Delta N1 or Delta Ng

[Jack Carson]

Can anyone enlighten me on the significance of using delta N1 as a limit on some Arriel engines? Also, how is this number computed? Is it a function of temperature or pressure altitude or both? :confused:

[Um... lifting...]

Jack-

"Measures the true Ng and corrects it electronically for changes in temperature and pressure because changes in temperature and pressure affect the air flow through the engine."

That is the answer to the exam question, so it must be right. In the Dauphin N2 & higher Arriel installations, what this does for you is permits you to use an arc on a gauge as your limit, regardless of ambient conditions. The "Delta" refers to the differential between (in the case of the N2 and N3 Dauphin, anyway) whatever you have set compared to Max AEO takeoff power. In other words, here are the max takeoff limits for the N3: Delta Ng, Torque, and T4 respectively. 0, 100%, 912C. The max continuous dual-engine limit in the N3 is -2.2. For OEI ops, max continuous is 0. 2-min limit is +1.2, 30-sec limit is +4.8.

The zero mark on the gauges (there's one for each engine) is at the 12:00 position, negative to the left, positive to the right, with colored arcs for the various zones, so if you're AEO, you operate on the left side of the gauge, if you're OEI, you can operate on the right. In the N2, it's not quite so simple, as the 0 mark is 30-minute OEI power and max takeoff AEO, but for the purposes of extracting yourself from a bad situation when a donk quits, it suffices. You just have to remember which machine you're flying and adjust before you reach your time limit. The two gauges are tied together for comparison as well, and when the Ng values disagree by more than 7%, there is a gong and warning light, and depending upon if it's a gauge failure or an actual engine failure, obviously your actions will vary and of course there's procedural protocol for each situation.

It's a bit odd to use at first if you're accustomed to using actual values for limitations, as is the dual needle torque gauge which has one needle to "1", and the other can be selected between "1" and "1+2", but not "2" alone. Once you get used to it, it's fairly intuitive. Actual Ng figures are displayed on a digital display in the lower portion of the gauge. The N3 also has a variable Nr law, which varies the Nr in different flight regimes and can be manually selected up to 365rpm (which is higher than the normal range) for maximizing performance. I was used to using actual values for Ng, and percentages for torque and Nr. In the N2 and N3, they use Delta values for Ng, percentages for torque, and actual Nr. Of course the French just do things differently, so what would you expect?

[Jack Carson]

Thank you for your quick response. We fly Agusta 109E's. The Agusta displays N1 as a vertical tape with digital N1 at the bottom. At the top of the display a digital delta N1 is presented. The delta N1 display does't even light up until we see 93% N1 on the tape. We typically operate at sea level with moderate temperatures between 0 and 30 degrees C. As such we never see reach our maximum continuous delta N1 range of -8 to -2.4. Our 5minute takeoff rating of -2.4 to 0 is never reached. In short the delta display remains blank most of the time.
Jack

[Um... lifting...]

Boy, you see what I mean about the differing gauge types? Our gauge, the analog steam gauge part is the Delta Ng (which comes off the peg well before you reach any critical range so you can see it coming) and the digital is the actual Ng. We operate offshore, so we're consistently heavy and hot and so are in the takeoff range just about each time we pull pitch to break ground.
I would be curious if your display has any colored warnings or ranges once you light it up (or is that on the tape)... tell you what, why don't you load up real heavy and pull a heck of a lot of power. Assuming you don't get the engineers chasing you around the hangar with a spanner as a result and nothing bad happens and you're not mad at me for the suggestion let me know how it came out!;)

[DeltaNg]

Sorry, Haven't a clue.

Sounds good though :E

[Um... lifting...]

Sorry, Haven't a clue.
Sounds good though :E
Caught out, then, eh?:ouch:

[Jack Carson]

Yes the box around the delta N1 changes color when a limit range is reached. The same goes for the N1 box at the base of the tape. In addition the tape rescales, the red line at the top of the tape, depending on the situation i.e. duel engine, OEI, start up. TOT, Torque, Nr, N2 all do the same.

[Um... lifting...]

Sounds a little less pilot-friendly to me, but I'd have to use it for a while to be sure. Interesting anyway, how the different manufacturers package what is essentially the same data.