Airfoil suggestions?

[LaGaffe]

Greetings,

Could anyone suggest suitable root and tip airfoils for a tapered wing with relatively low wing loading (12~13oz/sq.ft), design cruise cl ~0.5, cruise RE ~200000, tip chord ~2/3 of root chord, high max lift, suitable for traditional construction, and not too thin (should not look out of place on something trying to look like a scale model of a largish plane from the 30's)?

TIA
Soft Landings
Torbjörn

[DICKEYBIRD]

I have used the Selig S8036/S8037 root/tip combination on several scratch built planes of varying size/wing loadings and all flew very well. These airfoils were developed by Dr. Selig for the Top Flight P-47 and optimized for model aircraft Reynolds numbers. They both are 16% thick. Here's a pic of the S8037 which has a little more camber than the S8036. My impression of them is that they are very forgiving around the stall, seem to be very efficient and have low drag plus will fly inverted reasonably well.

My $00.02 worth anyway.

[Mike Taylor]

It is very interesting that you posted that particular airfoil. I also would vote for it.

I have been studying the old plans for a 60's KeilKraft SkyRay model, and that is the airfoil it uses. The model flew really well and spawned a bunch of flying wings all using this family of airfoils.

The model uses a 9 1/2% airfoild (6" at the root, .65" thick) root to tip, and up to 16% through the fuselage fairings. The model was Jetex-50 powered free-flight and went like a bat... I'm thinking a micro DF with RFFS equipment would be nice. Is KeilKraft still around by any chance?

The balance point indicated on the plan is at 27% MAC, and there is some reflex in the outer control surfaces.

[Sail 'n Soar]

DickeyBird,

Coordinates for the S8036 and S8037 aren't in my database. Can you forward them?

Thanks,
Gerry

[DICKEYBIRD]

Hi Gerry,

Do you use CompuFoil? If so, I have the coord.'s in the .cor format and can email them to you if you want. Or, go to:
http://www.aae.uiuc.edu/m-selig/ads/coord_database.html
Click on "S", scroll down 'til you find the airfoils and you can download them in the .dat format.

HTH!
Milton Dickey

[BMatthews]

I've been looking into airfoils for a duration model that someone on another website wants to build. Surprisingly enough the good old ClarkY comes out very close to the modern high zoot airfoils. I WANT the fancy new stuff to win but that darn ClarkY keeps snapping so closely at their heels that it's hard to ignore.

The fact that it's so darn easy to build doesn't help either..... :D

And it's certainly in the period for a classic looking design.

And bear in mind we are talking about the TRUE ClarkY here. Not some Florsheim size 9 shoe tracing like some people like to use.... :rolleyes:

[Sail 'n Soar]

Thanks for the web site. I wrote my own program prior to any of the commercial products. The .dat files are just what I need.

Gerry

[LaGaffe]

I must have been less than clear. The plane I'm looking for a suitable airfoil for is a fairly slow 1930s style flying boat cruising at 25mph with a root chord of about 276mm (almost 11"). High speed pylon turns would be somewhat out of character ;-)

Torbjörn

[Sail 'n Soar]

Torbjörn

I've developed successful designs - mostly gliders - using both the Eppler and the older NACA 4-series foils. I normally design for my cruise point at about a 2 degree angle of attack. The Eppler 207 and 209 have a .5 CL at about 2 degrees AOA. The 207 is a 12.04% foil and the 209 is a 13.72% foil. The Eppler 195 and 197 generate a .5 CL at around 1.5 degrees AOA. They are 11.8% and 12.8% foils. For the NACA foils, the NACA 2412 (12%), 2415 (15%), etc., develop a CL of .5 at a little over 2 degrees. The NACA 4412 has a CL of about .5 at 0 degrees AOA, but will need to be at a slightly positive AOA for minimum drag, which I assume you would want at cruise. With your cruise RE ~ 200,000, all should be viable candidates for your intended application.

In terms of tip vs root airfoil of ~2/3, with a constant foil and no wash-out, the maximum CL will occur at ~ 40% of the semi-span from the aircraft centerline. That is where the plane is likely to stall first. And since no wing is perfect, one side will stall before the other. Two to 3 degrees of washout will move the stall point to ~15%- 20% of the semi-span at stall improving the giving a more benign stall behavior while simultaneously bringing the induced drag at cruise to within 1-2% of ideal depending on the aspect ratio of your wing.

As long as you keep your foil thicknesses to within normal design practices, I'm not sure the effort of designing a different airfoil section for each wing station from root to tip would be worth the effort. Unless you are into aerobatics or intend to fly your '30ish plane inverted, washout is my prefered approach over playing with leading edge radii and varying sections. The results are easier to analyze and predict and less RE dependant.

Gerry

[LaGaffe]

Thanks. I have contemplated using NACA 3412 (root) and 4412 (tip) - inspired by Martin Simons: higher camber tip airfoil + washout. Btw, it's not a straight taper wing: a no taper centre section (about 1/3 span). But after staring at a zillion polars (computed & measured), I've decided on good (bad?) old Clark Y (slightly thincker at the tips). I haven't built a Clark Y wing so far and thought I could avoid it this time as well, but :-).

Torbjörn

[T. Lyttle]

Ah, just bite the bullet and use the ClarkY... there is a reason why it is still in use: it works. If you look at the flying boat airfoils (full scale) they are up to 25% thick, and no one is going to call you a liar if you miss the thickness by 10% ;) . Besides you are going to need all the PREDICTABLE lift you can get for a boat.

And KielKraft is gone I believe. I think it was absorbed by Veron (I went looking for the plant once), but since then I have seen nothing. Too bad, there were some great kits from that group; last 2 I built were the Deacon and a Tiger Moth freeflights. :( :( :(

[Sail 'n Soar]

Btw, it's not a straight taper wing: a no taper centre section (about 1/3 span).

That makes a difference. If I paste it successfully, the graph shows the CL distribution for a constant foil, no washout, straight to 1/3 span and tip to root ratio of 2/3 near at a max CL of ~1.2. I've assumed an aspect ratio of 6, although without washout, the profiles don't change significantly, only the max CL for a given AOA. Red boxe lines are the local section CL, blue boxe lines are the product of local section CL times local section cord, and green boxe lines are for ideal eliptical lift distribution. Note that the blue box line is not that far from ideal, i.e., near ideal minimum induced drag. The wing looks pretty good as is. A little washout gets rid of that local maximum away from the root.

Clark Y's work fine.

For what it's worth.

Gerry

[Sail 'n Soar]

Forgot to mention, vertical scale is CL (0 - 1.4), horizontal is fraction of span.

[LaGaffe]

Thanks - and a Merry Christmas and Happy Flying :)

[green66]

Anyone interested in comparing performance characteristics of airfoils should get a copy of the latest version (2.x) of Profili.

At just $10, it would be a gross understatement to say that this program is worth the money ==> Profili now incorporates the well-known XFoil program in a windows interface that's dirt simple to use (unlike the somewhat user-unfriendly nature of the standalone XFoil program). Because XFoil accounts for the effects of laminar separation bubbles, it is arguably the best program available for the analysis of airfoils at low Reynolds numbers. The $10 price tag might make that statement hard to accept.

A program like this is especially useful for doing objective airfoil comparisons by considering only the 2D flow behavior characteristic of the profile ==> avoiding the uncertainties inherent in wind tunnel data stemming from differences in tunnel construction, flow conditions, instrumentation accuracy/calibration, accuracy of test specimens, etc.

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