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Jet engine losing power

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Jet engine losing power

Postby kuifje09 » Fri Jun 26, 2015 1:04 pm

Could someone clear this up for me...

I have an jet-engine, on the ground it delivers about 900 lb, but when I am at 34000 ft, it has lost power to about 300 lb
Is that correct, or is there something wrong.
The higer you go, the less air to burn... but how do I know the amount of loss in power is correct ?
If you loose speed, you would loose power too... I have realy no knowlege of these engines.


EDIT: Even when taking up speed on the runway, does shrinks the power... When I expected just getting more power.
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Re: Jet engine losing power

Postby kuifje09 » Fri Jun 26, 2015 1:09 pm

This might be handy ? the power table from jsbsim:

Code: Select all
  <function name="MilThrust">
   <table>
    <independentVar lookup="row">velocities/mach</independentVar>
    <independentVar lookup="column">atmosphere/density-altitude</independentVar>
    <tableData>
          -10000       0   10000   20000   30000   40000   50000   60000
     0.0   1.2600  1.0000  0.7400  0.5340  0.3720  0.2410  0.1490  0
     0.2   1.1710  0.9340  0.6970  0.5060  0.3550  0.2310  0.1430  0
     0.4   1.1500  0.9210  0.6920  0.5060  0.3570  0.2330  0.1450  0
     0.6   1.1810  0.9510  0.7210  0.5320  0.3780  0.2480  0.1540  0
     0.8   1.2580  1.0200  0.7820  0.5820  0.4170  0.2750  0.1700  0
     1.0   1.3690  1.1200  0.8710  0.6510  0.4750  0.3150  0.1950  0
     1.2   1.4850  1.2300  0.9750  0.7440  0.5450  0.3640  0.2250  0
     1.4   1.5941  1.3400  1.0860  0.8450  0.6280  0.4240  0.2630  0
    </tableData>
   </table>
  </function>
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Re: Jet engine losing power

Postby Bjoern » Fri Jun 26, 2015 2:14 pm

Loss of thrust at higher speeds and altitudes is perfectly normal.

If you want to increase net thrust (thrust produced by the engine minus drag produced by the engine) at different speeds and altitudes, just increase the corresponding numbers in the tables you've posted.
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Re: Jet engine losing power

Postby daveculp » Fri Jun 26, 2015 3:08 pm

Jet engines lose thrust with altitude increase, and gain thrust with speed increase. Nearing mach 1 the engine will choke on shock waves unless the inlet has been designed to operate at supersonic speeds.
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Re: Jet engine losing power

Postby Jabberwocky » Sat Jun 27, 2015 2:01 pm

I have a little bit an impression, the MilThrust table created by aeromatic is a little bit too hefty when it comes to thrust-loss in altitude. I have to see whether I can find better data for a number of engines. But where?
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Re: Jet engine losing power

Postby Johan G » Sat Jun 27, 2015 7:36 pm

This website might be a good starting point: Nathan Meier's Jet Engine Specification Database.

Probably on a slightly less serious level, this page have a "toy" that might be fun to play with: NASA Glenn Research Center EngineSim Version 1.8a (beta version)
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Re: Jet engine losing power

Postby kuifje09 » Sat Jun 27, 2015 8:02 pm

Well, I also found some articles and lot of formulas, but it puzzles my mind.
I took the performance table as generated from the jsbsim site. So I thought it should be rather nice.
But when it comes to flying, then I see my plane has almost no power to get altitute at a decent time.

