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Old 12th November 2005, 02:38
Christer Bergström Christer Bergström is offline
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Aircraft performance curves

To the ongoing discussion on performance curves in the book section, I’d like to add my 5 cents:

Most of the published performance curves are unreliable, due to different reasons. Some of the most important are these:

Above all, nearly all performance curves are the result of clinical trials.
Secondly, many performance curves are highly unreliable because they are the result of test flights of captured, worn and often damaged (and repaired) aircraft where - due to the unused “enemy’s” lacking experience - the best simply wasn’t brought out of the aircraft. (See examples below.)
Thirdly, some performance curves originate from test flights with captured aircraft where we are not informed of the equipment (load) with which these tests were made. (See examples below.)
Fourth: One and the same aircraft had different performance due to different stages of engine wear, and depending on how much fuel and ammunition the aircraft carried. (Performance could also differ depending on polishing or worn paint, etc.)
Five: These so adored performance curves show the theoretically maximum speed attainable after a certain time in level flight, i.e. straight forward. Mostly the curves don’t say how long time it took the aircraft to reach that speed at level flight - which would be a very important factor to know. Since those performance curves are used to compare fighter planes, they are of fairly little use, because hardly any fighter combat is flown only at level flight.
Six: Curves for acceleration would be more useful, but then we would need lots and lots of tables showing acceleration from different speeds at different angles (dive, climb) and at various degrees of turning or recovering from turns, at various propeller pitch and trim applied, etc.
Seven: Two aircraft of the same type rarely had exactly the same performance, due to a multitude of reasons. (See examples below.)
Eight: How an aircraft is flown in combat differs from pilot to pilot - depending on how skilful he is to handle the aircraft in order to utilize its performance to a maximum - and that creates a huge difference between the best pilot in the best plane at the lowest weight from the worst pilot in the most worn plane at maximum load.

Now please don’t get me wrong. I understand that such realistic performance curves probably are not available, and I would not make a fool out of myself and demand that something should be published which I don’t know if it is available.

But I am questioning the strong demands for simple performance curves, as though these would be of any particularly great value.

The clinical performance curves which people here talk about might be amusing, and may give a weak hint, but for someone deeply interested in the technical details - someone who really is interested in the actual performances in real combat - they are of no greater use than any cursory school book in history is to a historian.

I will give you some examples which will show just how unreliable these performance curves are:

A Bf 109 F-4, test-flown by Messerschmitt at Augsburg, 29th November 1941 showed a performance which differed the following, compared with a test flight at Rechlin with the same aircraft type:

About 15 kph slower at sea level.

About 5 kph faster at 2000 m altitude.

About 5 kph slower at 4000 m altitude.

About 10 kph slower at full pressure altitude of 6200 m.

About 15 kph slower at 8000 m altitude.

The flight weight is with 2890 kg identical with the flights in Rechlin. Without knowing more details of the test planes, the differences cannot be explained.
http://www.beim-zeugmeister.de/zeugmeister/index.php?id=26&L=1

Then we have the calculated performances of the Bf 109 F-4, by Messerschmitt in Augusburg, 1 July 1942:

And again in comparison with the Rechlin speed measurements:


About 18 kph slower at sea level.

About 9 kph slower at 2000 m altitude.

About 8 kph slower at 4000 m altitude.

About 35 kph slower at full pressure altitude of 6200 m.

About 44 kph slower at 8000 m altitude.

http://www.beim-zeugmeister.de/zeugmeister/index.php?id=27&L=1

So if the span could be as wide as 44 kph (27 mph) even under ideal conditions - then of what use are these performance curves really? Other clinical tests have showed that the maximum speed at level flight for one and the same aircraft during the same flight (i.e. with the same degree of engine wear etc) could differ 30 kph between max load and nearly empty. Add that to the 44 kph above, and you get a span of 74 kph (46 mph). Now add the other factors, such as engine wear and different pilot skills, and you will start approaching reality.

