Aircraft performances

Gentlemen,

please forgive me to open a new can of worms ...

I try to make a small database with the performances of some WWII aircrafts. I downloaded a lot of things on the web (especially WWII Aircraft Performance ) and tried to make a synthesis of the data. Unfortunately the data is more than often different from one document to another.
I am interested in climb rate at low (SL) medium (10.000 fts) and high (20.000 fts) altitude and found sometimes some different figures :
P-38F : 3.300 / 3.600 / 3.300 fpm, 3.000 / 2.750 / 2.400 or even 2.900 / 2.700 / 1.800
P38J : 4.000 / 3.800 / 3.200 fpm or 3.100 / 3.250 / 2.900
Bf 109G-1 : 4.113 / 3.780 / 3094 fpm or 3.700 / 3.000 / 2.200 or 3.500 / 2.900 / 2.400
Fw 190A-3 : 3.100 / 2450 / 2.200 fpm or 3.050 / 3.280 / 2.900 or 3.200 / 3.300 / 2.300
etc.

What is amazing is that data comes from official documents, not second hand sources or test made by other air forces.

I understand that some factors may change climb rate :
- load
- version of the plane (ie Spitfire I, V or IX) and even sub version (different motorisations and / or tropical filter for Spitfires V and IX)
- engine regime
I always tried to select the same version of the plane, unloaded at MP (or WEP if available).

How do you explain this difference and can you help me selecting the "true" or "real" (if these words means something) values ?

Thanks for any help,

Francis Marliere

Francis - the variables include fuel, engine CONDITION, airframe condition, factors influencing air density (temperature, atmospheric pressure, humidity), instrumentation, airframe preparation prior to test flight (taping gun ports, fuel amount, ammunition or ballast, wing surface preparation such as wax and polish or sand with fine sandpaper), gross weight at takeoff, gross weight at altitudes per the tests executed, the rated Horsepower (with actual variance per engine condition), boost and rpm...

To name several that come to mind.

So, for each of the models of interest (i.e Bf 109F-4, Bf 109G-10) you need to look at the tests and carefully parse for Gross weight at take off, Hp, Boost AND if you can find it, the plots of Hp as a function of altitude and boost and RPM for the engines installed.

Depending on several factors the Engine parameters differ with respect to Hp output as a function of altitude. For two speed, two stage supercharger engines (i.e. Packard Merlin 1650-3 vs 1650-7) the Full throttle height for each is a.) different, and b.) Hp as a function of altitude, boost and rpm also differ. Consequently the test flight data at FTH for the P-51B-1 and -5 for high blower is at 29,000 feet whereas the P-51B-10 and -15 with the 1650-7 has FTH for high blower at ~ 24,000 feet. To compare the two you must look at the charts to pick of points of Speed attained at comparable heights in between FTH for high or low blower (i.e. SL, 5000, 10000, 15000, etc). In some cases one of those altitudes will match a FTH test result - but mostly, No.

If you wish to enter into the world of drag calculations the Hp vs Velocity plot, particularly at the altitude of top speed runs is a must.
The Hp, theoretical propeller efficiency (~.85 often used), and the Velocity is mandated to calculate Thrust.

Good luck - there is a lot of painstaking evaluation to be made to arrive at detail that you wish to use to compare for speed, climb, turn, acceleration, with particular specificity regarding the airframe, gross weight, mission (i.e full internal fuel and ammunition parameters) and engine parameters (Model of the engine, Hp vs altitude and RPM/Boost).

The weight and power (or at least boost/rpm if you don't have the power) are the most important by far, at least these should be included in your database.