Frog Mustfire

[Bob Cotsford]

Finally it's time to resume work on the Mustfire.

A bit of background:

With thanks to 'Gluehand' on RCG -

"'Mustfire' is one of our Swedish 'all-time-greats', still remembered and loved by many...
It was designed about 1960 by Jesper Von Segebaden.
First locally kitted, eventually also kitted by Frog and some other manufacturers.
A truly successful (and nice looking) design, rightly deserving its place in the 'multi' Hall Of Fame..."

It actually won the Swedish championships in 1964, 1965, 1966, 1967 and 1968.

Hales flogged it in the UK under the Frog banner as a 'Multi Radio Control Model' from some time in the '60s and it was reviewed by RCM&E in '67 - see my album if interested.

I bought this kit off Harry Maclean through the classifieds - I did actually think about it for at least 0.000009 second before firing off an email laying claim to it. Very many thanks Harry.

So far,

Old fashioned wooden bearers, note two holes in the bulkhead, top one for fuel pipes, bottom one for the Evolution 60GX ignition connections. Plus a drain hole, no noseleg here!

The tankbay hatch reveals the other tankbay hatch - for access to the electrics.

[Colin Leighfield]

Now you're talking Bob. When it comes to real flying machines, you can't do much better than a Mustfire! Love it.

[Bob Cotsford]

A couple more pics for today-

Snakes - can't be doing with wooden pushrods and 2mm wire ends flopping about so it's Gold'n'Rods for me. The end is supported by a wedge of 1/4" sheet to stop it flexing and add a bit more gluing area. I've pre-covered this area as I hate trying to get into all the nooks and cranies later

The radio bay is cavernous - designed for biiiggg Bonner Duramite reed servos and massive receivers with reed packs and amplifiers. That's a standard servo feeling lost and lonely in there

So the board is cleared and the glue blobs sanded off

 

So on to the wings - I have marked the ribs with datum lines as an aid when lining them up on the spars. The wing uses egg-box construction - a full depth sheet spar is slotted half depth at the rib stations, the ribs are slotted half depth at spar stations and the components key together in a self jigging assembly.

Considering the kit is potentially 50 years old the wood is in excellent shape, everything is usable. Some of the ribstock was a little soft resulting in a few die-crushed ribs, but they are good enough to use. The vertical spars are pre-slotted for the rib stations with nice snug fits.

There are one or two parts I won't be using - such as the 1/2" square undercarriage mounting blocks which really don't have enough meat for the U/C retaining screws and look at risk of splitting in a heavy landing. They will be replaced by 1" wide 1/4" and 1/8" ply laminates giving 1*3/8" blocks with no risk of splitting.

The rudder and elevator blanks are 1/4" balsa mated to the built up fin and tailplane with 3/8" deep hinge lines - I've built up replacements with 1/16" sheet over 3/32" ribs to flow the sections together properly, also increasing rudder chord by 30% and elevators by 25%.

Right, that'll do for an intro, the rest will come a little more slowly!

Edited By Bob Cotsford on 08/11/2013 15:25:05

[Martyn K]

Looking forward to see this get together. Did you start the kit from scratch Bob or was it already partly assembled?

I can see the MUSTang bit in the box artwork but not sure about the FIRE bit yet.

Looks like a proper build

Watching and learning..

Martyn

[Bob Cotsford]

it came as a bare kit Martyn, I built the fuselage and tail a couple of months ago and lost my building mojo. Instead I fettled some ARTF and second-hand models, upgraded the central heating and went flying, but that's all done with for now so it's back to the board.

No photos of the work to date so a quick rundown on the structure.

The fuselage is your basic box section built around the engine/tankbay assembly. The bottom half is built first, starting with the front two formers and the engine bearers, braced with 1/8" walls creating a snug box for the tank. The 3/32" lower sides get 1/8" balsa doublers from behind the wing to F1 and stringers back to the rudder post. The sides are attached to the tankbay assembly, pulled in at the rear and the remaining lower formers are added.

With this assembly off the board the upper former halves are attached, then the simple task of attaching the 1/4" * 2" upper sides. These are plain strip wood, and quite firm. They are also angled in to the top of the fuselage, so they should taper in width to mate with the lower fuselage sides as they come together at the back. No pattern is supplied so the choice is to make a pattern and spend time fitting the 1/4" planks or to use brute force. Seeing as the wood is 'firm' I opted for brute force - soak them in an ammonia solution overnight then strap them in place to dry. After soaking they flexed across the 2" side quite easily and when dry were a doddle to fit. Next plane down the upper sides to match the formers top line, and add spindle molded 1" top blocks.

