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Quote:
At the same speed, a lighter aircraft must descend more steeply so that the reduced weight vector provides enough effective thrust to counter the effectively-constant drag:
What he said ^ The force equation ^
The light aircraft will have MORE drag than the heavy in a, same speed scenario (like in the 767 case above).
The reduced weight will give the light aircraft a shallower AoA moving
it to the left side of the optimum airfoil drag bucket which means increased
drag and having to do a steeper descend to maintain airspeed.
If we do some calculations on the 767 chart, we find that the
Light acf has an average descend angle of -3,03 deg and the heavy acf -2,69 deg.
So the difference is only a 0,34 deg average descend angle.
Energy calculations don't really work well in an environment where there is energy loss. In cruise, the two B767 engines produce around 20.000 lbf of thrust which means thats the amount of drag you got. Thats alot
of friction (energy loss). It's like having a B737 engine at TO thrust sealevel pointing in the wrong direction... .
XPM