legoguy
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How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 3:30 am

After messing around on Flight sim for a while, it occured to me, how fast can an aircraft go in reverse thrust? What is the maximum speed ever reached? etc etc

Infact, can an aircraft with engines on the wings even go backwards with reverse thrust without the tail lowering and hitting the ground?

EDIT-obviously I ment how fast can an aircraft go backwards whilst on the ground. Thought I would correct myself before I got some smart ass comments  Silly

[Edited 2006-07-11 20:35:01]
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777DadandJr
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 3:47 am

Quoting Legoguy (Thread starter):
how fast can an aircraft go in reverse thrust?

I'm not sure of actually how fast it would go. Eventually, once you got rolling, the mass of the aircraft would have a lot to do with how much speed you could gain.

Quoting Legoguy (Thread starter):
Infact, can an aircraft with engines on the wings even go backwards with reverse thrust without the tail lowering and hitting the ground?

I believe any plane with reverse thrust can go backward. I don't believe that the tail would fall to the ground. The biggest threat would be sucking debris into the wing mounted engines. That would sure stop the plane in a hurry.

Russ
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swissy
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:21 am

Quoting 777DadandJr (Reply 1):
I believe any plane with reverse thrust can go backward. I don't believe that the tail would fall to the ground. The biggest threat would be sucking debris into the wing mounted engines. That would sure stop the plane in a hurry.

Coming to a stop would be a risky task......

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Birdwatching
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:29 am

Quoting 777DadandJr (Reply 1):
The biggest threat would be sucking debris into the wing mounted engines

Why would that be any more probable than normal?

Quoting 777DadandJr (Reply 1):
the mass of the aircraft would have a lot to do with how much speed you could gain

Why?

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micstatic
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:33 am

I've always heard 60knots is the target for coming off the reversers. I know they instruct pilots to keep their feet on the floot and off the brakes so they don't inadvertantly step on the brakes after powerback and send the tail to the ground.
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jetflyer
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:36 am

Quoting Birdwatching (Reply 3):
Quoting 777DadandJr (Reply 1):
The biggest threat would be sucking debris into the wing mounted engines



Quoting Birdwatching (Reply 3):
Why would that be any more probable than normal?

The engines would need to use high power setting to move the aircraft in reverse thrust mode, more than to taxi normally I should think, since only the air from the bypass duct is reversed. This increases the chance of damaging airport objects. Also, the air from through the reverse thrust doors is pushed forwards and outwards and would send any small objects from around the plane flying into the air which could be sucked back through the front of the engine.

Quoting Birdwatching (Reply 3):
Quoting 777DadandJr (Reply 1):
the mass of the aircraft would have a lot to do with how much speed you could gain

Why?

Well duh.... The aircraft needs a certain amount of thrust/weight ratio to even move, and the heavier the aircraft is, the more thrust it needs to move. The same reasons that mass affects takeoff acceleration, climb performance, etc..You need more of your thrust to move the aircraft when it is heavier than when it is very light.
 
777DadandJr
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:40 am

Quoting Swissy (Reply 2):
Coming to a stop would be a risky task......

That's true. Stopping abruptly while reversing would change the center of gravity and could certainly send the nose skyward.

Quoting Birdwatching (Reply 3):
Why would that be any more probable than normal?

Because it would be blowing even more debris forward. That is the reason why for the most part, only planes with tail mounted engines will do a powerback.

Quoting Birdwatching (Reply 3):
Why?

Because the more mass the object has, the te more momentum it will gain.

Russ
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legoguy
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:40 am

Quoting Birdwatching (Reply 3):
Quoting 777DadandJr (Reply 1):
the mass of the aircraft would have a lot to do with how much speed you could gain

Why?



Quoting Jetflyer (Reply 5):
Well duh.... The aircraft needs a certain amount of thrust/weight ratio to even move, and the heavier the aircraft is, the more thrust it needs to move. The same reasons that mass affects takeoff acceleration, climb performance, etc..You need more of your thrust to move the aircraft when it is heavier than when it is very light.

Im sure he ment momentum wise. Once the aircraft gets up to speed, it carries more momentum.

Quoting Micstatic (Reply 4):
I've always heard 60knots is the target for coming off the reversers. I know they instruct pilots to keep their feet on the floot and off the brakes so they don't inadvertantly step on the brakes after powerback and send the tail to the ground.