I just can recalculate the table and get some numbers, to get a decent climb rate. Just don't think thats the correct way.
But a reduction of power to 30 % at 30000 ft, is that normal ?
When I check nasa, indeed it should be correct.
http://www.grc.nasa.gov/WWW/k-12/Missions/Jim/thrust.gif
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Re: Jet engine losing power

Postby Johan G » Sat Jun 27, 2015 8:10 pm

Good thing the air is thinner and less "draggy" up there. "It's frightened of heights, I guess."* ;)

* Thanks for that one Waldo. :)
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Re: Jet engine losing power

Postby daveculp » Sat Jun 27, 2015 9:28 pm

If you have trouble getting to altitude then the problem is usually 1) too much weight for that altitude 2) improper climb speed/mach schedule , or 3) higher than normal air temperature. Without the manufacturer's climb charts it's hard to get it right.
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Re: Jet engine losing power

Postby Alant » Sat Jun 27, 2015 10:35 pm

Don´t worry too much. The loss of thrust is almost exactly equal to the reduction in parasitic drag as the air becomes thinner. Unfortunately the thinner air also results in a loss of lift, - so more incidence is required, thus producing an increase in induced drag.

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Re: Jet engine losing power

Postby Jabberwocky » Sun Jun 28, 2015 3:35 pm

I looked over the Nathan Meier site, Johan G. linked and if I resolve the formulas backwards, means from thrust and tsfc and range for certain planes using those engines in FG, I come up with higher values for many of them. At 30,000 ft I should have roughly 30%-40% loss (depending on the engine model, bypass ratios and so on), but the MilThrust table as generated by Aeromatic is for most engines at 30,000 ft already at 40% to 50% and from there it goes exponentially up. High performance fighter jets have at 40,000 ft already a loss of 60% to 80% with Aeromatic.
So, maybe there is an error in my math (indicating NASA is also math inept because they come to similar results) or Aeromatic leaves us a little underpowered.
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Re: Jet engine losing power

Postby japreja » Sun Jul 05, 2015 3:14 am

I can't imagine that JSBSim or FG has this effect "engine power loss that affects multiple models of commercial airplanes and engines". But maybe some evil developer decided to toss it in for more realism without telling anyone :twisted:

It might be a new feature and not a problem with your math models. You might want to check with the devs... Also a jet mechanic might be able to answer your questions. The only one I know of is on YouTube, AgentJayZ. He welcomes questions but is not able to answer certain questions unless they are asked directly on youtube and not on Google+ He will answer with a video response like the most recent post as an example.



If you ask him and he answers link the video :D
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Re: Jet engine losing power

Postby jam007 » Fri Mar 31, 2017 3:25 pm

Found this thread searching mystified by a possible lack of power for turbojet at high altitude in FG that I noticed.
There is a similar discussion in this blog post regarding X-plane. He is arguing that pure turbojets are modelled with too little trust at high altitude. The numbers for turbofan is very similar to those generated by Aeromatic for jets in FG (eg. in Viggen 37, Draken J35).
Image

Using EngineSim (linked to by Johan G above) also suggests that the turbojets might be underpowered at high altitude.

What do you think?
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Re: Jet engine losing power

Postby Clive2670 » Fri Mar 31, 2017 4:47 pm

japreja wrote in Sun Jul 05, 2015 3:14 am:I can't imagine that JSBSim or FG has this effect "engine power loss that affects multiple models of commercial airplanes and engines". But maybe some evil developer decided to toss it in for more realism without telling anyone :twisted:

It might be a new feature and not a problem with your math models. You might want to check with the devs... Also a jet mechanic might be able to answer your questions. The only one I know of is on YouTube, AgentJayZ. He welcomes questions but is not able to answer certain questions unless they are asked directly on youtube and not on Google+ He will answer with a video response like the most recent post as an example.



If you ask him and he answers link the video :D

He is a VERY knowledgable guy, he vids are very interesting to boot!
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Re: Jet engine losing power

Postby Necolatis » Fri Mar 31, 2017 5:05 pm

Hmm, I just did a comparison using an engine we actually have the real thrust curves for the F100-PW-200.

He is the real thrust values, as used in a 1979 F16 simulator:
Image

Here is the stats used to feed into the AeromatiC++:
Image

Here is the mil table from the Aeromatic 3.3.8 output:
Image

So just taking a random number (50000ft at 0.2 mach):
Real thust: 0.0954
Aeromatic thrust: 0.1408

Seem like aeromatic is actually giving a bit too much thrust..at least for 0.2 mach at 50000 ft.
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