Questions on Thrust

[uwochris]

[FONT=verdana, arial, helvetica]Hey guys,

I have a few questions on "thrust," and I hope some of you guys/gals can shed some light. (keep in mind I have no experience whatsoever flying any high-performance or turbine machines).

1. I don't understand why the "thrust available" curve when considering jets, is a straight horizontal line (i.e. y-axis= thrust in lbs, x-axis= velocity). To me, it seems that when a jet wants to go faster, the pilots must increase the thrust setting, thus implying higher velocity= higher thrust available. Obviously, I am misunderstanding something here- perhaps I am confused as to what exactly is meant by "thrust available" and how it is measured??

2. I do not understand why a jet engine has a low specific fuel consumption at higher altitudes. I understand that for the most part, prop a/c lose efficiency at higher altitudes due to the reduced air density; however, some of the books I am reading seem to imply that a jet engine is most efficient at higher altitudes- why is this? Why is fuel flow reduced at higher altitudes? Is it not true that in order to maintain a certain % of power, that you must increase your thrust setting as altitude increases, thus causing higher fuel-flow rates? For example, if you're flying a jet (737, A319, etc), does a thrust setting (assume N1= 65% and EPR= 1.7.... I just made up these numbers, as I have no idea what they may be in a real a/c) at 3000ft ASL have a significantly higher fuel flow rate for the exact same setting at a much higher altitude, like FL330? I don't understand the "why" behind all of this.

I appreciate any input. Thanks in advance, and merry Christmas![/FONT]

 

[VNugget]

1. That's because by "thrust available" they really mean "max thrust available." As in, you have the power set to the max and there is some magical external force keeping you at that speed.

 

[100LL... Again!]

Aerodynamics for Naval Aviators throws some light on it a little. ANA is a little cryptic, however, so you might have to read through it a time or two.

 

[VNugget]

I think you're confusing a couple of different issues. Specific fuel consumption is the ratio of fuel consumption to thrust. SFC does decrease with altitude, but only slightly, and for some kinda complicated reasons that I don't fully get.

I'm a little less sure about this one, but I think the real reason jet engines are so efficient at high altitudes, is a lot simpler. For any given TAS within normal cruising ranges, drag decreases (a lot) with altitude because of the lower air density. Since we need less thrust to maintain that speed (which is most likely limited by compressibility effects) we use less fuel.

 

[machaf]

[FONT=verdana, arial, helvetica]
2. I do not understand why a jet engine has a low specific fuel consumption at higher altitudes. I understand that for the most part, prop a/c lose efficiency at higher altitudes due to the reduced air density; however, some of the books I am reading seem to imply that a jet engine is most efficient at higher altitudes- why is this? Why is fuel flow reduced at higher altitudes? Is it not true that in order to maintain a certain % of power, that you must increase your thrust setting as altitude increases, thus causing higher fuel-flow rates? For example, if you're flying a jet (737, A319, etc), does a thrust setting (assume N1= 65% and EPR= 1.7.... I just made up these numbers, as I have no idea what they may be in a real a/c) at 3000ft ASL have a significantly higher fuel flow rate for the exact same setting at a much higher altitude, like FL330? I don't understand the "why" behind all of this.
[/FONT]

I'll attempt to answer your question: Jet engines are more efficient at high altitudes because they are burning less fuel because they are producing less thrust, but the true air speed is much higher at high altitudes than at low altitudes.


N1 is just the speed of the fan portion of the jet engine
N2 is the speed of the compressor section

EPR is engine pressure ratio. It is the ratio of engine output pressure to engine intake pressure. EPR of 1 means the engine is producing no thrust because intake and output pressures are the same.

Check out this website it should help
http://142.26.194.131/aerodynamics1/Performance/Page8.html

Check out Turbine Pilots Flight Manual. Excellent book.

 

[Phony Marconi]

The main reason jet airplanes need higher altitude to maximize their efficiency is because of the massive increase in true airspeed (TAS) at those altitudes.

Consider a jet airplane doing 250 knots indicated at MSL vs. at 35,000 ft.

Although not exactly the same, the fuel flows are nearly identical in each situation, however the TAS of the high-altitude jet will be almost double that of the sea-level plane. Thus specific fuel consumption (analogous to "miles-per-gallon" in cars) is much better (ie: more mpg's) at higher altitudes.

For a given IAS, the fuel flows will be the same regardless of altitude. Fan speed (N1) and other engine parameters will vary with altitude due to air density and temperature, but the main indicator of efficiency (fuel flow) stays the same.

 

[Mr Wu]

As with all engines the fuel/air ratio must be maintained at or near the stoichiometric ratio to produce the most power. Jet engines usually operate at higher altitudes and have to reduce the fuel burn to keep this ratio constant. Because of the lower air density at altitude the difference in the fuel burn at sea level and altitude is significant. It's much like leaning the mixture in a piston engine except that most turbine engines use a computer to automatically set the "mixture" thus making it more efficient. These "computers" vary from a simple bellows and needle valves to an electronic engine control.

 

[wmuflyguy]

1. I don't understand why the "thrust available" curve when considering jets, is a straight horizontal line (i.e. y-axis= thrust in lbs, x-axis= velocity). To me, it seems that when a jet wants to go faster, the pilots must increase the thrust setting, thus implying higher velocity= higher thrust available. Obviously, I am misunderstanding something here- perhaps I am confused as to what exactly is meant by "thrust available" and how it is measured??

You are confusing thrust available with the thrust required.


You are right to go faster you need to add more thust, but that is the Thrust Required curve. Where the thurst required curve meets the thrsut available, you have the fastest you can go.

 

[VNugget]

As with all engines the fuel/air ratio must be maintained at or near the stoichiometric ratio to produce the most power. Jet engines usually operate at higher altitudes and have to reduce the fuel burn to keep this ratio constant. Because of the lower air density at altitude the difference in the fuel burn at sea level and altitude is significant. It's much like leaning the mixture in a piston engine except that most turbine engines use a computer to automatically set the "mixture" thus making it more efficient. These "computers" vary from a simple bellows and needle valves to an electronic engine control.

Looks like I went off on a kind of a semantical tangent, nevermind.

But on a completely separate point, I want to remark that I've seen one of those mechanical fuel computers and they're anything but "simple." It was actually really really neat; it's this incredible little Rube Goldberg machine just packed with all sorts of bellows, gears, cams, flyweights, and other stuff. It was almost like a functional work of art.

 

[mar]

Bumped back to life

Chris--Did you get your question answered?

I wanted to bump this back to the top in case it was missed by a propulsion expert.

Personally I didn't respond because I wasn't too confident in my responses. I'd like to see a little more discussion on these points, however.

I'm still a little unclear on the "thust available" curve. You describe a straight line plotted against "thrust in lbs" and "velocity".

Re: "Velocity": is this TAS, free air stream velocity, ....(?)

I don't ever recall seeing this graph. Maybe you can cite a source or post a link.

As for jet engines burning less at higher altitudes, as someone mentioned, that's simply a fuel/air ratio problem. Less air mass, requires less fuel.

But I just re-read the last part of your question and you ask about efficiency. Why are jet engines more efficient at higher altitudes?

Efficiency involves time. So if you can get up high, go fast, and lower your fuel flow, you're increasing your efficiency.

I think I recommended this book years ago but if you didn't get around to picking it up I think you might want to take another look.

Flight Theory for Pilots
by Charles E. Dole
Institute of Safety and Systems Management
USC

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