Questions on Thrust

[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.