Wood as an aircraft airframe structure

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Wood as an aircraft airframe structure

Unread post by Origini » Mon Oct 18, 2010 1:35 pm

Good day

I would like to know why wood does not seem to be such a popular choice of material for aircraft construction in South Africa these days?

Why is it that most of the light aircraft that are sold these days are either made of alluminium or composite materials instead? Is it beause people think wood is an "old fashioned" material or is it just a concern that it can't stand up to moisture, dust or prolonged exposure to the open?

(Surely given wood's ease of workability that requires a minimum of special tools to produce a high quality finished product, the fact that it repairs relatively easily, is less prone to the effects of vibration and unlike some other materials does not develop fatigue cracks from repeated stress and vibration would lead it to be a logical choice for light aircraft manufacturing? In addition to that, today's glues and protective coatings can provide the wooden aircraft with a life that should span decades...)

Curious to hear your views!

Thanks
8)
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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Mon Oct 18, 2010 7:06 pm

For the simple reason that wood (nature's composite) is not a well controlled or manufactured to factory standards. When I built the L-18 I could find different hardness ans strenght in a single plank.
If you use 2024 allu or steel, you have a manufacteror's standard of weight, strength, bend radius ect to work to.With wood it is up to the builder to look for the piece that would work the best in that particular aplication.This does take quite a bit of skill I must add.
If a scarff joint is to be made, it is very importaint to look at the grain direction and grain runn-off to be acurate in the repair. Same with the repair of for example a splice joint in a spar. There are set down rules in the AC-43 that tell you how the slope has to be, the overlap of the ply webs ect.

You are correct that wood has no fatique life and with modern epoxy glues and varnishes it can be protected for ever.

I also think to get the amount of wood to be comercial will be to costly ans the trees takes decades to mature (up to 80 years for Sitca Spruce) and no one works that far ahead.

As for a material to work with ,it is my all time best. :D

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Re: Wood as an aircraft airframe structure

Unread post by Origini » Wed Oct 20, 2010 7:25 pm

Interesting... But that of course mostly applies to home-built aircraft! :wink:

But what if wood was used by a reputable aircraft manufacturer on a factory built plane? If the quality of the wood is regorously controlled, it is treated with the correct resins and varnishes and the whole structure is say covered in a modern composite body-shell, surely that would make for a superiour aircraft? :?:
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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Wed Oct 20, 2010 8:45 pm

Maybe so, but aircraft manufacturers are driven by profit. To get to that you need, amongst other things, to have a very streamlined manufacturing process. Probably wood does not fit in so nicely with modern (mass?) aircraft manufacturing?
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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Thu Oct 21, 2010 6:03 pm

bosvark wrote:wood does not fit in so nicely with modern (mass?) aircraft manufacturing?
Correct

T
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Re: Wood as an aircraft airframe structure

Unread post by RichieRich » Mon Oct 25, 2010 9:24 am

T. van Vuuren wrote:For the simple reason that wood (nature's composite) is not a well controlled or manufactured to factory standards. When I built the L-18 I could find different hardness ans strenght in a single plank.
If you use 2024 allu or steel, you have a manufacteror's standard of weight, strength, bend radius ect to work to.With wood it is up to the builder to look for the piece that would work the best in that particular aplication.This does take quite a bit of skill I must add.
If a scarff joint is to be made, it is very importaint to look at the grain direction and grain runn-off to be acurate in the repair. Same with the repair of for example a splice joint in a spar. There are set down rules in the AC-43 that tell you how the slope has to be, the overlap of the ply webs ect.

You are correct that wood has no fatique life and with modern epoxy glues and varnishes it can be protected for ever.

I also think to get the amount of wood to be comercial will be to costly ans the trees takes decades to mature (up to 80 years for Sitca Spruce) and no one works that far ahead.

