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Wood size

 
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simmor2



Joined: 29 Jun 2015
Posts: 13
Location: Murfreesboro, TN

PostPosted: Tue May 30, 2017 7:08 pm    Post subject: Wood size Reply with quote

I purchased some 1x6 rough cut to build a Piet. I cleaned up my first piece tonight. When I finished cleaning up the surface completely and cleaned an edge for table saw guide, my thickness came up to 7/8. I still need to clean up the other side.

I'm building the vertical stab first for a trial part and learning piece.

There are a few pieces that are 1x1 in size. Obviously, the best I can get is 1 x 7/8 or so.

I'm sure I'm not the only one that has hit this.

Is This acceptable or do I need to order some wood that is 2x 6 in the rough.

Would laminating 2 pieces to get the 1x1 acceptable?

Please share your thoughts.

- Rich


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glen



Joined: 17 Sep 2013
Posts: 144
Location: Oregon Coast

PostPosted: Tue May 30, 2017 9:26 pm    Post subject: Wood size Reply with quote

Hi Rich. Yes laminating is ok but you have to do it right. I wouldn't try to bend once laminated though. Use it for cap strips for ribs is even better. Check your wood supplier for S4S 5/4 then you can plane it to 1". Please don't even consider using it in place of 1X1 stock.
Stay with original materials call out unless you can justify materials substitution through empirical data or better yet check FAA publication 43-13
Quote:
On May 30, 2017, at 8:08 PM, Comcast <4rcsimmons(at)comcast.net> wrote:



I purchased some 1x6 rough cut to build a Piet. I cleaned up my first piece tonight. When I finished cleaning up the surface completely and cleaned an edge for table saw guide, my thickness came up to 7/8. I still need to clean up the other side.

I'm building the vertical stab first for a trial part and learning piece.

There are a few pieces that are 1x1 in size. Obviously, the best I can get is 1 x 7/8 or so.

I'm sure I'm not the only one that has hit this.

Is This acceptable or do I need to order some wood that is 2x 6 in the rough.

Would laminating 2 pieces to get the 1x1 acceptable?

Please share your thoughts.

- Rich






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Pocono John



Joined: 22 Aug 2011
Posts: 115
Location: Stroudsburg, PA

PostPosted: Wed May 31, 2017 8:25 am    Post subject: Re: Wood size Reply with quote

When you have a part not quite right or up to your standards, would it bother you during flight (just knowing it's there)? If yes, start over.

Even if an error is acceptable (by FAA standards), it might still irritate you. Probably the best thing I can say is, when in doubt, chuck it and start over. The more important the part is to structural integrity, the more precise I'll be. A 1/8" error on wood size is probably OK for the legs holding a seat up, but I wouldn't like it on critical flight components. By the way, here's some info on aircraft wood: http://naca.central.cranfield.ac.uk/reports/1931/naca-report-354.pdf


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aviken



Joined: 27 Sep 2013
Posts: 127
Location: arkansas

PostPosted: Tue Jun 06, 2017 7:45 pm    Post subject: Re: Wood size Reply with quote

I read a post by Forrest Lovely who said that people wonder how BP built his craft so light, and that he knew he sometimes ripped his one inch longeron pieces from a two by four and with the missing Kerf the boards wound up being 7/8 by 1 . Now I don't know just how much that change would multiply into measurement problems following the plans and all, But I do not think it would be un-doable. I also believe that a 7/8x1 would be plenty strong , and if I build another piet I will probably do that just to save a little weight. Just my piddling 2 cents worth.

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womenfly2



Joined: 31 Jul 2007
Posts: 224

PostPosted: Wed Jun 07, 2017 4:03 am    Post subject: Re: Wood size Reply with quote

I would only use aircraft graded wood an leave the longerons a full 1" x 1". In BP time the plane were very light because they were low cost builds dollar wise and material wise, no fancy electronics or other non-essential items.

Today's Piets are way over gross weight, including pilot and a passenger, added to that all the fancy full IFR panels builders embellish their plane with.

If you deviate from the plans remember you become a test pilot. The Piet was design to be a light flying airplane, simple in its nature and design. If you keep it like that you will have one great flying airplane. Just do as the plans say .... and for God's sake do not use a 2x4 Wink

WF2


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Dan Helsper



Joined: 28 Dec 2016
Posts: 15

PostPosted: Wed Jun 07, 2017 12:31 pm    Post subject: Wood size Reply with quote

I actually measured "The Last Original", and I can confirm the 7/8" dimension. I also noted at the time that Bernard used 1/16" tail brace cables. So those are the wires I used in NX929DH.

Dan Helsper
Loensloe Airfield
Puryear, TN


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Dan Helsper
Loensloe Airfield
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taildrags



Joined: 29 Dec 2009
Posts: 1548
Location: Medford, OR

PostPosted: Thu Jun 08, 2017 8:58 pm    Post subject: Re: Wood size Reply with quote

So I'm always the guy who wants to analyze things because I'm an engineer. Not taking a position one way or the other on the longeron size issue, just analyzing a little bit. Let's just say that the basis of design for the longerons is that they are of spruce, that they are 1"x1" in cross-section, and that we're interested in the strength of that member in bending. Spruce modulus of rupture is 9400 psi so we assume that's the maximum bending stress that the longeron will sustain. We can calculate the maximum bending moment that the longeron can take, M, to be equal to the modulus of rupture times the width of the longeron times the square of the height of the longeron divided by 6, with that result divided by 12 to get it in lb-ft of bending moment instead of lb-in. That calculates out to be a little over 130 lb-ft of bending moment.

If we make the parts to still be 1" high but only 7/8" wide, that drops to about 114 lb-ft of moment... a 12% reduction in what they can sustain in bending.

Some builders have gone to Douglas fir, which has a modulus of about 10,900 psi. If fir is used for those 7/8" x 1" longerons, then they will resist a bending moment of about 132 lb-ft and you've gotten back what you lost by shaving down the 1" to 7/8". It comes with a price though... Doug fir weighs more than spruce and you have to determine for yourself if the tradeoff is worth it.

As others have posted, unless you're willing to calculate stresses here and there and everywhere in your airplane structure, it's best to stick to the plans and only venture away if you want to do the homework. In the case of the longerons, they are not in pure bending in any one spot at any one load condition, so even if you check just bending in one axis, there may be other loadings that are worse than the one you evaluate.


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