The MiniMax plans call for 1/16 or 1.5mm plywood for the gussets. On some other designs with similar ribs they are using 1/32 or .8mm ply. Would it be ok to use the thinner ply on the Max ribs? Trying to save every ounce I can.
I would use 1 mm birch ply for rib gussets if I were trying to make Pt 103. I have worked with it and found it strong enough. 1.5 mm ply may be required for the heavier Maxes with higher wing loading.
On my second plane I changed all 1.5 birch ply in the wings to 1.0 except the spar sheer web , that means all rib gussets, leading edges, trailing edges and end strips. I also made test samples with 0.8 plywood rib gussets and it just didn't look strong enough to me.
On my second plane I changed all 1.5 birch ply in the wings to 1.0 except the spar sheer web , that means all rib gussets, leading edges, trailing edges and end strips.
I agree with George.
The weight savings are more significant when you substitute the leading edge 1.5 mm wrap-around ply with 1 mm ply, than when you substitute just for rib gussets. You use 6 ply panels for leading edge and each panel ( 17 x 45 inches) weighs as below:
1.5 mm domestic birch 22.3 oz
1.5 mm Finnish birch 19.3 oz
1.0 mm Finnish birch 15.4 oz
There are 6 panels in all, and the weight savings total to about 25 -28 oz when you you step down from 1.5 mm to 1.0 mm. ( the last panel towards the wing tip is longer by 3 inches)
If you substitute just for the gussets, the weight savings will be about 3 to 4 oz in total .
The Legal Eagle wing uses 0.8 mm ply for both the leading edge wrap-around and the rib gussets. However taking that as a cue can be misleading because the Eagle uses 3 mm ply for the spar web. The Eagle spar web has 3 circular cutouts of about 3 inches diameter ( 2.5 " for rear spar web ) removed from each station on the spar between ribs. So if we were to use 0.8 mm ply for leading edge of the Max wing, then we need to strengthen the spar web , to come to a fair comparison. Using 0.8 mm for leading edge may work in a Max without issues, though some safety margins may be erased which cannot be seen or evaluated in a regular flight.
Using 1.0 mm ply is a safer choice for rib gussets and leading edge wrap-around, assuming you are really trying to make weight under 254 lbs.
I have no engineering basis for my comment, but I built mine with 1.0MM ply leading edge. It was at the suggestion of a fellow who built multiple MiniMAXs, so I felt comfortable with the variation from plans. I used 1.5MM for the rib gussets, but if I were to do it over I believe I would use 1.0MM birch for them too.
As far as the spar web, I think the 1.5MM birch spar web would be lighter than the 3.0MM web, even with lightening holes, to cancel out the slight savings of the .8MM leading edge vs the slightly thicker, and heavier 1.0MM. Detailed calculations might not bear out that gut feeling, but I personally would go that direction. The 1.0MM leading edge ply I used is "tender" enough as is and .8MM would be even more delicate. There's a MiniMAX at the airstrip where I keep my MiniMAX and the .8MM leading edge on it is really "soft". Perhaps that particular LE was not carefully built, but the leading edge is very cupped between the ribs too.
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