A Stable Airframe
Part 2
By Greg Gremminger
Although the aerodynamic design and properties of a rotor can affect the degree of stability of a gyroplane, the major element affecting a gyroplane's overall stability is the stability of the airframe. This airframe stability can be defined according to two variable factors:
• How accurately and rapidly does the airframe attitude aerodynamically respond to a change in relative wind? Relative wind can change from either wind turbulence or from pilot commanded cyclic input that changes the flight path or speed of the gyroplane.
• How rapidly and in what direction does the airframe attitude respond to changes in rotor lift - or g load on the aircraft.
The aerodynamic stability of the airframe is somewhat intuitive. Essentially, the more it reflects the qualities of an arrow, the more the airframe attitude will track and respond to the relative wind. This quality includes the size and aerodynamic efficiency of the tail surfaces - vertical and horizontal stabilizers - relative to the airframe aerodynamic properties, lift and drag, forward of the CG. Essentially, the aft stabilizing surface volumes (area of the stabilizer times it's moment arm from the airframe CG) must be larger than the surface volumes forward of the CG - the larger the better. The Magni Gyroplane has extremely large and effective vertical and horizontal stabilizers. They employ efficient airfoil shapes and are located far aft of the aircraft's CG - long moment arm and essentially high tail volume. In addition to the airfoil shapes, the arrangement of vertical tip rudders on the horizontal stabilizer improves the effective area of both the horizontal and vertical stabilizing action. And the rest of the fuselage is designed for low drag and a constant and controlled center of pressure (Center of Drag) to maintain balance with the stabilizers and the rest of the moments acting on the airframe. Just looking at the Magni, it's easy to see the attention to aerodynamic cleanliness, from the large tail surfaces, to the low drag enclosure, landing gear and wheel pants.
Airframe response to g load is another important stability factor. Essentially, like any aircraft, the lift vector of the rotor (lifting element) must be aft of the CG. When this is the case, on any aircraft, a g load transient will cause the airframe to move in the direction to reduce that g load. This is the essential element of stability on any aircraft. On a "fixed wing" airplane, the loading limits established by the designer assure that the CG is forward of the lift vector of the wing. The aircraft designer specifies the aircraft loading so that the CG is properly forward of the lift vector to achieve the desired stability for that aircraft.
For a gyroplane, it is a different story. The rotor is not "fixed" and so the rotor lift vector and CG are able to move relative to each other. And, there are a number of "moments" acting on the airframe so as to cause the CG to move relative to the lift vector - possibly reducing the stability or even causing negative stability if the CG is forced aft too far - from nose-down moments on the airframe. Alternatively, if the CG can, through the balance of static moments on the airframe, be caused to be remain well forward of the rotor lift vector, the essential g-load stability will be enhanced.
The "moments" acting on the airframe to position the CG relative to the rotor lift vector are several. First, the rotor lift vector is, hopefully trying to rotate the nose lower - analogous to the wing lift vector on a properly balanced airplane trying to rotate the nose lower. Then there is the effect of the propeller pushing - possibly not directly aligned with the CG of the gyroplane for that particular flight. The CG may be different for different weight pilots or passengers, for different fuel, or possibly for baggage squirreled away somewhere on the gyro. Whether the prop thrustline is above or below the CG, causing a nose down or nose up moment, depends on the configuration of the gyroplane and possibly on the loading of the gyroplane on that particular flight. Then, there are moments from the rest of the airframe, fuselage, windscreen(s), landing gear, etc. possibly trying to force the nose up or down. And finally, hopefully, there is a large balancing moment from a horizontal stabilizer. The horizontal stabilizer must "balance" all of these other moments, under all conditions of load, power and airspeed, to keep the airframe level to the airstream and the CG properly forward of the rotor lift vector.
This g-load stability factor is essential because any transient and any pilot commanded pitch input will present g loads on the aircraft - and it is essential that the airframe pitch in the direction to reduce the effects of this g load transient. The aircraft should pitch in the direction to restore the g load back to one "gravity". If a g load acts to pitch the airframe in the direction that increases that g load change, the whole system is possibly "divergent" and is either very difficult or impossible for the pilot to stabilize. This would be somewhat akin to balancing a ruler on your finger - negative stability.
A gyroplane whose fuselage pitches in the correcting direction for a g-load transient, will tend to restore the gyroplane back to one gravity load. This would be akin to hanging a ruler on one end from your fingers - positive stability. (Neutral stability, which in a gyroplane would be quite squirrelly, is akin to balancing a ruler on a nail at the 6 inch point).
The Magni gyroplanes are aerodynamically balanced to maintain the airframe essentially level to the flight path - which positions the CG well forward of the rotor lift vector. The fuselage and other airframe moments are minimized and controlled so they can be properly balanced with the horizontal stabilizer moment and the other moments on the airframe.
