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SZD Cobra 17 (This needs looking at)

Posted: 04 Apr 2020, 17:17
by Chris Bowles
Hi all, this was going to be a long project, something to do when things got quite, I had followed the build thread, on RC Groups of Skyhawk, building the Cobra 17 back in 16/17, he later posted on SSUK, always liked the shape of the Cobra, so after Christmas started thinking about building one myself,
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in 1/3 scale, drew out a plan of the fuz, the fuz would be using the same construction as Skyhawk, ie planked balsa strip, so cut out the half formers, covered the edge with tape to stop anything sticking to it, and screwed them down onto 19mm MDF board, this was then clamped to the workbench, planking went along OK, a bit slow as the strips where only 6mm wide, but ended up with a nice half, flat and true, problem now was to make the 2nd half the same, formers removed, and reversed to make the 2nd half, after alot of cutting and glueing, used pva wood glue, I got 2 halfs that fitted, when the halfs where removed from the jig, I cut and screwed 3/4" sq wood on the edge where the formers were, 1, to stiffen the fuz, 2, to make joining easier, a much bigger landing area than just a 4mm lip, before the halfs where joined, I glassed the inside, with 180 tape and reinforced the fin with carbon and kevlar, I made sure that all the wooden braces, hadn't been glassed to the fuz, then joined the top first, wrapping double sided velcro around the fuz to keep it tight, after the top had dried, I was able to open the bottom enough to spread the glue on to the joint, again using the velcro to hold in place, after drying, the screws where removed from the wooden supports, and with a long bar, knocked out from the back, tipping the fuz up on its tail, all the supports fell out, next the outside was sanded smooth, then 80 cloth was epoxyed on the outside, followed by 160, then another layer of 80, this has giving a light but strong fuz
Chris

Re: SZD Cobra 17

Posted: 04 Apr 2020, 17:41
by Cliff Evans
Nice project Chris, Watching with interest.

Re: SZD Cobra 17

Posted: 04 Apr 2020, 17:48
by roo Hawkins
Will be watching. I have always liked the cobra myself and have been tempted to build one also. I do have kicking around a veron cobra fuz and plan. Good luck with the build. Roo

Re: SZD Cobra 17

Posted: 04 Apr 2020, 18:51
by Chris Bowles
Thanks Cliff,
Roo, back in the early70's, our Club held a pylon race, slope, we where flying Soarcerer's and the like, one member came up with the Veron Cobra, won everthing, since then always liked the model,
Chris

Re: SZD Cobra 17

Posted: 06 Apr 2020, 19:50
by Chris Bowles
This is where my build differs from Skyhawk 92, wings, rudder and tailplane will be made from foam, obechi and glass cloth, on the full size, the tailplane is all flying, my plan was to do the same, the foam cores where cut, covered in 160 & 80 glass cloth & vac bagged, I then made up a aluiminum bracket & bearing, fitting this to the top of the fin, the tailplane fitted, disappointed with the latteral movement of the tailplane, the tailplane is 900mm long, the bearing in the fin is 16mm, quickly made up a bracket to go over the outside of the fin, still quite a bit of movement, plus it look ugly, the only way I would be happy with the movement, would be to make a ali framework and use ballbearings, to heavy and time consumming Plan B, fixed tailplane and elevator, so sharp knife and cut off the elevator,
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cut the elevator to be top hinged, next problem, the center of the elevator is 17mm the tip 3mm, top hinging gives a nice curve when you move up & down, center hinging would solve the problem, but I had already cut the vee for top hinging 👹👹 Plan C, cut the elevator in half, and use 2 servos, a MPX connector was fitted to the fin to give 2 seperate supplies to the servos
Chris

