IAS and Altitude

[Turbot]

I am given to believe that aircraft have certain performance numbers that do not change with altitude - for example Stall and compression speeds, measured in Indicated Air Speed.
 
This has always boithered me in AH (it doesn't happen in WB (least I never had it happen to me) and I have never had it explained:

A p38 is shaking like crazy at only 280 mph IAS at 28k - this same shaking won't occur till 100 mph higher at 18k alt, again IAS.  Being as how indicated airspeed already factors in atmospheric density, why is this so?  

"...the drag force (D) decreases as the air density decreases. However, less drag means that we can fly faster, assuming the aircraft engine delivers the same amount of thrust. ... In summation, the decrease in air density that occurs as an airplane climbs to higher altitudes has three effects: 1) reduces lift, 2) reduces drag, and 3) reduces thrust. The propulsion effect is the most significant, and it is in fact engine performance that limits the maximum altitude that an aircraft can reach. "

http://www.aerospaceweb.org/aircraft/question/atmosphere/q0046b.shtml

Not knowing any better, I looked around on the web for quite sometime. I find nothing anywhere that would explain why compression sets in at lower IAS with altitude in real life.  I am of the mind at this point it is a AH game oddity?

[Zigrat]

temperature
m=sqrt(gamma*r*t)

[Nashwan]

Compression is related to mach speed, which isn't fixed to ias.

Edit: sorry, missed Zigrat's reply.

[Badboy]

Originally posted by Turbot
I am given to believe that aircraft have certain performance numbers that do not change with altitude - for example Stall and compression speeds, measured in Indicated Air Speed.
 
This has always boithered me in AH (it doesn't happen in WB (least I never had it happen to me) and I have never had it explained:

A p38 is shaking like crazy at only 280 mph IAS at 28k - this same shaking won't occur till 100 mph higher at 18k alt, again IAS.  Being as how indicated airspeed already factors in atmospheric density, why is this so?  

"...the drag force (D) decreases as the air density decreases. However, less drag means that we can fly faster, assuming the aircraft engine delivers the same amount of thrust. ... In summation, the decrease in air density that occurs as an airplane climbs to higher altitudes has three effects: 1) reduces lift, 2) reduces drag, and 3) reduces thrust. The propulsion effect is the most significant, and it is in fact engine performance that limits the maximum altitude that an aircraft can reach. "

http://www.aerospaceweb.org/aircraft/question/atmosphere/q0046b.shtml

Not knowing any better, I looked around on the web for quite sometime. I find nothing anywhere that would explain why compression sets in at lower IAS with altitude in real life.  I am of the mind at this point it is a AH game oddity?

Hi Turbot,

I’d like to expand on the responses you have already received, but firstly let me assure you that the behavior you are witnessing in Aces High is correct, and since you have quoted some numbers I’ll also take the opportunity to explain them.

As already pointed out, what you are observing are the effects of compressibility and they are related to Mach number. The speed at which these affects become apparent is known as the critical Mach number and generally varies between about M0.6 and M0.8.

Now the P-38 has a top speed at sea level that is close to M0.44 but at 30,000ft its top speed is closer to M0.6. So, at high altitude, any aircraft flying close to its top speed will be flying much closer to its critical Mach number than it would be at lower altitudes. That’s why these things become more noticeable higher up. Now let’s get specific and take the numbers you cited as a case in point.

You said that at 28k, you noticed the onset of Mach effects at 280mph indicated. Well, at 28k, 280mph is equivalent to Mach 0.645. I took the trouble to look up the airfoil used on the P-38 and it turns out that it had a critical Mach number of 0.647. Those figures are in agreement within about 0.3%, and that is as close as it gets!

Further testimony to the fidelity of the Aces High flight model.

To put that into perspective, you must remember that the aircraft can be safely flown above the critical Mach number if you can tolerate the effects of compressibility. The critical Mach number is where those things first become noticeable. I believe that the real P-38 was safe up to M0.67 and closer to M0.7 with the dive brakes extended. I also have a source that claims that the highest Mach number ever attained by a P-38 was M0.73, but that’s another (contentious) story altogether.

Hope that helps!

