Percentage RPM
04-04-2012 | 07:30 PM
#1
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Percentage RPM
I'm sure this question has been brought up plenty of times before, but could someone explain the concept of percent RPM on N1, N2 gauges? I have heard that 100% is intended to be the maximum rotation speed (since rotation speeds are so high in turbine engines), but if that is the case then why does the instrument go beyond 100% in most cases, and the green arc may sometimes even bleed a little over 100%? I'm surprised so little is explained in documentation or even in type rating classes (even the instructors are unsure in most cases!) especially when many pilots are transitioning from piston airplanes where RPM is actually denoted in revolutions per minute. Thanks.
04-04-2012 | 07:36 PM
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From: Curator at Static Display
For the same reason your speedometer in your car goes to 120, or your airspeed indicator goes beyond the redline of the aircraft.
Sometimes, the fuel controls don't work properly, or in a dynamic situation, allow rpm overshoot.
In other words, sometimes you need to know if the limit has been exceeded, and by how much.
Sometimes, the fuel controls don't work properly, or in a dynamic situation, allow rpm overshoot.
In other words, sometimes you need to know if the limit has been exceeded, and by how much.
04-04-2012 | 07:42 PM
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You want to be able to see if the system is in an over speed-pressure-voltage condition. It generally works that way for any system that has a max operational limit. In the case of RPM, if the gauge stopped at 100% you wouldn't be able to see if something has gone wrong and maybe it is actually at 105%.
Maybe somebody with more of an engineering background could chime in with details on how an upper gauge limit is decided in the design stage.
Maybe somebody with more of an engineering background could chime in with details on how an upper gauge limit is decided in the design stage.
04-04-2012 | 07:44 PM
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For the same reason your speedometer in your car goes to 120, or your airspeed indicator goes beyond the redline of the aircraft.
Sometimes, the fuel controls don't work properly, or in a dynamic situation, allow rpm overshoot.
In other words, sometimes you need to know if the limit has been exceeded, and by how much.
Sometimes, the fuel controls don't work properly, or in a dynamic situation, allow rpm overshoot.
In other words, sometimes you need to know if the limit has been exceeded, and by how much.
What about wartime limits? I remember hearing something about that when I was wrenching on vipers.
04-04-2012 | 08:24 PM
#6
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From: 737 CA
The limit isn't always 100%. The way I've always heard is 100% is the original design limit, and the other amount is what it can actually do. Say they upsize a turbine, it may bump the max limit from 100% to 105%.
Somewhat of an example: the original Space Shuttle engines were rated at 100% design thrust. Through a series of upgrades, they eventually gained some more power, allowing 104% of original design thrust continuous. This is the 'maximum power' that was used during climb.
Citation III's max N1 is 101.5%, the Dash-8 Q200 can go to I think 115% (we were limited to 105%) torque, etc. etc.
Somewhat of an example: the original Space Shuttle engines were rated at 100% design thrust. Through a series of upgrades, they eventually gained some more power, allowing 104% of original design thrust continuous. This is the 'maximum power' that was used during climb.
Citation III's max N1 is 101.5%, the Dash-8 Q200 can go to I think 115% (we were limited to 105%) torque, etc. etc.
04-04-2012 | 11:16 PM
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From: 747-400 FO
102.5%
The way it was explained to me was the same as a above. Initially, there was a design parameter. After building and testing the prototypes, P&W or GE would bump up what was allowed, often just to make it to the design resulting thrust. Remember, aviation has always been limited by the amount of thrust available.
04-05-2012 | 06:42 AM
#9
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From: Sr. VP of button pushing
Flyhayes, just think of it like horsepower. If it's a 100 horse engine, METO or MaxContin may only be 80. But everyone calls it a 100 horse engine.
We've got a max N1 of 117.5 and max N2 of 112.5. It's like cranking your amp up to 11.
We've got a max N1 of 117.5 and max N2 of 112.5. It's like cranking your amp up to 11.
04-05-2012 | 03:02 PM
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