Radiator drag

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109 data.

I. For what it's worth, French trials of 109E suggest the following figures:

http://kurfurst.org/Performance_test...formanceT.html

Bf 109E-3 at 5000m, OPEN radiators, 2400 rpm and 870mm Hg boost: 520 km/h
Bf 109E-3 at 5000m, CLOSED radiators, 2400 rpm and 880mm Hg boost: 570 km/h

If you line up the altitudes and look at the manifold pressures the French recorded, they used higher manifold pressure's when the closed radiator test was conducted.

That means the speed gap between open and closed due to radiator drag is not as wide as their results.