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Topic: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 20:30:34 and read 14969 times.

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

[Edited 2006-07-11 20:35:01]

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 20:47:50 and read 14915 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:21:30 and read 14847 times.

 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......

Cheers,

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:29:03 and read 14827 times.

 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?

Soren

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:33:40 and read 14807 times.

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:36:30 and read 14786 times.

 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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:40:44 and read 14767 times.

 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.

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

Russ

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:40:46 and read 14765 times.

 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?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:42:31 and read 14758 times.

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

because normally you have forward speed!

because for a constant power setting, if your heavier, you wont be able to go as fast as a light aircraft!

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:46:40 and read 14742 times.

 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??

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:48:26 and read 14726 times.

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

Another question would be, do either Boeing or Airbus ever test backwards speed with reverse thrusts when testing aircraft?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:49:00 and read 14725 times.

 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!

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:57:02 and read 14689 times.

 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?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 21:58:13 and read 14682 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 22:50:18 and read 14605 times.

 Quoting Legoguy (Reply 10):ever test backwards speed

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

Tod

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 23:22:55 and read 14575 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-11 23:28:39 and read 14575 times.

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.
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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:08:19 and read 14547 times.

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

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

I thought it might be a random useful thing to know   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?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:14:39 and read 14546 times.

 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

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:22:27 and read 14531 times.

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

Thanks for the reply!! It would be interesting to know if any aircraft have actually tried it.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:29:23 and read 14526 times.

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:32:30 and read 14526 times.

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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:39:35 and read 14517 times.

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?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 00:52:19 and read 14508 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 02:42:17 and read 14462 times.

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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 04:15:20 and read 14540 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 04:31:05 and read 14563 times.

 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.

2H4

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 04:38:35 and read 14665 times.

 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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 04:44:53 and read 14677 times.

 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.

~Vik

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 05:09:20 and read 14651 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 06:12:33 and read 14628 times.

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 06:50:13 and read 14603 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 09:07:58 and read 14556 times.

 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.

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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 09:24:11 and read 14546 times.

 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.

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 10:56:47 and read 14564 times.

 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.

Chris

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-12 15:09:05 and read 14472 times.

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-13 01:10:26 and read 14321 times.

 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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-13 18:26:34 and read 14224 times.

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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-13 18:52:32 and read 14259 times.

 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

b52murph

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-13 19:16:48 and read 14214 times.

 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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-14 05:15:19 and read 14138 times.

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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-14 18:58:29 and read 14073 times.

 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).

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-14 20:42:27 and read 14039 times.

 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.

Chris

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-14 22:09:37 and read 14054 times.

 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!

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-15 00:17:52 and read 14012 times.

 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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-15 00:52:40 and read 14041 times.

 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...

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2006-07-15 15:58:09 and read 13952 times.

 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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-08 13:43:30 and read 9531 times.

 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.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 03:37:53 and read 9109 times.

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.

PMK

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 03:57:58 and read 9092 times.

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

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 05:07:16 and read 9047 times.

 Quoting Legoguy (Reply 7):So switching from between reverse and normal thrust is where the danger of hitting the tail off the ground lies?

Not quite. Using the wheel brakes while going backward is the problem. Even above idle, I can't imagine abruptly going back to forward thrust would tip it. During that transition, the reverse thrust would have to go back to idle anyway due to the motion/function of the reverse levers, so the thrust change from reverse to forward would be gradual. Even if, for the sake of argument, you had attained a rediculous speed in reverse and went through idle immediately to take-off thrust, the acceleration imposed would be little different from the beginning of a normal take-off roll.

 Quoting Filton (Reply 18):The buckets send the thrust forward at and angle of 80deg to the horizontal

I can't quote any figs., but I doubt any thrust reversers of any kind get anywhere near diverting the thrust to 80 degrees to the horizontal.

 Quoting MarkC (Reply 20):I have seen test results of a JT9D-7R4 that made 35,000 lb of thrust in reverse.

Maybe, but if this thrust is exiting the cascades at, say, 45 degrees (for the sake of argument), it doesn't equate to 35,000 lb of reverse thrust.

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

Not only high maintenance, but the weight penalty wasn't viable bearing in mind how little difference they made to stopping ability. THBDF* (I'm trying to popularise a new abbreviation there!) but even a high level of reverse is a minor factor at low speeds compared to the contribution of the brakes. Years ago I tried selecting max reverse (737 CFM56) and no brakes after landing. The deceleration was pathetic! Having said that though, at about 60 knots when the reversers go back to stowed, it feels like a sudden gentle acceleration, but is of course a reduction in deceleration. (For the purists - yes, accel. and decel. are +ve and -ve values of the same thing).

