What Airfoil Should I Use?

Jackson T

Elite member
Having had both 2 and 3 piece 3m wings, I'd prefer 3 given the option. 1m sections are easier to work with, connections are lighter and the hardest loaded part of the wing is kept strong. Sure, weight moves out toward the wingtips, but a 3m wing will never be nimble in roll.

For 3 segment wings, AL joiners are fine. I'd cut the 3mm plate into two strips and sandwich a piece of hard balsa or basswood between them and epoxy the stack together. This would be mounted in the wing with the AL oriented vertically. You should have an I-beam effect in the direction you need, and will be more than strong enough.

Don't forget to include an indexing pin about 3/4 the way to the TE to absorb the twisting forces on the joint.

As for aileron connections, with servos mounted in the wings, you'd need extensions anyways. Get a pair just a bit longer than center to the break, and leave a small pocket right at the joint on each side for the connectors to live. With open bays between the ribs, the wire will have plenty of place to go as the joint is closed up.

Be sure, regardless of the number of joints to face each side of the joint with a thin sheet of harder wood than the typical rib (might get away with hard balsa, but I'd recommend thin ply or basswood). 1mm or thinner, if you can. Set the angle of the join with the balsa end-rib, and when you're happy with that, glue on the harder facing strip, precut to match the rib+sheeting. Naturally, you'll cover over these, so it'll look clean when you're done.

For joining, I'd recommend wrapping each side with a strip of packing tape (to protect the covering) then after joined wrap the joint with a strip of electricians tape along the joint -- the vinyl tape has just a bit of give and good grab. Lateral forces on the wing aren't that strong, so vinyl tape is plenty.
That joiner construction sounds good. The airfoil/spar layout leaves enough space for a 6mm high joiner. Would that be enough? What should I use for the indexing pin? I don't have easy access to other woods in thin sheets. Where do you get yours from? Would 3mm balsa work for the joiner face instead of ply if I can't find any? I was thinking packing tape as well to hold the wings together. Would the packing tape be enough to hold the joints together by itself? I've only ever used rubber bands to hold wings on. Would that work for this project as well, or is it too big?
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
That joiner construction sounds good. The airfoil/spar layout leaves enough space for a 6mm high joiner. Would that be enough? What should I use for the indexing pin? I don't have easy access to other woods in thin sheets. Where do you get yours from? Would 3mm balsa work for the joiner face instead of ply if I can't find any? I was thinking packing tape as well to hold the wings together. Would the packing tape be enough to hold the joints together by itself? I've only ever used rubber bands to hold wings on. Would that work for this project as well, or is it too big?

My gut feeling, for midwing, 6mm AL is pretty thin -- borderline, but probably on the OK side. If you can get steel strip-stock or rod stock in that size, do so.

As for an indexing pin, I'd lean toward a 3mm-ish rod-stock or hardwood dowel cut to about 3cm long. If you have any long screws with a solid shank near that size, you could chop them up for that smooth shaft. Glue one end into the center section and have a mating hole on the other side. sand the peg on the un-glued side a bit so the pin slips in relitively easily -- you don't want slop, but don't want to have to force it either.

I typically get my thin ply from one of the local hobby or craft stores. Not sure what you have in your area to recommend similar. Our common soft craft wood (heavier than balsa, but softer than most practical woods like oak or pine) is "basswood" . . . I wouldn't be surprised if it wasn't available in your area, but that a similar wood is probably available. HARD balsa would do, but it can be difficult to find at times . . . and sometimes you can't find anything but :p For 1.5mm balsa (what we call 1/16") you can generally see soft/hard patches by holding the strip up to an overhead light. See light clearly shine through, it's soft. See a light amber glow, it's medium. See none, it's hard. For this application, you need something relatively thin and durable to make a clean end-cap. For most of your ribs, ideally, you'd lean toward medium balsa, but your facings you just want a bit more durability. All else fails, hard balsa will do, but if you can, you want something just a little harder than that.

If you want to use only packing tape, that should still work fine. I'd still wrap the ends of the wing with packing tape separately to give the joining tape layer something to hold on to that it won't rip up when removed -- the acrylic glue common on packing tape sticks far too well to covering materials . . . but ironically, not as well to itself. the first layer of tape gives a bed for most tapes to grip lightly and release easily when pealed off after the flight. In flight, the lateral forces on the wings are very low, so you just need a strip all the way around to hold the joint flush.