So switching from between reverse and normal thrust is where the danger of hitting the tail off the ground lies?
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Matt72033
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:42 am

Quoting Birdwatching (Reply 3):
Why would that be any more probable than normal?

because normally you have forward speed!

Quoting Birdwatching (Reply 3):
Why?

because for a constant power setting, if your heavier, you wont be able to go as fast as a light aircraft!
 
Matt72033
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:46 am

Quoting Micstatic (Reply 4):
I've always heard 60knots is the target for coming off the reversers. I know they instruct pilots to keep their feet on the floot and off the brakes so they don't inadvertantly step on the brakes after powerback and send the tail to the ground.

i hope your not implying they get up to 60 knots on powerback??  Wink
 
legoguy
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:48 am

So no one actually knows what the maximum backwards speed of an aircraft is  Silly

Another question would be, do either Boeing or Airbus ever test backwards speed with reverse thrusts when testing aircraft?
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micstatic
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:49 am

Quoting Matt72033 (Reply 9):
hope your not implying they get up to 60 knots on powerback?? Wink

I'm not, but that would be thrilling!
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Matt72033
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:57 am

Quoting Legoguy (Reply 10):
Another question would be, do either Boeing or Airbus ever test backwards speed with reverse thrusts when testing aircraft?

what would be the point?
 
2H4
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:58 am




Quoting Legoguy (Reply 10):
Another question would be, do either Boeing or Airbus ever test backwards speed with reverse thrusts when testing aircraft?

If so, I need to know where to buy tickets.




2H4


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Tod
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 5:50 am

Quoting Legoguy (Reply 10):
ever test backwards speed

That would put one's tiller control skills to the test too.

Tod
 
TheSorcerer
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 6:22 am

Quoting 777DadandJr (Reply 6):
Because it would be blowing even more debris forward. That is the reason why for the most part, only planes with tail mounted engines will do a powerback.

Just to add to that, IIRC a lot of turboprops can do powerbacks, the Jetstream 31 for example, one A/C i know that powerbacks would pose problems for is the Dash 8, it's got something to do with the nose gear, sorry i can't really elaborate.

Dominic
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flymatt2bermud
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 6:28 am

Take it from me guys, tricycles were not intended to go backwards. At least not fast and I ended up getting some serious sidewalk rash about 45 years ago when I learned my lesson the hard way....I was 4.  Angry
All aircraft have the potential to move backwards if they have reverse thrust however many airframe and engine manufacturer's have limitations prohibiting such. There is great potential for problems, as mentioned above I think FOD and a potentially damaging and aircraft and engines top the list. Any reversing should be planned.
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legoguy
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:08 am

I was wondering where my thread had disappeared to!!! I thought it was deleted for a minute!  Sad

Quoting Matt72033 (Reply 12):
what would be the point?

I thought it might be a random useful thing to know  Smile I thought they tested every possible manourve possible with the aircrafts limits in testing...and reversing backwards is within the limits.

And this reminds me of another question!!! Im not sure If I should ask it in a new thread or ask it here.

Is there any other airliner that has flown upside down in a test flight? I saw a documentary and it showed a 707 flying upside down. Anyother airliner flown upside down before?
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Filton
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:14 am

Quoting Legoguy (Thread starter):
how fast can an aircraft go in reverse thrust?

Well, lets make some assumptions...

- You are in Concorde (nice bucket thrust reversers)
- You can apply 'reverse' thrust up to 10,000 lbs (44.5KN) before the buckets fall off
- The buckets send the thrust forward at and angle of 80deg to the horizontal
- Your Concorde is fully loaded and weighs 185,000 kgs.

Trig:
Total reverse thrust = 2 x 44500 x cos(80) = 15500 N

x 4 engines = 61800 N of reverse thrust.

In theory, you can go as fast as the amount of drag this much thrust will overcome, which is quite rapid.

However, lets assume the limiting factor will be runway length. Lets say you have 1 mile (1600m) to hand before you need to get on the brakes.

A 185,000 kg Concorde will accelerate backwards from rest at 0.334m/s (or 0-60mph in 1.4 minutes).

So, in one mile, you will be going SQRT(2 x 0.334 x 1600) = 33 m/s or 74 mph.

