HP-18 Center Stick Project

This was another slow-motion weekend.

In addition to working on the center stick, I was also supposed to get together parts for a batch of 18-576s, and also get materials for some 18-143s. Well, the steel for the -576s didn't arrive. It was back-ordered. So I transferred that part of the order to Dillsberg. Then I found out I'd run out of aluminum rod for the 143s. Sigh.

On the center stick, I did a bunch of work to restore the stock aileron and flap cam systems. These aileron system had been modified for some (unknown) amount of differential, and somehow the builder needed a modified flap cam and cam follower. I've been slowly collecting (and having fabbed) replacements, and this weekend I put them all together. As part of this process, I also did the final installation of the P-P tubes and P-P tube guides for the center stick.

When it was all together, I found that there was hard contact between the bolt that attaches the aileron P-P tube (the one that runs down the flap cove) to the 18-418 bellcrank and the 18-417 flap torque weldment. It only occurred at positive flap settings, but it was a firm contact that limited the total aileron travel.

Earlier, when I was designing the bellcranks for the aileron system, I'd done a pretty rigorous analysis of the aileron system, showing the linear and angular displacements of the various elements. And that analysis was the basis upon which I set the aileron stops on the stick mechanism under the knee hump. This contact that I was getting between the bolt on the bellcranks and the 18-417 was ocurring within the travel limits on the stick. So either I had mis-analyzed the original parts of the system, or I had mis-fabricated the new parts, or the original builder had mis-fabricated some old parts.

What I did to go forward is to fabricate a new set of larger 18-418 bellcranks. The hole for the P-P tube is 1/4" further from the pivot axis to give more clearance between the bolt and the 18-417. I placed the other hole 3/16" further from the pivot to keep the ratios of the two distances the same. Now there's no contact between the aileron system and the 18-417 throughout the entire range of all possible aileron positions and flap settings.

And, BTW, the contact between the bolt and the 18-417 in the earlier configuration might explain why that particular ship seemed to have poorer than standard (even for an HP-18) aileron response.

Anyhow, fabricating those new bellcranks was pretty quick and easy. Getting them installed and hooked up took a maddening two hours in which anything I assembled had to get disassembled to turn bolts around, add or subtract washers, etcetra ad nauseum. Yuk. I even had to make new bushings for the flat bars that connect the centerline and outboard bellcranks - a pretty mean feat using only the shop's drill press and a die-grinder.

I also did a little more work on the gas spring for the landing gear counterbalance, and that's all finished. In the process of turning a bolt around for better clearance, I figured out how to keep the strut compressed during installation. It works great, and I think that anybody would be pleased with the improvement in system smoothness, and the added simplicity (or more appropriately, the subtracted complexity) over the spring + pulley system.

I also got the hydraulic disk brake system finalized, plumbed, filled, and bled. After adjusting the system stops a little, the brake seems firm and positive.

As with last weekend, the next big thing is the trim system. Last weekend, I experimented with torsional springs (like the anti-sway bars on your car) made of .063 and .093" wire. I'm pretty sure that both those wire sizes are too small, so I went ahead and ordered some .110 and .118 to try out. I also worked out the connection between the trim spring and the pitch P-P tube. The trick is to make something that is firm enough to carry the trim loads, yet frangible so that the pilot can either override the system or tear it loose in case something happens to cause it to try to jam the pitch system.

Next weekend, I'll do more work on the trim system and start figuring out a bunch of details like the pedestal mount hardware and the rudder pedal adjustment. The rudder pedal adjust has an interesting complexity to it - the position of the adjustment knob is relative to the position of the pedals. With the pedals all the way back, there's several inches of slack that leave the knob wandering around in the cockpit. With the side stick, it's OK because the knob on the base of the pedestal is nowhere near the stick. But with the center stick, that leaves potential for the knob to jam between the stick and something else. I'll probably end up putting in a bungee to take up the slack (I've seen that done on factory ships like DGs), but the question is, where?