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Why undercambered airfoils?

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Why undercambered airfoils?

Old 06-14-2024, 05:36 PM
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Default Why undercambered airfoils?

Many years ago I built a glider called the Caprice from a kit made by Keil Kraft, it was my first sucessful model. Fast forward 60-odd years and I have built another one from plans downloaded from the net, put in some RC gear and it flies very nicley from a "Hi Start" launch. Until of course I decided to upgrade the launch elastic and a blustery day caused a major failure of the wing (I recall that during my aeromodelling career I had many such disasters due to pushing my luck!)
Anyway I am rebuilding the wing. My son came by and saw what I was doing and said "why is that wing section curved underneath, I thought airfoils were flat on the bottom, you know, the air has further to travel, increased speed, drop in pressure etc etc."
I explained that for slow flying gliders an undercambered airfoil has some advantages, lower stalling speed, higher lift. If you are stooging around looking for thermals the ability to float at low speed has some advantages.
Inevitably the response was "why" "how" ?
Any body want to comment? I know that an undercambered section has a higher CL with the penalty of a higher CD and that their efficient speed range is limited, but as to how and why I am at a loss, it would seem that making the lower wing surface curved would have the effect of making the air's path UNDER the wing be closer to the path OVER the wing, so what gives?
You can get an idea of the airfoil section by downloading the Keil Kraft Caprice PDF (I can't post a link sadly)

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