Sounds reasonable. Good luck steering though.
 
legoguy
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:22 am

Quoting Filton (Reply 18):

Well Mr Filton. I have to hand it to you. Nice calculations there! We all know however that not everything works out as good as it does on paper  Silly

Thanks for the reply!! It would be interesting to know if any aircraft have actually tried it.
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MarkC
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:29 am

I have seen test results of a JT9D-7R4 that made 35,000 lb of thrust in reverse. The same engine did 47,000 forward. Goes to show you how large the contribution of the fan is to total thrust (JT9's do not have core reversers).

So, if you don't care about the FOD (min recomended reverser speed is 80 kts), and the landing gear can take it, you have quite a bit of power available.
 
n8076u
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:32 am

Reverse thrust can be used to accomplish push back on certain aircraft, at certain places by certain airlines. One of note is the DC-9/MD-xx. However, the fact that the engines are up high on those particular aircraft makes the possibility of FOD ingestion far less of a problem.

Also, an aircraft like the DC-9 with aft-mounted clamshell type reverser deflectors, which deflect all of the engine's exhaust, will be more successful in being able to move backwards with reverse thrust than say a 747-400 with fan-only cascade reversers. Also, having a fan-only reverser allows the core of the engine to negate some of the reverse thrust from the fan because the core always provides forward thrust.

On some aircraft with under-wing mounted engines that have cascade-type fan reversers, the reverse thrust may disrupt the engine's incoming air at higher power settings if the aircraft is not moving or moving too slowly. This may result in surging. There is also a high possibility of FOD ingestion during these same conditions.

Chris
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legoguy
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:39 am

There is a lot of mention of FOD (foreign object debris). Is the risk of FOD higher just because the aircraft is moving slower than it would at take off speeds?
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n8076u
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 7:52 am

Quoting Legoguy (Reply 22):
There is a lot of mention of FOD (foreign object debris). Is the risk of FOD higher just because the aircraft is moving slower than it would at take off speeds?

Yes, at slow speeds, the reverser's exhaust can blow something that is on the ground up into the air where it will be sucked in by the incoming air. At higher speeds, the airflow from the forward movement of the aircraft cancels out some of the forward-blown reverser exhaust, so it won't affect objects as far out in front. It also means that if something does happen to be blown up at higher speeds, the aircraft will pass by it before it can even get close to the inlet.

Chris
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MesaMXORD
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 9:42 am

All I can say is clam shell reversers work alot better than cascades for backing up. You can get them going fast but as mentioned there are issues with ass ending it when you go to stop
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vikkyvik
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 11:15 am

Quoting MesaMXORD (Reply 24):
All I can say is clam shell reversers work alot better than cascades for backing up. You can get them going fast but as mentioned there are issues with ass ending it when you go to stop

I believe that's because clamshell reversers reverse the bypass AND core flow, correct (like the 732)? Cascades only reverse the bypass flow. (I might have my reversers mixed up)

With wing-mounted engines, reverse thrust should actually produce a nose-down moment. Braking while going backwards, however, will produce a nose-up moment.

If you gained some speed going backwards in a tricycle gear aircraft, you introduce the possibility of a ground loop. From what I remember, a ground loop occurs in non-tricycle gear aircraft (can't remember what non-tricycle gear aircraft are called), during forward motion, when the rear end will swing around to the front. Similar to how, in reverse, your car wants to turn sharper and sharper, whereas in a forward gear, your car wants to return to going straight.

In addition to FOD, you typically don't want a jet engine to reingest its own exhaust (which is one reason that reverse is only used down to a certain speed).

Just to add a bit more: once you get an airplane moving, it shouldn't take as much thrust to keep it moving (the static friction in the wheel bearings is higher than the kinetic friction). You'll also have to get to a reasonably fast speed before your air drag really becomes a major part of your overall drag (I don't know actual speeds, but I'd guess maybe 30-40 kts).

Feel free to correct any of the above.

