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Apr 06, 2013, 04:40 PM
AndyKunz's Avatar
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Comet Sparky Build Thread

Back at the start of Building Season my club ( decided to do a One Design for our annual Builders Contest in May. The model chosen was the old Comet Sparky. About 15 of us ordered kits from Bob Holman Plans and I know several of the guys are building their kits. A couple weeks ago I found out we were going to have a visitor from out of state coming, and I gave him my un-started kit, so I'm building this from plans.

Well, sort of.

It's been a long time since I did a vintage design like this. Those rounded wing tips scream "laminate me!" even though the plans show the standard kit tips made of several sections of sheet balsa. How could I resist?!?! So in this thread you'll see me changing lots of stuff as I build my RC conversion Sparky.

I started by printing out 2 copies of the Comet plans I got from a guy online, then cutting one up to use for parts, construction, etc. I started with the fuselage sides, following the plans exactly except for the tail motor holder, for which I substituted another vertical square stock. Both sides are built at the same time with nothing between them. I've found this makes it much easier to build identically, accurately, and simply.

After that dried overnight, I removed the framework and sanded the outside edges to flatten them and remove any little glue/paper gussets. Next I added the nose doubler sheet (1/16" instead of the 3/32" needed for rubber).

While that was drying, I turned my attention to the laminations. I stripped a handful of 1/32" scrap sheet down to 1/8" wide and then boiled them for several minutes. While that was going on I cut up another set of plans and made forms to mold the laminations (just a normal corrugated box was sufficient). Since I didn't have another sheet of foam to work on, I did the tail stuff first and then followed by the wings. In the photo you can see several strips cracked, so I used my wife's turkey roaster for the next batch of strips and discarded the broken ones.

The laminating is done by pinning the form down to the foam board with a layer of clear plastic wrap under it (the backing from MonoKote or UltraCote is better than Saran-wrap type stuff, but I didn't have any available today). After the wood has gotten pretty soft from boiling, wrap a hot, wet piece around the shape using tension to keep it from cracking. Pin this in place. Get out another strip and lightly squeegy the water off between your fingers, then dip your fingertip in wood glue and smear/squish/squeegy it in. Pull this piece around the first and pin it in place. Do this until you have all the layers in place - I used 4 on the stab, wing tips, and sub-find and 3 on the vertical fin.

By the way, these strips should have plenty of excess length! Mine are usually 4-5" extra.

After all the layers are in place, push them down so they're all firmly against the foam board - the tops may be uneven.

While that was drying I cleaned up the excess wood on the nose.

More next time ...


PS - I'm looking for some 1.25" Comet wood wheels. Got a set?
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Apr 08, 2013, 07:23 AM
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RAFguy's Avatar
Andy that is looking really nice, you always do great work. I was hoping to pick up my wood this weekend at Toledo but by the time I got away from the booth, all the balsa I needed for this was gone. Order is going in today so hopefully I can start soon.
Apr 08, 2013, 08:25 AM
AndyKunz's Avatar
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I've got plenty of spare wood, I'd be happy to share.

Apr 08, 2013, 03:15 PM
Registered User
Nice project. I built one a while ago and I love it.
Only plane I have that came out nose heavy. Could use a larger rudder too.
Apr 09, 2013, 12:34 AM
Cookies Good - Bacon Better
NoMatta's Avatar
Watching the Sparky come together. I've built at least a dozen Sparkies and Phantom Furies, both kit and scratch. Just a really nice relaxing school yard flyer.
Apr 10, 2013, 12:16 AM
AndyKunz's Avatar
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Thanks for posting the photo. Great inspiration!

I wasn't able to get to sleep tonight so I took some photos of what I did (not much). I also cut out the other formers.

Apr 10, 2013, 01:41 AM
Registered User
ColinNZ's Avatar
Subscribed. Andy if you can't get any wheels there quite easy to make with a dremel.

Apr 10, 2013, 10:56 AM
AndyKunz's Avatar
Thread OP
Nice job on your wheels.

