PDA

View Full Version : Compressability



Daiichidoku
08-08-2005, 08:23 AM
hello, just a simple question


why is the P 38 the ONLY plane in FB to feel the effects of compressability?

please remove this from the P 38 or add it to every plane that can achieve the neccesary velocity to enable this event

i can dive a P 47, P 63, 190 D9 etc to over 900kph, from 15,000 down to under 5000 with NO compressability effects...WHY, please?

P 47 D30s were fitted with dive braves...WHY?
i certainly dont get compressed diving the Jug

i would like to know, please, Oleg, why this is, and if there will be anybody addressing this issue, to kill P 38 compress., or apply it to ALL other types

Kocur_
08-08-2005, 09:05 AM
AFAIK P-38 didnt really suffer from compressibility in general, assuming I understand it correctly as all the negative phenomenons related to high pressure of air flowing airframe at high Mach numbers.

AFAIK P-38 problem was that very locally on its wing airflow was very fast, much faster than it would seem basing on entire planes speed. Still not the compressability in general was the problem, but fact that point of center of lift moved forwards causing nose to be not liftable, beyound certain speed.
Dive flaps werent air brakes but surfaces that disturbed airflow and thus moved point of center of lift backwards.
This locally faster airflow over wing to cause above effect, was unique feature of P-38 design, among other WW2 fighters AFAIK.

Daiichidoku
08-08-2005, 01:47 PM
Originally posted by Kocur_:
AFAIK P-38 didnt really suffer from compressibility in general, assuming I understand it correctly as all the negative phenomenons related to high pressure of air flowing airframe at high Mach numbers.

AFAIK P-38 problem was that very locally on its wing airflow was very fast, much faster than it would seem basing on entire planes speed. Still not the compressability in general was the problem, but fact that point of center of lift moved forwards causing nose to be not liftable, beyound certain speed.
Dive flaps werent air brakes but surfaces that disturbed airflow and thus moved point of center of lift backwards.
This locally faster airflow over wing to cause above effect, was unique feature of P-38 design, among other WW2 fighters AFAIK.


and this happened when diving a P 38 starting from below 20,000ft?

p1ngu666
08-08-2005, 02:09 PM
Originally posted by Kocur_:
AFAIK P-38 didnt really suffer from compressibility in general, assuming I understand it correctly as all the negative phenomenons related to high pressure of air flowing airframe at high Mach numbers.

AFAIK P-38 problem was that very locally on its wing airflow was very fast, much faster than it would seem basing on entire planes speed. Still not the compressability in general was the problem, but fact that point of center of lift moved forwards causing nose to be not liftable, beyound certain speed.
Dive flaps werent air brakes but surfaces that disturbed airflow and thus moved point of center of lift backwards.
This locally faster airflow over wing to cause above effect, was unique feature of P-38 design, among other WW2 fighters AFAIK.

thought centre of lift went back, thus pitching u nose down?

Kocur_
08-08-2005, 02:19 PM
thought centre of lift went back, thus pitching u nose down?

Arrghh! http://forums.ubi.com/images/smilies/35.gif
My mistake! Of course I meant that at high speed centre of lift moved backwards at high speed.


and this happened when diving a P 38 starting from below 20,000ft?

AFAIK any alt. It all depends on speed.

249th_Harrier
08-08-2005, 04:56 PM
Compressibility is when local airflow over any part of the aircraft reaches the speed of sound. This causes a shock wave to form which can interrupt the normal function of control surfaces. It occurs on any aircraft. P-38 had a very low critical mach, it reached compressiblity at 0.61 x speed of sound. By comparison, p-51 critical mach is 0.8 x speed of sound.

NonWonderDog
08-08-2005, 09:55 PM
I hate that confusing bastardized definition of "compressibility." It just makes no sense. Compressibility is the fractional change in density per unit pressure, equal to 1/K. Any other definition does nothing but confuse the engineers. You don't want to confuse the engineers, they're the reason you're up in the air in the first place.

Compressibility effects are due to the ability of air to be compressed (imagine that!). They're not really distict effects at all, just excuses to make up for the fact that simple aerodynamics is all based on a lie. At low speeds, this is OK; we can safely pretend that air is incompressible and flows nicely around airfoils and gives us nice little equations to predict flight. But above about 200 mph you have to fudge a little to keep the equations correct. To make matters worse you start to get shockwaves when you get close to mach 1 ("close" depending on the plane), which really throws everything off. Centers of lift change, you get wave drag, maximum lift is dramatically different, yada yada yada.

