commercial aircraft reverse thrust questions

Factual Questions
1

The thrust reverser mechanisms on commercial aircraft engines don’t appear provide much of a forward velocity component to the exhaust plume; depending on what aircraft you’re looking at, the exhaust angle appears to be somewhere around 60 degrees from straight forward, meaning that the engine could at most provide a reverse thrust of around 50% of its forward thrust (cos(60deg)=0.5). Also, on high-bypass turbofans like those on the A320, it’s not clear that the engine’s main exhaust is being redirected by the reverser; it appears that the reversers are only redirecting the bypass air. I understand the lion’s share of thrust comes from the bypass air, but if reverse thrust is already being limited by the angle of the redirect, then I’m wondering just how much reverse thrust is typically available.

So there’s one question: if an engine provides X pounds of forward thrust, what percentage of X is generally available as reverse thrust?

The other question: can the reversers be deployed while the engines are already operating at full thrust? For example, in a rejected takeoff, can the pilot just deploy the reversers without idling the engines first, or does he have to idle, deploy reversers, and then power up?

2

To answer the last question, info received from a guy that does functional testing of 737 engines, the throttles have to be at full idle to deploy the TR’s.

3

First the simple answer–Yes, on modern engines thrust must be at idle and the aircraft on the ground to unlock the reversers. After they are deployed the engines can be spooled to supply “thrust”

Next for the amount of reverse:

You are correct in that in modern bypass engines only the bypass air is reversed. This accounts for around 75 to 80% of thrust put out by the motor. This of course varies slightly between designs, but its a good ballpark. Some older straight turbojet and low-bypass engines did have clamshell reversers that did redirect 100% of the air (see the MD-80 series for a good example).

You are also correct that the air is not redirected at a perfect 180 degree angle, so it is not a true “reverse” but more of a forward redirect. As far as the amount and effect of that, it is beyond my knowledge and a question for the engineers.

Some other notes on reverse thrust–

It is surprisingly ineffective at slowing the aircraft. It certainly helps, but the biggest benefit is from cancelling the forward thrust more than actually reversing. In fact, the Cessna Citation CJ series has a set of “thrust attenuators” (small flaps behind the engines that block forward thrust) in place of reversers…they are surprisingly effective.

Some airlines have the policy to only pop the buckets at idle reverse and not accelerate the engines after landing (unless needed) to save on wear and tear. The theory is that it is cheaper to replace brakes than to incur extra cost of increased reverser overhaul, engine wear and fuel expense for little benefit. Others want full reverse and minimal braking until needed. Just depends on what parts are leased vs owned and what the accountants say is cheaper. Pilots of course always have the option to use full reverse if needed.

Reverse thrust is aerodynamically most effective at high speeds. This is kind of works against you. In a short runway situation, by the time you touch down, wait a second or two for the aircraft to go into ground mode and allow reverser deployment, select reverse, the reversers to unlock and deploy and finally a few seconds for the engines to spool back up, the spoilers and heavy braking have slowed the aircraft significantly thus negating much of the benefit. Of course, every bit helps, especially on a short or contaminated runway.

Reverse thrust is generally not included in landing and rejected takeoff distance calculations. Any benefit is a bonus.

Hope this helps.

4

I guess this is as good a place to reveal a small portion of ignorance that rattles around in my brain. I can’t accept the fact that reversers work in the first place. Here you have the engine generating forward thrust by throwing air towards the rear. Now, just because you change the direction of that air, how does that negate the forward thrust that has already been generated?

5

I’m surprised it works at all.

Seems to me when the engine is spooled up it is providing thrust forward (the fan blades are pushing air backwards so Newton’s equal and opposite applies and the plane is pushed forward). Then add in the air going through the compressor which is not redirected at all also providing thrust.

Then the air is reversed and applies some measure of braking. Since not all the air being redirected is in the exact opposite direction the braking effect would seem to be even less than the acceleration effect.

