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Monday, October 3, 2022

Doug's Domain

Doug Vetter, ATP/CFI

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Engine Test Stand

Fabrication and Break-In Process

I experienced my first engineering difficulty on the project when I attempted to use the Tatone steel/aluminum test stand I purchased for the SX's break-in period. The stand would not accept the SX due to the height of its crankcase. Although the stand was marketed to handle "through .90 size engines", the engine mounts clearly sat above the stand mounts by about 4mm. I thought about building-up the stand mounts or engine mounts but this was determined impractical because the stand clamp screws and locking pin (part of the stand that helps prevent fore-aft movement of the engine) were too short. I instead tried to use the Dremel with a stone abrasive drum to remove some of the stand material near the base of the engine, but the metal was too thick and clogged the drum.

My solution was to start from scratch and fabricate a wood stand (lesson #1: if you can make it yourself, even if it takes more effort than you wanted to devote to it, do it). I took a 1.5 ft. long piece of 2x4 and cut two 1" blocks from it to form the new mounting base for the engine. After playing with the resulting pieces, I determined that I would need to mount the engine off to the left side of the base 2x4 with shaft facing forward so that the exhaust header and pipe could clear the wood base on the left side of the engine. I also predrilled the blocks to prevent splitting and affixed them from the top and bottom each with two 2" long drywall screws. The resulting structure was rather strong. In retrospect, I should have used pressure-treated lumber for this, but I made due with materials I had on hand.

Roughly three hours later I had the stand screwed together, the engine mounted, and the entire exhaust system and fuel bottle fitted to the stand. The only thing left to do was drill and tap the exhaust pipe (a Miniature Aircraft #3966) for muffler pressure to the test stand fuel bottle so mixture settings generated on the stand could be translated into ballpark settings in the heli. While I thought about how to destroy a perfectly good pipe by drilling a hole in it, I milled about the local Builder's Square (Home Depot clone) in search of a few tools and materials. I happened to find a set of saw horses, made of metal with a wood (2x4) top on sale for $15 and a couple of well-made 4" C-clamps on sale for $5.50 each. Perfect, I thought, for suspending the test stand. The horses easily fold, so I plan to use them, mated with a small piece of plywood, as a workbench/table at the field after the break-in period.

I finally found some J.B. Weld Bond metal epoxy and decided it was time to destroy a perfectly good tuned pipe. Although I figured I could drill the tap almost anywhere (except directly across from its intake port), Mark Johnson suggested I drill my MA#3966 pipe on the side opposite to its intake approximately 1" toward the end of the pipe (exhaust port).

While I was at the local hobby store I decided to pick up a few Great Plains fuel pressure taps just in case I damaged the tap provided in the MA kit. When it came time to do the drilling, however, I noticed the threads on the MA tap were closer to machine-type than a self-tapping type, as well as larger than those of the GP taps. Rather than risk oversizing the hole, I decided to use the GP tap first, and this turned out to be the best choice. I measured the tap's base thread thickness with my calipers, found the nearest size in my drill set, and proceeded to use the Dremel at moderate speed to carefully drill the pipe.

Then, I mixed up a small amount of J.B.Weld, loaded the tap in a hex driver, placed a small amount of Weld on the threads, and used it as a threading lubricant. The tap went in beautifully and the Weld created a perfect seal around the tap and its included fiber washer. I noticed that the tap "seated" properly...the threads did not strip. Perfect. Many thanks to Mark & others on the heli list for the suggestion to use J.B. Weld.

Engine Break-In

After a productive day of building indoors one sunny weekend, I decided to take the test stand and OS 61 outdoors to begin the break-in process. This session revealed one minor flaw in the test stand design. I noticed that the vibration, which appeared to be the result of a resonance between the engine and test stand at 3/4 throttle and above, loosened the C-Clamps fairly quickly (no matter how tight I made them!), so the fix was to torque the clamps such that their handles overlapped each other. This allowed me to wrap some vinyl electrical tape around both handles, which prevented them from turning and loosening...no matter how much the stand vibrated. Part of this vibration was ultimately traced to a slightly out-of-balance propeller, so I placed it on the high-point balancer and used 320 grit emery cloth to carefully and uniformly sand the edges of the heavy blade to bring it into perfect balance.

All in all, I believe my first experience with the OS 61 was positive. The engine initially only required two blips of the starter to fall into a smooth idle and it performed beautifully with the O.S. setup-manual-specified carb settings. As recommended in the manual, and backed up in RCHM, I warmed the engine for approximately 30 seconds and then began the process of cycling the throttle from high to low RPM every minute. During the entire time, I closely monitored its mixture settings and throttle response, and as much as my instincts told me to at least try to fiddle with the settings, common sense told me that this was unnecessary...it had simply run beautifully.

After nearly two tanks, however, my starter loosened and threw the prop nut out of my peripheral vision and into the adjacent grass (!). I spent about a minute looking for it, but figured this was a sign meant to convince me to pack it up for the day, so I decided to abandon the search.

