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KrasniyYastreb
04-12-2004, 03:11 PM
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.]

KrasniyYastreb
04-12-2004, 03:11 PM
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.]

04-12-2004, 03:21 PM
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.

pinche_bolillo
04-12-2004, 05:46 PM
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.

04-12-2004, 07:15 PM
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>

04-12-2004, 07:29 PM
Bug has been written up and mailed to Oleg. http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif

I suggest following the results in ORR,
http://forums.ubi.com/eve/forums?a=tpc&s=400102&f=63110913&m=557101943

KrasniyYastreb
04-12-2004, 10:56 PM
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 http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif) 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.

clint-ruin
04-13-2004, 12:26 AM
Very nice testing work Cosmokart, good to see.

http://users.bigpond.net.au/gwen/fb/leninkoba.jpg

04-13-2004, 01:13 AM
<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 http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif) 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 http://ubbxforums.ubi.com/infopop/emoticons/icon_smile.gif

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).

tttiger
04-13-2004, 01:53 AM
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 http://ubbxforums.ubi.com/infopop/emoticons/icon_biggrin.gif

ttt

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

-- Chuck Yeager describing "The Best Airplane."

04-13-2004, 03:30 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by tttiger:
This is getting way beyond picky (OCD gone ballistic)...<HR></BLOCKQUOTE>

You are entitled to your opinion, but the BI-1 compressibility bug is hardly "picky". If you don't understand all this, just leave it alone. We don't need ignorant comments, thank you.

The BI-1 begins to encounter compressibility at 0.62 Mach at sealevel, and at 0.84 Mach at 5000 meters. Something is very wrong with the flight model, and it probably affects other aircraft as well.

Loco-S
04-13-2004, 10:34 PM
Interesting, vintage info passed to American Pilots on WW2....http://ubbxforums.ubi.com/images/smiley/10.gif
http://mywebpage.netscape.com/kurbalaganda/PIF1944-compressibility.jpg

http://mywebpage.netscape.com/kurbalaganda/Loco-S.gif
Armis Bela, non venenis geri

04-14-2004, 02:47 AM
Excellent find, Loco.

The problem with the current FM is that compressibility seems to vary with IAS, rather than Mach number.

M_crit does not vary with density. For instance if a particular aircraft encounters "mach tuck" at 0.75 Mach at sea level, it will have the same experience at 0.75 Mach at 8000 meters.

Loco-S
04-14-2004, 06:31 AM
here I have a couple of PDF files to standardize the testing of In game AC, and a nice reading, enjoy:

http://mywebpage.netscape.com/kurbalaganda/ac90-89a-Amateur+testing+of+Aircraft.pdf

http://mywebpage.netscape.com/kurbalaganda/High+Altitude+flight.pdf

http://mywebpage.netscape.com/kurbalaganda/Loco-S.gif
Armis Bela, non venenis geri