Airfoils on Foamies

[Butch777]

I posted this else where but got little responce, so I hope to get more here.

I'm familiar with airfoils in general, Undercambered, the symmetrical, the semi-symmetrical, the flat-bottom ClarkY type, and Flying Wings reflexed airfoils.

But I've noticed that some of my foamie parkflyers use a different style.
It looks like most on top, but the bottom starts out like a symmetrical but goes undercambered half way back.

My E-starters, MiniMags, EPP flying Wings and a few more have this type.
It seems easier to put flat sheet ailerons, like depron on these types of airfoils.

So I'm wondering what the pros or cons are with this type.
I'm building a few different types of EPP planes and this type looks interesting. Thanks for any info, Butch

[JetPlaneFlyer]

Butch,
There is nothing 'foamy specific' about that airfoil; it looks reasonably 'normal'.

Airfoils are not really divided into undercambered, symmetrical, semi-symmetrical and flat-bottom 'sub-groups'. these are just catagories that modellers have dreamed up.
All airfoils have two basic properties.. thickness and camber. Depending on the 'curve' of the camber line and how thick the airfoil is, and where the thickness is placed, you can get a whole range of different airfoil shapes.

[Butch777]

So what would be the pros or cons of this type of airfoil, compaired to just a typical semi-sym found on RC plane?

[JetPlaneFlyer]

It should give higher Clmax (higher lift coefficient) than a typical 'semi-symmetrical' airfoil so it should slightly fly slower before it stalls. It may also be a bit more efficient so enable the model to fly on less power.

Down side is inverted performance will be worse and top speed may be down slightly.

Steve

[Butch777]

Ok, That makes sense. Thanks

[VeeOneRotate]

A simple way to put it would be; undercambering the lower surface acts like putting flaps down, you get more lift, and more drag.

[Texas Buzzard]

If the original poster was asking about airfoils for a 28" to 36" wing span common foamy then the flat plate airfoil will do the job.

During the past 2 years I have scratch-built about 14 flat plate foamies of Depron 6mm. All have the flat wings and all but one was succesful. The unsuccesful one was a delta with about a 50 degree sweepback. The CG was too far aft and since it was a pusher with the battery way up front it still was tail heavy - it wasn't because of the flat plate wing.

[JetPlaneFlyer]

If the original poster was asking about airfoils for a 28" to 36" wing span common foamy then the flat plate airfoil will do the job.

I'm not sure what the span was, it's not really relevant. The the original poster was asking about the actual airfoil used on his existing model.. Not about if he could replace it wath a flat plate ;)

By the way Butch if you are still reading... Your airfoil looks like an Eppler 197 but with a thickenned TE to make it suitable for 'foamy' construction.

Steve

[Butch777]

The airfoil is on a Multiplex MiniMag.
I've looked at some Eppler airfoils recently and your right, its very simular.
The reason I say "Foamie" airfoil is, so far, out of the over 300 Rc planes I've owned, I've only seen this type of partially undercambered airfoil on foam planes or maybe composite. It seems it would be hard to get a iron on covering to stick to just the ribs in the undercambered area. Butch

[ciurpita]

but isn't this type of shape, where it is concave or nearly concave near the trailing edge, pretty common? if you take the chord line of a symmetrical airfoil and bow it upwards, moving the top and bottom surfaces with it, won't you end up with this type of airfoil?


i also have a general airfoil question. in general i'll accept that thicker airfoils have more drag, and that airfoils need to have thickness to be able to construct a wing. but why do some people say thinner airfoils are better in some applications? (RN?)

if thinner means less drag, does that mean a zero thickness airfoil has the least amount of drag?

doesn't airfoil thickness affect max Cl?

[Flight Engineer]

looks a little bit like a super critical airfoil. not really important in a foamy. also i would hazard a guess that it's quite rear loaded. i.e the cp is quite far aft.