Monday, June 6, 2011

KR-2S Elevator Trim Tab 01

This installment covers the build of my elevator trim tab.

In this first shot you can see I have already cut the tab out almost to plans. I chose to run the hinge perpendicular to the inner rib to keep the gap fine and ensure no binding. You can see the exit of a cable outer that was already embedded in the elevator. I tried to reuse this but the cable would have had too tight a bend. Have decided to embed a Mac servo in the horizontal tail to maintain a straight run and shallow angle.

This is roughly where I will place the servo. This will be done later when I flip the fuselage over again.

Cut back the foam ready to bond in a false rib.

Here the rib is trial fitted, It is made from an offcut from the instrument panel, Carbon/Ineggra sandwich.

Both ribs in place and the hinge tube is dry fitted. I got 2 brass tubes from the hobby store, one 8mm and one 7mm both with 0.5mm wall thickness. They nest in one another perfectly, a little silicon grease and there will be no play (important to stop flutter).

Here's a closer shot of the inner tube in place.

Here is the whole assembly. The two shorter sections will be bonded in the elevator, the inner tube will be free to rotate.

Here I have made a control horn out of 2 pieces of light angle bonded into a "T".

I have bonded the control horn in a pocket under the skin with epoxy and flox. I have also bonded in the hinge tube.

And this one shows the finished tab. I have filled between the ribs with micro/epoxy and sanded to a smooth radius. This will fit into a similar radius formed in the elevator later. You can also see 2 brass washers to ensure there is no lateral movement to cause binding.

Now to finish the elevator. One side of the opening is on the real rib, the outer side needs a false rib. The plans call for you to just embed the outer rod in the foam and glass over, I think this is much stronger.

To get the 12" inner tube out you need to remove the rudder (if it was already fitted like mine was) and drill a small hole in the opposing rib.

The 2 smaller lengths of outer tube can now be bonded into the elevator ribs ensuring perfect alignment till the flox sets.

The finished tab, works superbly. All I need to do now is to mount the servo and the real control rod.

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