alpinelakespilot2000
Well Known Member
A builder friend related a suggestion that I thought I'd run past the list...
Once the slider frame is shaped to fit the profile of the fuselage, Van's instructs you to bend an additional "pre-load" of 1/2" into the forward bow and (something like) 3/8" in the rear bow to accomodate the canopy's tendency to pull the frame back out.
However, it seems that in doing this, the builder is intentionally introducing stress in the canopy that might some time down the road lead to stress cracks. Since there has been a lot of discussion on the forum about stress cracks that showed up later, and how to avoid them, I've been following the threads pretty closely.
Enter my friend: he indicated that he's known a builder or two who shaped the frame to fit the contour of the fuselage (as per plans), but did not bend any additional preload into the frame. Instead, the builder temporarily "fixed" the desired width of the front and rear bows by stretching 2x4's across the bottom. He then clamped the canopy to the frame and let it sit on top of black paper out in the very hot summer sun. The result is that the plexi gradually softened and molded to the canopy frame; not vis-versa. Then, once the canopy was riveted to the frame, no stress on the plexi remained.
Is there, perhaps, any merit to this suggestion? Is there any harm in trying it before defaulting to Van's approach? I've got some nice 90 degree days coming up pretty quick to test it, but know next to nothing about the characteristics of plexiglas, so thanks for any thoughts.
Once the slider frame is shaped to fit the profile of the fuselage, Van's instructs you to bend an additional "pre-load" of 1/2" into the forward bow and (something like) 3/8" in the rear bow to accomodate the canopy's tendency to pull the frame back out.
However, it seems that in doing this, the builder is intentionally introducing stress in the canopy that might some time down the road lead to stress cracks. Since there has been a lot of discussion on the forum about stress cracks that showed up later, and how to avoid them, I've been following the threads pretty closely.
Enter my friend: he indicated that he's known a builder or two who shaped the frame to fit the contour of the fuselage (as per plans), but did not bend any additional preload into the frame. Instead, the builder temporarily "fixed" the desired width of the front and rear bows by stretching 2x4's across the bottom. He then clamped the canopy to the frame and let it sit on top of black paper out in the very hot summer sun. The result is that the plexi gradually softened and molded to the canopy frame; not vis-versa. Then, once the canopy was riveted to the frame, no stress on the plexi remained.
Is there, perhaps, any merit to this suggestion? Is there any harm in trying it before defaulting to Van's approach? I've got some nice 90 degree days coming up pretty quick to test it, but know next to nothing about the characteristics of plexiglas, so thanks for any thoughts.
