This was reported a while back. I did my own testing and from my results, I didn't get the impression that the effect was as simple as RAF planes are indestructable in collisions and Luftwaffe planes are made from balsa.
It seemed to me like the collision calculation was taking into effect relative speed during collisions and delivering more damage to planes with higher speed and or energy. It's admirable that they're trying to simulate a different level of damage based on point of collision, speed, etc. Not to mention you have to have collision detection on a massive scale with a damage model this complex. Delivering more damage to the faster plane may even be an attempt at discouraging this kind of behaviour (in which case, stop trying to ram our planes!
). I may also be entirely off base and wrong about the whole thing.
RAF Hurricanes and Spitfires tend to be travelling slower than their Luftwaffe counterparts in these collisions mostly, I've found, and therefore might seem to be taking less damage overall if my above diagnosis is correct.