Ummmm can we change the compression code?

[coyote]

I was at 35k in P47 and made a discovery, the red hash mark on the air speed indicator (ground speed) is controlling compression!

I dove on a 109 and compressed at UNDER 400mph IAS , but my ground speed was 500+

ALSO it seems like the steeper the dive angle the low the compression speed is.......... THIS IS GETTING OLD

[Citabria]

high speed buffeting would be helpful too since it was a warning of the onset of compresibility

[Fishu]

P38 has the same thing, people excuse it as 'mach tuck' or something like that.

[funked]

Coyote, (I say this without sarcasm) get a fluid mechanics text and look up "Mach number".  That's what dictates the onset of compressibility effects.  It's got nothing to do with IAS.  

The Mach number "Ma" is your true airspeed divided by the speed of sound.  So Ma = 0 means you are stopped and Ma = 1 means you are at the "sound barrier".  In between, there is a critical Mach number at which compressibility effects begin on a given aircraft.  

At higher altitudes the speed of sound is MUCH lower, so you get a higher Mach number for a given indicated airspeed.  In a plane like the U-2, there is only a few knots between the stall speed and Ma = 1 at the plane's operating altitude.

Also, if you are getting compressibility effects in the P-47D-30, try using the dive flaps.  That's what they are they for.  Republic would have called them "compressibility flaps" but that was a few too many syllables for the intended audience.

[This message has been edited by funked (edited 08-14-2000).]

[This message has been edited by funked (edited 08-14-2000).]

[Citabria]

he means that the TAS is screwed up by wind thus making it ground speed that determines compression in AH

[funked]

What wind?  Red hash mark is TAS as far as I know, good old "v" in fluid dynamics land.

[This message has been edited by funked (edited 08-14-2000).]

[flakbait]

Funked, what he's saying is this:

He goes to 35k, dives and hits 380 mph IAS. Yet the TAS mark, which is red, shows over 500mph. He's entering compression due to the TAS reading at 2?k and 500+mph, NOT the IAS as it should be. Indicated Air-speed would show you, at a given altitude, when you would enter compression. Right now, for some reason, True Air-speed is showing when you enter compression. TAS is only good for figuring ground speed, IAS is how you tell how fast you're going at altitude. If you hit 500mph IAS, then you should barely be able to move the stick since air is moving over the aircraft at 500mph. If you hit 500mph TAS, nothing should happen. Why? TAS is how you measure ground speed of an aircraft, not air-speed.


Flakbait

[RAM]

Flakbait, and I think that what funked says is that ,as sound speed decreases with altitude,at 32K there is a much less IAS required to compress than at say 10K,as at 10K there wont be any problems with match number, as it is lower than at high altitudes...

But as the TAS increases with altitude it may seem that the speed used to calculate the compression is the TAS instead of the IAS, but that is not the case.

Did I got that right funked?  

[This message has been edited by RAM (edited 08-14-2000).]

[funked]

Yes RAM you are right, you may have a cookie from my jar.  

TAS (True Air Speed) is the red hash mark.  TAS is how fast your plane is actually moving relative to the air around it.

IAS (Indicated Air Speed) is the white needle.  IAS comes from an instrument that measures the difference between static pressure and pitot pressure.  It is calibrated for the air density at sea level.  At sea level the instrument will give IAS roughly equal to the TAS of the plane.  If the instrument is used in air of lower density than sea level air (like at higher altitudes) the instrument will give a reading lower than the TAS.

Mach number, as I explained above, is what determines compressibility effects.  Mach number is the TAS divided by the speed of sound.  The speed of sound decreases with increasing altitude.

So it is a double whammy.  As you go higher, your TAS becomes greater for a given IAS.  Also as you go higher, your Mach number becomes greater for a given TAS.  The combination of these effects means you can reach a critical Mach number at a suprisingly low IAS reading at high altitudes.

[coyote]

Well that sounds great,but I just dont buy it.... Unless you are gona post the numbers, what speed of sound is at 30k? Still over 600mph I believe... what "mach" numbers various aircraft compress at... then we can test your theory....

ONE THING I DO KNOW, DIVE ANGLE IS AFFECTING COMPRESSION........ WHY???????? shallow dive, you hardly ever compress, same spped in steep dive, its lock up time fast

[funked]

Check this out:  
 http://www.lerc.nasa.gov/WWW/K-12/airplane/atmosi.html

The speed of sound is about 675 mph at 30,000 feet.  

America's Hundred Thousand indicates a maximum safe dive speed for the P-47D of .82 Mach.  So that would be 554 mph TAS at 30k.  IAS would be 340 mph.

Dive angle shouldn't make a difference.  If that's really happening, I can't think of a good reason for it.  I'll check it out and see if I can get some numbers.

[This message has been edited by funked (edited 08-14-2000).]

[Zigrat]

Funked is right, listen to him  

Mach # varies with alt because of temperature changes in the atmosphere.

[RAM]

 

Originally posted by Zigrat:
Mach # varies with alt because of temperature changes in the atmosphere.

Huh? I thought it was because the drop on pression at high altitudes...

[funked]

That too RAM.   Pressure, density, and temperature are all related.

[This message has been edited by funked (edited 08-15-2000).]

[715]

Mach number is related to temperature only, not pressure.  It is your true speed divided by the speed of sound.  The speed of sound is the speed of air molecules, which is proportional to the square root of temperature and independent of pressure.  It's cold at high alt so the speed of sound drops (cold molecules move slower than hot ones).  With a lower number in the denominator the Mach goes up for a constant speed in the numerator.  Once you near speeds equal to those of the local air molecules you form shocks that mess up the aerodynamics of the plane.  (This occurs at Mach numbers below 1.0 because some of the air flowing over parts of the wing is doing so at speeds above your average speed.)  The pressure only enters in via the TAS vs IAS: the instrument that measures air speed, the pitot tube, is dependent on pressure and is calibrated at sea level.  At high altitude there is much less pressure and the pitot reads "low".  So it's TAS and altitude (temperature) that determines Mach, not IAS.

715A