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dihedral break in middle of center section

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pictsidhe

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Hello
I'm trying to follow this interesting post
I take this opportunity to ask you a question
I take the example of the KR2 and its 3-part spar
I'm assuming that the spar caps thicknesses need to be increased at the connecting fittings compared to a one-piece spar to account for the spar being drilled.
Do you know how we can calculate or estimate the necessary increase in thickness?
thank you very much
Dominique .... sorry "google" translation !
Design of Wood Aircraft Structures, ANC-18 Bulletin
 

Hot Wings

Grumpy Cynic
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Do you know how we can calculate or estimate the necessary increase in thickness?
This is how I approach this:
Calculate the loads at the point of interest. This isn't 100% needed but it lets you double check your assumptions against the existing structure. If there is a big difference someone likely made a mistake. Without this step you can use the existing structure as the base point.

For wood you then need to design the metal fitting. This will establish a starting point to calculate the increase in wood area. At this point you should have pretty good idea of the loads that need to be transferred to the wood. You may need to adjust the bolt diameter, number of bolts and spacing.

Using the starting the dimensions of the metal fitting look up the material strength numbers of the wood being used. A good source of this is the Evans Lightplane Designers Handbook.
Go through the calculations for the wood. If everything gives the factor of safety needed you are finished. Most likely you will have to increase the spacing and diameter (or number) of bolts because of the wood's reduced strength compared to the ideal metal fitting.

Recalculate - redesign - until the numbers work.

The process is similar for metal and composite.
 

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raymondbird

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This is how I approach this:
Calculate the loads at the point of interest. This isn't 100% needed but it lets you double check your assumptions against the existing structure. If there is a big difference someone likely made a mistake. Without this step you can use the existing structure as the base point.

For wood you then need to design the metal fitting. This will establish a starting point to calculate the increase in wood area. At this point you should have pretty good idea of the loads that need to be transferred to the wood. You may need to adjust the bolt diameter, number of bolts and spacing.

Using the starting the dimensions of the metal fitting look up the material strength numbers of the wood being used. A good source of this is the Evans Lightplane Designers Handbook.
Go through the calculations for the wood. If everything gives the factor of safety needed you are finished. Most likely you will have to increase the spacing and diameter (or number) of bolts because of the wood's reduced strength compared to the ideal metal fitting.

Recalculate - redesign - until the numbers work.

The process is similar for metal and composite.
Great stuff guys. Really appreciate all of your expert input and on a final note: I just received Jim Maske's book "Composite Design Manual" and it is excellent. Everything I was wanting to know is in there. Wing design for dummies it could be called. Will be cutting my carbon spar into three pieces tomorrow.
 

Lendo

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raymondbird, Good first move- and although at first blush Jim Marske's book seems very good, there are things I might suggest. Firstly derate the Rod Compression strength to 200,000psi and Tensile strength to 250,000 psi (as some have done) or make them both 200,000 psi, to keep the Centroid in the middle of the Spar. His earlier books quoted Lab test values, other suggest real world vales may be less. I can only suggest what has been suggested to me. Also watch Tip deflection, Jim adjusts that with the Compression figure.

The Rod count is good for the Root but must be adjusted for the elliptical Lift Distribution, Jim suggests adjustment every foot for an even deflection.
In addition, I would suggest using the Cantilever Buckling formula for the WEB wraps, or perhaps just doubling them.

If using Carbon Wraps on the WEB you will find the wing much stiffer, however there needs to be some flex to give a smoother ride, when it's bumpy.
Just confer with Jim he's always happy to give additional advice, he has a lot of time in this game. I've always found him to be very helpful - where he can.
Hope that helps :).
George
 

blane.c

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Files gift from Fritz.

Shows how Evans was planning VP-3 wing.

You can see dihedral change from center "flat" section to "5 degrees" on outer panels. Outer panels are removable.
 

raymondbird

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Nov 29, 2008
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Belleville, Ontario Canada
raymondbird, Good first move- and although at first blush Jim Marske's book seems very good, there are things I might suggest. Firstly derate the Rod Compression strength to 200,000psi and Tensile strength to 250,000 psi (as some have done) or make them both 200,000 psi, to keep the Centroid in the middle of the Spar. His earlier books quoted Lab test values, other suggest real world vales may be less. I can only suggest what has been suggested to me. Also watch Tip deflection, Jim adjusts that with the Compression figure.

The Rod count is good for the Root but must be adjusted for the elliptical Lift Distribution, Jim suggests adjustment every foot for an even deflection.
In addition, I would suggest using the Cantilever Buckling formula for the WEB wraps, or perhaps just doubling them.

If using Carbon Wraps on the WEB you will find the wing much stiffer, however there needs to be some flex to give a smoother ride, when it's bumpy.
Just confer with Jim he's always happy to give additional advice, he has a lot of time in this game. I've always found him to be very helpful - where he can.
Hope that helps :).
George
Great advice and much appreciated. Will have to poke Jim again as no response to my last email. Hey, is that why the european sailplanes seem to use mostly fiberglass wraps on the webs? I thought it was for toughness.
 
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