Mach trim:
During high speed flight, an aircraft is subject to certain changes in control and stability. One such change is the rearward movement of the wings center of pressure causing a nose down pitching moment.
At supersonic speed the use of trim tabs etc has little effect on the pitching movement of the aircraft.
Mach trim can be achieved in two ways:
- One way is to effect the center of gravity of the aircraft by using the fuel as the trimming medium. If the center of gravity is to far forward giving a nose down pitching moment fuel can be transferred rearwards, thus restoring the aircraft to normal flight by raising the nose. This type of trim is normally carried out by flight crew, guided by instrumentation that indicates aircraft cg at all times.
- The more popular method of mach trimming is to use a mach trim system. From design studies, the point at which the pitch down conditions occur is known to the aircraft designers. Say for instance, it occurs in a certain aircraft at mach 0.85. By having sensors and drive motors coupled to the pitot statics system a signal will be generated to the mach trim system at mach 0.85. This then sends on signals to the automatic flight control system, which in turn will carry out small trim changes to the horizontal stabilizers or the all moving tail plane to counteract the tendency to tuck under.
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