My question is, Brett talking about stiffening stab. and such. Do you do this with CF or C grain wood ? 
Does CF work better in compression or just use CF vail on the outside? 
I am building the LA Heat and it has a lot of tail. Any input would be of great help. Ron.
Depends on a lot of things. My stabs are balsa/foam very much like the old CSC design used on dozens of east coast models, including the "arc" airfoil. Airfoils are stiffer than flat, all else being equal. I use 1/32 *medium" A or B-grain wood with epoxy as the adhesive. On the previous airplane I double-covered the stab with .2 oz/ sq. yd graphite mat and nitrate dope. This led to a of pinholes, but it was *stiff*. On the current airplane I double-covered with OO silkspan and nitrate. That's not quite as stiff but still pretty good. NEXT time, I will do the same except add .2 oz graphite at the glue line between the foam and the wood. Just apply the epoxy to the wood, add the graphite, and put on some more glue to wet it out, then squeege out the excess. I don't think you can do a lot better than that without exotic materials and molds.
But that's not exactly what I was concerned with. After some examination, I found that most of the flex was in the aft fuselage around where the stab mounts to the fuse. In particular, the fuse sides below the stab tended to bend along the grain line when you twisted it in roll, and also "skewed" when rolling (since the top of the fuse was a lot stiffer than the bottom). It should have been obvious. The rest of the fuse was pretty good. After some fiddling, I decided to add 1/32 balsa doublers on the inside of the fuse sides below the stab mount, with the grain running vertically (perpendicular to the fuse sides), and make sure everthing was tied into the fuselage formers. Since the pushrod has to go in there too, I left the formers short of the stab and then "boxed" the formers with some 1/16 balsa on the top, and reinforced it with 1/4 triangle stock. I also left only a very minimal hatch opening, and allowed for pushrod adjustments by making a tool to turn the pushrod and leaving a hole for it along the rudder hinge line.
The net result was a rather startling improvement in the stiffness of the tail. Of course, I also used my typical closely-spaced fuse formers with triangulation between them, and heavily reinforced pipe tunnel and bottom block. If it all sounds heavy, that's not quite accurate, as the current airplane ended up well below the design weight and ended up needing a pretty large chunk of lead in the tail to get the right CG. It may have cost an ounce or so over minimal structure, but, I am convinced that this (and some of the other similar approachs throughout the airplane) is ABSOLUTELY essential to get the kind of bulletproof repeatable performance I wanted. I can't say it's perfect but I have been pretty happy with the results on both of the last two airplanes. It's a lot more important than the airfoil or the wing loading.
I would also add that the *paint* is pretty important to the process. Mine is painted with epoxy colors and car clear. These are both very hard, and somewhat brittle, coatings. I noticed a considerable increase in the rigidity of the stab and aft fuse when the colors cured, and even more with the clear. It got detectably stiffer since I first flew it (and then left it in a hot car while driving, and out on the pavement at the NATs). It's a bunch stiffer than with just nitrate dope and low-shrink butyrate or butyrate with plasticizer is not a lot better than Monokote, at least over solid surfaces.
Bottom line is I am perfectly happy to give up 2-3 oz for the additional structure and I think anyone that has seen it fly will admit that it's at least a competent flier.
Brett
Topic: Stiff tail feathers (Read 10133 times)