I completely realize I am missing something here since clearly these are put on jet engines and are used to slow the plane. I just do not know where I am going wrong.

6

Yeah, what he said.

7

If the bypass is 75% of the thrust, and it is redirected by 120 degrees (i.e. exhaust direction is 60 degrees from direction of motion), then this still represents 70%*cos(60 deg) = 38% of the engine’s thrust in the reverse direction. More than enough to counteract the 25% forward thrust.

8

But the bypass fan has already imparted 100% of its thrust to the plane forward (it seems to me). As soon as a blade pushes air backwards towards the exhaust and thrust reverse the air has already imparted the forward thrust to the blade.

Try it this way.

Forget about the thrust reverse.

If we only consider the bypass fan turning we see the forward thrust imparted at the blade itself regardless of what comes next.

9

For a simple theoretical analysis, you can treat the engine and everything that happens inside it as a “black box”, meaning you don’t know/don’t care what’s going on inside; you only care what’s happening at the edges, where your enigmatic black box interacts with the rest of the universe.

For a jet engine, that interaction can pretty much be boiled down to two fluid momentum streams: intake air entering with momentum of a certain magnitude and direction, and exhaust gas leaving with momentum of its own magnitude and direction (you could also factor in the mass of the fuel stream added to the exhaust, but this is pretty trivial compared to how much air a high-bypass turbofan moves).

The magnitude and direction of the net thrust from the engine is related to the vector sum of the momentums entering your black box minus the vector sum of the momentums leaving it. A running engine produces forward thrust because the exhaust stream exits with a lot of front-to-rear momentum, while the intake air enters with relatively little front-to-rear momentum; the result is a net forward force on your black box. By redirecting thet exhaust stream so it points forward, you can develop a lot of reverse thrust because now your black box has no front-to-rear momentum leaving it; instead, the exhaust stream is now departing with a good bit of rear-to-front momentum.

Your thrust reverser could even direct the exhaust stream in a completely radial direction - up, down, sideways, but not forward/backward - and you’d still develop some reverse thrust because of the front-to-rear momentum entering your black box with the intake air (and no front-to-rear momentum leaving with the exhaust stream). That would basically be the same as standing on the wing and holding a big sheet of plywood in front of you (or a parachute behind you).

10

Can we though?

I mean, isn’t it important to know where the thrust is imparted and in what direction? We have two separate pieces here. The bypass fan and the thrust reverser and they are doing different things.

Basically what I said just above in post #8.

11

Yes. If you can get ahold of a fluid mechanics textbook, you’ll see the same explanation of how to approach the analysis. Look at the second picture on this page, and you’ll see that dotted-line boundary forming the edge of the box, and the mention of momentum associated with incoming/outgoing fluid streams. It may be that there are more than two fluid streams that cross the black-box boundary - example, even with thrust reversers redirecting the bypass air, the central engine exhaust is still directed to the rear, in which case (together with the intake air) you have three fluid streams crossing the black-box boundary - but once you’ve accounted for all the fluid streams and the momentum they each enter/leave with, you can calculate the net thrust of the engine. Doesn’t matter if you’ve got a hair dryer, an open-air propeller, a ducted fan, a water-squirting fire hose, a rocket, or a mysterious engine that makes the air do nine corkscrews and then bats it left and right five times each before sending it out the back.

Looking back on what I’ve written in this and my previous post, “black box” was a poor choice of term on my part, since it’s typically associated with the data/voice recorders on an aircraft. Instead, the better terminology, the one typically used by engineers is “control volume.” Nonetheless, the idea is the same, i.e. what transpires inside the box is irrelevant, and we only care about what happens at the boundaries.

12

Are you saying that, if we could make the air, once it’s in the engine, somehow magically disappear (okay, let’s say it streams out of the engine in all directions equally – just stick a sphere with lots of holes in it on the back of the engine) – then a plane, starting motionless on the ground, would start to move forward? That is, the mere **sucking in of air **would pull it forward – enough, even, to get some speed going, and maybe even get airborne?

