[Franek Grabowski]

Bill

Franek - you are right about yaw issues in dives from .75 to .83
But this discussion is about level deck speeds in the .55 to .6 range.
In a dive you have to feed rudder to keep the nose from wandering but it was not a divergence issue as 'highly unstable' which it wasn't.

I just wanted to point out that there were some aerodynamical differencies between the two types. Certainly the wetted area was different for both, especially on the fuselage.Of course, the question is, how it affected the aircraft performances, being in turbulent airflow from propwash. (I hope it is still in English)

Aero engineering is a complex field. Getting precision estimates of performance, pre flight or wind tunnel testing for a new airframe and geometry was a combination of arcane combinations of empirical factors (i.e "e" and "propeller efficiency" and even various components of parasite drag like wheel wells, new antennae, bomb racks, etc.).

Yes, indeed, but we often omit several such components, like a parasite drag of riveting and panel lines, sometimes multiplying results by a specific factor. Then we have a interference drag, which we may get approximate or measure on wind tunnel test, but it is rather hard to predict. We can also simplify the model and do not calculate drag of antennae, etc. either omitting it or adding a constant figure, depending on influence on a total drag.

Having said this, this discussion and Crumpp's presentation and arguements are about Parametric studies on the same airframes with one variable changed.. so we don't have to screw with "e", AR, predictions about flow separation, etc.

Yes, indeed, but the problem is with estimating influence of one variable - change of weight caused by fuel consumption - on overall aircraft performance. I tend to agree with Graham, based on my aerodynamical knowledge with very limited practice, that it is minor.

On the other hand when flight testing the same airframe, with low fuel versus full combat load - same engine, same airframe, same boost, same pilot, same geography and easy to calculate variations for Temp and Altitude

Well, we will never enter the same river to be exact. But of course we can use an approximate and do the comparison. Let's say the pilot is able to put an aircraft into same conditions in air but weight. The question is, how we would measure change of weight of flying aircraft, and how we will calculate airspeed? I think accuracy of measurements is a key problem.

The P-51. For the answer to the obvious question compare the Drag of each airfoil and airframe to get the "aha". Speed and range.

Oh yes, indeed. Heavier aircraft, powered by the same engine, but with better aerodynamics is substantially faster. On the other hand, the lighter one, Spitfire, is generally a better climber and able to fly higher than any Mustang. Different parameters should be considered.

Franek - I respect Graham's knowledge but Crumpp in my opinion based on my own academics and practice is entirely correct.

Yes and no. Graham in my opinion correctly pointed out that the change of weight caused by fuel consumption of an average WWII fighter is just too small to be considered important factor while discussing the maximum speed. Theory is generally correct but it has some limitations, especially if we discuss profile of the mission.

The question for the example discussed is ~ 10mph significant for the 6% increase in weight? It would be for me, particularly if my Mustang was in the lower range of performance in the production series..

I bet it would be much more important for you, to have it smoothly polished, with no dents, and to have fuselage tank empty in case of dogfight, to avoid stability problems. Let's not forget one thing as well. We discuss actual performance of aircraft in combat. talking about maximum speed of Mustang, let's not forget nobody would enter dogfight with full fuselage tank and wing tanks. Mustang entering combat over Normandy and over Berlin had similar weight and it would not affect your performance. You would not fly to both locations at your maximum speed as well.

That said, I am curious of the mentioned performance comparisons of Mustang, and wondering if it is the same thing I have seen some 10 years ago. No comments, till I see the stuff.