[Crumpp]

Graham's original point was just that effect of the fuel state is small at high speeds near sea level in this kind of planes.

No Grahams original point was in response to my statement:

Crumpp says:

Weight very much affects our aircrafts envelope. It has a direct relationship with velocity.

Graham then says:

No. Weight affects the envelope, indeed, but the direct relationship is wrong.

Which is not correct.

It certainly does have a direct relationship with velocity.

You have tried to morph my position into something of your own invention.

My contention is:

While weight effects are smaller at high velocity, they are far from insignificant. Weight certainly makes a difference not matter what portion of the envelope we examine.

Does velocity change when we add weight? YES

Does this change in velocity signal a reduction in the entire envelope of the aircraft no matter how small the specific change in velocity? YES.

All these changes are accounted in my demonstration ie higher Cl, therefore higher Cdi and Cd and further slower speed and therefore even higher Cl and higher thrust and so on.

Wow. You don’t get it. Of course they are “accounted for” when determining the specific performance.

What they do not account for is the magnitude of the affect of weight. To do that, we need a parametric study.

Your contention:

This is also why the parametric study can't be used because we don't have a constant key parameter.

The idea we need a steady parameter to hold the parameters steady is laughable.

It is complete horsepucky of your own invention. The whole point of a parametric study is to isolate affects in a complex system of changing parameters.

The airfoils key relationships of L/D ratio to AoA are held constant to determine the affect of weight in isolation:

V2/V1 = SQRT(W2/W1)

V1 = 300KEAS

V2 = ?

W1 = 10000lbs
W2 = 10500lbs

V2 = {SQRT(W2/W1)}*V1

V2 = {SQRT(W10500lbs/10000lbs)}*300KEAS
V2= 307.4 KEAS

If we hold angle of attack constant, we must increase our speed 7.4KEAS or 8.5 mph if we add 500lbs weight. Well one just might think that is insignificant. It certainly isn't gong to make much difference in what we can we catch or run from.

Now lets look at it from the Power required relationship in our fictional aircraft. Using standard BGS formulation for a power producer:

Pr1 = 2000thp

Pr2/Pr1 = (W2/W1)^3/2
Pr2 = {(W2/W1)^3/2}*Pr1
Pr2 = {(10500/10000)^3/2}*2000thp

Pr2 = 2680thp

Or a 34% increase in the amount of power required! That 4-5 KEAS in reality represents a very significant reduction in the designs power available.

All the best,

Crumpp