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Reverser and braking

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Reverser and braking

Old 11th Nov 2019, 21:51
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Reverser and braking

Hi Guys,
i am looking for the theoretical part of how reverse thrust slows done an aircraft ( is it true that it is aerodynamic braking by creating like a drag wall with redirected the thrust flow) and how to calculate the speed until it is efficient.
Additionally how carbon brakes work. How to calculate the brake energy and why it depends so much on the right temperature. Do auto brake settings have a certain fix deceleration rate by certification requirements or different from aircraft type to aircraft type. And additional Information concerning this topic also appreciated.

Many thanks in advance
uzintheair is offline  
Old 12th Nov 2019, 05:34
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Here is a video on how carbon brakes react at different temperatures;

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Old 12th Nov 2019, 07:37
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Older airplane engines reverse the “core” thrust in a mechanical way:




More modern ‘high bypass’ engines reverse the Fan air





What both systems have in common is that they are stowed somewhere around the 60-80kts speed range as the chances of FOD ingestion (from runway contamination) increases with lower speed and warmer (if not hot) air is directed forward and ingested in the engine which increases air intake temperature which increases pretty much all engine related temperatures.

Auto brake settings 1-2-3-4-Max work at increasing deceleration rates.
So auto brake 1 the least and Max the highest deceleration rate. Using spoilers/speedbrakes and thrust revers provides braking action so a deceleration rate which means the brakes don’t have to work as hard.
So the more “thrust reverse” you use the lesser the brakes need to work to achieve the deceleration rate required by the auto brake setting.
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Old 12th Nov 2019, 09:53
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I know that there’s a weight penalty, but I much prefer designs that deflect the core air as well. My first jet had that design, and they were highly effective. I’d almost never use the brakes on landing.
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Old 12th Nov 2019, 10:28
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The original PW JT9 engines on the 747-100 series had bypass and core reversers. The hot, core reversers were so unreliable they were quietly modded out and forgotten.
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Old 12th Nov 2019, 11:05
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Absolutely the stopping of the aircraft should be done by
the brakes,...then there is the potential added benefit of
using reverse thrust. this use may not always be wise, such
as when the a/c has a bit of drift angle on a slippery surface,
or when a very long stopping dstance is available, in which
case a low brake usage will not always benefit carbon
brakes which perform and last better with strong use. Brakes
are designed to stop the plane under the most adverse and
extreme conditions, that's their job. Of course on a slippery
runway (no cross wind) reverse thrust is a major added benefit.
That comes with a weight/maintenance/stressful use price.
(The A380 dose not have it outboard as the FOD risk is not
worth that price) Brakes are designed to stop planes and are
a mature successful device. Thrust reversers are complex,
maintenance heavy, and engine sress cycle cause. What's
good about using no brakes and using reverse? Ask your
engineer. Brakes are simple, reliable and easy to replace.Tires too.
RR Trent operators will well know the fan duct damage that can
occur with harsh use of reverse...very expensive!
(Note, I've written this as a long haul heavy operator, when auto
brakes are an important SOP. Narrow bodies at most airports
may well never need their brakes, so lucky them!)
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Old 12th Nov 2019, 13:15
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I picked this up from a very old thread by a gentleman of the name Henri Spencer:

In most thrust-reverser designs, little or none, despite the name. The
key purpose of the thrust reversers is not actually to reverse thrust, but
to kill thrust by sending the exhaust off in a useless direction -- more
or less sideways in most modern designs. The braking effect comes from
the fact that the engine is swallowing very large amounts of air, creating
a considerable drag on the engine, without producing any compensating
thrust. Almost all of the braking comes from intake drag, not from the
forward component of the exhaust.

>> ...The braking effect comes from
>> the fact that the engine is swallowing very large amounts of air, creating
>> a considerable drag on the engine, without producing any compensating
>> thrust. Almost all of the braking comes from intake drag, not from the
>> forward component of the exhaust.
>

Viewed from the engine's point of view... As the incoming air is
compressed in the inlet (and compressor), it is also decelerated to quite
low speed within the engine. The engine very nearly brings the air to a
halt, creating a great deal of drag on the engine. After adding some
heat, the engine then expands and accelerates the air through the exhaust
nozzle (and turbine), creating thrust.

Using the terminology loosely, the net useful thrust of the engine is
nozzle thrust minus inlet drag. Nozzle thrust and inlet drag are both
typically several times the net thrust; an engine with 20klb of net thrust
may well be generating 100klb of nozzle thrust and 80klb of inlet drag.
(This is one reason why the net-thrust/weight ratios of jet engines are so
puny compared to rocket engines, which are all nozzle and no inlet.)

So killing the nozzle thrust while retaining the inlet drag instantly
gives you a *lot* of braking force, and it's not really necessary to
divert the exhaust forward.
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Old 12th Nov 2019, 14:04
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Not completely true.
https://engineering.purdue.edu/~prop...s/reverse.html
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Old 12th Nov 2019, 20:54
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Thank you guys for sharing your knowledge! Appreciate reading it.

Anyone has a mathematical approach to the reverse brake force ? I always understand better with numbers, during the ATPL theory you are only clicking questions to get the license but with some airliner flight experience you wanna understand it more in my case.
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Old 14th Nov 2019, 03:33
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Originally Posted by JeroenD View Post
I picked this up from a very old thread by a gentleman of the name Henri Spencer:

... an engine with 20klb of net thrust
may well be generating 100klb of nozzle thrust and 80klb of inlet drag.
(This is one reason why the net-thrust/weight ratios of jet engines are so
puny compared to rocket engines, which are all nozzle and no inlet.)

So killing the nozzle thrust while retaining the inlet drag instantly
gives you a *lot* of braking force, and it's not really necessary to
divert the exhaust forward.
No. Just ... no.

Dave Therhino is offline  

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