what causes this?

[stx44]

So I built this whilst I was waiting for my Lancer to arrive- its called a "Lil sister" and is from plans downloaded from the net.

It flies a good distance, but under the initial power run, Im getting a lot of rolling side to side (think of a hammock rocking back and forth). Trying to work out what is causing it- I initially thought it might be turbulence, but it was pretty calm today. It will roll to about 30 degrees, then right itself. It tends to settle down after the initial burst of power is gone (ie from about 50% motor time onwards, all is good).

Ideas?

[David Lewis]

Sounds like Dutch roll. Can be caused by too much sweepback and too little rudder area.

[stx44]

Thanks David- I think your right on the money.

Im going with it being the short fuselage (ie small rudder moment) combined with the small(ish) rudder area.

Not sure I can fix it short term, but it flies ok anyway- and hey my Lancer arrived, so the next project is underway anyway!!

[zoomie]

+1 on Dutch roll .

The rudder/fin area could gradually be enlarged by tack-gluing narrow strip(s) of thin dinner plate foam to the rudder trailing edge or top of the rudder/fin. Strips about 1/8" wide would be a good place to start. If there is no improvement, add about 1/16" more at a time. You may not have to add very much to obtain stable flight. When you reach the desired amount of area, you can build a new replacement rudder/fin for the model.

Important to recheck your C.G. and rebalance if necessary whenever you add area this way (more weight back there).


zoomie

[stx44]

I added a small (half inch) extension to the rear of the fin and did a rebalance.

Have only had the chance to try it out the last few days- thing goes great now!

Thanks for the remote diagnosis- it was spot on!