N1 vs. N2

[uwochris]

Hey guys,

Is it possible to have a low N1 reading and a high N2 reading, and vice versa? If so, what may be the cause of this?

It's my understanding that the low pressure compressor and high pressure compressor spin independently of one another; however, it just seems that N1 and N2 readings should always be similar to one another.

All comments welcome,

Chris.

 

[JAFI]

chris,

Sometimes snow or other obstruction can jam the N1 blades so you can have a high N2 and no (zero) N1 with the engine running. (something to look for during engine start - can damage the engine due to reduced air flow [excessive heat]). On a PT6 engine you can have a stuck prop (Np) and the engine running.

If the engine is shut down in flight you can have a higher N1 (windmilling blades) than N2 (not windmilling as fast as N1 or jammed to read zero).

-----------------------------------
Be careful with "Blanket statements" about aircraft and engines. Some are designed differently and a procedure that is OK on one can damage another. Be specific.
-------------------------------------

JAFI

 

[Saabslime]

As far as I know, with a free air turbine and the engine running normal, N2 (gas generator) will always be higher than N1.

 

[KeroseneSnorter]

On most jet engines, if N1 and N2 match, you are probably overtemping the ITT. Under the right conditions (temp, alt.,etc)N1 and N2 get real close, but you do not see it much.

The values you see depend a great deal on the type of engine you are talking about.

 

[avbug]

No particular relationship exists between N1 and N2,or N3 for that matter.

 

[DrewBlows]

How do you know if N1 and N2 are matched? I would think that 100% N1 is not necesarily the same as 100% N2, because they may have different limits. Anyone know if this is true?

 

[mar]

That's a true statement

I would think that 100% N1 is not necesarily the same as 100% N2, because they may have different limits. Anyone know if this is true?

On the CF6-50:
Max N1 = 118.5%
Max N2 = 109.5%

The two "spools" are of physically different sizes therefore it makes sense that one would complete more RPMs than the other in the same amount of time.

Specifically, N2 is smaller (in diameter) than N1 so it's logical that it spins faster.

But in cruise, you'll see both set to values in the high 90s. (%)

From the FOM: <<"TACHOMETER" The engine tachometer system measures the speed of the low speed N1 rotor and high speed N2 rotor and provides the speed to indicators in the cockpit. N1 speed is a means of monitoring engine power output, engine condition, N1 rotor integrity, and N1 rotor overspeed. N2 rotor speed is a means of monitoring engine starting, engine condition, N2 rotor integrity, and N2 rotor overspeed. The system consists of an N1 tachometer transmitter (fan speed sensor), an N1 indicator, an N2 transmitter (core speed sensor) and an N2 indicator. The two transmitters are AC generators whose frequency is directly proportional to rotor speed. The N1 transmitter is mounted on the fan case at the two o'clock position. The N2 transmitter is located on, and driven by, the oil system lube and scavenge pump.>>

It's pretty simple actually. On start you monitor N2 as it's easier to spin the assembly with the smaller diameter.

Once started (at low altitudes) you get most of your power from the high-bypass ratio fan (N1) so you set your power with that.

Piece of cake.

 

[bafanguy]

uwo,


As Mar said, N1 & N2 are going to run at different speeds, hence, different %'s at most power settings. I took your question to be more related to abnormal circumstances where N1 is out of whack vs a normal N2, or the opposite, right ?

P & W put out a small book called, "Malfunction Analysis from the Flightdeck", P&W OI #192 ( my copy is circa 1968 ), for axial-flow, twin spool turbojet engines. It addresses some of the cases to which you refer. There are too many variations on a theme to list here, but they seem to be related to anything from abnormal bleed air, to fuel control adjustment, to blade damage/contamination, to cracked diffuser case, to bowed nozzle guide vanes, and will be accompanied by additional indications in EPR, EGT, and/or F/F in many cases depending on the phase of operation.

Then there's always simple instrument failure if only one parameter is bad. While jet engines are very simple to "operate", I don't think the same can be said when you start talking about "theory".

Sorry if I confused your simple question. Now, I'll go put my P&W OI #192 back in the basement where it belongs.

 

[mar]

Buck Rogers

P & W put out a small book called, "Malfunction Analysis from the Flightdeck", P&W OI #192 ( my copy is circa 1968 )...

Wow, they had jet engines back then?

