FlyUtah

I have a "classic" P-120 with idle RPM and MAX set at 35K and 123K respectively. The turbine is consistently overshooting 123K, (up to 124,500) and undershooting idle, (down to 33,000). The over/under isn't just happening during rapid acceleration or slow down, but rather when the throttle stick is at a constant position.

I haven't noticed this with my other P-120. Has anyone else seen this before?

Cheers,
Mark West

Thud_Driver

My Feedback: (1)

There's pretty much only two things that will cause that. One is an erratic rpm sensor and the other is the pump.

I once caught a bad rpm sensor when every time I leaned over the engine and blocked the sun, the rpm would change. Sounds funny, but I took my hat and covered the engine and took it away while the engine changed rpm each time. That engine also experienced a 10K over speed shutdown due to the sensor problem. The rpm sensor on that engine is an IR Led transmitting thru the hole in the spinner nut to a IR Led receiver diode.

I'd bet on the pump not being as good as it once was. Those numbers aren't great but also not that bad. Where the problem arises is if the engine rolls back too far when you pull the throttle back to idle in the pattern or on final, the ECU may turn the engine off. I'd have to go look, but as far as I remember, the cutoff is 29K.

SushiHunter

How is the rpm sensor set up on a turbine?

Thud_Driver

My Feedback: (1)

On JetCat's, it's either the optical sensor as above or a Hall effect magnetic sensor. Both work just like the tachometer you might have for a prop airplane. ECU sees the pulses from the sensor and does the math in software. There's nothing the user can service or calibrate.

SushiHunter

Yes, I use the optical sensor (futaba SBS-01RO) on my prop plane. However, the manufacturer is saying that the same optical sensor can also be used with a turbine power plant. Interesting to me is that the manufacturer indicates that in order for the optical sensor to work properly, it needs to be utilized in an environment which is lit (has light) and within roughly 6 inches of the prop (turbine blades if applied to that type of power plant. How would this be accomplished on a turbine? The optical sensor that I'm using on my prop planes have a max rpm rating of 300,000, which it was supposedly designed that way so it could also be used with a turbine.

Thud_Driver

My Feedback: (1)

Frankly, I don't know. Likely, you would need to eliminate the FOD screen to use it and it's too sizeable to have inside the FOD screen or real close to the inlet.

I suggest you take your question to the main Jets forum so we don't hijack the OP's thread.