critical altitude dataref

[MartinJ]

I am currently working on the engine performance of the twin-jet I am developing. Therefor I wanted to modify the engine's critical altitiude by altering the respective dataref called acf_critalt. When I load the aircraft in X-Plane the dataref initially shows the critical altitude I entered in PlaneMaker. But when I change the dataref's value there is no effect on the engines. Do you guys have any ideas why it is not working? 

 

best regards

Martin

Edited February 27, 2014 by MartinJ

[alfletcher]

I ran some tests regarding this topic some time ago and I think X-Plane does not model very well engine performance.

For exemple you can put a C172 a 30,000 ft and the engine run as good as at sea level (just a slight decrease of power output).

If you find something else, just let us know.

 

Alan

[tkyler]

Critical altitude is specifically for propeller aircraft.   Use the "Jet Curves" settings to tweak jet engine performance at altitude.

 

TomK

Edited February 27, 2014 by tkyler

[tkyler]

nvrmind

Edited February 27, 2014 by tkyler

[MartinJ]

Thank you very much. I will take a look at it.

[MartinJ]

I checked the jet curves today and they are indeed very helpful. Unfortunately I still experience a problem with the aircraft's performance.

 

Concerning the jet curves I noticed that at high altitudes and fast speeds, e.g. Mach 0.8 at FL 390, the thrust ratio is hardly influenced by the engine's bypass ratio. In other words: the engine's performance at high altitudes and fast speeds can't be adjusted using the bypass ratio. So if it is not the bypass ratio that can be adjusted to achive an appropriate N1-value according to the aircraft's manual (given a suitable maximum thrust setting at sea level) the way to go is to manipulate the aircraft's drag. According to the manual of the aircraft the N1-indicator should read about 93 % at FL 390 for a weight of 130 t and a cruising speed of approximately Mach 0.8. Initially I had to apply 108 % N1 to reach the given speed. Hence I reduced the aircraft's drag stepwise to reduce the necessary thrust. Now the situation is that the fuselage's frontal area is set to 0.0 and the drag coefficients of the wing's and empennage's airfoils are set to minimum, which should result in a minimum drag. But even with these settings the N1-indicator still reads about 100 %. Since I can neither increase the aircraft's static thrust nor reduce the aircraft's drag any further I have no idea how to achieve the given N1-settings. That is why any ideas from you guys are highly appreciated.

 

best regards

Martin

[Tonka]

Adjust the Tmax dataref with a plugin or in Gizmo.

 

Adjusting thrust AND drag makes for a bit of a nightmare in tuning performance, but sometimes you just have to keep bashing away at it.

 

The jet curves were next to useless for the engines i am simulating, FWIW.

 

edit; sim/aircraft/engine/acf_tmax

Edited March 2, 2014 by Tonka