 Quoting Bri2k1 (Reply 41):but a high-bypass turbofan is going to suck in more air at idle than will ever be "RAM"med in at any airspeed.

Really?

 Quoting N8076U (Reply 42):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.

Good point!

Regards - musang

* This Has Been Discussed Before!

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 05:52:59 and read 9027 times.

 Quoting MarkC (Reply 20): 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).

This statement cannot be true.

JT9D-7R engine full fwd trust 48.000 lbs (lowest certified value)
Bypass ratio = 5 ,so 5/6 of the thrust is fan thrust and 1/6 is core engine thrust.

Total Reverse at full T/O thrust (not certified) would be theoretically :

48.000 X 5/6 = 40.000 lbs fan thrust, but minus the FWD core thrust = 8000 lbs (no core reverser installed)

So max reverse thrust possible = 32.000 lbs. ( but in real live 100% fan reverse is impossible.)

Max. fan reverse with 45 degrees cascades.= 50%

In real life we have 20.000 - 8000 = 12.000 lbs reverse thrust with engine thrust set at T/O power.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 06:20:40 and read 8953 times.

Nitpicking, but wouldn't that be 1 / (SquareRoot(2)) = 0.707 (approx 70%) for 45deg, not 50% ?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 06:48:41 and read 8937 times.

 Quoting Jetlagged (Reply 43):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!

Jajaja, classic stuff!

He would have to be in a car, with the "Follow Me" sign posted in reverse!
ÃÂÃÂM WOÃÂ¢ÃÂÃÂÃÂ¢ÃÂÃÂOÃÂªÃÂÃÂ», I wrote it in reverse, but A.net seems to have a few 2010 decennium bugs.

Problems, problems problems.
I have a suggestion: why not push planes back?
Small (low design) powerful trucks that go under a plane and connect to the nose gear for push back or even for towing!
Just an idea.

### "I am always on the Run"###

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 06:49:06 and read 8935 times.

 Quoting DH106 (Reply 52):Nitpicking, but wouldn't that be 1 / (SquareRoot(2)) = 0.707 (approx 70%) for 45deg, not 50% ?

True, but 50% is probably nearer the truth as there are losses in the cascades to consider. Also in reverse the engine is limited to a lower N1 and thus a lower maximum thrust than is available at takeoff power. Boeing simulator data I've seen shows that at 90% N1 net reverse thrust of a JT9D-7R4G2 is 4295 lb at zero speed and around 11000 lb at 140 kts. Clearly the sim data might be inaccurate at low speed as reverser is not meant to be used below 60 knots. The data also shows how reverse thrust of a big fan engine is much more effective at touchdown speeds, reducing rapidly as the aircraft decelerates.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 07:42:52 and read 8911 times.

I have no idea how this happened, but that reply wasn't posted by me! My username is all lower-case; is it possible that they are permitting users with the same characters, just different case? I certainly hope not...

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 08:04:07 and read 8905 times.

 Quoting DH106 (Reply 52):Quoting 747classic (Reply 51): Max. fan reverse with 45 degrees cascades.= 50% Nitpicking, but wouldn't that be 1 / (SquareRoot(2)) = 0.707 (approx 70%) for 45deg, not 50% ?

You are totally right, I was still dreaming.

In this example max. possible (not certified) reverse thrust would be approx. 28280 - 8000 = 20280 lbs.
However the actual max reverse thrust is limited by 90% N1 and far lower, even at high speeds, as replied by Jetlagged.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 09:19:19 and read 8865 times.

 Quoting jetlagged (Reply 54): at 90% N1 net reverse thrust of a JT9D-7R4G2 is 4295 lb at zero speed and around 11000 lb at 140 kts.

Excellent! Good to have some hard numbers to support our opinions!

Regards - musang

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 09:36:18 and read 8880 times.

That reply was made 3 years 7 months ago. Are you sure you haven't just forgotten about it? You may have logged on using capital letters when you made that post... all those years ago.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 13:45:10 and read 8766 times.

 Quoting David L (Reply 58):Are you sure you haven't just forgotten about it?

If I'd forgotten about it, how could I be sure? Just kidding. Yes, it does sound just smart-assed enough to have been written by my past self.

But to the point: Yes, I am sure that ram air cooling is not an important consideration in turbofan engine operation. The very first stage of the engine, before the fan blade, is a diffuser. The goal of this is to lower the entropy of the incoming air as much as possible so the engine will have maximum efficiency. The air cooling the jet core is propelled by the fan blades; ramming it into the engine for cooling is not a consideration.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 21:11:16 and read 8647 times.

