Airfoil for low Re swept wing?

[JetPlaneFlyer]

I posted =this in the 'Flying wings' forum but got little response, maybe I'll get more interest here:

I’m thinking about designing a small lightweight tailless swept wing to compete in a free flight duration event for models powered by small (6g thrust) Rapier L1 rocket motors. The target weight for the model including motor case would be 8g or so and I think I can build a 16” span down to this weight with stick and tissue construction. Sweep will be about 30 deg, the rocket motor will be mounted on a central pylon located on the CoG and it will have tip mounted fins, pitch trim will be achieved by wing twist combined with low Cm airfoil.. think mini F3B glider.

Obviously such a small and light model will operate at very low Re numbers, maybe 20,000 or less. I’m looking for airfoils that will have good performance but also have close to zero Cm. I’ve played around with Profilli Pro and I’m finding that the usual F3B type airfoils when operated at very low Re don’t perform very well, plus their Cm goes negative. In Profilli a thin symmetrical airfoil appears to be about as good compromise as anything else though perhaps in the real world there are airfoils out there that are known to work well under these conditions…

Does anyone have any recommendations?

Thanks
Steve

[MarkusN]

Don't know the profile designations, but hasn't Mark Drela done some work on profiles for DLG tails? They should operate in similar conditions, though not quite as low.

Controlling pitching moment at Re this low will be a b!tch, though. Cm graphs usually loock very bouncy when you get to Re this low.

[nmasters]

You're getting down into large insect Reynolds numbers. A sheet of paper or 1/32 balsa will make as good a wing as a symmetrical section foil at that Re. Since the Cm is a function of the mean line you could take the mean line from a reflexed section and make a form to clamp sheet material onto and harden it with dope or something. I made a 4” sailboat with a lateen sail last year and used this technique to make the sail. I made the sail from typing paper hardened with acrylic spray paint to set the camber. I used a 1 gallon paint can and rubber bands as the form and clamps. The boat can't tack but it's steered around in the tub with a hair dryer so that's not a problem.

--Norm

[creyes123]

Using my calculator (RCAdvisor.com), I found some potential candidates:

EH 2.0/9.0
NACA 0008
NACA 0010
NACA 63(2)-215 MOD B
NACA 63-210

I just added a tip to the website on how to speed up this type of search.

[raptor22]

As you have found, airfoils that work well at normal model speeds suck at really low RN. This holds especially true for flying wing sections because the reflex causes a sharper pressure rise that encourages separation.

I would try a generic thin low camber section with some reflex added. XFoil isn't very accurate at these small scales, but if you run the results for some undercambered NACA airfoils you will see that you the airfoil isn't very critical.

The 4305 and 2205 in the polars below are thin medium and high cambered airfoils respectively. The others include a quickly modified 2205 and a "real" flying wing airfoil. Notice the jagged lines indicating that the results are less than perfect.


--Alex

[HugePanic]

how can you garantee that your build airfoil has the correct shape??

you should consider that your airfoil might (depending on you building technique) be significant different from you theoretical one.

if this might be the case, you should consider a simple airfoil, that is easier to build and/or a airfoil that is more forgiving to build failures.

[JetPlaneFlyer]

how can you garantee that your build airfoil has the "right" form??

you should consider that your airfoil might (depending on you building technique) be significant different from you theoretical one.

if this might be the case, you should consider a simple airfoil, that is easier to build and/or a airfoil that is more forgiving to build failures.
This is a good point and one I'd not completely overlooked. The actual airfoil achieved with stick and tissue open frame construction will only be an accurate rendition of the chosen airfoil at the rib positions, this is a fundamental limitation of my chosen build method... At the low Re numbers we are talking about i dont think it makes much difference to be honest.


I'd considered a flat depron sheet wing, and this may be the best solution, however depron does not really appeal to me, I like old fashioned stick and tissue.

[BMatthews]

For free flight at this size Thin Is In. And a sharp leading edge to help promote turbulant flow over the upper surface.

I'd be looking at using a built up version of the classic hand launch airfoil. And for the reflex it'll require at the tips check and see if there's still a web site up for the Paoli flying wing. He used a clever trick of cutting away the tip airfoils and gluing it back on so that the tips have the rear of the airfoil glued on upside down and that gives the tips the washout you want without a lot of airfoil planning.

Because it's a swept wing you don't actually require a self stabilizing airfoil but it helps to have one with a farily low Cm. The classic HLG airfoil with the curved front and flat rear will fit that bill nicely. In fact now that I think of it the cracked rib with added turbulator spar airfoil method would be just the thing for you. It'll sure keep the weight down! I'm not sure it'll work well with the cut and flip method though.

[Texas Buzzard]

Thornberg of "Bird of Time" fame ( he was the world's Champion then) had dabbled in lots of glider & F.F. ships. Here is waht I read from him in an article about 3 years ago. [ Thornberg is not much for the theoretical - he uses what works for him]. The KISS principal wins out most of the time - besides who can build a wing within 3% + of- of airfoil specs? I can't.

Lacking a diagram, this description will have to suffice. Assuming we are using 3/32" thick balsa and the chord is about 4" cut the outline. Then 30% (or 35%) back from the L.E. cut from root to tip and separate the two pieces. Pin 1/16" x 1/16" to board at the ends. Cover it with seran wrap. Now join the two 3/32" pieces right over the 1/16th sq. Let glue cure. You now have a workable airfoil in the 4" chord range. This was used years ago.