As far as rubber bands . . . all depends on where and how many. Bands can hold the tips on, but building an aerodynamically clean locking mechanism may be tricky. For wing-mounting, I've held on a 3m wing before with bands (6-8 of our #64 bands worked reasonably well), and I've held them on with hard-points (pegs and wing-bolts). The hard points are easier to work with come assembly/disassembly time, but the bands are more forgiving when you catch a wingtip on landing.
 

Jackson T

Elite member
My gut feeling, for midwing, 6mm AL is pretty thin -- borderline, but probably on the OK side. If you can get steel strip-stock or rod stock in that size, do so.

As for an indexing pin, I'd lean toward a 3mm-ish rod-stock or hardwood dowel cut to about 3cm long. If you have any long screws with a solid shank near that size, you could chop them up for that smooth shaft. Glue one end into the center section and have a mating hole on the other side. sand the peg on the un-glued side a bit so the pin slips in relitively easily -- you don't want slop, but don't want to have to force it either.

I typically get my thin ply from one of the local hobby or craft stores. Not sure what you have in your area to recommend similar. Our common soft craft wood (heavier than balsa, but softer than most practical woods like oak or pine) is "basswood" . . . I wouldn't be surprised if it wasn't available in your area, but that a similar wood is probably available. HARD balsa would do, but it can be difficult to find at times . . . and sometimes you can't find anything but :p For 1.5mm balsa (what we call 1/16") you can generally see soft/hard patches by holding the strip up to an overhead light. See light clearly shine through, it's soft. See a light amber glow, it's medium. See none, it's hard. For this application, you need something relatively thin and durable to make a clean end-cap. For most of your ribs, ideally, you'd lean toward medium balsa, but your facings you just want a bit more durability. All else fails, hard balsa will do, but if you can, you want something just a little harder than that.

If you want to use only packing tape, that should still work fine. I'd still wrap the ends of the wing with packing tape separately to give the joining tape layer something to hold on to that it won't rip up when removed -- the acrylic glue common on packing tape sticks far too well to covering materials . . . but ironically, not as well to itself. the first layer of tape gives a bed for most tapes to grip lightly and release easily when pealed off after the flight. In flight, the lateral forces on the wings are very low, so you just need a strip all the way around to hold the joint flush.

As far as rubber bands . . . all depends on where and how many. Bands can hold the tips on, but building an aerodynamically clean locking mechanism may be tricky. For wing-mounting, I've held on a 3m wing before with bands (6-8 of our #64 bands worked reasonably well), and I've held them on with hard-points (pegs and wing-bolts). The hard points are easier to work with come assembly/disassembly time, but the bands are more forgiving when you catch a wingtip on landing.
If I used steel, would I be able to shorten the joiners from 40cm to 30cm (15cm either side), to save weight? steel is almost 3 times heavier than AL, so unless I can shorten the joiner a bit I might stick with AL and hope for the best. I think I'll use hard balsa for the end cap. Is the end cap for structural reasons, or durability? I think I'll use rubber bands for the wing. Could you show me some photos of your hard-point mounts? I'm curious about how they're made.
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
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If I used steel, would I be able to shorten the joiners from 40cm to 30cm (15cm either side), to save weight? steel is almost 3 times heavier than AL, so unless I can shorten the joiner a bit I might stick with AL and hope for the best. I think I'll use hard balsa for the end cap. Is the end cap for structural reasons, or durability? I think I'll use rubber bands for the wing. Could you show me some photos of your hard-point mounts? I'm curious about how they're made.

3 times the weight, but three times the yield strength too.

Tradeoffs.

So what are we balancing here? Strength of the spar itself (will it be enough to resist snapping?) and how much force can be transferred between the spar joiner and into the spar's web (does it have enough area that the joiner doesn't knife it's way out through the wing). Anything beyond that is a question of how strong the wing is in general, not of the joint.

