Obviously they do but I am missing some of the physics of it.
I am thinking of the reversers that flip backwards behind the engine to redirect the exhaust towards the front of the plane thus slowing it down.
I would think at best this would be a zero sum game. As far as I know the jet has sucked in air, combusted it and pushed it out the back thus producing thrust. Redirecting the thrust just cancels the thrust of the exhaust leaving the engine. So it would seem just idling the engine would have the same effect.
Is there some way that thrust is not applied till the exhaust has totally left the engine nacelle such they redirecting it works? Doesn’t make sense to me but that is why I am asking since it apparently does.
In a modern jet engine most of the air passing through the engine by-passes the combustion stage. In these types of engine e.g. RB211 the thrust reversers then divert the bypassed air (sometimes called cold stream air or fan air) forward.
Not sure that is the same thing. That seems to be talking about how material is drawn into the engine and not canceling the thrust.
In this case I am thinking the engine has already provided forward thrust. The combustion chamber and fan blades out the back of the engine push the plane forward. Once the exhaust leaves the engine we have a metal clamp (thing) that redirects the flow towards the front of the plane. To my mind these two effects would roughly cancel each other out (I’d actually think the net forward thrust would be a tad more). As a result idling the engine would produce the same result.
But the by-passed air still goes through the fan blades up front (I thought) gaining thrust there. Again you are just turning around air that has already done its job moving the plane forward.
Not trying to be obstinate. Just not understanding it obviously.
This is the kind of thrust reverser I am thinking about. Notice exhaust gas has already left the engine before hitting that (and that does not look like a hi-bypass engine although I am no expert).
No, because the thrust reverser is part of the same system - the air hasn’t left the system (and therefore caused a reaction to it, as a whole), until it has left the system as a whole - and it does this in a forward direction, causing reverse thrust.
Could you express the question a different way? I’m not understanding why it matters that the flow is redirected. I mean, the flow isn’t over until the flow is over, is it? I think there needs to be some clarity drawn between “directed” and “redirected”. Isn’t it the case that the flow is simply directed toward the front?
See my Post #7 and the picture there. Looks like the reverser is a foot or so behind the exhaust of the engine. I am just guessing the forward thrust from the engine has already been applied to the plane as soon as it is an inch out of the engine. Maybe that is where I am wrong. Where does the plane derive its forward motion from?
The fan air only goes through one stage of the compressor ( strangely called the fan If you were to look in the intake once the air has passed this fan it has a clear run through the by-pass duct. It is barely compressed and is just accelerated by the fan. It is this air which is then diverted.
Just to add - if the exhaust gas could be considered to have left the system before hitting the thrust reverser, then those cartoon devices consisting of a raft with a sail, driven by a fan mounted on the raft would work. They don’t.
Yes. If the net effect of the system is to push mass (gases) forwards - regardless whether some components of that sum happen to be going backwards - then the reaction results in reverse thrust.
You’re right, that’s not a high bypass engine. A small amount of air is bypassed but the majority is compressed and combusted. These are older types of engine like the Pratt and Whitney JT 8 and are in the class of low bypass engines and all the air passing through the engine is diverted.
Oddly the example by Mangetout of the cartoon with the fan on the raft sorted it for me.
Thanks guys.
As an aside in the picture I linked in Post #7 there looks to be an intake at the front of the tail of the plane. Any idea what that is for? Air for the passengers maybe?
Take a given hunk of air going through the engine, with a mass of M. The engine accelerates it backwards at -V. The delta v is therefore -V and the air therefore undergoes a change in momentum of -MV. This exerts an equal and opposite reaction on the engine ie imparts momentum to it of MV. However, the chunk of air then hits the reverser. The reverser stops the air from going backwards at -V and sends it forwards at V. That is, it undergoes a delta v of 2V and therefore a change in momentum of 2MV. This exerts an equal and opposite reaction on the reverser ie it imparts momentum to the reverser of -2MV. Since the engine and reverser are connected, the net change in momentum imparted is -MV.
Or to put it in simpler terms, the engine just (1) blows the air backwards. The reverser (1) stops the air moving backward and (2) sends it forwards. Therefore it doesn’t just cancel the thrust, it cancels it and sends it back the way it came.