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Holtzauge
02-22-2007, 12:48 PM
Are there any plans to model compressibilty effects in PF? Or failing that, what are the plans for BoB in this area?

It would be great if this could be considered. Personally, I'm very satisfied with the graphics as they are in PF and if it is a question of one or the other I would vote for a more realistic flight model in BoB that incorporates compressibility and mach effects rather than improved graphics.

Below is a comparison of dive performance for a Fw190 A8 done with a C++ simulation code I have hacked together as compared with flown values in PF V4.04.

The C++ program models, amongst other things, subsonic drag rise due to compressibility and propeller efficiency at different heights and speeds. I have chosen the Fw190A8 because there is a lot of data on max speeds, climb rates etc around that can be used to calibrate the modeling in the code. I have used 1.42 ata boost and a weight of 4300 Kg for the A8 in the simulations. This gave me a top speed of about 545 Km/h at sea level and 643 Km/h at 6.2 Km so I believe I am pretty close in the modeling. Climb rates also agree well with original data from Focke-Wulf.

As can be seen, there is a large degree of agreement between the PF and C++ simulated IAS from 250 Km/h up to about 500 Km/h. At 500 Km/h at 8-8.5 Km altitude the Mach reaches 0.68 and there should be a substantial increase in drag due to compressibility effects as the C++simulation shows.

However, PF does not seem to model this. I did a test where I removed the drag rise in the C++ code but left in the decrease in propeller efficiency due to high prop tip speeds. The tip speeds reach around M=1.1 in the C++ simulation. This is shown by the dark blue "C++ no mach" curve. If this is compared with the PF IAS curve it seems to indicate that the loss of propeller efficiency at high speeds in not modellled in PF either.

I have also included the max allowable IAS from the Fw190 Bedienungsvorschrift LDvT2190 A-1 bis A-8 for the altitudes 9,7,5 and 4 Km altitudes correlated to the green PF flown altitude axis. As can be seen, the PF IAS values are way above the figures allowed by the Fw-190 Pilots Operting Handbook.

My conclusions from comparing the curves below is that compressibility effects are not modelled and there seems to be no or to low loss of propeller efficiency at high speeds.

Are there any plans to adress these issues in PF or BoB?

http://img.photobucket.com/albums/v210/Holtzauge/Fw190A8divePA1.jpg

Holtzauge
02-22-2007, 12:48 PM
Are there any plans to model compressibilty effects in PF? Or failing that, what are the plans for BoB in this area?

It would be great if this could be considered. Personally, I'm very satisfied with the graphics as they are in PF and if it is a question of one or the other I would vote for a more realistic flight model in BoB that incorporates compressibility and mach effects rather than improved graphics.

Below is a comparison of dive performance for a Fw190 A8 done with a C++ simulation code I have hacked together as compared with flown values in PF V4.04.

The C++ program models, amongst other things, subsonic drag rise due to compressibility and propeller efficiency at different heights and speeds. I have chosen the Fw190A8 because there is a lot of data on max speeds, climb rates etc around that can be used to calibrate the modeling in the code. I have used 1.42 ata boost and a weight of 4300 Kg for the A8 in the simulations. This gave me a top speed of about 545 Km/h at sea level and 643 Km/h at 6.2 Km so I believe I am pretty close in the modeling. Climb rates also agree well with original data from Focke-Wulf.

As can be seen, there is a large degree of agreement between the PF and C++ simulated IAS from 250 Km/h up to about 500 Km/h. At 500 Km/h at 8-8.5 Km altitude the Mach reaches 0.68 and there should be a substantial increase in drag due to compressibility effects as the C++simulation shows.

However, PF does not seem to model this. I did a test where I removed the drag rise in the C++ code but left in the decrease in propeller efficiency due to high prop tip speeds. The tip speeds reach around M=1.1 in the C++ simulation. This is shown by the dark blue "C++ no mach" curve. If this is compared with the PF IAS curve it seems to indicate that the loss of propeller efficiency at high speeds in not modellled in PF either.

I have also included the max allowable IAS from the Fw190 Bedienungsvorschrift LDvT2190 A-1 bis A-8 for the altitudes 9,7,5 and 4 Km altitudes correlated to the green PF flown altitude axis. As can be seen, the PF IAS values are way above the figures allowed by the Fw-190 Pilots Operting Handbook.

My conclusions from comparing the curves below is that compressibility effects are not modelled and there seems to be no or to low loss of propeller efficiency at high speeds.

