Horizontal Stabilator Push-Pull Tube

Go ahead and do it. Read the title of this posting aloud: Horizontal Stabilator Push-Pull Tube. I’ll bet everyone within earshot is looking at you real funny now, aren’t they? But what the heck; how many times in your life will you get to say, “Horizontal stabilator push-pull tube?” 🙂

Over the last few weeks, I have been assembling the push-pull tube which will run then length of my Bede BD-4C airplane. At the front of the plane, it connects to the torque tube at the base of the control sticks. At the back of the plane, it connects to the horizontal part of the tail. Push the stick forward and the back of the tail (the stabilator) moves down, which will make the plane dive. Pull the stick backward and the stabilator moves up, which will make the plane climb.

The push-pull tube is actually two tubes, which meet in the middle at a bell crank. The front tube is 3/4 inch diameter steel. The back tube is 2 inch diameter aluminum.

The front tube has to run through the landing gear box. Now there are plenty of pieces on the BD-4C which I mess up, and make again, and it is no big deal because they are made out of small pieces of aluminum. At worst, I end up wasting an hour of my time. The gear box is different. If I mess it up, it would probably take me a couple of weeks to get the old box out, fabricate a new one, and install it. To say that I was nervous about drilling holes in my gear box would be an understatement.

I measured very carefully (and several times) and then drilled 1/4 inch pilot holes through the gear box. Then I ran a string from the torque tube (at the bottom of the control sticks) back to the bell crank. Imagine my relief when the string shot right through the middle of both holes! (Click on the photo to see it large enough to see the string.)

Once I knew that the pilot holes were properly positioned, enlarging them to 1 inch in diameter was pretty much stress free. Then I temporarily attached the steel push-pull tube to the torque tube. In testing the full range of motion, I discovered that I needed to elongate the holes slightly so you will see in these photos that the holes are a bit more than 1 inch tall.

Fabricating the back half of the push-pull tube was a challenge. It is a 2 inch diameter aluminum tube, about 7 feet long. (By the way, I weighed this tube on a postage scale and it is just 20.6 ounces!) I used a hydraulic shop press to flatten the ends so that they could be attached to the “metal clad” rod ends.

Here is the front of the tube, which will attach to the bell crank in the middle of the plane.

And here is the very back of the tube, which will attach to the horizontal stabilator.

If you look closely at this picture, you can see the aluminum push-pull tube connected to the bell crank and running out the back of the fuselage, ready to connect to the stabilator.

I have some work to do on the back of the fuselage and on the weldments which will support the stabilator. Once that is done, I can permanently install the push pull tube and connect it to the tail.