Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Orange on 18/09/2022 18:19:11

Title: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: Orange on 18/09/2022 18:19:11

Hello, when an airliner engine (turbofan) switches to thrust reverser mode, it increases its speed and redirects the air it sucks forward to create a forward thrust to slow down the plane.

During this, the engine sucks in air, does this suction create a forward thrust? or in other words does this suction pull the engine forward and thus the whole plane?

If so, that would negate the slowing of the plane caused by the air being redirected forward, right?

Thanks!

Title: Re: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: evan_au on 18/09/2022 21:50:01

Air which has been mixed with fuel and ignited has a slightly higher mass (the added fuel), is hotter and moving faster than what went in the air intake.
Momentum = Mass x Velocity, and both have increased
So the redirected air can reduce the momentum of the plane.

Title: Re: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: Orange on 18/09/2022 23:21:52

If I trust this video, the air that has been mixed with fuel and ignited is not redirected forward.

Title: Re: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: alancalverd on 19/09/2022 00:06:10

But the redirected bypass air is still hotter and travelling faster than the intake air, and instead of being expelled backwards, which would transfer forward momentum  to the airframe, it is being expelled forwards and thus transferring rearward momentum to the airframe.

Title: On Re: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: paul cotter on 19/09/2022 09:06:12

On low bypass engines, eg early 737, two sections of the low pressure nozzle detached, exposing the high pressure nozzle, and swung around to form a deflector largely behind the high pressure flow. On current high bypass engines all I have ever seen is deflector plates for the bypass flow. I have never seen anything behind the high pressure nozzle but then again I may not have been in "the right place at the right time".

Title: Re: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: alancalverd on 19/09/2022 10:20:55

Funny how the engine has evolved from propellor, via pure jet, back to "mostly propellor" for subsonic flight!

With a pure prop, you can reverse the pitch angle to bring the aircraft to a spectacular halt.

Title: Re: Principle of an airliner engine thrust reverser (turbofan engine)
Post by: Orange on 19/09/2022 21:47:27

Thanks for the explanation !

So the forward momentum caused by engine suction is less than the rearward momentum caused by the air being redirected forward.