Are there any plans to adress these issues in PF or BoB?

http://img.photobucket.com/albums/v210/Holtzauge/Fw190A8divePA1.jpg

Viper2005_
02-22-2007, 12:59 PM
http://forums.ubi.com/images/smilies/agreepost.gif Excellent work! http://forums.ubi.com/images/smilies/agreepost.gif

In my experience this problem applies across the board, and the present modelling is especially inaccurate with regard to the P-38, which suffers control problems at low level even at very low Mach numbers, and yet seems to have little trouble at high altitude even at quite high Mach numbers.

I doubt this will be fixed in FB, but I certainly hope that BoB gets realistic compressibility effects!

Holtzauge
02-22-2007, 01:29 PM
Thanks for the positive feedback Viper, I spent a lot of hours hacking the code (1600+ lines counting comments) so some kind words are always welcome http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

I too have my doubts that these issues with compressibility will be fixed but seeing that PF is such a good sim I want to give the feedback anyway and maybe Oleg and his team will consider including it in the future. I think it would be great if they did.

However, in the end, I guess someone is finazing all this and want's to get returns on their investment. There is probably more of a demand for better graphics than there is for improved FM from the silent majority who puts food on the table for everyone at Ubisoft.....

JG4_Helofly
02-22-2007, 06:52 PM
I realy hope that BOB will have things like compressibility in it's FM. At higher speeds it become extremly important like it is shown on the graph.

Please Oleg don't forget such important things.

CMHQ_Rikimaru
02-23-2007, 09:38 AM
http://forums.ubi.com/images/smilies/agreepost.gif

Viper2005_
02-23-2007, 09:56 AM
Having looked more closely, I think that the model presented may be a little extreme, since 800 km/h TAS at 6500 m is only Mach 0.70.

I would expect the 190A8 to reach at least Mach 0.75 (its reported tactical Mach number according to Eric Brown in "Wings on my Sleeve") under these conditions, and probably faster - perhaps as high as Mach 0.80 given that this is a simple dive test, and the the only demand placed upon the control system is holding a constant dive angle.

Holtzauge
02-23-2007, 12:26 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Viper2005_:
Having looked more closely, I think that the model presented may be a little extreme, since 800 km/h TAS at 6500 m is only Mach 0.70.

I would expect the 190A8 to reach at least Mach 0.75 (its reported tactical Mach number according to Eric Brown in "Wings on my Sleeve") under these conditions, and probably faster - perhaps as high as Mach 0.80 given that this is a simple dive test, and the the only demand placed upon the control system is holding a constant dive angle. </div></BLOCKQUOTE>

Viper, the dive I modelled is at a quite moderate angle: only 45 degrees. I think M=0.7 is quite respectable at this angle. If I increase the dive angle to 87 degrees I get M=0.77 at 7.5 Km altitude which should be closer to the terminal value for the Fw 190 A. Lower down the mach in the simulation goes down due to the higher density of the air and the increase in speed of sound.

Unfortunately, I lack drag rise data on the Fw 190 so I actually modelled the Fw 190 drag rise data based on the P-51 drag rise data from NACA report 916 for want of better input. I will update the model when/if I get hold of better drag rise data on the Fw 190. On the other hand my point with this post is not to argue if Fw 190 terminal mach is 0.77 or 0.8 but rather to address aspects of the modelling in PF that are obviously lacking.

Right now the planes seem to be blissfully oblivious of compressibility but when Oleg has introduced compressibility into the PF FM I look forward to interesting discussions based on tracks exploring the terminal machs of P47, P51,P38 and Me109 etc. http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

Viper2005_
02-23-2007, 12:45 PM
No, 45º is a pretty steep dive - here is R/L data for a Spitfire in a 45º dive, reaching a peak Mach number of 0.89:

http://www.spitfireperformance.com/sd2011.jpg

Holtzauge
02-23-2007, 02:31 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Viper2005_:
No, 45º is a pretty steep dive - here is R/L data for a Spitfire in a 45º dive, reaching a peak Mach number of 0.89:

http://www.spitfireperformance.com/sd2011.jpg </div></BLOCKQUOTE>

Viper:
1) The Spitfire dive starts at almost 40000 ft not 10000m
2) The Spitfire has a 2 stage supercharger giving high output power at high alt at the beginning of the dive when it counts and before it reaches altitudes where the increase in air density will slow it down. To reach high mach you need to do it up high, above 7 Km. Notice that the Spitfire reaches highest mach at about 29000 ft.
3) The BMW801 engine is reknowned for having low power at high alt, i.e. there is not much other than gravity pushing the Fw 190 down at high alt
4) The Spitfire is well known for it's high terminal mach due to the very thin wing section used.
5) The Spitfire is already at M=0.84 at 10000 m. The Fw simulation starts here at M=0.4.

