I understand this is difficult to explain without actually seeing, but why would I not want to move the pivot point up to the green circle as opposed to keeping it below as the original?

I have highlighted the top of the jet tube in red, it is really close to the servo and the designed attachment method (two bolts) almost touch it.

I agree Dick, I don't like it either!

Plywood for the spars is totally inadequate.
If the pivot bearings aren't pulling out of the fuselage when the tails have failed, the plywood has failed.
Nice thick walled CF cubes for that application.

Changing the design is a great idea. But there is a fairly good reason to not move it to the location you indicate with the green circle if you wish the pivot to be slightly ahead of .25C MAC. Because that's nowhere near .25C MAC.

If you look closely at Tall Paul's pictures, see the two CF spars. One of them would be better for the new pivot location as it points almost exactly ahead, but close to the 25% location on the MAC.

BTW, if you were to use the aft CF tube in Tall Paul's righthand plan as the pivot for the stabs, you would simplify the work needed for your redesign. Since you're working at the rear of the airplane, you'd benefit from a very slightly lighter structure. And the hingeline of the stabs would be reasonably close to the .25C of the MAC. Since you're not bound by anything at all, simply place it slightly farther forward than in that picture or not.

The geometry from servo to stabilator -sucks -
make a couple of cardboard pieces to duplicate servo arm and stabilator arm- and using a piece of wire as linkage -- move servo thru 100% of rotation - check corresponding movement of stabilator

I can't help myself, had to butt in. The tail assembly would be a lot simpler if the root sections of the stabilators were perpendicular to the axle. Why not build out that area of the fuselage to "square things up" first? I would use as large a C/F axle as you can get in there and stuff it with birch dowel. Then use tempered aluminum knitting needles to pin the stabs to the axle in a couple of spots. The axle can ride in maple bearing blocks that have been greased. I use a similar set up in a 1/2A sized propjet and DH is right, use the inner hole of the servo arm and go with a nice long control arm on the axle.

Hopefully this drawing is good enough to get the idea......

Thanks Guys, interesting idea's. Can't use the carbon fibre as this is a BAE Hawk, the tail plane is canted downwards as per the photo.

I would have thought that the plywood bearers are strong enough as there is very little movement when I try to twist the two halves. Obviously they need finishing of and then glass cloth added to strengthen everything.

The wood supplied for the root is awful, hence not going ahead with it, I just fitted it to see how it all worked.

On the made up tail plane, the 25% MAC line is shown in red and the original pivot positions are shown in green and the bolts go through the washers. After all the discussion, I was going to make the tail plane pivot close to the aft bearer as that is very near to the 25% MAC line. Some extra bracing would strengthen things up as well.

At the very least, change the plywood spars to oak.
The download on a horizontal is very high.
Half the material in plywood has no strength in the proper direction to resist the bending load.
The whole thing is rocking back and forth on the low pivot. That is awful!
It's quite difficult to get the scale look and performance with a wood structure.

Thanks very much. I will look in to that.

Actually, since the structure is very close to completed, there is something else you can do to strengthen it without resorting to rebuilding.

Carbon fibre comes in many shapes. And so does glass cloth. Matter of fact, glass cloth basically can BE shaped quite easily.

If you've got a dremel, it would be fairly easy to route out for CF reinforcement. And glass cloth pieces would help those plywood spars quite a bit.

Good idea, I was wondering about fibreglass for reinforcing the wood as it is extensively used elsewhere in the model. Adding a bit of carbon fibre wouldn't be difficult.

I was going to do a stress test on some spare wood first, as usually marine 7 ply is very strong. I had never thought of oak before.