 Quoting N8076U (Reply 42):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.

Temperature rise across the fan can be upwards of 100 degF...it's quite easy to surge an engine if it eats its own fan air.

 Quoting 747classic (Reply 51):Bypass ratio = 5 ,so 5/6 of the thrust is fan thrust and 1/6 is core engine thrust.

This is only true if the core and fan exhaust have the same exit velocity, which isn't generally true. As a back-of-the-envelope shortcut it's not bad, but the thrust split is not the same as the mass flow split.

 Quoting bri2k1 (Reply 59):The very first stage of the engine, before the fan blade, is a diffuser. The goal of this is to lower the entropy of the incoming air as much as possible so the engine will have maximum efficiency.

At best, the diffuser maintains constant entropy (a theoretical ideal)...there is no component within the engine that decreases entropy of the incoming air.

 Quoting bri2k1 (Reply 59):Yes, I am sure that ram air cooling is not an important consideration in turbofan engine operation.

Agreed.

Tom.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-10 21:32:02 and read 8634 times.

Is the conveyor belt also going backwards ... and at the same speed?

Jimbo

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-11 08:03:22 and read 8531 times.

 Quoting tdscanuck (Reply 60):At best, the diffuser maintains constant entropy

Right, as usual. I suppose I should have phrased it "minimize entropy generation."

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-11 20:17:46 and read 8422 times.

 Quoting Legoguy (Reply 10):So no one actually knows what the maximum backwards speed of an aircraft is

All we need now is a myth to go with it - and we can submit it to Mythbusters and watch them back a 747 around the desert at high speed by remote control.

I'd fly in to see that..

So - what is the myth?

Did you hear about the 747 pilots that left the thrust reversers deployed and ended up getting a speeding ticket on Pena Blvd?

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-14 06:06:35 and read 8178 times.

 Quoting rcair1 (Reply 63):I'd fly in to see that..

Me too………
You deal with the plane, while I deal with Carry!

 Quoting tdscanuck (Reply 60): Temperature rise across the fan can be upwards of 100 degF...it's quite easy to surge an engine if it eats its own fan air.

Tom, how much water can a modern engine deal with?
I believe they do test this, correct?
If I were to spray a few cubic meters of water at a running engine, what is the worst that could happen?

### "I am always on the Run"###

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-14 10:16:17 and read 8153 times.

 Quoting alwaysontherun (Reply 64):Tom, how much water can a modern engine deal with?

Lots. I'm not sure what the exact figure is, but it's equivalent to flying through a monsoon. It's a *lot* more than you'd think the engine can handle.

 Quoting alwaysontherun (Reply 64):I believe they do test this, correct?

 Quoting alwaysontherun (Reply 64):If I were to spray a few cubic meters of water at a running engine, what is the worst that could happen?

It would depend if it came as a nice spray, or a slug. Worst that can happen is you'd flame out the engine...on most modern engines, the engine would kick on the igniters (if the flight crew hadn't done it already) and restart when the water cleared.

The most famous case I know of on this is the TACA 110 flight: http://en.wikipedia.org/wiki/TACA_Flight_110

Obviously, mother nature can provide more water than the engine can handle if you really piss her off.

Tom.

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-02-16 06:09:45 and read 7966 times.

 Quoting tdscanuck (Reply 65):the TACA 110 flight:

Surely someone must have filmed the take-off? Love to see that.

here's the only photo I've ever seen of the aftermath.

http://www.flightglobal.com/pdfarchive/view/1988/1988%20-%201821.html

Regards - musang

Topic: RE: How Fast Can An Aircraft Go With Reverse Thrust?
Posted 2010-03-02 15:03:40 and read 7446 times.

Interesting!

 Quoting tdscanuck (Reply 65): It would depend if it came as a nice spray, or a slug. Worst that can happen is you'd flame out the engine...on most modern engines, the engine would kick on the igniters (if the flight crew hadn't done it already) and restart when the water cleared.

Okay, but in the TACA case you posted this obviously could not be done!
Well done by the crew, landing on grass!

 Quoting tdscanuck (Reply 65):Obviously, mother nature can provide more water than the engine can handle if you really piss her off.

Yep, Mother Nature is a powerful lady. She can make us feel very small sometimes.

 Quoting Musang (Reply 66):Surely someone must have filmed the take-off? Love to see that.

Yep, wouldn´t mind a bit off that!!
A 737 dodging the potholes, not something you see every day!!