QuickTalk 8 - COMPOSITE NEWS
- Category: Q-Talk Articles
- Published: Monday, 28 February 1983 06:11
- Written by Jim Masal
- Hits: 1758
/Harold Little (#476) of Bradyville, Tennessee has been an active compositer for several years. Credits include his association with three Variezes and one Quickie project. With several of his local builders, Harold has formed the Little Aircraft Factory (LAF) to investigate various fiberglassing techniques. Just over a year ago, he sent the following report and several samples to Burt Rutan for inspection. Mr. Rutan's comments have been paraphrased and a final synopsis given. -Ed./
LITTLE AIRCRAFT FACTORY REPORT:
As the exterior finish is the first item seen by a builder (and his friends), it is apparent that all builders are going to apply the best finish that they can. To enhance the finishing process with the least possible weight penalty, an alternate surface preparation method is suggested. That method employs the application of a thick slurry as the final step in the layup of any surface that will be finished. All other steps remain unchanged.
The Varieze and Quickie plans stress the application of the very minimum of finish, as show plane finishes are heavy. Such a finish will be aerodynamically adequate, but will be far from show quality. It is my belief that very few builders will be satisfied with such a finish.
A review of the Educational Section of the plans will show that light sanding of a cured surface is required prior to the application of any additional resin, fillers, primers, etc. It is also noted that the sanding must be done with care to prevent the destruction of the top strands of the fiberglass weave. Therefore, it is not possible to dull an entire cured surface to affect the best possible adhesion of the subsequent resin application. In the Finishing Section, an application of dry micro to a cured airfoil surface is indicated to build up low areas, followed by sanding to the correct contour. In those areas requiring just filling of the weave, an application Feather Fill, extended with microballoons, is indicated. However, any surface filling is required to be preceded by a thorough sanding to dull the entire surface. That, of course, is the Catch-22, as it is essentially impossible to completely dull a surface without damaging quite a bit of the weave.
Based upon this review, a method of surface completion was formulated and it was determined that two test pieces would be constructed. The first sample would be a Quickie canard Styrofoam section, the second a fuselage urethane section. The Quickie plans would be scrupulously adhered to in the fabrication of the samples.
Upon completion of the layup of the bottom of the canard sample, the entire surface was peel-plied. The peel-ply was applied over a well-squeegeed surface, using peel-ply that was pre-epoxied. The peel-ply was cut to length, laid on a Saran covered area of the construction table, had epoxy squeegeed on, then the peel-ply was placed on the canard. The peel-ply was squeegeed into place, removing any excess epoxy.
Upon cure, the piece was inverted and the upper surface, layup was completed. The surface was well squeegeed, then a thick slurry was applied to the entire surface. The slurry was spread with a squeegee to assure uniform distribution, but no attempt made to remove excess slurry. Peel-ply, pre-wet with slurry, was then applied to the surface and squeegeed down uniformly with no attempt made to remove the excess slurry. A part of both surfaces was taped, after cure to preserve the as-cured surface from subsequent finishing operations.
Both surfaces were sanded with a spline as directed by the plans. The upper surface contour does not match the template, but it was decided not to attempt to match the contour, as this would cover a section of the as-sanded surface and prevent a complete evaluation of the test sample. Sanding was terminated upon exposing the top of the weave. Another wrap of tape was applied to preserve the as-sanded surface.
Feather Fill, extended with microballoons, was applied to both surfaces as suggested by the plans. Upon cure, the Feather Fill was sanded to achieve the smooth surface required for priming. Again, a wrap of tape was applied for surface preservation.
Primer was then sprayed on both surfaces and sanded per the plans. Spot and glaze putty was applied to the areas showing imperfections, and the surfaces were once again sanded, reprimed and sanded, where necessary. A final wrap of surface tape was then applied. The canard sample was then sprayed white and the tapes were removed to complete the piece.
One side of the fuselage sample was laid up with three plies, as is the Quickie fuselage side (forward of the seatback bulkhead). The opposite side of the sample has the single ply layup as found on the fuselage side aft of the seatback. Both sides were constructed exactly as required by the plans, except for the final resin application, which was accomplished similar to the upper surface of the canard. However, no peel-ply was used on either surface, the slurry was simply smoothed out with a squeegee. A flox corner and single ply BID tape was applied to the edge of the sample piece for edge protection. All finishing steps after final resin cure were performed exactly as the upper canard surface.