The instructions have a delightful paragraph regarding the cowl are - 'build up the cowl from the supplied balsa or mold a fibreglass cowl to suit'. Thanks Frog, that's really helpful. OK, bolt on the engine, tack the nose ring to a spinner backplate with some 3/32" packing and bolt to the engine. Use miscellaneous likely looking block, 1/2" sheet and scrap to fill in between F1 and the nosering, remove the engine and you have something resembling a Neanderthals' club. Never mind, I enjoy a session with the razor plane and sanding block.

Tailplane and fin - just your basic sheet over rib structures - see prior note re the supplied 1/4" sheet elevators and rudder that would leave a step

[Bob Cotsford]

ps - the 'fire' bit is from the elegant semi-elliptical wing!

[Martyn K]

Ah yes - but with a clipped wing..

[Bob Cotsford]

High speed PR clipped wing jobbie?  Maybe I should do it like that pink PR version?

Edited By Bob Cotsford on 08/11/2013 16:03:24

[Martyn K]

Yep pretty in pink... ( I dare you)..

On second thoughts don't - its in the UKCAA section and people will have even more doubts about our sanity.

I think a nice manly white red and black or blue will be OK

Martyn

[Bob Cotsford]

Orange and white with black cheat lines is favourite at the moment - sort of Hardley Ableson colours, nice and macho

[Gazza58]

Great looking aeroplane that has passed me by. The tail surfaces look very elegant. As for the cowl instructions, I seem to remember that most kits of that era did little more than give you an outline of where the plan drawer expected the finished model to end up. Control line Phantoms and even Junior 60's were the same. Just blocks of balsa that you were expected to graft into an acceptable shape. The living room, in those far off days, being covered in sawdust. Fortunately, dad was an avid aeromodeller and he was onside. Poor mum used to sit there with a fine sheen of dust settling on her as she watched Ena Sharples on our 12 inch Bush television.

Good to see an old kit be formed into a flying machine after all these years. Old kits seem to be sooo expensive these days and kept as ornaments or even investments rather than made for their designed purpose.

I wish you all the best with your endeavours and look forward to this thread developing. I'm liking the wedge snake support and the idea of covering the tricky bits before assembly. Yet another dodge for the little grey cells.

Gazza

[Bob Cotsford]

Thanks Gazza, you are right with the instructions which is why it puzzles me when even ARTFs get criticised because they omit to mention some blatantly obvious (to me at least) point - have we lost so much initiative and inventiveness in the computer age?

The instructions for the Mustfire cover 3 sides of A4-ish paper, with one side being devoted to a description of the model comparing reeds and propo setups. Then again it was considered as only suitable for advanced flyers at the time, which meant advanced builders as there were no ARTFs.

While it's fresh, the suggested mods for 'propo' users were to increase the chord on the rudder and elevator, reduce the dihedral to half that shown and reduce control movements.

It'll be interesting to see how it performs on a 10cc petrol 2 stroke which should put out about 1hp, much the same as a Merco 61. The 68" span model is spec'd for .49 - .61 glow motors with a.u.w. being between 6 1/2 and 9lb.  With about a pound less radio gear to carry I'm hoping for around 7lb or so.  The lower end of the range is unlikely as some of the supplied wood is - er - not light?

Useful feature - the plan is on 2 sheets, and on the reverse of the wing sheets are patterns for all diecut and sawn parts.

Edited By Bob Cotsford on 08/11/2013 16:47:31

[Gazza58]

I actually tried moulding a fibreglass cowl once in the 70's. It looked good, weighed an absolute ton, and I think the model balanced perfectly on it's CoG. Before the engine was put in. The model has probably long gone but I bet that indestructible cowl still exists somewhere.

As for initiative and inventiveness, we maybe had access to more experienced modellers in those days. Today, only wonderful things like the electric interweb net and this site in particular teach us some old dodges.

ARTFs get criticized in the mags (not RCM&E of course) because there is so little to complain about. The models are almost perfect. I even see complaints about the plastic pilot being the wrong size. When we lived in that shoe box at the bottom of the lake we made our own.....

Gazza

[Bob Cotsford]

Undersized pilots? Nah, in our day they were just undernourished

[Bob Cotsford]

With all the reference lines drawn on the ribs and spars I used a 16mm spade bit in the pillar drill to put the holes for the cable run in the ribs inboard of the ailerons:

Next it was time to start juggling 10 ribs and 4 spars trying to get all the reference marks lined up with the LEs and TEs in straight lines. Having accomplished this I went to make a cuppa, which is when Pimple the younger tom decided he wanted to know what I'd been doing and jumped up for a look-see... bummer!

Of course a startled cat doesn't carefully pick his way back off the workbench!

It could have been worse as the outer rib trailing edge sections will be cut off as part of the ailerons anyway, and the inner rib bits can just be glued back on without too much trouble. I'll replace the tip rib as the balsa was a bit spongy and it lost a segment from between the rear spars.

