If you make the aileron chord greater to gain room for a larger aileron spar, it becomes aerodynamically advantageous to continue the concave shape on the ailerons. And it could make it difficult to design the aileron spar with enough torsional stiffness. You might need a larger rudder, or move the rudder farther aft to improve its leverage. Yet another disadvantage is it increases adverse yaw from the ailerons. Some designs compromise the ideal under-camber shape to allow room for a rear spar, by only making the very aft 1/8 or so of chord concave. So you have to be careful not to add weight in the process.Īnother disadvantage is the under-camber doesn't leave much, if any, room for a rear spar. And you'll need pretty closely spaced ribs to keep the fabric from dipping between them. One disadvantage of adding under-camber is all the extra work in fabric covering. And it makes the plane glide much better. And it helps the plane climb well with low horsepower, which would help your plan. That airfoil is a Gottingen 387 on top, with my own design on the bottom.Ī deep under-camber like that improves your stall speed by at least 3 mph. Ideally, the included angle should be as small as possible, like the airfoil I used on the U/L in this picture. The air flows smoothly down gaining lift while decreasing drag. With under-camber, the included angle is much less, so the air doesn't collide. The greater the included angle of the trailing edge, the greater the inefficiency. In addition, there is inefficiency because the air flowing off the bottom collides with the air flowing off the top. But with an under-camber, the air is directed down adding lift. With a convex under surface, like a Clark Y, the rear part of the airfoil curves up toward the trailing edge, so it doesn't force air down much. Now picture the air flowing under the wing. The air flowing over the top is angled down a lot because of the shape of the airfoil. Picture the air flowing off the back of your wing. depending on how deep the under-camber ,and wing loading, and other factors. Much faster than that, and you can lose efficiency. As you get up near your proposed cruise speed, the gain would be small. In climb you'll still get significantly more lift and efficiency, that translates into a better climb rate. So near stall speed, you'll get much more lift and efficiency. At U/L speeds, the loss of efficiency of using a flat bottom is pretty insignificant.īut you can gain a lot of low speed efficiency by adding under-camber. Many, if not most U/Ls have a flat bottom airfoil just to make construction simpler, and you can save weight if you design for it. I have experimented successfully with adding undercamber on two of my designs.
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