Re: SZD Cobra 17

Posted: 14 Apr 2020, 12:10
by Chris Bowles
The wings are going to be made out of XPS foam, and made in 4 sections, for ease of transport, first need to make spars, these are made from 2mm ply, cut just under full depth, 2 to each spar, they are then covered in carbon cloth, tubes are made for the wing joiner, by rolling glass cloth around the joiner.
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the wing is cut to take the spar,the piece of foam that was removed, will be used later, glue the wing joiner tubes onto 1 of the ply spars, the foam is now cut and fitted to the ply, the top and bottom 2mm smaller than the ply, the 2nd ply spar is lined up and glued, thin epoxy and fairing compound mixture is poured around the wing joining tubes to level them up, carbon tows are wrapped around the ends of the spars, then carbon tows are laid along the length on top of the foam, this is then topped with balsa, when dry the spar is glued into the wing, and the balsa shaped to the wing profile

Re: SZD Cobra 17

Posted: 14 Apr 2020, 19:37
by Jolly Roger
Nice work Chris. Did you cut your own cores?

Re: SZD Cobra 17

Posted: 15 Apr 2020, 10:03
by Chris Bowles
Hi Rog, yes I cut my own wings, started in the 60's as a quick and easy way to get back in the air

Re: SZD Cobra 17

Posted: 20 Apr 2020, 16:40
by Chris Bowles
After glueing in the spar, the next is the covering, the first thing to do is put plastic sheeting on the bottom panel, this is to stop the core sticking to the bottom panel then place the wing core on top of the plastic, then carbon cloth is placed over the root and tip joiner area, also where the servo for the airbrake goes, this is wetted out first with epoxy, then 160 gram glasscloth is placed over the wing, and epoxy applied, carbon tows are the laid along the trailing edge, obechi is placed on top, then thin mylar sheet, that has had release agent applied, goes on top the obechi, paper towel is then put on top of the mylar, the reason for the paper,
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is any epoxy that might get onto the top of the mylar, it soaks into the paper and is easy to remove, no paper, makes it very difficult to remove any epoxy that has got onto the top, even with release agent, ask me how I know! plastic sheet is then placed over the paper towel, finally the top panel, then check that everything is lined up and in place, tape the whole together and pop it in the bag
I use painters plastic sheet for the bag, sealed with silicone, the plastic is cut to size first, when the wing goes in, silicone is piped around the edges, sealed up and the pump started, I pull down to -05, have found that this is more than enough, normally keep the wing under vacuum for about 10hrs

Re: SZD Cobra 17

Posted: 25 Apr 2020, 19:27
by Chris Bowles
For the tip panels, layup was slightly diffirent, because the tips have the ailerons, peelply was placed on the hinge line to act as a living hinge, so we have the core, carbon cloth around the wing joiner/spar and servo position, then 160 cloth, carbon tows on the trailing edge, then the obechi veneer, then 80 cloth the extra layer of cloth on top of the obechi really stiffens up the wing and aileron, this is topped with tissue paper, the tissue paper will give a smoother finish, compaired to the pin hole finish of the 80 cloth, next the mylar, paper towel, plastic sheet and the top panel, then pop it in the vacbag

Re: SZD Cobra 17

Posted: 25 Apr 2020, 20:24
by Jolly Roger
How exciting! Bet you can't wait for tomorrow morning. :)

Re: SZD Cobra 17

Posted: 08 May 2020, 16:50
by Chris Bowles
While I waited for the epoxy to fully cure on the wings, poor excuse, cut and shaped 1/4" ply to form the cockpit frame, this was glued in, then glassed, small magnets where glued in to retain the canopy, a piece of foam was shaped to form a mould for the tail wheel, glass cloth was wrapped the mould, when cured, the foam can be removed, leaving a nice housing for the tailwheel, and no muck going up into the fuz,
using my patio as a grid reference, the fuz was lined up, the wing joiner tube and wings fitted and lined up,
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using a piece of string taped to the fin, square tape measure and eye, when happy the tube was tacked in place, the wings removed, and the tube fully fixed, next the wing roots where placed over the wing tube,wings refixed, incidence checked, and the roots glued in place, then balsa and filler was placed in the gap

Re: SZD Cobra 17

Posted: 08 May 2020, 23:28
by Jolly Roger
It's coming along really nicely Chris. You've got a lovely finish in the fuselage.

