Subject: RE: Airplane reverse thrust

I am going to start by explaining a little about momentum, although some of you readers already know all about that. But for a complete explanation of reverse thrusters I must resort to momentum. Momentum is a property that we compute as the product of mass times velocity, and since velocity has a direction then momentum has direction, too. In a closed system (not influenced by outside forces) the momentum is constant, which is what we call conservation of momentum. That means that if one part of the system has its momentum changed, usually by a change in velocity, then some other part of the system obtains the opposite momentum, again usually by a change in velocity, and typically in the opposite direction from the velocity change of the first part of the system.

Remember that the thrust that gives acceleration to a jet is caused by the change in momentum of the air that passes into and then out the back of the jet engine. That is, in order for the hot gases to obtain momentum for exiting quickly out the back of the engine, the engine must obtain the same momentum (with opposite direction) toward the front of the jet.

When reverse thrusters are deployed vanes partially block the exhaust gases coming out the back of the engine, and some of the exhaust gas becomes diverted (to directions which are mostly up and down) from the back of the engine. Some of the exhaust gases, therefore, obtain a change in momentum, and it is the reverse- thruster vanes which provide the change in momentum. Therefore the vanes obtain momentum toward the back of the jet, since the engine and gases can be considered a closed system and so momentum is conserved, and since the vanes are attached to the engine and the engine is attached to the remainder of the aircraft, the aircraft is braked, or accelerated toward the rear.

I recommend that you do some searching on the Internet for resources related to Aviation Training. I found some interesting sites by searching using Lycos with the terms 'Flight School' and, separately, 'aircraft engineer'. Use your ingenuity and try to find some good reference sites.