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I've been searching the internet for awhile and haven't really come across a good answer for this question. So:

I have a basic understanding of the principles of flight (thrust, lift, drag, etc). But one thing that has been confusing me is how exactly a thrust reverser on a high-bypass turbofan engine works. So from what I understand, the large fans at the front of the engine will provide the majority of the thrust. Air coming through the inlet nozzle produces an action-reaction pair on the fan blades to produce this forward thrust. So upon deploying the thrust reversers, the air that has already produced a forward thrust on the fan blades now moves back through the bypass duct to strike the thrust reverser, thus redirecting the air forward by creating another action-reaction pair to slow the aircraft.

So it just kind of seems to me that the thrust reverser just cancels out the thrust from the fan blades. Or is the air coming off the fan blades is accelerated so that by the time it hits the thrust reverser the forward directed thrust is greater than the thrust generated from the fan blades, thus producing a net reverse thrust?

Or maybe I'm just looking at this wrong and need to think of the entire engine as a closed system so that the air pushed backward from the fan doesn't produce an action-reaction pair (and thus thrust) until it exits the engine?

I'm just kind of confused on the specifics of the physics of the thrust reverser (where the action-reaction acts).

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  • $\begingroup$ Welcome to Av.se! Good question. Consider that forward thrust is much more about air being pushed out at high speed/pressure/temperature, than it is about air being sucked in. The vector of that exhaust (aft - forward thrust, or forward for reverse thrust) is much more significant than the pseudo thrust vector created by pulling air into the inlet. I'm sure others will expand on that concept in their answers. $\endgroup$ – Ralph J Jul 9 '18 at 18:58
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    $\begingroup$ I wasn't necessarily referring to the air being sucked in, but more to the fact that thrust is generated by the fan blades pushing air molecules back and the action-reaction that comes with that. So after an air molecule gets pushed back by the fan blade (thus pushing the airplane forward), it travels back in the bypass duct until it hits the deployed thrust reverser (TR). At that point it hits the TR and is deflected forward, thus pushing the airplane backwards. But this backwards push needs to be greater than the forward push at the fan blades in order to have a net "reverse thrust". $\endgroup$ – Hot_Doug Jul 9 '18 at 20:03
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    $\begingroup$ Imagine a (working...) shotgun with the barrel sporting a 180° bend. In which direction does the recoil act, and why? $\endgroup$ – Cpt Reynolds Jul 10 '18 at 9:21
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If you want to separate the effects of the fan and the reverser, the reverser is not just decelerating the fan air flow. It is redirecting the air outwards, but also forwards. This means there is a force applied on the reversers to partially reverse the direction of air flow.

You can also look at the system as a whole. When operating normally, the fan accelerates air aft, providing thrust. But with thrust reversers, that air ends up going outwards and forwards, providing a net reverse throust. You can think of it as a form of thrust vectoring. It's the resulting direction and velocity of the air that determines the thrust and its direction.

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  • $\begingroup$ So let's just say that the instant that a mass of air reacts with the fan blades a forward thrust of 1000 Newtons is produced. This air then travels back through the bypass duct until it runs into the thrust reverser panels. In order to produce a net "reversed" thrust, this air must react with the thrust reverser and produce over 1000 Newtons of rearward thrust. So what I'm asking is, how is it that the air reacting with the thrust reverser can produce a higher magnitude thrust than what it does coming off of the fan blades? $\endgroup$ – Hot_Doug Jul 9 '18 at 17:26
  • $\begingroup$ Some or most of the air coming off the fan blades gets redirected from aft to forward and out. That air is no longer providing forward thrust. $\endgroup$ – selectstriker2 Jul 9 '18 at 17:34
  • $\begingroup$ Yes, but the forward thrust is happening directly at the fan blade. So as I was talking about above, the instant that the air reacts with the fan blade, forward thrust is produced. Down the line, that air then hits the thrust reverser. That air hitting the thrust reverser doesn't stop the forward thrust from acting at the fan blades. $\endgroup$ – Hot_Doug Jul 9 '18 at 17:46
  • $\begingroup$ Correct; and so the deployment of the thrust reversers could result in the engine being ripped apart. But the net effect, after the short start-up transient is that air is entering the engine at rest, and leaving in the forward direction. Momentum added to the air equals momentum lost by the engine/wing/aircraft... $\endgroup$ – DJohnM Jul 9 '18 at 18:07
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    $\begingroup$ Have you seen someone propel themselves on a skateboard by using a CO₂ fire extinguisher? Imagine what happens if you replace the nozzle on the extinguisher with one that's U-shaped. The CO₂ comes out forwards instead of backwards, so it propels you the opposite way. $\endgroup$ – Dan Hulme Jul 9 '18 at 18:42
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You make a fundamental mistake in the original setup that I think is the source of your confusion.

