Originally Posted by
hans brinker
Ahm no. No. NO.
Imagine a balloon filled with helium with a weight attached to it floating around. Cut the string, and the weight falls down and the balloon shoots up. That is what happens when an aircrafts releases its payload in flight. Either the aircraft goes up, or the load factor is reduced. But absolutely never will the load factor increase from a decrease in payload.
That analogy doesn't work. Or if you do want to stick with this: When you cut the string, the lift on the balloon is appreciably more than its weight (I know, it should be mass...) which equates to a L/w of more than 1. If you were to be inside that balloon, you would feel that as a perceived G-force.
Originally Posted by uplinker
If a wing is providing a lift force to carry a certain weight, and that weight is removed; the stress and strain in the wing structure will be reduced.
Not exactly, it will change, but it's a bit too simplistic to call that a reduction or any kind of beneficial effect without knowing the specifics of that change. Others have commented on the W_wing and other relations. Let's look at what happens inside a wing. I can't provide the shear and moment diagrams for these wings right now but think of the wing as a simple beam, supported at the inboard end by the fixings to the fuselage (I'm ignoring the strut for now). When that wing is being used (prior to the drop), there is a force, trying to bend the beam upward, which is distributed over the entire span and we call that lift. The structure weight of the wing and any other bits (fuel inside it) are pulling the wing in the other direction. These forces do not cancel each other out as the lift force is significantly higher than the others (half the total weight) and therefore there is a distinct bending moment and other forces on the attachement fittings. From this also follows that the beam itself needs a distinct amount of rigidity and strength to cope with these forces on it. If a significant load is dropped from the fuselage, the attachment load will change rapidly. Combine this with a pull-up that will increase the lift force on said wing and the attachment fittings will see a rapid change of stresses/moment.
As pilots, we are all trained to respond to configuration changes and other effects, so bearing in mind that I have never done any flying like this, the normal reaction of the pilot on dropping a load from a hopper would be to push the stick forward, decrease AOA and thereby keeping the aircraft on its level flight path. That avoids the aeroplane moving up, although you can still get a momentary flight path deviation of course. In this case (we're veering away from the theory again, apologies) the drop and pull-up appear to be (difficult to see on this video) timed pretty close together and that's why I started wondering about the drop and the pull-up ganging up on an unsuspecting bit of metal in the airframe.