Well once again, we have a good story of how much ?fun? it is to build an RV-3. As much as an art as it is a science, our latest ?tale? involves a mystery of a mismatch between the vertical and horizontal stabilizers?..
We have been attaching the Vertical Stab this weekend, and ran into this interesting problem ? once we had bolted the rear spar to the back of the fuselage, the forward spar is about 3/8? back from the tab that sticks up from the Horizontal Stabilizer forward spar to join it. We traded some notes with the engineering guys at Vans and they are OK with a thick spacer and four AN3 bolts, so the problem is solved ? but the mystery remains. {BTW ? when I call Vans guys on the RV-3, I generally am not looking for a ?factory position? on anything ? the truth is, no one there but Van has built one, and that was a LONG time ago. Rather, I call them for an independent sounding board for engineers familiar with the general design. It relieves them from feeling that I am depending on their answers to be ?official??}
The Problem:
Here?s the thing ? we didn?t build our tail feathers, so we have been backtracking and measuring through the drawings to see if there is an error. The placement of the horizontal stabilizer on the fuselage is dependent on where the vertical attachment bars (for the rear spar) are located ? they are located on the F-310 bulkhead. That bulkhead is in turn set relative to the aft bulkhead (F-311) by the tail wheel mount weldment. The Vertical Stab aft spar is bolted to that aft bulkhead, so that that point can be considered as a ?zero point? for measuring the assembly. In our case, the distance between the F-311 and F-310 is exactly per drawings, and the distance between the HS spars is perfect (10.5 inches). So we know that the HS forward spar (and its tab) is where it should be relative to the aft spar of the VS. So the question then was the distance between the VS aft and forward spars. What we found is that on the Vertical tail drawing, there is a measurement that basically sets the distance from the aft spar to the bottom of the forward spar ? it is given as 14-11/16?. There is a change note bubble by it ? and when you check the change notes, it says that it was previously 15?. That is almost the exact distance of our discrepancy (5/16 - approximately 3/8?). The change to the drawing was done in 2001.
Now the fellow that built our tail (how many folks are lucky enough to get a QB tail on a -3?!) was on his fifth RV. Everything he has built has been to about 1/64? tolerance to the drawings. He obviously built this exactly to the drawings, but it doesn?t fit the HS, which is built to the drawings. So it?s our current theory that the Horizontal Stabilizer and Vertical Stabilizer drawings don?t currently match. The question we have, of course, is if any of the other current builders out there have run in to this mismatch. Since most tails are built long before the fuselage ? and you need the fuselage to know how these are going to fit together, it is a good thing to know when you are building. As I've stated many times, building a -3 is an adventure - you have to enjoy solvign little puzzles like this!
Oh - Our fix was to make a spacer that measured 3/8? (built from two pieces of 3/16?x1 - ?? bar), measured carefully for edge distance for four AN3 bolts, and match drilled to join the two spars. I then drilled some lightening holes that fit with proper edge distances to keep from getting the CG too far aft. Not a hard fix.
The Fix:
The Spacer:
Paul
We have been attaching the Vertical Stab this weekend, and ran into this interesting problem ? once we had bolted the rear spar to the back of the fuselage, the forward spar is about 3/8? back from the tab that sticks up from the Horizontal Stabilizer forward spar to join it. We traded some notes with the engineering guys at Vans and they are OK with a thick spacer and four AN3 bolts, so the problem is solved ? but the mystery remains. {BTW ? when I call Vans guys on the RV-3, I generally am not looking for a ?factory position? on anything ? the truth is, no one there but Van has built one, and that was a LONG time ago. Rather, I call them for an independent sounding board for engineers familiar with the general design. It relieves them from feeling that I am depending on their answers to be ?official??}
The Problem:
Here?s the thing ? we didn?t build our tail feathers, so we have been backtracking and measuring through the drawings to see if there is an error. The placement of the horizontal stabilizer on the fuselage is dependent on where the vertical attachment bars (for the rear spar) are located ? they are located on the F-310 bulkhead. That bulkhead is in turn set relative to the aft bulkhead (F-311) by the tail wheel mount weldment. The Vertical Stab aft spar is bolted to that aft bulkhead, so that that point can be considered as a ?zero point? for measuring the assembly. In our case, the distance between the F-311 and F-310 is exactly per drawings, and the distance between the HS spars is perfect (10.5 inches). So we know that the HS forward spar (and its tab) is where it should be relative to the aft spar of the VS. So the question then was the distance between the VS aft and forward spars. What we found is that on the Vertical tail drawing, there is a measurement that basically sets the distance from the aft spar to the bottom of the forward spar ? it is given as 14-11/16?. There is a change note bubble by it ? and when you check the change notes, it says that it was previously 15?. That is almost the exact distance of our discrepancy (5/16 - approximately 3/8?). The change to the drawing was done in 2001.
Now the fellow that built our tail (how many folks are lucky enough to get a QB tail on a -3?!) was on his fifth RV. Everything he has built has been to about 1/64? tolerance to the drawings. He obviously built this exactly to the drawings, but it doesn?t fit the HS, which is built to the drawings. So it?s our current theory that the Horizontal Stabilizer and Vertical Stabilizer drawings don?t currently match. The question we have, of course, is if any of the other current builders out there have run in to this mismatch. Since most tails are built long before the fuselage ? and you need the fuselage to know how these are going to fit together, it is a good thing to know when you are building. As I've stated many times, building a -3 is an adventure - you have to enjoy solvign little puzzles like this!
Oh - Our fix was to make a spacer that measured 3/8? (built from two pieces of 3/16?x1 - ?? bar), measured carefully for edge distance for four AN3 bolts, and match drilled to join the two spars. I then drilled some lightening holes that fit with proper edge distances to keep from getting the CG too far aft. Not a hard fix.
The Fix:
The Spacer:
Paul