There is rellevent recoil factored into the game. To what degree of accuracy, I'm not sure.
If a person has a He111 on the ground for example, and fires the nose MG, the aircraft may shudder a little, but will remain stationary. So the game consider's the He111's mass in relation to the caliber and force of discharge of the nose MG.
The same goes for my aformentioned Bf110G2 with the additional cannon/MG. Sitting stationary, the nose group will move the Bf110 backwards, but add to that, the twin Mk108s, and the '110 really rolls backwards!
Next time I have IL-2 loaded, I'll run a few tests and see what the effect of recoil has on the airspeed of the aircraft.