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Aug 09, 2020, 05:18 PM
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MKellyvich's Avatar
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Build Log

Zaic Utility Special Build


I was browsing Outerzone last week looking for plans of some older racing planes. In my search of "Special" up popped Frank Zaic's "Utility Special" from the May 1937 Popular Science (https://outerzone.co.uk/plan_details.asp?ID=1819). I was just about to wrap up a dime scale Miles Sparrowhawk and thinking about what to build next. This seemed like a perfect summer build - a few weeks construction during the hottest days here in San Antonio and something new and different to fly in the nice weather this fall. It also fills some holes in my Flying Aces fleet, letting me dip my toes into the 2-bit+1 and Old Time Rubber Fuselage events.

The model has a great look to it, and at 24" span it won't be a monster to cart around or eat up my entire rubber budget (unlike my long-term 56" Commodore, waiting patiently for my modeling ADD to come back to it).

So, I've printed out appropriate sections of the plan and have things rolling along. Built the fins and stab this morning, laminated the first set of longerons this afternoon and sanded some 1/32" sheet down to 1/64" (!) to laminate stock for the 2-layer circular formers.

Mike
Last edited by MKellyvich; Aug 09, 2020 at 09:24 PM.
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Aug 09, 2020, 06:38 PM
B for Bruce
BMatthews's Avatar
That's a really cute one! This will be fun to watch.

I find I don't get much chance to fly the big stuff any more. Even our local free flight field is a once in a while thing and if the wind is up even a little it's at best it's a minute to minute and a half sort of field before crossing large drain ditches becomes a requirement. There's much to be said for models in the 18 to 24 inch span range that can be flown well in local sports fields when they are not being used for other things.
Aug 13, 2020, 03:47 PM
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Thanks Bruce, there's a lot to be said for making flying convenient, which smaller models often do.

Spent the first part of the week on the fuselage. The laminated formers are still ridiculously thin and fairly flexible, but at least they don't crack. Once the parts were notched and marked I set them up in my (somewhat cumbersome) fuselage jig. Put on the top keel and top four stringers, then flipped the shell and jigged it back up for the bottom half, this time using thinner crosspieces to fin under the topside stringers. Adding the stringers is a bit tedious since there's very little gluing surface where the stringers meet the formers, but it went smoothly. Once this assembly has had a few hours for the thinned Duco cement to fully cure I'll pull the fuselage out of the jig, add the last two stringers, then clean up the front end and trim/glue the stringers at the aft end.

Mike
Aug 14, 2020, 07:25 PM
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Got the fuselage out of the Iron Maiden last night, did some sanding today and fitted the tail. I like the way the stab integrates with the fuselage and how the ends of the stab are outboard of the fins (like a P-38). Wings are next. Tapered, no spar - this will be fun...

Mike
Aug 15, 2020, 05:54 AM
I like real wooden aeroplanes!
Sundancer's Avatar
That fuselage is a work of art Mike
Aug 18, 2020, 09:51 PM
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Thanks Sundancer - it's got a few wiggles in the stringers but it'll do. Limited progress so far this week due to back-to-school activities with the family. I did get the LE stock cut and tapered and made some jigs for sanding the rib ends and the dihedral joints. The jigs are just scrap pine cut to the appropriate angles on a mitre saw. I also made up a couple sheet sanding jigs using music wire and scrap poplar to sand 1/16" stock to 1/20" and 1/32" stock to 1/64" and used them to sand the rib stock and forward fuselage sheeting down to size.

Hope to cut the ribs and assemble the wing panels tomorrow.

Mike
Aug 20, 2020, 12:17 PM
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Got all the wing parts cut out yesterday. I spent a couple hours fiddling around with the bamboo tip reinforcements. The plan shows 1/32" square bamboo wrapped around the LE, tip sheeting and TE. After a couple of tries I was able to split some skewers to get roughly 1/32" square bamboo strips. I've bent bamboo before using a soldering iron, but not to the tight radius of this wingtip. After some experimentation I found that if I soaked the strips for an hour or so in water they would heat up and make the tip bend without cracking.

Last night I fitted the tip ribs and sheeting, then this morning I installed the rest of the ribs. The rib templates on the plan get a bit long toward the tips, but that's better than too short. Sanding jig made it easy to correct the rib length and get the angles right where the ribs meet the LE and TE. I'll glue on the bamboo after rough-sanding the wing panels.

