Snowbird 18mm RCRG

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TheAviator

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I mentioned in another thread that I crashed one of my E6-powered gliders. Well, I was able to salvage about 37" of the 45" wing. With that salvaged bit of wing, I've decided to make an 18mm glider capable of boosting on B6, C3.4, C6, and D2.3 motors. I'm hoping that I can use it for C-RG at NARAM-57 in July.

Unlike the Fireball, this build is mostly from scratch excepting the salvaged wing. I'm using a 7mm carbon arrow shaft as the boom and will be cutting new balsa tails. I'm also going for absolute light weight, so the wing is permanently mounted to the glider. I'm also using a Parkzone 3-channel brick out of a Vapor indoor flier. Several fliers had good luck with them at NARAM last year in C-BG, so I figure why fix what ain't broken.

Pushrods will be .015" music wire in bondable teflon housings. I'm going to try to run them internally to reduce drag as much as possible.

Overall my total mass budget comes to 75g without motor, which puts the glide weight at a beefy 85g. This is a little heavier than some other C-RGs I've seen, but I also have a fair amount more wing area at a whopping 187 square inches. Most others top out just over 100 squares.
 
First order of business was to prepare the wing for bonding. I beveled both roots at 12.5* with a 220grit sanding block to give adequate dihedral for good lateral handling. After that, I joined the wings together at the bottom surface with a piece of wide packing tape. Most packing tapes don't stick to epoxy, which is exactly what I was looking for here. The tape allows easy handling of the wing when the epoxy is ready to cure on you.
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The only other easy pre-prep of the wing was digging out a small slot between the top and bottom spar at the root. When the wing is loaded, the top spars go into compression and the bottoms tension. Because of the dihedral, this puts the center of the wing under pretty strong compressive loads. Digging out a small area between the spars at the root and filling with thickened epoxy adds a ton of compressive strength here, increasing the load at which the wing will fail (all other things equal anyways.)

To join the two halves together, I mixed a 2g batch of 15 minute epoxy and added a little West 404 filler (colloidal silica) to add some toughness to the resin. A good amount was firmly pressed into the spar slots, then a thin coat smeared on the rest of the wing root. Two Testor's spray cans gave just the right amount of dihedral, so I propped the wing tips up on them, placed a weight in the middle to give a modest amount of compression and left it to cure.
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The final order of business for the evening was attaching the wing to the carbon boom. The undercambed wing meant that to get a good glue joint, I needed a good filler block between the wing and boom. I built up the proper thickness of balsa from a piece of 3/16" and two pieces of 1/16". I let the 1/16" overhang a little to center it on the boom. Using this to my advantage, I sanded the bottom of the pylon with a piece of 220 grit wrapped around the boom. This gave a good fit to the boom for gluing.

