N1 vs. N2

[TIS]

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...

Hey now, quit talking as if these are old news! Some of us still fly GII/GIIIs. There's something to behold in the sound of those Speys breathing hard in your ear as you pass through about 2.35 EPR on takeoff, their roar just beginning to reflect in a serious way from the runway you're trying to leave!

They've tried to shut me up with hush kits but I can still make as much noise as I used to by pushing the power up. Nothing like reminding the local residents why the property values near the airport keep going down!

TIS

 

[TurboS7]

I used to fly 20 series Lears and the 727. Now I fly one of those sissy jets.

 

[TurboS7]

I just bought a Dodge Ram Pickup with 5.9 liter engine with headers. I try to replace the noise I miss on the 727 and the Lears, at least my neighbors know when I am home.

 

[enigma]

Chris, In general assuming a normal engine, the relationship between N1 and N2 remains fairly predictable.

Hey guys,

Is it possible to have a low N1 reading and a high N2 reading,

Yes, but (assuming that the problem is not in the indicating system) the problem most likely is just high N2 because of a weak high pressure turbine. In that case, the N2 spool has to turn faster in order to compensate for it's lack of efficiency. In other words, the high pressure turbine has to turn faster than normal in order to acheive normal power. A weak N2 could be the result of a previous overtemp that caused abnormal high pressure turbine blade wear. I've seen over-heated turbine wheels that had lost a significant amount off of the end of each blade.

and vice versa? If so, what may be the cause of this?

It is possible to see higher than normal N1. I've seen it in JT8D-219's that were suffering from Bleed air controler or valve malfunctions. If a low pressure stage bleed valve is opening at high power settings, it will "unload" the low pressure stages and they will spin faster than normal. In the case I mentioned, a low pressure bleed valve was opening at just about take-off power. When it popped open, N1 jumped about three percent above normal, (It went from 92 to about 95 percent) and you could hear the engine sound change. It almost sounded like a soft compressor stall, as there was a definate "whomp" when the bleed opened.

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.

You would be correct in seeming that they will always be similar. Both increase with increased power settings and the attendent increased fuel flow. The rates of change in RPM are not linear, but do remain basically linked to each other. There really are waaaaaaaaaay too many variables here to try and get more detailed, but you are generally on the right track.

enigma

 

[enigma]

bump,

Darn, I was hoping that someone had something else to add just in case I am wrong on one of my suppositions.

enigma

 

[avbug]

As a general rule, when one changes, the other changes. How much isn't particularly relevant, as there exists no specific relationship between the two. Certain manufacturers will set one or the other as the primary reference source. However, for a given N1, N2 may be doing many things, depending on numerous variables. Is it possible to have one or the other high, and the other low. You betcha. Density altitude and airspeed, as well as thrust lever position all play major factors in that relationship, but either may be high, or low.

 

[enigma]

Avbug, can you think of any situation/malfunction that would cause abnormally low, or high, speeds? I can only think of the ones I related earlier, turbine damage or a bleed valve/controller problem.

enigma