thrust of airliners

[Dreamlinerwannabe]

Hi aviators,

When we quote the engine thrust of a jet, for example, 777-300er, ge90-115b, we say 115,000 lb thrust.

What thrust is this figure saying? I went to Boeing website it says maximum thrust, but I went google it so many answers.... What does it mean?

Thanks!

[tdracer]

When we quote thrust numbers, we generally quote the max rated takeoff "Sea Level Static" thrust. Boeing has something called "BET" or Boeing Equivalent Thrust - that has certain installation assumptions added in. Generally the numbers the engine company quotes are 'ideal' - and don't include losses from the nozzle and inlet. The nozzle and inlets on subsonic airplanes are usually extremely efficient - better than 99% - but that can still be ~1,000 lbs. on a GE90-115B :rolleyes:

The other part that many people don't understand is that turbofans start producing less net thrust as soon as they start moving - the thrust decreases or 'lapses' with forward speed (the gross thrust does go up, but not as fast as in the inlet 'ram' drag). The effect is not small - by 80 knots total net thrust can be down by 10%. The thrust lapse with speed is typically higher with higher bypass ratios - pure turbojets suffer little thrust lapse with speed (which is why aircraft that are designed to be supersonic tend to have either pure turbojets or very low bypass turbofans).

The other aspect is thrust decreases with altitude - pretty much as a direct function of the drop in ambient pressure.

As a result, something like a GE90-115B is producing a small fraction of that thrust at a 39k cruise condition.

[pattern_is_full]

The simplistic explanation is that a thrust of 115,000 lbs is just enough to support a weight of 115,000 lbs (including its own weight) directly against the pull of gravity.

Place 96,640 lbs of, let's say, iron weights on a platform. Attach to this "payload," with 100 lbs of cables, the GE 90-115b, pointed straight up like a space rocket. Since the engine itself weighs 18,260 lbs, and the cables weigh 100 lbs., the total weight of the engine, cables and weights is 115,000 lbs.

Run the engine up to maximum throttle, and it will just exactly support the weight of itself and the extra weights and cables (115,000 lbs total) against the pull of gravity. You can remove the supporting platform and the engine will hover, holding up the total weight - without descending, but also without climbing.

Add 1 pound to the weights to create a total weight of 115,001 lbs, and the whole thing will sink slowly - the weight is greater than the thrust. Reduce the total weight by 1 lb to 114,999 lbs and it will rise slowly under the engine's "excess" thrust of one pound.

Now, that is just a thought experiment in an "ideal" world. As tdracer points out, in the real world you'd run into all the losses he mentions. In addition, modern computer-controlled (FADEC) engines are often artificially limited to a specified thrust by software that limits rpms and fuel flow.

You'll also note that - thanks to the magical device called a "wing" - the 775,000 lbs of a 777-300ER can be "flown" with only 230,000 lbs of engine thrust. Unlike our thought experiment, the engine thrust does not have to support the vertical weight of the plane directly. It simply has to move the wing fast enough horizontally to provide "lift" that equals or exceeds 775,000 lbs.

[Wally Mk2]

Some amazing well thought answers here guys (ya gotta get a life!:E) But in real simpleton (me) terms chock the engine/machine stick a vertical weight measuring device behind it much like a large set of bathroom scales & blast away....what the screen reads is what the thrust is, provided there's anything left of the screen:-) Or you could just tie the engine/plane to a monster set of fish scales & let the beast pull it's heart out:-) All other complex factors as mentioned here are worked well b4 us drivers push up the thrust levers or is that power levers?:)
As Mr Newton once said .............for every action there is an equal & opposite reaction.........Eg you hit me in the face & I'll hit you right back:E
You stand on a set of scales & it says 100kg (fat bugga) so yr producing a 100kg's of thrust:-)

Like as has been mentioned the book figures are at S/L under certain conditions, what you get on the day when yr push those levers fwd is anyone's guess just like the T/Off weight of a plane at any given time, no one knows exactly what it is!:-)

[nicolai]

Does anyone have (or can refer to a source for) some example figures of static S/L thrust compared with thrust at speed and altitude?

For example I think it would be useful to illustrate, say, thrust for a GE90-115B static at sea level, 150kt at sea level, 250kt at 5000ft, etc, up to typical cruise speed and altitude.

[VinRouge]

as an aside, is there any way of converting E.P.R. into thrust with some other variables (eg outlet area, air density, mass flow etc)

Just interested in what our thrust is in the cruise.

[WeekendFlyer]

Easiest way to do a quick estimate is to use the Lift/Drag ratio. In steady level flight (i.e. constant speed, height and direction) Lift = Weight and Thrust = Drag.

Typically, the weight at the start of cruise = 0.95 x takeoff weight. This accounts for the fuel burnt during takeoff and climb.

In the cruise, apply L/D = W/T and rearrange:

T = W / (L/D)

L/D in the cruise for most jet transports is in the region of 15, so in simple terms thrust required will be Take of Weight x 0.95 / 15

Look at the A320-200 as an example: MTOM = 78000kg = 764.92kN

Maximum sea-level static thrust = 240kN total for the two engines

764.92 * 0.95 /15 = 48.44 kN, which is about 20% of the rated sea-level static thrust for the 2 engines together. I suspect for most modern commercial jets the % rated thrust figure will be similar.

[tdracer]

which is about 20% of the rated sea-level static thrust for the 2 engines together. I suspect for most modern commercial jets the % rated thrust figure will be similar.

20% is a pretty good approximation - I generally figure cruise thrust to be somewhere between 10% and 30% of rated thrust (accounting for light vs. heavy, low altitude vs. high altitude, etc.)

[lomapaseo]

as an aside, is there any way of converting E.P.R. into thrust with some other variables (eg outlet area, air density, mass flow etc)


Yes as long as you compute the EPR at multiple stages along the length of the engine.

Be prepared for some negative numbers though

[Tarq57]

And is the fuel flow at cruise around about 20% of the sea level figure for similar engine rpm?

I suspect it will be very close to that.

Which also would appear to go some way to answering the op's other question in this sub-forum.

[No Fly Zone]

Hot Damn! The several responses posted here have answered some questions that I've been harboring for years. Engineering than math qualified, I've never worked with these subjects. Thanks, multiple responds, you've helped a lot of us. Perhaps that is why this is such a fun space!!

[FE Hoppy]

Don't forget the ram effect!

[flyboyike]

Don't forget the Fetzer valve!