Jet Engine FSX
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Thread: Jet Engine FSX

  1. #1
    Frippe123
    Guest

    Jet Engine FSX

    Hello

    Problem: The real aircraft is supposed to reach close to mach 2 at high altitude. But in the sim it reaches less mach at high alt than at sea level. Im happy with the sea-level mach(around 1.2) and speed. but is it a way to make it go faster at high altitude?
    I use airwrench and i see "Mmax", is it possible to edit that value for my needs?

    Thanks Fredrik

  2. #2
    Good question! If it does not work out that Airwrench (it may work, Sparks?) it should be possible to edit the airfile table (430) with regards to the value of mach drag. At altitude generally equivelent Mach numbers result in a lesser true airspeed as the speed of sound decreases with decreasing temperature. This is not of such a magnitude as to cause the max speeds differences, which are a result of the overall domination of aerodynamic drag due to the thick air at seal level, and the reduced effect of this at altitude, replaced by Mach drag as the biggest factor. You aircraft is designed to be relaitivly efficent at high Mach numbers, allowing high speed (at least for a little while) in it's optimum environment.

  3. #3
    Altitude is a variable in both the thermodynamic engine thrust and aerodynamic drag equations. In theory, jet engine thrust decreases more with altitude than drag; and since FS runs on those equations, the maximum attainable TAS will decrease with altitude in FS. When measured by mach number, maximum attainable speeds for jet aircraft do not vary much with altitude from sea-level up to within a few thousand feet of the service ceiling where lift induced drag starts becoming a significant factor.

    When there is a significant difference between the sea-level and high altitude Vmax specs for a jet aircraft, the lower sea-level spec is probably not limited by the engine. It's more likely a service limit imposed to keep the wings from melting off. The only way to get the flight model to reach mach 2 at high altitude (without resorting to behind the scenes programming with gauges, fsuipc, or simconnect) is to design the flight model to reach mach 2 at sea-level.

    If this makes sense, I'll go on to explain how to do it with AirWrench... :ernae:

  4. #4
    Right now sonic booms may be heard as the Arctic Airforce Games are underway.

    At altitude, the turust vrs power equation which is related to true airspeed is more favorable. FS is capable over a wide range of parameters, but not perfect at all.

    As always, Jerry's explanations are very welcome.

    Cheers: t.

  5. #5
    Frippe123
    Guest
    Ok, thanks, am i on the right track if i change the drag-critical mach, i dont want to change the subsonic drag?

    Thanks Fredrik

  6. #6
    Quote Originally Posted by Frippe123 View Post
    Ok, thanks, am i on the right track if i change the drag-critical mach, i dont want to change the subsonic drag?

    Thanks Fredrik
    AirWrench will set all of the drag coefficients to appropriate values if it has the correct input parameters for the performance and the engine.

  7. #7

    High performance jet flight models

    Let me start with the assumption that the maximum advertised air speed for a jet fighter requires full afterburners, and the maximum level flight airspeed without afterburners is always lower.

    Every modern high-performance fighter is capable of supersonic speeds, but very few can go supersonic without afterburners. Those that can are usually advertised as having ‘super-cruise’ abilities.

    As far as the flight model is concerned, it doesn’t really matter if the aircraft has super-cruise abilities or not. The point to bear in mind is that there is a difference in performance with afterburners on and afterburners off, and we need to model both conditions.

    Flight model development for a jet fighter should start with estimating Vmax with afterburners off.

    If the aircraft isn’t advertised as having ‘super-cruise’ abilities, then it follows that its maximum speed without afterburners is less than mach 1. If the aircraft is capable of super-cruise, then Vmax without afterburners will be a little over mach 1. If you don’t know for sure, it’s ok to make a reasonable estimate – at the end of the day, the exact value won’t matter all that much.

    Just for argument sake, I’m going to use the following F-4E Phantom specifications as an example:

    Twin engines, 17900 lb thrust with afterburners, 11110 lb dry
    Wing area = 530.0
    Wing span = 38.3
    Aircraft Length= 63.0
    Empty weight = 29,500
    Max gross weight = 61,651

    Performance: Maximum speed 1430 mph at 36,000 feet (Mach 2.21), 914 mph at sea level (Mach 1.19). Cruising speed 585 mph.

    (to be continued...)

  8. #8
    Cruising speed is 585 mph, so I’m going to guess Vmax without afterburners is 625 mph at 36,000 ft (mach 0.94), so that’s what I enter for Vmax on the Specs tab. On the Engine tab I enter 36000 for Vmax Altitude, 11,100 for static thrust, and 1.0 for Mach Scale. (For the air file aficionados, Mach Scale sets the upper mach limit in sections 1502, 1504, 1506, and 1507)

    Two remaining parameters on the Engine tab, Intake Area and RAM Air Gain, determine the value AirWrench calculates for zero lift drag. The button labeled ‘Repair Jet Engine’ was recently added to allow AirWrench to calculate values for these two parameters that will produce a zero lift drag coefficient appropriate for a jet aircraft.

