Spitfire IIa prop pitch

[41Sqn_Banks]

This is because the Spitfire II has a CSP (constant-speed propeller). The lever in the cockpit doesn't set the propeller pitch, it sets the desired engine RPM. The constant-speed unit/governor tries to keep the desired RPM by adjusting the propeller pitch. Of course the constant-speed unit/governor only allows to set RPM values that don't overstress the engine.

So if you set the RPM lever to the forward position the highest possible RPM is maintained. In combination with full throttle this will give you the highest possible engine output power and therefore the highest possible speed*.

*I assume the at highest possible engine RPM the propeller is spinning with an RPM where the propeller has the best efficiency. This is achieved by choosing a suitable propeller reduction gear.

Note that there are other aircraft that have a CSP, e.g. the Hurricane I Rotol and most (if not all) german bombers have a CSP.