Originally posted by MinnFlyer
1. If Bernoulli's principal has anything to do with creating lift, it is minimal at best.
So why are the "Sporty" models using fully symmetrical airfoils? Mainly for speed. If, for example, you want to do a "Split 'S'" (From straight and level flight, roll halfway to inverted, then add "up" elevator causing the plane to come down in a half loop and pulling out right side up going 180 degrees from your start heading) on the downside of the Split S, you pick up airspeed, now, when you try to pull out, a flat bottom wing going so fast will have a tendency to "Balloon" at the bottom causing a straight and level pull-out to be very difficult to maintain. With a fully symmetrical wing, the extra speed results in very little extra lift.
While downwash is the creator of lift, it needs a mechanism by which it transmits its reaction to the wing surface. This is done by pressure of air molecules. Since the change of velocity incurred by turning the free stream downwards will alter the pressure, the Bernoulli equation can be used at any point to calculate the pressure distribution. Bernoulli's law is absolutely correct at any point on the surface, where it goes wrong is when people use it as an explanation of lift with the wing somehow being one side of a venturi tube!
It is not true that flat bottom wings zoom with extra speed. The YAk52 full size that I fly has a flat bottom wing yet is considerably better at aeros than the symmetrical wing Robin 2160. The Robin zooms immediately on an increase in speed whereas the Yak is much more neutrally stable. And that is the crucial word - stability. Stability is all about tail volume ratios and static margins, the wing section has no variable in the equations used. You are used to flat bottom models zooming because it is used mostly on trainers and they have a lot of stability. It is the stability that causes the zoom, not the section. Symmetrical foils tend to have blunter noses so have less sharp stalls, and they perform as well either way up, that is why they are popular.
Harry