Shortround6
Major General
I am not talking about the wooden fixed pitch prop.But there was little possibility that any UK fighter would have to fight with a wooden fixed pitch prop. The question was what variable pitch prop do you fit, which depends on how much power the plane has, which depends on things like is 100 octane fuel available.
The 2 pitch prop, some times called two speed, and often called variable pitch (sounds much better than fixed pitch) but all the same propeller only offered (most of the time) two pitches.
There is a set of directions on how to operate a switch to get pitch settings in-between two limits but I am not sure all planes had that.
In any case one of the test planes describes how the propeller was set up so that at take off 2850rpm and 6 1/4lbs boost the prop was at 32 1/2 degrees pitch (at 42in radius) and at about 2000ft with a speed of 170 IAS the prop was changed from fine to coarse pitch and the speed increased to 185 IAS while the engine rpm dropped to 2070rpm at 6 1/4lbs boost. 185IAS was considered the best climbing speed in coarse pitch.
It doesn't have much of anything to do with 100 octane fuel. What you want for coarse pitch is the prop pitched to give you the max speed at 3000 rpm (engine speed) at 17,000ft or above. IF you reduce pitch to give you better climb down low (under 17,000ft or so ?) you will either over speed the prop at the higher altitudes or have to throttle back a bit. The 100 octane fuel only gives you more power several thousand feet below FTH.
See this report.
Spitfire Mk I K.9793 Trials Report
Spitfire Performance Testing, wartime flight trials and reports of Spitfire aircraft. Spitfire Mk I data.
www.spitfireperformance.com
even with the two pitch prop when climbing they never got the engine over 2440 rpm which is severely going to limit boost and power output.
A constant speed allowed 2600rpm and 6 1/4 boost at any altitude below 11,000ft.
Later they approved a climb rating of 2850rpm and with the impeller spinning faster the 6 1/4 lbs could be held higher.
In combat you could use the full 3000rpm.
Maybe
If you over rev the engine at too a low a speed your prop will be slipping in the air and not giving good thrust. The constant speed prop is supposed the adjust the pitch setting to keep the prop from over speeding for the conditions. Like letting you use 3000rpm and 6 1/4 lbs boost at 170 mph while climbing in fine pitch and then keeping the prop pitch increasing while the speed and/or altitude increases.
Anything you do with a two pitch prop is going to be a comprise.