Once the engine was shutdown, I removed and examined the glow plug, which I found in good condition; the end of the glow plug wire nearest the piston appeared slightly golden in color...something I'd expect given the nature of the device. I also tipped the engine stand 90 degrees to the vertical to allow any oil to drain out of the pipe. Since I had never run a fuel with synthetic oil before this day, I was a unprepared to see as much oil as I did, but I've been told this is normal, as well as highly desirable during the break-in process.

I called GS Hobbies the following weekend and found that they had exactly three OS-61 prop nuts in stock. I bought all three...not only in case I reencountered misfortune, but because I hated the thought of driving 35 minutes to pick up a single nut. :-) While I was there, I picked up a package of throttle links and some control rod to fabricate an extended throttle control mechanism for the testbed. As well, I decided to invest in a few assorted MA metal balls and large plastic ball links to aid in construction of the heli. Then, of course, I gazed upon the die required to thread control rods, a neat fuel header tank kit, and some additional fuel line, so these items found their way into my bag as well. $75 later, I walked out fully equipped to continue construction of the heli and configuration of the testbed. That, I figured, was the most expensive nut I ever purchased. :-)

By the end of the day I had reached the point in the heli's construction where I needed to install the engine, so I figured I ought to run a tank or so through it before dark so I could continue construction the following weekend.

After a tank of stable operation, I pinched the fuel line to kill the engine (don't need any fuel in the carb to produce an accidental start while I play around the prop!) and refueled. I went over the entire testbed for any cracks or potential instability and pulled the plug for a brief examination, I reconnected the fuel line and glow plug heater and attempted a restart. No dice. I advanced the throttle slightly. Still no dice. Advanced the throttle to near midrange (and held it there, while reaching *VERY* carefully around the prop) and got the engine to fire briefly, but as soon as I brought the throttle down to a smooth idle, lowered the starter to the ground, idle, and walked around to the rear of the stand where I could better manipulate the throttle, the engine died.

I then checked and double-checked the current mixture settings with those provided in the operator's manual and tried to start it again. After I went through the above procedure one more time and was only able to get the engine to run with the glow heater attached (and then not very reliably at that!), I figured that the engine crankcase was flooded with fuel and/or oil and decided to wait until next weekend to give it another try. I cleaned the unit up, ran about a dozen drops of after-run oil through it to protect the bearings and crank from any fuel/oil that may have been left behind, and called it a day.

The next weekend I decided to replace the plug with another OS#8 and try again. The engine fired normally and ran well; so well, in fact, that I was able to put four tanks through it that day, leaning it out gradually, tank by tank, and by the last tank (9th) had it running still on the rich side with plenty of smoke, but with a lot more power and responsiveness. Since the last three tanks produced predictable results, I decided to end the break-in process here. I removed the 61 from the run-up stand and cleaned it up for installation in the heli.

The aforementioned problem (hard starting, unstable mid-to-high range operation, inability to run without plug heater attached) seemed to result from a combination of effects:

  1. Bogus plug. Although it still looks good to me, I think I may have damaged it in some way. Some on the list have speculated that the fuel may have deposited a glass-like substance on the plug that may have inhibited its ability to function correctly. Replacement of the plug allowed the engine to run without the plug heater but it didn't solve everything.
  2. Very rich idle mixture setting and prolonged time spent at idle, which may have allowed fuel/oil to pool in the crankcase, thus radically affecting the mixture as throttle position and airflow through the engine varied. High end needle probably kept a little too rich for the wx conditions but otherwise near target.
  3. My lack of experience with the 61.

At the end of the break-in, my 61 had the following mixture settings:


The OS61 runs acceptably smooth once it's leaned out for normal operation. Power is substantial and response is good. The 61 isn't the shaky mess people make it out to be. Yes, it has very high compression, and it produces a ton of power as a result, but once properly leaned and connected to a balanced mass, it runs quite smoothly.

The synthetic oil of Cool Power 15% contributed to a challenging, but manageable and safe break-in period. Seems as though the synthetic oil doesn't burn off at rich (low temperature) mixture settings as well as castor and I think it tended to pool in the crankcase during prolonged idle, which radically affected the mixture setting as throttle position (and airflow) were changed. All things considered, Cool Power ran quite cleanly. I do recall running straight castor years ago, and we had trouble finding the engine through all the grime after only a tank or two. :-) Restoration of the engine's original luster required only a simple wipedown with paper towels and some fuel.

As the 61's bearings are susceptable to rust and pitting to a greater extent than other OS products, the 61 requires its operator perform a shutdown ritual; pinch the fuel line to clear the carb and engine of any unburned fuel, put a few drops of after-run oil through the plug hole (directions warn about doing so throught the carb...apparently, the oil can cause some carb seals to prematurely deteriorate), circulate the oil with a few blips of the starter, and then turn the crank backward against compression to close off both intake and exhaust ports. This procedure has been verified by a fellow 61 owner who has experienced no problems with the 61's bearings after more than 10 gallons of fuel. Yes, the procedure is an official PITA, but I believe it will contribute to the long term health of this rather expensive engine.

A final thought on the break-in process. Had I not placed the engine on a properly configured test stand and become used to its peculiar operating characteristics PRIOR to installation in the heli, I surely would have driven the ship in during its first flight. Do yourself a favor...protect your investment and break-in your engine on a test stand!