I am trying to select and airfoil for my A-level Dt project, the build thread can be seen [link=http://www.rcuniverse.com/forum/m_8547717/anchors_8547717/mpage_1/key_/anchor/tm.htm#8547717]here[/link].
The details and requirements of the design are a 40" or so wingspan with a tapered 2 piece wing (much like the sebart katana and sukhoi 30 size wings)
the model must be able to perform 3D aerobatics well, and also be able to perform smooth aerobatics to a reasonable standard, like the EF vanquish and sebart katana 30. I have been looking at different airfoil types but Im really not sure which one would be best for this project, seeing as its quite a crucial part of the models flight characteristics and overall performance I figured I would ask here.
I have the program Mac foil so all I need is the data file for the particular airfoil type and I can plot the foam cutting templates from that.
So in short which airfoil type do you think would be best suited to my project, being tapered, able to perform hardcore 3D yet fly reasonably smoothly aswell.

Thanks in advance
Michael.

You can't go wrong with an NACA 0015. Works all the time for me. Dan.

Ok thanks, I think thats just the kind of thing I was looking for, any advise on a sensible ratio of taper, so the cord of the root compared to the cord of the tip?

Michael

Anything you choose will be a compromise. At this size for some truly gut wrenching 3D stuff your optimum choice would be a flat plate airfoil. But if you need to be able to do the smoother and more open maneuvers then I'm thinking something more in the NACA0012 range or a Selig 8022 would be nice. But you're asking for two different performance tasks that have conflicting requirements. For good 3D you want to be able to stall the airfoil on demand to stop the airplane in mid air. For that the flat plate is king and anything else will be more stall resistant and thust harder to actually drag to a sudden halt.

Actually in my own flying with a flat plate aerobat the bigger more open maneuvers were actually pretty smooth. Where it'll all fall apart is during the higher speed portions where the extremely wide control surfaces needed to do truly great 3D stuff will begin to flutter. THAT is where the airfoiled wing with the greater thickness to give it more stiffness and strength comes in.

I know it seems low tech but the flat plate IS a valid option. And if your desire for 3D is serious then it's the best option in many respects.

Larger 3D models are getting away with "proper" airfoils but if you look at them the airfoils they are using these days often tend to be a thinner % thickness than a few years ago. And on a couple of models I've seen pictures off the airfoil's high point was noticably farther back than was normal a few years ago as well. Both of these traits help these larger models to act like they have a flat plate airfoil in the typical larger sizes.

As mentioned flutter is the enemy. If you truly want this design to have a really wide speed range that your dual mode requirement suggests then I'd recommend a Naca 0009 and sharpening the leading edge as required to achieve the "stall on demand" that you want for true 3D work. At the same time the typical 3D wide ailerons will be really wide and flutter prone. To combat that I suggest a slop free drive to somewhere near the middle of the control surface to mazimise the support and at the same time arrange for mass balances to bring the balance point of the control surface itself forward and close to the hinge line. Same with the tail surfaces. And all the movable surfaces need to be light but skinned with a layer of some sort that emphasises stiffness. Otherwise the segments of the control surfaces further from the control horn and mass balace will still flex and flutter on their own.

You can't beat a constant chord wing for ease of construction and flyability especially when it comes to landing. If you want to try a tapered wing, I have attached two models I have designed in the past with tapered wings. The orange trimmed plane has a taper of 75% and the Macchi is 66%. I really can't tell the difference exept on landing. I have to keep the speed up a little more. Good luck with your design. Dan.

Here are the images. Dan.

Thanks Dan and B, Yeah I figured that getting both good 3D and smooth aerobatic performance from one airfoil would be tricky. The smooth aerobatic performance doesn't have to be to f3a standard, I was more looking for relatively smooth flight characteristics, nothing to special. I will look at the NACA 0012 and Selig 8022 as they seem to be just what I'm looking for. I will measure the taper ratio on my friends sebart katana 30E and will use something similar to that. I plan to inlay the wing with layers on carbon tow to create a spar, also I'll skin the surface with a light weight glass cloth to help keep the wing nice and stiff.

