Jet Engines, their Curves and the missing Thrust

[VentZer0]

Hi,
I am currently modding the J404 Engine to fit it to my SU27 replica (might release as part mod eventually).
I noticed something weird when I was editting the engine curves and trying them out ingame and this is true for Stock KSP engines aswell. The static thrust of the J404 is 130kN, @ Mach 2.5 it is supposed to be 219.5kN okay that is as advertised when you try it out. However when I look at the curves the thrustmultiplicator of the J404 at mach 2.5 is 3.5. 219/130 is a mere 1.8-ish of the thrust that the curve says. I thought the atmospheric curves play a role in this and might explain the vastly lower thrust, but nope even with a constant atmospheric curve the engine still is not producing the amount of thrust.

What variable am I missing? What else is playing into the calculation of the thrust? I really don't want to make totally ridiculous thrust curves and have multiplicators like 6 or higher in there.


(I hope this is the correct subforum to ask this)

[AeroGav]

What about weight?  How much does the weight of the panther compare with its real life counterpart?  Also i doubt the ram air thrust gain is realistic of any jet engine yet made.  3.5 times the static thrust at mach 2.5?   Don't think even the J-58 did that.   Most jet engines loose thrust every knot you go above mach 0.  They gain a bit of efficiency with alittude (colder air) but that's it.

[Streetwind]

@VentZer0, are you looking at the curves as shipped by the stock part, or are you defining your own curves and are confused that you are not getting the result the curve you made should yield?

Because in the second case, you may be running afoul of missing tangents. Stock curves usually (not sure if always) have proper tangents, but curves you make yourself generally don't. And then the curve goes and does something different from what you thought it would do. You can read about it here:

 

[VentZer0]

10 hours ago, AeroGav said:

What about weight?  How much does the weight of the panther compare with its real life counterpart?  Also i doubt the ram air thrust gain is realistic of any jet engine yet made.  3.5 times the static thrust at mach 2.5?   Don't think even the J-58 did that.   Most jet engines loose thrust every knot you go above mach 0.  They gain a bit of efficiency with alittude (colder air) but that's it.

Doesn't matter I was fitting the engine to the plane performance. With stock engines I wasn't able to achieve 1400km/h at sea level, so I upped the thrust for the engine to achieve this speed. Upper in the Atmosphere I tweaked the atmCurve to reduce the Thrust at altitude so it doesn't go beyond mach 2 at 7000m. I am aware that the engine is not realistically running, however afterburining turbofans gain some thrust at speed again, because of the ram pressure effects at high speeds. Stats of the AL31 : weight 1.57tons, thrust 145kN (wet), max speed ~2500km/h in the SU27 (but that is airframe limited) my craft is very accurately modelled. In earlier version it could do cobras without thrust vectoring ;D

Again now that I use FAR the curves can be reduced to realistic values. Somehow FAR got rid of this weird problem.

7 hours ago, Streetwind said:

@VentZer0, are you looking at the curves as shipped by the stock part, or are you defining your own curves and are confused that you are not getting the result the curve you made should yield?

Because in the second case, you may be running afoul of missing tangents. Stock curves usually (not sure if always) have proper tangents, but curves you make yourself generally don't. And then the curve goes and does something different from what you thought it would do. You can read about it here:

 

I use the floatcurve editor already. The curve I had was having 10 times thrust at mach 1 to only achieve about 2 times the thrust at mach1. It is like there is something limiting the thrustcurve.
I installed the dev build of FAR and revised my curves for the engine.

To illustrate the problem:
http://imgur.com/a/cIoLu

Edited February 20, 2017 by VentZer0