concord977
Topic Author
Posts: 1224
Joined: Fri Jan 02, 2004 1:43 pm

### Thrust Reverser Effectiveness?

This is a simple (maybe naive) question.

Are thrust reversers really effective in slowing aircraft? Can they stop an airliner without the use of wheel brakes?

As a passenger, I always hear the reversers come on but never feel any slowing of the aircraft until the wheel brakes are applied.
No info

rendezvous
Posts: 534
Joined: Sun May 20, 2001 9:14 pm

### RE: Thrust Reverser Effectiveness?

Well a plane will slow down without thrust reversers or brakes, it's just not as fast. Thrust reversers are effective, but they're only a supplement to the wheel brake system. They're most effective at high speeds, and not as much once you slow down.

MITaero
Posts: 485
Joined: Mon Jul 28, 2003 8:00 am

### RE: Thrust Reverser Effectiveness?

>They're most effective at high speeds, and not as much once you slow down.

Why is this true? (I'm not sure)

phollingsworth
Posts: 635
Joined: Sat Mar 06, 2004 6:05 am

### RE: Thrust Reverser Effectiveness?

>They're most effective at high speeds, and not as much once you slow down.

Why is this true? (I'm not sure)

Simply F=ma.

Thrust = mass flow rate * (Velocity out - Velocity in) + Exit Area * (Exit Pressure - Ambient Pressure)

The last part of the equation (pressure) is zero for almost all commercial aircraft (the exhaust velocity is subsonic or sonic, therefore information at the exit plane commingles). Therefore, the thrust comes from the delta in velocities (the mass flow doesn't change appreciably through the engine). Many reversers only 0 the effective exit velocity in the longitudinal axis (others actually reverse it). Consequently, T = m*(0 - Vi). Therefore, as Vi goes to 0 (plane slows down), T goes to zero. The same trend holds true is Ve is negative, except the zero speed thrust is a finite negative number, i.e. power backs.

darkblue
Posts: 227
Joined: Thu Sep 11, 2003 10:27 pm

### RE: Thrust Reverser Effectiveness?

Another reason why thrust reversers are more effective at high speeds than at low speeds is reingestion. At low speeds, bypass air is redirected forward and can be reingested. This air is extremely turbulent and will generate more losses in the inlet and at the fan than if clean air only is pulled in. You many even stall the compressor.

Also, if there is a slight tail wind, hot core exhaust can even pulled around the inlet and if reingested, would produce large losses of efficiency not to mention the effects of the higher temperatures on the different engine components.

prebennorholm
Posts: 6582
Joined: Tue Mar 21, 2000 6:25 am

### RE: Thrust Reverser Effectiveness?

Another reason for the seemingly not very effective thrust reversers may be that the flight crew is not using them to their full potential in order to reduce noise.

At my neighbor airport CPH for instance it is forbidden to use thrust reversers due to noise restrictions. What we see there is that most often the thrust reversers are engaged and the engines spooled up to little above idle, only for making a faster response in case of a problem on the wheel brakes. In such a case the use of reversers is of course permitted.

Maybe that is also what you experience?

In the good old days when noise was less talked about, then SAS would sometimes almost pull the tail off their DC-9s, and the thrust reverser really made your seat belt do its duty.

But then I also think that the clampshell reversers on those JT8Ds were more effective than the cascade reversers on modern high bypass ratio engines.

Those old JT8D reversers were also much lauder. First time passengers would often begin screaming of fear for their life when the reversers were engaged. Fortunately the engines were so laud that you couldn't hear the screams.
Always keep your number of landings equal to your number of take-offs

7574EVER
Posts: 462
Joined: Sun Jan 04, 2004 4:47 pm

### RE: Thrust Reverser Effectiveness?

First time passengers would often begin screaming of fear for their life when the reversers were engaged. - Prebennorholm

LOL. I remember the first time i saw those reversers engage on a 737-200. I was about six or seven years old at the time. Man, I nearly crapped myself when I saw two big pieces of metal separate from the engine. I thought the damn airplane was falling apart!
Right rudder....Right rudder...Come on, more right rudder....Right rudder......Aw forget it, I quit!!

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