Spar dimensions

[jCandlish]

Re: Spar dimensions

Though my timber poaching days in Oregon are long over, I suppose buying oversize lumber and planing it to size is the (for profit) bussiness lumber companies are in.

I just also want to add an unsubstantiated rumor I've heard; the reason Martin quit building guitars with Eastern Spruce tops is that they were outbid for the last big stand by the Ohio Match Company.

Cheers
jCandlish
.

[Acroeric]

Re: Spar dimensions

I sure am glad I left that .001% chance of being incorrect- I looked at my data and found references to 3/4 as well. Still looking for the 25/32 reference- may have been on the Swick plans. The clipwings I built used 25/32. . . . .

As for the wood ribs on the clipwing- I used 1/4 x 5/16 capstrips with the 5/16 being the vertical direction. There is another wood rib swick flying with 1/4 x 1/4 using an o-320 with no problems. . . . .. I just wanted the additional material for impact resistance when hitting your head under the wing.

[crispy critter]

Re: Spar dimensions

Eric,
Don't sweat it,I'm wrong all the time.If you don't believe me just ask my wife....she'll tell ya.hahaha

[gmackay]

Re: Spar dimensions

Sorry to keep asking all these questions, but what about the 1/16" plywood? Should I use birch 45 or 90 degree? - Greg

[Don Eide]

Re: Spar dimensions

On my T-craft the spar plates have 90 degree grain.

Don

[gmackay]

Re: Spar dimensions

This may be another stupid question, but just curious.........why did Taylorcraft mill down the butt end of the spars anyway? Seems like it would weaken a very critical area. I guess the plywood doubler protects the spar from being crushed by the wing attach bolts, but why not add to it rather than take away?

[Jim Herpst]

Re: Spar dimensions

The laminate (scarf) actually adds strength when embeded as it is on the butt end. Think of it like a solid piece of wood which when laminated (key word as in not simply applied over the top ) any way the embebded laminated piece of plywood acts as another "layer of strength" (my words for describing this) which the wood does not have as a non laminated solid piece. In truth laminated wood is genarally stronger as one is not relying on the grain in one piece of wood where it is common thread throughout the board but rather it is reinforced or "backed up" by another wood with different grain. Not a aerospace designer answer eithor I know but no matter not a dumb question .

[gmackay]

Re: Spar dimensions

Thanks Jim, that helps, but I guess I'm just slow. I can understand how a laminated spar is stronger, when the laminates are full length pieces with no cuts. Like you say, the cross-grain can add strength. But when the spar is cut (milled down), it just seems like that would weaken it.

Reason I'm curious is that I'm wanting to build a clip-wing and obviously want the strongest spar. Mike Swick didn't mill down the butt end, but instead glued the doublers on top, then filed down part of the doubler for the attach fitting to fit. I don't know which method is better, and wonder what other builders were doing.

Thanks again for your response.

[Don Eide]

Re: Spar dimensions

Now I have a question.
I have a set of clipwing plans.
Where on the plans does it say to file down the doubler for the attach fitting?

Swick uses T-craft part #'s for the wing attach fittings and for the compression struts and states "Wing attach same as standard Taylorcraft 85 - 100hp". To me that means he milled down the spar ends and put on the plywood plates same as T-craft but used the 85-100hp larger bushings.
If he didn't mill down the spar, the T-craft compression strut won't fit. Right?

Don

[gmackay]

Re: Spar dimensions

Don,
That's why this is so confusing to me. The plans don't say to file anything down, but I asked Mike about it and that is what he told me he did on his.

I haven't even thought about the compression strut, that's a good point. That's why I wanted to know what other builders are doing. Maybe I misunderstood Mike. It sounds like the best way is to stick to the orginal milled down design.

Sorry if I've created more confussion, I'm confused myself and looking for advise.

[crispy critter]

Re: Spar dimensions

Greg,
If you do what you were saying,glue the doublers on original thickness then file out a place just big enough for the butt fitting to fit into then all your doing is destroying the strength of the doubler because you will be removing a big piece of it all the way down to the original spar thus elimiating the purpose of the doubler all together.Stick to what t-craft did and mill it down.

[Jim Herpst]

Re: Spar dimensions

Ok . I will try again. When you mill down the spar and then replace the milled portion with the different grained wood which is bonded to the spar you have a part that is stronger (inspite of having milled it down) to a stronger piece than you had orginally. You yourself said you can see how the strength increases with the different grain. Point being the cut down of the spar is MORE than made up by the new piece of plywood in its place. Remember that the glue portions are also actually stronger than the orginal wood itself. (You will often see this when a splice is made on a spar(In one specific area...not the length of the spar by the way) Any how if damaged again you will usually see the glued portion will not fail as easily as the orginal wood it self. NOW compound this where you have put in this piece of plywood with its stronger different grain and presto you have arrived at alogical conclusion.

By the way , I believe you did misunderstand Mike when he discussed this with you. YOU NEVER CUT OUT WHAT YOU PUT IN . That indeed defeats the purpose.

I hope this helps in some way .

[gmackay]

Re: Spar dimensions

Got the picture. Thanks everyone for all your help.

[jgerard]

Re: Spar dimensions

If you were to run a bolt through one 1" piece of wood/metal ect. you only get the sheer strength of the bolt for each parting surface. If you were to run the same bolt through four 1/4" layers(plys, or what ever ya want to call them) now you have just quadroupled the sheer strenth of the bolt/joint. More layers = smaller bolt, stronger joint. Look in a aircraft mechanics hand book for the specifics on this subject. I think? I am explaining it correctly.....

Jason
N43643

[Acroeric]

Re: Spar dimensions

The 25/32 reference I was so sure I remembered seeing was a figment of my imagination. . . . . dang it- I know there is something that measures 25/32 on a Tcraft. . . . . . . . ;-)