C.L. Chennault wrote on Sep 21
st, 2008 at 7:51am:
The Clark Y is well known as a good all around airfoil among aero modelers. It was also used on the Spirit of St. Louis. Interesting bit of aviation history.
A little background.
When I was just a wee small Terribleist, ca.-1960 or so, all the hot guys were talking about how they were moving away from the Clark Y. Somewhere in there at the end of things, Keith Laumer (remember Keith Laumer?
How to Design and Build Flying Models, a seminal work full of
charming designs--he went on to be a pretty important science-fiction writer. Me, I
started in sf, and then went the other way. I'm not famous.) commented about Clark Y having had its day among the hotshots, but the air didn't care much. I remembered that statement.
Now, the real Clark Y is a semi 'foil with a little Phillips entry running into a flat bottom about 30% back, and it's about 12% thick. Pretty thick for rubber, though, and guys were going to a "modified" Clark Y, just slicing the semi off the bottom at the end of the nose radius, or just at the meanline and tweaking the rose radius. This gives more like 10%. One of the compelling reasons for lopping off the bottom was pure unreasoned fear: some part of the airfoil
curved the wrong way. "What? thee's minus-lift on the bottom taking away the plus-lift on top? How's
that help me? Nooooo, can't have that, nosirree..." The same sentiment is applied to washout now, or dihedral, or anything that looks like it removes effective lift while affording real stability.
According to the real snobs, rubber scale in our size range likes maybe 6% foil with a sharp edge. That's what the snobs say. Well...then a buddy of mine will make his ribs 9%, thinking that the tissue inbetween the ribs will sag down to 6% and he's in the ballpark. But that sharp LE depends on tightly-controlled attitude or it'll start to lose boundary layer adhesion, so it's for a model that already has hella longitudinal stability. I dunno about you, but I build what I love, not always what's got the Optimized Planform, soooo...
When I grew up and became the F/F imp I am, another buddy and I started whittling down our favorite-shaped airfoil; based on the 'foil of a great sport flyer, the Pacific Ace/Black Bullet (all our airframes can be traced directly to these two models!), we muddled about for a few years and have settled in to something that looks pretty much like a skinny Clark Y, with the entry back and a more-regular top camber, breaking a little further back. Now, this may not be the best-lifting 'foil ever nor the lowest-drag one, but it's forgiving through a wide range of conditions, which is what you get in the Real World.
And if you look around, just about all the non-undercambered 'foils in popular use today are no more than two glances removed from the ol'Clark Y, just back down in the ~9% range. And, given the chaos induced by all the lumpsbumpsbells&whistles of Scale models, I personally think the air's already dealing with too much else to give a fig for the fine-tuned sophistication of the airfoil. Even if you managed to pack such accuracy into your rib-cutting.
So that's where I'm at with this.
Michael