13

If the air is only being sucked from the front of the engine, and exiting in every direction equally, then yes, you will have a net thrust pushing the entire engine forward.

14

Okay, thanks. Does this provide a significant percentage of the overall thrust in a real engine? If it’s half of it, then I would think you could get airborne in a typical plane, no? (Not with much safety margin, mind you.)

15

When Lambert Airport in St. Louis built a new terminal section in the 1980s, they were quite proud that they had graded the gate ramps several degrees uphill so departing jets could use their thrust reversers to back out, rather than needing a tractor to push the aircraft back.

While things might have improved in the last 30 years, what this suggests to me is that the thrust reversers are so weak they can’t reliably push a passenger jet backwards from a standing start on level ground.

16

the DC-9/MD-80 series with the low-bypass engines could do a “powerback” from the gate. years ago I used to see Northwest’s DC-9s do that once in a while. but the reversers of those engines were quite different from the ones used on modern high-bypass engines. here’s a video of an MD-8x doing one:

17

See below chart from FAA document “Takeoff Safety Training Aid – Section 2 – Pilot Guide to Takeoff Safety”. Figure 20, page 2-32. The data shown is typical for all high bypass engines. At 90% N1 engine speed and 150 knots true air speed, net reverse thrust is about 40% of max normal engine thrust.

Complete document: http://flightsafety.org/files/RERR/TakeoffTrainingSafetyAid.pdf

18

The problem is that air isn’t sucked into the engine from just straight in front of the intake; it’s sucked in from pretty much every direction near the intake. You can see this when the engines are running while there’s water on the pavement, as in this video. (another good one here). The presence of the ground makes that vortex point toward it; without any ground near the engine (imagine it’s 100+ feet above the ramp), that downward-pointing vortex would likely disappear, and the inlet flow would exhibit a lot more radial symmetry. So if air is coming into the intake from pretty much all directions, and it’s being deliberately exhausted in all directions, you won’t develop much forward thrust.

On a related note, the fact that an inlet draws in fluid from all directions around it (instead of just parallel to the orientation of the inlet) is the reason pop-pop boats can generate a net forward thrust: very little thrust when sucking in water, and a good pulse of forward thrust when farting it out.

Things look a little different when the engine is moving forward through the air, because now (from the engine’s perspective) it looks like air really is coming straight into the intake. Except the engine isn’t pulling that air in, the air is being crammed down the engine’s throat by the slipstream. If you scatter the exhaust stream in all directions at equal velocity (relative to the engine itself, which again is moving forward through the air), then you end up generating reverse thrust (really just aero drag).

19

Got it, thanks. I suspected as much – and that the answer (for getting up to speed while on the ground) would be related to how nozzles at the backs of engines are small holes that direct air and exhaust in a very specific direction.

And, as you said, the air DOES go in “straight” once you’re up to speed and airborne, by which time my question (about GETTING up to speed and airborne) no longer applies.

20

For most airliners with high-bypass turbofans, around 40%. Which is a lot and worth having available. It can be higher if the core flow is reversed too, like on the C-17, but that extra equipment weighs more and doesn’t pay for itself in airline operations, even if it does in short-field combat-zone service.

No, the power lever has to come back to idle for the reverser locks to be released, then it can go back to max once the reversers are fully in place (reverse can be at part power too). They aren’t made to take the load while still in transition.

Some airports allow that, some don’t - some have plenty of glass in line with where the reversers will blow debris. There are safety issues, too - the crew also does not have visibility to the rear, and the ground or ramp controller may not be able to see there easily either, while a tug driver can see where he’s going.

Yes, they can, even with a level ramp. The grade at STL just makes reverse backup take less. It does take quite a bit more thrust, in either direction, to start rolling than to keep rolling.