B Model Elevator Trim Tab

[DuncanK]

I've read some previous posts about elevator trim tabs, but....
I have installed a conventional trim tab to the inboard portion of the left elevator of my B Model Sonex. It is just less than half the elevator width by about 2.5 inches. With flaps retracted it works fine (although it is not very sensitive), but it will not hold the nose up with full flap. So I built a new tab which is twice as big (i.e. same hinge point but it protrudes nearly 3 inches behind the original trailing edge of the elevator). I also increased the length of the tab lever arm, so there is more trim tab angle available.
Qualitatively, with full nose up trim, the full flap stick forces are about the same. That is, doubling the trim tab area and increasing its angle has had no noticeable effect.
Does anyone have similar experience or a reason why there is no improvement?
Given little or no change in stick force, the moment generated by both tabs must be about the same, (and not very much). Does it suggest that there is little (or separated) airflow over the inboard trailing edge of the elevator (at least with full flap)?
Has anyone made a full width trim tab for the elevator? If so, did it work and what was the size?
Has anyone tried vortex generators on the horizontal stabilizer or the elevator?
DK

[GraemeSmith]

Ignore the trim tab for a moment....... Just to check:

Can you hold the nose up with the stick at full flaps? ie - do you have the elevator angles set correctly and thus the elevator authority needed (even if you have to heave with arms of a gorilla!). If NO - Might be your elevator angles, you might be too far forward in CG. I would think you need to resolve that/those first.

The trim tab would then follow.

[DCASonex]

On my A model flipping the lever to full nose up when putting on about 15 degrees flap gives near neutral stick force and still very easy to hold when going to full flap for landing. Doubt if I would ever want set trim to fully balance full flap condition even if could do so. Busy enough when landing my tail dragger to not want to bother with that. Also, the small wire that actuates the trim tab will only take so much force so would probably have problems with that if using a larger tab.

David A.

[DuncanK]

Thanks for the replies.
Can I hold the nose up with full stick? Yes.
I did not mean to imply that the stick forces are huge, as I wrote, the trim tab will not balance the stick forces at full flap. I land with full flap and although I'm holding a nose up force, I don't think about it, its not a large force.
The fact that it is not a large force really gave rise to the questions in my post. Basically, why does the trim tab fail to take out the force?
My larger tab is about the same area as the tab on my RV6, the RV6 trimmed out the full flap force with no problem. The RV6 elevator does have a balance weight under a fairing which extends forward of the elevator hinge. Therefore, the pitch stick forces in the RV6 are all due to the aerodynamic force on the elevator, in the Sonex B, the force is the aerodynamic force plus the force from the weight of the elevator. That is, In the hangar, the Sonex stick is always full forward (there is no spring attached to my pitch stick system), in the RV6 it balanced at any angle (again, no spring). I think I will get a spring balance (fish scale) and measure the force to hold the stick neutral when on the ground (ie due to the weight of the elevator), plus I'll measure the force in the air with full flap and full nose up trim, (repeat with new and original trim tabs).
Is my CG too far forward? No.
Is my elevator angle is correct? Yes
But even if it was wrong, why would doubling the trim tab area have little or no effect?
My tab is electrically actuated, hence I am not concerned about the strength of the Sonex tab wire. If the force on the tab was too large, the little electric motor would stall, it doesn't, after landing the actuator is fully extended.

[GraemeSmith]

I don't know what Sonex recommend for the B. My Legacy elevators are balanced with a bungee acting on the control rod - fitted under the seat pan.

[GraemeSmith]

I don't know what Sonex recommend for the B. My Legacy elevators are balanced with a bungee acting on the control rod - fitted under the seat pan.

[DCASonex]

Might just be that your actuator does not move the tab enough to provide compensation past a certain degree of elevator deflection, or at maximum angular deflection it just produces turbulence.

David A.