3 Needle Engine Tuning Guide (OS91hz)
Something I would have appreciated while I was on the steep learning curve of “rc nitro heli engines”, is a simple to follow engine tuning guide. This assumes you want your engine to run at optimal performance suitable for 3D flying. The reality here is optimal performance is very close to breaking point with these engines.
One analogy used to describe it would be model rocketry.
For this I’m going to base the tuning on the 3 needle OS91HZ engine but generally I’ll try to keep it generic.
Set your needles to factory setting and start the engine up. Bring the heli into a hover for a minute for so in order heat the engine (it wont be full operating temperature but warmer), then land and spool down.
- Idle needle (governor OFF) - if when you now spool up the heli splutters or hesitates (while it’s initially spooling up) you are rich and need to lean the idle needle. In order to ensure you are not too lean find the “splutter point” and then wind back until it’s smooth on spool up, it should relatively close.
- Main needle (governor OFF) – is active when the carb is fully open so you need to test the main needles effectiveness during a climb out with full collective/throttle deflection. There is a bit of an art to testing this needle, what you are looking for is a good climb out rate which also produces plenty of smoke but not dripping from the exhaust. Also the engine temp needs to be warm to hot but not melting. You should be able place your finger on the back-plate for about 5 secs easily.
- Mid needle (governor ON) - the mid need is really all about recovery and transition and in many ways it’s what makes the engine feel like it has power. My technique for tuning the mid needle is with a governor enabled, I’m not saying that’s 100% correct but it works for me. After the head has loaded the governor will open the throttle in order increase the head speed and once that is achieved it will drop back into the mid range. Now if the mid is reloaded again quickly (in a high power move) and is badly tuned the recovery of the engine/head speed will be slow and the heli will appear to bog. I use an aileron “tic toc” to load the head and then make decisions on whether to increase richness or lean based on the amount of smoke, temperature and power.
- General flight test – once I think I have tuning close I’ll fly normally (3D) for a few minutes to bring the engine up to operating temp under normal flight conditions. And as long as the engine is not too hot (test via the back plate) and it’s producing good power then generally I say it’s tuned.
These days I tune my engine once before every session (excluding the idle). Mainly because I’ve toasted a few engines, for the practice but more importanly for the power. I’m taking between 1-4 tanks to get it right..trust me it’s worth it.
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