You state that the thrust from the fan is an action/reaction pair on the fan itself. That is fundamentally wrong. It is the action of the air accelerating out of the rear of the engine that is producing most (???) of the thrust due to the fan.

Forget the fan for a second and consider a turbojet. In this case the compressor stages are slowing the air coming in, and if there's a ramped intake, it's slowing it as well. All of this is actively removing momentum from the aircraft. And yet the aircraft moves! Why? Because the acceleration of the air at the back of the engine makes up for all of this.

So forget all the stages and what-ifs. Thrust is the net difference between the input air momentum and output air momentum. The air starts at the front of the engine at velocity X, and exits the reverses heading (somewhat) forward at >X. Don't overthink it!

p.s. I should point out that if you watch films of early jets that had landing parachutes, you'll notice they always drop them while they're near the end of the runway. If they don't, the net thrust is zero and they couldn't taxi.

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There are several ways to describe thrust, which amount to the same thing. Very broadly, it's described by Newton's third law: if A moves forward, B moves backward with equal momentum. On a closer level, it comes from pressure differences across the surface of a body. Even closer, you'd be looking at viscosity and velocity, laminar and turbulent flow, boundary layers, analytical equations.

But all these ways come to the same result: if one, applied correctly, predicts result X, the rest can't predict an opposite result. In the most basic view, the reverser's thrust comes from pushing air forward. As long as the ultimate result is air being accelerated in some direction, however it happens, the thrust will be in the opposite direction.

If you want to get into the mechanics (which isn't quite necessary), the exhaust produces high pressure between the engine and the thrust reverser. This pressure acts on the thrust reverser's inner surface. The resulting backwards push is indeed greater than the fan's forward thrust.

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There are a series of picture here where you can see how the bypass air is redirected forward to help slow the plane down

https://upload.wikimedia.org/wikipedia/commons/d/df/F-GTAR_Air_France_%283698209485%29.jpg

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  • $\begingroup$ This doesn't answer my question about the physics of the action-reaction forces. $\endgroup$ – Hot_Doug Jul 9 '18 at 17:28
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You are correct; the air being accelerated rearwards by the fan blades of a turbofan does exert a large forwards reaction force on said fan blades (this is, in fact, how the vast majority of a high-bypass turbofan's thrust is produced). And, if the thrust reversers merely slowed the air back to a halt, they would produce no net slowing force.

However, since a turbofan's thrust reverser is physically attached to said engine, either directly or indirectly (via the nacelle and/or wing structure), the forwards reaction force exerted on the engine via its fan blades is also exerted on the reverser... which (when deployed) then exerts this same force on the air which hits it. Then, as the air cannot pass rearwards through the structure of the reverser, the reverser drags the air forwards with it until the air escapes out the front and sides of the reverser.

A deployed thrust reverser would still slow down the airplane even if it were mounted on an empty nacelle with no engine contained therein, since you're basically dragging a bucket through the air open end forwards (this is easier to visualise with older-style "target"-type reversers, but it's still valid even for the newer types; you just have to imagine a toroidal [doughnut-shaped] bucket). Mounted on an empty nacelle, this bucket takes air at rest and accelerates it forwards, generating a backwards reaction force on the bucket (and, thus, the airplane). Put an engine in the nacelle (any kind - all that matters for our purposes is that it accelerates the air rearwards), and the air's journey becomes more complicated, as the air is first accelerated rearwards... but, in the end, all this means is that:

  1. The air hits the reverser - and, conversely, the reverser hits the air (thank you Newton) - much harder than it would have sans engine (this is how the forwards reaction force on the engine is still completely cancelled out).
  2. As the mass flow rate through the engine is far greater than it would be through an empty nacelle (which is why a running turbofan has to actively suck air - and, occasionally, other things, such as tumbleweeds, baggage containers, or ground personnel - in from in front of it, which an empty nacelle doesn't), there is, correspondingly, more air per unit time hitting the reverser behind a running engine, and, thus, being redirected forwards by it, than would hit a reverser behind an empty nacelle (this is how, when the reversers are deployed, a higher throttle setting produces more braking force off the reversers than a lower throttle setting does).
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