Two questions..?

[Thunderbird8]

Hi

Can someone point me in the direction of a good explanation of what N1 is, and what it means from a pilots perspective, rather than just a technical explanation?

Number two, ditto for a tutorial on FLCH?

Thanks.

[jwenting]

N1 is simply the rotational speed of the front fan of a turbofan engine.
It's a way to measure engine power in relation to the maximum power the engine can put out.

FLCH is no more and no less than a button on some autopilot consoles that tells the computers to compute and execute a rapid economic change to a new set altitude (usually used only in descent).

[Thunderbird8]

Thanks - that's helpful.

So, if I may, the N1 button on the autopilot does what specifically when pressed?

[vulcan]

If there is an N1 button on the MCP then there will be some way to set a value for N1. This could be done by the FMC or possibly set manually.
When the button is pressed the engines will spool up to that preset value.

In many 3rd party airliners programming the FMC will set an N1 limit and have some way of activating auto throttles on take off. Activating the auto throttle effectively tells the electronic engine controls to spool the engines to that N1 limit, just the same as if you had pressed the N1 button.

FLCH Commands the auto pilot to maintain the current speed but climb/descend to the altitude set on the MCP. It changes pitch & thrust to maintain the speed.

So if climbing, the engines will spool up to climb thrust and the aircraft will pitch up to climb. As the speed falls the pitch angle will reduce to maintain speed.

HTH

[Thunderbird8]

Thanks for your help Vulcan.

[NikeHerk67]

To anyone who cares.
I certainly don't mean to disagree with anyone here, however; to say N1 is the speed of the turbofan in jets is not exactly correct....perhaps some clarification, for the sake of accuracy, should be made here.

This is from the Microsoft Learning Center.

N1 is the "percent" of maximum rpm of the low pressure turbine shaft of the engine. N1 is the value that best correlates to the power output of the engine. This is the value you set by moving the throttles to adjust your target airspeeds.

Note: I emphasized percent.

More specifically, 100% N1 is the "safe operating speed" of the turbofan, measured at the shaft that turns it and is not any specific speed in RPM. (It's not like Mach 1 being the speed of sound for instance). It's a varible like V1, V2, Vr etc is for various aircraft......take the 737-800 in FSX as an example. The engines can be spooled up to 117% N1 or more, but if a pilot made it a habit of going over 100% they'd hear about it from the airline mechanics and the front office for sure.
Also N1 can vary depending on the size of the turbine engine.......usually higher for smaller engines. For instance in a large jet engine, say the 747 for example, the speed of the turbine shaft might be 5000 RPM where, on a smaller jet engine, like the Lear, that speed might be 8000 RPM. I'm not stating exact speeds, just using these as examples to illustrate the differences in speed that N1 can be.

To say the MSFS learning center is 100% accurate would be an over statement. In one location they state that take off is 100% N1, but in the lessons they say 95% N1. Also, the maximum weight given on the reference chartes is for the real 737-800 and not the one in FSX.

Here's what they say in the lessons portion.

Takeoff Weight 174,200 pounds (79,016 kilograms)
Flaps set to 5
Takeoff Power 95 percent N1
Speeds V1 Vr V2 V2+15
150 knots 154 knots 162 knots 177 knots
Climb Power 90 percent N1

Somewhere in the design of the MSFS 737-800 there was a breakdown of communications. They use the takeoff weight as being 174,200 pounds, which is exactly correct for the 738 in the real world, however in FSX it's only 155,500. In order to load the 738 to that gross weight, the flight officers have to weigh about 250lbs, the crew about 2000, and each class of passengers 9999 pounds. Perhaps it is because they didn't add baggage, which would have made it possible.

Interesting though, is the fact that the 738 in FSX is modeled accurately enough that the V speeds can be accomplished in the so called over-weight configuration.

I find comfort in that fact, since I'm a purest at heart.

[jwenting]

N1 is the speed of the fan, noone stated it's measured in RPM however
As you say it's measured as a percentage of the maximum rated speed of the fan.

[InsyleM]

This topic was discussed at great lengh here
http://www.flightsim.com/vbfs/showth...highlight=outs
Hope it helps
-Jonathan

[skylab]

N1 this, N1 that.....doesn't anyone ever talk about BMEP any more ?!?! Or MAP? Oh, I guess I'm showing my age, ah?

[InsyleM]

N1 this, N1 that.....doesn't anyone ever talk about BMEP any more ?!?! Or MAP? Oh, I guess I'm showing my age, ah?

BMEP: Brake Mean Effective Pressure
This is the portion of the mean pressure in the power stroke of a recip engine that is converted to useful work at the crankshaft flange.

IMEP: Indicated Mean Effective Pressure
This is the acual mean pressure in the power stroke of a recip engine.

IMEP - BMEP = Lost to overcomb friction of the engine, to drive the cam shaft, oil pump accessories ect.

MAP: Manifold Absolute Pressure
Absoulute pressure in the induction manifold behind the throttle valve, this is direectly porportional to the power output of the engine.

SO how did I do for not looking any of that up in my books?
-Jonathan