# dihedral break in middle of center section

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#### pictsidhe

##### Well-Known Member
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
HBA Supporter
Log Member
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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Merci
Dominique

#### raymondbird

##### Well-Known Member
HBA Supporter
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

##### Well-Known Member
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

HBA Supporter

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#### blane.c

##### Well-Known Member
HBA Supporter

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

##### Well-Known Member
HBA Supporter
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.

#### raymondbird

##### Well-Known Member
HBA Supporter

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.
Love it! Too bad it didn't get finished. Thanks!

##### Well-Known Member
HBA Supporter
given that people who come on here routinely ask about wing fold options, would it have been possible to build an Evans wing with a Corsair type wing fold?

#### Doran Jaffas

##### Well-Known Member
With the exception of the military and a few rather Stout versions of folding wings I personally am not in favor of them. They add weight and anywhere there is a movable part there is a weak link. I know the thought unfolding the wings was to save on the hanger and be able to trailer the airplane back but again you run into issues of extra wear and tear on landing gear that is not meant to be hauled that far at the speeds are usually being trailered or hauled at. The alternative to that is a few smart folks have custom built trailers to haul their steeds back and forth to the airport and that definitely takes care of the wear and tear on the landing gear and bearings and other parts. But the folding wing design unless it is really well thought out and is used minimally to me is just not a super idea. If someone is going to use a folding wings make sure they stick with the tried and true mechanisms that have been around for several decades. if owning an airplane means you cannot hang it then maybe you need to rethink your finances although I know some hangers are getting astronomical and price range,, in some cases not much less than a mortgage on the house then there would be the exception to the rule. I am fortunate in my hanger is $200 a month and that is including a$25 month electric bill that I pay. good luck to everyone out there still looking at the folding wings but better luck and maybe increasing your finances so you do not have to use those.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
It is way easier to make the centersection straight, then bend the wing for dihedral at the connection to the outer panels. The bend is then in the fittings, and you do not need to force a curve into the caps, which can be tough to achieve well in a built up metal spar.

Laminating veneer to make a wooden cap or composite fabric caps with a bend is not a big deal. I have written about how we did composite elsewhere on hba.com. For wood consult Epoxy Works magazine and The Gougeon Brothers on Boat Construction both from Gougeon Brothers.

If you are considering Marske methods, you can curve the rods easily, but the radius of bend should be kept large and rod size small - bending puts in residual stresses, which "uses up" some of the strength - which makes the spar more of a U than a V through the fuselage. Lots of example composite spars where they overlap and thus have a foot or two of spread between cross bolts. Once you are thinking composites, you can probably just do the wings the way MANY sailplanes are done with the two spars overlapping each other through the fuselage and mounted with two pins on main spar and two more aft. Almost as simple a mount as a one-piece wing, but then you have to build wings that are b/2 + 1' (single seat or tandem) or b/2 +2' (side-by). I do not know if that works in your space...

Billski