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Thread: IF critical_altitude= Doesn't equal Aircraft Ceiling, then what does?

  1. #1

    Lightbulb IF critical_altitude= Doesn't equal Aircraft Ceiling, then what does?

    I was trying to take off from Alejandro Velasco Astete International Airport (ICAO: SPZO)in Peru using two different aircraft. The field's elevation is around 11,000 feet. The first aircraft was Milton's Shupe's Beechcraft Model 18 with a supposed service ceiling of 26000 ft. In the Aircraft.cfg file the critical_altitude=8000 is set for this aircraft. In the Douglas RD46 NATS for CFS2 that same setting is set at critical_altitude=20000 for an aircraft with a 24000 ft service ceiling. Since the critical_altitude= setting seems to effect only turbocharged aircraft, I'm wondering what might have happened today. The Beech took almost all the 11,000 foot runway to take off. So I reloaded the Flight to load a larger aircraft. As soon as I loaded up DC-3, the engines conked out and refused to start! Nothing could get that to turn over, even using Crtl-E or the M++ commands. Eventually I finished the session in a Learjet 23 which flew fine!
    After shutting down FS2002 and restarting it, I loaded up the Douglas R4D6 at Meigs just for giggles! That bird lit up and took off like a bat out of hell! I didn't touch anything either. At this point, I'm assuming that the ceilings must be in the .Air files?

    Thanks!

    ChristopherT

  2. Default RE IF critical_altitude= Doesn't equal Aircraft Ceiling, then what does?

    Hi Christopher,

    at greater altitudes the air becomes thinner, which automatically makes the fuel-air mixture richer. Thus, for non charched piston engines to operate normally, you'll have to decrease the mixture. Some of these engines will still run with too rich a mixture - albeit with a strongly reduced performance (like your Beech 18), ohers will even refuse starting up (DC3).

    For the same reason you generally have to reduce mixture when flying at greater altitudes in order to get optimum performance.

    I hope this helps ...

    Alex

  3. #3

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    It does help! Luckily, my search of South America is done for now!
    I'm going to dig up the keystrokes for the mixture and test that out!

    Thanks,Alex!

    ChristopherT

  4. #4
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    ChristopherT,

    Ctrl + Shift + F2 = Decrease Mixture

    Ctrl + Shift + F3 = Increase Mixture

    The trick is to decrease the Mixture just until the engine(s) start to sputter, then increase just enough to get rid of the sputter. Remember that Prop Pitch and MP will need to be re-adjusted for best performance.

    It took me a little bit to get it all figured out but WHAT a difference it made!

    Alan

    "I created the Little Black Book to keep myself from getting killed..." -- Captain Elrey Borge Jeppesen
    i5 M450 2.4/4GB RAM/ATI RADEON 5000HD 1GB GPU/500 GB HDD/WIN 7 PRO 64/FS9 CFS CFS2

  5. #5

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    Used for turbocharges engines.
    CPU: I7 4790K @ 4.5 ghz, GPU and CPU water cooled
    GPU: Gigabyte GTX 970
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  6. #6

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    Critical altitude is an engine performance specification, for supercharged engines. For any given power rating, critical altitude is the highest pressure altitude at which that power is available. In the simulator world, the critical altitude in the config file is usually for takeoff power. In the real world there may be critical altitudes for takeoff/goaround, maximum continuous, climb, and maximum cruise power ratings. Model designers have to work these other engine performance limitations into gauges or operating procedures.

    The service ceiling is a specification for the aircraft as a whole. With some assumptions about weight (because ceiling is weight dependent), it is roughly an altitude that can be maintained at a maximum cruise or maximum continuous power setting, with some margins for safety. The ceiling is actually defined by available climb performance, which changes throughout a flight as fuel is burned off.

    I would not expect any aircraft to take off at altitudes near the service ceiling (because it will usually be heavy with fuel, and need an excess of power to accelerate and climb).

    An aircraft with a higher critical altitude for takeoff power may maintain full engine performance at some high altitude airports, but it will still need more runway to take off at those airports because the aircraft has to be accelerated to a higher ground speed to reach the indicated airspeeds needed for takeoff, climb, and flight.

    Both the Beech and the Douglas are mechanically supercharged, but not turbocharged. FS2002 does not very well model the performance of mechanical superchargers, so the engines are treated as if they were supercharged by the particular type of turbocharger that the simulator models.

  7. #7

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    Quote Originally Posted by ViperPilot2 View Post
    ChristopherT,

    Ctrl + Shift + F2 = Decrease Mixture

    Ctrl + Shift + F3 = Increase Mixture

    The trick is to decrease the Mixture just until the engine(s) start to sputter, then increase just enough to get rid of the sputter. Remember that Prop Pitch and MP will need to be re-adjusted for best performance.

    It took me a little bit to get it all figured out but WHAT a difference it made!

    Alan
    Thanks, Alan! I'll give it a whirl the next time I'm up!

    ChristopherT

  8. #8

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    Thanks, Tatest! That was an excellent reply too. I'm saving that one for later!

    ChristopherT

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