I was crazy confused about thrust reversers for the last couple days. I think a kinda conservation of momentum thing. I was originally thinking that there was a force on the turbofan blades pushing the engine forward, and an equal and opposite force on the thrust reverser, equating to a zero sum force. If I think about the momentum of the air changing, however, I realized that the air does not give all it’s momentum instilled on it by the turbofan to the thrust reverser, it actually gives more, because it is not slowed to a stop, it is redirected in the opposite direction. So if there was no friction, and was redirected in the complete opposite direction it could theoretically produce 2x the force backwards, which, after subtracting the 1x forward force on the turbofan, would produce a net 1x full thrust in the reverse direction… This article is pretty legit, however the sign changes were a little confusing to me for what ever reason: http://www.engr.sjsu.edu/nikos/courses/AE160/pdf/M5-Thrust.Reverser.pdf
The thrust reverser reverses the thrust. It is a simple application of Newton’s Laws.
While it is certainly true that the ejection of the rapid and hot stream of air out the back of the jet engine causes that engine to want to move forward very quickly (along with everything it is attached to…such as the airplane), the thrust reverser is ALSO attached to that airplane.
Because the reverser reverses the direction of the thrust by physically redirecting it, it provides the equal and opposite reaction that prevents the aircraft from accelerating forward (this imposes large strains within the engine structure) and diverts all that exhaust in the forward direction. In other words, neglecting losses, it changes the momentum of the thrust stream from +p to -p and the structure of the engine soaks up the strains associated with doing this. Thus, the net thrust causes the airplane to accelerate backwards.
Also, I see a continuing misconception on this thread; the fact that the engine exhaust is blowing against anything is totally irrelevant. This “blowing against” other air, or a barrier, or anything else, has nothing to do with why the engine (and hence the plane) moves.
If this “blowing against” idea had any merit, then rockets would not work outside the atmosphere because there is nothing to “blow against”. It is strictly a conservation of momentum/conservation of energy thing.
The exhaust is ejected at a high speed in one direction, the equal and opposite reaction takes the engine that generated that exhaust in the opposite direction.