I wanted to what a 'reflex camber' is. Is it basically a symmetrical wing? what are its properties.

From : Don Stackhouse

It's where the trailing edge portion of the wing is bent up, to give the airfoil a nose-up aerodynamic pitching moment, or in aero engineering terms "a positive Cm" (i.e.: moment coefficient).

The moment coefficient describes how much the airfoil wants to pitch up or down on its own. Most non-symmetrical positive-cambered airfoils want to pitch nose-down, so their Cm is negative. If you pitch the nose down and increase airspeed, the aerodynamic pitching moment increases, causing the plane to pitch even more nose-down, increasing airspeed even more, which causes even more nose-down pitching, which increases airspeed even more... The opposite happens if you pitch up and start reducing airspeed. Obviously this is statically unstable, or more specifically "statically divergent in pitch. That's bad.

A symmetrical airfoil normally has a moment coefficient of zero, because its camber is (by definition) zero. If it wasn't, it would not be symmetrical. It's mainly the camber that causes the pitching moment. Note, there can be exceptions at very low Reynolds numbers (i.e.: low airspeeds, low air densities, and/or small chord lengths) if funny little quirks like boundary layer separation bubbles start to make the airfoil appear unsymmetrical to the air molecules. However, in general a symmetrical airfoil has a Cm that is for all practical purposes zero. In that case, if you change airspeed, there will be no change in the aerodynamic pitching moment, and the airplane will stay pointed in its new direction. This is neutral stability, the plane goes wherever you point it.

In the case of the reflexed airfoils, the trailing edge is bent up. The overall camber is still positive, so the airfoil still lifts upwards at positive angles of attack. The reflex has only a small effect on the airfoil's overall lifting qualities. However, since the trailing edge is quite far from the airfoil's aerodynamic center, it has a lot of leverage to work with, so it has a very strong effect on the aerodynamic pitching moment. By giving up a small amount of lift, it can change the Cm to a positive value. With that, an increase in airspeed from pitching the nose down will cause a nose-up effect. That will bring the nose back up towards the original pitch attitude, making the airfoil naturally stable.

Martin Hepperle has a good discussion of this subject on his website at: http://www.mh-aerotools.de/airfoils/index.htm

including some diagrams of non-reflexed and reflexed airfoils. He also has some good general advice on his site about the design of flying wings. We also have a fair amount on this subject in the "Ask Joe and Don" section of our website: www.djaerotech.com

A word of caution: Flying wings can be as efficient or even more efficient for certain applications than a conventional layout with a tail. However, they are much more difficult and complex to design properly, tend to have a much narrower range of efficient flight conditions (unless the designer is either very clever and very knowledgeable, or else very lucky), and in general tend to be much less forgiving of engineering mistakes. If you're planning to design a flying wing, first learn everything you can about conventional aircraft design, as well as the peculiarities of flying wing design. If you don't want to rely on blind luck, you'd better be ready to do some serious learning. Designing a flying wing correctly is not for the novice, nor for the faint-of-heart.

Don Stackhouse
DJ Aerotech