that does affect gyroplane maneuverability - equally in stable and unstable gyroplanes - is the overall weight of the gyroplane. The inertia of the whole aircraft makes a heavy gyroplane a bit less maneuverable than a relatively light gyroplane. However, the rotor would be sized proportional to the weight of any gyroplane. The rotor power to change the flight path (rotor power is very powerful and responsive compared to a wing - elevator/aileron system), even for heavy gyroplane, is very impressive, relative to a light gyroplane. Because a light gyroplane can therefore be a bit more maneuverable, it is therefore even more important that that gyroplane be stable and forgiving and self-correcting to uncommanded maneuver inputs.