~Vik
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2H4
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 11:31 am




Quoting Vikkyvik (Reply 25):
can't remember what non-tricycle gear aircraft are called

Tailwheel or conventional-gear aircraft....though, these days, they're anything but conventional.  Wink




2H4


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ilikeyyc
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 11:38 am

Quoting Vikkyvik (Reply 25):
I believe that's because clamshell reversers reverse the bypass AND core flow, correct (like the 732)? Cascades only reverse the bypass flow. (I might have my reversers mixed up)

This is correct. examples: the JT8 on the 732 are clamshell and reverse everything. the CF34 has cascades and only bypasses fan flow. But now that I think about it, I remember being told that the RB211 used on the L1011 used cascades and also reversed core airflow. That statement could be wrong, though.
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vikkyvik
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 11:44 am

Quoting 2H4 (Reply 26):
Tailwheel or conventional-gear aircraft....though, these days, they're anything but conventional.

Thank you! Can't believe I couldn't remember that. Then again, the humidity is like 90% and I just played an hour and a half of tennis....yeah....that's my excuse.  Wink

~Vik
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n8076u
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 12:09 pm

Quoting Vikkyvik (Reply 25):
believe that's because clamshell reversers reverse the bypass AND core flow, correct (like the 732)? Cascades only reverse the bypass flow.

Some aircraft with clamshell reversers (731, 732, 727, DC-9, MD-XX) do reverse both the fan and core exhaust. But there are exceptions. The DC-8 50 and 61 series with JT3D engines did have clamshell reversers, but they only deflected the core flow, as they had a seperate setup to reverse the bypass air.

Yes, cascade reversers like those found on modern high-bypass engines only reverse the fan bypass air. Normally there is no core reverser, but if there is, it is a completely seperate mechanism.

Quoting Ilikeyyc (Reply 27):
I remember being told that the RB211 used on the L1011 used cascades and also reversed core airflow.

I have a book on the L1011, and in it is a photo of the #2 engine with a deflector for the core exhaust. But this was a seperate mechanism from the cascade reverser for the bypass air. From what I remember reading, it was found to be more trouble than it was worth, and was deleted at some point.

The 747 originally had a core reverser setup on the JT9Ds, but it was later deactivated, as the amount of maintenance the system needed far outweighed any benefits it provided.

Quoting Vikkyvik (Reply 25):
Just to add a bit more: once you get an airplane moving, it shouldn't take as much thrust to keep it moving

This is very true. It takes very little to keep the aircraft moving once it gains some momentum, but initially "ungluing" it from its static position takes quite a bit. I have been told that when using reversers to push back in the DC-9, it is sometimes helpful to taxi forward a tiny bit and then throw it in reverse, as it is easier to get it moving back that way, rather than from a standing start.

Chris
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bohica
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 1:12 pm

At an airshow at ADW there was a demonstration of the C-17. After it landed it deployed the thrust reversers and left them deployed. The plane stopped and then backed up approximately 2000 feet down the runway. I'm guessing it got up to 15-20 mph. At the end of the reverse procedure it stowed the reversers throttled up and took off. BTW the C-17 has cascade reversers for the fan and clamshell reversers for the core. The demonstration showed that it is possible to reverse a plane under the right conditions.
 
n8076u
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 1:50 pm

Quoting Bohica (Reply 30):
After it landed it deployed the thrust reversers and left them deployed. The plane stopped and then backed up approximately 2000 feet down the runway. I'm guessing it got up to 15-20 mph.

That must have been quite a sight to see!

I would guess with a military cargo aircraft, there's the possibility of landing in all sorts of places other than large, well-equipped airfields. The ability to be able to back up without any ground support would be a huge advantage, not to mention the added stopping power the full reversers would provide on shorter fields.

Chris
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FredT
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:07 pm

Quoting 777DadandJr (Reply 6):
That's true. Stopping abruptly while reversing would change the center of gravity and could certainly send the nose skyward.

The center of gravity remains where it is at. However, the forward acceleration tilts the total force acting on the aircraft so that the resultant falls behind the mains.

Quoting Birdwatching (Reply 3):
Why?

A good question, which didn't get many good answers. The mass itself does not affect top speed. It only affects the acceleration while getting there.

a = F/m

The one thing which is changed by the aircraft being heavier is the rolling resistance. The tires will be compressed more, meaning more friction in the ground/aircraft interface.

I'd say that weight isn't such a huge factor as people seem to think here. The distance and time required to achieve a given speed are very dependant on the weight though.

Cheers,
Fred
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DH106
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 4:24 pm

Quoting FredT (Reply 32):
The one thing which is changed by the aircraft being heavier is the rolling resistance. The tires will be compressed more, meaning more friction in the ground/aircraft interface.

