Spit/109 sea level speed comparisons in 1.08 beta patch

[NZtyphoon]

Originally Posted by Crumpp

Do some math....

The RAE chart is at 12,000 feet and was taken off one data point. It did puzzle me as our radius and other data aligns. It puzzled me until I stated getting into the details of the chart.

According to that chart, the Spitfire Mk 1 is capable of reaching 340mph (+) at 12,000 feet on 1050 bhp.

The RAE graph found in AVIA 6/2394 is a performance estimate from September 1940.


A flight report from March 1940 gives the power at 12,000 feet:



And lists the Vmax for the type as 326 mph TAS.

http://www.spitfireperformance.com/n3171.html

The AVIA 6/2394 does not fit the only +12lbs estimate we have for level speeds.

http://www.spitfireperformance.com/s...-rae-12lbs.jpg

This estimate shows 359mph TAS at 12,000 feet.

And what do we see on the 109 data? Estimate on chart = 1,200 Bhp at 2,400 rpm 15,000 feet, TAS 340 mph + at 12,000 which Crumpp, conveniently has ignored, whereas the true output was about 960 ps 2,300 rpm at about 3,500 metres

http://www.wwiiaircraftperformance.o...oct40-pg22.jpg

So Gates was also using an unusually powerful 109 for the chart as well. I suspect it was probably an experimental high-altitude 109E.

Question is what data did Crumpp use to compile his chart? There's no engine rating shown, no take of weights, nor anything else to indicate on what basis Crumpp's "calculations" were made. For any proper analysis Crumpp's chart is totally useless.

[Crumpp]

There's no engine rating shown

The data is listed in the thread.

Here it is too, right off the spreadsheet:

Spitfire Mk I

Aircraft Data
weight 6050lbs
Power 990bhp
Level speed 247KEAS
Propeller efficiency 0.8
Wing area 242 sqft
wing efficiency 0.85
Dynamic pressure 206.8101695psf
Aspect Ratio 5.6
Mass 187.8881988 ft/s^2


Bf-109E-3

Aircraft Data
weight 5580lbs
Power 990bhp
Level speed 269KEAS
Propeller efficiency 0.85
Wing area 174.9 sqft
wing efficiency 0.85
Dynamic pressure 245.2915254
Aspect Ratio 5.77
Mass 173.2919255 ft/s^2

[Crumpp]

And what do we see on the 109 data?

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.

[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

[NZtyphoon]

Originally Posted by Crumpp

The data is listed in the thread.

Here it is too, right off the spreadsheet:
Bf-109E-3

Aircraft Data
weight 5580lbs
Power 990bhp
Level speed 269KEAS
Propeller efficiency 0.85
Wing area 174.9 sqft
wing efficiency 0.85
Dynamic pressure 245.2915254
Aspect Ratio 5.77
Mass 173.2919255 ft/s^2

Interesting how the Bf 109E-3 loading chart http://www.wwiiaircraftperformance.o...ladeplanes.jpg shows the weights as 2,608 kg (5,749 lbs) fully loaded for combat while, without ammunition for training flights, the weight is 2,532kg (5,582 lbs)...