Video and still photos clearly show the trim tab on Jimmy Leeward’s modified P-51 air racer Galloping Ghost breaking off before the tragic crash sequence began at the Reno Air Races last week. Reliable eyewitnesses also saw the trim tab depart. Why is the trim tab failure such a potentially important piece of evidence in the search for a probable cause of the accident?
Trim systems are important on any airplane because they are used to neutralize control forces across the airspeed operating range of the airplane. Trim is also used to compensate for forces caused by various CG locations, and to remove control force caused by an out of balance condition such as more fuel in one wing than the other. Extension and retraction of wing flaps also generates pitch force changes in most airplanes.
But pitch trim takes on an even greater importance in a racing airplane because the airplane is flying at the edge of, or more likely beyond, its original design airspeed.
Among the many decisions an airplane designer must make is setting the angle of incidence of the wing. The angle of incidence is the angle of the wing compared to the centerline of the fuselage, and more importantly, compared to the angle of the horizontal stabilizer.
Since a wing produces lift by flying at a positive angle of attack, most airplanes have a positive angle of incidence for the wing. With the wing angled up compared to the fuselage, the fuselage can remain close to level in cruise while the wing is flying at a positive angle of attack.
Because stability requires the center of lift of a wing to be located forward of the center of the wing chord, an increase in lift causes the wing to pitch up. To compensate for the up pitching moment created by wing lift, the horizontal tail is angled to create a down force to keep the airplane in balance.
As airspeed increases, so does lift. Additional lift creates more nose-up pitching force, so the horizontal stabilizer must generate more down force to keep the airplane in balance or in trim, as pilots would say. If the stabilizer is fixed and non-moving, the only way to generate more down force is to push the trailing edge of the elevator down. It is the function of the elevator trim tab to push the elevator down to compensate for the added wing lift of increasing airspeed.
At high airspeeds the pilot moves the trim system to deflect the trim tab up, which generates the force to push the elevator down. When an airplane is flying within its original design airspeed envelope there is enough force from the trim system to balance pitching forces from the wing.
But when an airplane is flying at the extreme fast limit of its design, the trim system is operating at its limits to push the elevator down and keep the wing from pitching the airplane up.
If a trim tab fails when the airplane is flying at high airspeed, all of the force the tab was generating would be instantly transferred into the flight controls, slamming the stick back with extreme force. The result would be a violent nose-up pitching of the airplane.
Nobody knows yet what caused Jimmy Leeward’s airplane to go out of control at Reno, but that’s why possible failure of the pitch trim system is being examined so closely.