Just kidding

Now that you mention it, Chris did ask about seemingly abnormal operations. I sort of missed that.

I'm still having problems with RTFQ.

Good post.

 

[DrewBlows]

Are N1 and N2 percentages based on the same speeds? In otherwords is 100% N1 the same RPM as 100% N2? What kind of RPM's are we talking about?

 

[bafanguy]

Are N1 and N2 percentages based on the same speeds? In otherwords is 100% N1 the same RPM as 100% N2? What kind of RPM's are we talking about?

DrewB,

No...

No...

I'm gonna tell ya...

This is coming off the top of my head ( precluding another trip to the basement ):

For the JT8D-7/9/11/15( don't hold me to this )...max N1= 8600 RPM +/-, and max N2=12,250 RPM +/-. And, I did this without a net, Boys & Girls.

Go ahead, ask me about the R-2800 and troubleshooting a pinhole leak in E chamber of a pressure-injection carburetor. I won't reply, but you can ask.

 

[mattpilot]

Go ahead, ask me about the R-2800 and troubleshooting a pinhole leak in E chamber of a pressure-injection carburetor. I won't reply, but you can ask.

What does the big red button do?

 

[LJ45]

many different engines

Can't make generalizations...too many different engines and limits out there!

 

[avbug]

Troubleshooting a pressure carburetor? Open a new can and put on a new carburetor.

The R2800 is a fine engine.

Drew, you keep asking the same question, over and over, and you keep getting the same answer. Over and over. See the relationship there?

There is no particular relationship between N1 and N2.

Typical operating speeds are in the 33,000 rpm range. A little faster than your typical 0-200.

 

[RJones]

Man ... who fricken cares! N1/N2 are specific to that engine and the limits are in the fricken manual. If they don't look right hand it over to maintenance!

 

[bafanguy]

Troubleshooting a pressure carburetor?

av,

Yep, believe it or not, that's the kinda stuff we were taught in DC6 ground school. Of course, now I couldn't tell you how to save my life. That level of knowledge was definitely more than anyone needed (other than a PFE or MTC).

As far as DrewB asking the question; maybe he just wanted to know the answer. As long as some TCE doesn't try to hold him responsible for stuff like the nice-to-know info...

Checked my old Douglas DC9 manual, Limitations Section: for the JT8D-9 engine ( just an example) 100.1% N1 = 8,600 RPM and 100.0% N2=12,250.

Just talkin' shop, RJones.

 

[bafanguy]

Wow, they had jet engines back then?

mar,

Yep...jet engines that produced billowing clouds of think, black exhaust, burned fuel as if mankind had no tomorrow, rattled the windows of dumbells who bought houses near airports then whined about the noise, and probably contributed to the near extinction of snail darter...

MANLY JET ENGINES !!! Not like the nancy-boy engines that get 85% of the thrust from the fan stage and make a noise like feedback from a cheap guitar amp...

 

[TurboS7]

Since when did Mar start talking about N1, N2. I am used to hearing BMEP, MP, Manifold inches, injected vs. non. Geezz, what it the world coming too. Avbug, there is a relation between N1 and N2. There are normal values based on normal operations, they are not exact as they vary with altitude etc, etc. We always check once the engine is stabilized for the normals values plus or minues 5%.If the values were not with in the parmeters I would shut the engine down and investigate. There just might be a ground guy stuck in the intake.

 

[DrewBlows]

Yep, believe it or not, that's the kinda stuff we were taught in DC6 ground school. Of course, now I couldn't tell you how to save my life. That level of knowledge was definitely more than anyone needed (other than a PFE or MTC).

Heres what I was taught in ground school.

The engines are PW306B. They produce 6050 lbs each. If something goes wrong the FADEC will take care of it.

Now you all know as much as me about the engines on an FRJ.

 

[Dumbledore]

There's another way of saying this...

No particular relationship exists between N1 and N2,or N3 for that matter.

...that I think is actually more accurate.

There is indeed a VERY specific relationship that exists between N1 & N2 and any other spools involved - as long as we're talking about a particular type of engine and that engine is in good working order, properly maintained. Abnormalities in the basic operating parameters of each are a key diagnostic tool.

It is correct also to say that these values operate within a range of values for each spool and it is variation from that range that might be a tip-off to a larger problem looming ahead.