Then of course the viscosity of air has to play its part. That's where it gets really confusing. Airflow doesn't really follow the shape of the plane, it follows the shape of the boundary layer. The boundary layer isn't necessarily the same shape as the plane, its shape changes as viscosity changes, and it can become turbulent, to boot.

So saying "compressibility is only present on one plane" makes no sense whatsoever. Maybe only one plane has canned critical mach effects, but compressibility is either modelled or not. If it's not, the sim will never be accurate at medium to high speeds or at high altitudes, although it might be close.

....it's probably not modelled in much detail.

Kocur_
08-08-2005, 10:03 PM
Thx!
What took you so long? http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Daiichidoku
08-09-2005, 08:20 AM
ok then


Oleg, would you please nix the "canned critical mach effects" that are placed upon the p 38, or apply them to all types that can achieve this....and all with the proper alt and speed parameters

Hetzer_II
08-09-2005, 09:11 AM
http://forums.ubi.com/images/smilies/cry.gif

JG5_UnKle
08-09-2005, 11:02 AM
<cough> Bi-1

Daiichidoku
08-09-2005, 02:32 PM
Originally posted by JG5_UnKle:
<cough> Bi-1

ah, thank you...i stand corrected

i forgot about that silly rocket...compress. CERTAINLY IS on that type...

but cmon, i can dive a Jug over 1000kph no problem, no compress.

Kocur_
08-09-2005, 02:43 PM
Its because over certain part of P-38 wing airflow was significantly faster than planes speed. Each plane wings are different, arent they...Im affraid its was unique P-38 feature http://forums.ubi.com/images/smilies/16x16_smiley-sad.gif

p1ngu666
08-09-2005, 07:09 PM
think it was making the wing thicker, to hold more fuel that gave the p38 more compressability that most others.

also at lower alt the air is thicker, so u have more drag, so u may not be able to get into the compressability with certain planes...

some planes just get very stiff at high speeds, yak9D got stiff at 450kph, @ 6000-7000metres ingame...
my odd coop, if u was wonderin what i was doing that high in a yak http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Kocur_
08-09-2005, 10:00 PM
Umm seems to me that controls stiffness is a different issue, not really related to compressability at high mach number. I mean controls get stiffer simply because faster airflow mean that the greater forces on control surfaces counteract pilots force on stick/pedals.

249th_Harrier
08-10-2005, 02:12 PM
Originally posted by p1ngu666:
think it was making the wing thicker, to hold more fuel that gave the p38 more compressability that most others.

also at lower alt the air is thicker, so u have more drag, so u may not be able to get into the compressability with certain planes...

some planes just get very stiff at high speeds, yak9D got stiff at 450kph, @ 6000-7000metres ingame...
my odd coop, if u was wonderin what i was doing that high in a yak http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Typhoon also had compressibility issues due to thick wing (it used the same wing as the Hurricane) and high speeds at high altitude. Tempest solved this problem with thinner wing. P-38 always had the same wing, fuel capacity was increased by relocating the turbo intercoolers. Compressibility is a worse problem at high altitudes because the speed of sound is slower at higher altitudes. That is why the key metric for compressibility is critical mach. P-38: 0.61 vs 0.8 for p-51. I suspect the Me-262 had the best critical mach due to swept wing design...

Daiichidoku
08-10-2005, 04:18 PM
the p 38 shouldnt be able to reach its critical mach at under 20,000 ft, or so i believe...one can dive a 38 from 12000 ft in FB and compress...silly!

berg417448
08-10-2005, 05:17 PM
Originally posted by 249th_Harrier:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by p1ngu666:
think it was making the wing thicker, to hold more fuel that gave the p38 more compressability that most others.

also at lower alt the air is thicker, so u have more drag, so u may not be able to get into the compressability with certain planes...

some planes just get very stiff at high speeds, yak9D got stiff at 450kph, @ 6000-7000metres ingame...
my odd coop, if u was wonderin what i was doing that high in a yak http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Typhoon also had compressibility issues due to thick wing (it used the same wing as the Hurricane) and high speeds at high altitude. Tempest solved this problem with thinner wing. P-38 always had the same wing, fuel capacity was increased by relocating the turbo intercoolers. Compressibility is a worse problem at high altitudes because the speed of sound is slower at higher altitudes. That is why the key metric for compressibility is critical mach. P-38: 0.61 vs 0.8 for p-51. I suspect the Me-262 had the best critical mach due to swept wing design... </div></BLOCKQUOTE>

The sources I've checked report that the Me-262 critical mach was .86 or .87.