Wood as an aircraft airframe structure

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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Tue Oct 26, 2010 9:58 pm

T. van Vuuren wrote: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

Thanks Theuns, your advice is much appreciated (also on another thread). Will probably at some stage contact you and "pay for your time" as Noel suggested.

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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Wed Oct 27, 2010 6:45 pm

:)

T
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Re: Wood as an aircraft airframe structure

Unread post by Dusty Eagle » Sun Jan 23, 2011 11:51 am

If you do not have access to a router as Noel says, over lap the lengths of timber to be joined, clamp and mark them and when you cut through, both cut surfaces are mirror images of each other and give a close fitting joint.
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Re: Wood as an aircraft airframe structure

Unread post by Vaughan » Mon Jan 24, 2011 1:19 pm

In response to the original question:

A good example of the benefits of wooden aircraft construction is the Robin DR-400 / 500, a 4/5 place single-engine French aircraft which is extrememly prolific in the UK and Europe. The DR-400 which is ICAO designated the DR40 first flew in 1972 and is still available new as a factory-built aircraft with a range of engines from 115hp through to 200hp including the 155hp Thielert 2.0s diesel in the Ecoflyer version. www.ceapr.com

I own ZS-JSV which is a 1979 model DR-400/180 Regent powered by a 180HP Lycoming 360 motor and fitted with a Hartzell Constant-speed prop.
The DR-400 has a fabric-covered wooden airframe. The wing-spar is a compound box-spar manufactured from plywood and composite resins, the wing ribs are of the lattice type and made entirely of wood. The fuselage is a box structure manufactured from plywood and composite resins. The cowling is made from composites / fibre glass. Steel brackets are used for additional support where necessary (for example at the wing and undercarriage mounting points) and the flaps and elevator trim-tabs are made from aluminium, all other control surfaces are fabric-covered wooden lattice. The firewall is made from plywood and covered with aluminium sheet on the engine side.

The empty weight of the standard DR-400 180 is 562 kg / 1236 lb and it has a MAUW of 1100kg / 2420 lb leaving 528kg / 1160 lb for fuel, pax and baggage. ZS-JSV is fitted with a CSU, a long-range 50 litre fuel tank (in addition to its standard 110 litre main and two 40 litre wing-root tanks for a total of 240 litres) and is also fitted with a glider tow-hook. This raises the empty weight of this particular example to 604kg and thus leaves 500kg for lugging pax, baggage and fuel.

At full power at 4000’, the DR-400/180 will cruise at 147kts (40 l/hr), 139 kts at 75% power (32l/hr) and 127kts at 60% consuming less than 7 imperial gallons / 28 litres per hour for a 7 hour-plus endurance and a range of over 1700 km with a 45 min reserve whilst still carrying 355kg (that’s 4 pax!) in the case of JSV.

Why do I mention all of this? Go and have a good look at the numbers for other aircraft:

PA28 Archer: 180HP 128kts, empty weight of 767 kg and max useful load of 395 kg, range 967km with a 268kg load.
The C172 Skyhawk: 180 HP 126kts, empty weight of 767 kg and useful load of 347 kg, range 1289km with 203kg.
The C182 Skylane: 230HP 150kts, empty weight 900kg and useful load of 511kg, range 1700km with 274kg),
even the composite Cirrus SR20 200HP 155kts, at an empty weight of 945kg and useful load of 441kg, range 1454km with 288kg.

Draw your own conclusions as to the benefits of wooden construction in aircraft. They are very light and very strong. The wood also transmits less engine noise through the airframe as it has certain damping properties and there is no fatigue limit.

Whilst I do not for one minute suggest that you compare the Robin to say, a Cirrus SR20 in terms of looks, build and features, in terms of price and usefulness when it comes to speed, payload and range…..it beats the Spam-cans hands-down.

Do I love my wooden Robin…..? You Bet! There are some drawbacks which I shall get to later. I'll also post some pictures of the structure.

