Don't forget to check your spam folder if you don't get an activation email after joining!!

SZD Cobra 17

Let us all watch your new project progress.
Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

SZD Cobra 17

Post 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,
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
Attachments
20200112_161627.jpg
20200118_111504.jpg
20200131_095949.jpg
20200206_152703.jpg

User avatar
Cliff Evans
Posts: 176
Joined: 29 Dec 2019, 15:13
Location: Bristol
Contact:

Re: SZD Cobra 17

Post by Cliff Evans »

Nice project Chris, Watching with interest.

roo Hawkins
Posts: 820
Joined: 18 Mar 2015, 20:12
Location: Northamptonshire

Re: SZD Cobra 17

Post 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

Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

Re: SZD Cobra 17

Post 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

Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

Re: SZD Cobra 17

Post 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, 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
Attachments
20200321_124549.jpg
20200321_124846.jpg

Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

Re: SZD Cobra 17

Post 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.
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
Attachments
20200306_090921.jpg
20200306_091014.jpg
20200321_103929.jpg

Jolly Roger
Posts: 389
Joined: 30 May 2015, 20:35
Location: Sutton Bank, North Yorkshire

Re: SZD Cobra 17

Post by Jolly Roger »

Nice work Chris. Did you cut your own cores?

Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

Re: SZD Cobra 17

Post 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

Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

Re: SZD Cobra 17

Post 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, 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
Attachments
20200321_103901.jpg
20200321_124445.jpg

Chris Bowles
Posts: 90
Joined: 18 Mar 2015, 16:12
Location: South Devon

Re: SZD Cobra 17

Post 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

Post Reply