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Old 13th November 2005, 14:40
Christer Bergström Christer Bergström is offline
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Join Date: Dec 2004
Posts: 352
Christer Bergström
Re: Aircraft performance curves

Since you admit the relative weakness of the simple performance curves (another one, not mentioned here, being that most of them are published without any source reference), I still fail to understand for what they are so useful. Knowing about the unreliability of the performance curves, such information can be explained with less inaccuracy in a few lines of written text - like you just did yourself, Juha:

At low altitude the LF V was clearly faster than the F. V.
Jukka used a quantitative method, explaining that a performance graph table requires less space than covered by a cigarette box. Well, the eleven worda above require even less space than a cigarette box, and contains less inaccuracy. I would also prefer a praiseworthy description as Juha's below rather than one of those ordinary and unreliable performance curve graphs:

The max speeds of Fiat G.50s test flown by FAF at 5000m varied between 403 and 430km/h, most important reason to variations was the airscrew used. These tests were flown with 790 mmHg boost pressure not with the max permissible temporary overboost pressure of 890 mmHg but Finns calculated that 430km/h with 790mmHg means circa 445 km/h with 890mmHg. Fiat’s figure for max speed was 484km/h at 5000m. So FAF Fiats were 40 – 60km/h slower that the manufacture claimed. And from a speed graph flown by G.50 FA-31 one sees that the difference to Fiat’s figures was biggest between 2500 – 7000m and smallest around 2000m.
We have heard people here talking about their technical knowledge. I don't say that aircraft performance curves is totally superfluous, but it requires technical knowledge to be able to use those curves properly, and I find it surprising that no one of those who have such high thoughts about their own technical knowledge have discussed the weaknesses of these ordinary performance curves. I would say that to someone without the high level of technical knowledge which has been implied by members on this forum, a table with performance spans like in Juha's example above is of far greater use.

Also, the value of performance curves may differ from aircraft to aircraft. For the Me 262, which based its success on high speed, a comparison between average performance curves of the Me 262 and the P-51D could be quite useful.

However, for the Me 109 and its most common tactic of "boom & zoom" attacks, the initial climb rate from various accumulated speeds would be more useful in order to compare its effectivity against enemy aircraft like, for instance, the Airacobra I.

In a prolonged fighter combat which took the shape of a turning combat - which in WW II was carried out at speeds of about 350 - 450 kph - a fighter aircraft's top speed at a certain altitude is of fairly little value; in such cases we rather need to know the competing aircrafts' roll rates, initial turning rates, sustained turning rates, and initial rates of climb.

I am afraid that the staring at performance curves and simple speed statistics (like 391 mph or 387 mph) has led the focus away from those more crucial facts for the understanding of an aircraft's relative value.

Besides, I particularly wonder for what there is such a great need of performance curves for a bomber or ground-attack aircraft. Here we have talked about various top speeds of fighter planes at low or high altitudes. But for what do we need to know a Hs 129 ground attack aircraft's different average level flight top speeds at clinic tests at 17,000 ft, 22,000 ft, 24,000 ft, and 28,000 ft respectively?
All the best,

Christer Bergström
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