1. #1
    This is a small detail, but I wanted to see if Oleg had modelled it. I did some "flight testing" in the BI-1 in the original Il-2, too see if the compressibility effects would start at different speeds on Summer and Winter maps. (Sound travels slower at colder temperatures, so for a given speed your Mach number should increase.)

    Alas, the sharp downward pitching moment of the Bi-1 appeared at 750 km/h both during winter and summer. It should have been at about 725 km/h during the winter.(If the summer speed is correct). Though doubtful, can someone tell me if its different in FB?

    P.S. Test was performed at 1000 m, straight and level flight, default loadout.

    [This message was edited by KrasniyYastreb on Mon April 12 2004 at 02:26 PM.]
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  2. #2
    This is a small detail, but I wanted to see if Oleg had modelled it. I did some "flight testing" in the BI-1 in the original Il-2, too see if the compressibility effects would start at different speeds on Summer and Winter maps. (Sound travels slower at colder temperatures, so for a given speed your Mach number should increase.)

    Alas, the sharp downward pitching moment of the Bi-1 appeared at 750 km/h both during winter and summer. It should have been at about 725 km/h during the winter.(If the summer speed is correct). Though doubtful, can someone tell me if its different in FB?

    P.S. Test was performed at 1000 m, straight and level flight, default loadout.

    [This message was edited by KrasniyYastreb on Mon April 12 2004 at 02:26 PM.]
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  3. #3
    Unfortunate to hear that, KrasniyYastreb.

    I have not checked to see if the BI-1 and P-38 compressibility effects occur at proper airspeed in AEP, but I sure remember that problem in FB and earlier versions. I will check it out tonight!

    One good thing-- the relationship between indicated airspeed (IAS) and true airspeed (TAS) does appear to obey proper physical laws, including the effect of temperature as well as altitude.

    Some people have observed that control stiffening does not seem to follow IAS or Mach, but I have not investigated it closely yet. It could be a simple bug in that parts of the flight model are tied to TAS when they should instead be related to IAS or perhaps Mach. Maybe the developers could grep for all functions that depend on TAS, and double-check that TAS is really the correct value for each function.
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  4. #4
    havent noticed any change in summer vs winter, but then again I rarely get a plane going that fast. I could be wrong, but a/c seem to stall more easily for me in summer maps vs winter maps. thats how it seems to me anyway. not a significant difference though.
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  5. #5
    Summary first, for those who do not wish to read the whole test writeup:

    BI-1 compressibility is not correctly related to IAS *or* Mach number.

    BI-1 "shake" test. Performed on Crimea map with 100% fuel load and perfect weather, at 1200 noon. I used the Crimea map because in this community Crimea has become sort of the accepted standard map for flight testing. Note however that the Crimea wx is not equivalent to a '76 Standard Atmosphere, so some conversions were necessary to calculate Mach.

    Test procedure: Place BI-1 at test altitude using FMB. Accelerate BI-1 gradually in level flight until aircraft begins to shake and note airspeed. Speed up and slow down past the shake threshold several times to confirm. Note: full "tuck under" does not occur until quite a ways beyond the initial shake speed.
    <pre class="ip-ubbcode-code-pre">observed data:
    altitude, m / observed temperature, C / shake airspeed, kmh (true)

    100 / 25 / 775
    1000 / 19 / 811
    2000 / 12 / 852
    3000 / 6 / 895
    4000 / -2 / 948
    5000 / -9 / 995
    6000*</pre>

    * BI-1 cannot reach "shake speed" in level flight (max. speed at 6000m is approx. 1015 kmh)

    Mach calculated with Digital Dutch Standard Atmosphere calculator,
    http://www.digitaldutch.com/atmoscalc/

    <pre class="ip-ubbcode-code-pre">Calculated speed of sound:
    altitude, m / observed temperature, C / Speed of Sound, kmh

    100 / 25 / 1246
    1000 / 19 / 1234
    2000 / 12 / 1219
    3000 / 6 / 1206
    4000 / -2 / 1188
    5000 / -9 / 1173</pre>

    Note that Crimea atmospheric conditions are on average 10 degrees warmer than a standard atmosphere.

    <pre class="ip-ubbcode-code-pre">Final Mach calculations:
    altitude, m / Mach number at which BI-1 begins to shake

    100 / 0.62
    1000 / 0.66
    2000 / 0.70
    3000 / 0.74
    4000 / 0.80
    5000 / 0.84</pre>
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  6. #6
    Bug has been written up and mailed to Oleg.

    I suggest following the results in ORR,
    http://forums.ubi.com/eve/forums?a=t...13&m=557101943
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  7. #7
    Nice work cosmokart! Very nicely conducted and organised. Looking at the implications of your test data, this bug/error is not as trivial as I thought.

    A comment on the temperature data. I presume those were calculated(since I did not see a thermometer anywhere in BI-1 cockpit ) However we don't know if temperature changes are even modelled. But even if the speed of sound in the game remains the same through all altitudes, the test results still indicate an increase in mach number.
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  8. #8
    clint-ruin's Avatar Senior Member
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    Very nice testing work Cosmokart, good to see.


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  9. #9
    <BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by KrasniyYastreb:
    Nice work cosmokart! Very nicely conducted and organised. Looking at the implications of your test data, this bug/error is not as trivial as I thought.

    A comment on the temperature data. I presume those were calculated(since I did not see a thermometer anywhere in BI-1 cockpit ) However we don't know if temperature changes are even modelled. But even if the speed of sound in the game remains the same through all altitudes, the test results still indicate an increase in mach number.<HR></BLOCKQUOTE>

    Thanks, hope the tests prove to be helpful

    About temperature - the He-111 has an analog thermometer in the cockpit. Look straight up, and you can find it on the overhead panel next to the fuel gauges.

    As far as I can tell, temperature absolutely affects indicated airspeed reading. I'm always careful to check the outside air temp before dialing up my TAS in the He-111. (By the way, cardboard flight computers are very handy!)

    Temperature *may* also effect aircraft engine performance and cooling as well, but those effects might instead be more simply linked to the map, rather than the local temperature at any particular altitude.

    Anyway... I'd love to see similar test results for the P-38, since I have a hunch that it too suffers from botched compressibility. However the P-38 can't exceed critical Mach in level flight, which makes flight testing quite a bit trickier. Hopefully if this is indeed a fixable bug, Oleg can identify what's wrong by the BI-1 tests and then apply the same solution globally to all aircraft (including the P-38).
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  10. #10
    tttiger's Avatar Senior Member
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    Gee, while we're on temperatures, why don't we work on the accuracy of density altitude as well? You obviously get more speed on winter maps. That's easy to notice on takeoff runs. But is it too fast or too slow? Is there more lift as well? Too much lift? Too little lift?

    This is getting way beyond picky (OCD gone ballistic)...it is what it is, so just fly the missions and enjoy yourselves

    ttt

    "I want the one that kills the best with the least amount of risk to me"

    -- Chuck Yeager describing "The Best Airplane."
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