Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Training Aircraft
Date: Tue, 2 Jan 2001 17:29:42 GMT

In article <9v435t4rljaa52c9ss1vi2pb4o7vcbnj2g@4ax.com>,
Matt Maurano  <maurano@GARBAGEuclink4.berkeley.edu> wrote:
>>No, but it does deploy its thrust reversers during the descent. The thrust
>>reversers - deployable metal leaves used to divert engine exhaust slightly
>>forward during landing roll-out on jets as a deceleration aid...
>
>Does anyone know approximately what percentage of the engine's thrust
>is diverted forward?

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.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Training Aircraft
Date: Wed, 3 Jan 2001 15:16:21 GMT

In article <3A529F38.C83FB5A8@sympatico.ca>,
Keith Beckley  <keith.beckley@sympatico.ca> wrote:
>> ...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.
>
>...Where is this
>drag coming from - I guess I didn't quite understand what 'intake drag' is?

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.

>...How does this compare to using the tire breaks on a typical landing?

The big advantage of thrust reversers is that they don't rely on wheel
traction, which means that they work fine on a wet or icy runway.
(Indeed, they work better in cold air, because cold air is denser and
gives better engine performance.)
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Training Aircraft
Date: Wed, 3 Jan 2001 20:28:58 GMT

In article <3A535B25.1421D061@canada.com>,
Jim Mantle  <jmantle@canada.com> wrote:
>> The big advantage of thrust reversers is that they don't rely on wheel
>> traction, which means that they work fine on a wet or icy runway...
>
>Though in an aborted take-off, [commercial] pilots are instructed to
>STOMP on the breaks, then reverse the engines. The big advantage is that
>the brakes will start their effect immediately...

Yes, one big wart of thrust reversers is the time needed to engage them,
and their relatively slow control response in general (because big engines
don't throttle up and down quickly).

>1. When the thrust reversers are engaged the engine thrust is diverted
>from straight back to at least vertical, and with the effect I see on
>the water on the runway, there is some significant amount forward as
>well. Is there any braking effect from having a "wall" of exhaust (even
>though air is fluid not a solid)?

Nothing much, because the "wall" isn't rigidly connected to the engine,
and so the extra air it sweeps up doesn't exert significant extra drag.

>2. If the thrust reverse is from inlet drag, why does the pilot advance
>the thursters after engaging the reversers? Is inlet drag related to
>throttle position?

Correct -- it's more or less proportional to the amount of air the engines
are swallowing, which is dictated by the throttle setting.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Shuttle Training Aircraft
Date: Wed, 3 Jan 2001 15:18:01 GMT

In article <3A52ACFA.CE08F691@cfl.rr.com>,
Steve Wachowski  <swachowski@cfl.rr.com> wrote:
>> ...Almost all of the braking comes from intake drag, not from the
>> forward component of the exhaust.
>
>I'm sure that is all correct. However there are some reversers that do
>just that. I've seen quite a few DC-9's, MD80's, etc. back themselves
>away from the terminal gate when no tug was available for a pushback.

Yes, I've seen that too.  That may be why the reversers tend to have a
*slight* forward angle.
--
When failure is not an option, success  |  Henry Spencer   henry@spsystems.net
can get expensive.   -- Peter Stibrany  |      (aka henry@zoo.toronto.edu)

From: Mary Shafer <shafer@orville.dfrc.nasa.gov>
Newsgroups: sci.space.shuttle
Subject: Re: Shuttle Training Aircraft
Date: 04 Jan 2001 13:22:08 -0800

henry@spsystems.net (Henry Spencer) writes:

> In article <3A529F38.C83FB5A8@sympatico.ca>,
> Keith Beckley  <keith.beckley@sympatico.ca> wrote:

>> ...How does this compare to using the tire breaks on a typical
>> landing?

> The big advantage of thrust reversers is that they don't rely on
> wheel traction, which means that they work fine on a wet or icy
> runway.  (Indeed, they work better in cold air, because cold air is
> denser and gives better engine performance.)

However, airliner stopping performance on wet or icy runways is
determined _without_ using thrust reversers.  This is, of course,
because thrust reversers don't always work and thrust reversers are
not used in some engine-out scenarios.  Thus, airliners can usually
stop better than the certification testing shows, which is how pilots
flying big airplanes manage to stop before the end of the runway when
they get confused and land at a little GA airport with a 5,000' runway
instead of the big airport with a 12,000' runway.

--
Mary Shafer  Senior Handling Qualities Research Engineer
shafer@orville.dfrc.nasa.gov
NASA Dryden Flight Research Center, Edwards, CA
Of course I don't speak for NASA