Both semi-sym & sym typically have more drag than a flat bottomed airfoil.

In order to generate the equivalent lift of a flat bottomed airfoil of the same thickness, aspect ratio and area, the semi & fully sym airfoils must operate at a higher AOA, resulting in more induced drag.

The fighters of WW II typically had flat bottomed airfoils. They had an absolute need for speed & agility.

It is also worth noting that all-out aerobatic models employ thick fully sym wings, not only for all-around agility, but also to slow them down in the down lines.


A flat bottomed airfoil also doesn't necessarily balloon any more than either one of the sym types. In level flight, all three must develop the same amount of lift to support the same weight. The Flat bottomed airfoil will be flying at a near zero AOA, while the semi & sym types will be flying at a higher AOA. The initial gust response will actually favour the flat bottomed airfoil, but not very noticeably. However, it is possible that the flat bottomed type will decelerate a bit more slowly & thus retain gust-induced excess lift a little longer, but again not to a particularly noticeable degree.

In cases of flight with all three wings at a perceptible positive AOA, gust response is slightly different. The flat bottomed & semi-sym types tend to pitch up more strongly (the semi-sym a bit less) relative to the fully sym wing.

I suspect that you are being mislead by the way that these wing types are typically employed. Flat bottomed airfoils are typically employed in low wing-loading, high dihedral, large area applications, while sym & semi-sym types are usually employed in higher performance models and typically have higher wing loadings and lower dihedral angles. These types may be faster and will certainly be more resistant to gust perturbations -- but it ain't necessarily the wing.


The real advantage of flat bottomed wings for trainers is their greater lift capability. Although they are no more stable, they allow lower take-off/landing speeds and a greater maneuver margin to accommodate inadvertant control inputs.