I'd say that weight isn't such a huge factor as people seem to think here. The distance and time required to achieve a given speed are very dependant on the weight though.

Good answer FredT.
I think in reality there would be a 'break out' force required to start the a/c moving that would be greater with heavier weights.
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n8076u
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 5:56 pm

Quoting DH106 (Reply 33):
I think in reality there would be a 'break out' force required to start the a/c moving that would be greater with heavier weights.

When pushing back an empty 747, it was very easy, even with a narrowbody tug. With a fully loaded 875,000lbs 747, it was sometimes almost impossible to get it to start rolling, especially should you happen to be using one of the older widebody tugs. Once moving, it wasn't so bad, but then stopping it was another story.

An otherwise empty 747 with maybe 40,000lbs fuel, during taxi, will roll on its own, with no throttle beyond idle needed. Brakes off, there we go. In fact you need to watch how much you use the brakes to keep from overheating them, as the thing gets away from you pretty quickly. Same aircraft with 300,000+lbs fuel but otherwise empty won't even move without giving it some throttle, lots of throttle. But once it is moving and you back the throttles to idle, it keeps itself going pretty well once it is rolling fast enough. But brake application isn't as frequent, as it just doesn't accelerate very much when it is this heavy.

Just thought I'd throw out some real world examples to go along with the theories.  Wink

Chris
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bri2k1
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Jul 12, 2006 10:09 pm

IIRC, those air carriers who perform powerbacks instruct crews to place both feet firmly on the floor and stop the reversing only with forward thrust. That mitigates the ass-sitting tendency of, say, a fully loaded MD-80. Or DC-9.
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MarkC
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Thu Jul 13, 2006 8:10 am

Quoting Bohica (Reply 30):
BTW the C-17 has cascade reversers for the fan and clamshell reversers for the core.

The F117 has cascade reversers for the fan, and the same type of blocker door / cascade design for the core contained in the nozzle. Not a clamshell design. The nozzle assembly bolts to a stock PW2000 TEC. Quite a bit different than the stock honeycomb shell.

The C17 is a high wing plane, and FOD under reverse is not as much of a concern. Actually, its almost non-existant.
 
MesaMXORD
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Fri Jul 14, 2006 1:26 am

All the engines I play with are rear engine high mount so you just have to watch your readings so the engine doesn't start ingesting hot air. RR AE3007A Is the only engine with clam shell reversers I have dealt with it just seems to me that cascades dont deflect the reverse thrust forward as much as the clam shells CRJ compared to ERJ
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b52murph
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Fri Jul 14, 2006 1:52 am

Quoting Bohica (Reply 30):
At an airshow at ADW there was a demonstration of the C-17. After it landed it deployed the thrust reversers and left them deployed. The plane stopped and then backed up approximately 2000 feet down the runway. I'm guessing it got up to 15-20 mph. At the end of the reverse procedure it stowed the reversers throttled up and took off. BTW the C-17 has cascade reversers for the fan and clamshell reversers for the core. The demonstration showed that it is possible to reverse a plane under the right conditions.

For security reasons, I won't divulge a location...but...this is standard procedure for getting out of the C-17 parking spots at a certain location. I have also seen them do it here--and believe me, it makes a racket!

IIRC, the C-130 can also back up with reverse thrust/props, but to stop the reverse, the pilot must put the engines back into forward thrust. Standing on the brakes will stand the plane on it's butt  Wow!

b52murph
 
vikkyvik
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Fri Jul 14, 2006 2:16 am

Quoting B52murph (Reply 38):
IIRC, the C-130 can also back up with reverse thrust/props

You do indeed remember correctly. They demonstrated this at an airshow at BED that I went to years ago.

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
sanjet
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Fri Jul 14, 2006 12:15 pm

Aircraft like DC-9's can do power backs and back up pretty quickly and is certified to do so. Most aircraft are not as the problem is that the engines can overheat pretty quickly if you use RT under 60kts due to lack of RAM air which is one of the biggest problems.
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Sat Jul 15, 2006 1:58 am

Quoting Sanjet (Reply 40):
engines can overheat pretty quickly if you use RT under 60kts due to lack of RAM air which is one of the biggest problems.

That might be true on an air-cooled piston engine, but a high-bypass turbofan is going to suck in more air at idle than will ever be "RAM"med in at any airspeed. The rotating fan blades would impede any such ram air, at any rate.

