Spit/109 sea level speed comparisons in 1.08 beta patch

[TomcatViP]

no again. Sry. But you are in inviscid and incompressible.

Those eq are valid only bellow Mach 0.3. Btw 0.3 and O.6, this can do a nice guess-estimate if you had a coeff. Above 0.6, you can't rely on this way for calculating perfs.

Remind that 0.X is the LOCAL maxima of speed.

Thx however for the details you pushed here.

REM: If you had used your calculation to estimate the time of accel from Stall speed to 200mph, I won't have said anything (if you have added a coef in 2Pi*Alpha). Or the cruise speed (WWII).

~S

[JtD]

OK Tomcat, if he's got it all wrong, then why does it work reasonably well with about all other WW2 aircraft in the same speed league? He's clearly got a point, 500 at sea level and 575 at 5000 m don't add up with the power levels given. Personally I'd be most curious to have that sorted, preferably to a point where the test results make sense.

Can you guys try to be constructive and solve that problem?

[TomcatViP]

I am

[JtD]

Great, so what's your explanation? I seem to have missed it.

[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.

[MiG-3U]

Originally Posted by TomcatViP

no again. Sry. But you are in inviscid and incompressible.

Those eq are valid only bellow Mach 0.3. Btw 0.3 and O.6, this can do a nice guess-estimate if you had a coeff. Above 0.6, you can't rely on this way for calculating perfs.

Remind that 0.X is the LOCAL maxima of speed.

Thx however for the details you pushed here.

REM: If you had used your calculation to estimate the time of accel from Stall speed to 200mph, I won't have said anything (if you have added a coef in 2Pi*Alpha). Or the cruise speed (WWII).

~S

Well, Mtt used exactly same rough calculation as Holtzauge to estimate the speed at different power. When the speed difference is around 10-15kmh, the changes in induced drag, propeller thrust and drag coefficient due to compressibility are so small that the error is less than 0.5kmh. That is certainly good enough, given that most data here is without compressibility corrections.

However, field is open for you interpretation, of course.