radiators have no influence on speed

[Matt255]

I think you will have great difficulty testing this without logging data.

All you need to do is trim the plane to fly straight with radiator fully open. then close the radiator and trim again if necessary (doesn't seem to be necessary on the Spit btw?).

As long as you do that, you can easily check if the setting of the radiator flaps has any influence on airspeed. Takes 5 minutes for each plane. You can use that wonder-woman view to make it easier.