Wow, a popular topic for discussion....who'd have thought aerodynamics could be so interesting....
I think the example that Thunderball 2 gave about the engine noise device is much the same principle. John T's autorotating helo rotors are similar. In both cases the item is extracting energy from the flow to generate a force.
The statement "winglets produce thrust" should not be taken as saying they generate a force without the right conditions. What it means is that
at the design condition the winglet extracts energy from the tip vortex - energy which the engines have placed there through overcoming the drag of the rest of the wing - and uses some of that energy to create a small forward force component.
If one were to consider the whole wing, then I agree; the effect of a winglet is to reduce the drag force on the whole wing by some small amount. No questions.
But if I JUST look at the winglet then the local aerodynamic forces acting ON THE WINGLET ALONE are in a slightly forward direction (at the right conditions). You may prefer to call it "negative drag" rather than "thrust", if "thrust" implies a primary source of motive power; I'm using "thrust" to mean ANY forward acting force on the aircraft.
There was, and still is, one bizjet manufacturer who stridently purpoted that the winglets produced a forward thrust vector. Wind tunnel analyses that I studied seemed to verify this point, but.....there's no free lunch.
I wonder how far away that data is?
Actually, to extend the analogy, there
is such a thing as a "free lunch" - if one is prepared to sift through the garbage discarded by a restaurant, one may in fact assembled a small snack. Winglets are like that; you're using wasted energy from the tip vortex and re-using it. The costs come in things like weight, cost, etc. But aerodynamically they are, indeed, a "free lunch".
Come now people. If you were to mount and infinately lare winglet on an airplane and put it out in a field someplace and wait for a 100 mph wind to come along the winglet is not going push that airplane just because it reduces vorticies at the wingtip. All the winglet would do is lower the wind speed at which point the aircraft would lift off the ground.
Actually, what will also happen is that the plane, if free to roll, would move back more slowly with the winglet because the OVERALL drag would be lower. Clearly some new, extra force is pushing in the opposite direction to the "normal" drag. That force is caused by, and acts at, the winglet.
Of course, the plane would never actually roll
forwards, because there isn't enough energy in the air to provide enough force to be extracted by the winglet.....unless....
Put the same plane in the field, free to roll. Install monster winglets and blow high energy air at the winglet FROM THE DIRECTION THE VORTEX WOULD BE. The winglet, acting like a sail, will extract the force it does, as usual. With no forces acting on the rest of the airframe (we only blow at the winglet) which way do you think the plane will roll, forwards or backwards. You all know MY opinion
- this forum needs an "en-garde" smilie
-