I just realized couple of days ago that I had never known why helicopters were stable, and avoided tipping over. When you look at a helicopter, it creates an intuitive impression that it is stable, because the center of mass is below the rotor. It seems as if it the helicopter's body is hanging from the rotor, so that if the helicopter was going to tip over, then it would do it about an axis which intersects the rotor, and because the center of mass is below the rotor, therefore the helicopter remains stable. This thinking is wrong, however. Objects which are in the air unsupported, always behave as if they were hanging from the center of mass. If they tip over, they will tip over about an axis that intersects the center of mass! So if the rotor is not perfectly directed away from the center of mass, and a physical rotor will never be perfectly directed away from any point, the rotor's thrust will create a torque that tends to force the helicopter to tip over. So why are the helicopters so stable then? I succeeded in coming up with one explanation, and it is that helicopters don't tip over easily for the same reason why spinning tops don't tip over. The rotor has lot of angular momentum in it, and it keeps the helicopter stable. If the helicopter would tip over, it would do it very slowly, like spinning tops fall very slowly too, and hence pilots have plenty of time to adjust the direction of the rotor. I'm almost convinced that this gyroscope explanation is the right one, but there are some things which still bother me. There are also helicopters, whose rotors don't contain large angular momentum, because they have two rotors rotating in opposite directions. Like these ones: http://en.wikipedia.org/wiki/File:Spanish_Army_Chinook.jpg http://en.wikipedia.org/wiki/File:Kamov_Ka-50_MAKS_2005.jpg Why are these helicopters stable too?