Why are thrust reversers ineffective at low speed?

[Hudson]

I should know the answer - but I don't. Experimenting in the B737-300 simulator with demonstration two-engine aborts using solely maximum reverse thrust (and no other means of retardation), it was obvious that at high speed (135 knots V1 abort) the reverse thrust caused impressive decelleration until around 90 knots when the rate of decell. began to decay. Although the runway was 7500 ft long, we went off the end at 45 knots with full reverse still applied. In brief the decelleration in full reverse was practically non-existent from around 80 knots down.

This ties in with what is seen in the simulator with a high speed abort on one engine where by the time the remaining engine is wound up to full reverse, the RTO system has already brought the speed down to around 80 knots. It is easy to be fooled into thinking the reverse is effective simply because it makes lots of noise. Of course, on a wet runway any reverse becomes a plus because of less efficient wheel braking.

What is the reason for the relative lack of reverse thrust decelleration at the lower speed range?

[Checkboard]

Force (Thrust) = mass times acceleration.

As the mass is constant, if the acceleration is reducing, then the thrust from the thrust reversers must also be reducing at lower speed.

-> speculation : as the aircraft's airspeed reduces, the ram effect reduces, reducing thrust. Slowing below 80-odd knots, the thrust reverser plume actually runs forward of the intake, starving the engine of air, and significantly reducing the engine's thrust output. Acceleration (decceleration) reduces accordingly.

[411A]

Conversely, reverse with propeller-driven aircraft is very effective at slow speeds.
Have flown several heavy jet aircraft where reverse just made noise (lots), without much noticable effect at high speeds, older B707 straight pipe (non-fan) models come to mind.

[Bottle Fatigue]

Checkboard,

I'm not sure you've got that first bit right. If we assume that the engines are producing constant reverse thrust force throughout the manouvre ( yes I accept your second contention ) you have a constant negative acceleration.

So although the engines may not be producing as much total thrust at low speed, I would imagine the reversers would still be pretty effective. Some aircraft can reverse taxi using the reversers.

In fact, I recall being told that VC10 pilots used the reversers during taxi, and they had some failures due to the total reverse cycles thus being greater than the units were designed for. (I have no confirmation of this by the way, perhaps someone who knows can enlighten us)

[None]

"So although the engines may not be producing as much total thrust at low speed, I would imagine the reversers would still be pretty effective. Some aircraft can reverse taxi using the reversers."

A while back we were taxiing in on a slippery, snow-covered ramp in an MD-80 type. There were no taxi lines visible. We approached the gate at a 90 degree angle, turning when the ground marshallers signalled to do so. In the turn, we came upon an area where the nosewheel could not get any traction, and braking action was poor (deice fluid in that area). With ineffective nosewheel steering, we began to slide toward the wingtip of a parked jet. Both TRs were selected to IDLE reverse, then to slightly more than IDLE reverse, and the jet stopped on the spot. We shut down and let them tow us in the remainder of the distance. This might not have occurred if our turn had been more shallow, however, it pointed out a situation where TRs on some jets can be useful even at slow speeds.

[Notso Fantastic]

Can you always rely on the reverser effect being fully represented in the simulator? I have suspected for years that they don't deccelerate you at the proper rate as stopping in the sim always seems so difficult! I don't believe the thrust changes with speed below 100 knots appreciably. The gas path speeds are so high that the aircraft forward speed is pretty irrelevant.
I flew the VC10 for 6 years. We did not use reverse in the taxi. It's always been well known that it creates more wear than it is worth.

[Eckhard]

It is worth pointing out that:

a) Reverse thrust is limited to a percentage of maximum N1/EPR;

b) Most modern fan engines don't reverse the 'hot' air from the core, only the 'cold' air from the fan; and

c) The exhaust plume is angled forwards at about 45°, so that only about 70% of the thrust is effective in decelleration.

If we assume that the factor in a) is 80%, that in b) is 66% and that in c) is 70%, we can see that 'full reverse' for a 23.5K CFM engine is only about 8.7K or 37% of the total forward thrust available!

I realise that this does not explain the change in retardation noticed at 80kts, but it shows how relatively puny the reversers are. Still like to have them on a wet/icy runway though.

[Intruder]

OTOH, 37% of 62,000# x 4 on a 744 is a bit more than "puny"... How would you like 91,000# pushing YOU around? :ok:

[norodnik]

Somewhat relating to this thread.

Have been to DFW many times and it is common practise for the AA MD-80's to back out from the gate using reverse thurst. They all do it on a regular basis.

MD-80's have the bucket type reversers, so they may be more effective than the side door variant on the 737.

Also, given the wear on the engine, one would assume its cheaper for AA to do this than employ someone to push back in the more normal manner.

Pratt engines used to be more hard wearing (robust!) than RR or GE (but less responsive) so maybe the additional wear is not that bad ??

