Monday, October 12, 2015

Engine Assembly Continues

Engine Assembly Continues

Over the last few months, the engine has come together slowly as time allows. As a final act in my career as a Delta pilot, I decided to bid the Boeing 777.

School for the airplane systems began in July, but this is not the type of school we had in the past. All the systems training is on a “thumb drive” which must be completed on your own time. Once the formal school begins, there is one day of systems review, then the test (used to be an oral exam for a new type rating. . .no more), then a series of simulators. Of course, I was still a B767 captain until conversion to the new category and I was due recurrent training on the 767 in June. So. . .I had to put down the books and study for recurrent training. Then five more Europe trips on the 767 before school started in Mid-July. It was a very busy summer. The Travel Air had to take a back seat to the real job. Now, with training and qualification complete, it’s back to where my heart is.

Also, the summer heat this year has been unrelenting, forcing a slower-than-normal work schedule. But progress was made and the old engine is coming together.
 With the front case halves joined, the accessory case was filled with the gearing that will drive the magnetos, the fuel pump (more on that later), the tach drive, the scavenge and duplex oil pumps, the starter input, and the generator.

The cam ring, which lifts the roller-tappets is laid into place and held radially stationary by it’s shaft which has teeth that mesh into teeth on a rear-facing shaft.

Hard to imagine, but to time the valves on this engine, the end nut on this aft-facing shaft has to be loosened, and the teeth meshed correctly based on top-dead-center of the #1 piston. This baffled me, how an otherwise well-designed engine would incorporate such a Rube Goldberg method of setting the all-important cam timing! Oh well, the shaft was held in place with that hefty nut finger-tight until the timing procedure would be accomplished. With all the gears in place, it is time to assemble the back side of the engine. (This is all taking place in June, by the way. It has just taken this long to post!)
Once everything is assembled in the accessory case, it’s time to join the aft case to the engine assembly. Used the paper (?) gasket and Tite-Seal on the joint and snugged down the nuts onto the studs.
This big nut tightens the end of the cam shaft against the teeth that engage it onto the main shaft. This is important and it also needs a snap ring which will later keep the nut capture plate in place. This is also important since any slippage of the shaft teeth would remove the timing set between the cam shaft and the engine. Disaster. I listened to a couple of stories about this happening and the result was sudden stoppage, of course, but mitigating this potential problem requires some thought. More on that later.

Once the rear of the engine is complete, it is time to start installing pistons and cylinders. This part is the easiest, but it is time consuming. Lube up a piston, place the rings in the lands, place the piston on the rod, insert wrist pin and caps, lube a cylinder, compress rings, slide cylinder onto piston pushing the ring compressor down toward the case, remove the ring compressor, continue to push cylinder toward the case while aligning the studs with the holes in the base of the cylinder. This is done seven times. Then each cylinder has twelve hold-down nuts threaded onto the studs and torqued. Then a “pal nut” is placed onto each stud and turned down until it contacts the nut. Then it is tightened 1/4 turn to jam it next to the nut. This is 1940s technology style nut safetying. Of course, the last cylinder was a problem. While tightening the cylinder base nut, I noticed that it never got tight…just kept on turning. Yep, the stud was being pulled slowly out of the case. This necessitates removing the cylinder, removing the problem stud and either replacing it with an over-sized stud, or enlarging the hole and placing a “heli-coil” in via an interference fit. This latter procedure was the one we chose. A heli-coil is like a piece of metal tubing which is threaded internally. This is inserted into a hole where a damaged internal thread has been drilled out to a specified diameter. Jim’s friend, Mike Conners did the heli-coil for us. I wasn’t present for the procedure, but it usually involves heating the area around the hole while cooling the heli-coil with dry ice and alcohol. It is then dropped into the hole, and as the two parts equalize in temperature a very tight fit is accomplished. I returned from a trip to find (happily) that the new thread was installed and a new stud was ready to screw into the engine case, this time with hi-temp thread locker added. The last cylinder was finally affixed to the engine and looking at the almost finished engine was a great relief.
Me and the Perfesser

Now, with the basic engine assembled, it was time to “time” the cam. On most (flat) engines this involves gearing the crankshaft and the camshaft together somehow, usually via a timing chain or belt with certain gear teeth in the proper position. With a radial engine, there is no camshaft, but a cam RING. Each lobe on the cam ring lifts valves at the proper time, but to ensure the proper timing, the cam ring must be set into position by placing the #1 piston at top dead center on the compression stroke and disengaging the cam from the engine. This is where the big nut on the back of the engine comes in.
With this nut carefully loosened, the cam ring is rotated to a point where no lobe is opening a valve on #1 cylinder. There is a long procedure to ensure the proper position, and once achieved, the nut is re-tightened which meshes the teeth in the shaft of the cam ring with the teeth on the main crank shaft. The nut is then properly torqued and a locking plate is secured around it and held in place by a snap ring which must be locked into a groove on the shaft. The ring did not appear to be well fit into this groove, so some light sanding and persuasion were used to ensure a good fit. If this snap ring ever fell out, the nut could loosen, separating the serrated shafts and the timing of the cam to the crank would be lost with almost certain engine stoppage to follow. In fact, I heard a story about two incidents that occurred that caused engine failure on takeoff and subsequent aircraft damage. Both were due to improper placement of this little snap ring. The ring must underlie five of the six points on the hex nut, and the fit must be secure. This we made damn sure of.

Once the cam is set, the valves must be set, each intake and exhaust to .010”. Usually, this is a very easy procedure, but on this big radial engine, they must be set and then checked through three complete rotations of the engine. Tight clearances must be loosened back to .010” and loose clearances left alone.

Airframe Work

When not working on the engine, I squeezed in some work in my shop making some items that I've been dreading. The stringers—the long wood strips that hold the fabric and shape the fuselage—were completed on the top of the fuselage, but the sides and bottom have yet to be completed. I've studied other Travel Air projects and noticed that the side stringers were held in place by little semi-circular wood pieces with slots cut in to hold the stringers. Attaching them to the fuselage tubing did not look very substantial on any of them, so I'll try to come up with a different method. But the stringer "mounts" will be standard items.

The size of the stringer mounts happened to be about the same size as a large tuna can, so this was used as a guide for drawing the semi-circle.

Once drawn and cut, the slots were all cut together using my radial arm saw.

These should do just fine.