No Mig Sry but Mach nbr is the local mach, not the plane frwd speed.

At 15% thickness, the speed is in high subsonic when the plane is flying around 600kph.

At this local speed, drag effects are not linear and raise sharply.

Moreover, I think I was one of the first to put the dK/dt=SIGMA(P) eq around on forums. So don't nurse me with it. Thx in advance.

Remind simply that this give only the max speed any increase of pow will give to a plane. At high speed, this is not linear.

Generaly speeking:

Incompressible theo apply only for M<0.3
Btw M0.3 to 0.8 effecst are sharp and results vary btw 6% to 100%
Over 0.8, you hve shock waves and you need to apply according theo.

Of course many guys rely only on incompressible, simply because they never really to get into the others case :eek:

It's also much cheaper, computationally speaking, to use subsonic incompressible flow theory and then apply a linear compressiblity correction.

I imagine this is what IL2FB did, though I'd be surprised if their correction factor was at all realistic.

Hoerner gives 10% rise on Cd at mach 0.55 in his analysis on the Bf 109G, steep rise starts around mach 0,6. We are talking here about 15kmh speed differences around mach 0.4 (460-500kmh at sea level) so we can safely assume that there is less than 2% difference in the Cd due to compressibility and that causes much less than 1kmh error in the calculation.

I don't think that anyone is relying on incompressible theory here, just giving a quick estimate of speed change due to power change at good enough accuracy. You can, of course, point out that compressibility is not accounted (nor Cl, prop efficiency etc.) but can you prove that there are significant errors?

BTW that calculation method has been criticized earlier in this thread, actually well before Holtzauge used it and you posted your stuff.

@Mig

Always the same, always bragging ... When did I post my "stuff" ? Tell me.

If you gonna be insulting be precise, accurate and honest..

Your Holtzauge did not post detailed calculation or discuss his method. I pointed one source of error. Tht's it.

This is the basis in Sciences. Put it down and submit to what ppl think.

ANd you just hve to open a NACA chart to see the drag rise for the specific airfoil. You don't hve to pick bit of info here and there. It's free and available.

But, huh, tht's what hurt the dark internet genius as always.

And by the way, 10% of drag raise (outsourced from my memory) is equal to what a fully open rad will do on Seversky P-35.

Tht's not negligeable my dear and will certainly not impact the max speed for only 1km/h

Boring...

@Doggle : you are right of course

Post #516

Exactly the same calculation is used on the Mtt test on the prototype V15a to calculate speed at higher power at sealevel (blatt 5) posted several times here. I used the same calculation for demonstration on the post #448 and noted that it's a crude, unaccurate and partially wrong way to calculate it. However, it's good enough for small differences.

Yes, of course. But why do you complain about Holtzauge's calculation now but not about Mtt calculation which has been here much longer (or mine which was posted 3 weeks ago)?

Actually I'm refering tests of entire airframe, tunnel and flight tests. And yes, many are available. For Hoerner's Bf 109G analysis, get his Fluid Dynamics book. Some are freely available, here is one for few aircraft including Spitfire I. Below is also a Cd/mach number chart from that report attached, you can see that up to Mach 0.5-0,6 there is rather minimal drag rise due to compressibility.

Hoerner's number 10% is for entire speed range from mach 0.3 up to 0.55 ie speed changes 250-300kmh while we are talking here about 15kmh speed difference around mach 0.4, that means that drag rise is certainly certainly less than 2%, probably less than 1%.

No. Again lack of modesty in your assumption. Take a look on Youtube searching TcViP and related variables. You'll see that I posted that last time almost 3 years ago. Good Hunt :cool:

...Obviously you won't but that show you how personal arguments hve nothing to do on a forum. Even if that behavior seems rather popular Lol

Small diff... Yeah. But not small diff at max speed for this kind of plane !

All this story abt the 109E not reaching 500Kph is ridiculous. Juts like other big themes fairly popular here. Nothing new here so you can past those line.

Didn't I say local mach number ???? Again when your plane is traveling at Mach 0.5, your wing see a peak of high subsonic speed for a 15% airfoil section. Same with the fuselage, especially the bottle neck effect at the rear par. Yes, the 109 was very well streamlined but your calculation need to be repeatable to other type

Compare to what ?

Wil hve a look to your doc cited as refs. hoping that I won't lost my time wit another bunch of lifting line and alike theo.

If you are interested in the matter you shld read the books from Karman directly.~

A good one and easy to reach (amazon - very cheap):
Aerodynamics - Selected Topics in the Light of their Historical Development, Cornell University Press, Ithaca, 1954

The university of Warsaw hve also a very good list of PDF doc posted on theit website. Doing some Google searching you sld found it easily.

Well, I rely on what you have posted on this thread. At the post 519 you suggest that the calculation is ok "to estimate the time of accel from Stall speed to 200mph". But that is actually not true because the Cl and the hence Cd changes in large degree during acceleration as well as thrust. However, for a quick estimate for a small change as discused here, the calculation is good enough.

The testing of a whole airfame accounts all these and we know, based on Hoerner and other sources, that around Mach 0.4 the compressibility effects are minimal in the case of Bf 109G, there is no reason to believe that the Bf 109E is much different because the wing profiles are almost the same as well as the fuselage.

We can safely use the same rate of drag rise as given by Hoerner for assuming Cd change from 460 to 475kmh. And because the speed change is so small and we are around Mach 0.4, we can safely assume that the Cd rise is just a small fraction of that 10% given by Hoerner, probably less than 1%.

Ok 3 post and nothing new.

I hve been tricked liked that alrdy for hundred of pages. End of conv. for me.

Here is the link to the book I cited for the reader that would be interested to get a quick snap of what are aerodynamics forces, as told by a Jedi master:

http://www.amazon.com/Aerodynamics-H...+Topics+karman

Much better read than hanging here, sadly.. :evil:

May the Force (of flows) be with you.

If you were right, the Mach related drag increase on the 109E would happen only somewhere between 498 km/h and 537 km/h, as the top speed in first charger gear (537 km/h, 2150m, 1115 hp) and in second charger gear (575 km/h, 4850m, 1100hp) are in perfect agreement.

A plane doing 498km/h at 1018hp at sea level, should do around 555km/h at 1115hp at 2150m and 600km/h at 1100hp at 4850m, give or take a percent or two.

These figures happens to be pretty much spot on Bf 109F performance, which either means it did not suffer the E model Mach effects, or it means the problem with the data is elsewhere.

Google searching you sld found it easily
http://www.qmmv.info/12.jpg
http://www.qmmv.info/13.jpg
http://www.qmmv.info/14.jpg