You should try the Concorde reversers, not only do they make an incredible noise, they seem hugely more effective than any other type.

[simfly]

This might be a way of understanding it, though 'm not qualified to do so!! To slow down, there has to be a drag force. If we double our speed, drag increases four times, thus the opposite will occur on decceleration. So should adding a drag force (reverse thrust) at a higher speed (ie 140ish kts on landing) not have more effect than it would at a slower 40kts, but still be effective enough to get the aircraft to roll backwards if not removed?

Adding airbrake at 400 kts would show significant decceleration, add airbrake at 10 kts, you wouldn't see much.

[Mad (Flt) Scientist]

One factor to bear in mind - and I don't know how it applies for the case of the original post - is that the reverse thrust schedule (or the N1/EPR schedule, it comes to the same thing) with airspeed may be playing a part.

Some engines allow the use of differing levels of max N1 in reverse mode according to airspeed, due to concerns about engine gas reingestion and/or aircraft direction control issues.

So it may be that the loss of effectiveness at lower speeds was caused by a FADEC scheduling you towards reverse idle as you slowed down.

[PAXboy]

Picking up on 'None' using extra TR when starting to slide onto the stand and the comments on VC-10 ...

In a similar thread last year, I recall a comment from a ground handler on a VC-10 in icing conditions many moons ago. It went along the lines of,
"The VC-10 started sliding towards the terminal, apparently with no grip! There was big noise from back end of VC-10 and VC-10 stopped moving in a second!!"

VC-10?
My first and true love. Aged nine on LHR~JNB, December 1965 and I wish I still had my Junior Jet Club Log Book :{

[BigJETS]

Like simfly mentioned: I would imagine that the drag on the airframe at high airspeed is also part of the equation. But certainly ram pressure creating more thrust should be the biggest factor.

[Hudson]

Thank you for all the replies. Also BIK_116.80 - many thanks for the erudite explanation. Years ago, there was a Boeing statement that it was OK to use reverse idle (B737-200) where tailwind taxying was involved. This was an invaluable technique where long taxying distances often caused increased wheel brake temperatures. With no brake temperature gauges it was impossible to accurately judge the amount of heat build up. If wear and tear figures were available for thrust reverser operation, we never saw them.

[sprucegoose]

From "handling The Big Jets" by D.P.Davies.

Reverse thrust is much more effective at high aircraft speeds than low speeds for two reasons. Firstly, the net amount of reverse thrust increases with speed because the acceleration imposed on the (constant) mass flow is greater. This is because the aircraft forward speed is additional when in reverse thrust as opposed to subtractional when in forward thrust. Secondly the power produced is higher at higher speeds because of the increased rate of doing work. In this context it means that the kinetic energy of the aeroplane is being destroyed at a higher rate at higher airspeeds.

And so on. I knew the book would come in handy for something.

I recall that reverse thrust on propellers is reduced at slow speeds simply because as the forward momentum of the aircraft decreases so does the angle of attack of the airflow over the prop and thus the amount of "lift" generated by the blade is reduced.

[Checkboard]

Bottle Fatigue, "If we assume that the engines are producing constant reverse thrust force throughout the manouvre ( yes I accept your second contention ) you have a constant negative acceleration."

The observation (also stated in most manuals) was that you don't have a contant negative acceleration! Hence, you can't assume a constant reverse thrust! I know that the reversers are still effective, and I have reversed an aircraft using the reversers myself - the point is that they are not as effective as at speed.

norodnik: Bucket reversers are slightly more effective than high bypass cold stream reversers, but that isn't why you don't see 737s using power-back. The higher wear mentioned earlier is due to dust and sand ingestion kicked up by the reverser flow, and tail mounted engines with their intakes over, or close to, the wings are more protected from this than wing mounted engines. Conversely tail mounted engines can have vibration troubles (i.e. cause cracking) with the empenage at high reverse.

simfly: There is a big difference between drag and reverse thrust. We are not considering the effect of drag devices here, such as speed brakes or spoilers. Reverse thrust isn't proportional to the square of the speed.

Mad (Flt) Scientist: Some engines limit the thrust output in reverse, but I don't know of any that schedule reverse thrust with speed.

BIK_116.80: Dynamic pressure formulae don't relate to thrust. Thrust, (or reverse thrust, for that matter) doesn't "push[es] against the force returned by ambient air ", in the same way that jet blast deflectors raised behind military aircraft on carriers don't increase take-off thrust, and a Saturn V rocket exhaust doesn't "push against the Earth" on take-off. All of these engines are reaction engines, and operate according to Newton's third Law of Motion - i.e. throw something out the back, and you will be thrown forward a bit.

sprucegoose, Gotta love D.P. Davies!

