28 Jul 2007, 08:15

Certainly the air load from drag will be higher at 800/SL, but that is such a trivial load for the structure, that it isn't even considered when determining the thickness of skins, spars, longerons, etc. Maneuvering loads from turns, rolls, sideslips, etc. are not necessarily highest at the highest airspeed. For example, take an F-16 in a 9g turn at 600/SL compared to 800/SL. Total lift is the same, Gross Weight x 9g. But the distribution of the lift (wing, fuselage, tail) is quite different. As speed is increased beyond 600 kt, wing twist reduces the wing angle of attack and wing lift. Since total lift must stay the same, the airplane angle of attack increases a small amount and the fuselage lift increases. So you have reduced wing lift and increased fuselage lift at 800 kt.

The actual structural load on the wing and fuselage is the sum of lift and inertia. The wing inertia is wing weight x 9g and the fuselage inertia is fuselage weight x 9g, both acting down. Wing lift (up) is much larger than wing inertia (down), so at 800 kt, total wing load is reduced. Fuselage lift (up) is much less than fuselage inertia (down), so total fuselage load is also reduced at 800 kt.

I have neglected the horizontal tail lift effects for simplicity - it changes things a little, but not the concept. Also not included is the effect of the wing lift shifting inboard, which reduces wing bending moments at 800 kt.

Not all component loads are reduced at 800 kt. Horizontal and vertical tail maneuvering loads are maximum there.

This is a simplified explanation of a very complex topic, but I hope it clarifies things a bit for you. I appreciate your interest and your question.