In the context of this discussion, I just got the latest Flying
Magazine, and there's a letter from someone who is asking how clamshell
thrust reversers work.
The question asks, "In modern jet aircraft, most have a reverse thrust
capacity. I can see how turning the exhaust direction in the nacelle
produces reverse thrust. But how does a clamshell reverser work? To me,
when the clamshell deploys, it forms a reverse cone behind and
unattached to the exhaust of the engine. The engine ... produces forward
thrust. The clamshell diverts those gases after they have left the
engine back in the opposite direction. My question: Why aren't those two
forces offsetting? Why does a clamshell reverser work when the gases
have already left the engine?"
As JD likes to say, accounting for the forces is sometimes difficult.
Accounting for the momentum is often much simpler. This is a perfect
opportunity for JD or someone to write in and explain.
The forces *are* offsetting, as any forces in opposite directions must
be. But the reversing force is greater than the pushing force.
If you look at the engine+reverser (with a big airplane attached) as the
system, you only need to look at the momentum leaving that system, which
is in the forward direction, so the airplane is receiving a backwards
momentum increase. If you look at the engine+airplane and
reverser+airplane as separate systems, the engine exhaust has a certain
delta-V, but the reverser exhaust has a larger delta-V because the gases
had to change direction.
It's the same thing we've been discussing with the waterjet.