One area that needs work for most helicopter pilots is engine mixture; especially with the O.S. SX 61H where most people run the engine overly lean. A smooth running engine is critical to solid performance, even to the point of sacrificing overall power. It cannot be stressed enough that smoothness from your motor translates to easier setup, particularly in the areas of tail rotor mixing and gyro gain.
To adjust this engine correctly, first close off the mid range needle completely and open the main needle one-and-a-half turns. To perform this adjustment requires that the engine be at full power, so either yourself or a helper should be comfortable with the helicopter and capable of autorotating!
The trick is to fully adjust the main needle mixture prior to altering the midrange, and to complete this task requires several extended full power climb outs. Hover the heli and smoothly go to full power while observing and listening to the engine. There should be a visible exhaust and the engine should sound consistent and "happy."
If you fail to get into the hover because of an overly rich mixture then adjust the main needle 1 click at a time and try again, or if it is overly lean during climb out (little exhaust trail) open the needle 1 click and try again.
Do not rush this adjustment and expect to make a use of a full tank of fuel to complete it. You may find that an adjustment in pitch at full throttle is necessary to prevent either an over-speed of the head or laboring of the engine.
When the last adjustment has been made, settle into a hover. With the midrange needle closed the SX will be lean. The midrange setting is less sensitive than the main and can be adjusted an eight to a quarter of turn at a time; however, for the first adjustment, land and open half-a-turn. Hover again and observe the exhaust trail which should be highly visible.
The engine should be running smoothly without any harsh or hard sound; additionally, there should be a nice exhaust trail visible in the down wash. The SX will let you know if the mixture is off by an audible cycling, almost as if it is alternating between 2 and 4 cycle running. Additionally, your tail will kick as if your gyro gain was too low, but there is not enough gain in a gyro to offset a rough running engine. If adjustment is required, land and make alterations 1/8 of a turn at a time, then repeat as often as needed. The idea here is to end up with a smooth running engine for those precise hovering maneuvers.
Once you are satisfied with both the needle settings, fly a couple of circuits to ensure the engine is fully warmed up. Perform an autorotation and listen to how the engine falls to idle. If everything is correctly set, the engine should fall cleanly to its idle state. Usually, prolonged operation above idle before falling off is caused by too lean a mixture. Open the midrange an eighth and try it over, or open the idle screw approximately 30 degrees. I have found that the idle mixture cam to be useful only for setting the mixture at idle and unlike other engines is ineffective in the midrange due to the mid-range mixture needle on this engine.
Personally, I like to run my SX on the rich side in the hover and use my transmitter mixing to open the throttle to compensate. In fact, my flight mode #2 (3D) looks more like a "W" than a "V." My settings are: one-and-a-half turns open for the main and five-eighths of a turn on mid range, the idle needle is set for a reliable idle.
If you would like to try running your SX engine a little on the rich side, then use an Enya #3 or O.S. #8 plug. Both of these plugs tolerate a rich mixture and will ensure a smooth running motor. The down side is that these plugs are around $6-$8 each, but they are long lived. The K&B plug is also an excellent plug, but does not over the breadth of needle adjustment that the Enya or O.S. plugs provide, and so your mixture settings must be more precise.
Changing fuels will have their effect on the mixture setting, so to preclude continuous fiddling always use the same brand and nitromethane content of fuel. Many flyers are using 30% nitromethane, but unless you absolutely need the power, $22 per gallon is too much to spend for the relative gain in power. I use 15% and am more than satisfied how my SX pulls my 11lb Tsurugi around.
The only recommendation I make on fuels is that it be either 100% synthetic or a combination of synthetic and castor. The castor based fuel is fine, but the oil gets absolutely everywhere and the castor itself produces a varnish. A synthetic oil may not have the high temperature flash point that castor has, but if you are burning the oil (black exhaust residue) you are too lean! Plus, the synthetic gives you slightly better power, but not so that you would notice in your helicopter.
Perhaps this will have been of some assistance to you and your helicopter. It may not be the only to adjust this engine, but I have been pleased with this fool proof method.
Happy hovering….
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Copyright © 1998 Chris Berardi. All rights reserved.
Revised: November 25, 1999.