Other interesting wooden aircraft, the Alpi 200 and 300 Pioneer…
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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Mon Jan 24, 2011 7:12 pm

The only BIG thind with especially wood frames , is that you must keep the water away from them!
With modern epoxies this is not a problem aslong as you remember to seal every place you attach a bracket wit fastners.Doesn't help to seal the whole thing only for moisture to go into a bolt or screw hole #-o
The other nice thing of a wood plane is that the repairs are less complicated than with allu, tube or tupperware. It just takes a skilled person to know what he is doing, as most repairs are only attached using glue. ei a splica joint in a spar.The AC-43 does however give exact rules on how to do this.

A wood plane will also float if you have to ditch in the water oneday 8-[ In the L-18 I will go head over heals though with the UC digging in :lol:

T
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Re: Wood as an aircraft airframe structure

Unread post by Vaughan » Mon Jan 24, 2011 9:45 pm

Some pictures showing the internals of JSV.
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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Wed Jan 26, 2011 10:47 pm

Dusty Eagle wrote:If you do not have access to a router as Noel says, over lap the lengths of timber to be joined, clamp and mark them and when you cut through, both cut surfaces are mirror images of each other and give a close fitting joint.
I've got a router. used it to cut all the ribs for the wings and tail planes, trim the bulkheads to dimention etc. I am going to build a "fixture" as Noel suggested to hold the strips of wood in and route it to make the scarf joints.

These will be laminated in an 11' jig?? I made with 34 clamps. Used it to clamp laminations for the wings' compression struts and it worked fine. Hope to attempt the first spar soon.[attachment=0]029.JPG
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Re: Wood as an aircraft airframe structure

Unread post by noelotten » Thu Jan 27, 2011 8:02 pm

Bosvark,
That is a magnificent laminating press! Well done sir! =D> =D> =D>

Do yourself a favour! Get 2 lengths of square or rectangular steel tubing to place between the clamp jaws and the timber. This will distribute the clamping force evenly across the full surface of the timber .... and it will also help to keep the timber straight! It saves material and effort.

Noel
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Re: Wood as an aircraft airframe structure

Unread post by Mauler » Sat Jan 29, 2011 5:36 pm

I've often wondered if meranti timber, which is commonly used for window and door frames etc is suitable for aircraft construction. What say you experts?

I was also thinking we should get Julius Malema interested in homebuilt aircraft. Perhaps we could persuade him to build a wooden one. 8)
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Re: Wood as an aircraft airframe structure

Unread post by Theuns v V » Sat Jan 29, 2011 7:56 pm

Howzit Mauler. I did a few "non clever" tests on SA pine, Oregon pine ,Meranti and Saligna when I started the L-18 many moons ago.
I would basically get the weigh of the wood and see what it would take to fail it,to get a strength to weight ratio.
The Oregon pine did very well, SA pine less well and the Meranti did not fare well atall :? It must have something to do with the way the fibers line up in the wood or something more clever than I am.

Except for the real aircraft wood like Spruce and Douglass fir, the Saligna fared very very well in the tests and it has been used to build planes in SA and Auz for decades.The trick with Saligna is that you need to select the planks carefully as they vary in weight dramatically :shock: . It is also not the prettiest of woods to look at or even work with, it has a very interwoven fiber, but this makes it very strong for it's weight.

The other thing with any wood is to get it at a reputable dealer that will be sure about the moisture content of the wood. You want no less than 8% and no more than 12% . The light colloured Saligna works like a charm and is a fraction of the price of other "certified" woods.

Just another IMPORTANT thing.......if you have to do a repair with wood, ie splice in a new piece of spar, stick the to same tipe of wood used.You can not join a piece of Saligna to Spruce, then rather replace the whole thing if need be !!!
You could however join sub assemblies of different woods if you want to build the tail from spruce or similar.

