Foamboard Skystreak 32

[martin collins 1]

Been mulling over building a foamboard model of the good old Keil Kraft version, i have an balsa kit for the 26" c/l version but a plan was published in the August 1986 Radio Modeller mag for a 36" R/C version, my foamboard one will be based on that but with a flat bottom wing for speed of build as this is being popped in whilst another foamboard build is ongoing (see my Carvair thread). I intend the airframe to be built in two or three days thus not disrupting the other build too much. If anyone else fancies joining in and building one please do, the more the merrier. Mine will be electric on a 3s pack, i have them in various sizes so will go with what i need for balance. Here is a copy of the plans and build article for the balsa 36" i found online.

Skystreak_32_oz11935.pdf Skystreak_32_oz11935_article.pdf

Edited January 9, 2023 by martin collins 1

[martin collins 1]

Pic for inspiration, both built by Jim Carss on these forums, balsa ones though not foam. 

[martin collins 1]

Started this afternoon, wing is now well on the way, foam and thin ply spars glued to lower skin to dry overnight, top skins curved to shape. Servos and top skin will be glued on tomorrow morning and i will start on the fuselage once done. Update again tomorrow evening, this will be a quick one, main structure will be finished by the end of the week.

[Guest]

Martin

That pic brings back a few memories,built those two back in 2010,great little models and STREAK they did,os15 in the red one and I can't remember the electric set up in the blue one

Blue one met an early end at the hand of my hooligan pal but the red one lived a long life,had a bit of a job fitting the gear in the red one but we got there in the end,( see pic )

Jim

[martin collins 1]

Hi Jim, yes looks well packed in there! Are they the 32" version, if so would be interested if you know what the flying weight was, this is going to come out very light using the foam.

[Guest]

Martin

Never checked the weight,can't have been much,it flew very well. Was the 26" if I remember correctly

When it was time to say goodbye to it I stripped out the gear and took  it to the local tip and it did a lovely flat glide to the rear of the skip.

Jim

[martin collins 1]

Some progress pictures, wing servos fitted and left wing top skin glued on, right skin will be glued this evening and left to dry overnight. Tailplane and fin produced and the basic fuselage structure finished, i have the top hatch with the canopy to do later today, for speed i am going to try three thicknesses of 5mm foam board glued together then sanded to shape when dry, i have a pop bottle lined up for the canopy, hoping to just cut it out without the need to heat and re shape 🤞 

[John T]

Having splashed out on some A1 size foam board from Hobbycraft (£12 for 4 sheets) I thought I'd join in the fun with this one.

As I've never built a foam board model before this is going to take me quite a bit longer than Martin as I'll be learning how to use the stuff as I go along.

I'd seen a bit about "Armin" style wings on line so I thought I'd give that a go to start with. The first step was to cut out the developed shape of the upper and lower wing surface.  The photo shows this with the red hatched areas indicating where I wanted to remove the paper on the inner surface.

 

Following the advice given in another thread I removed the paper with the aid of an iron.  Next I bevelled the trailing edge of the lower skin, using a bit of 3mm piano wire against it as I didn't want to make it too thin. I also cut a channel at the leading edge fold point to prevent the curve being too chunky.

 

 

In the next photo I've added the spar, including a sub spar set at 90 degrees to the wing root. When I join the two wings together I'll slap a bit of ply in there as a wing brace.

 

 

Just before folding the top skin over I ran a bead of Gorilla Glue Clear down the leading edge slot to toughen it up a bit. I used UHU Por for the other joints and this was the result.

 

 

After cutting the aileron hinge bevel I'll need to chamfer the trailing edge underside down. Then the next trick will be to see if I can make the other wing half match up.

Fortunately for me Martin is posting lots of pictures with ideas that I'll shamelessly nick to use in my own build 🙂.

[martin collins 1]

Great job John, good start there, is yours a 32" as well? I decided to try the Vitrex for the wing skins on this one to see how strong the wings were once all glued together, some of the fuselage  formers are from foamboard where the strength is needed but Vitrex formers at the rear to save some weight. The fuselage sides are foamboard with the paper skin removed on the inside, What i like about the use of the foamboard/depron is it is cheap material to experiment with. Look forward to watching yours come together 😄

[John T]

Yes, I've based it on the old 32" span Radio Modeller plan.

On 12/01/2023 at 19:37, martin collins 1 said:

Are they the 32" version, if so would be interested if you know what the flying weight was

The magazine article says that the all up weight was 20 oz, and the engine, tank and radio gear accounted for about 12 oz of that.

[martin collins 1]

Beginning to look like a Skystreak now! Wings and tailplane glued on and will dry overnight using Gorilla clear glue, will work on the battery hatch now, should get that finished today and the fin will go on tomorrow. Will order up a motor on Monday, i have an ESC that will do the job. Nose will be made from an offcut of Recticel insulation board i have hanging round and will be held in place with a couple of magnets. Going to try to cover the finished model with model covering film with the iron set on a low setting, fingers crossed, i have tried it on some scrap foam and it seemed to work..........watch this space 😝

[andrew exton]

 

 

Hi Martin

dont know if you have seen this but might be of interest 

 

andy

[martin collins 1]

Thanks Andy, looked up the dusted stuff, might be worth experimenting with a roll at some point in the future, i have a load of Profilm here though, some of which goes back more than 15 years! Hoping to cover this with some of the scraps to use it up, will pick the most visible colours as this might get small quickly!