Firstly, the net amount of reverse thrust increases with speed because the acceleration imposed on the (constant) mass flow is greater. This is because the aircraft forward speed is additional when in reverse thrust as opposed to subtractional when in forward thrust.The Ram effect is considering increased thrust from increased mass flow through the engine, while he is talking about momentum change - which I think is a bit of a red herring, as he is ignoring the attendant momentum drag, but anyway...

Secondly the power produced is higher at higher speeds because of the increased rate of doing work. In this context it means that the kinetic energy of the aeroplane is being destroyed at a higher rate at higher airspeeds. While true, this statement is also a bit of a red herring, in that we aren't looking for efficiency or the rate of destroying kinetic energy, but decceleration.

Power = Work done, divided by time.
Work Done = Force (in this case reverse thrust) divided by distance.

If the reverse thrust were constant, then the amount of force produced would be contant for each metre that the aircraft travels down the runway during the landing roll.

The Work Done by the reversers would be constant over each of those metres, but at the start of the landing run (i.e. at high speed) you are covering more of those metres in each second - so the engines are providing more power then at the end of the landing run (i.e. at low speed). As the fuel is burnt according to time, the fuel used over each metre is less at the start of the landing roll, than at the end (which is one of the reasons why jet aircraft are more efficient at high speed.)

However (before you get all excited ;) ) Jet engines are (more or less) "constant force" engines, rather than "constant power" engines, and the acceleration is still related to the force, not the power. The rate of decceleration provided by the reversers (number of braking 'G') would still be constant throughout the landing roll, if the reverse thrust remains constant.

In short, if the statement "The reverse thrusters are more effective at high speed" is true,
and "more effective" means a higher rate of decceleration,
then the thrust provided must decrease with speed. :D

[Shore Guy]

Reverser design must be a player also…we are one of the few 757 operators that have both P&W and RR engines in our fleet. The Rolls reversers just make noise and provide little reverse thrust. The P&W reversers will lean you forward in the seat, even at low speeds (book says idle reverse below 80K).

Because they are not required for certification or any performance data, I’ve never seen a truly usable “reverse thrust” chart, indicating reverse thrust force. My guess is that the airframe/engine manufacturers have this data, and it would be interesting to see if anyone has access and could post some info.

[GlueBall]

With contaminated ramp surface and the tug's wheels spinning, it is always necessary to start at least two engines and apply at least idle reverse!

[Hudson]

If I recall correctly, the Lauda Air B767 accident over Thailand some years ago was caused by one thrust reverser deploying during the climb at around 28,000 ft and mach 0.76? - and at night.

I understand that the crew had less than 5 seconds in which to shut down the engine before they lost complete control. It was an almost impossible situation to be faced with. Presumably the reverse thrust plume that destroyed the lift over the affected wing was phenomenal at that speed. If the event had happened at the same IAS at a lower altitude, would the effect of the reverse thrust on the aircraft behaviour have been changed significantly?

[West Coast]

Increased chance of FODing a motor using TR's at low speeds.

[Mukka]

If I may make an analogy - what would you expect to have the biggest effect on speed - opening a car door at 5 mph or 70 mph?

No equations necessary!

[Checkboard]

Mukka - there is a difference between drag and thrust.

[Mukka]

Checkboard,
I think I've fallen into the trap of thinking that reverse thrust pushes against the surrounding airflow, the push being more effective at higher speed.

I thought the propulsive efficiency of the jet engine is greatest when there is a small difference in speed between the jet exhaust and the surrounding airflow, because the "push" is most effective then.
This doesn't make sense any more though!

Are you saying that thrust decreases with speed because the massflow through the engine decreases?

[Snakum]

A question about activating reverse thrust in large aircraft ...

It appears one would go to idle thrust at touchdown (mains), and following nose-wheel touchdown (I'm assuming a squat switch) the levers are pulled up and back to activate thrust reversers with the engines at idle.

Is this the correct GENERAL sequence?

Is it common to spool the engines back up while in reverse thrust (other than using RT for pushback) for short runway, for contaminated runway, or other possible over-run situations?

Just curious ...

Minh

[Checkboard]

It appears one would go to idle thrust at touchdown (mains),
Well, usually at some height above touchdown, like 30 feet or so...
and following nose-wheel touchdown (I'm assuming a squat switch)Actually it varies, on the 733 it is at the earlier of 10 feet on either radalt, or air/ground logic (i.e. squat switch) activation, and the thrust levers at idle (the reverse levers won't rise unless the thrust levers are at idle.
the levers are pulled up and back to activate thrust reversers with the engines at idle.Yep.Is it common to spool the engines back up while in reverse thrust This depends on company procedures. Using reverse thrust puts a little stress on various aeroplane bits, so some companies ask for idle reverse, unless the situation demands othersie (like a wet runway). An interlock will hold the piggyback levers at the first detent until the reversers are at least 60% deployed, after tat you can pull the levers back to increase reverse thrust. The amount of reverse thrust permitted to you varies with the aircraft and engine combination.