IIRC Sitka Spruce is only about 8% lighter than Saligna, but you can bring this down to "size" as the Saligna is about 14% stronger for the same weight.

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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Mon Jan 31, 2011 7:40 pm

Noel, coming from you, I take it as huge compliment :D but I have to thank you for it!
I cracked my head figuring out how I am going to build it and then saw on the background of a photo you send me of the spars you laminated something similar. I then broke my back welding the clamps (and found out that "made in China" welding rods are made only to fck you around)

I thought of using two pieces of meranti to distribute the pressure.

My final problem is, what do I do with the "Monster", as my wife calls it, once the spars have been made :?: :? I would probably have no further use for it, but to dismantle it would break my heart :(
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Re: Wood as an aircraft airframe structure

Unread post by noelotten » Mon Jan 31, 2011 8:16 pm

bosvark wrote:Noel, coming from you, I take it as huge compliment .......
............... My final problem is, what do I do with the "Monster", as my wife calls it, once the spars have been made :?: :? I would probably have no further use for it, but to dismantle it would break my heart :(
Build another aerie! :lol:

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Re: Wood as an aircraft airframe structure

Unread post by oldtimer » Tue Feb 01, 2011 7:48 am

Build another aerie!
I fully agree with Noel!!!
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Re: Wood as an aircraft airframe structure

Unread post by Jayson v Schalkwyk » Sat May 09, 2020 3:15 pm

bosvark wrote:
Wed Jan 26, 2011 10:47 pm
Dusty Eagle wrote:If you do not have access to a router as Noel says, over lap the lengths of timber to be joined, clamp and mark them and when you cut through, both cut surfaces are mirror images of each other and give a close fitting joint.
I've got a router. used it to cut all the ribs for the wings and tail planes, trim the bulkheads to dimention etc. I am going to build a "fixture" as Noel suggested to hold the strips of wood in and route it to make the scarf joints.

These will be laminated in an 11' jig?? I made with 34 clamps. Used it to clamp laminations for the wings' compression struts and it worked fine. Hope to attempt the first spar soon.[attachment=0]029.JPG
Bosvark, many of the photos in this thread were lost. Any chance you could post a photo of your laminator?
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Re: Wood as an aircraft airframe structure

Unread post by bosvark » Thu May 21, 2020 4:52 pm

Jayson v Schalkwyk wrote:
Sat May 09, 2020 3:15 pm
bosvark wrote:
Wed Jan 26, 2011 10:47 pm
Dusty Eagle wrote:If you do not have access to a router as Noel says, over lap the lengths of timber to be joined, clamp and mark them and when you cut through, both cut surfaces are mirror images of each other and give a close fitting joint.
I've got a router. used it to cut all the ribs for the wings and tail planes, trim the bulkheads to dimention etc. I am going to build a "fixture" as Noel suggested to hold the strips of wood in and route it to make the scarf joints.

These will be laminated in an 11' jig?? I made with 34 clamps. Used it to clamp laminations for the wings' compression struts and it worked fine. Hope to attempt the first spar soon.[attachment=0]029.JPG
Bosvark, many of the photos in this thread were lost. Any chance you could post a photo of your laminator?
Hello Jayson. Sorrie for the late response. I haven't been on Avcom for some time.

I think the moderators remove photo's to save space. That photo is one of a lot which was on a memory chip in my camera bag and got stolen with my car. I will check. The whole "laminating press" as the late Noel Otten called it is now the base of my building table. I clamped a chip board top onto it.

The clamps were welded up from rect. tubing, angle and threaded rod thru a nut used for the screws. The base is made of three beams, 210mm x 45 mm. Bolted together with with pieces of treaded rod and spacers in between. (Welding the 34 clamps (spaced 100mm apart) necessitated placing the old oil-bath welder in a tub and running water through it to keep it cool)

I took a cellphone photo from the one end which should give you an idea of what I am talking about. If you send me your nr I will WApp it to you.
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