Milt,
Apologies for the thread creep but if people have an interest in flight control systems and I can answer their questions ....
Stitchbitch,
I am obviously too transparent!
Rigga,
The aileron gear change was used to give increased roll performance during landing although it could be used for take-offs in strong crosswinds also. The roll rate that is generated when deflecting the ailerons is a function of, essentially, indicated airspeed. The faster you fly, the higher the roll rate for a given aileron deflection. At high speeds the maximum aileron angle has to be limited for several reasons. The twisting moment on the wings may result in excessive structural loads, the sideslip caused whilst rolling could exceed the fin strength limits and the aircraft could depart. Also, the gearing between the stick and the ailerons is dictated by the available stick movement whilst ensuring that the aircraft is not too sensitive in roll such that low roll rates can be generated when needed. However, once this gearing has been set for the high speed case there may be insuffucient roll control on the approach, especially for a crosswind landing. Therefore, some aircraft have an incease in stick to aileron gearing at low speeds. In the Buccaneer this was manually selected by the pilot. In the Tornado, it is selected automatically when the flaps are greater than 15 deg (in the 'full CSAS' normal control law).
Back to the P-51, it had 3 different aileron gearing settings which were changeable only by maintenance action. The high gearing did put large loads on the wings and I am not sure how much it was used for combat sorties. The middle gearing was the normal position and the low gearing was, I believe, used when carrying heavy underwing stores because of the excessive roll inertia that would be generated at high roll which made it difficult to stop a rolling manoeuvre.