choosing wood

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fly2kads

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Not bad, overall. Looks like this is a teaser for their full-length video (for sale)? I am curious about their reasoning for cautioning against laminating a spar. It's acceptable if you pay attention to proper alignment of the grain of each layer.
 

Tiger Tim

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Check with your sign-off authority before committing to laminated spars. I know of a Tiger Moth wing that failed pre-cover inspection because a spar had been laminated, even though it was in accordance with factory instructions for laminating a spar.
 

BJC

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Check with your sign-off authority before committing to laminated spars. I know of a Tiger Moth wing that failed pre-cover inspection because a spar had been laminated, even though it was in accordance with factory instructions for laminating a spar.
What is the rest of the story?


BJC
 

Tiger Tim

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What is the rest of the story?
A Moth has eight wood spars in it, there was enough good wood available on hand to do seven solid spars and one built-up. The de Havilland instructions for lamination were engineered for just such an occasion and were followed to the letter. Inspector didn’t like that there was an odd one out at the pre-cover (the DH ‘STC’ for lack of a better term was presented to him) so while while three wing panels were covered right away the fourth was disassembled and a good blank for the eighth solid spar was sourced and machined before reassembly. I don’t know whatever happened to the laminated spar, it could have been sold to someone who could get it approved or maybe it was cut up to be used in other projects (rib cap strips and such) as often happens.

Point is, wood construction is getting to be somewhat exotic nowadays so make sure your guy is willing to sign off what you’re making.
 

don january

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Choosing wood has become a very challenging part of my build but I have found that if you follow proper grain run out and other regs. required it becomes much easier to achieve your mission. You can take a 2"x2" stick and may feel very strong at that size but when ripped to say 3/16"x 5/8" you can feel the strength just by hand pressure. I can't count the many pieces that were thrown into the burn pile after the feel of the piece was not to par in my view. I can't express enough how each and every stick of wood should be hand tested and meet your standards. How many builders have grabbed a stick from the pile that was ripped to size and went straight into the rib jig or fuselage sides without even being bowed by hand to see if a crack is heard or a splintering is seen to start ? Wood is such a beautiful type of construction to do and at times it's a shame to see so much not pass the needs for a aircraft build.
 

Aerowerx

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Choosing wood has become a very challenging part of my build but I have found that if you follow proper grain run out and other regs. required it becomes much easier to achieve your mission. You can take a 2"x2" stick and may feel very strong at that size but when ripped to say 3/16"x 5/8" you can feel the strength just by hand pressure. I can't count the many pieces that were thrown into the burn pile after the feel of the piece was not to par in my view. I can't express enough how each and every stick of wood should be hand tested and meet your standards. How many builders have grabbed a stick from the pile that was ripped to size and went straight into the rib jig or fuselage sides without even being bowed by hand to see if a crack is heard or a splintering is seen to start ? Wood is such a beautiful type of construction to do and at times it's a shame to see so much not pass the needs for a aircraft build.
Of course it will be weaker when cut to smaller sizes! The strength is proportional to the cross-sectional area. It will also depend on the grain orientation.

If the wood originally met the specifications then it should still be OK when ripped to smaller pieces.

There is currently a build thread where the guy graded his own wood. It met the rings-per-inch spec. It met the angle-less-than on the end grain spec. But IMHO you should not use a board that meets the minimum on both of these at the same time!
 

don january

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I'm glad I have passed the stick wood needed and now I can focus on the plywood needed to cover craft. Very good point Aerowerx I guess it's best to bend the wood in both directions.:)
 

Aerowerx

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Keep in mind that wood is not uniform, obviously. You might have a board that is find in one area but not in another.
 

don january

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Yes Aerowerx it really is a PAIN when you need a 12 inch piece and you fiddle with it and Snap it lets go at 10.5 inches. While building my tail group I had a assortment of different lengths that failed the stress test some got used and some kept my bottom warm near the fire:rolleyes:
 

Aerowerx

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Yes Aerowerx it really is a PAIN when you need a 12 inch piece and you fiddle with it and Snap it lets go at 10.5 inches. While building my tail group I had a assortment of different lengths that failed the stress test some got used and some kept my bottom warm near the fire:rolleyes:
What do you mean by "lets go at 10.5 inches"? Do you mean your hands were 10.5 inches apart?

And what force were you applying? The wood may have been in spec, but you were exceeding it.