Badboy

[Holden McGroin]

Originally posted by Turbot

 
I find nothing anywhere that would explain why compression sets in at lower IAS with altitude in real life.  
 

I was at Black Rock Desert Nevada in '97 to see the British team break the mach barrier in their car, Thrust SSC.  They did 763+ to go M1.01, because BRD is maybe 4000 feet alt..  At sea level, they would have only needed to go appx 680, as air density is the big factor in mach.

[Turbot]

Originally posted by Badboy



Hi Turbot,

I’d like to expand on the responses you have already received, but firstly let me assure you that the behavior you are witnessing in Aces High is correct, and since you have quoted some numbers I’ll also take the opportunity to explain them.

As already pointed out, what you are observing are the effects of compressibility and they are related to Mach number. The speed at which these affects become apparent is known as the critical Mach number and generally varies between about M0.6 and M0.8.

Now the P-38 has a top speed at sea level that is close to M0.44 but at 30,000ft its top speed is closer to M0.6. So, at high altitude, any aircraft flying close to its top speed will be flying much closer to its critical Mach number than it would be at lower altitudes. That’s why these things become more noticeable higher up. Now let’s get specific and take the numbers you cited as a case in point.

You said that at 28k, you noticed the onset of Mach effects at 280mph indicated. Well, at 28k, 280mph is equivalent to Mach 0.645. I took the trouble to look up the airfoil used on the P-38 and it turns out that it had a critical Mach number of 0.647. Those figures are in agreement within about 0.3%, and that is as close as it gets!

Further testimony to the fidelity of the Aces High flight model.

To put that into perspective, you must remember that the aircraft can be safely flown above the critical Mach number if you can tolerate the effects of compressibility. The critical Mach number is where those things first become noticeable. I believe that the real P-38 was safe up to M0.67 and closer to M0.7 with the dive brakes extended. I also have a source that claims that the highest Mach number ever attained by a P-38 was M0.73, but that’s another (contentious) story altogether.

Hope that helps!

Badboy

Thanks for taking the time to look into this in such detail.  I had never heard of critical mach before.

[Turbot]

Originally posted by Holden McGroin


I was at Black Rock Desert Nevada in '97 to see the British team break the mach barrier in their car, Thrust SSC.  They did 763+ to go M1.01, because BRD is maybe 4000 feet alt..  At sea level, they would have only needed to go appx 680, as air density is the big factor in mach.

This is the way I was thinking too.  Higher = faster.  I had not heard of wing designs and the critical mach factor related to them before and this is all way above my head.

[Seeker]

"this is all way above my head"

No it's not; it's just unfamiliar

Mach is the speed of sound. It's used as a reference point because air starts to behave funny at around the speed of sound (why: I dunno! )

Air and water are both fluids with similar behavior; but it's easier to visualise water:

You know how you can slowly draw your hand through water with hardly making a ripple; but if you do it fast you get whirlpools and splashes? That's what's happening to your aeroplane. At about critical mach; those whirlpools and spashes start to move away from the nose and towards the tail of the plane; and that's when the plane starts to misbehave.

Imagine that at "normal" speed there's a big whirlpool coming off the front of the windscreen and hovering reasonably stable just after the cockpit. As the plane gets faster, that whirlpool moves backwards. Seeing as a whirlpool is a vacuum; what happens if the whirlpool moves so far backwards that it sits above the elevators? Firstly, the tail plane gets sucked up into the whirlpool (nose down pitch movement); secondly; because the elevators are now sticking up into a vacuum, and not air; they don't do much (think of the 109's high speed "solid stick").

That's how things like dive flaps and spoilers work; not by making the airplane more streamlined; but by deflecting those whirlpools and spashes away from the control surfaces where they do no harm.

The reason Mach numbers are used is that air's behaviour is constant with regard to the speed of sound. So you can say with confidence that (for example) the P77 can dive safely at Mach 0.99; and be correct regardless of actual speed, indicated speed or altitude. It's how close to the famous "sound barrier" the plane can fly that's the constant; every thing else is a variable.

