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

For sale by IAP, Inc.
PO Box 10000
Casper, WY 82602-1000
(800) 443-9250
(307) 266-3838
(307) 472-5106 fax

 

[wmuflyguy]

http://142.26.194.131/aerodynamics1/

Found this website
There is some good stuff under aircraft performance

 

[FN FAL]

There's no such thing as thrust; the jet engine creates a low pressure area in front of the engine, which in turn sucks the aircraft through the air.

 

[uwochris]

Mar,

The link that wmuflyguy posted is a source that I saw this graph- here is a link from one of the pages: http://142.26.194.131/aerodynamics1/Performance/Page8.html.
If you scroll down to the section "Jet Engine Efficiency," you will see the graph with the horizontal line. It also states there that jet engines are more fuel efficient at lower temps, and when operating at higher RPMs.

Also, the book "Flight Theory for Pilots" also shows this graph and does offer somewhat of an explanation (at least I don't fully understand it) on pages 82-85.

Charles Dole explains it as follows: " Turbojet engines are rated in terms of static thrust. The a/c is restrained from moving, and the thrust is measured and converted to sea level conditions. It can be equated by:
T-avail= Rho*A*V1(V2-V1), where A= cross sectional area of engine, Rho= density of air, V1= inlet/flight velocity, V2=exit velocity. As the a/c gains velocity (v1), the mass airflow increases, but the acceleration through the engine decreases (v2 is nearly constant). The thrust available is nearly constant with airspeed, and is considered to be constant in this discussion."

Also, the graphs on the following pages show that specific fuel consumption (i.e. fuel flow per lb of thrust) decreases as RPM increases and also that it decreases as altitude increases.

 

[Stifler's Mom]

You guys give me a headache. :smash:

 

[mar]

It's all becoming clear.

Ok, thanks for the link.

I think I understand your question better now.

But do *you* need more discussion on this or has everything fallen into place?

 

[uwochris]

But do *you* need more discussion on this or has everything fallen into place?

I can honestly say my understanding on this complex topic has increased, so I definetly have learned a lot. There is one somewhat related issue which I was thinking about too, which I would like to address.

Based on that Thrust-available formula I provided, it is clear that, all else equal, a higher air density gives better engine performance. Also, on the web link that I posted, the author mentions that jet engines are more efficient at colder temps.

While discussing TSFC and efficiency in this thread, it was mentionned that jets are more efficient in higher altitudes because of less drag and a higher TAS (i.e. this is in relation to maintaining the stoichemetric ratio of fuel/oxygen).

When considering colder air, it is generally fair to assume it is more dense. If it's more dense, does this not mean more drag? And if there's more drag, don't the engines need a higher thrust setting (thus, more fuel flow) to offset the drag?

I guess I am trying to think about the trade-off in this case:
- Colder air= higher density= better overall engine performance.
- colder air= higher density= more drag= decreased performance.

Is it fair to assume that the benefits of flying in colder/denser air are far greater than the negative effect of higher drag?

 

[mar]

Let me get back to you on that one.

<drops head on desk>

 

[Phony Marconi]

Cold air is MUCH better at altitude

When considering colder air, it is generally fair to assume it is more dense. If it's more dense, does this not mean more drag? And if there's more drag, don't the engines need a higher thrust setting (thus, more fuel flow) to offset the drag?

I guess I am trying to think about the trade-off in this case:
- Colder air= higher density= better overall engine performance.
- colder air= higher density= more drag= decreased performance.

Is it fair to assume that the benefits of flying in colder/denser air are far greater than the negative effect of higher drag?

Uwo:

Maybe this will help....

Consider a turbofan climbing through 29,000 ft ( a very high altitude for the CRJ....very poor climb performance ) in 2 different scenarios:

Scenario 1) Temperature ISA +15 (about -28F)...a typical summer day.
Scenario 2) Temperature ISA -5 (about -48F)....very rare cold at altitude.

The limit on engine thrust is core temperature. With each scenario, check out what happens to the engine temps as climb power is set to maximum allowable temperature:

Scenario 1 has much lower air density for the fan to work on to generate thrust. Thus, it has to spin faster to generate the same thrust as scenario 2. But no more fuel can be handled by the engine since it is already at max temp. So climb performance suffers due to lower thrust.

Scenario 2 offers the fan a thicker medium against which to work. Thus more thrust. In fact, the jet can maintain a higher airspeed in the climb, thus keeping more airflow to cool the core. This would allow for slightly higher fuel flows (and thus even more thrust) at max climb temp.

As far as efficiency, you are absolutely right....Higher temps mean less drag and lower fuel flows, but the performance suffers to such a degree as to limit the aircraft to lower altitudes (unable to climb) and thus burn more fuel....a perfect catch-22!

Consider the ultimate low-density situation: Outer space. If the engine could run there, it would burn fuel, yet produce zero thrust from the fan, since it would be just spinning without a medium against which to push. Efficiency: ZERO.

 

[mar]

I'm tryin' to narrow it down a little.

Is it fair to assume that the benefits of flying in colder/denser air are far greater than the negative effect of higher drag?

Hi again Chris. I think Phony Marconi gave a good scenario...better than I could've done.

I've been trying to narrow down my response because I when I read your question I want to answer with a discussion on ISA, optimum altitude, and other flight planning issues. But there are books about that stuff and I don't carry them with me.

Basically the only issue I have with your statement above is your use of the term "far greater."

I don't think it's accurate to give it that much weight. The truth is, like you say, it's a trade off (for efficiency) but not just between temp and pressure (density) but also weight and winds.

The truth is, nobody really gives it much thought--except for the performance engineers who design the various performance charts.

As pilots, we're mostly concerned with "optimum" altitude and that's based on weight. So as fuel is burnt and the airplane becomes lighter, it's allowed to climb. As mentioned before, the benefit is higher TAS and lower fuel burns.

Obviously the numbers will vary a little as the temperature deviates from standard (ISA) but this issue in and of itself isn't critical *until* you're in some sort of a max performance situation (The RJ that stalled at FL410 is a perfect example: The airplane was capable of maintaining that altitude but *only* under a certain combination of weight and temperature. They didn't have the temperature that day).

I tried to stay on topic and hope I didn't ramble too much. That's the best I can do without a chalk board. But check this out. It's an engine simulator. I found this link on pprune.

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

You can choose an engine type (I chose the CF6 because that's what I fly) and you can select the data to be displayed graphically. You can also choose to modify speed, power and altitude (just like you're flying) and then watch how temperature (EGT) and pressure within the engine vary with different inputs. It's kind of fun for about 5 minutes.

Keep the questions coming. I need the homework.