For the strength, 3x6mm of Al (oriented in the good direction) makes me nervous. I've bent AL wire that thick before. it's not easy, but it can be done by hand . . . barely. Steel? Not so much. Can you get away with it in a composite sandwich? Proooooobably . . . but it would make me nervous.

As for keeping the joiner from cutting it's way out, the governing parameter in this failure mode is the surface area the force is distributed across -- the area of the top/bottom of the joiner. width x length. If you make a fairly good composite sandwich from your joiner, not only will it resist bucking failure modes in the metal, it will also increase that width.

What you are left trading against is weight for length, and every cm you shorten it boosts the pressure against that wood. My gut feeling says 150mm/side is still good -- that's about how long the joiner is in my BoT . . . but it's also about twice as tall. The height doesn't play a role in the force transfer, only in the overall strength, so yeah, a stack with 3x6x300mm steel in the composite sandwich should be fine.


Forces in the wing structure are shared through the joiner and indexing pins. end-caps keep the socket holes from splintering in use and add a bit of hangar rash resistance. Keeping that in mind, since it's butted up against another rib, you might consider cutting the end-cap cross-grained to the end rib it's glued to for the ply effect.


As for hard-point mounts, generally you sink one or more T-nuts or similar captive threaded nut into the fuselage near the trailing edge, then have a indexing peg around the leading edge that sockets into the wing-saddle. Only thing left is to beef up the hole in the wing where the bolt goes through to keep the wing from tearing away. Slip the indexing peg into the hole, then slip a bolt through the reinforced hole in the wing into the captive nut, tighten, and Viola! your wing is firmly fixed to the fuselage. Some wings forgo the indexing peg for a second (or third, or forth) bolt, and while this works, it's a pain to keep everything aligned until you have at least two bolts in the wing.

It's a fairly common way the wings are mounted on a balsa airframe . . . and I've got at least a half dozen wings/fuselages designed like this within sight . . . and I have no idea where my camera is :p I'll see if I can get a few up in a while.
 

Merv

Site Moderator
Staff member
....i am wanting to cut a wing for a slope soarer from a block of eps, with my hot wire...
If you just want to have a decent fun flyer. The standard Clark Y is a good choice. Make a thinner one, maybe 9-10% of the cord in thickness.

I’m not going to be of any help if you are wanting to make a competitive flyer.
 

Matthewdupreez

Legendary member
If you just want to have a decent fun flyer. The standard Clark Y is a good choice. Make a thinner one, maybe 9-10% of the cord in thickness.

I’m not going to be of any help if you are wanting to make a competitive flyer.
nooo, not at all competitive. i just want to slope for fun... i have never flown any rc plane before.... i just want to fly for fun and have a decent flight time... 30 min? or is that to much to ask?
 

Phin G

Elite member
Merv,

I've yet to see a sim or wind tunnel run on a FT wing, so . . . Dunno. Performance from just an airfoil itself is a bit over the map. A Clark-Y is an easy foil to build and performs all-around well, but high performance? No. How inferior is the venerable Clark-Y to a well optimized wing? A lot. If I hadn't gotten sucked into DLGs and seen what the difference a foil can make between meh, impressive and unbelievable, I'd have a hard time believing how good it can get way out on the tail of that bell curve . . . which is a fancy way of putting "the more I know, the more I know I don't know", and "how good it can get" is way out out into unbelievable.

Balsa flies better . . . but how much of that is the better foil from cut ribs vs. generally stiffer design? Still hard to say, but all things forced to equal, on the low end of balsa (sporting a TLAR or Clark-Y airfoil) I expect it'll fly "noticeable but only a little better" than a folded foam wing. Start optimizing the airfoil to the task, and it progresses down that tail adding performance and multiplying the cost.

In the end, if I take a folded foam DLG wing to a contest it would be as a gimmick (which I'm not above 3:) ), but it will give me more than a few enjoyable flights. An old-school DLG sporting dated foils will push past it with ease . . . and be put to shame by the latest up-and-comers. Each step has it's joy in flight, and each is roughly 5-10 times the last in cost. How far you want to push it will be tempered by your tolerance for the price of entry.

. . . or in other words, how much better you want it, always depends on how much more you're willing to pay. Personally I find FT has found a sweet spot in that price/fun ratio, and I expect that's a rather popular opinion around here ;)
My sisters uni has a wind tunnel. I wonder if i could just...
 

