# Open Sided Box Spar

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#### ktfiles

##### Well-Known Member
Ref: Building a Pietenpol

http://www.cpc-world.com/photos/Spar.jpg

This spar was built for a Pietenpol Aircamper. I believe it is a Jim Wills modification to the plans for PFA approval in the UK. It is my understanding that all Pietenpols in the UK have this type of spar construction.

Can someone point me to reference material specific to this type of spar construction? Is it better than similar construction of a closed box with ply on both sides?

Thanks CL

#### Dana

##### Super Moderator
Staff member
A closed box will be stronger in torsion, but depending on the structural configuration (wing struts, etc.) this may not be necessary. What is the standard design (as opposed to the mod shown here)?

-Dana

HBA Supporter

#### TFF

##### Well-Known Member
The first that used this closed was the Fokker D7 1917 and it was tank strong. Open might be strong enough, but I like the closed.

#### ktfiles

##### Well-Known Member
A closed box will be stronger in torsion, but depending on the structural configuration (wing struts, etc.) this may not be necessary. What is the standard design (as opposed to the mod shown here)?

-Dana
Guessing they don't like simple in the UK. Is there a strength advantage or is it just weight savings? Cost also would be a factor.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
You should read up on beam theory and then do the calculations for yourself. It is one chapter of Timoschenko and Gere is my preferred source, but there are others. When you calculate I and y/I for each of those spar sections, you can see for yourself. I is section moment of inertia (bending stiffness due to shape of the beam) and M*y/I is max stress in spar, if you know M. You can also compute shear stress in the glue line if you know both M and V at this section.

Billski

#### Vigilant1

##### Well-Known Member
Guessing they don't like simple in the UK. Is there a strength advantage or is it just weight savings? Cost also would be a factor.
Maybe the PFA (LAA) sees an advantage in an improved ability to inspect the glue joints and the wood?

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Terrific book on mechanics, which is what you gotta get a handle on for figuring out structures. Lots of good stuff applicable to airplanes. And we can help you with how to apply the stuff you will find in there. If you have an engineering college near you, you might be able to do better on a used copy, or maybe get an earlier edition.
be
The big thing about beams is EI. E is material characteristic, and I is shape characteristic. Bigger means less deflection and usually lower stresses.

Other stuff, like trusses, combination of materials, torsion structures, beam deflections, and on and on. Good book.

Billski

#### ktfiles

##### Well-Known Member
I found out one reason for the open side construction. In the UK they want to be able to inspect the inside of a box spar for dry rot. An enclosed box spar denies inspection access.

#### Autodidact

##### Well-Known Member
I found out one reason for the open side construction. In the UK they want to be able to inspect the inside of a box spar for dry rot. An enclosed box spar denies inspection access.
That makes a lot of sense. I suppose it would be a little lighter potentially than a routed or solid spar and could make better use of hard to get (in Britain) and expensive sitka spruce?

Just by itself, with no ribs or other structure, the spar would want to twist when a load was put on it, but with the constraint of the ribs, etc., that would be a non-issue. And a single shear web is more efficient than two of them - but only in bending and not in torsion. The Fokker box spars were for cantilever wings and needed to resist torsional loads as well.