Airfoils that work at very low Re

[GeeW]

I am looking for recommendations for an airfoil suitable for a 0.9m span (36" ish) wing with a wing chord of 100mm (4"). I anticipate the weight to end up 100gm (4oz).
I have Profili but 2200 plus airfoils is a lot to wade through :eek: so a couple of pointers to start me in the right direction would make my day. :)
TIA
Gordon

[DT56]

Wing, as in flying wing, such as a plank or a chevron?

Powered or unpowered?

Flying speed?

[MCarlton]

Agreed, we need to know what the model IS, and what the performance envelope is going to be.

Also, some idea of construction would be handy too, there are some sections which might be appropriate if you can build them accurately, but which would be pointless if you are using a less accurate building technique.

Actually thats one of my bugbears about wing section selection, on a 4" chord, can you REALLY build the wing accurately enough to represent a particular section properly?

I would also hesitate to call your spec VERY low Re, unless your expected flying speed is also very low.

[GeeW]

To clarify slightly (!) the wing is part of a conventional glider with the tailplane at the back. I'm open on build technique as I'm happy with foam,balsa,framed or jedelski.
At this weight it is not going to be a windy weather flyer. Flying speeds will be 15 to 30mph (ish) max. Planning on piggy-backing it on a parkflyer or aerotowing. Have experience both ways. I'm kind of guessing that I'll end up in small HLG neck of the woods but without the launch design issues.
I started by looking at the Bug which was popular a few years back.
I am also looking at the Kolibri but I cannot accept that an 'Aquilla' type airfoil is any good at these kind of values of Re. (its bad enough at Re 100,000 to 150,000), although the rest of the design looks pretty sound.

Gordon

[MCarlton]

If you're not needing a low drag section for a fast launch, then I would be tempted to go with something like SD7037, possibly MH32 or dare I say it, a thinned Clark Y (maybe 9%).

Other options might be an SD4083, which was initially designed for DLG, but makes quite a nice soaring section.

Others may have a different opinion however.

Construction wise, I would stick with a traditional ribs and spar wing. It may be beneficial to fully sheet the wing to maintain as much section purity as you can.

[Wendi Smol]

To clarify slightly (!) the wing is part of a conventional glider with the tailplane at the back. I'm open on build technique as I'm happy with foam,balsa,framed or jedelski.... Flying speeds will be 15 to 30mph (ish) max.
Gordon

15 to 30 mph -- if that, probably less. Re on average will then be around 30000. The only suitable "profile" will be a cambered plate or a Jedelsky.

All the best

W.

[MCarlton]

I get the Re. to be a bit higher than 30,000?

6380 x 22 (ft/sec at 15mph) x 0.333 (ft chord) = 46739

6380 x 44 (ft/sec at 30mph) x 0.333 (ft chord) = 93479

(ref: http://www.rcgroups.com/forums/showthread.php?t=804551)

Still low, but not as low as, say, a free flight floater glider, or even a peanut scale indoor FF model, which fly just fine on a conventional section wing.

[mlbco]

Subscribe to this publication and you'll find many airfoils designed for this Re range:

http://www.freeflightquarterly.com/

Steve

[JetPlaneFlyer]

For low Re 'thin is in'... Thinner the better pretty much. Camber also should not be too great, maybe 4-5%

maybe an Eppler E63?

[Shedofdread]

Have a look at the airfoils used for the tips of DLG gliders. The Drela ones would be good place to start.

Low camber, thin and with a forward max thickness pos.

S

[Wendi Smol]

I get the Re. to be a bit higher than 30,000?
6380 x 22 (ft/sec at 15mph) x 0.333 (ft chord) = 46739
6380 x 44 (ft/sec at 30mph) x 0.333 (ft chord) = 93479
Still low, but not as low as, say, a free flight floater glider, or even a peanut scale indoor FF model, which fly just fine on a conventional section wing.

