Spit/109 sea level speed comparisons in 1.08 beta patch

[IvanK]

Originally Posted by Crumpp

Again, it just calls into question the validity of the RAE estimate. I just figured it was a given the RAE would not have the best data on the German aircraft.

They did a lot of estimating off very few data points. The CLmax for both aircraft closely matches the full flaps CLmax and not clean configuration.

Well clearly you haven't read AVIA 6/2934 They had reasonable data on the aircraft in question. AVIA 6/2934 is based on actual flight test of a BF109E3 in RAF hands.

Here is AVIA 6/2934 summary of turn performance based on Flight tests and calculation :



So the RAE determined the opposite to you based on flight test and calculation.

[Kurfürst]

Originally Posted by IvanK

So the RAE determined the opposite to you based on flight test and calculation.

To me it seems RAE determined something entirely different than Crumpp's calculation... (turns at and only at minimum turn radius vs. Crumpps calculations over the speed range)

[IvanK]

The RAE chart on its own shows sustained G over the complete speed range at 12,000ft altitude.
Its the a similar but more detailed chart to Crumpps.

[Kurfürst]

Originally Posted by IvanK

The RAE chart on its own shows sustained G over the complete speed range at 12,000ft altitude.
Its the a similar but more detailed chart to Crumpps.

RAE's calculation also using estimated/guessworked stall speeds, Clmax and rather questionable power values for both the Spit and 109 (the latter probably understood with the effect of engine thrust). That's the problem with these charts in general - there's such a margin of error with the base values, that the results are all over the place. (estimated) Propeller effiency can vary results by 5-10% alone, drag values are unknown, the wing's oswald effiency factor is unknown (directly shifts the results, since its a multiplier in the equation), Cl max is unknown.

Just to make it clear I don't doubt the Spit had a sustained turn advantage at lower speeds, but OTOH I am pretty sure the situation reverses at higher speeds (for the 6 1/2 lbs version) at lower altitudes, since the 109E has both less drag and more power.

I am also curious about the effect of the two speed prop on turn capacity. Having 990 HP at the prop shaft is nice, but its all for naught if the two pitch prop can't properly convert it into thrust at turning speeds.

[IvanK]

The RAE chart shows a Spitfire sustained turn advantage across the entire speed range from the Lift limit through to the max 1G sustained speed of around 340mph.

i.e. if the "Angle of straight climb" (Ps=0) for both the Spitfire and BF109 were overlayed on the same chart the Spitfire angle of straight climb would be above the 109 line from the Lift limit through to 1G Vmax. So at any speed in this range the Spit can sustain a higher G according to the RAE .... but not according to the Crumpp plot ... at any speed.

Both aircraft in this chart having similar values of 1g Vmax at the charted altitude.

There is no mention of prop type in the AVIA report for either the Spitfire or the 109. I take your point on propeller efficiency though ... that is touched on in another AVIA report (AVIA 6/13805) in which the RAE believe the 109 and Spit prop efficiency was essentially the same at 10,000ft with the 109 around 3% better at 15,000ft.... though with caveats.

[Glider]

Just one observation. Janes gives the Merlin III with 100 octane as 1,310 hp at 9,000 ft, not 990 hp, which would make a difference

[Osprey]

All that extra power from 12 fewer litres. According to some of the logic here I conclude, mathematically, that the DB601 was rubbish.