Mike
Aug 22, 2020, 12:44 PM
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After sanding the topside of outer wing panels to shape, I glued the bamboo tip reinforcement on using thinned Duco. Let that sit overnight, then sanded the tips and lower surfaces. I really like how that thin sliver of bamboo has stiffened up the tip, and how the taper of the wingtips creates a couple degrees of washout. Now to sort out the center section - this will be interesting as the LE is a bit larger than the gap it's supposed to go into between the stringers on the fuselage...

Mike
Aug 23, 2020, 02:40 PM
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Got the wing center section sanded, with a lot of test fitting to make sure the LE and TE profiles matched between the center section and outer panels.

As noted previously, the wing center section LE was taller than the opening for it between the stringers. The LE dimensions on the plan fit between the centerlines of the stringers, but didn't account for the thickness of the stringers. After thinking it through, measuring and comparing to the plan, I used a bit of acetone to pop loose the glue joints on the stringer under the wing so it could be moved, and I notched the LE at the upper stringers about 3/32". With that I was able to slide the LE and TE through the stringers and adjust the notches until the wing sat level (in the pictures the right and inboard center section ribs aren't glued yet).

Couldn't resist blocking things up to see what it'll look like with the wings on it.

With wing placement solved, I removed the wing and stab in preparation for sheeting the nose and aft fuselage.

Mike
Aug 23, 2020, 03:06 PM
I like real wooden aeroplanes!
Sundancer's Avatar
What a delightful and delicate little airframe Mike - almost seems a shame to cover it!
Aug 24, 2020, 01:25 PM
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Thanks Sundancer - it is a tidy little bird, isn't it?

The plan shows a 3/16" overhang of 1/64" sheet around the forward bulkhead, forming a well in which to place the noseblock. My fumble-fingers would undoubtedly tear that up stuffing in the noseblock, so I wrapped a strip of well-soaked 1/32" sheet around a marker to make a reinforcement ring for the nose. I also glued some 1/16" square strips between the stringers at the second bulkhead to give a solid surface for the aft edge of the nose sheeting. Per the plan, I sanded ~1/64" off the forward section to let in the sheeting and smoothed out the transition between the forward bulkhead and the nose ring so the sheeting would lay flat.

Nose and tail 1/64" sheeting went on in quadrants, glued with thinned Duco. The tail sheeting was a bit fiddly towards the pointed end, but came out fine and sanded up nicely.

Rather than using a hook in front of the stab as called for on the plan, I added peg mounts behind former #8. This roughly centers the motor about the CG, and gives right at 12" hook-peg.

Next: landing gear!

Mike
Aug 27, 2020, 11:26 AM
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Landing gear is done. Turned a set of wheels on the Shopsmith, cut, glued, carved and sanded the wheel pants. The plan shows the landing gear wire mounted using metal staples on a laminated plate glued between the third and fourth formers back from the nose. I sanded some stiff 1/32" c-grain down to about 1/64" and laminated three layers of it to make the plate. I used twist-tie wire for the staples.

Mocked up the gear mount with a cutoff from the laminated sheet and was satisfied with how it located the gear wires, but was a bit concerned that the staples would pull through the sheet. I added a short length of bamboo toothpick on the opposite sides of the plate from the gear wires - with this everything was nice and solid. The article says to "tightly wrap rubber bands" around the joint between the front and rear gear wires. Given the short life of rubber bands in the Texas sun and heat, I substituted heatshrink tubing for the rubber bands.

After fit-checking everything I punched holes for the staples in the plate, glued on the toothpick segments, then glued the plate into the fuselage. I pushed the staples in through the bottom, then used forceps to twist them down from the top side - only cracked one stringer in the process... After a final alignment check I twisted the staples tight. I'll check alignment one more time after the wings go on before folding down the twisted ends and putting a daub of Duco over each staple.

It looks really cool with the cigar angled up on the gear. I was going to fit the wing today but instead I think I'll do the noseblock and carve the prop - I want a picture of the fuselage with the prop and spinner on it before cluttering up the lines with the wing.

Mike
Aug 27, 2020, 04:33 PM
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Those photos deserve some lyrical comment but that's beyond me I'm afraid - just beautiful.
Aug 27, 2020, 08:59 PM
I'd rather be flying.....
JeffMac's Avatar
Just WOW.....

Best regards,

jeff
Aug 28, 2020, 04:08 AM
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and WOW again from me, Mike. What I like in particular is that this seems to be a perfect example of the 1937 period "modern look". Streamlining for speed, cigar shape etc.


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