Get real young friend!!

with a total weight of 100 gram and 9sqdm of wing area your terminal velocity is probably 15 - 20 mph. Unless you put some balast on it, that little glider will not come anywhere near 30mph.
And please have a close look at peanut scale wings -starving horses come to mind but not any recognisable "aerfoils" (but that's why they work at all).
There is quite a lot of literature on subcritical airflow available now - go on read a bit.

W. :mad:

[fnev]

I am backing up Wendi on that one…

To me, the best choice would be a Jedelski. I have a lot of experience with this section and it will surprise a lot of people on the performances side even at (relatively) higher speeds (15/18 m/s) and heavier wing loadings (up to 70g/dm2 or even 80g/dm2). An added bonus is the ease to build a wing with such profile and its incredible ruggedness when build properly.

[MCarlton]

I got it wrong it seems.

My apologies, but I just plugged the figures cited by the OP into a Reynolds number calculator and went from there.

And please have a close look at peanut scale wings -starving horses come to mind but not any recognisable "aerfoils"

At the risk of raising your ire again, I would disagree to an extent, judging by plans such as this one;

http://www.onerateads.com/images/EXPRESS.JPG

Such a wing section might not be the most efficient for the Re. but it does work. In the same way that a Clark Y profile worked on a KK Conquest modified for single channel micro RC, with a span of 30" and a chord of about 3".

Just because something is not absolutely aerodynamically optimised doesn't mean it won't work. One has to consider ease of building and the like.

Anyway, I'll apologise for my ignorance of subcritical airflow.

[Airboatflyingshp]

Gee wee - Gordon contact me mate through the E mail - I have a small sloper very fast simple sheet skin V. agile and a Dh Swallow type thing.
I have plans and article, if you want.....and a headbut piggy back single channel micro glider.
Regards Clive

The Jed wing isnt a bad idea ive seen a glider but wings were drilled to lighten and skinned in light transparent film top and bottom to fill in the foil so to speak It was an InterX experiment the owner was pleased.

[BMatthews]

There's a huge host of airfoils to choose from in the free flight world. However while the little Vega is cute I would not use that airfoil. It's thick to better conform to the scale model design but it would be terrible for a 4 oz 36 inch span glider.

What you're after is either one of the Drela tip airfoils or to go with one of the more highly cambered free flight model airfoils.

The Jedelsky wing is not a good plan due to the amount of wood it uses. Unless that wood is very carefully picked from light contest stock it'll push your model over your 4 oz target assuming there's radio gear going into it. Far better to stick to a built up wing.

At 4 oz the "penetration" ability will be highly compromised and I tend to agree that even in a dive a 4 oz model will be luck to reach 30 mph. So you may as well enhance the slow speed flight and opt for a known good free flight airfoil.

Some options to look at would be;
Benedek BE6306 or BE6453.
Solokov
All of these are from the Profili2 program list of airfoils.

I've used the Solokov airfoil on a couple of my old free flight models. It worked very well in the two designs. However it's not an airfoil for fighting back upwind so it would be for calm to very light breeze days. But MAN does it float.

[kristjane]

To clarify slightly (!) the wing is part of a conventional glider with the tailplane at the back. I'm open on build technique as I'm happy with foam,balsa,framed or jedelski.
At this weight it is not going to be a windy weather flyer. Flying speeds will be 15 to 30mph (ish) max. Planning on piggy-backing it on a parkflyer or aerotowing. Have experience both ways. I'm kind of guessing that I'll end up in small HLG neck of the woods but without the launch design issues.
I started by looking at the Bug which was popular a few years back.
I am also looking at the Kolibri but I cannot accept that an 'Aquilla' type airfoil is any good at these kind of values of Re. (its bad enough at Re 100,000 to 150,000), although the rest of the design looks pretty sound.

Gordon

If you are thinking of my Kolibri original with V tail, there was not used Aquilla airfoil. It is my own airfoil. It fies nice. Here is video
http://www.youtube.com/watch?v=0fUh8Cf8GWw&feature=fvsr

For new Kolibri 2009 I used GOE 602 MOD (5% camber and 12% thickness)
Here are plans for Kolibri 2009 (http://www.fileden.com/files/2007/5/9/1062957/Kolibri%202009%20final.zip)