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Spruce or Obechi for a wing spar?

It's for a Q-Tee Wing

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Kit Durham20/06/2018 09:34:26
9 forum posts

Hi all

Building a Q-Tee, I have heard that oils contained in spruce can cause glued joints to fail over time so am wondering as to whether obechi might be a better bet.



Former Member20/06/2018 10:39:31

[This posting has been removed]

Peter Christy20/06/2018 11:44:16
1769 forum posts

Agreed! I've been using spruce for spars for decades without any problems. I only use balsa spars on lightweights.



Bob Cotsford20/06/2018 13:08:44
8380 forum posts
463 photos

Isn't obeche generally a short grained wood prone to cross grain fractures? The reason spruce works so well is thar iirc it has a long straight grain structure making it ideal for long straight runs such as wing spars. There is a school of thought that rates cyparis as being a better material but I've always found spruce does the trick ok. I do suspect that half of the 'spruce' sold is really cyparis but what's in a name?

Former Member20/06/2018 13:25:38

[This posting has been removed]

PatMc20/06/2018 13:36:51
4379 forum posts
524 photos

Spruce has been used in the aircraft industry since the Wright Bros, has cyparis ever been used ?

Nigel R20/06/2018 13:49:31
3724 forum posts
582 photos

Neither - rock hard balsa, twice the thickness of the spruce, always worked for me - never experienced a spar failure.

Levanter20/06/2018 14:14:29
882 forum posts
437 photos

Hi Kit
Bob Cotsford has the description right and I would not use Obechi for a spar for the reasons he gave.
Commonly used for veneers on foam wings because it machines easily.

brokenenglish20/06/2018 15:06:28
539 forum posts
29 photos
Posted by Levanter on 20/06/2018 14:14:29:

Hi Kit
Bob Cotsford has the description right and I would not use Obechi for a spar for the reasons he gave.
Commonly used for veneers on foam wings because it machines easily.


onetenor20/06/2018 16:26:04
1901 forum posts

Yes but it still splits easily

Gary Binnie20/06/2018 18:33:48
520 forum posts
45 photos

Not heard of problems gluing spruce before, I guess it's possible. The only other wood I have known to be used for spars (apart from balsa) is basswood, common in American kits.

Spruce spars can be designed for weight saving, particularly important in large gliders. I have a jig in my workshop so that I can plane spars to a taper for tip panels.

Another idea is to use spruce at the root end of the wing panel then splice to balsa further out.

My dad said that Obeche was used by modellers during WWII when balsa was used for Mosquito production and that any models he built didn't fly very well!



Gary Binnie20/06/2018 18:52:09
520 forum posts
45 photos

Spruce spar, reinforced at the root using tapered sections.

picture or video 029.jpg

End result, reinforced with ply and balsa webbing.

picture or video 150.jpg

Splicing from spruce to balsa mid-span.


Credit belongs to Chris Foss who drew the plans for these gliders.

kc20/06/2018 19:03:15
6414 forum posts
173 photos

Spruce is the stuff to use. However it is possible to replace balsa spars with 'pine' as sold by B&Q - they sell 6mm sq etc in the moulding section. But beware it may be variable width or thickness in some places and you need to avoid the finger jointed part coming near the root. ( you need to look carefully but each length has several finger joints where they joined the timber. Not a problem if out towards the wing tips.)

The pine is no cheaper than spruce from a model supplier but it just may be easier to go into B&Q if you are far from a model shop.

Ramin is a good substitute for spruce but it seems to be less common than in previous years ( something to do with chopping down rainforests )

Note that if making a 50 to 52inch span model it may be possible to use just two 48 inch bits of spruce instead of four 36 inch lengths cut to 24 - saves money!   48 inch spruce is available from Balsa Cabin etc.

Edited By kc on 20/06/2018 19:11:23

Former Member20/06/2018 19:10:08

[This posting has been removed]

Gary Binnie20/06/2018 19:21:42
520 forum posts
45 photos

I have a Screwfix wallpaper pasting table (cost a whole tenner!) made of pine, smells lovely in the car! I use it to avoid bending down at the flying field, back is not good.

To put the spars I posted above into context here are the gliders they turned into, Hi-Phase and Centi-Phase.

picture or video 223.jpg



jrman20/06/2018 19:28:03
374 forum posts
3 photos

Cyano is sometimes reluctant to give a good bond to spruce but can be improved by rubbing baking soda on to the joint before introducing the cyano.

John Stainforth20/06/2018 20:16:10
329 forum posts
39 photos

The most remarkable structure for a model wing I have seen recently was on a 40 lb F-16 turbine jet that had crashed. One of the wings was intact (but the owner had thrown this is the trash along with the rest of the wreck), which felt as strong and inflexible as steel. I was intrigued to see how such a strong wing was constructed so I cut it open. I was astonished to find that it had no spars and no ribs except for the root and tip ribs, and was made of 1/16" balsa sheet that had been fibreglassed on both inner and outer surfaces and moulded to the tapered airfoil shape, i.e., a pure monocoque. The only additions were some carbon fibre cloth on the inboard side that had been wrapped into a half tube which was filled with something like Gorilla Glue - I suppose as a spacer.

PatMc20/06/2018 23:23:37
4379 forum posts
524 photos
Posted by Percy Verance on 20/06/2018 19:10:08:

Castor oil was also used in aeroplane engines at the time of the Wright Brothers, but is deemed unsuitable these days. It's called progress I think.

How does that "analogy(?)" work ?

Manish Chandrayan21/06/2018 03:58:36
630 forum posts
73 photos
Runny, me thinks
Kit Durham21/06/2018 08:57:47
9 forum posts

Thanks all, all advice much appreciated.

I think it will be spruce.



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