I was hoping to use real ones from Comet but that looks like it won't happen. I asked my Dad to spin some from walnut on his lathe instead. Those will be NICE!

Apr 10, 2013, 07:47 PM
Visitor from Reality
Cool model Andy. Those walnut wheels will really set her off.

Apr 11, 2013, 09:42 PM
AndyKunz's Avatar
Thread OP
I got a little more done tonight. I put cross pieces in down the length of the fuselage, top and bottom. On the bottom nose I left one out because I will be putting the hatch there, large enough for access to both the AR6400LBL brick and 200mAh battery.

The landing gear mount is different than the plans. Rather than making a sandwich of formers and wire, I made it similar to what we have on many larger RC planes. The cross brace is hard 3/8 x 3/16 balsa with a groove cut into it. The wire fits into the groove, a right-angle bend at the end serving to lock it in place and to allow it to torque when landing. Two small servo mount screws lock the gear in. The big advantage to me is the ease of replacing the wires, and the ability to finish the model without having to fight the legs.

This change necessitated some diagonal bracing to help spread the stress across the model better. As you can see, the diagonals extend up to the wing root as well as forward toward the motor. In the area directly behind the gear mount I built an X to help keep the mount from breaking out backwards.

I also added the outside formers, and will be notching them for the 1/16" sq stringers probably tomorrow evening.

This little plane is a joy to build! Lots of fond memories coming back from my pre-teen days!

Last edited by AndyKunz; Apr 12, 2013 at 09:29 AM. Reason: Typo correction
Apr 12, 2013, 09:16 AM
Übung macht den Meister..
Deuce's Avatar

Apr 12, 2013, 08:08 PM
Registered User
R Zielinski's Avatar
Forgive me if you've already solved this, but have you thought about balancing this guy? The lifting tail puts the CG far back, and that big long nose designed to hold a long rubber motor is going to have a chunk of magnets and steel way out front. I would think you'd want to put the radio and batteries way back behind the landing gear so that you won't need 5 oz of tail weight added.
Apr 13, 2013, 09:06 AM
AndyKunz's Avatar
Thread OP
Yes, in fact I did. The tail is not going to be a lifting tail on mine, so the CG will be forward. The brick will be just ahead of the CG, above the LG cross brace. The hatch as you may see is not all the way forward, but one section back (immediately in front of LG).

I wasn't going to bring this up until I got into the tail construction, but thanks for pointing it out.

Apr 13, 2013, 07:11 PM
AndyKunz's Avatar
Thread OP
Today's weather was not conducive to flying, so I did a little playing in the shop.

I started by using my Dremel with a cutoff wheel to cut notches in the formers. This lets me get them straight (I have wavy stringers!) without having to stress the wood. The front notch as you recall was pre-cut. This is so that it is very tight. The others are not as tight nor deep, but that's because I don't want the covering to touch the formers. It saves me from scalloping them later.

While that was drying, I made the tailfeathers. The laminated pieces are now totally dried out and hold their shapes perfectly. It was just a matter of putting them on the plans and fitting some bracing inside.

As you can tell, the internal structure bears no resemblance to the plans. That's because this is RC, not FF. In the verticals you can see I made the rudder large and put in a little counterbalance. The fin and rudder will be separated after final sanding.

Meanwhile I added the stringer on the rear, using tape instead of pins to hold things together. You can see here where I removed the top of the former; later on I will fix the taper up to it, again working to keep the covering off the formers. I also finished the final ones on the sides of the nose.

The next work on the fuselage will be to prepare a mount for the receiver. All this work on the tail today is because I want to prepare pushrods and supports for them inside the fuselage, and confirm the estimated balance as noted in the previous message.

Tomorrow is the Lord's Day. See you at church!

Apr 13, 2013, 08:20 PM
Registered User
ColinNZ's Avatar
Looking good so far , you should be able to turn on a dime as you have plenty of rudder and with that sub fin it just looks right or maybe i'm just showing my age .

Andy it was already tomorrow here when you posted .

Last edited by ColinNZ; Apr 13, 2013 at 08:28 PM.

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