Matthewdupreez

Legendary member
nooo, not at all competitive. i just want to slope for fun... i have never flown any rc plane before.... i just want to fly for fun and have a decent flight time... 30 min? or is that to much to ask?
and i am wating for my electronics to arrive and i am impatient so i want to start flying this coming weekend
 

Merv

Site Moderator
Staff member
... i have never flown any rc plane before....
I’d recommend you start with a known design. Something like the Tiny Trainer. We will be able to give you far more guidance with a design we know than with an unknown design. After you learn to fly, then go wild and design your own plane.

The Tiny Trainer has a lot of glider like features. Under the right conditions, I’ve flown my Versa for 30 minutes.
 

Matthewdupreez

Legendary member
I’d recommend you start with a known design. Something like the Tiny Trainer. We will be able to give you far more guidance with a design we know than with an unknown design. After you learn to fly, then go wild and design your own plane.

The Tiny Trainer has a lot of glider like features. Under the right conditions, I’ve flown my Versa for 30 minutes.
can i opt for a simple soarer?
i don't have very strong winds here so larger wing should soar easier
 

Merv

Site Moderator
Staff member
can i opt for a simple soarer?
i don't have very strong winds here so larger wing should soar easier
Sure, the Simple Soarer is another great choice.

For me, the best days to thermal have lots of sunshine and a moderate wind, 5-10 mph
 

Jackson T

Elite member
hey there guys. i know this is an old thread, but i am wanting to cut a wing for a slope soarer from a block of eps, with my hot wire... so any aerofoil design is possible.
what airfoil would be the best for the slope soarer..
expected wingspan 1.7 to 2m
@Craftydan @Jackson T @TooJung2Die @Merv @Fluburtur
The Simple Soarer might fit the bill nicely. If you want to scratch build something, as long as there is enough wind you should be able to fly for as long as the battery lasts with any half decent airfoil. The clark Y would do fine, but the SD7037, MH32, and the AG35 are probably a bit better. The AG35 has a flat bottom from 20% from the leading edge to the back, so it's quite convenient. If you haven't already heard of it, Airfoil Tools is a great website with polar curves for many model and full scale airfoils, and includes a list of similar airfoil designs for each airfoil. Good luck, and let us know how you go!
 

Matthewdupreez

Legendary member
The Simple Soarer might fit the bill nicely. If you want to scratch build something, as long as there is enough wind you should be able to fly for as long as the battery lasts with any half decent airfoil. The clark Y would do fine, but the SD7037, MH32, and the AG35 are probably a bit better. The AG35 has a flat bottom from 20% from the leading edge to the back, so it's quite convenient. If you haven't already heard of it, Airfoil Tools is a great website with polar curves for many model and full scale airfoils, and includes a list of similar airfoil designs for each airfoil. Good luck, and let us know how you go!
will do.... so which of these aerofoils would be the best.... i don't mind if it is a complex shape the shape possibilities are endless. which one would be the best for long slow flights in a moderate wind... aerobatics are not important just a docile flyer
 

Jackson T

Elite member
will do.... so which of these aerofoils would be the best.... i don't mind if it is a complex shape the shape possibilities are endless. which one would be the best for long slow flights in a moderate wind... aerobatics are not important just a docile flyer
I'd personally go with the AG35, but they're all pretty equal in performance anyway. There are other more experienced builders that may have better ideas, we'll see what anyone else says.
would some one tell me what all these charts mean.
http://www.profili2.com/eng/dett_profilo.asp?Id=2703
Alpha is the angle of attack, i.e. the angle between the wing and the oncoming airflow. Cl is called the lift coefficient. It is a function of the shape and angle of attack of the airfoil, and is proportional to the lift force produced by the wings. Cd and Cm are the drag and moment coefficients, respectively, and are also a function of the airfoil shape and angle of attack. The drag force of the wing is proportional to the Cd, and the downwards twisting force of the wing is proportional to the Cm. The graphs show the value of these coefficients at different angles of attack. the Cl/Cd graph is a graph of the glide slope vs alpha of the wing by itself (ignoring the drag of the fuselage).