If you had said the engines will suck in hot exhaust gasses, you would have been partially right, as they could contribute to heating, but this is not the limiting factor.

If you were talking about Ramjet engines, well, it's pretty obvious they don't work in reverse.

The reason wing-mounted turbofan-equipped aircraft don't power back is the risk of FOD ingestion is too high.

Quoting 777DadandJr (Reply 1):
The biggest threat would be sucking debris into the wing mounted engines.



Quoting Jetflyer (Reply 5):
the air from through the reverse thrust doors is pushed forwards and outwards and would send any small objects from around the plane flying into the air which could be sucked back through the front of the engine.



Quoting 777DadandJr (Reply 6):
Because it would be blowing even more debris forward. That is the reason why for the most part, only planes with tail mounted engines will do a powerback.



Quoting N8076U (Reply 23):
at slow speeds, the reverser's exhaust can blow something that is on the ground up into the air where it will be sucked in by the incoming air



Quoting N8076U (Reply 23):
at slow speeds, the reverser's exhaust can blow something that is on the ground up into the air where it will be sucked in by the incoming air



Quoting Vikkyvik (Reply 25):

In addition to FOD, you typically don't want a jet engine to reingest its own exhaust (which is one reason that reverse is only used down to a certain speed).
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Sat Jul 15, 2006 3:42 am

Quoting Bri2k1 (Reply 41):
That might be true on an air-cooled piston engine, but a high-bypass turbofan is going to suck in more air at idle than will ever be "RAM"med in at any airspeed. The rotating fan blades would impede any such ram air, at any rate.

I agree, an engine ingesting its own exhaust gas is not good, but with a typical high bypass engine, it will only ingest the fan air, which isn't hot anyways. There is, however, a problem with the reverse airflow disrupting the incoming air at high power settings.

If overheating because of a lack of ram air was a problem, you would never see any high-power run ups being done with the aircraft parked in one spot.

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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Sat Jul 15, 2006 5:09 am

Quoting Legoguy (Reply 22):
There is a lot of mention of FOD (foreign object debris).

FOD stands for Foreign Object Damage, i.e. damage done by foreign objects. Foreign Object Debris (debris from foreign objects?) makes no sense.

Before breaking the land speed record in reverse, I think some good rear vision equipment might be necessary. Once you get above walking pace, the powerback marshaller is going to have a hard time keeping up!  gasp 
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Sat Jul 15, 2006 7:17 am

Quoting N8076U (Reply 42):
There is, however, a problem with the reverse airflow disrupting the incoming air at high power settings.

For an engine that is more surge prone than others (not mentioning any names), this is more of a concern than FOD. Airflow disruption to the engine can cause a surge at low ground speeds. Depending on the severity, it could lead to an expensive overhaul.
 
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Sat Jul 15, 2006 7:52 am

Quoting MarkC (Reply 44):

For an engine that is more surge prone than others (not mentioning any names)

Well, we know it's not the JT8D, given the (sadly, historcal) practice of powering back "Super" 80's and DC-9s... Smile
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Sat Jul 15, 2006 10:58 pm

Quoting Bri2k1 (Reply 45):
Well, we know it's not the JT8D, given the (sadly, historcal) practice of powering back "Super" 80's and DC-9s...

Even with a JT8D, if you apply full reverse with the aircraft stopped or moving slowly you run the risk of compressor stall due to exhaust gas re-ingestion. Powerback involves using partial reverse thrust, only just enough to get the aircraft moving.
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Mon Feb 08, 2010 9:43 pm

Quoting Legoguy (Reply 10):
Another question would be, do either Boeing or Airbus ever test backwards speed with reverse thrusts when testing aircraft?

I believe they do.

The 717. To about 4 knots. For AirTran.
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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Feb 10, 2010 11:37 am

In regard to the C-17, at the Nellis Air Show I too saw the C-17 demonstration. The announcers of the team stated that the reverse speed of the C-17 appeared slow however that was due to the size of the aircraft. The actual reverse speed was approximately 45 MPH.

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RE: How Fast Can An Aircraft Go With Reverse Thrust?

Wed Feb 10, 2010 11:57 am

Also the Saab Viggen was famous for backwards taxiing with reverse thrust :

http://www.youtube.com/watch?v=zpxz_-ELi6U
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