I was glancing through the hundreds of airfoils for my next homebuilt combat plane and quite frankly I'm just not sure which one would be good. I thought about copying teamseaholms avenger until I read that CG range is really small and if your off just a touch it can be disasterous. Any of you airfoil buffs got a suggestion?
Would simply a fully symetrical foil be an overall good way to go with the spar area back say 1/3 of the way? I like the fully symetrical idea due to the fact that it's easier to cut out of the foam but I don't know where to put the spar section. I need to keep speed and turning ability high on my list of needs but overall flyability is also important for my slow thumbs. So far I've just designed this plane in my head but I just got the foam for the wings so building is just a few days away.
72" span
12" root cord
6" to 8" tip cord
straight leading edge
1 1/2" thick at root
1" thick at tip
1 1/2" ailerons
approx 28" fuse
.32 size motor
Wide CG range
Thanks

assuming you'd prefer the model to not snap easily in tight turns and stalls i would suggest going for a 12% NACA 0012 at the root and NACA 0018 at the tip (12inch root and 8inch tip)

so the wing is equally thick all the way through.

If you want more root strength than a 12% airfoil can provide then just go for a constant percentage thick wing - e.g. NACA 0015 all the way through.

The spar should be positioned at the thickest point of the airfoil I think its 30% on the NACA 00 series symmetricals.

If you want a tighter turning aircraft you might want to consider a cambered airfoil (non-symmetrical)... I've used Blanchard WB135 recently on a lightweight funfly aircraft and its a gentle stalling, low drag, and reasonably thick airfoil (13.5%). http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1149

i would assume the CG problem is probably related to inadequately small tail area rather than airfoil choice. I suppose if the CG is already far back and you push it even further it could be the straw that breaks the camel's back

An option for a high lift symmetrical could be some of the eppler airfoils designed for full size aerobatic aircraft -

e.g.

eppler 472 http://www.nasg.com/afdb/show-airfoil-e.phtml?id=182
eppler 473 http://www.nasg.com/afdb/show-airfoil-e.phtml?id=183
eppler 474 http://www.nasg.com/afdb/show-airfoil-e.phtml?id=184
eppler 475 http://www.nasg.com/afdb/show-airfoil-e.phtml?id=185
eppler 479 http://www.nasg.com/afdb/show-airfoil-e.phtml?id=186

however i don't know how they'd operate at the low Re's of your 8 inch tip at a flying weight good for a .32 motor.

I've read that they snap easily but can generate a lot of lift but are quite draggy.

hope this helps! good luck and tell us how you get on!

On my recomendation some buddies used the 474 on their control line fast combat models a few years back. The 474 was fast and highly resistant to stalling.

I've seen a lot of the RC scale combat models that used flat bottom airfoils. From this I gather that more of the flying for this type of model involves pulling positive with little negative G maneuvers? If so then there's a good case to be made for a cambered airfoil of some form. Something like the Eppler 195 or 197. Or that Selig 8035 that is good for higher Cl's.

72 inch span seems like a lot of model for a .32 for scale combat. Or is this an open design flying wing style?

It needs to be a fast airfoil. I'm probably wanting to much (ya know my cake and eat it to) We had our first combat event a month ago and after watching my foamies that I build go up against 11 other guys I realized that I NEED to redesign my planes. I love the way my foamies fly for all around fun type flying but for combat they leave ALOT to be desired. I've been building these with the very scientific "that looks about right" method. They are to slow. Don't turn tight enough, and several other things. I just need to start by choosing a good airfoil then design a plane around it. I could just buy one but I like the building/designing part.
Let's go back to the simple fully symetrical foil. If I go with this, what effects does spar placement have? If the thickest part (spar) is close to the leading edge it will produce alot of drag,,,(right?)
How far can I move it back and what will that do? What will a tapered (thickness wise) do 1 1/2" at root/ 1" at tip. Is this a faster design due to thinner profile?

All else being equal....

[ul][*] Thinner is faster but thinner also stalls sooner[*] The more forward the max thickness (within reason) the more delayed the stall. Further back stalls easier[*] Camber in the airfoil raises the max lift coefficient you can achieve before stall occurs
[/ul]

In looking over Profili I found that there is an Eppler 472 that is 12% with the high point at 17.5%. That may be a good compromise. It'll tend to delay the stall and thus avoid high speed snaps in tight loops and turns but should be thin enough to let the model fly fast. The gain in lowering drag you would get by going to an airfoil with the max thickness further back than 25% would be offset much more by the reduction in stall resistance IMHO.

Don't forget light weight is also important. Lighter models loose less speed in turns and also accelerate faster out of the turns.

Scale combat models use 25's on models as small as 36 inchs. That's why I was wondering why you spec'd 72 inches for the .32. I would suggest that something in the 48 inch span with about 400 to 450 sq inches would be fine. The wing loading will allow fast turns and good acceleration if you can keep the weight down to about 2.5 to 3 lbs.

I'll also repeat that if you find that more of your combat flying so far involved pulling positive G than pushing negative G then you're a prime cadidate for an airfoil with a bit of camber (think semi symetrical here). The gains in delaying the stall during positive G may easily offset the loss in negative G turns. Just set the elevator travel limits so you don't get as much down elevator as up. You should have the elevator throws set such that the model does not try to force the airfoil past it's best Angle of Attack in any event. If pulling full elevator at medium speeds makes the model over rotate and forces the model into a tight but slow loop then you've got too much elevator throw.

if this is a flying wing design then you need to chose an airfoil with no or low pitching moment. e.g. a reflexed section or a symmetrical.

If it's a tailed design then you are not restricted by that requirement.

IMHO the speed of your aircraft is more likely to be affected by powerplant/prop or weight issues.

Also wing aspect ratio has a big effect on total drag - especially when you are in a high lift area of flight like a tight turn. Low aspect ratio wings will be lighter for a given strength and they will roll faster (very important in combat) but will also create more induced drag than higher aspect ratio wings.

Roughly speaking a wing with aspect ratio 5 with RG15 a good sailplane airfoil has about the same drag as a wing of aspect ratio 7 with a draggy eppler symmetrical airfoil with thickest point forward at 20%.

this is of course a generalisation but its worth looking at aspect ratio when you are trying to minimise drag. However you need a model that can roll fast and turn tightly more than a low sink rate high Lift/drag wing.

Keep the weight down as far as possible and get your engine tuned to the max. If you are using a tailed design then couple elevator to ailerons for tighter turns.