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What Airfoil Should I Use?

Jackson T

Active member
#21
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
Moderator
Mentor
#22
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

Active member
#23
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
Moderator
Mentor
#24
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.