The purpose of the polished stainless steel section on the inner surface of the wheel doors is for the tyres to slide on during emergency extension of the undercarriage. For an emergency extension, a "T" handle is pulled on the lower sub panel below the instrument panel which releases the hydraulic system pressure and, IIRC, also releases the wheel door latches. The gear lever is then selected "down", releasing the uplocks and both the mains and tailwheel extend under gravity, hence the tyres sliding on the wheel doors, pushing them out of the way. It is ops normal, after engine shut down on the ground, to pull the "T" handle to to relieve the hydraulic system pressure and consequently the reason why you will normally see a P-51 with the wheel doors open when parked.
Last edited by mightyauster; 29th Jan 2014 at 12:05.
Reason: left a vital word out...