Extract of LurkerBelow post
Matt believes the cause of the crash was due to The Galloping Ghost having a CG too close to the aft limit which resulted in pitch instability.
During qualifying Matt watched Galloping Ghost from inside the cockpit of Furias and could not believe how much trouble Leeward was having in keeping the Ghost in a stable pattern around the course.
Thus far people have been concentrating on the trim tab as the initiator of events, where as perhaps its failure was a consequence of events, as with the extension of the tail wheel and the loss of its gear doors.
It's understandable why a racer would want the CofG as far aft as possible - less drag and hence more speed. How far aft is aft enough though?
As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point. The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and may require some form of augmentation (computers) to be flown with an acceptable workload eg relaxed stability fighters F-16.
The result of moderate instability might be a flyable aircraft, but the workload goes up. If you pulled back on the stick the aircraft will pitch up and slow. But if you let go of the stick the nose will continue to pitch up, since a positive pitching moment will remain. It will require a push force to maintain a climb angle, not the mandated pull. If you pitched down and let go, the nose will tend to tuck under. You’d have to apply a pull force to hold your dive angle, not the mandated push. That’s how the Spirit of St. Louis behaved.
Manoeuvring stability, like static stability, is influenced by CofG location. However, when the CofG is aft and near the neutral point, then altitude also has a significant effect. Since air density has a notable impact on the damping moment of the horizontal tail, higher pitch rates will result for the same elevator deflections as altitude increases. From the pilots perspective, as altitude increases, a pull force will result in a larger change in pitch angle, which translates into an increasing angle of attack and g. While a well-designed flight control system, either mechanical or electronic, will reduce the variation of stick force with CofG and altitude, it is very difficult to completely eliminate the variation due to design limitations.
For example, for the same control surface movement at constant airspeed, an aircraft at 5,000 ft experiences a higher pitch rate than an aircraft at sea level because there is less aerodynamic damping. The pitch rate is higher, but the resulting change in flight path is not. Therefore, the change in angle of attack is greater, creating more lift and more g. If the control system is designed to provide a fixed ratio of control column force to elevator deflection, it will take less column force to generate the same g as altitude increases. What was the DA at Reno at the time?
Note the increased work load. Matt is quoted as observing Leeward having difficulty flying the course. High work load due aft CofG? Perhaps the workload just got too much and the aircraft caught him out. Time will tell.
Even the experts can get the CofG wrong. An SR-71 was lost on a test flight when the aircraft pitched up uncontrollably and disintegrated. The initiating event was an unstart, but the loss of control was the result of having the CofG too far aft.