Building curvy fuselages? - RC Groups
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Nov 29, 2007, 03:04 PM
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adamtc's Avatar

Building curvy fuselages?

Im currently working away on a nice ben buckle great news design and have hit a problem.

Most vintage stuff Ive built has been well designed with the fuselage construction involving building 2 identical sides that are then braced with formers etc. This has always relied on at least one of the longerons being flat to aid a square build. You can then add formers to create a curve later.

This design involves 2 large sides with nothing but 2 long curved longerons reaching a point in front of the stab. The only flat area is the wing seat. 2 attempts later and no matter how I try I end up with warps. I use a SLEC jig but its not helping as its very difficult to sight down the fuselage to check all 4 longerons are in parallel and following the same curvature.

Any advice?
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Nov 29, 2007, 03:12 PM
USA'd ex Brit
toesup's Avatar
Originally Posted by adamtc
The only flat area is the wing seat. 2 attempts later and no matter how I try I end up with warps. I use a SLEC jig but its not helping as its very difficult to sight down the fuselage to check all 4 longerons are in parallel and following the same curvature.

Any advice?
The wing seat is the clue...

Put the two sides on the plan upside-down so the wing seat coresponds to the plan. Using scrap balsa, make up packing pieces that keep the longerons the correct distance up from the board...
Using a set square, gradually pull the sides in together, making sure that they are at right angles to the building board..

It might help to wet the outsides of the longerons (jar of water and a paintbrush) to get them to curve a little easier..
Nov 29, 2007, 04:27 PM
Registered User
i know it is too late for this model , but for future , it is essential to choose same stock for all longerons ( even to extent of stripping sheet ) to ensure matching stress on curving in to avoid stiff side pulling over soft longerons.

It might help to put in temporary cross bracing while glue sets to assist keeping fus straight ( keep them in if not seen)

Nov 29, 2007, 09:42 PM
Culper Junior
adamtc, are you referring to the upper corner longerons? If so another idea is to slit the longerons where the curve is the greatest and then glue the slit closed with the wood pinned in it's final shape. I.E.- slit the longeron (looking at it from the side) from the wing trailing edge back about 1/2 way to the stab, but not all the way to the stab. Put glue between the slit halves, pin down , and continue on. Hope I made myself clear. Jim
Nov 29, 2007, 10:57 PM
B for Bruce
BMatthews's Avatar
Yep, this is definetly a time where you build the main box portion upside down using the wing saddles pinned directly down to the plan. What I like to do from that point is addin the cabin area cross pieces and then using some right angle triangles I block the sides up square before glueing the main cabin cross peices into place. A temporary diagonal bracing strip isn't out of the question at that point either. From there I pull the rear together using triangles again to ensure they meet at the center line and I would also add a packing shim back there to lift and support a "low side" so the ends meet. If the one side is badly out of shape you may need to add some pushed in place diagonal jacking braces as needed to force the bad side to conform. Later on you can likely remove then or decide if the fuselage requires some extra bracing to be left and duplicate as required to make it look like it was planned that way.

Some water mixed with ammonia brushed liberally on the wood while clamped back onto the side view may hellp a lot as well. Let dry overnight. The ammonia really helps the wood give while its there and it's hard again when it drys away.
Nov 30, 2007, 04:28 AM
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adamtc's Avatar
Cheers all.

John where are you in Soton? Im near Fair Oak, Hedge End way. Small world.

Toes up, thats what I tried .. twice. The problem is being able to sight both longerons at the same time. I can watch the main longeron but not what is effectively the upper longeron.

I have decided to retry but having built the fuselage differently, namely extend the small cabin longeron all the way to the stab to create a smaller initial fuselage box section that I can then add to.
Nov 30, 2007, 01:50 PM
Registered User
I am round the corner in dibden purlieu - fly at beaulieu - as u say - its a small world and getting uncomfortably smaller by the day
re your problem, I still think it stems from unmatched wood for the longerons as a stiff/hard longeron side will always pull round a side built with less stiff wood , unless held in a jig while glue dries AND cross bracing added to stop it pulling back.
From the photo of the model, I cannot see a former under the trailing edge - if there is not , I would add either a former or X bracing at this point and another X 1/2 way to front of tailplane.
Nov 30, 2007, 03:27 PM
B for Bruce
BMatthews's Avatar
Adam, perhaps the issue is that you are trying to cut and fit the cross pieces while the sides are setup? If so then it's no wonder you are doomed to difficulty. For a model of this sort I would suggest you need to precut the cross pieces from the top view complete with their approximate angles (mark the fronts with a small dot or line or whatever). Then during the gluing in of them you can do a final fit for a minimal glue line by very slightly sanding the faces to fit closely. Doing it that way along with some big triangles clamped, weighted or bolted to your building board to act like a jig should hold things in place and in line.

