Airfoil for slow flying trainer???

[Butch777]

I've got several EPP trainers that I'm messing with. Most are 24'-28' and a 9mm EPP wing with a slight airfoil sanded into it. They fly ok but don't glide at all. Also the airfoil needs to be able to handle some wind as well.

I'm also flying a slofly wing from Slofly.com, that I put a twin tail and half fuselage on. At 6ozs, with a CustomCDR single, it flys very well in the wind and the street, so I asked the the designer about the airfoil and he didn't want to give it out, but said it was a reflexed airfoil. But I'm wondering that if because I put a tail on it, that a different airfoil may be better??
Any ways, I have a Feather cut and bow, I just need a airfoil to start with.
The cord is kinda wide and so I don't want to go too thick, plus I don't want to add too much weight. Heres a pic, Hope you Guys can help. Butch

[vintage1]

The key to a good glide is low wing loading and low drag.

At our reynolds numbers airfoil shape is (almost) irrelevant.

For a trainer, a sort of 10-15% flat bottomed section drawn to look 'just like everyone elses' is probably as good as it gets.

Get the stall speed down under 15mph if you can, and put some washout - 2 degrees - in the wing as well, and you will have a docile well gliding plane that doesn't snap and drop a wing if you over cook it in landing..

A full body rather than profile will probably help drag, as will some iron on film over the foam.

[BMatthews]

Well... up to a point that's true.

Generations of slow flying free fight models have shown that while the airfoil shape for thin airfoils is quite tolerant within reason that the camber and leading edge is quite critical and makes a big difference in how the wing performs at slow speeds.

Think of a sail on a sail boat, It's thin but it has camber. It's that camber that allows the wing or sail to guide the air in a smoother change of direction and generate more lift (actually a higher lift coefficient) before a stall takes place.

But if you arc or camber the depron sheet to get a better slow upright flying speed and delayed stall then when it's inverted it'll act poorly. And from the looks of that model inverted is important.

Another option is that within reason a thicker airfoil stalls later than a thin one. So cutting out a proper shape symetrical airfoil of about 12 to 14% will delay the stall to a higher lift coefficient and you end up being able to fly a little slower.

If I'm wrong about your desire for inverted then try adding about 14 to 3/8 of an inch of camber to the wing. Keep the sharpening of the leading edge as well but perhaps make it a bit more blunt. It's more of a wedge than a rounding. However you could use the present leading edge shape at the trailing edge.

[Butch777]

For this trainer, I'm not worried about inverted flying. But on the other hand, I am worried about flying in the wind.
Also, I'm trying to get a airfoil rather than a flat plate, because of hopeful better flying qualities and a airfoiled wing would be thicker, so a bit stiffer.
I have several EPP planes, by Potensky and Freeair, that don't use CF rods in the wings. But there wings are mostly semmeterical.
So I like to find a airfoil that is not too thick(because of weight and drag), but thick enough to be stiff with one CF rod or no CF rod. And have good slow flying qualities. Thanks, Butch

[BMatthews]

Well. for that you just cannot go wrong with the good ol' Clark Y. Not some arc topped flatt bottomed shoe shape but the proper ClarkY as intended by the fellow that designed it.

If you don't have any airfoil software that will show it I suggest downloading the free version of Profili2 from www.profili2.com . It has a lot of basics and will print your single airfoils just fine. It's got a bunch of neat stuff locked away as well that you can unlock for about $15US.

[Butch777]

It seems that if I use the Clark Y airfoil, it would be too thick.
What would be the effect of narrowing the airfoils a little?
Thanks, Butch

[macr0t0r]

The Clark Y is good. For gliding, the critical part is getting a good taper to the trailing edge. If you have extra-thick ailerons that are blocked off, you'll have no pressure recovery, which will have a very draggy effect. Gotta make it tear-shaped.

- Jim

[Ollie]

Use an AG 36 airfoil. See:
http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm

[BMatthews]

If you think that the ClarkY is too thick then the next step is to go for one of the more free flight like thin and undercambered options.

The secrets of slow flying are light weight and higher camber values in the airfoil. If you want to use a thin airfoil then you end up with a concave lower surface. There's no way around that.

Look up something like the NACA 6409 and 4409. Also something like the Benedek 6356 and 7457 airfoils for examples of what these look like.

If you don't have it already then download and install the free version of Profili 2 from www.profili2.com . It'll let you view and print out hundreds of options.