Small amount more progress, fin added and the battery hatch sanded to shape, will make the wood motor mount plate and ailerons next and will give it a weigh tomorrow with all the gear to see what it is looking like weight wise.

[John T]

I've not exactly been rushing this build, but a bit more progress has been made.

Slightly to my surprise, the second half of the wing ended up a pretty good match to the first one.

 

The original control line models wing had the top half of the wing constructed flat on the building board using ribs split from the leading to the trailing edge. The lower part of the ribs were then added after removing it from the board, resulting in a flat wing when viewed along the trailing edge. However, to avoid that slightly "droopy" look I thought I'd sand in a small amount of dihedral at the root so that the top of the wing was roughly flat. I should also help avoid catching a wing tip with my dodgy landings.

I added a ply dihedral brace before joining the wings together with Gorilla Glue clear.  Before I finally assemble the model I'll add some tape at the join.

 

The tail surfaces were next.  To achieve a tapered trailing edge I used the method suggested by Richard Wills in a different thread of cutting a 'V' and then sticking it together with UHU Por. I wasn't sure what to do about the leading edge, particularly on the fin which had curves in both directions.  I finally unearthed some strips of bamboo from an old place mat that I had in my "it might come in useful one day" stash which is easy to bend to the desired shape with steam, or after soaking in hot water.

 

 

Due to altering the wing section I've had to make the fuselage slightly deeper so it aligns correctly with the leading and trailing edges of the wing.  To compensate I've made the fuselage 1/2" longer, and I'll move the elevator hinge point back by 1/2" to match.  The original elevator was way bigger than needed for an R/C model anyway.

I couldn't resist putting the bits together to see what it looks like so far.

 

 

The next step is to see if I can manage to bend the rear decking to shape without splitting it...

[martin collins 1]

Looking very good John, will post an update on mine with some pics tomorrow evening............

[martin collins 1]

Not much done over the last week, hopefully get it covered tomorrow.

[John T]

It's looking good Martin.

Just out of interest, what glue did you use for bonding the layers together for the hatch?  I'm planning to do the same thing on my version, but both UHU Por ang Gorilla Glue clear seem a bit rubbery and I wondered how cleanly they would sand.

[martin collins 1]

I used the Gorilla glue, it has left a ridge, if i was doing it again i would probably use aliphatic as it sands better. 

[John T]

Thanks Martin.  Aliphatic is my "go to" glue when building balsa models, but I wasn't sure if it would work with foam board.

Following your suggestion I used some to glue a couple of scrap pieces together and left it for 6 hours to dry. The bond seemed ok so I tried sanding it which, as expected, gave quite a clean result.

With a bit more force I broke the joint apart and found that near the edge the bond was pretty good and some of the foam had ripped away rather than the glue failing.  However, further in the glue hadn't set and was still liquid.  I guess the foam is too impermeable to allow the glue to set.

I think what I'll do is use aliphatic near the edge of the bonded area and UHU Por in the middle as a sort of belt and braces approach, although it's probably unnecessary just for a battery hatch.

[martin collins 1]

Foam board Skystreak is completed, very pleased with the finished item, it weighs 15oz ready to fly with a 1250 3s lipo pack. I wanted the covering to make it look like the name suggests.....Skystreak and i wanted it very visible in the sky, hopefully i have achieved both. Weather this week looks pretty blowy here so i will keep an eye on the forecast and make sure i have a willing hand to give it a first launch. There is only a small amount of 5mm foamboard in this build, fuselage sides and bottom, ailerons, wing spars (with plybraces), fin, tailplane and front fuz formers. Rear top decking is 3mm insulation depron and wing skins, front top decking and rear fuz formers from Vitrex 5mm flooring depron. So about a 3rd of a Hobbycraft board and about half a Vitrex board plus some scraps. Airframe cost under a fiver in materials 🤣  🤣

 

 

[John T]

Great job Martin, it looks really smart. I think you've achieved in your aim of making it very visible!

I'll hopefully get mine finished by the end of the week.

[Eric Robson]

Well done Martin, a good looking airframe for the cost of a sheet of 1/4" balsa.

[Graham Davies 3]

Great job Martin.

 

How did you get on with the covering? I presume it's a standard film covering? I'm toying with the idea of painting the foam and then covering with laminating film. That's for the next project...

 

Graham

[Graham Davies 3]

One thing Martin, you might want to consider a little 'skid' ahead of the elevator horn. It looks like a plastic gear servo and I can see that getting stripped if the horn catches the ground on landing.

 

Just a thought...

[martin collins 1]

Covering went on great Graham, better than i had anticipated, i have a couple of irons, i used the one with the `sock` for the main covering and the old `naked` one set hotter to do the edges, it was just a matter of finding the right temperature on some scrap foam with a bit of the covering material which is Chrome Profilm. The flouro stuff is some REALLY old Solartrim, most of the stick had gone as it is probably nearly 20 years old but i ran the sock iron over it and it reinvigorated the glue. Under the flouro is black Fablon to make the orange pop a bit more. Now i know the covering goes on foam ok i will use it for any of the models that need a glossy finish and use the B&Q mix and match pots for anything needing matt paint. The elevator horn has more ground clearance than you think as the fuselage sweeps up towards the rear.