The top of the pylon was sanded to fit the underside of the wing. Finally I quickly mixed a small batch of 5 minute epoxy and held the wing in the proper position for 10 minutes. What can I say, I'm a human clamp. Anyways, the result is as shown. Next orders of business are to create the tail feathers and radio pod.
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So, I've gotten a fair amount done in the past week. Foremost on the list was to cut a set of tail feathers; that is a horizontal stabilizer and fin. These were both cut from light-ish (7#) balsa from my contest stash. You can see both of them cut with markings for centerline and hinge line.
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I prepared the boom for the elevator by wrapping the glue point with a single layer of 0.75 oz/yd^2 fiberglass to prevent splitting during the next step. I used a sharp 1/16" drill bit and drilled two holes in the boom about 1/2" apart. Using a sharp razor blade, I opened a slot for the elevator pushrod. The two holes at the end theoretically should reduce the stress concentrations and help prevent further propagation of any cracks in the boom.
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Not pictured here, I made a stabilizer pylon in much the same way as the wing pylon. Because I wanted the elevator horn in trail with the pylon, it had to be a little longer than perhaps it should be. We'll see whether or not this will come back to bite me when I fly it. I glued the pylon on with some 15-minute epoxy and weighted the boom with a bag of beans to keep everything in place. A small 0.75 oz/yd^2 patch of fiberglass wrapped the boom and pylon locking everything in place.
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I forgot to mention it earlier, but both the fin and stabilizer were airfoiled with 100 and 220 grit sandpaper on aluminum t-bar sanding blocks. The fin is shown here after airfoiling. For both surfaces, I applied two light coats of 66% thinned dope. The highly thinned dope does a good job of penetrating the balsa and sealing it. It also means that the dope is mostly volatiles that evaporate off, leaving a lighter surface. They were finished with 400 and 600 grit paper to knock down any prominences left by the dope.
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Both the elevator and rudder were cut free from their respective surfaces with a sharp razor blade and the hinge line sanded at a 35* angle to allow travel in both directions. I hinged both with silver mylar tape on both sides to create a durable hinge line. This is the same technique used on the Snowball/Fireball that has lasted 20+ launches, so I figure why fix what ain't broke.
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The fin was attached much the same way as the stabilizer, except that it was attached directly to the boom. I cut a slot at the appropriate height in the fin and inserted that part of the fin into the boom to stabilize it while the epoxy cured. Once that was finished, I applied another fiberglass wrap and drilled the boom for the pushrod exit.
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And here she is with her big brother! I have the balsa radio pod dry fit to the boom for size. As you can see, she isn't that much smaller, 900mm to 1150mm span. However, the Snowbird is looking to come in under 90g glide weight whereas the Fireball is over 160g. Should be good on C motors, and if I am daring, I may try an Aerotech D2.3 reload flight.
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I don't have pictures for it yet, but I installed a pair of 0.060" G10 control horns and 0.015" music wire pushrods in bondable teflon housings. I did my level best to ensure that the pushrods were as free and slopless as possible, but it seems that the small Spektrum brick is unable to adequately move the control surfaces. This is not entirely unexpected; I have a back-up plan.

In addition to the brick, I also have a pair of Spektrum A2010 servos and a Spektrum AR6310 nanolite receiver. These come at a weight penalty of about 5g, but I'll take it if it means that I can control the glider! I'll be using these in conjunction with a 150 or 200 mAh single cell lipo for power.
 
I got a lot of work on this glider finished this weekend. The radio pod is mostly complete, servos are installed, and the rest of the radio gear fits, so it's pretty much a glider at this point. As expected, the CG is about 35mm aft of where it needs to be; the empty motor casing and rocket pod will help to bring it forward to the appropriate point.

Pushrods and control horns are seen here in the first pic. I hand cut the horns out of 0.060" G10 and shaped them as necessary. The holes were started by spinning a #11 blade and finished by pushing a piece of pushrod through the hole. This ensures that there is no slop between the horn and pushrod. I used this same technique to drill new holes in the servo control horns for the other end of the pushrods.
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The new radio pod was built up out of 1/8" and 1/4" contest grade balsa. Mostly my design process was: fit the servos, make sure the radio gear fits, and shave as much weight as you can. The two formers encapsulate the servos, and the rear former is notched to accept the front of the tailboom. I tried SIGment glue, and I must say that it sands much better than CA in this application. The seams are not really noticeable unless you look very closely.
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I glued the pod on with some 15 minute epoxy and fixed the alignment with some balsa shims and a couple of paint cans to hold the tail straight. Full disclosure: I should have put the servos in before gluing; the pushrods made it difficult to get them in. In any case; the servos are installed and everything fits nicely in the pod.
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And finally another glamour shot with her big brother. All that's left is to finish the top of the radio pod and add a rocket pod and then it will be time for some flight testing!
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I did the final couple of items necessary to wrap up the build, and I completely blew through my mass budget. Weight with no motor is at 95g! (With motor, 106g.) However, most 900mm DLGs I've seen are spec'ed at 120-140g, so I won't complain too much. We'll see how it actually flies sometime when the weather in Cleveland isn't 5*F with windchill around -25!

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