Trim Tabs

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Trim Tabs

Post by jchisolm »

Finishing some wing issues and got to thinking about the trim issues. I'm putting in a G3X with autopilot so I'm not too concerned about aileron trim as I'm a big fan of letting the autopilot fly XC while I look out the window. In my view the AP would take care of the aileron trim in level flight.
I'm only installing 2 axis so rudder trim will still be necessary and elevator trim for non-AP work.

I've been looking at the various RV setups and it seems Van's has gone to the bias spring approach and not supplying any hinged trim tab. I like the simplicity and that you dont have all that hardware inside the elevator or rudder you have to try and balance out. My worry is the servo force and spring forces needed to hold a CT tailed Express in trim for landing or what I might need for rudder trim. I know there are several production aircraft that use the bias spring method. I've also see some manual trim "kits" for expermentials that use the springs.

I would appreciate any input on what kind of stick forces you might have for a un-trimed landing configuration and how (or if) you have rudder trim and what is your setup.

I could build the trim tabs and take that approach but I like the clean look of a no trim tab control surface.
Joe Chisolm
Express CT builder
Marble Falls, Texas
Posts: 10
Joined: Tue Mar 14, 2017 2:49 pm

Re: Trim Tabs

Post by n49ex »

I have the standard trim tab with the MAC servo, and it works very well. Plus with the G3X AP, it operates the servo to null out the elevator forces.

I don't have any experience with bias springs, but intuitively my sense is that they may reduce some forces but can never compensate completely for all speeds and loadings.

There are two considerations I would suggest when going with the standard tab (BTW, some folks put tabs on both side - I find this totally unnecessary):
1) I needed to add about 1/2" extra width to the given elevator fiberglass in the counterweight area to be able to balance - and found it out after already painting. Paint adds considerable weight and Wheeler did not anticipate it enough in this place.
2) It is extremely important to minimize slack in the trim tab - servo connections and hinge, for flutter avoidance reasons, as the trim tab is not counterbalanced. So, I suggest using a piece of carefully selected diameter rod for the servo attachments, drill the rod yourself for cotter pins, rather than using standard 1/8th" pins; and, though not specified in the instructions, include screwed attachment for the hinge at the elevator side, so you can replace worn hinges. Also, again this is a place one can use precision rod slightly over the 3/32" standard pin - the less initial slack, the less the wear.

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