As for a material to work with ,it is my all time best. :D

Theuns
Very interesting info, Theuns. Maybe you can shed some light on the process used by De Havilland in WW2 for the Mosquito? I believe unskilled labour was one of the benefits derived from using wooden construction. Not disparaging your comments but simply want to know if this process was not conducive to mass production? Also, the pilots who flew the Mosquito found it to be a rugged, strong and safe aircraft. Any info?
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Re: Wood as an aircraft airframe structure

Unread post by RichieRich » Mon Oct 25, 2010 9:32 am

As an aside, in the '80's the Merensky Institute conducted tests on South African Saligna and found it to have the exact same characteristics and performance in terms of weight, strength. etc as imported Aircraft grade Spruce. At the time I spoke with the late Ton Maneschijn, who had already retired from the CSIR, and his son Anton, who holds a degree in Aeronautical Engineering and was working on a number of projects for the then Atlas Aircraft. Both of them had designed and built a number of projects and Ton was at the time in charge of all the Approved Persons in SA. Both these gentlemen were of the opinion you could safely replace spruce with saligna with no risk at all. The benefit obviously being in a much lower price for the saligna. Is this not well known or do SA homebuilders consider everything imported and expensive as the only 'safe' way to build?
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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Mon Oct 25, 2010 6:20 pm

I must say that spruce is a much nicer wood to work with than saligna/glue gum. The spruce has a very clean straight grain vs. the saligna that has a more interwoven corce fiber to it. Not a problem really, just know what to look out for.

The saligna is actually slightly stronger for the same weight than spruce.

As for the Mozzie, they were built in large concrete moulds.A layer of 3mm ply then 19mm balsa then 3mm ply again was glued together inside the moulds.This formed a very tough yet light and stiff composite shell. (clever these ould toppies hey!) All the controlls, formers ect vere installed and then the 2 halves were joined, just like a big model.

The wings (45' span) were made the same and each side had only 7 ribs.

This was truely a remarkable plane and could outrun even the Fw-190's.

The Herculese (spruce goose) was not made from allu, but from a plywood that was injected with a heated glue of sorts to make a sube composite. The allu for the plane was just not there at the time.

God's composite.............just can't beat it :D

T
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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Mon Oct 25, 2010 10:12 pm

How is total grain slope measured? In an article by Victor Boyce? he states the following:

"The total combined slope is determined by taking the square root of the sum of the squares of the two slopes"

And in an example given in the article, slope one = 20, slope two = 23 Total combined slope is thus 15.
But the squares of slope one = 400 and slope two = 529 so sum of the two slopes = 929 and the square root of 929 = 30.47

Or have I got the cat by the tail with my poor command of maths?? :? :? :?
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Re: Wood as an aircraft airframe structure

Unread post by noelotten » Mon Oct 25, 2010 10:15 pm

bosvark wrote:Maybe so, but aircraft manufacturers are driven by profit. To get to that you need, amongst other things, to have a very streamlined manufacturing process. Probably wood does not fit in so nicely with modern (mass?) aircraft manufacturing?
Not so! If the aircraft were designed around "plywood" as the base material, the manufacturing process can be as streamlined as any metal process. Plywood can be easily routed and even "water-jet" cut to a high degree of accuracy ... and quickly! The main difference, (if 6there is one), would be in joining the timber components. Gluing and setting times compared to riveting or welding.
RichieRich wrote:As an aside, in the '80's the Merensky Institute conducted tests on South African Saligna and found it to have the exact same characteristics and performance in terms of weight, strength. etc as imported Aircraft grade Spruce...... ........ The benefit obviously being in a much lower price for the saligna. Is this not well known or do SA homebuilders consider everything imported and expensive as the only 'safe' way to build?
Saligna has some other "nasty" peculiarities. It is stronger than either Spruce or Douglas Fir (Oregon), but it is heavier than Spruce. The grain is fairly straight, but it is inclined to be more coarse than either Spruce or Douglas Fir. Saligna is a more difficult timber to "dry" and, it has a tendency to "case harden". It is difficult to get it to dry uniformly throughout. It is not as easy to work and glue as either Spruce or Douglas Fir. In the hands of an expert wood-worker it is a great timber!

One major setback with Saligna is finding long clear lengths. That is like trying to find "Rocking Horse <<moderated - language>>). That is not in itself a major problem! I am an advocate of the use of "laminated" timber for aircraft construction. Laminated timber is stronger, more stable and .... safer!

I agree with Theuns. Give me a timber aerie! They will be flying long after the plastic ones are turned into shopping bags!

