Spit/109 sea level speed comparisons in 1.08 beta patch

[TomcatViP]

Really did you read at least what I wrote on the last page?

(Edited on request).

[JtD]

Yes I did, assuming you are referring to this one. But it only states a few principles of aerodynamics, doesn't quantify them or provides a calculation illustrating that if this is taken into account, the numbers add up. I agree on the trend, but I doubt that they will account for the full difference.

Thank you for editing your post.

[MiG-3U]

Originally Posted by JtD

Yes I did, assuming you are referring to this one. But it only states a few principles of aerodynamics, doesn't quantify them or provides a calculation illustrating that if this is taken into account, the numbers add up. I agree on the trend, but I doubt that they will account for the full difference.

Thank you for editing your post.

Well, the logical problem with Tomcat's argument on Holtzauge's calculation is that Mtt calculated that claimed 498km/h at sea level exactly same way as Holtzauge. So if we prove either of them seriously unaccurate, the other is just as unaccurate.

However, generally there is no steep rise on the Cd until mach numbers well over 0.6 so the error caused by compressibility is very small given the speed differerences around 15kmh talked here.

[TomcatViP]

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

[CaptainDoggles]

Originally Posted by TomcatViP

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

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.

[MiG-3U]

Originally Posted by TomcatViP

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

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.

[TomcatViP]

@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

[MiG-3U]

Originally Posted by TomcatViP

@Mig

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

Post #516

Originally Posted by TomcatViP

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

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.

Originally Posted by TomcatViP

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

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)?

Originally Posted by TomcatViP

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.

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.

Originally Posted by TomcatViP

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.

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%.