Hatz Biplane Association

Thanks for that explanation.
I think I will have 4 holes to plug, and also will weld the bottom, where the two tubes end. I will feel good about that.

As a comparison, the CB-1 plans show just two plug welds just above the rudder horn. There is a difference. The CB-1 plans show the tube over top the rudder leading edge tube and it appears to be completely welded around the top of the added short tube. They also may be welded together at the bottom. Some CB-1 guys would know about that.

I think the reason for the extra tube is to add strength because of the twisting forces caused by the turning the rudder into the wind with the rudder horn. If the two aren't welded together in enough places it wouldn't do much good to have the inner tube. Theoretically it would be better to have a thicker walled tube for the entire rudder leading edge. On the other hand the strength (and weight) isn't required for the entire rudder leading edge.

The leading edge of the rudder gets a 27" tube inserted into it on the bottom (plans page 13) and plug welded.
I am wondering about the reason to plug weld more than in one or two places. The plans seem to indicate 9 holes to be plug welded, three times in three vertical station along the doubling tube.
Any engineers out there?