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Eric Witherspoon
12-17-2011, 09:59 AM
Is there a rationale for which side of the elevator gets the trim tab? For my Sonex, it is shown on the left side. Since these airplanes run engines that turn either direction, I don't believe the plans call for us to switch the trim tab based on prop rotation. I suspect it's on the left because the lever is on the left, and the cable just runs down the left side. But by the time the cable is under the tail, it would only be marginally longer to run it out the right side - not a big weight hit or complication to the mechanism or anything. In fact, I have seen a photo of a Sonex with the trim tab on the right side (or it might have been one that I saw in person - or maybe I saw it and took the photo...)

Anyway, the airplane I am working on now shows the trim tab on the right side - similar idea - the lever is on the left side of a tandem cockpit (so center mounting isn't a reasonable option), then cable down the left side of the fuselage, then at the back end the cable crosses over to a right-side tab location. This one runs a Jabiru in tractor configuration, same as what I have on the Sonex.

I suspect trim tab location is such a minor part of the overall situation that location doesn't matter (left side or right side elevator). But I was curious if anyone had a rationale for one side over the other.

Now that I think about it, I'll have a look at the cable routing - they may loop the cable out the right side to give a better bend radius as it runs from being anchored on the left side, curves out spanwise along the RH elevator, then turns back aft again to reach the trim tab which is about halfway out on the elevator. The Sonex tab is as inboard as it can get, so there is not much angle / bend at all to the cable...

MPerkins
01-04-2012, 11:36 PM
The side of the trim tab is pretty much a function of mechanical convenience factors (as you point out), and not so much aerodynamic ones. To keep bending moments low for stress and flutter reasons, it should be mounted as near to the main elevator actuator as possible.