Airfoil selection for light electric twin
 

Hi,

For a class project on autonomous aircraft design, construction, and testing I need to select an airfoil for a light, twin electric. Some quick background:

The main challenge will be designing the autonomous control system. The only data we get is GPS at 1Hz, no gyros or accelerometers allowed. This means that the plane will need to be very stable, both statically and dynamically.

The challenge is to take off by handlaunch, then circle to maximum altitude, then land at a designated point. The power system is two small electric motors, each puts out about 60 grams of static thrust. The control board is also preselected for us, along with a receiver and some micro servos. The planes will be about 3 feet in wingspan, with foam wings made on a computer controlled foam cutter. Plenty of outboard dihedral for spiral stability, probably some small winglets for additional stability too. The whole thing will probably weigh around 300 grams, not really sure about that yet. The propulsion/battery system is sized so the planes will probably barely climb, so the flight envelope won't be anything like an overpowered acrobat. Attached a pic of an example of an earlier class. Their req'm'ts were different, but a general idea of sizing, etc. can be seen.

I was hoping some people who have experience in this flight regime might have some suggestions for airfoils that would be good.

Thanks,
Jamie

RE: Airfoil selection for light electric twin
 

I've used a NACA 2412 before, with a transition to a 2415 at the tips since it has a gentler stall than the 2412. Both have fairly decent lift/ drag ratio's and are capable of Cl of around 1.5. The flat bottom makes it pretty easy to make also ( one of the reasons that I went with it.)

RE: Airfoil selection for light electric twin
 

I suggest you do some searching for airfoils for very low Reynolds number. What you will find is that it is hard to beat the "curved plate". The flat plate is also very good at small cord lengths and low dynamic pressure. These both have advantage in easy to build from bluecore or Depron foam. The main reason for a thicker airfoil is it better supports a cantilevered structure.

RE: Airfoil selection for light electric twin
 

If you have access to a foam cutter, look toward the airfoils used on discus and hand launched gliders. Mark Drela has developed many successful foils for this type of thing and since you are looking for light wingloading applications with efficient lift characteristics at low Re #'s, I would check those out. You can use more simple foil designs and they will also do the trick...but if you have the tools use 'em. Having seen many RC college competitions over the years, the guys who win build simple, practical planes and usually have RC experience. Over engineering it will have the opposite effect.

BTW, winglets don't necessarily add stability, sometimes the opposite in yaw stability (picture wind gusts from the side). If you model it after a free flight glider or other stable model plan-form you'll be just fine. :)

Mark Drela's X-foil

http://raphael.mit.edu/xfoil/