Hi Butch,
>I'm not that much surprised buy the effect of polishing/filling, i have seen reports where aircraft under that conditions were 40km/h faster than ones in usual combat conditions.
That seems like too much to stem from filling/polishing alone.
This is from a US Navy F4U test:
"The principal changes in drag included sealing and fairing the wing fold hinge line, removal of the tail hook, carefully fitted cowling, and a faired and smoothed, but not polished skin.
The total speed gain, as a result of drag reduction alone, in this airplane is estimated to be 8 MPH at the airplane upper critical altitude."
As NACA Report 824 indicates that polishing has little effect on an aerodynamically smooth surface - though it helps to maintain a (filled and sanded) smooth surface - I'd say that you won't get much more than these 8 MPH in total, and not all of this is due to surface finish.
It was compared to another F4U (with a different propeller so conclusions aren't perfectly safe), but the smoothed-skin Corsair displayed a speed advantage of hardly 4 MPH above upper critical altitude to the other Corsair "with a surface finish in rather poor condition but with the tail hook removed." The "rather poor" surface finish was considered typical for aircraft "after moderate service".
I've read several anecdotes about surface polishing, which is usually attributed with much greater effect than this, but one episode told by a German crewman pointed out that after all the work they had spent on the aircraft, they were quite disappointed that the results fell far short of their expectations.
Maybe the 40 km/h were the difference between a factory-fresh aircraft and a combat veteran? If aerodynamic shape is the determining factor, one over-G pullout might suffice to distort the skin enough for such a large difference even if the surface still appears smooth to the unarmed eye.
Regards,
Henning (HoHun)
