[drgondog]

[quote=Graham Boak;70211]

Originally Posted by Crumpp

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

V2/V1 = SQRT(W2/W1)

[End quote]

No. Weight affects the envelope, indeed, but the direct relationship is wrong. For aircraft of this vintage, it is probably true that induced drag has a linear relationship with weight, but induced drag is only part of the story. The effect of weight on speed varies with altitude, because it varies with induced drag only, and induced drag increases with altitude (at any given speed). At low altitude and high speed, the drag term is dominated by the zero-lift drag. This is made up of skin friction drag, profile drag and excrescence drag, none of which vary with weight.

Induced Drag varies as the square of CL, and for a 5% weight increase - CL varies 5% if speed constant. CL varies directly with Weight for level flight.

At max speed and power settings the same ship at 1.06 W (a P-51D versus B-15) using same engine and TO fuel weight, same aerodynamics but heavier frame by 500-600 pounds can not achieve the lower weight max V.. so AoA increases slightly to get new CL which must increase by 6% to maintain equilibrium.


For high altitudes and low speeds, the effect is reversed, as the drag is dominated by the lift-induced drag, and the top left corner of the envelope is notably smaller at higher weight. The bottom right is hardly affected.

I made my living doing these sums. There's a lot about aircraft performance I never knew, but I think this is fairly basic.

The rest of the posting is sound, and a very useful addition.

Graham, what you say about low speed/low altitude versus high speed/low altitude is true relative to discussions about relative induced drag and parasite drag

But whether the extra weigh is due to increased fuel, or internal load, or more structural weight for same basic airframe, the Velocity change for same power settings and the increased AoA resulting in a higher CL results in the SQRT (Wheavy/Wlight) change in V