Elevator trim tab set up

[rodsmith]

I would like to get an idea of how much the trim tabs should deflect, either in degrees or trailing edge deflection. I have set up my trim tabs so with the elevators neutral, and the trim wheel centered between full up and down the trim tabs are also neutral, Having used the wood strips to profile the stabilizers, I can't move the trim wheel as far as when I initially set it up. The bearing contacts the surface of the stabs and limits travel. On the trim tab actuator I have multiple holes for attaching the bearing. With the elevators neutral I can get 1", 1 1/2", or nearly 2" trailing edge deflection up and down depending on which hole I attach it to. I think less is better to limit sensitivity but want to make sure I have enough movement to cover any situation. As a side note and I realize there are a lot of variables, where do you set your trim for take off?

[jaredyates]

I would start with the tab in trail when the elevator is neutral, that is to say, center the elevator and then match the tab trailing edge to the elevator's trailing edge. That will get you in the ballpark. Then I would rig for minimum deflection, which is the outermost hole on the tab. Tab deflection also impacts pitch sensitivity because of the servo action.

[Nev]

Adding to what Jared has said (lowest hole on the trim tab arm), I also needed to bend the arms further on mine for clearance when the elevators are deflected up (full back-stick), otherwise the arms touch the elevator hinge and limit full travel of the trim tab.

Trim setting for take-off is CG dependent and therefore also related to engine selection (weight). On mine with an IO540 (empty CG around 10.5"), with a typical 2 POB load and take-off CG around 12-14", trim is set slightly aft of neutral for take-off, and for landing it is full aft (and wishing I had more). At forward CG there is continuous "fiddling" with the trim wheel during flight to relieve flight loads on the control stick.

With an aft CG (typical of 4 POB), I set trim slightly forward of neutral for takeoff, and slightly aft for landing. With an aft CG there is very little trim wheel adjustment needed during flight.

During a go-around at forward CG, I only apply partial power, then re-trim, and reduce flap, before applying remaining power. If full power is applied initially, the nose-up pitching moment is difficult to control.

The picture below shows the new arms that I've just made with rod end bearings at both ends (4-place B model kits only had the aft bearing). Graham Johnson kindly did the welding for me.

67CD9623-AD50-40BD-BC91-CD8667461F76.jpg

[rodsmith]

Thanks Nev, my trim rods don't have as much bend as yours, but they do clear the structure in all situations, I built mine with bearings at both ends also.

[nellie2110]

Nev,

Did you change the length when you remade the arms, or did you use the same length just with a greater bend.

[Nev]

My original trim arms needed to be bent as pictured above for adequate clearance, so when I made the new set, they were a carbon copy of the original ones with the exception of the additional rod end bearing at the forward end. So to answer your question, there was no change in length, just the change in bend. If you decide to change the bend, just do it in very small increments because a very small adjust can have a large downstream effect.

So far it's been an excellent mod. The only reason I made new ones (instead of simply welding another rod end bearing to the existing ones) was so there was no down time, and I now have a spare set.

[Mark Goldberg]

How much the trim tabs should move is a question heard often Rod. Setting the system up as designed and described by everyone above will get you a good functioning system. I might differ in one way from some of the comments. You need more nose up trim than nose down trim. I doubt anyone ever goes full down trim. But full nose up trim on every landing (except at aft CG) is on every flight. So if you want to bias the system to maximize nose up trim and at the same time losing a little nose down trim - that will not hurt you at all. Mark

[rodsmith]

That is great information Mark, and makes perfect sense.

[Nev]

+1 on Marks comment above. I would add that if the empty CG is well forward (14" or less, more typical with an IO540 etc) and you are flying light in the forward CG range, it's very advantageous to have ballast in the aft cargo area. I normally carry a 50lb survival/spares/overnight container (at the recommendation of others on the forum).

Yesterday I reminded myself of why I normally carry it, by leaving it behind, and life got quite exciting on short finals.