Aer Lingus From Ireland, joined May 2000, 1508 posts, RR: 0 Reply 1, posted (11 years 6 months 1 week 3 days 21 hours ago) and read 2153 times:
Well reverse thrust is selected by the pilot by means of two "minny throttles" forward of the "main" throttles, after the main gear has come in contact with the runway surface. In some airlines, eg Air France, reverse thrust can only be selected when the two main gears and the nose gear have made ground contact
Modesto2 From United States of America, joined Jul 2000, 2549 posts, RR: 7 Reply 2, posted (11 years 6 months 1 week 3 days 4 hours ago) and read 2134 times:
If you're sitting in front of the wing, you can look at the engines and see the reverser doors open. They slide towards the aft of the aircraft, exposing the insides of the engine. And when is it engaged? Well a clue...it's that really loud sound you hear, a second or two after touchdown. Some aircraft have "bucket" reversers. These aircraft, such as the MD-80, have two panels that close together, instead of sliding doors.
AF_Guy From , joined Dec 1969, posts, RR: Reply 3, posted (11 years 6 months 1 week 2 days 23 hours ago) and read 2113 times:
Aer Lingus and Modesto summed it up pretty well, Jon. Something to remember is that the thrust is NOT directed exactly forward (i.e. 180 degrees from the reverse axis). Once the aircraft touches down, electrical switches on the landing gear enable the thrust reverse control system to engage. The pilot brings the engines to forward idle, and pulls up on a set of smaller levers forward of the actual throttles. This causes the "buckets" or "sleeve" (see below), to swing back into the engine's thrust stream. Once the pilot sees that all engine thrust reversers are engaged, through a set of indicator lights, he advances the normal engine throttles to bring the engine power up and the reverse thrust force higher.
On the T-Tail aircraft, thrust reversers are large "buckets" of metal that mechanically swing back and "clamshell" into the exhaust stream of the engine, deflecting the thrust maybe 10-20 degrees max forward. These "buckets" are actuated either pneumatically (using air bled from the engines) or hydraulically (using a self-contained hydraulic system).
On underwing engines, the principal is the same but you really can't see the reversers working. Usually a sliding sleeve on the engine slides aft, which closes off the buckets inside the tailcone, deflecting the exhaust to the sides and forward, but nowhere near directly forward, more like 10-20 degrees....
Some of the myths of thrust reversers are:
(1) the engine stops, and spins in reverse, directing thrust forward (NO! not even close)
(2) the compressor blades in the engine are pivoted to direct the thrust forward (NO! although some engines do have variable stator vanes, these are used to schedule the airflow through the engine)
The aircraft doesn't have any sleeve that slides back but rather for little "traps" that open on each engine. My guess would be that the way these "traps" are placed they are also creating some drag that would help speeds down the a/c, some kind of engine-mounted spoilers, what do you guys think?
VC-10 From United Kingdom, joined Oct 1999, 3656 posts, RR: 42 Reply 5, posted (11 years 6 months 1 week 2 days 20 hours ago) and read 2092 times:
One thing AF_Guy didn't mention was the Throttle lever interlock. You can select more the Rev idle unless the the reverser is fully deployed and, conversly, you can't select more than Fwd idle unless the reverser is fully stowed. The is achieved by a solenoid operated mechanical interlock on the throttle levers
Aer Lingus From Ireland, joined May 2000, 1508 posts, RR: 0 Reply 6, posted (11 years 6 months 1 week 2 days 19 hours ago) and read 2083 times:
Yes Nicolaki,
They do create a miniamal amount of drag to assist slowing the aircraft down but my guess is that its so small they have a negligable effect
CPDC10-30 From United Kingdom, joined Feb 2000, 4744 posts, RR: 31 Reply 8, posted (11 years 6 months 1 week 2 days 5 hours ago) and read 2067 times:
The most fun aircraft to watch the thrust reverse in action is the 737-200 series. Almost the entire rear casing of the nacelle seperates and swings backwards...quite a sight when you're next to the wing!
Soku39 From United States of America, joined Nov 2000, 1797 posts, RR: 12 Reply 10, posted (11 years 6 months 2 days 11 hours ago) and read 2030 times:
Then how does the reverse thrust on an A320 work it goes out o the side?
Skystar From Australia, joined Jan 2000, 1363 posts, RR: 3 Reply 11, posted (11 years 6 months 2 days 3 hours ago) and read 2015 times:
The value of reverse thrust, relative to forward thrust is actually quite low. As said by many on this thread, it's somewhere between 10 and 30% of forward thrust, rule of thumb puts it around 25%. The main point of reverse thrust is that it removes the high level of residual thrust when the engine is idling (how many of you [esp. FS fans] know that an A320 will taxi with about 23N1, little more than idle?). Actually, I think landing distances are actually calculated without the use of reverse thrust.
Reverse thrust is particularaly effective at high speeds, however, as hinted by the AF reverse thrust policy, it does present some problems. Stability, specifically steering ability can be reduced by reverse thrust - especially on a wet runway.
In regards to the A320 with its reverse thrust seemingly coming out of the side, well it's better than nothing - there will be a little reverse component. But, the main thing is, it removes that "powerful" residual thrust component at idle. Note how the A320 with CFM56s has the same reverse system as its bigger cousin, the A340.
Out of interest, aircraft such as the DC8 were certified to use reverse thrust inflight - on the 2 & 3 engines (inboard). They were effectively airbrakes.
At the end of the day, nothing stops a plane better than its own brakes.
F-WWAI From Andorra, joined Dec 1999, 131 posts, RR: 0 Reply 12, posted (11 years 6 months 1 day 23 hours ago) and read 2017 times:
some simple physics on thrust and reverse thrust:
gross thrust is massflow multiplied by exhaust speed
FG=Mdot x Vexh
Intake drag (or inlet momentum) is massflow multiplied by aircraft airspeed
FR=Mdot x Va
the thrust pushing the aircraft (in steady level flight it equals the aircrafts drag)
is FN = FG-FR
let's now stay with an old NON-BYPASS single flow engine:
If we want to create reverse thrust, we turn around a part of our FG.
Usually, T/R's are limited to reverse thrust of about 40 per cent of the FG.
But our reverse thrust equation becomes interesting because the inlet momentum and the reverse thrust have the same vectorial direction:
FNrev= -0.4xFG-FR, that give a big force to stop the a/c. This negative net thrut is high at high speeds due to the definition of FR and rather low at low speed due to FR tending to zero while 0.4 FG stays constant.
Different concepts of T/R's:
bucket or target type (military, old single flow engines, etc.)
cascade T/R's with translating sleeves (modern high bypass engines)
pivoting doors / petal type doors ( modern high bypass engines)
T/R's may reverse the FAN flow only or both FAN and CORE flow.
Most modern high bypass engines on civil aircraft reverse fan flow only. The above equations have then to be adjusted to include the split between FAN thrust and CORE thrust, the latter not being reversed ...
Most efficient T/R in service today is the one on V2500 on A320. Though it only reverses the fan flow, the common nozzle concept of this engine leads to overexpanding the core nozzle flow in reverse thrust condition, thus the core thrust tends to zero in this codition.
T/R's are selected as soon as the aircraft touches ground, they take about 2-3seconds to open, engines re-accelerate and provide thrust. At about 70 kt indicated speed, the pilot reduces the reverse thrust to idle to avoid hot air re-ingestion.
T/R's are also not used at very low or zero speed in order to avoid sand and gravel to be ingested (FOD), that's also why aircraft cannot usually be backed on reverse thrust.
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