Performance of the Fw 190A on the Deck?

[Harri Pihl]

Originally Posted by Crumpp

You are confusing the forumla's used to construct our power and thrust curves as well as the contribution of induced drag and drag other than from lift. These do not couple the affects of weight changes.

No, I use very basic polar calculation to determine the speed change when the weight changes.

The difference between mine and your approach is that I calculate the changed induced drag due to weight change (which means that Cl/AoA change is accounted) while the formula you use, assumes constant Cl/AoA which is a wrong approach for this particular case.

[Crumpp]

The difference between mine and your approach is that I calculate the changed induced drag due to weight change (which means that Cl/AoA change is accounted) while the formula you use, assumes constant Cl/AoA which is a wrong approach for this particular case.

Of course we are holding CL constant. We are demonstrating the affect of a weight change. If the aircraft does not have that quantum increase in power available, then it must reduce performance to the point Pr = Pa.

A 50% increase in weight represents an 83% increase in power required at the same CL. That is far from insignificant.

It is not a very large change in velocity but it represents a very significant reduction in the aircrafts envelope. For Gods sake, fly a plane with and without passengers to look at your performance or one with 1/2 fuel as opposed to full tanks.

All the best,

Crumpp

[Graham Boak]

Crumpp: There is no point in doing calculations using this equation of simple ratios. It contains no effect of altitude, which is relevant to the effect of weight on speed. It would be true if all the drag was lift-induced, but it is not. It would be more appropriate for addressing the stall speed, if in the inverse sense. There it is induced drag which is the dominant term, and zero-lift drag that can be neglected. I assume that you are working to some derived system of equations, which do not hold true for this case. It is always better to go back to the basic principles of lift, drag and thrust (power).

Your comments on power drop is puzzling, as there would be no power drop. Maximum speed is when thrust = drag: over such small differences (yours or mine), inputs such a propellor efficiency and engine power will not change so thrust is constant. The only change is drag: I repeat that the only component of the drag that changes with weight is the lift-induced drag. The size of the change in the induced drag is of the order of that I quoted.

Drag is dependent upon the square of the speed. I think I see what you mean about "power of drag" being to the cube of speed, but it is not a concept I'm familiar with, nor necessary in this case. I think you are trying to say that to regain 8.5 mph (on a Typhoon sl top speed around 300 mph) would take an extra 34% of power - even on a cube law this is overstated as only some 9% would be required (1.03 cubed).

Harri: your calculation shows an even smaller change than mine, but given the respective assumptions we are clearly in the same ballpark. My estimate of 15% induced drag may be out by some percent, but equally I don't think that any Typhoon actually achieved 2200hp at its top speed, which I think you may be overstating. Perhaps some of the later Tempests did. Actual performance of the Sabre is not defined very well in the references I have.

Nick: the main effect of the loaded case is the additional drag of the bombs and carriers.

[Harri Pihl]

Originally Posted by Crumpp

Of course we are holding CL constant. We are demonstrating the affect of a weight change.

Well, I and Graham are demonstrating how much the increased weight reduces the top speed; increased weight naturally changes the Cl/AoA at top speeds. However, you are demonstrating how much the speed (or power) should be increased to keep Cl/AoA constant.

Basicly you are using the wrong approach for this particular case; we are interested about the speed change due weight change and that also means that the Cl/AoA changes.

[Harri Pihl]

Originally Posted by Graham Boak

Harri: your calculation shows an even smaller change than mine, but given the respective assumptions we are clearly in the same ballpark. My estimate of 15% induced drag may be out by some percent, but equally I don't think that any Typhoon actually achieved 2200hp at its top speed, which I think you may be overstating. Perhaps some of the later Tempests did. Actual performance of the Sabre is not defined very well in the references I have.

I just used some quick and dirty values out of hat for the demonstration; the purpose was just to show the correct approach for this kind of calculation.

[Crumpp]

Basicly you are using the wrong approach for this particular case;

Baloney. You are arguing that the speed reduction is insignificant but you fail to see the entire picture.

The small change in speed represents a very significant reduction in the aircrafts performance envelope. It's all tied together and you cannot separate the affect.

That aircraft's entire envelope is reduced.

No I am not "wrong". You wish to separate that which cannot be.

In fantasy land in a realm with no significance to the real world, weight affects are insignificant because top speed is only reduced a small portion.

In reality, weight affects are very significant and the number one concern of most designers.

Two wrongs do not make a right.

All the best,

Crumpp

[Crumpp]

Bottom line is the idea we can classify weight affects as insignificant because in a very short sighted manner we only percieve a small velocity change is fundamentally flawed.

[Crumpp]

The difference between mine and your approach is that I calculate the changed induced drag due to weight change (which means that Cl/AoA change is accounted) while the formula you use, assumes constant Cl/AoA which is a wrong approach for this particular case.

Harry PHil,

I am very familiar with "your technique" btw. It's not new or original. In fact I use it in a spreadsheet I constructed to predict aircraft performance. I would be glad to share that spreadsheet with you.

However you still have to go back and couple the affects of the power reduction. Only this time we have to do it manually. The end results are exactly the same.

The standard BGS formulation presented in numerous aerodynamic and engineering text I posted in this thread works just fine at illustrating the affects of weight. The end results are exactly the same.

An increase in weight represents a significant reduction in performance.

All the best,

Crumpp

[Harri Pihl]

Originally Posted by Crumpp

Baloney. You are arguing that the speed reduction is insignificant but you fail to see the entire picture.

What I and Graham have pointed out is that for these kind of aircraft the speed reduction is small at high speed near sea level. And we both do that correctly by using the polar and iteration method which accounts needed higher Cl/Aoa.

Your approach assumes constant Cl/AoA which is not a correct approach for this particular case because with given available power the plane can't maintain constant AoA at higher weight.

Originally Posted by Crumpp

I am very familiar with "your technique" btw. It's not new or original. In fact I use it in a spreadsheet I constructed to predict aircraft performance. I would be glad to share that spreadsheet with you.

It's not "my technique" but very basic use of the polar; Graham does the calculation with the very same principle and anyone with sound basic knowledge would use this technique. And I teached you to use it in Ubiforums some time ago:

http://forums.ubi.com/eve/forums/a/t...6131040055/p/1

You might remember that then you claimed this technique as "misuse of formulas" so I afraid that the sensible discussion is impossible in this case as well.

[Crumpp]

You are about to be ignored. I know very well what the formula's are for as I am degreed in Aeronautical Science.

Once more I am pilot and aircraft owner so I see these affects first hand every time I fly my family on a trip. It's not some academic exercise.

Now go back and find where I said anything about there not being a relatively small top level speed reduction due to weight affects. No one has ever claimed anything but that. Graham is the only one who has tried to narrow the issue to that one subject.

Instead of having a nice conversation, we are dealing with your lack of understanding on how a parametric study works to determine the affect of something. Parametric study is the most common and accepted way of determining the cause, effect, and magnitude.

My issue has always been that Graham's original post:

Actually, fuel state is not that important with regard to maximum speed.

Is not true because one cannot separate the affect as Lift, drag, and Angle of attack all have a fixed by design finite relationship. If the only affect of adding weight was small reduction in top level speed, then we could say it was insignificant.

It is not insignificant. That small drop in level speed represents a very significant reduction in the aircrafts entire maneuvering envelope.

In no way, shape, or form can we say that weight does not matter and has little affect on the airplane.

All considered it’s a healthy concept to keep in mind: the flying weight of your aircraft directly affects its performance. Each time. Every time. No exceptions.

http://www.principalair.ca/article-weight.htm

All the best,

Crumpp