At the me 109 lair web site there is a doc with Me109G dive tests results from 10.7 Km in which the dive angle is given at 70-80 degrees and the max mach is 0.8. Also, remember that the Fw 190 A8 has an ETC bomb rack to drag around. So until I see some solid data on the Fw 190 that can be used to calibrate where the drag rise actually occurs I would lean more towards the Me 109G figure than that for the Spitfire starting at 40000 ft with a 2 stage supercharged engine pulling it earthwards http://forums.ubi.com/images/smilies/16x16_smiley-wink.gif

Viper2005_
02-23-2007, 04:21 PM
I'm not expecting the Fw190 to reach Mach 0.89, but I think that the current assumption of Mach 0.70 may be a little pessimistic - I would expect it to be capable of reaching its tactical Mach number of 0.75 in a 45º dive from 10 km, but that's just opinion. I agree that starting at 10 km puts your 190 at a considerable disadvantage vs the Spitfire data I posted.

What happens if you fly your 190 simulation along the same profile as the Spitfire?

GR142-Pipper
02-23-2007, 09:19 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Viper2005_:
http://forums.ubi.com/images/smilies/agreepost.gif Excellent work! http://forums.ubi.com/images/smilies/agreepost.gif

In my experience this problem applies across the board, and the present modelling is especially inaccurate with regard to the P-38, which suffers control problems at low level even at very low Mach numbers, and yet seems to have little trouble at high altitude even at quite high Mach numbers.

I doubt this will be fixed in FB, but I certainly hope that BoB gets realistic compressibility effects! </div></BLOCKQUOTE>As a small tangent in this thread, IMO it would be an improvement to simply remove the P-38's compressability code because it's pressently grossly inaccurate (i.e. there wasn't any comnpressability in the real world at this altitude and below) and the clear majority of the in-game fighting is done at 20,000 feet and below. Simply remove the code until it can be done in a more accurate manner.

GR142-Pipper

Viper2005_
02-23-2007, 11:03 PM
IMO since the game cannot realistically deal with compressibility effects the P-38 should be allowed to fly like all the other aeroplanes in game.

Actually, since Mach number is a very simple function of TAS an air temperature I am surprised that IL2 does not model its effects more accurately...

Holtzauge
02-24-2007, 05:23 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Viper2005_:
I'm not expecting the Fw190 to reach Mach 0.89, but I think that the current assumption of Mach 0.70 may be a little pessimistic - I would expect it to be capable of reaching its tactical Mach number of 0.75 in a 45º dive from 10 km, but that's just opinion. I agree that starting at 10 km puts your 190 at a considerable disadvantage vs the Spitfire data I posted.

What happens if you fly your 190 simulation along the same profile as the Spitfire? </div></BLOCKQUOTE>

I tried to run the Fw 190 C++ simulation from 12.2 Km altitude but the code just hangs. Probably has to do with that I am starting the run from an altitude above the A8's ceiling of 10.5 Km. Anyway, as I said before, I do not have the detailed drag rise data on the Fw 190 so I made an assumption based on P-51 data. It may well be so that your guess of M=0.75 is right. Since the drag rise reassembles a virtual wall it all has to do with where the drag rise is placed on the mach axis. Lacking accurate data it's difficult to place this within M=0.05.

However, as I said before, my point with the original post was to highlight that compressibility effects are missing in the PF code and that I would like to make push for getting this in.

Holtzauge
02-24-2007, 05:46 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by GR142-Pipper:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Viper2005_:
http://forums.ubi.com/images/smilies/agreepost.gif Excellent work! http://forums.ubi.com/images/smilies/agreepost.gif

In my experience this problem applies across the board, and the present modelling is especially inaccurate with regard to the P-38, which suffers control problems at low level even at very low Mach numbers, and yet seems to have little trouble at high altitude even at quite high Mach numbers.

I doubt this will be fixed in FB, but I certainly hope that BoB gets realistic compressibility effects! </div></BLOCKQUOTE>As a small tangent in this thread, IMO it would be an improvement to simply remove the P-38's compressability code because it's pressently grossly inaccurate (i.e. there wasn't any comnpressability in the real world at this altitude and below) and the clear majority of the in-game fighting is done at 20,000 feet and below. Simply remove the code until it can be done in a more accurate manner.

GR142-Pipper </div></BLOCKQUOTE>

I agree completely. PF should begin with modelling the drag rise and propeller efficiency effects at high speed. These are the first order effects of compressibility. Modelling stick force and controllability due to compressibility should come after that. In addition, if one starts to tamper with controllability it's important to get it right.

JG14_Josf
02-26-2007, 08:23 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">These are the first order effects of compressibility. </div></BLOCKQUOTE>

What order is the reduction in CLmax caused by compressibility effect?

