harleym
Oct 04, 2008, 01:46 PM
Here is a "how to" for those with a twisted panel:
For whatever reason, the left tip section of SGP #3 had some washout, causing a left turn tendency. In earlier attempts to remove it, really hot water over toweling was applied and left overnight twisted in the opposite direction. This initially worked, but over several weeks the washout slowly returned due to internal stresses created by the twisting.
Experimenting with an old salvaged tip section, it was found that the bottom CF/glass skin could be peeled away forward from the hingeline without damaging it.
This led to the idea of progressively peeling off the bottom skin of the tip to a point where it might be readily flexed straight. It was found to be easily straightened with about 1/3 of the skin behind the LE still left attached, including on the endcap.
Reattaching the skin with epoxy in the vacuum bag was the initial thought, but with the skin still attached up front, access to roll on epoxy was too limited. Pouring it and trying to scrape it thin with a credit card, etc. would be awkward and could get messy.
Weight of the overall epoxy was an issue and, under vacuum, excess would ooze out and fill between the main panel and the hinged aileron.
I had a can of thin, water-based Weldwood Contact Cement for attaching veneers, counter top laminate, etc. To separate the surfaces, from 1/16” balsa I made some triangular shaped separators to vertically jam between core and skin. With a 1” brush, I applied the cement to both surfaces except near the hingeline where I wanted to reattach with epoxy.
When dry, thin cardboard “slip strips” were placed between the separators and the separators removed. With the panel supported inverted in the top bed, it was weighted down at its outboard end and pressed down by hand inboard. The strips were removed one by one and the skin pressed with a roller to the core.
To protect bottom paint, while applying epoxy along the hingeline, the area was coated with paste wax. With a thin glue knife, quick epoxy was applied a few inches at a time between skin and core weighted down flat with a steel bar and left to cure. The work was supported inverted just ahead of the hingeline with a second steel bar so the aileron could dangle out of the way. Oozing epoxy was picked up with the glue knife to help keep things clean. What little got on the waxed paint was easily peeled off.
Results: A cosmetically neat and strong fix that eliminated the washout and the residual stress that might cause it to return.
For whatever reason, the left tip section of SGP #3 had some washout, causing a left turn tendency. In earlier attempts to remove it, really hot water over toweling was applied and left overnight twisted in the opposite direction. This initially worked, but over several weeks the washout slowly returned due to internal stresses created by the twisting.
Experimenting with an old salvaged tip section, it was found that the bottom CF/glass skin could be peeled away forward from the hingeline without damaging it.
This led to the idea of progressively peeling off the bottom skin of the tip to a point where it might be readily flexed straight. It was found to be easily straightened with about 1/3 of the skin behind the LE still left attached, including on the endcap.
Reattaching the skin with epoxy in the vacuum bag was the initial thought, but with the skin still attached up front, access to roll on epoxy was too limited. Pouring it and trying to scrape it thin with a credit card, etc. would be awkward and could get messy.
Weight of the overall epoxy was an issue and, under vacuum, excess would ooze out and fill between the main panel and the hinged aileron.
I had a can of thin, water-based Weldwood Contact Cement for attaching veneers, counter top laminate, etc. To separate the surfaces, from 1/16” balsa I made some triangular shaped separators to vertically jam between core and skin. With a 1” brush, I applied the cement to both surfaces except near the hingeline where I wanted to reattach with epoxy.
When dry, thin cardboard “slip strips” were placed between the separators and the separators removed. With the panel supported inverted in the top bed, it was weighted down at its outboard end and pressed down by hand inboard. The strips were removed one by one and the skin pressed with a roller to the core.
To protect bottom paint, while applying epoxy along the hingeline, the area was coated with paste wax. With a thin glue knife, quick epoxy was applied a few inches at a time between skin and core weighted down flat with a steel bar and left to cure. The work was supported inverted just ahead of the hingeline with a second steel bar so the aileron could dangle out of the way. Oozing epoxy was picked up with the glue knife to help keep things clean. What little got on the waxed paint was easily peeled off.
Results: A cosmetically neat and strong fix that eliminated the washout and the residual stress that might cause it to return.