Fishtailing Extra, Why?

[Robin]

Comp Arf 3m Classic Extra, fishtails a wee bit at high speed and if I'm not gentle with the rudder around neutral.
Any ideas on how to sort this and if possible whats going on aerodynamicaly so perhaps this can be cured as opposed to overcome if you get my drift!

cheers

[murry]

I installed a clear plastic door edge guard to the trailing edge of my rudder

[AHP]

	Quote: Originally Posted by murry
	I installed a clear plastic door edge guard to the trailing edge of my rudder

That will help, but in my experience, it's caused by a lack of servo power. My buddy's 40% TOC Yak would wag its tail at anything over about half throttle. He tried the clear plastic paper binder strips and it helped a little. Adding a few more servos fixed it.

[vuster]

Not having enough servo power is one of the problem - the real problem is that the rudder counter balance is a bit too big on this plane.

I had 4 x 5945MG on mine when I had the 3m Extra awhile ago.

Ryan

[dmcquinn]

My 35% Yak would do this at high speeds. There was a slight slop in the pull-pull system. Basically, I had it set up so that the wires got slightly tighter when the rudder was deflected and at neutral the wires were a little loose. I'm talking about maybe 1 mm of slop (less than 1/16 inch) at neutral. But correcting this fixed the tail wag. I have two 5955TG servos so there is enough holding power.

[Cracky]

A couple of years ago I was at a fly-in and a guy had a 3M C-arf Extra and it had fuel tubing on the trailing edge of the rudder. I asked why it was there and was told that the rudder was too "clean" and thin at the trailing edge and would wag. Adding the tubing took out the wag. FWIW thats what I was told.

[Neogenesis]

My SD 33% Yak wags something horrible......it's got a 5955 on pull pull that is so tight you can strum them. It wags at anything above 3/4 throttle. I've been toying with the idea of trimming the top of the rudder "ie reduce the counter balance" this winter.

I think I'll try some of the above solution before I try that though.



Neo

[Plane Wrappers]

Vuster has the most correct answer. The counter balance is too large on the rudder and also the trailing edge is to thin and the air flow cavitates coming off the rudder. The simplest fix is to slice a peace of large fuel tubing straight down the center, slide it over the trailing edge and wick medium CA down the inside of the tube.
Then let it dry overnight.
Had the problem on both of my superXtras with 4 - 8611'a on them and this solved the problem.

[Robin]

Sounds good, Im using three 5955 at 6.5 volts so servo power should be OK initialy tried two which was not quite enough to knife edge loop, two would hold the rudder over if put in at lowish sped at the top but not enough power to pull the rudder across at speed, three seems very good. So we shall upset the airflow at the rear and stick something on it!! cheers guys

[Digger3D]

Thanks for the info.

[DA2000]

I flew the 3M for two seasons with 4 Futaba 9152's. I tried the fuel tubing, which worked, but the Plastsrut 3/32 right angles, one per side, worked best.

http://www.hobbylinc.com/htm/pls/pls90003.htm

I tried several different size "fences" on the back side of the rudder to experiment a bit. The larger the fence, the more locked in the rudder was, BUT rudder authority is reduced. The trick is to go as small as possible.

After two years, I decided to modify the rudder like I saw on Don Sczur's 3M. Cut 2.5 inches of the lower counter balance off and glue it to the fin. If you think you might want to try it I might be able to dig up some pics. Now it's rock solid at the highest speeds. It sounds a little scary, took me several months to get up the nerve. The whole mod took about and hour and was well worth it.

My old airframe has nearly 2000 flights on it, a friend in my club is still flying it.

Daniel