I am designing an electric, 4-channel r/c plane and I was wondering what airfoil I should use. Also, what factors should I consider in choosing the airfoil. With an electric craft, should I focus on high lift or low drag or a mixture of both? This is my first time in designing an aircraft of this complexity, so any other suggestions would be greatly appreciated.[8D]

To help we need to know a bit more about this model. Is it supposed to be able to fly fast or slow? Upright all the time or some inverted or 50-50? What motor and battery combo? Trainer, sport or full on 3D capable?

Well, I was planning on making it a slow flight trainer that spends moth of its time upright. I want to make it extremely stable in the air. So stable that even if you tried to flip it, it wouldn't invert. I'm not sure yet about the motor, but it will probably be around a speed 500. I also was looking at around 7-8 cells for the batt pack. But everything that I just said it still up in the air. I just started on the design phase and haven't made many decisions, except for the slow flight trainer.

OK then. First off the airfoil will only help it fly slowly. The stable part is more up to other elements. Smaller control surfaces with limited travel angles will pretty much ensure that the response is gentle. Don't make it TOO gentle though or you won't be able to respond to upsets from turbulence or be able to pull out of a "mistake" that happens a bit close to the terra firma.

Probably the best all around'er is still the ClarkY with it's great low speed and gentle stall. It's also quite efficient so you'll use less electrons per mile than with some others. If you REALLY want to go slow then consider something with more camber to the airfoil. Some of the pollywog'ish options with undercamber. Harder to cover but they can help extend the low speed end of the spectrum. I have a couple of old timer models that use these sorts of airfoils and can recomend the Gottingen 501 at 12% thick. The performance of this one is great on my Flamingo even with the typical covering sag. With some turbulator spars it would probably be even better. The NACA 6409 floats well but has a bit of a sharp stall so it's probably not what you really want. But basically for very low speed you want something in the 4% camber region. Some of the newer airfoils from Selig may work better but many of them require very accurate building and sheeted surfaces to see any advantage.

Oh, another that would work very well for "reall world" construction and covering methods is the Carl Goldberg section used on the old Comet Sailplane. I think, from looking at Profili, that it's the Goldberg G5. I've seen an overweight electric version of the Sailplane with this airfoil fly at a HORRENDOUS wing loading and the darn thing still floated like a feather. It seemed to have gentle stall manners as well.

To ensure the stall is gentle and does not drop a tip at all be sure to build in some washout at the tips. This can either be done at the building board or, if the wing structure is flexible enough, warped in using the covering. Generally -2 to -3 degrees leading edge down at the tips works well. These airfoils all mean that this model is not going to be happy upside down for long. In fact you'll probably find that it just won't do at all. But positive maneuvers like inside loops, rolls and chandelles will sure look nice.

Good luck with doing your own design. It's the best part of this hobby IMHO.