Aircraft performance curves

[Christer Bergström]

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?

[Jukka Juutinen]

You need rate of roll data? Then what is a better way to present that data than a curve showing rate of roll on the vertical axis and the IAS on the horizontal axis? You can easily have several curves at various stick forces on the same graph making comparison easy. Or what is your objection to curves this time?

[Juha]

Christer
the beauty of graphs is that one sees at glance roughly how much faster than Spit F. V LF V was at certain level, up to what level it was faster and how must faster F. V was above that at any given altitude. To explain that in writing one need a table which is more difficult to gasp.

Usually there are at least a speed graph, time to altitude graph and rate of climb graph for a certain type/version of a/c in books.

Beauty of a speed graph is that at level flight max speed is attained when max. thrust possible to develop with the engine-propeller combination (plus exhaust stack jet thrust) is equal to the drag. From that one can deduce much, if the thrust is smaller a certain amount because for ex. engine wear, propeller pitch problem, propeller wear because of stone hits etc, the max speed is lower by certain amount which is possible to calculate. Same way if the a/c skin is carefully waxed the drag of the plane is a little bit smaller and the plane is a certain amount faster. All is pure physics, not simple physics but anyway calculable.

There are books which explain the basics on all this, certainly also in Swedish.

I agree that the performance graphs might be more important for understanding fighters than ground attack planes.

HTH
Juha

PS as Jukka wrote, one can presents many phenomes by graphs, they are like compressed info.

[Graham Boak]

It is perfectly true that some of the available information is not fully defined, or of doubtful origins, but this does not lead to the conclusion that all should be discarded. A comparison of several such datasets - the more the better, surely? - will soon lead to discovering which are realistic and which are not. However, even the best can be viewed as a measure of the capabilities of the aircraft, if idealised compared with service conditions.

The development of an aircraft cannot be understood and appreciated without understanding the technical changes that did or did not happen, that were or were not possible at the time. Some part of this is most easily expressed in terms of these fairly simple curves, annotated as necessary.

From these charts can be obtained basic drag data, which when coupled with engine power, weight and lift can be used to produce all the intermediate climb, dive and acceleration points you suggest. Perhaps you are not aware that there is a simple conversion between climb rate and acceleration?

Every aircraft ever built would take a little while to accelerate the last few kph/mph to meet a true Thrust=Drag level speed. (Early high subsonic jets possibly excepted, due to the rapidity of the drag rise.) I don't see the value of this comment.

Comparisons are also best made using these charts. The relative performances of the Spitfire Mk.V, LF Mk.V, Fw 190A and Spitfire Mk.9 can be shown very easily on such a figure, especially the differing variations with altitude. To express all this in words would take many sentences, and would lack the clarity of the picture. Comparative areas of combat advantage are clear, with their key points of changeover.

ideally, it would be best to see the full flight envelope in terms of Specific Excess Power contours, but this technique was not available at the time. Some studies have been done to generate this but apart from one on the P-40 vs Zero I can't bring any to mind. Modern sources rather lack the knowledge available in the original aerodynamic offices. Without this we can only do the best we can with the data available, knowing that the air has not changed, and neither have the laws of physics. The more data, the better the job.

[Kutscha]

A picture, err graph, is worth a thousand words of text.

[Christer Bergström]

No, Graham, these graphs shouldn't should be completely discarded. As I said, 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.

Also, 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?

I am always out looking for good WW II combat aircraft data, and would love to learn e.g. which is the source to the Bf 109 F-2 performance curves which Franek has access to.

[Christer Bergström]

Sorry for "bumping", but maybe Jukka and Franek missed my questions?

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?

Which is the source to the Bf 109 F-2 performance curves which Franek has access to?