Parabolic curve (http://www.flightlab.net/pdf/8_Maneuvering.pdf)

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At least you can for speeds to about Mach 0.3. Above that, compressibility effects take over, CLmax declines, and the slope of the curve decreases. </div></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At speeds above roughly Mach 0.3, CLmax begins to decrease. Mach number depends on altitude, so indicated Va increases with altitude because you have to go faster to generate equivalent lift at the lower CLmax. </div></BLOCKQUOTE>

Compressibility and incompressibility (http://www.centennialofflight.gov/essay/Dictionary/Compressibility/DI136.htm)

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Generally, for theoretical and experimental purposes, gases are assumed to be incompressible when they are moving at low speeds--under approximately 220 miles per hour. The motion of the object traveling through the air at such speed does not affect the density of the air. This assumption has been useful in aerodynamics when studying the behavior of air in relation to airfoils and other objects moving through the air at slower speeds.
</div></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">However, when aircraft began traveling faster than 220 miles per hour, assumptions regarding the air through which they flew that were true at slower speeds were no longer valid. At high speeds some of the energy of the quickly moving aircraft goes into compressing the fluid (the air) and changing its density. The air at higher altitudes where these aircraft fly also has lower density than air nearer to the Earth's surface. The airflow is now compressible, and aerodynamic theories have had to reflect this. Aerodynamic theories relating to compressible airflow characteristics and behavior are considerably more complex than theories relating to incompressible airflow. The noted aerodynamicist of the early 20th century, Ludwig Prandtl, contributed the Prandtl-Glaubert rule for subsonic airflow to describe the compressibility effects of air at high speeds. </div></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At lower altitudes, air has a higher density and is considered incompressible for theoretical and experimental purposes. </div></BLOCKQUOTE>

http://www.tfd.chalmers.se/~ulfh/gas_dyn_h/lecture_notes/14-perturbation/img014.gif

Prandtl-Glauert rule (http://www.tfd.chalmers.se/%7Eulfh/gas_dyn_h/lecture_notes/14-perturbation/sld005.htm)

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem. Perturbation theory is applicable if the problem at hand can be formulated by adding a "small" term to the mathematical description of the exactly solvable problem. </div></BLOCKQUOTE>

What is the higher order of compressibility effect?

Is it this:

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At high speeds some of the energy of the quickly moving aircraft goes into compressing the fluid (the air) and changing its density. </div></BLOCKQUOTE>

Does that occur above 220 (Mach .03) slightly and gains effect' requiring more energy to compress air and resulting in a very large energy cost required to compress more air to higher pressure?

What altitude is considered low altitude where air is no longer considered compressible?

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At lower altitudes, air has a higher density and is considered incompressible for theoretical and experimental purposes. </div></BLOCKQUOTE>

Isn't that a simple function of Density Altitude?

Deviation from Newtonian hydrodynamnics (http://72.14.253.104/search?q=cache:w7kr70BgVjcJ:www.aethernitatis.net/articles/fluidic/A_Case_for_a_Fluid_Substrate_-_04_14_2006.pdf+Prandtl-Glauert+rule+density+altitude+incompressibility&hl=en&ct=clnk&cd=1&gl=us)

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">the inverse square root factor represents the extra resistance due to high speeds that is not accounted in the incompressible scheme </div></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">At this point of the discussion, it should not be shocking to the reader to find out that there are indeed several other cases apart from the "mass increase" equation where the abstract mathematical relations that are part of mainstream physics are seemingly modelling a real, physical, fluid sub-atomic substrate, a medium that gives a meta-physical causal structure to our relatively abstract concept of space. Indeed, in an era of science where the causal modelling of physical reality has been entirely neglected while the affinity to the appealing power of numerical predictions via curve-fitted quantitative relations51has been progressively increasing to a state where unexplained "Laws", "Principles" and empty empirically-tuned mathematical formalisms are indeed most of what is left at the core of current physics, it is reasonable to expect some of these relations to actually quantify the physically existent fluid that the previous parts of this article evidenced as real, even if orthodoxy fails to acknowledge any relation at all. As it was previously explained, this stems from the fact that while there are plenty of people who can do the math, they are incapable of understanding what the math means since there is no qualitative correlation with sensorial imagination; a connection which is absolutely necessary to make sense of the model. </div></BLOCKQUOTE>

Well...I just want to know the altitude at which energy begins to be used up compressing air above .3 Mach., and, if that is a higher order effect.

One other question: Will the higher mass airplane have more energy to burn in the process of compressing air?

JG14_Josf
03-09-2007, 09:31 PM
&lt;BUMP&gt;