Thursday, May 10, 2018

Flight Controls

Once the stringers were fabricated, they had to be removed while I made flight control cables. Actually, the only cables made today were the elevator control cables, but access to the inside of the fuselage frame required removing some recently fabricated items.
On the Travel Air, the elevator control cables are made in pairs. From the book “Travel Air–Wings Over the Prairie,” by Ed Phillips, the idea for this redundancy came from Walter Beech, who insisted on dual elevator control cables in the event of cable failure. Lloyd Stearman agreed. He had most likely witnessed a cable failure at some time and was determined that such would not affect the ability to control the airplane, after all, this was 1925 and the state of the art for aircraft materials and design was in it’s infancy. So two cables are attached to the stick above and below the hinge point, running aft to the elevator control horn.
Making four cables for elevator control was not a problem, but routing them to the control horn revealed obstructions in the form of the extra tubing welded into the fuselage frame not present on the original.

Using string for a guide, I had to devise a pulley arrangement that would provide clearance from frame parts throughout the full throw of the stick, not only fore and aft, but side to side as well.

The first bracket was made from 4130 steel plate 1/8” thick drilled to accommodate two 3” pulleys, but they proved to be too small for the amount of cable deflection required. Typical of my work—too much trial and error. The larger 5” pulleys did the trick. The smaller pulleys will be used for rudder cables. Of course, this necessitated making a new bracket as well.

The pulleys are held in place by a piece of spruce with two 3/4"x3/4", .063" extruded angles held in place with four tubing clamps. The chromoly pulley bracket is attached by four 3/16" AN bolts and lock nuts. There is a spacer between the two pulleys to provide the proper alignment of the cables.

The cable used for the controls is 1/8” diameter 7x19 wire rope. This is comprised of seven strands of 19 strand rope all woven spirally. It is fairly flexible and 1/8” cable has a tensile strength of about 2000 lbs. I am fortunate that when a neighbor moved, he left me a box of cable of this type with right-hand thread studs swaged to one end.

Swaging is a process that uses a specialized tool to compress a steel end of some sort (stud, eye, or fork) onto the cable. So much pressure is generated by the tool that the end is actually extruded into the cable slightly, making for a very strong attachment, equaling or exceeding the cable strength. Each of the cables that had the 1/4” threaded studs swaged to the end were long enough to use for the elevator cables. I only had to attach an eye on the other end at the correct length.

I used the “Nico-Press” system for these ends. This requires a much simpler tool. The cable is run through a copper sleeve, wrapped around a thimble and routed back through the sleeve.

The Nico-Press tool is then used to compress the copper into the cable just like the swaging process does with stainless steel, but requiring far less squeezing force. Three compressions are required to complete the joint. A go-no go gauge is used to check that the sleeve was sufficiently compressed to make the joint meet specifications.
The loose end of the cable is then trimmed. Later, the whole joint will wrapped with rib-lacing cord to look period correct.
Once all four cables were made, the swaged ends with the stud, turn buckle, and fork were attached to the stick and the Nico-Pressed ends were attached to the control horn at the elevator.

The cables were the correct length, but some adjustments must be made to center the stick with the elevator flush with the horizontal stabilizer. I just wasn't getting full up elevator with the stick back as far as it would go, so the only way to correct that (can't make the cables longer) would be to re-make the little steel tangs on the stick. With these tangs a little longer, I should be able to adjust the elevator travel with no problem.
The only issue left to correct is the area where the cables criss-cross. Some rubbing is evident, and intolerable. More head-scratching.