Thanks
Michael

Spent some time gettingthe best compromise on larger models
the BIG factor was strength to weight
so - the best for me was a 12% root and a 9% tip and the taper ratio was amost 2-1
Effectively a pretty low aspect ratio.
As for the "curve?" (airfoil).
take yer pick- I ran out of ideas on that - an easy shape to make and still not hav flex is important
so I usually made the LE a small radius (strong ) went to a blended high point at -oh 25-40% of MAC and then straight line to a blunt te a long as it didn't twist easily, I never could find a dimes worth of difference

So-looks good , survives threshing about. for really small stuff - flat plate -no question about it tho some who don't fly em will disagree. (n come up with some old formulas etc-to try and make their point )
From these to the previously mentioned 12/9% stuff - the compromises were for strength
The hard point for some to swallow is that the shape used on MOST aircraft evolves from strength needs - not some L/D calculation.
It's all necessary but strength comes first
max strength with min weight then drag
got it?
With the advent of Depron - we got extremely high strength to weight -cheaply -and tis allowed for a LOT of cut n try which prooved the point.
I still get guys asking " yeh but if you could streamilne that flat plate - wouldn't it help?
In the manner n size of use
the answer is simple
No
There is yet another approach
a big fat airfoil say 18%- made in an extremely well thought out fashion for min weight -
this setup is pretty numb to AOA and has been used on control line stunt for 60 years I know of - and on a truckload of flip n flop lawn darts etc..
My ERATIX is like this and is a lot of funto fly mainly because it is supe light - 600 squares and 3.75 lbs all up.

Strong and stiff is always good but watch for weight buildup. At this size and for optimum aerobatic performance weight is truly the enemy. Look at the weights of the flat foamies when in ready to fly condition and figure that you're going to want to keep things to around that sort of weight. Also if using foam in a manner other than in a flat plate it is actually darned heavy. If you're adamant about a built up shape and must use foam then do consider using a folded over construction of 3mm Depron with a few 3mm Depron ribs and a 3 mm spar that is eggcrate notched to the ribs. A small diameter carbon rod (like .050) can then be sandwiched between the full depth spar and the outer skins to work as the spar system.

For diagonal strengthening some internal or external diagonals of carbon tow strands would provide a lot of stiffness. With this makeup all you need to bond the two fibers would be some water based polyurethane varnish. Also with the depron skins and internals there's no need for the heavy glass cloth and resin outer coating.

The other option would be a full on vacuum bagged wing with the glass/epoxy outer skin. But in that case to avoid an excess weight buildup from all the foam in a thicker section I'd suggest going with an 8'ish % airfoil to cut down on the foam volume.

Or.... the old curmudgeon that I am... there's always the old tried and true balsawood option. Ribs with balsa leading and trailing edges, spruce or balsa/carbon laminated spars and then work in a full geodetic XXXX format of carbon tow diagonals. Finally tissue and dope or use one of the light parkflyer films. Do the same sort of built up structure for the control surfaces as well. This would be both light and stiff but it would look "old fashioned" but what the heck. It'll work and work well while being light.

Yeah, I have outlaid a weight guide to try to stick of an AUW or around 1.25kg, this is the same as the sebart kat of the same size. I will weigh all the materials to get a good idea of what is possible, and also weigh the components during each stage of construction to see how close/far off the guide I am. I really like the Idea of the depron skinned wing and will look into that. I have been thinking and come up with an idea for the wing and would appreciate some feedback on it, it may well be really stupid but it seems to make sense to me. The idea is that if I were to foam cut the wing I would put a template of the NACA0012 at the root end of the foam and a smaller NACA0009 at the tip end of the foam block, I figure this would create both a tapered cord and a tapered wing thickness whilst giving me the best of both airfoils. One issue I thought of is the fact that this may create a wing that is more likely to tip stall which would not be good. Not sure if this is the way to go about it but was just an idea. I was also thinking about cutting lightening holes into the blue foam with a dremel router attachment , could always skin the wing with 2mm depron after than. Well at the moment these are all just ideas and would appreciate your thoughts on them.

Michael

600sq inches - 8 ounces inc srvos

Thats awesome but Im not sure if something like that would be strong enough for my design. Is my idea about using two different airfoil templates crazy, or would it work. Also would those particular airfoils work well together or would they give the wing weird stall characteristics. I think I will use that depron wing idea for a non school ap project Im working on, seems perfect for that kinda stuff.

Michael

forget the stall theory stuff
In this size and configured in the % discussed - all that counts is wing loading
-

Dick's right, at this size unless you really go out of your way to bodge things up then tricky stalls is not an issue in this size range. I'd also reconsider your "goal" of 1.25 kg. That's far too heavy at that size to make a good aerobatic model. You'll want to drop that down to around .8 kg. It should not be that hard to do at all. Even the chunky EPP foamies are coming in at around .6 kg in the 0.9 meter span region.