RESPONSE FROM RUTAN AIRCRAFT:
We have seen quite dramatic differences in part weights before finish, even though they appear similar. Excess resin in the layup (indicating inadequate use of a squeegee) can be a culprit in weight excesses. The videotape on construction emphasizes the "squeegee-stop-inspect for bump" method. A show plane finish starts with accurate foam core preparation and accurate layups. If done correctly, a Varieze finish will weigh less than 9 lbs. for show quality.
It is agreed that sanding to dull a glass surface prior to filling the weave is a problem. Shot blasting works very well, but requires careful builder judgment. We disagree, however, with your use of slurry after the layup and excessive peel-ply. Slurry and resin are HEAVY compared to Feather Fill/micro. Also, peel-ply has "memory" which lifts areas of the layup and results in excess resin being required. This gives a FALSE impression of a smooth contour. The weave is filled smooth, but the overall contour will have waves that have excess resin. This is a heavy lamination practice. DO NOT peel-ply the entire surface!
The strongest structure is with an intimate contact of piles - obtained only with the correct resin ratio, not excess. Resin rich is not as strong as correct glass-to-resin ratio.
Again, the biggest single contribution to a lightweight, but show quality aircraft is very accurate core preparation, plus good quality glasswork and minimal finish. This is verified by Norm Ross and Fred Keller's Variezes - both used this method and both won Grand Champion at Oshkosh.
FINAL COMMENTS FROM HAROLD LITTLE:
As noted, Mr. Rutan disapproved the use of slurry for surface filling. Burt's conclusions, in my view, were not related to any structural deficiencies that might arise from a final slurry application, but rather from a weight and sanding standpoint. I cannot debate the weight comment, as I have no basis for judgment but I can concur with the sanding comment. Slurry is somewhat more difficult to sand than microballoons added to Feather Fill. And I do agree that peel-plied surfaces are much harder to sand than Feather Fill. Intuitively, it is undoubtedly heavier.
Since receiving Burt's reply to our report, I have constructed my own Quickie's canard with extensive use of peel-ply. I have carefully observed the surfaces prior to application of the peel-ply and subsequent to the resin cure. There is no visible or tactile evidence of the peel-ply "memory" described by Burt, nor of any lifted glass or resin bumps. However, this may be due to the method that we employ when using peel-ply.
As for Burt's "squeegee-stop-inspect for bump" method, we have no clue what this is. It would certainly be advantageous to review the Long-Eze educational section and his video tape to learn this method.
Burt's other comments concerning a very accurate foam core contour yielding the lightest aircraft is quite accurate. So is his comment on the use of judgment in sanding a glass surface. Also the fact that a well squeegeed surface with the least amount of resin to obtain the correct ply bonding will have the best strength.
As most builders have heavier aircraft than factory RAF aircraft, then my conclusion is that what builders might normally consider a dry surface is an adequate resin application. But how dry before being too dry? It's rather risky finding out. Just as judgment and caution are required, they only come with experience. And since none of Burt's aircraft have ever had a structural failure due to inadequate resin application, it is safe to say that we are still all wet!
/Several builders wanted to know more about the T-tail modification for the Q2 developed by Garry LeGare. To give you an idea of the construction involved, we have included Bruce Patten's (#2512) own Reader's Digest version of the mod. -Ed./
"You're going to scoop out a pocket high on the right side of the vertical fin about halfway through the fin to accommodate the arm on the trim tab axis and the cable return pulley. But first you layup a 2 BID cover plate directly on the masked-off right side of the fin, drill a 3/4" axle hole through the cover plate and fin, then flox a phenolic bearing onto each side of the fin over the axle hole. Make the pocket. Install the tab axle and cable return pulley. Route a groove down the right side of the fin and drill down through the last bulkhead for the two Nylaflo cable runs. Repair with 2 BID tape. Run cable to the nylon rope on the trim wheel. Pop rivet on the cover plate. Hot wire and lay up the trim tabs, then slip them on the axle and pop rivet in place. Installation time is said to be an honest six hours."
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