Question about Nh and Nl

[dashflyer]

This has been bugging me.

Flying the Dash 8 with PW120A's, PW123's, and PW123C's. On the 123's we have the Nh and Nl gauges. It usually seems like the Nh is always higher than the Nl.

Every now and then I see one that has a Nl higher than the Nh by a few percent.

How come? Is this normal/harmful?

 

[avbug]

Think about your airspeed and power combinations when you see these relationships change, and that answers your question. Harmful? No. Not merely because one number indicates higher than the other at any given time.

There's a natural tendency, if you're used to manifold pressure vs. propeller RPM, to try to draw some kind of parallel, and that would be wrong, here. Probably the greatest tendency is to go back to the old "oversquared" philosophy regarding piston engines (which yours obviously ain't)...but even in pistons the oversquare rule of thumb is nothing more than a myth...and it has no bearing or relationship to turbine engines.

What moves your prop in terms of speed and load is a function of your power (torque) compared to airspeed...what's loading the prop? Go fast enough with a low enough setting, and the prop isn't loading up at all. This changes the relationship between the various speeds. The Nh, your gas compressor speed, is an indication of what the engine is needing to do to accomplish what it's accomplishing.

The P&W 120 is a free turbine engine; there's no set relationship between the gas generator speed and what the engine is accomplishing with respect to torque and N1. Sometimes the engine must work harder to produce the same torque value, and N1 is a function of where you set the prop speed. Climb high and hot, you may be working that engine a lot harder to produce the same torque, and the value may be considerably higher. Considering your airspeed, OAT, IAS (or TAS as appropriate for your monitoring program), and the load required to produce them, you'll see varying relationships when any parameter is changed. With N1 expressed as a percent just as Nh expressed as a percent, the relationship between the two depends entirely on the environmental factors and what's being demanded or required of each at any given time.

For now, just remember that no set particular relationship exists between the two; they're subject to change as they need to, which is characteristic of the free turbine engine

 

[dashflyer]

Thanks!!!