This is why modern Jets have Mach meters. The pilot doesn't care how fast he is in absolute terms; he wants to know how close to the plane's structural limit he is; and that's governed by Mach; not speed nor altitude.

While the above is technicaly inaccurate as hell; I hope it helps!

[Turbot]

Thanks seeker I am getting more education today.  akak also had a few links that point out specifics regarding the P-38 and this issue.  Before posting I did a search of the archive and there was nothing on this, so for the sake of the archive :

NACA Report 646 Effect of Compression on an airfoil 1939 http://naca.larc.nasa.gov/reports/1939/naca-report-646/
NACA TN-543 The Compressibility bubble http://naca.larc.nasa.gov/reports/1935/naca-tn-543/
Compressibility Error http://142.26.194.131/aerodynamics1/Basics/Page7.html

Compression in P-38 http://www.p-38online.com/cmprs.html


Ack-Ack
479th FG

--------------------------------------------------------------------------------
I pulled out a few comments I found interesting:
--------------------------------------------------------------------------------

"Flight engineers wanted the test pilots to go past 300 mph above 30,000, which was not normally done. Mattern and Bircham declined to perform the tests because they thought the engineers were being too aggressive. "

"If the P-38 was traveling at 500 mph, the airflow over the wing was approaching the speed of sound"

"This problem created many rumors, especially in the ETO (where combat missions were normally above 20,000 ft., which is where compressibility is encountered)"

"The P-38 would be destined to encounter this problem because of the 1930's style of a thick wing to accommodate the amount of fuel needed.  Johnson openly admired the Spitfire's wing design, but as good as it was as a fighter, it did not have the range for long-range escort duties like the P-38. "

"Lockheed designers developed a special kind of flap that would be incorporated into later P-38 designs. Tony LeVier was selected to perform initial testing of this new type of flap. The flaps were supposed to be deployed prior to entering a dive. The flap was not designed to slow the aircraft down, "

It certainly seems like HTC did their homework on this - better than anyone else so far at least in regard to this issue.  It has been a long time since I flew AW or WB/WB3 but this effect was not modeled (to my knowledge and experience flying there)  IN fact the dive flap behaves totally wrong in those other games as it is indeed very much a brake in both of those sims.  

cudo's again to HTC for accuracy in flight model.

[dtango]

guys.  Great explanations!

Tango, XO
412th FS Braunco Mustangs

[Holden McGroin]

Mach is the speed of sound. It's used as a reference point because air starts to behave funny at around the speed of sound (why: I dunno! )

Air and water are both fluids with similar behavior;

[/B]

The biggest difference between the two fluids is compressability.  At slower speeds, the air particles can flow out of the way much like water, but as the velocity gets close to sonic, the air gets more dense, where water is virtually incompressable.  

As this compressing starts, the mach shock wave forms on different parts of the airplane at different times, because the speed of the wind over the surface of the aircraft skin is not constant, varying with the shape of different parts of the a/c (aircraft).

Neglecting the propeller tips for now, shock waves tend to start on the top of the canopy, at wing roots, and anyplace else on the a/c that requires the air molucules to get out of the way of the moving a/c the quickest.

These shock waves, starting at different times and different places were what WW2 a/c designers had yet to understand.  When a shock wave interacted with a control surface, strange things occured.

The prop tips, because the are travelling in a helical route through the atmosphere, hit mach way before the rest of the a/c, and that is why propeller a/c seem to have a maximum indicated speed a significantly below the speed of sound.

[Seeker]

"The prop tips, because the are travelling in a helical route through the atmosphere, hit mach way before the rest of the a/c, and that is why propeller a/c seem to have a maximum indicated speed a significantly below the speed of sound."

So the prop tips are trans sonic? Seeing as the prop is a collection of aerofoils it's self; do props suffer from compression effects? or is the shape clean enough to to provoke eddies and swirls?

[Holden McGroin]

Yeah, the prop tips hit the transonic region first, and thereby lose efficiency.  That's why the sonic 'barrier' hasn't been broken by a prop driven aircraft in level flight.

[Holden McGroin]

Cool photo of the helical path of the prop tips.

[Witless]

Hi,

What an excellent thread, nice work guys

Cheers..........Witless/Trikky