Design > Stunt design

Stiff tail feathers

(1/2) > >>

Ron Merrill:
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.

Brett Buck:

--- Quote from: Ron Merrill on July 21, 2006, 11:46:16 AM ---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.

--- End quote ---

  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

Ron Merrill:
Brett, congratulations on your nats win. Thank you for your advice on the stiffing of the stab area. I will stiffen the area around the stab, also will double paper the stab and elevators. Ron.

Eric Viglione:
Ya know, I wasn't able to really put my finger on it or prove it, :! but I think you've hit on something I've wondered about.

Ever since I've gone to adjustable controlls, I've widened the back end of my stunt ships by inserting a layer or "shim" of 3/16 or so in between the fuse sides when I pinch the back end together, (for more clearance for the hardware) plus use X braces plus I create a saddle to sit the stab on to, to overcompensate for my (perceived) worry about the access hatch weakening the structure, ending up with what I thought might be an over-built aft fuse.

The planes flew notably better and more consistent, especially in wind or heavy turbulence. Wondered if it was just the controls, but my old bent wire w/cf tube over it scenario was plenty rigid. The side benefit of a stiff boxed aft fuse must be part of the equation. Another side bennie is that the fuse is less likely to twist and induce stab tilt when the finish is applied since it is extra rigid.

Since I can't detect any appreciable weight gain from this, I think I'll keep doing it just this way, thanks for the confirmation.

PS: Way to go Brett! Well done!

EricV

Randy Powell:
This is an interesting topic. Seeing how others deal with essentially the same issues. My latest plane is obscenely light (for me anyway), but probably the stiffest construction I've ever achieved. And most of it comes (sort of tangentially) from what Brett was talking about. The top and bottom blocks are molded 1/16" balsa with very light carbon cloth attached with diluted epoxy to inside during the molding process. The fuse sides are 3/32" but have diagonal sheet reinforcement (diagonal grain 1/32" balsa) running from the CG to the stab. I used 1/64" plywood under and around the stab. Not much flex in the fuselage. In addition, the fuselage is pretty wide for the size airplane. This also allows for a stiffer construction.

Each plane I build, I learn tricks from the last one. Learn what I can dump and what needs to be beefed up (not much). The one rule I've learned that is inflexible. Air weighs less than structure,.

Navigation

[0] Message Index

[#] Next page