Noel
The words "cheap", "aviation" and "safe" cannot, in my opinion, be used in the same sentence; not unless you add the word "not"! ... John Howse 1947 - 2010
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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Mon Oct 25, 2010 10:47 pm

One major setback with Saligna is finding long clear lengths. That is like trying to find "Rocking Horse <<moderated - language>>). That is not in itself a major problem! I am an advocate of the use of "laminated" timber for aircraft construction. Laminated timber is stronger, more stable and .... safer!

I agree with Theuns. Give me a timber aerie! They will be flying long after the plastic ones are turned into shopping bags!

Noel[/quote]

I have started building a VP1 and plans to use saligna. What you are stating her has been my main worry, specifically for the wing spars and I have been wondering about lamination. Are there any suppliers out there who does such "custom" lamination??

And secondly, how thick should the laminations be and should it be laminated in the width or the breadth?
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Re: Wood as an aircraft airframe structure

Unread post by RichieRich » Mon Oct 25, 2010 10:54 pm

Theuns and Noel I really appreciated your comments re the Saligna. These were things I did not know about. You gentlemen are clearly very knowledgeable on the subject. This is why this particular section of Avcom really appeals to me. Keep sharing the knowledge and more people will take up the really rewarding hobby of homebuilding!
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Re: Wood as an aircraft airframe structure

Unread post by noelotten » Tue Oct 26, 2010 5:55 pm

bosvark wrote: I have started building a VP1 and plans to use saligna. What you are stating her has been my main worry, specifically for the wing spars and I have been wondering about lamination. Are there any suppliers out there who does such "custom" lamination??

And secondly, how thick should the laminations be and should it be laminated in the width or the breadth?
I have a set of VP1 plans somewhere, (can't find them right now so I am relying on my memory). The Spars for the VP1 are built up from Plywood "webs" and solid timber for the "caps" and compression members.

Ideally, all load bearing members such as spars and spar caps, etc., should be laminated in such a way that the laminations are perpendicular to the direction of the load, (have a look at a laminated roof truss .... the same applies!) Roof trusses are made up of "short" lengths which are finger-jointed together in order to get the length you require. Finger jointing is not approved for aircraft construction! One has to "scarf-lap" the timber to effect a suitable joint. This is simply putting an "angle" on both pieces of timber to be joined that will increase the gluing area and minimises the "butt-joint" effect that one gets with a typical "finger", "dove-tail" or "F" type joint which is commonly used in Joinery work.

A "scarf" joint typically has a 12:1 up to 15:1 ratio for the mating angles. In other words, if your timber is 10 mm thick, the length of the glued surface will be minimum 10 x 12 = 120 mm long. It takes a bit of practice to make such a joint accurately, but it is possible even with simple hand tools. If you have a Router, it is easy enough to construct a "fixture" to hold the timber at the right angle and machine it. If you are worried about doing it yourself, I am sure the likes of Theuns van Vuuren will be willing to "sell" you some of their time.

As for the thickness of the laminations, the simple rule is the thicker the strips are the lower will be the cost. The thinner the laminations the higher will be the cost. The "down-side" is that the thicker the strips, the less chance you will have to find the flaws in the timber. My advice is laminations of 15 - 20 mm is quite acceptable for most aircraft. 10 mm would be perfect even for a highly stressed spar such as one would use for an aerobatic aircraft.

Noel
The words "cheap", "aviation" and "safe" cannot, in my opinion, be used in the same sentence; not unless you add the word "not"! ... John Howse 1947 - 2010
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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Tue Oct 26, 2010 7:43 pm

I could also not find the correct length wood for the spars or longerons when I built the L-18 Cub ( in Sabie nogall :roll: ), so I simply made good old fassioned scarf joints as per the AC-43 regs. I did several tests (even with joints tht were only 50% the minimum specs) and every time the wood failed outside the joint. BUT - stay with what the plans say!!!

To make a scaf joint in something like a 15mm by 25mm stringer, you will take either the with or thickness of the wood and multiply by a factor of 15 to get a 1/15 slope.Cut the wood with that angle (both pieces) and do the final shaping with a hand plane.Constantly check the fit and if there is no gap, you can glue and clamp.

