Can I shorten the fuselage on a Chrysalis to fix CG problems?

I followed your suggestions for the Chrysalis by removing a TE shim and adding 3/32" washout to the inner wing panels. I also checked my CG again and found that it was a bit behind the rearward range limit so I added a bit of nose weight. On saturday I had a chance to fly in pretty calm conditions. Launches were a bit higher than before with no noticeable height loss during the roll over. Flying it faster to keep the fuse level did seem to help, although I'll need to practice to keep it in thermals at higher speed. I was flying with a friend who has a Climmax which is about 1 oz heavier but also has 4 servos. He was out performing me a bit, but not by much. Thanks for the help in getting it trimmed. I think that it will perform much better now.

My Chrysalis turned out a bit heavy, mostly because of tail heaviness. It weighs around 12 oz. My wing with Monomkote transparent weighs 3.8 oz, and my radio weighs 4.2 oz ( I could reduce this by .7 oz but needed the nose weight). I built it with the narrow fuse and the conventional tail, cut lightening holes in the surfaces and only applied covering (micafilm)over the holes. I applied a couple of light coats of Krylon clear to the fuse and balsa parts of the tail. I don't think that this added so much weight but maybe it did.

At any rate I was considering shortening the fuse by 2 inches. Do you think that this would seriously affect its performance or do you think that I'd be better off trying to sand the tail and rear fuse as much as possible or by changing to the V-tail to reduce weight? Thanks
Ray

From Don Stackhouse:

12 oz. is rather heavy for a Chrysalis unless you're using standard size radio gear. Your wing weight sounds like it's right where it should be. Krylon (even clear) can be rather heavy, so that could be part of the problem. Monocote over lightening holes is about the same to slightly heavier than a really light Varathane finish, so you may or may not have some possible savings there if you build a new tail.

If you shorten the tail you will probably increase any dutch roll tendencies, reduce pitch stability, reduce the allowable c/g range (particularly at the aft end, which could force you to use more nose weight) and reduce dynamic stability (damping) in both pitch and yaw. For dynamic stability the relationship is with the square of the tail moment, so your 2" reduction in tail moment would result in about a 15% reduction in dynamic stability. Not huge, but enough to be noticeable. The model will certainly be flyable, but not as gentle or stable. You will also lose some rudder and elevator authority.

Do you have all the servos mounted well forward in the nose? Joe built a narrow-fuselage v-tail Chrysalis with an 80 mah battery in the nose, a pair of FMA S-80's stacked one on top of the other just behind it, and a Hitec 535 Rx with the case off behind those. The entire installation required about half the length of the nose compartment, with nothing under the wing. It balanced fine.

I suspect you possibly might have also overbuilt something in the tail. Too much glue in the joints back there can create plenty of c/g problems. It doesn't take too many stray patches of epoxy or c/a to add up to a lot of noseweight.

One other little weight saver back there (which I haven't had to use yet myself, but I plan to on a Monarch 'D' I'm trying to find time to finish) is to use a pushrod for an antenna. Get some of the tubing from the lightweight Sullivan cables (the ones that use the 1/32" steel cable), and run some 1/32" music wire pushrods through them For serious weight savings you can run .020" wire, but you will need to c/a the holes in your control horns and re-drill them. Measure the length of one pushrod and cut that much off of your receiver antenna. Solder the tip of the remaining antenna to the fwd tip of the pushrod. This eliminates about 2.5 feet of heavy, ugly wire from the tail.

Going to a V-tail will save some weight provided that you build it light and efficient(structurally that is). Round the fuselage corners generously, till you can see about a .030" wide stripe of the corner of the spruce longerons showing. Do a neat and tidy job of glassing the dihedral joint in the center, join it to the fuselage with enough but not too much glue, don't go overboard with the fiberglass. Use nice, thin laminating resin, not 5 minute epoxy or c/a (c/a is surprisingly heavy if you use enough of it). Fit some nice balsa filler pieces between the fuselage and the tail, don't just wipe in some heavy filler. Cut the 1" wide glass tape in half and use 1/2" wide strips to reinforce the joint between the underside of the tail and the fuselage. Do a good, light Varathane finish (2-3 very thin coats), and use 1/2" wide 3M600 tape or a 1/2" strip of Ultracoat for the hinges. Above all, be careful with the paint. All you need is enough to protect it from water, and where you fly you might not even need that!

I'd recommend that you try to get as much weight as possible out of the tail, and all weight as far forward as possible in the nose, before chopping the aft end off of the fuselage.

Good luck, and as always, please keep us posted!

Thanks,

Don Stackhouse @ DJ Aerotech