I agree and add the following. 1/8" plywood is 12 pounds for a 4 X 8 ft sheet.. I haven't measured, but I don't think there is more than 18 pounds of plywood in the whole KR. There really isn't any weight to save here
The biggest problem with glass is that it will need to be so thick to get the buckling resistance you need that it will weigh at least as much as the wood. Glass does have good tensile strength, but very low buckling and torsional strength for an equivalent weight. Quoting some excerpts from Low Power Laminar Aircraft Structures by Alex Strojnik, "Plywood is the single most efficient aircraft material we know. It combines the highest shearing/buckling resistance with the lowest density. ... In an extensive study, directed at analysing the suitability of material for structural stressed skins several years ago, A. Puck (Some Examples of the Design and Construction of Highly Loaded Sailplane Parts in FRP, OSTIV Publication 1960) came to the conclusion that while theoretically the bidirectional fiberglass fabric + epoxy offer the highest shearing strength for the weight -today this honor would go to carbon fiber- it is the plywood that possesses the highest resistance (shearing and torsion) against wrinkling. By far." Now if you did a sandwich construction with foam between the stringers and glass on both sides you would increase the buckling and bending strength a lot, but you would need something other than urethane foam because the glass will tend to tear off of the foam under bending , torsion, and buckling loads. You would need to use a higher density, and heavier, core. You will probably have a finished weight more than with the plywood by the time you are done, and by the time you load it up with fillers to get it as smooth and flat as the plywood it will certainly be a lot heavier and take you three times as long. Sorry to disagree with you Justin, these are just my two cents. Good luck with your project and keep us posted. Mark Langford wrote: >Justin wrote: > >>I dont feel that plywood is a good choice. I know it is proven for many >>years but why not make it stronger and most likley lighter using >> >fiberglass > >>to get a perfect smooth finsh? It's not as if im changing the airframe to >>accept the fiberglass, it is still a stock KR2S airframe unlike the other >>fiberglass boats which make complex shapes. >> > >So, there have been something like 1500 of these built, and not ONE of them >has ever had the plywood mysteriously sheer off and create a crash, but you >don't think it's good enough? The plywood isn't there just to keep the wind >out. It's an integral part of the structure. If nothing else, it acts as a >gusset. > >> It's actually not much work to use this fiberglass skinning. I draw on >>the foam and cut it out, it will take me about an hour to do the whole >>fuselage and then applying the glass (2 people) will be about another >> >hour > >>X 2 (inside and out). Sanding the foam will take a few hours. >> > >I think you're seriously mistaken here. It can't get any easier to finish >something than plywood is. You start with a smooth, flat surface, that >really only requires a little sealer and fine sanding and it's ready for >primer and paint. You must not have had much experience with fiberglass, >but the surface will be far from straight, and the pinholes will eat your >lunch trying to fill them all. My guess is it'd take at least three times >more effort to skin it with fiberglass then with plywood., and the strength >is still in serious doubt... > >Mark Langford, Huntsville, AL >N56ML "at" hiwaay.net >see KR2S project at http://home.hiwaay.net/~langford > > > >_______________________________________________ >see KRnet list details at http://www.krnet.org/instructions.html >
