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rcgldr
#10
Jul16-09, 05:19 PM
HW Helper
P: 7,110
Quote Quote by Brendon_agius View Post
So an airfoil cannot achieve greater lift force than drag force
I never stated that. In the case of high end gliders, such as a Nimbus 4T:

http://en.wikipedia.org/wiki/Schempp-Hirth_Nimbus-4

the glide ratio of the entire glider is 60:1, which corresponds to a lift to drag ratio of 4:1.

If the Nimbus weight was 1500lbs, and acheived 60:1 glide ratio at 60.0083327547 mph, the decent rate would be 1 mph, the forward speed would be 60mph, and the power consumed would be only 4hp (4 hp = 1500 lbs x 1 mph / 375 conversion factor). The power input is 4 hp from gravity, and the glider in turn transfers the energy gained from gravity to the air at the rate of 4 hp.

What I stated was that an airfoil can't produce thrust from an apparent headwind. Continuing with the glider example, imagine there's a an updraft of 1 mph. The glider can then maintain level flight at 60 mph forward. The updraft of 1 mph is the apparent crosswind, which the wings divert into thrust sufficient to propel the glider forward at 60mph. At this speed the thrust from the wings is equally opposed by the drag, which is related to the apparent headwind, so the glider ceases to accelerate once at 60 mph. The wings cannot generate any thrust from the apparent headwind of 60mph. If the updraft ceases, and the glider maintains horizontal flight, it slows down (until it can no longer maintain horizontal flight) because the wings can't generate thrust from the apparent headwind, only from an apparent crosswind (the updraft in this case).

The text at some sailcraft web sites make the implication that the faster the glider moves forward, the more thrust (forwards force) the wings can produce, which is false. In the case of a sailcraft, the total force will increase, but not the component of this force in the direction of travel (thrust). Getting back to the glider, the amount of thrust the wings can produce is related to the apparent crosswind (in this example the updraft) only; the apparent headwind is an overhead, and actually reduces the thrust produced as forward speed increases due to drag. What these web sites don't make clear is that the role that the apparent headwind plays is that it and the apparent crosswind are diverted with sufficient "upwind" component against the "true wind" to slow down the true wind (in this case the updraft) and extract energy from it.