Re: SZD Cobra 17

Posted: 09 May 2020, 10:07
by roo Hawkins
what a fantastic looking job you are doing. looking nice. roo :D

Re: SZD Cobra 17

Posted: 22 May 2020, 16:24
by Chris Bowles
a bit more progress, have made the canopy frame, this was cut from 6mm ply, then 4x4mm ply was glued on the edge to give a larger glueing area, canopy cut, glued and painted, the canopy I used is for the 1/3 scale ASW15, also have made a retract unit, the full size came with a fixed wheel,
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but a retractable wheel was a option, have found that a fixed wheel model, can sometimes be a bit of a pain to store or transport, so went for the flat bottom version

Re: SZD Cobra 17

Posted: 22 May 2020, 19:52
by Barry_Cole
Nice looking retract, but please use a servo that moves 180 degrees. Then there is no load on the servo at either end, and it will stay locked down much more reliably.

8-) 8-) 8-) 8-)

BC

Re: SZD Cobra 17

Posted: 22 May 2020, 20:06
by Chris Bowles
Hi Barry, the standard servo works just fine, with the retract locking in both up and down, being a homemade unit the movement will be different from a commercial one

Re: SZD Cobra 17

Posted: 14 Jun 2020, 14:58
by Chris Bowles
Been a while since the last post, but have been plodding on, got the airbrakes fitted and working, freed up the ailerons, living hinge, primed and painted the wing, before I cut the hole for the retract, I took a mould of the bottom of the fuz, because the fuz is quite thick 4/5mm, having doors that thick dosen't work very well,
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also you can hack out the hole, knowing that you can cut and fit new doors, fitted all the wiring, and installed the rest of the radio gear, balanced the model, then weighed it, came out at 12.5kg should be able to remove some weight after test flying

Re: SZD Cobra 17

Posted: 14 Jun 2020, 19:59
by B Sharp
That's looking really good Chris. I look forward to hearing about the maiden.
Brian. :)

Re: SZD Cobra 17

Posted: 14 Jun 2020, 20:49
by Chris Bowles
Thanks Brian, will let you know, one way or another, just need a weather window, easterly on our site is not good, it's a race with the tug, who gets to the strip first

Re: SZD Cobra 17

Posted: 14 Jun 2020, 21:30
by chris williams
Now that's super-pretty...! (My favourite colour, too)

Re: SZD Cobra 17

Posted: 15 Jun 2020, 09:55
by Chris Bowles
Thanks Chris, it is a nice colour, but don't know if I would use it again, found out the hard way, that the base coat needs to be almost perfect, any slight change in colour shows through, no matter how much yellow you put on
Chris

Re: SZD Cobra 17

Posted: 15 Jun 2020, 10:05
by chris williams
I have recently discovered that white primer is an excellent base for yellow to combat the poor opacity ;)

Re: SZD Cobra 17

Posted: 07 Jul 2020, 19:20
by Chris Bowles
Today managed to have a successful maiden, had the C of G well forward for the first flight, by the end of the day, had removed 280grm from the nose,
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back to where I thought the Cof G should be, better safe than sorry

Re: SZD Cobra 17

Posted: 07 Jul 2020, 20:27
by Peter Balcombe
Well done Chris. :D
What’s next?

Re: SZD Cobra 17

Posted: 07 Jul 2020, 20:50
by SedB
Nice, good to hear/see he flies well!!
Looking at the forward section, gives me the feel of a glider version of the Bell X-1 :D

D

Re: SZD Cobra 17

Posted: 07 Jul 2020, 23:11
by Cliff Evans
Well done Chris.

Re: SZD Cobra 17

Posted: 08 Jul 2020, 10:28
by Chris Bowles
Thanks for that, as for what's next, a rest and hopefully some flying, the Cobra was intended as a long term project, but the virus changed that, but at least
it gave me something to do during lockdown