Compressibility modelling
 

When diving under power aircraft speeds should be limited by compressibility effects. If not then aircraft that utilize vertical tactics like BnZ will get an unfair advantage over aircraft that employ angles tactics.

If airspeeds in the dive are not limited by compressibility, then the energy retention will be too high and an aircraft doing a BnZ attack will end up at to high altitude following a dive and zoom.

I don't know how/if this is modelled in the sim but I think this could be tested implicitly by comparing airspeeds from a test dive in the sim with the attached C++ simulation chart. The chart contains two graphs:

One is for the Spitfire Mk1 at +6.25 boost with compressibility modelled, both in terms of compressibility drag rise and also reduction in propeller efficiency due to Mach effects. The other has no compressibility correction whatsoever, either on drag rise or propeller efficiency. Both start from 23,000 ft altitude and 336 mph TAS.

In IL2 there was something called "devicelink" where one could extract this type of info and do a comparison. Don't know if this or something similar exists here. An alternative is of course simply to test dive and read off the speed at 2.5 Km altitude 61 s into the dive and see how this compares with the sim. If there is no way to read out TAS in the sim then I suppose one could convert the IAS cockpit reading if this is reliable enough.

I don't have the hardware to run this sim so I cannot test myself but I'm curious to know how the sim handles high speed dives.

OH MY GOD! Please stop to talk about unfair advantages in this game!!!

Aircrafts don't employ angle tactics: the pilots do. Nothing prohibits pilots to BnZ in an Hurricane... except their own noobness

It's just too difficult to track planes under your ship and too easy to spot aircrafts over you head... and this is really helpful for guys who don't care about tactics...
But NO... lets put in disadvantage the ones who actually do fly in the smart way!!!



Anyway I agree about compressibility modelling, but it's not a priority at all.

But when it comes to unfair advantages, I guess you want the same for angles fighters as well right? Otherwize there could be a Spitfire with a turn time of 21 s at 20,000 ft if Crumpp has his way :)

The Spit dive is just an example: The Me109 C++ simulation looks just the same. If no compressibility then too high speed in the dive and I assume everyone wants as close to historical performance as possible right?

I understand that there are more pressing mods to be done. While I have not simulated the Hurricane, the Spitfire top speed numbers looks way to low. I get 280 mph for the Spitfire for +6.25 boost and about 475-480 Km/h for the Me109 at SL 1.3 ata and I guess that would be a higher priority to fix.

First, I want to point out that my rant was referred to the "aircraft that employ angles tactics" statement. I go nuts when I read something similar...

Without a realistic environment around the planes it's dangerous to model some aspects that would totally ruin the combat aspect of the simulator.

Since, you know, there are guys that actually try to simulate combat tactics... if these are not practicable then what's the reason to fly this sim? For its "not really complex" engine management? To enjoy the historical speed of the plane at altitude ("Look I'm reaching the same speed of the real plane, best moment in my life!!!")?

It's just bad enough that fatigue is not modelled and every virtual pilot can sustain high G accelerations for dozens of minutes...

If the guys who follow the main rule of warbird dogfighting ("Altitude Is Life") can't even gain advantage from that (BnZ) then be sure the game will die very quickly. 1C don't even need to model altitude over the 10K feet since the "BnZ is not allowed" rule found in some IL2 servers would not be forced by those loser admins, but by the game itself.

Without a realistic world around the virtual pilot the single aircraft performance becomes far more important than the player own skill... infact in these weeks there are too much "X is too slow by 20km/h; we can't fight!!!" threads for my taste.

Can't wait the day a WW2 sim could actually simulate a realistic aircraft/engine dynamic quality (wearing)...

I'm not arguing to introduce something to upset the balance of gameplay.
I used to fly IL2 a lot back when I had more time and in that sim there were things that were off that really affected gameplay. I think the elevator authority on the Me109 was one example. IIRC this was off in IL2 in the sense that it seemed more connected to TAS than IAS.

I still think it would be good to get everything on the table (also the stuff you mentioned) but compressibility is one component there as well. As I said, I guess there are more pressing fixes to do to get the balance in gameplay right but that does not lessen my curiosity as to how the currently modelled planes behave in high speed dives :)

I want most things modelled too. Compressibility is one of these, of course.

And I don't want an arbitrary game balance: do I have to fly in a crap plane? I do it and I'll try to fly it in the historical way. (for example I16 vs 109).

The thing I don't want is implementing something that heavily affects the combat simulation over more important things.

Because in real life "Good Tactic" >>>>> "Raw Performance"...

