How's this for a puzzler: Prop/trim interplay.

[Skyhook]

Plane: RV-4 with Lyc O-360
Initial prop: Sensenich 2-blade FP wood.
New Prop: Catto 3-blade

When the swap-out was finally ready to fly, I noticed the ball on the T&B was out to the right by about 1/2 ball diameter requiring constant right rudder input which increased with airspeed. I installed one of Avery's wedge rudder trims and that brought the ball almost center, but now I have a heavy right wing. If let alone, the plane quickly enters a downward spiral of increasing bank, airspeed, etc.

Now, with the 2-blade Sensenich, the plane flew hands-off, straight and level. I am tempted to go back to the Sensenich if the trim situation cannot be addressed.

I could use some ideas and commentary on this.

[hevansrv7a]

As the torque increases there should be an increased tendency to roll left, the opposite of the prop's rotation as seen from the cockpit. In mine and in my old C-150, there was only one speed at which rudder trim was neutral and this agrees with the above concept. If the prop alone is causing this, then I would hazard a guess that there is a significant difference in effective pitch between the two props. If not, then I'd carefully review the outside possibility that something else had changed. When I switch between my Prince 2-blade and my Catto 3 blade, even with different pitches I do not get the kind of difference you are describing.

Good luck with that.

[videobobk]

Another random thought that probably means nothing: If there is significant difference in weight (and I think there is) it may in some way be related to shifting the CG aft a little. If you like the performance of the Catto, you might try some of the heavy wing fixes.

Bob Kelly

[Skyhook]

Originally Posted by hevansrv7a

As the torque increases there should be an increased tendency to roll left, the opposite of the prop's rotation as seen from the cockpit. In mine and in my old C-150, there was only one speed at which rudder trim was neutral and this agrees with the above concept. If the prop alone is causing this, then I would hazard a guess that there is a significant difference in effective pitch between the two props. If not, then I'd carefully review the outside possibility that something else had changed. When I switch between my Prince 2-blade and my Catto 3 blade, even with different pitches I do not get the kind of difference you are describing.

Good luck with that.

I have had other acft upon which I'd changed props and experienced nothing like this.
As to something else having changed-- nothing has and that's why I am scratching my balding pate looking for some input from some with more savvy.
Thanks.
(Puzzled in NY)

[Skyhook]

Originally Posted by videobobk

Another random thought that probably means nothing: If there is significant difference in weight (and I think there is) it may in some way be related to shifting the CG aft a little. If you like the performance of the Catto, you might try some of the heavy wing fixes.

Bob Kelly

Bob, I think you are correct in that there was some weight difference, but my A&I who helped said it was minimal when the longer bolts, thicker crush plate were taken into account.
Monday I will phone Craig Catto and see what he may think of this conundrum. I admit to being more than a bit confused.
Thanks for the thoughts.

[Skyhook]

I just had another thought on this..
Is it possible that the prop wash on the HS is different enough to produce that rolling tendency?

Maybe others here with a similar swap-out could offer some thoughts??

[osxuser]

I've heard of this before switching from a two blade Hartzell to a three blade MT on Glasairs and Lancairs. Consider that it takes less torque to bring the lighter prop up to rpm and (slightly less) to keep it there, that has to go to more effective pitch. Also consider most three blade Catto's are shorter blades, which puts a faster column of air in a tighter radii around the fuselage.

How much has speed been effected? If you are gaining speed, even out of trim, and want to keep the Catto, simply go through the 'heavy wing' proceedures in the FAQ and fix the airframe problems so it flys straight again. If you are losing too much airspeed because of the uncoordination, and don't think you can get it back, go back to the SenSen...

[Skyhook]

TO and 'sweet spot' cruise performance figures are way better than the Sensenich 2-blade.
My 55% normal cruise went from ~145kts to ~152Kts (unchecked by GPS).
The TO is much quicker, I gained important gnd clearance (2-blade was the airport's weed eater).
Now considering it all, I think after I call up Craig, I'll try to get hands-off st. & lvl once more via aileron, whatever adjustments.

Thanks, osxuser, for the input, it is much appreciated.