Give me a generally good airfoil for a slow, .40 size model

[Accu157]

I know it's a little more complex than that, so here's the goal:

This model is going to be light for a .40 size plane
The airfoil needs to be good with half length flaps and half length ailerons
The plane needs to be able to slow down well, not quite a crawl, but needs to be good for STOL (no surprises like an unusually sudden dropout in lift at low speed).

... that's about it. I'm not too picky about airfoils for a powered model, other than I do not want it to behave like the usual flat bottom airfoil used by most trainers. This isn't going to be a trainer, but it's going to be a nice quality model. Throw something out there. :)

[JetPlaneFlyer]

A good old Clark-Y takes some beating and would meet all your stated requirements. Nice and easy to build too. The only area a Clark-Y may be found a little wanting is for high speeds and inverted performance (but you don’t list these as requirements)

A Clark-Y could well be the airfoil used on the trainers you're thinking about (for it's safe stall and good low speed characteristics)... but a trainer flies 'like a trainer' not because of it's airfoil but because it's set up to have lots of stability.

[vintage1]

Totally agree. I would have said go undercamber, but with inboard flaps to slow it down, no point.

Only thing to add is put a couple of degrees of washout in it to make it a complete pussycat when turning near the stall speed.

[biber]

And even the real fullsize Lo 100 aerobatics glider is equipped with the Clark-Y.

Note that most models that claim to have a Clark-Y don't have a real Clark-Y but something remotely similar to that, only much worse.

biber

[vintage1]

Note that most models that claim to have a Clark-Y don't have a real Clark-Y but something remotely similar to that, only much worse.

biber

In this country, "Clarks" is a manufacturer of shoe.

A Clark-Y is what you get when you draw round the edge of that shoe :D

Frankly, I've drawn up so many flat bottomed sort of airfoils, and built planes from them that fly well, that I no longer care..with covering sag, inaccuracies in building, it really makes very little difference exactly where the peak of the curve is, or its exact shape.

What seems to count for more is the camber, the chord to depth ratio, and the overall change of those over the span. Plus washout.

Unless you are making a sheeted and glassed wing to very high accuracy, there doesn't seem to be much real difference at sub 6 foot spans, between one e.g. 15% chord flat bottomed section and another..

[biber]

Of course a glow engine in front of the model will pretty much even out any airfoil choices.
In fairly decent gliders you notice much more differences in the performance and characteristics of airfoils.
So yes, the basic airfoil parameters do count in all cases in the first place, but if you go deeper into it, you'll notice the original Clark-Y is a very decent airfoil.
An oldie but still goldie.

biber

[coosbaylumber]

I have built a few S.T.O.L. aircaraft, and none of which use anything resembling a Clark-Y or flat bottom airfoil. All of which use a bit or extreme undercamber.

This was written about in some edition of M.A.N. back in the 1980's when experiments were mentioned as to suitable airfoils.


Wm.

[vintage1]

Undercamber (or flaps) nets you a very low stall sped at the expense of terrible high speed drag. It also worsens the rapidity of stall onset in my (limited) experience..

Flaps is better..the question then becomes which airfoil works best with flaps and still has a decent medium speed performance.

Ive looked at a full size STOL plane to model and it looks very much like Clark Y with really massive flaps..

[Accu157]

A good old Clark-Y takes some beating and would meet all your stated requirements. Nice and easy to build too. The only area a Clark-Y may be found a little wanting is for high speeds and inverted performance (but you don’t list these as requirements)

A Clark-Y could well be the airfoil used on the trainers you're thinking about (for it's safe stall and good low speed characteristics)... but a trainer flies 'like a trainer' not because of it's airfoil but because it's set up to have lots of stability.

Ah. I should clarify. :) I don't like the constant need to adjust the elevator for all speeds of flight. I like the relatively steady pitch at a wide range of speeds. But I will settle for anything at this point. This model might max out at 55 mph with a shallow dive and full power, but it will hardly fly at that speed the whole time. On to read the next 5 replies.

Clark-Y it is. :) BTW, I'm after near-STOL abilities. I know that with the wing loading this plane will have, it will not be a problem.

Any thoughts about the other clark airfoils?
http://www.ae.uiuc.edu/m-selig/ads/coord_database.html#C

[biber]

Ah. I should clarify. :) I don't like the constant need to adjust the elevator for all speeds of flight. I like the relatively steady pitch at a wide range of speeds. But I will settle for anything at this point. This model might max out at 55 mph with a shallow dive and full power, but it will hardly fly at that speed the whole time. On to read the next 5 replies.No need to adjust elevator to different speeds for any common airfoil, if you just dial in a decently moderate stability margin.

biber

[vintage1]

..or add downthrust

[JetPlaneFlyer]

Ah. I should clarify. :) I don't like the constant need to adjust the elevator for all speeds of flight.

As Biber said and I hinted at in my original reply.... This characteristic is due to the generous stability generally built into trainers, it's absolutely nothing to do with the chosen airfoil.

Clark-Y still looks a good bet.

Steve

[BMatthews]

If you want to keep it simple and don't have the need for a higher speed range then I'd suggest you stick with a thicker undercambered airfoil.

Options that I've had personal results with are the Eppler 201, Selig 4061 and Gottingen 501. The E201 actually has a nice speed range but can slow down nicely for a pretty darn decent low speed approach. The G501 excells at low speeds and has an OK "get up and go" but not as good as a Clark-Y by any stretch. But if comes with "built in flaps". My Bird of Time was modified to use a Selig 4061 and it slows down nicely but doesn't have any issues at all with flying fast. A nice easy to deal with stall as well.

Having said this if you want to use flaps then just stick with a true shaped Clark-Y. It's a seriously underrated airfoil that can get up and move but can also slow down on demand and the shape is easy to build. And if you add flaps then you're building in some serious lift and drag all at the same time to deal with STOL issues.

If you slow way down to near stall you'll be able to deploy the flaps with little elevator input needed. You'll need a little down trim at first but then it should slow down and re-trim to stable speed but slower. If you can do it some mixed in down elevator will reduce the ballooning tendency with no effort from you other than a little programming time to set up the volume of the mix.

For a truly no fuss no muss setup just go with a higher camber option in the first place and don't plan on any speedy flying.

[BMatthews]

Ah. I should clarify. :) I don't like the constant need to adjust the elevator for all speeds of flight. I like the relatively steady pitch at a wide range of speeds. But I will settle for anything at this point. This model might max out at 55 mph with a shallow dive and full power, but it will hardly fly at that speed the whole time. On to read the next 5 replies.

Clark-Y it is. :) BTW, I'm after near-STOL abilities. I know that with the wing loading this plane will have, it will not be a problem.

Any thoughts about the other clark airfoils?
http://www.ae.uiuc.edu/m-selig/ads/coord_database.html#C

I missed your requirements the first time through.

The G501 flying with a slightly larger than normal stabilizer and a slightly rearward CG will trim out in a way that'll avoid the need for more than the odd bit of stick pressure or a few clicks of down trim here and there.

It's going to be tough to avoid the need for trimming the elevator with your requirements though. It's just the nature of the beast that if you deploy flaps that you'll need to either trim the elevator or use a mixing transmitter to add down trim for you as the flaps come down.

If you choose to use something like the G501 it'll slightly reduce the need for trimming but it'll still be there. But you'll soon get the hang of using a little forward stick pressure for a dive or a hair of back pressure to hold the nose up in the glide. Shifting the CG back will reduce this and with any luck may even all but eliminate it. But no guarantees on this.

[FANMAN]

The MH32 is also a good airfoil. I've used it in many designs, including a small indoor capable lear jet! -)