Instead of bending it by hand, support it at both ends and hang a bucket from the middle. Add gravel to the bucket until it snaps, or reaches its specified load limit. You may find that you are throwing away perfectly good wood.

I assume that the plans you are building from specified a certain type of wood and the dimensions? You should be able to find the published specs for that type of wood and calculate the force required to break it. Only then will you know for sure if the wood is good or not.
 

don january

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It broke at 10.5" length and I started with a 15" piece. I personally do not like or depend on the bucket method myself to me that would be Ok if the product was stationary and did not take flight loads and vibration/hard landings etc. I will be the first to admit I may over stress the wood many times more then what is needed and in truth it is very hard to follow some chart that has been made for each and every piece of wood to construct with tho they are a great reference to follow. I first look at the finished product rather a rib or tail post and estimate the curvature the wood needs to bend and I try and double the distance or at least a third more distance and if a small piece then my ears and feel will give me a good idea on just how far she wants to go. Straight pieces such as tail post or HZ spar then the old bucket test will get you in the right neighborhood of the woods strength but unlike metal wood can be deceiving in it's true ability you must remember it once was a living thing and now is dead. Then when you get into warping with steam and water its another thing in it's self. As far as plans they came from a 1959 design from the UK. Spruce /Ash /Beech/balsa is common on these plans.
 

Aviacs

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If the wood originally met the specifications then it should still be OK when ripped to smaller pieces.
Keep in mind that wood is not uniform, obviously. You might have a board that is find in one area but not in another.
So...... "look before you leap because he who hesitates is lost?" :)

Not only might small section pieces be ripped to contain defects that are acceptable in the large plank but could fail the scantling; also ripping wood usually relieves stresses that cause the 2 new parts to warp, bow, or twist, or all 3. In long pieces, possibly not uniformly. So if the smaller section pieces are cut oversize and straightened (jointed & re-squared), the grain run-out might not pass anymore in a section of a longer length, though it might be fine in short pieces.

smt
 

MadProfessor8138

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Isn't building with wood sort of a leap of faith from the very beginning ?
You can abide by the set guidelines and test sample pieces to destruction to get a general idea of how the wood will perform but theres still no guarantee that one piece will be the same as the other because each piece is individual.
Unfortunately,you can't test every piece of your material to destruction because you would have no material left to build with.
So,you're taking it on faith that one piece will act the same as the next as long as it falls within the set standard guidelines for wood construction.

Kevin
 

don january

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Kevin: as you probably know on a all wood build, one piece of wood usually is supported by another such as the boat of the fuselage is covered with a light ply and corner blocks are supporting the glue joints. Grain direction is a big player in the strength of the finished product much like a all Alum. plane is covered over the bulkheads with alum. skin. You are totally correct In my opinion that a leap of faith is involved and to me that is why the hand and feel of each piece is important for the builder. Say you have a 11 foot longeron and that would be a trick to get a accurate load test by bucket and weight but if a person slides it along in their hands and attempt to bend in both directions and twist I have found you can actually feel the weak areas and worse yet that's were you'll find the breaking point where you'd never thought it was a bad zone of the wood piece. I found it to be odd that on the Taylor-mono there is no diagonal support pieces from top longeron to opposite bottom longeron to help in twisting of fuselage under flight loads.
 

TFF

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You don’t test the piece, you test the batch. You need enough excess to have a test part. You have to infer that the rest is homogeneous. Just bending it does no good. If it needed to bend a lot, most are going to steam it. There is evidence that it’s stronger if you don’t, but do you need that extra little bit? Wood is probably the wrong material for that kind of design.

If I’m building a model airplane from scratch, I will bend the wood in hand for a stiffness check. Then I match stiffness of the woods. You don’t want to pair a stiff with a looser piece if they are meant to do the same thing. If I mismatch it it’s either on purpose or no choice.

Grading your own wood is the leap of faith. If there is any hardware store wood that’s good, it’s going to be the minimum side of the scale matching parameters to airplane wood. It’s not the juicy meaty pieces. It’s not the wood you walk by and say, that’s some pretty wood. At least I have never seen anything in the last 20 years.
 

Aerowerx

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...It’s not the wood you walk by and say, that’s some pretty wood. At least I have never seen anything in the last 20 years.
Although on occasion I look at the wood at the big box store, and think:"If I set my table saw at 47 degrees, rip that beam, and then square up the sides it would be a nice piece of Douglas Fir." But I would be throwing away 70% of it.
 
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