[Graham Boak]

The Rotol was lighter than the DH prop but the percentage difference in total weight would be small, and thus have little effect on the stall speed. More significant would be other changes being introduced at much the same time, such as the armoured windscreen and rear seat armour, so it would be helpful for you to know the mod standard of the individual aircraft concerned, rather than just taking one specific value. A correction for weight is straightforward, as I suspect you know.

There would also be a secondary effect due to the movement of the cg with these extra masses, possibly resulting in more/less trim and thus a higher/lower maximum lift coefficient, but I suspect this is can be neglected - it usually is!