A big first step to a nicely lined up fuselage is that your side frames need to match when placed over top of each other. If not then you've got a problem right there that needs to be corrected somehow. If they do line up then the erecting of the basic box should go relatively straight forward if you're using some sort of jig or even some big triangles nailed to your building board so that the base of the verticals lines up with the outside line in the top view.

I feel like a lot is being confused in the translation here..... Any chance of some pictures of your jig with the sides set up in it?
Last edited by BMatthews; Nov 30, 2007 at 03:32 PM.
Dec 01, 2007, 02:27 PM
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adamtc's Avatar

I think I know thee considering Im the chairman at Beaulieu. Large red/blue vintage floater with foam ribs is your model if Im right?

Mr Mathews,

Like you idea of precutting. Ill give it a go. I suspect the clue is getting 2 longrons of identical stress and density. I tried using spruce as well to hold a better curve but only for the lower ones not the top.

Ill take some pics onc mk3 is in the jig.
Dec 02, 2007, 04:48 PM
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You got me, Adam,

I have Vic Smeed's Mamselle original at 36", a 72 " and a 108" which is the red and blue one. The crutch fusalage design is one of the easiest to build altho it still has the problem of keeping the crutch straight till you get formers and stringers on.

The balsa req'd for the ribs was considerable and I had a go at foam ribs using sandwich method and hot wire - real easy - foam was a 2 sheet of 1" foam from Wickes - capped foam with soft 1/2" by 1/16" to keep heat of foam when shrinking solartex - it also smoothed out imperfections on the hot wire cut.

the 36" sits with an albon dart (0.5cc) in it and is free flight - an activity I no longer indulge in - but the other 2 are regular fliers and if I can work out how to post a picture I would put up a nice shot of the 108" in the air.

Clikcking on the image insert brings up a "script prompt" whatever that is - now worked it out - see picture

not sure who you are, Adam - what do you fly?

Last edited by John38; Dec 02, 2007 at 05:09 PM.
Dec 03, 2007, 03:40 AM
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adamtc's Avatar
1/4 scale pup, flair hannibal, yellow and black ( red) zephyr, flair puppetter, mick butlers old majestic major.

Chap with glasses....
Dec 04, 2007, 06:25 AM
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think I place you now
funny how we all fly together but names dont allways stick - or is it me ??
Dec 16, 2007, 02:40 PM
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Hey Adam, how's that great news of yours coming along? I'm building a great news at the moment, just finishing the wings before I attempt the fuselage. Any tips or pictures would be most appreciated.
Dec 17, 2007, 04:48 AM
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adamtc's Avatar

abandoned Im afraid. Wings went perfectly OK , but as you can read, my snags were with the fuselage and pre cut parts. Just not enought time to spend re designing it. Depending on where you measured the plan, you got 3 different measurements for each part!.

My set of plans are actually poor quality photocopies and if you look down the length of the fuselage with your eye to the plan, you will see the lack of a straight line anywhere!!! On my copy you can see the wobbles created by the photocopier.

The pre cut parts were so poor, I abandoned them. I recut new stuff from the plan but them found that the profile sections on the plan were again, way out. Bens redraw seems to be OK in places but way out in others. My main former is 5 mm wider than the plan and the notches are odd intervals. Keeping it all symetrical whilst working it out has proved a faff.

I managed to build the fuselage by building the sides in smaller sections than the plan showed. I extended the lower cabin longerons all the way to the rear and building a smaller box section, so to give me a flat longeron to build from. I then added the cabin and rear section but without the curved top longeron. I ran straight pieces instead to prevent the curve pulling the rest of the fuselage out of shape.

I could stick at it but the lack of time is stopping me from enjoying the build.

If you do stick at it, be aware that the tail ribs are notched incorrectly from the plan and its unclear how the fillets for the rudder are actually installed!!

So much for home grown kit manufacturers!!! Ive gone and bought a Balsa USA kit.

I hope this doesnt put you off, its just Im finding it hard to trust the plan and having to do all the building freehand.
Dec 17, 2007, 10:06 AM
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vintage1's Avatar
Somebody send me a plans scan.

It's pretty enough to be worth a redraw.

If you want it slimey, thats OK by me

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