Detailed Material on Specific Arriel Engines, Delta Ng

charlicopter · February 7, 2010

Hey all, I'm studying AS350 BA, B2 & B3 systems and I've gotten around to the engine at this point.

Anyone know where I could get some rich technical and operational information on these engines? Digital manuals? Etc...

Additionally, regarding Delta Ng:

I believe that I have a grasp of the function of the gauge but I need some guidance on the finer details:

Basically it is an N1 analog that interfaces with a computer. The computer takes readings of pressure and temperature and applies an algorithm to N1, weighting it against the ambient conditions and presenting N1 on the dial adjusted to relevant values. The digital N1 underneath the analog is unadjusted and is used during startup and as a cross reference so that you can verify the Delta Ng values.

In a way, I imagine that a manifold pressure gauge in an R-22 could be redesigned to do the same thing: Manifold pressure indirectly tells us how much power the engine is producing by measuring the vacuum pressure in the intake. We can sort of deduce the power of the engine by knowing what maximum power is possible at wide-open-throttle at specific air densities and interpolating. But since air density is variable, then assumed power produced at a specific manifold pressure is variable. Hence the chart to inform us of specific MAP-Power relationships.

If we wanted, we could make a Delta-MAP gauge that would take temp and pressure and bias it against MAP to tell us directly how much power the engine is projected to be putting out...

So this is where my understanding breaks down... Turbines are different. In a turbine I don't fully comprehend what happens to the speed of the engine as DA changes. At various DA scenarios, Ng will change for a given amount of power. If the engine moves to a higher DA, then it seems to me that it must turn faster to compress and burn additional air in order to maintain power... Since I understand Ng (or N1) to be important for structural reasons (namely centrifugal) then it follows that the Ng limits do need to be considered at different DA's (mainly higher DA). BUT DON'T THE STRUCTURAL LIMITS ON THE COMPRESSOR AND TURBINE STAY THE SAME? Why do we have to adjust the indicator dial to compensate for DA? In other words: Won't we hit the structural Ng limit at the same RPM regardless of DA?

RtrCFI · February 9, 2010

Let me see if I can take a stab at this. DISCLAIMER: I don't have mountain experience in helicopters. My reply is based on my time at sea level in the AStar, as well as operating a turboprop (Cessna Caravan) up at 18,000. So, I may be wrong. Also I apologize if I ramble about stuff you already understand.

So, first off, the Delta Ng gauge. To understand the DNg gauge, please study the instrumentation on the Astar BA, B2, and B3 models. The BA simply has the three "power limit" gauges: Torque, Ng, and ITT. You probably know this, but any of these items can be your power limiting factor on given days. When I flew the Astar, it was torque in the winter and Ng in the summer. We never had one temp out, but it happens in the newer Caravans. Now, the catch to all this is you not only have to scan all three instruments "simultaneously" but max Ng changes based on density altitude. (As you figured out) This is similar to an LMP placard in a Robby or Schwiezer. So, in the BA, we had a little flip up chart on the dash that allowed us to calculate the max Ng for the pressure altitude and temperature.

In the B2, they introduced the DNg gauge. As you said, this is a computerized performance calculator. -3.5 is always max continuous power (upper limit green arc) and 0.0 is always max takeoff power. (red line) Think about how different it would be to fly the two gauges. On the early model, you had to calculate the max Ng for that day, and store it in your memory. Any red lines on the gauge have to be compared with performance numbers for that day. And oh yeah, don't forget the other two gauges. With the DNg, red line is always max. Easy. As you said, if you NEED to know the actual max Ng for that day, you can press the button on the ground. However, I never saw a practical use for this beyond the system test.

Even better is the B3 and the FLI (First Limitation Indicator) display. A computer screen shows one giant dial (as opposed to three little ones. The computer decides which limitation you are closest to (torque, temp, speed) and gives you values based on your "first limitation". This system is so easy a Cirrus pilot could use. (Sorry, couldn't resist)

Moving on to the why. This is more hearsay and less verifiable data. Think piston for a second. The best example of this is a Schweizer C model. (Not Cbi) A C model has a max hp rating of 190, but the engine is "de-rated." That is, the engine itself is capable of producing more than 190, but the other components (short-shaft, transmission, rotor, etc) can't take it. On a cold day, you could pull more MAP than the LMP chart indicated. You are exceeding a limitation. The engine could do it, but who knows for how long, and what other components are being compromised. However, on a high, hot, heavy day you might not be able to obtain 190 hp, despite full throttle and collective. The density altitude reduces the ability of the engine to produce power, just like a turbine.

Now apply this to a turbine. Lets say that the turbine itself could sustain Ng speeds of 110%. (Arbitrary number) However, if they were to spin at these speeds, they would produce more horsepower than the engine/aircraft are certified for. Simply put, it's not the speed of the turbine that is driving the Ng limitation, it's the resulting power that these speeds produce. Make sense?

Back to pistons. On a 300C you may have an LMP of 24" on a cold day at sea level. However, at 100 degrees you are authorized to pull "FT=Full Throttle" and pull collective until the rotor decays, often with MAP settings of 26-27". What happened? Is the engine somehow able to withstand more suction on a hot day? Nope. But, those higher MAP settings won't produce the maximum HP limitation, due to the increased DA.

Again, it's not the actual Ng speed being limited, but the engine power production that results from that Ng speed.

Finally, try and get a hold of a mechanic's inspection procedures for a limitation exceedance. (Not a word) That will clue you in to what components can't stand power over max. For instance, if you exceed LMP in a Schweizer, the mechanic is required to inspect the tail rotor, TR attachment bolts, drive shaft, etc.

IN SUMMARY:

-In both a piston and a turbine, power production goes down as DA goes up.

-At higher DAs, more power is required to produce max horsepower.

-It is this max hp, not the actual Ng (turbine) or MAP (piston), that limit what the pilot can do. (On a de-rated engine)

-The DNg gauge automates calculation, and simplifies interpretation.

I hope that I'm right, and that it helps. Please give a big shout-out to John Simone at Ellington.

Edited February 9, 2010 by RtrCFI