Jet Engine Question

Citationkid · December 11th, 2007, 22:19

There is probally a simple answer to this, but how is it that the N1 can be greater than 100%? How can the N1 spin (if that is even what it is measureing) at greater than 100%.

Thanks,

Alex.

Doug Taylor · December 11th, 2007, 22:22

100% of the rated power I figure.

BobDDuck · December 11th, 2007, 22:37

And engines are often derated.

I had a windshear recover in the sim and "max power" gave me 105%.

Citationkid · December 11th, 2007, 22:43

I sort of understand where you two are going. Here is what I do not get. The CJ2 cruises at an N1 setting of about 102%. I just do not understand how it could be 2% greater than the fastest it is supposed to spin. I understand being the engine being derated, but shouldn't the redline still be 100%?

Thanks,

Alex.

Stone Cold · December 11th, 2007, 22:53

Quote:

Originally Posted by Citationkid

I sort of understand where you two are going. Here is what I do not get. The CJ2 cruises at an N1 setting of about 102%. I just do not understand how it could be 2% greater than the fastest it is supposed to spin. I understand being the engine being derated, but shouldn't the redline still be 100%?

Thanks,

Alex.

Don't overanalyze it...that's what the engineers have already done...hence the 102%. The sad part is that the CJ2 cruises at 102%. We curise at 90% usually, at 40,000'-41,000' at .77 mach. I know it's not the fastest, but if Lear ever gets a new wing, we'll be up there with the Citation X.

B767Driver · December 11th, 2007, 23:10

Quote:

Originally Posted by Citationkid

but shouldn't the redline still be 100%?

Thanks,

Alex.

You would think...but that's not how jet engine compressor speeds are measured. 100% is not maximum compressor speed. It's just 100% of some arbitrary number (it probably has some reason to it...I think I knew at one time, but I can't remember now.)

Using that arbitrary number for a baseline measurement of compressor speed...the redline (max) might actually be 105% or so of that baseline number.

TFaudree_ERAU · December 11th, 2007, 23:13

Quote:

Originally Posted by B767Driver

It's just 100% of some arbitrary number (it probably has some reason to it...I think I knew at one time, but I can't remember now.)

Does it have to do with a standard atmosphere?

B767Driver · December 12th, 2007, 08:32

I went back to an old engine book I had and might have the answer.

Unlike piston powered airplanes, there is a large difference in the actual compressor RPM of many different types and sizes turbine engines. Using "percent RPM" it makes it possible to use approximately the same RPM value for approximately the same power setting on a large variety of engines.

Percent tachometers are designed to read 100% when the tachometers drive shaft is turning at 4200 RPM. To find the actual RPM, divide 4200 by the tachometers drive gear ratio...let's say it's .112. 4200/.112 would yield a compressor RPM of 37,500 RPM when the tach was reading 100%.

So...the 100% value is not a function of engine limitations...it is derived using the tachometer's calibrated value and drive gear ratio.

Hope that helps.

Citationkid · December 12th, 2007, 22:30

Quote:

Originally Posted by B767Driver

I went back to an old engine book I had and might have the answer.

Unlike piston powered airplanes, there is a large difference in the actual compressor RPM of many different types and sizes turbine engines. Using "percent RPM" it makes it possible to use approximately the same RPM value for approximately the same power setting on a large variety of engines.

Percent tachometers are designed to read 100% when the tachometers drive shaft is turning at 4200 RPM. To find the actual RPM, divide 4200 by the tachometers drive gear ratio...let's say it's .112. 4200/.112 would yield a compressor RPM of 37,500 RPM when the tach was reading 100%.

So...the 100% value is not a function of engine limitations...it is derived using the tachometer's calibrated value and drive gear ratio.

Hope that helps.

Thanks a lot, that's about what I found also.

Alex.

Jeff_S_KDTW · December 14th, 2007, 18:03

Quote:

Originally Posted by B767Driver

I went back to an old engine book I had and might have the answer.

Unlike piston powered airplanes, there is a large difference in the actual compressor RPM of many different types and sizes turbine engines. Using "percent RPM" it makes it possible to use approximately the same RPM value for approximately the same power setting on a large variety of engines.

Percent tachometers are designed to read 100% when the tachometers drive shaft is turning at 4200 RPM. To find the actual RPM, divide 4200 by the tachometers drive gear ratio...let's say it's .112. 4200/.112 would yield a compressor RPM of 37,500 RPM when the tach was reading 100%.

So...the 100% value is not a function of engine limitations...it is derived using the tachometer's calibrated value and drive gear ratio.

Hope that helps.

That's interesting. Thanks.

WalterSobchak · December 14th, 2007, 19:04

http://youtube.com/watch?v=akaD9v460yI Spinal Tap has your answer.

BajtheJino · December 14th, 2007, 20:10

Quote:

Originally Posted by WalterSobchak

http://youtube.com/watch?v=akaD9v460yI Spinal Tap has your answer.

Good call.

December 11th, 2007, 22:19	#1
Citationkid Senior Member Join Date: Aug 2003 Location: Delaware Posts: 1,298 Blog Entries: 5	Jet Engine Question There is probally a simple answer to this, but how is it that the N1 can be greater than 100%? How can the N1 spin (if that is even what it is measureing) at greater than 100%. Thanks, Alex. __________________ My airliners.net pics

December 11th, 2007, 22:22	#2
Doug Taylor Agent Smith Join Date: Dec 1969 Location: KSDL Posts: 39,396 Blog Entries: 1	Re: Jet Engine Question 100% of the rated power I figure. __________________ Doug Taylor http://76school.flyblog.com (old!) http://30west.flyblog.com (updated 11/28)

December 11th, 2007, 22:37	#3
BobDDuck Old Skool Join Date: Sep 2003 Location: Left Seat Posts: 5,267	Re: Jet Engine Question And engines are often derated. I had a windshear recover in the sim and "max power" gave me 105%. __________________ TBJC08 The Gear Monkey The Gear Monkey Store

December 11th, 2007, 22:43	#4
Citationkid Senior Member Join Date: Aug 2003 Location: Delaware Posts: 1,298 Blog Entries: 5	Re: Jet Engine Question I sort of understand where you two are going. Here is what I do not get. The CJ2 cruises at an N1 setting of about 102%. I just do not understand how it could be 2% greater than the fastest it is supposed to spin. I understand being the engine being derated, but shouldn't the redline still be 100%? Thanks, Alex. __________________ My airliners.net pics

December 12th, 2007, 08:32	#8
B767Driver Old Skool Join Date: Sep 2003 Posts: 1,830	Re: Jet Engine Question I went back to an old engine book I had and might have the answer. Unlike piston powered airplanes, there is a large difference in the actual compressor RPM of many different types and sizes turbine engines. Using "percent RPM" it makes it possible to use approximately the same RPM value for approximately the same power setting on a large variety of engines. Percent tachometers are designed to read 100% when the tachometers drive shaft is turning at 4200 RPM. To find the actual RPM, divide 4200 by the tachometers drive gear ratio...let's say it's .112. 4200/.112 would yield a compressor RPM of 37,500 RPM when the tach was reading 100%. So...the 100% value is not a function of engine limitations...it is derived using the tachometer's calibrated value and drive gear ratio. Hope that helps. __________________ A self described gym rat. "I got next."

December 14th, 2007, 19:04	#11
WalterSobchak Senior Member Join Date: Oct 2007 Posts: 503	Re: Jet Engine Question http://youtube.com/watch?v=akaD9v460yI Spinal Tap has your answer. __________________ 4 forces of flight: Stall, Spin, Crash, & Burn

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode