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Trouble understanding N2 limitations and control of N2 during flight 250-c20

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Trouble understanding N2 limitations and control of N2 during flight 250-c20

Old 9th Aug 2015, 16:18
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Trouble understanding N2 limitations and control of N2 during flight 250-c20

A bit embarassed to be asking this question, but I can't seem to find good answers elsewhere. And I'm trying to avoid tearing up equipment.

I'm having an issue understanding exactly what the N2 limitation is for the 250-c20b is. I've looked at a few different manuals from helicopters with the same engine and they all say something slightly different.

The Jetranger manual says:

Power on:
Max: 100%
Min: 97%

An army manual for the OH58 says:

Max: 100% and 110% for 15 seconds during autorotation. I'm assuming they mean practice autorotation, but how is that even going to happen as you roll the throttle off?


I am flying an OH58 in illinois.

The issue I am having is that for the ag (spraying) work I am doing I am required to fly the helicopter rather aggressively. If I set the governor just below the red line, every time I pull out of a field and let some power out, the N2 takes off and climbs past redline, if it set the governor at the bottom of the green, it does a little better, but on windy days the n2 still climbs past red line when pulling out of a field, but now the N2 will droop well below green (NR still in green) when raising the collective and asking for more power during the dive back into the field.

I have also tried letting out my power about 3/4 of the way through the field and slowing down, helps a little, but not much, and I've also tried not touching the collective at all, and flying a little slower so that I don't need to reduce pitch at all while turning. This is also a little better, but the N2 will still go past red during the climb out of the field.


I am being as ginger as I can with the old girl and I still can't seem to keep the N2 stable. Unless I'm just thinking about this too hard. I've watched others doing the same job or similar jobs like hauling xmas trees and you can hear the engine doing the same thing I am describing.

Maybe I just don't understand the limitation correctly and what I am describing is acceptable because you are letting power out and not trying to drive the rotor system above 100%? I thought at first maybe the governor is just slow to adapt but we have 5 aircraft and they all behave about the same.
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Old 12th Aug 2015, 02:30
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The limitations are only for steady-state operation. Read the transient limits. You can safely and legally exceed 100% for a few seconds. I don't recall the exact time for a C20B off the top of my head, but it's more than you should need to get the RPM back to 100%. Reducing RPM is not something I would recommend.
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Old 12th Aug 2015, 04:56
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This is what I have written down for the Jetbox, it's a few years old though so it may have changed, but I think it answers your point.

TURBINE (N2):​Maximum 100% Power on
​Minimum 97% Power on
​Minimum 95% Power on <3000lbs 5 sec max
​Transient (max 15 sec) ranging from 107% at 32% torque or less to ​103% at 100% torque
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Old 12th Aug 2015, 05:25
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Max: 100% and 110% for 15 seconds during autorotation. I'm assuming they mean practice autorotation, but how is that even going to happen as you roll the throttle off?
If the auto is started by dumping the lever before rolling the throttle, the sudden unload will result in N2 climbing, until the governor reduces the fuel in an attempt to get the RPM back in limits. The N1 might be almost at idle, but the rotor being driven by the airflow has completely unloaded the N2, so it might go high.

Often, also, you don't get a needle split, and the N2 will follow the rotor RPM up into the red range in turns and flares.

Check that you have a Bendix FCU, if you are running a Chandler Evans (CECO) it will always drag its feet when it comes to maintaining RPM.
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Old 14th Aug 2015, 06:30
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The JetRanger and the OH-58 may have similar silhouettes but they are two very different beasts for two very different purposes. The former is built and operate to military standards while the latter is built, certified and operated to civilian standards. It therefore follows that you will find differences - sometimes significant differences - in the respective flight manuals.


Having said that, you say you fly an OH-58 and therefore you should be using the flight manual for this type - ignore the JetRanger flight manual limitations.


The problems you describe regarding main rotor over- and under-speeding would certainly seem to indicate a problem with the power turbine governor and/or the N2 rigging - this should be the area of focus to start your troubleshooting.


As Ascend Charlie has said, first determine if your engine has a Bendix or Chandler Evans fuel control system. If it's a CE system ensure that the PTG is properly bled.


For both systems check for slop/backlash in the entire N2 linkage and ensure the rigging is correct. In a properly rigged and functioning system the over- and under-speeding of the main rotor should be minimal - despite flying the helicopter "rather aggressively". Let us know what you find.



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Old 22nd Aug 2015, 15:12
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Thanks everyone for the replies.

Gomer Pylot: This is what I originally thought, and I have been flying accordingly for years now. But I heard in passing that using the transients on purpose was not legal/approved for any system, and is what prompted me to make this thread.


I looked at the fuel control in a few of the aircraft and they have Honeywell data plates on them.
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Old 24th Aug 2015, 01:29
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NEVER NEVER be emabarrassed to ask a question where Aviation Safety or Aircraft Operational Issues are the topic.
Better to be looked at as a dummy than to be one. We all started at Zero hours of flight time. And Yes I still ask dumb questions.
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Old 26th Aug 2015, 01:27
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You're not using the transient limits on purpose. That would be pulling transient torque deliberately, or some such. You make the control inputs necessary to make the machine do what you need it to do, and if the N2 moves, so be it. You cannot deliberately change the N2 outside limits if the governor, fuel control, and linear actuator are set correctly. The N2 just does what it does when power is changed. That's why the transient limits are given.
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Old 10th Sep 2015, 17:31
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I am curious if anything has changed or what you have done about this...


I also fly Ag in an OH-58 and a 206B3... The OH-58 I fly is actually very stable with regards to the N2 fluctuations, although I have cyclic issues that make it a little wild at times...


I don't have as much time in the 206B3 but the one thing I did notice right away while flying it a little is that the N2 runs all over the place compared to our 58. Just like you said, I set my N2 just at the redline or a hair below and try to leave it there as much as possible. When I am lowering power I get a rise on the N2 that goes up to about 102%, but I am gentle with it because I feel like if I get too aggressive its going to spike a lot more... when I come back and land I find that the N2 still gets sucked down quite a bit and sometimes out of the green... Nr is within green but I still get a little nervous when I see that N2 droop so much... It does come back, but 3-5 seconds seems like an eternity while I am waiting for it. Our mech says is no big deal and that I need to just let the gov do its thing...


anyway I am just curious if you have come too grips with it or if you have figured out something that might be going on to cause the fluctuatuions...


on a side not, how big are the loads your taking off the truck and what kind of torque you pulling? I am curious cause I feel like I am pulling the guts out of the thing everytime I come off the truck...as carefully as I can lol
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