Pimple and I did make up when he finally stopped hiding at the top of the garden around supper time.

One thing I did learn, it really needed tabs to aid rib alignment, it's a very fiddly job without them!

Edited By Bob Cotsford on 12/11/2013 09:54:01

[Martyn K]

You can add flaps if you want instead of gluing it back together. It may help slow it up a bit when landing - or even couple the flaps with the elevators like Phil Kraft did with Slik-Fli. A nice transmitter mix to think about

Martyn

Edited By Martyn K on 12/11/2013 09:56:56

[Bob Cotsford]

That's an idea, but as the #2 rib would need to be part of the fixed TE under the fuselage I'd have to get the cat to do another run to get the right ones!

Seriously, I doubt a lightly loaded model such as this would benefit from flaps with it's semi-symmetrical section. The Slik Fli was a later generation model IIRC, more heavily loaded, faster flying and having a fully symmetrical section.

What in theory is a simple solution to building a straight, light wing has turned out to be a bit of a pain. In years long gone by I used egg-box construction on a few models, and with a single capped mainspar creating a full I beam it's a quick and easy method to get a strong wing but the curvy edges and extra spars on this one make it awkward to align.

[Martyn K]

Yes - I can imagine. I have never built an eggbox wing before so trying to manipulate it does looks tricky. I was going to suggest making a foam jig, but the double ellipse may be difficult to achieve. However, some form of LE and TE support jig nailed to the board may help.

From the photos, the section looks symmetrical by the way.

Martyn

[Bob Cotsford]

OK, back on track as I have a couple of hours at home. I just had to tell myself to stop puss-footing around, it's a toy aeroplane not a precision instrument, the odd flaw is not a cat-astrophe!

With the 1/8" and 3/16" sq spar caps in, everything was lined up and a straight edge strapped on to hold everything - straight - what else? A quick measure that all rib reference marks are 1" above the board and I gave in to modern technology. The instructions say to run a fillet of balsa glue into the joints - ie no face to face adhesive joints! Thin CA wicked in gets a full depth bond, there's no going back now.

Now here's a thing, all this time I've been reducing pictures to 800*600 to get them to upload - even that failed earlier so I tried sending one at a time - success. Now I've tried 1600*1200 and they will upload 1 at a time. All that time I've been faffing around with Gimp and I just needed to send them singly.

[Bob Cotsford]

Bit more progress - the other wing panel has been assembled, so I cut some new dihedral braces for the recommended reduced dihedral. The design uses just two 1/16" ply braces for a 68" span, which seems a bit mean. I added one extra for the LE to brace the wing dowels - the plan shows rubber bands round 3/8" dowels but I'm going for a bolt on version - and an extra one between the caps of the main spar.

Next job was to insert some conduits for the servo cables, strips of gift wrap did the job here, inserted by wrapping them round a length of dowel then letting them unfurl once in place. Thanks for that idea Peter M.

It's all a bit delicate at the moment with rib ends sticking out everywhere, I'll be glad to add some sheet covering to protect it.

Edited By Bob Cotsford on 16/11/2013 15:16:27

[Bob Cotsford]

Some more progress. With the two panels joined the first job was to fit the LE and TE sheet to the underside. This is fitted around the oversize spars which will be needed later.

That locked in the rib ends which were a liability up to this point. The gap is the split for the aileron.

Next, the aileron hinge blocks, ply horn mount and wing bolt support blocks were fitted after which the wing was weighted down on the board one panel at a time with the spar extensions holding the panel straight while the LE and TE top sheet was added.  This creates a rigid structure by closing off the boxes.

With both sides done I added the replacement U/C blocks and rib doublers, then the cap strips and centre section sheeting. You can see I've fitted thin card funnels to guide the servo extensions through the top skins into their gift wrap paper guide tubes. I'm undecided which servos to use for the ailerons, the Corona 229MG thin wing servos or Corona 339MG minis. The 229s are mounted using double sided tape and/or the screws through the three lugs. They don't use grommets so I'm wondering whether they would be affected by vibration when screwed in.

That just leaves the outer LEs and tip blocks to fit before cutting out and facing the ailerons - once I've stopped pithering about worrying which servos to fit!

Edited By Bob Cotsford on 29/11/2013 16:58:12

[jeff2wings]

Looking very nice Bob ,that's what you call a real modellers model ! I used Hitec HS 125 MG 'S with screws and a layer of 1/8 sponge rubber ,no probs so far ( and that's with a shaky old Merco up front )

[Colin Leighfield]

I think you're going to love this one Bob. They don't get better than a Mustfire!

[jeff2wings]

How very true ,flies loverly square loops and very graceful stall turns and makes my landings look good !

[Colin Leighfield]

I clearly need one.