I use Epidermix 372 epoxy ,but there are MANY products on the market.Just chat to AMT in Kempton, they make epoxies :)

Never sand a joint to final stage, the dust can clog up the wood fibers and lead to a weak joint.Always make a test block with a new bach of glue and remember to keep a log of the bach #'s

I also laminated the spar caps of the cub as I was not totally happy with the idea of not being able to "see" inside the wood caps.The spar is made up of an upper cap (20mm by 70mm IIRC) and a slightly smaller lover cap with A grade ply web.
I cut strips of 15mm by 20mm to do the cap laminations and stagered the individual length scarf joints as not to have them all in the same place. Even if there was to be a "vrot kol" in one strip, the other will bear the load.Just check the wood carefully.

Remember, epoxy likes to have a slightly corse surface to stick to, so I usually scrape the area to be glued with a 32 tooth hacksaw blade to make fine scrapes in the glue face.

Also remember that the grain run-off for any wood is a min of 1/15. This means that if you were to look at a spesific line of grain, it must run from the top of the plank out the bottom of the plank in no less than 15 times the with of that plank, if that makes any sence to you :? :lol:

Laminating to wood is really not hard, just time consuming, but atleast you can be sure it is all fine.OR just buy spar quality spruce and save a lot of work :wink: I was to cash straped to do this when the $ was 13 rand!

Dont forget woods like Douglass fir and clear Oregon pine, they are also very good and easier to work.

Hope it helps a little.
I will try and find pix of how I laminated the spars and post it. NB!! if you laminate spar caps, do it against a straight edge on the tabe, otherwize you could laminate beautifully curved caps for a frencs Flying flea! :lol:

A tip on glueing, don't clamp it so tight that the wood starts to creek, actually leave some glue in the joint! #-o

T
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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Tue Oct 26, 2010 9:52 pm

noelotten wrote:
bosvark wrote: I have started building a VP1 and plans to use saligna. What you are stating her has been my main worry, specifically for the wing spars and I have been wondering about lamination. Are there any suppliers out there who does such "custom" lamination??

And secondly, how thick should the laminations be and should it be laminated in the width or the breadth?
I have a set of VP1 plans somewhere, (can't find them right now so I am relying on my memory). The Spars for the VP1 are built up from Plywood "webs" and solid timber for the "caps" and compression members.

Ideally, all load bearing members such as spars and spar caps, etc., should be laminated in such a way that the laminations are perpendicular to the direction of the load, (have a look at a laminated roof truss .... the same applies!) Roof trusses are made up of "short" lengths which are finger-jointed together in order to get the length you require. Finger jointing is not approved for aircraft construction! One has to "scarf-lap" the timber to effect a suitable joint. This is simply putting an "angle" on both pieces of timber to be joined that will increase the gluing area and minimises the "butt-joint" effect that one gets with a typical "finger", "dove-tail" or "F" type joint which is commonly used in Joinery work.

A "scarf" joint typically has a 12:1 up to 15:1 ratio for the mating angles. In other words, if your timber is 10 mm thick, the length of the glued surface will be minimum 10 x 12 = 120 mm long. It takes a bit of practice to make such a joint accurately, but it is possible even with simple hand tools. If you have a Router, it is easy enough to construct a "fixture" to hold the timber at the right angle and machine it. If you are worried about doing it yourself, I am sure the likes of Theuns van Vuuren will be willing to "sell" you some of their time.

As for the thickness of the laminations, the simple rule is the thicker the strips are the lower will be the cost. The thinner the laminations the higher will be the cost. The "down-side" is that the thicker the strips, the less chance you will have to find the flaws in the timber. My advice is laminations of 15 - 20 mm is quite acceptable for most aircraft. 10 mm would be perfect even for a highly stressed spar such as one would use for an aerobatic aircraft.

Noel
Thanks Noel for the info and advise.

The VP1 spar is a simple, solid piece of wood. Just a nice old fashioned plank! And quite skinny too - Fwd spar 4 1/4" by 3/4" and the aft spar 3 3/8". At least it gets "doublers" for about 29" where the stuts attach. Don't know how that is supposed to carry the static load test loads! For the sake of my health I will rather laminate it as you and Theuns have explained. Don't want to have to buidld new wings after the static load tests nor would I want a wing to part company when somewhere between the clouds and mother earth.

Eddie
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