If I recall correctly, it just means that joints ( butt joints usually ) on the spar webs must align with the center of RS-9 verticals. Some vertivcals are RS-5, but only 2 verticals on each spar are RS-9 pieces. So any butt joint on spar web must be centered and glued on cener of these RS-9 verticals.

I had only one butt joint on each spar, but depending on what length of plywood you have for the spar web, up to 2 butt joints per spar are OK, provided the butt joints are centered / glued on the RS-9 verticals.

You already know this, but grain on spar-web-ply must run length wise on the spar for desired strength of spar.

Hope the kids are enjoying the build process.

Edit:
The pictures show each spar from 2 sides.
The line going across the spar cap is for showing RS 9 vertical’s distance from wing root etc.
That line across spar cap is NOT a butt joint in the RS 9 spar cap.
And yes, spar web covers FULL front side of front spar, and FULL rear side of rear spar.

Here's an old thread on load testing of wings / spars.

http://www.lonesomebuzzards.com/cgi-bin/forum/Blah.pl?m-1395289456/s-0/highlight-spar+web/#num0

Thanks! The wing is just shy of 12'. The material kit came with the plywood which appears to be the correct width and maybe 4' long. If I understand correctly, the edges of three pieces I'll use per spar must "butt" each other at the center of RS9's. The lines on the drawing going into the cap strip were confusing to me. I'll check the grain of the plywood. Kids having a good time. Ten of them, two per one hour session. With breaks in between, my day runs 6:45.

You can check photos of our progress here: https://imageevent.com/eaa839

Pocono John

Keith103 said:

"You already know this, but grain on spar-web-ply must run length wise on the spar for desired strength of spar."


I'm not so sure of this. The web works in compression, i.e. the loads are perpendicular to the spar.  Plywood is stiffest when loaded endwise, as in a web with vertical grain.

Of course, you could argue that the laminates alternate, so there are always some in the right orientation.

Cut a small test piece, try bending with & then across the grain.

Thin ply is usually supplied in 4' width, hence 2 butt joints/spar.


Bruce

I had ordered 8' x2' birch ply of 1.5 mm thickness from Aircraft Spruce,
and cut length wise strips. Didn't even check the grain at that point,
but discovered later it turned out to be right. I had only one joint in each spar.

If it is a 3 layer ply (which I think is what I got ) then the two outer pieces
usually run lengthwise on a 8x2 sheet, and the only sandwich layer running perpendicular.

Other members may wish to clarify more about the grain,
as it is still not a well understood topic.

If you're asking about the dotted lines, those denote what is "hidden" on the back side of the part in the drawing. The dotted and solid lines through the spar caps are showing to location of the we plywood splices. When I built my MiniMAX I could only get 4' X 4' 1.5MM plywood, so my spar webs had 2 splices in each spar. Maybe I'm giving info that you already knew, so please disregard if that's the case.

The bottom drawing of the last drawing is actually a bit messed up in my opinion. The zig-zag line depicts that the plywood has been "peeled away" to show what is behind where it would be, but the dotted line in the member you have the red lines drawn to looks to maybe be a drafting mistake.

Lynn  

Thank you Lynn. Thanks to help from Keith103, I was able to understand. It appears the indicated line on the drawing is where two pieces of ply web "butt" at a vertical (RS9) member.

Regarding the grain direction, I'm no expert, but in one of the books from Tony Bingelis, he states if the designer indicates grain direction on the plans, then follow the plans.

John

Text extracted from the document “minimax stress analysis”

4. Spar Analysis:
Spar analysis is done in accordance with Reference 8.
In order
to determine the maximum allowable compressive stresses in the spar caps, the material modulus of rupture is modified according to the spar cross section (per Reference to determine the maximum ultimate stress in the compression flange. The opposite flange is then checked for the negative compressive load, and dimensions modified if necessary. These ultimate stresses are multiplied by 2/3 to determine the maximum allowable stress.
The
calculated stresses are then compared with these maximum allowables. A doubler was added to the rear spar as a result of preliminary calculations.
Figure C-2 and C-3 show the calculated
spar bending loads versus allowable loads.
Maximum shear load for the front spar occurs at the +4.4 G symmetrical load, and is calculated to be 293 pounds in the web just inboard of the strut attach position (72.75 inches from aircraft centerline).
From Ref. 10, section 2.7, the shear web
is calculated to support a shear load of 262 pounds without buckling. Since shear buckling of thin plywood is an inexact science, a test was performed (in addition to the original structural static test) on a representative shear sample identical in dimensions to the spar web.
This sample resisted a
total shear load of 375 pounds before the test fixture failed.
The sample itself did not fail. The leading edge D-section is not loaded in torsion, and is held essentially rigid (in twist) relative to the spar by the wing and strut structure.
Therefore,
the D-section can be assumed to support part of the shear load of the main spar. Based on the deflection of the spar web for a given load, the D-section is calculated to support 46 percent of the load of the shear web, or 172 pounds, for a total of 547 pounds.
This provides a safety factor of 1.87.
The rear spar shear web dimensions preclude buckling failure, therefore, the maximum shear load depends on the basic shear strength of the 1/16 inch plywood. The maximum shear load for the rear spar occurs at Va with 1/3 aileron deflection.
This
load is calculated to be 350 pounds just inboard of the strut attach position. Ultimate shear strength of this section is 637 pounds, for a safety factor of 1.82.