How to repair a broken spar

[Tony Patman]

Following a crash of my Tutor 40 a couple of weeks ago, I'm getting to grips with repairing the damaged wing.  A hedge impact broke through the balsa skin of the leading edge between two ribs and severed the upper spar, which appears to be 1/4" balsa, at the very edge of a rib.  There doesn't seem to be a solid leading edge, indeed.  The back of the D-section was also cracked, so the shape of the wing is currently relying only on the lower spar.  If I can rejoin the broken spar, replace the back of the D-section and reskin the leading edge, I think all will be airworthy again.

So, oh gods of balsa, any tips?  I was going to try a splice for the spar, carefully cutting a very shallow V shape along it and epoxy-gluing a new section into the V, positioning the V such that part of the depth of the original spar which is still glued into its slot in the rib remains.  The spar seems to be the critical structural element here: the other bits are trivial.  So, am I on the right lines?

[Bob Cotsford]

sounds a good plan to me, nice long angle on the splice, and  I'd probably add some light 1/16 webs as insurance, glued to the rear faces of the top and bottom spars.  Don't gorget to balance the wing after recovering it.

[Bruce Richards]

Tony

I can't see the break but if its quite clean I would join the existing spar as best I could. Then enlage the slot in the rib front and back and add a doubler (same depth as the spar but half the thickness) front and back of the spar glued with PVA (white glue) and clamped. Clean off excess glue befor it drys.

 Hope this helps.

Bruce 

[Former Member]

[This posting has been removed]

[Tony Patman]

Thanks chaps.  The break is near the root, erm, three or four ribs out from the middle from memory.  The fact that the stresses are higher near the root is why I'm here asking!  I thought of repairing using a piece of hardwood rather than a piece of balsa; and I note Bruce's suggestion to add a doubler too, which seems wise.  I had hoped to avoid removing all the covering and replacing the whole spar.  I know what I need to do: take a picture and post it on here!

[Guest]

Tony,

you said the spar is broken next to a rib - if so then you need to cut the spar out into the next bay and splice in a new bit . this also enables you to glue a doubler piece under each splice to strengthen joint. If there is not a lot of space between top and bottom spars, then fill the whole space in with doubler.

then as has already been said web between the top and bottom spars both sides vertical grain.

 if you are 3 to 4 bays out then repair should be ok

john

[Tony Patman]

Upon closer inspection, there's more damage than I thought

The trailing edge spar is cracked at the inboard aileron hinge position, which is hard to get to without more substantial destruction of the wing; and I suspect just filling the crack with epoxy would not be satisfactory.  Hmm.  I might shelve it and focus on getting my new model balanced and in the air.

[Erfolg]

What you need is help from George Stringwell, he does not seem to be about these days.

I am sure his approach would have been along theses lines.

The spar /spars carry most of the flight loads, the incorporation of a "D" section leading edge both aids torsional resistance and a stressed skin, relieving some load from the spar.

His repair approach always seemed to be a  planned strategy to leave the wing as strong or stronger. His first act would be

• to get the wing flat and true on the building board
• tack the main peices back together
• If possible glue permanantly the broken/cracked spar in place.
• If not possible remove a section and splice a new peice back in place (at least 6 to 1 taper).
• In either case the splices/break is the weak link. A sub spar of similar proportions would be attached to the original spar (making and positioning the sub spar  as in- obtrusively as possible). Remembering that that outer fibres carry most of the stresses.
• Shear webs would be reintroduced as appropriate.
• Where the "D2 section has failed, an appropriate area would be removed. Which may be bigger than you would like. Making sure that a scarf joint for both ends can be reasonably accomodated. A peice of sheet balsa larger than the removed area would be cut.
• The new sheet would be used to as a template to finally cut through the original top sheet.
• The under sheet removed to let the new sheet to be perfectly inserted, then glued.

It is a simple set of operations, sketches would clarify some of the actions (but not really possible hear). Repairs do not take as long as my description (well almost).

George would have continued this way until a near perfect repair was acheived, then all sanded and recovered. He had an excellent column and his books were not half bad either.

I know some modellers where major repairs are indistingushable from the original structure and covering

Erfolg

[Tony Patman]

Thanks so much, Erfolg: excellent, detailed advice.  What's a "scarf joint"?

[Erfolg]

Hmmmm Tony

You really need one of those books from 1970s which seem to have disappeared, and the mags no longer seem to cover this area. Typical Books , that should be available for a £1 or so on the internet second hand are

• Thermal Soaring, George Stringwell
• Radio Controlled Soaring, Dave Hughes
• Radio Controled Guide, Norman Butcher
• Radio Controled Scale Aircraft, Gordon Whitehead.

I personally like Stringwell and Whiteheads books. Must be time for an update by Chris Golds (very adventerous builder, lots of techniques/wrong mag) or maybe David Ashby  could be persuaded.

Any way what is a scarf joint? Simple in concept, imagine two pieces of wood, square in cross section (do not need to be), length, whatever you want. One end of each piece is made into a wedge shape, they are identical in angle (therfore in length). The peices of wood are now aligned together to make one straight length, ie wedge face to wedge face. If they were know joined on the faces of the wedges, we would have one continous single piece.

In principal the longer the wedge or taper, the better will be the strength of the  joined pieces. For wood most people would consider a length of taper to width of material should be no less than 1:6. A similar idea is often used to join sheet, but the joint will look like a pinking shear joint (my other hobby is dress making, joke).

Dependant on the sizes many technigues are available. I personally allways try to make one piece first, then to use it as a template to cut the second piece, to form a perfect joint (if only). I generally use PVA for this type of work, as it will fill small gaps and allow time to adjust. Sharp scappel is an essential.

Erfolg  

[Tony Patman]

Thanks so much for all your help.  I can't wait to see some pics of you in your frocks