Anyway I don't think the elevator authority issue was about TAS or IAS: it was modelled like a loss of control surface effectiveness instead of a simple stick heavyness... in the game it was impossible to operate the stick with the strength of 2 arms like the real pilots did... they roughly modelled a pilot's strength related aspect without taking in account the pilot's stamina/fatigue modelling.

I hope it will be possible in CloD's future.

Holtz. I don't know exactly if your saying Airspeed should be limited by compressibility, or if your saying at which Compressibility actually becomes noticeable to causing a runaway dive / increasing speed / decreasing control.

Or are you meaning loss in engine performance? Cause your right, its going to decrease performance, but in a dive, your still going to increasing speed from the simple physics of gravity, only acceleration would be affected. But it won't act like a "Dive Break," that's how I see you trying to explain it.

Thats pretty much correct. TAS / IAS really isn't the issue. From my understanding (8 years real pilot, however no real experience in this area other than study, and never will because I'm not a test pilot and testing this can be suicide) TAS / IAS compressibility affects don't slow down the aircraft / reducing speed of dive capabilities.

The real key to compressibility stall is the atmospheric conditions via what altitude your at. Higher alt= less dense air. I say this from the fact that most stories I've read about WW2 compressibility stalls, they always start out with the pilots typically above 25,000 ft. They get into a dive, and as they go faster the laminar flow of the high alt / less dense air around the wings and control areas is so reduced to the point that the pilot looses control / has a runaway dive. At which point the only thing they can do is chop power, but your speed is still going to increase because your diving.

Your only able to regain control of the aircraft when it decreases down to denser air density returning laminar flow to the aerodynamics of the aircraft. However by this time your either A) Going so fast and theres no return of laminar flow that controlability doesnt return. B) The aircraft begins or already has broken up from structure failure of approaching the speed of sound.

There is one story I remember reading about Robin Olds, in a P-38. Started a dive at bomber escort alt (think 30,000 range) Got into a compressibility stall and literally didn't pull out of it till a few 100 ft above the ground. The P-38 is one of the most notorious WW2 aircraft with this aerodynamic design problem.


But seeing as most aircraft in CLOD at the moment are not modeled correctly above 18,000, I see it as only being a miniscule problem till this is fixed, then worry about compressibility FM problems.

The compressibility issue the simulation shows is limited to the speed aspect and controllability is not modelled. The lower curve takes the compressibility drag rise and propeller efficiency loss due to speed and Mach effects into account while the higher curve assumes constant propeller efficiency and a "flat" Cdo which does not start to rise as the speed goes up. The problem is if compressibility is not modellled then any aircraft in the sim will build up too much speed in a dive meaning it will zoom too high in the pullout. This would work to the advantage of so-called BnZ tactics in the sim since energy retention would be higher than IRL.

Mach effects on compressibility are an interesting but complex subject but the C++ simulation posted does not model any impact on controllability due to Mach effects which is beyond the capabilities of the code which is limited to performance comparisons only. As you point out the P-38 was especially susceptible to this phenomena but the Mach "tuck" effect did affect the other types as well and it was more of a question at which Mach the problems started to appear. However, I think modelling this is even more complex and I think it would be a significant step in the right direction just to get the flight performance aspect of compressibility in to begin with.

While it was some time ago I looked into it, I still believe that the Me109 elevator control modelling in IL2 was off because it modelled loss of control authority more due to TAS than IAS: At low alt you were OK at certain IAS but at high alt you lost authority at the same IAS even though the Mach number was quite low. Since I have not flown the Me109 here in CloD I have no idea how this is modelled. Hopefully it has improved.

This is a good thread.

Manu, I don't believe that holtzauge was implying that using BnZ is unfair; only that aircraft whose performance was limited by compressibility effects would see an unfair advantage, I.e more speed and "e retention" than they would have historically had.

This is why some 1946 servers banned the 190D9, because it was possible to fly it and a few other aircraft through the sound barrier, since compressibility was not modeled.

Holtzauge, it would also be helpful to be able to see the source code for this c++ solver so we can audit it. Otherwise, and no offense intended, from our perspective we have only the word of an anonymous forum poster that the simulation is accurate. Ihope that makes sense.

We do not even know if compressibility is modelled or not. We do not have real world test figures for correct high speed dive behaviour, and a random poster's colored graphs based on cluster of guesswork figures and labeled '109' or 'Spitifre' hardly make a comparison basis.

It makes an interesting theoretical discussion but the whole thread is absolutely useless for checking the FM's validity. Unless someone would suggest the developer's should rely on hobbysts guesstimations of performance.