ooooooh, handbags.....

To get back to the graph posted by IvanK it show us that the drag effect of the rad flap in the free stream is negligible. Most of the drag come from the free stream of air impacting the actual radiator surface inside the duct (the duct being the encasing sheet metal).

Lowering the flap increase the exit area allowing a greater mass of air to circulate trough the radiator. Since the geometry of the rad system is fixed, in the inlet portion, the air is less decelerated and then impact the rad at a higher speed. This increasing the drag of the system (but adding more cooling effect obviously).

Concerning the FM of CoD, it could be interesting to use this same value for the Spit.

And so we should not start splitting hairs. There are considerably larger incorrect level speed figures in the Hurricane at the moment (nearly 60mph @ 20,000ft?). 5mph might matter in a tail chase but in terms of combat performance it means almost nothing. It may be more significant at lower altitudes though.

Agree 100%