Hi all

first post

i recently built a seagull Harmon RocketIII. This is my 4th plane in my fleet and have had it flying (6 flights so far) pretty good in slightly windy conditions. It seems to have a tendency to fishtail (wag its tail) when above half throttle.

I cant seem to work out whats causing it. all my linkages are working correct and there is no slop. ailerons and elevator are hinge taped.

I am running a OS 50sx with a 11X6 master airscrew scimatar prop. Other than the fishys in flight (it tracks straight on the runway) there are no other problems with this beautiful model.

any help on this problem would be greatly appreciated. thanks

Daniel

This is sometimes the result of incorrect trim set-up. If you have the rudder incorrectly trimmed & you have compensated with the ailerons to get it to fly more-or-lesss straight, it could result in tail-wagging.

Okay i will double check that when home from work, thanks britbrat. I only have about 2 clicks of aileron trim and about 3 of elevator from memory.

So is it caused by just the motion of the plane with a out of trim rudder? or propwash over the out of trim rudder?

Did you have a cross wind when you were flying?

Wow ! I just looked up the Harmon Rocket l. Thats a beautiful airplane. I found the one one Horizon. Does the plane look as good as the picture ?

With that kind of set-up error, the control surfaces are essentially fighting each other & there is a fair bit of vortex-shedding from the offset controls. As vortices break away & re-form/re-attach the oscillatory motion results.

That is not necessarily your problem, but it is a possibility.

That's called Dutch Roll. The full size V-tail Beechcraft Bonanza has a distinct Dutch Roll in rough air. Usually it means there isn't enough lateral area to damp out the yaw motion. The Bonanza seems to get it from the V tail plan. At least one early Boeing 707 airliner crashed because the captain failed to correct Dutch Roll properly. The 707s got a yaw damper after that. Jets with swept wings have built-in Dutch Roll. Passengers in early 707s spent a lot of time airsick until they put the dampers on all the jets.

The point is that fast, slick planes like the Rocket are more likely to have some Dutch Roll. Trim and alignment can play a roll, but the roll may be built into the design. If the alignment and trim are right, I'd consider taping a temporary ventral fin of stiff cardboard or balsa (to the bottom of the tail cone, underneath) and see if it cures the roll. If I remember the shape of the Rocket tail cone correctly, it could be short on lateral area.

A lot of Corsair models have dutch roll as well.

My Funstar also fishtails. It fishtails no matter what direction I'm flying with respect to the wind. This is not a fast, slick plane - it has a very thick, constant-cord wing. The fuselage is about medium sized for a 40-sized plane - maybe a little longer than most. My rudder pushrod setup has no slop. On its maiden flight it required very little trim of any kind to give me straight & level flight. I've seen other Funstars fly and they don't exhibit this fishtailing problem. I'll check whether I have ailerons working against rudder, but I doubt if that's the problem since so little trim was required. Anybody got any more ideas on what might cause an otherwise well-flying model airplane to fishtail on straight and level flight?

==Mike==