While looking through different models many times under the airfoil type it will say "flat bottom type". Does this truly tell me anything of the characteristics of the airfoil?

For example even though I have many flat-bottom trainers that are slow and cannot penetrate, I also have other gliders that use both thick and thin flat-bottom airfoils that are very fast on the slope and penetrate very well. Is there really any behaviors that are consistent to all flat-bottomed airfoils? Or is it just an easy to chew word that is slapped onto all beginner craft since beginners don't know any better?

From : Don Stackhouse

For the most part, the latter is correct.

Airfoils are something that modelers, especially sailplane flyers, seem to obsess about. However, the details of the airfoil taken by themselves, outside the context of the rest of the design, are virtually meaningless. "Flat-bottomed" is even more vague, and tells almost nothing of value except that the model might not fly as well inverted as it does upright. Even that isn't a certainty.

It's about like saying "He wore black shoes". They could be black high-top basketball shoes, black low-top running shoes, black sandals, black Oxfords, an unusually large pair of ladies' black spiked heels, a pair of black Army boots, or any number of other possibilities. It tells you essentially nothing about the person, their abilities, or even what sort of activity they were dressed for. Taken by itself, out of context, it's essentially useless information.

I can (and have) designed airplanes with outstanding low speed performance and handling that used what would probably be considered "racing" airfoils. I've also designed airplanes with outstanding high speed performance that used what would normally be considered low-speed (even "flat-bottomed") airfoils. There were good reasons in each case for using those particular airfoils, and it would be impossible to make a valid evaluation of the design without properly considering those reasons. Your own observations corroborate this.

Even if the information listed the airfoil as an "SD 7037", or an "MH 32", and even if we assume the manufactured airfoil adequately matched the wind tunnel test model's shape and Reynolds numbers, it still doesn't mean anything without considering the aerodynamic needs of the aircraft, and how the effects of planform, twist, flight condition, etc., vary along the span and what resulting local demands are made on the individual airfoil shapes along the span. The airfoil that is ideal at the root is usually not the best choice for the mid-span location or the tip. Reynolds numbers aren't the same (even for a constant-chord wing, if you're looking at the aerodynamics during a turn), the required lift coefficients aren't the same. Even the supposedly "ideal" elliptical wing does not behave in practice the way its oversimplified theoretical analysis does.

Airfoil specs tend to be mostly buzzwords. Any change on any parameter in an airplane design will have "ripple effects" on every other parameter of the design. To understand the design, you really need to look at all of the specs together.

Don Stackhouse
DJ Aerotech