An article at virtualpilots.fi should give the “performance curve believers” more to consider:

” When talking about the Messerchmitt 109 performance, we must take into account that many western sources are simply wrong. They are based on original wartime allied test flights flown with damaged planes, or with such equipment that the planes do not represent a normal fighter variant. Also western sources often fail to quote the used power setting. Was the engine runnign on continuous, 30 minute or 5 minute power? Western performance numbers (US/FAF/RAF) are always quoted with maximum power settings. Luftwaffe standard was to test all climb and level speed performance with the 30 minute setting, which really gives a more "real life" performance. (. . .) A good example this are the 109 F-4 tests.

”Bf 109 F-4 . . . Practically all its performance reports stem from a single British test flown with a damaged airplane with derated engine. All other test "reports" are copied from this one test. (. . .) The American test, “Combat Evaluation Report Nr. 110” for the Bf 109 F, 7th February 1943”, are only a compilation of the British test reports sent to the USA and no American flight tests were flown with F-4s. And to top it, the transferred report is riddled with errors in converting the numbers and drawing the performance curves. For example the reported climb rate is the British climb time for 16,500 feet converted to 15,000 feet. Also in the American summary are existing further serious transfer errors.”

http://www.virtualpilots.fi/en/feature/articles/109myths/

Very interesting! Now compare with this - you can check the source for yourself:

“In the book "War Prizes" by Phil Butler is told, that the Bf 109 F-4 with the built number 7232 made a wheels up landing in Beachy Head on 20.5.1942. After replacement of the damaged engine the plane was flown on 24.10.1943 at the first time by British pilots. Measurement results of test flights with this plane are not documented, another Bf 109 F-4 was never captured.
Where did the British performance data from August 1942 then came from? Most probably the real flown measurement results of a Bf 109 F-2 were taken as base and extrapolated due to the engine card of the DB 601 E. It has to be mentioned, that the measurements of the Bf 109 F-2 performance were faulty, since the engine was troublesome during all tests and had to be repaired several times. The British authors of the Bf 109 F-2 test report had pointed out clearly, that because of this the measured performances should only be seen as minimum performances.”
http://www.beim-zeugmeister.de/zeugmeister/index.php?id=32&L=1

Back to virtualpilots.fi:


“Comparitive Trials between Me109E and British Fighter Aircraft, RAE (?), 14 August 1941
Here we have two interesting reports. They're actually a 1941 report from tests conducted in September of 1940 from an aircraft that was captured by the French in 1939. (. . .) The 109 tested is claimed to be "Me 109E-3 Werk-Nr 1304" which is documented to have been captured. However, there is some discrepency as to WerkNr 1304 actually being an Me109E-1. So what have they tested? E-1? E-3? E-4? Did they test one of the crash landed, damaged planes? (. . .) Another problem is with the test itself, when compared to a Spitfire. Overall the accuracy of the test suffers from the fact that it was flown with a crash landed plane wirh a worn, several years old engine producing less power than usual. It was then flown against a brand new Spitfire with a 1940 engine. As shown by the test data, the turns were made in the 120mph range which is too slow for the 109 slats to be deployed, which doesn't compare the maximum turning abilities of each aircraft. (. . .) The French flew their test with the same Me 109 E-3 (E-1?). The test results are not available in English, but to author's knowledge their recorded performance numbers are higher than in the British tests.


(. . .)

“The Bf 109 G-6 model's performance numbers are usually quoted from a flight test flown by mr. Brown. The actual plane was a 109 G-6/U2, which is a three cannon night fighter variant with night fighting equipment. Authors now take these numbers, drop away the information that it was a 3-cannon night fighter and voila, we got weak performance numbers for the G-6. The 3-cannon night fighter G-6 made 621 km/h in 30 minute power setting. A clean G-6 does 635-640 km/h with 30 min setting and 650+ km/h with 5 min WEP setting. So you can see that the wing cannons not only decreased speed, but they decreased the climb rate, roll rate and overall agility of the plane.”

http://www.virtualpilots.fi/en/feature/articles/109myths/

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Christer Bergström

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