    After clicking the ‘Repair Jet Engine’ button, AirWrench will suggest alternate values for Static Thrust, Vmax Altitude, Vmax and Intake Area. In this case, I’ll click the ‘No’ button for Static Thrust, Vmax Altitude, and Vmax to keep the values I’ve already set, but I accept the suggestion for Intake Area (5.555 sq ft).

    Here’s what the Thrust Available/Thrust Required’ chart looks like (for FSX) without afterburners. The Thrust Available curve crosses the Thrust Required curve at mach 0.94.

  9. #9
    The final step is setting up the afterburner table to achieve maximum airspeed with afterburners. The values needed in the afterburner table will depend on the version of FS, but the process is still the same. (FSX in this example)

    Click the ’Reset Afterburner Table’ button and make sure the number of ‘Afterburner Stages’ is at least one.

    Put a check mark in the Display with Afterburner Thrust box below the ‘Thrust Available/Thrust Required’ chart. Now adjust the values in the ‘Afterburner Table’ until you get the Thrust Available curve to cross the Thrust Required curve at the correct Mach number.

    Here’s an example of how the Thrust Available/Thrust Required’ chart and ‘Afterburner Table’ look with afterburners. The point where the Thrust Available curve crosses the Thrust Required curve, at about mach 2.2, is Vmax with afterburners.

  10. #10
    Thanks for the explanation Jerry!

    Cheers: T.

  11. #11
    After you've been through it once or twice, you realize it takes longer to explain it than to do it. :costumes:

  12. #12
    Frippe123
    Guest
    Thank you Jerry, when i followed your example i first thought that my aircraft was really f-uped, i had to have a scalar of 7.2 in the afterburner to reach mach two, but then i changed from "metric"to "us-mph" and the scalar could be adjusted to a more normal 2.1 to reach mach 2.
    The mph and the kts are the same why is the metric curve so different?

    Thanks Fredrik

  13. #13
    Hi Fredrik,

    I'll have to look into what's going on using metric units. Everything is converted to US units on input, so maybe I missed something. Anyway, thanks for pointing that out.

    Jerry

  14. #14
    Problem found and fixed in version 72, which is now available for download. As always, the update is free. Just install it over the current version, no new activation required.

    The program was converting Thrust Required to metric units for display, but not Thrust Available, so the graph was displaying drag in newtons and thrust in pounds.

    Again thanks for the reporting the problem... :ernae:

  15. #15
    Frippe123
    Guest
    Hi again, new question:
    Is there a way to set the idle rpm for a jet engine? The n2 rpm is 11850(this is what i got from books about the aircraft im not sure its the n2 rpm but its the only setable in the sim) the value the gauge uses is (A:General eng1 rpm,rpm) and with this value the idle rpm is close to 8000. Ive seen on pictures etc tat the real thing is around 5000rpm. is there a way to fix this? and does anybody know if it will impact on something else? thrust etc.?

    Tanks Fredrk

  16. #16
    Quote Originally Posted by Frippe123 View Post
    Hi again, new question:
    Is there a way to set the idle rpm for a jet engine? The n2 rpm is 11850(this is what i got from books about the aircraft im not sure its the n2 rpm but its the only setable in the sim) the value the gauge uses is (A:General eng1 rpm,rpm) and with this value the idle rpm is close to 8000. Ive seen on pictures etc tat the real thing is around 5000rpm. is there a way to fix this? and does anybody know if it will impact on something else? thrust etc.?

    Tanks Fredrk
    You can set N2 (max) on the Engine tab, but you can not change the percentage of max RPM AirWrench uses for idle. N2 idle should be around 66%, and N1 around 35%. Changing max N2 rpm to another value won't change anything other than what you see on the gauge.

    FS uses three tables to control two engine speeds (N1/N2). I'll consider adding idle speed adjustments as a future upgrade, but I'm not enthusiastic about adding that much more complexity to the program because it just makes the program that much harder to use.

  17. #17
    Frippe123
    Guest
    Ok, well theres atleast one who´d like that.
    Would adding idle adjustment change the charachteristics of the engine a lot or is it that there would be to many parameters for the jetengine to be user friendly?

    Thank Fredrik

  18. #18
    Quote Originally Posted by Frippe123 View Post
    Ok, well theres atleast one who´d like that.
    I'll put it on the list of things to do...

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