Hot Temps, Weak Engines, and Heavy Loads

[Medhawkdriver_25]

Here is a hopefully thought provoking questions pertaining to Rotary Wing aerodynamics, power management, and aircraft/engine performance.

If an aircraft is limited by temperature (T5, TOT, TGT, etc…Let’s call it T5 for now), will increasing rotor rpm by 2% (as is the case in many aircraft in “Cat A” mode) INCREASE or DECREASE T5 for a specific maneuver? (e.g. hover IGE, hover OGE, takeoff from a confined area, etc.)

Discussion: If you are temperature limited, ability to takeoff is in question, and you have a rather wide range of possible “normal” Nr, say 100%-107%, are you better off at a higher RPM or lower, and why?

[spinwing]

Mmmm ...

Why are you asking this ? ....

If you are temperature limited, you are temperature limited at whatever Nr you choose .... you may well be MORE temp limited if you use 102% because you are demanding more speed from the Nf section.

Use of 102% Nr is generally to allow the pilot more inertia in a rotor system to counter Nr droop should a power unit fail during a critical flight regime such as Cat A departure, Winching or external load operations.


:confused:

[HeliComparator]

I'd say definitely go for the highest available rrpm. The Nf or N2 turbine acts as a dam to the hot gasses, the slower it goes (for a given fuel flow) the more damming effect and the higher the T5 - same heat input, slower air flow rate. Although the torque would go up slightly at lower rrpm, the overall thrust is ~ torque x rrpm and is likely to be less at lower rrpm.

HC

[Mast Bumper]

I venture to claim that this discussion is pointless unless we know specific aerodynamic properties of the rotor blades.

Take a step back for a moment and think about what "100% Nr" really means. Some aircraft 100% meas they are at their most efficient in a hover, others are more efficient at high speed. In the case of an S-76, you get more lift per shp at 100% in a hover, but the normal Nr is 107%. If I remember correctly, the D model will have variable Nr to take better advantage of varying lift-to-power requirements as well as reduce the noise signature.

[Shawn Coyle]

Back to basics:
Power is a function of torque times rotor RPM times a constant.
In this case, power is limited by TOT. Increasing rotor RPM will mean that you will have lower torque and a lower collective pitch setting. No more power going to the rotor.
Power is needed to overcome the drag on the blades, and lift is a useful byproduct.
You might have a lower blade pitch setting with the increased rotor RPM, but unless there's some significant reduction in CD with CL remaining the same or not going down the same amount you're not going to gain anything.

[HeliComparator]

OK Shawn, back to even more basics, lets leave aside any rotor efficiencies and just look at the power output from the engine's N2 shaft where T5 is limited. I maintain that there isn't constant power available regardless of N2 rpm - in a "reductio ab absurdum", taking the N2 towards zero, the torque would not tend towards infinite (even assuming there is no surging etc), so clearly there is an optimum N2 where power from the engine is maximised for a given limiting TOT. My gut feeling is that the power would be greater at higher N2 rpm, since fuel flow could be increased for a given T5 (because of greater air flow). The limiting factor may be the stresses and structural integrity of the N2 turbine wheel and / or the way the turbine guide vanes are tuned.

But not an engine thermodynamic expert so don't shoot!

HC

[Medhawkdriver_25]

Thank you gentlemen (and ladies?) for your responses. This is great stuff!

The aircraft I personally am dealing with IS indeed an S76...A model. I was intrigued to read that an S76 rotor system will develop more lift per shp at a hover at 100%Nr. I am relatively new to the "76" and my training was far from comprehensive, which is why I am left posing questions like this one. One point that WAS brought up in training, though, was that the reason for reducing Nr to 96% after establishing a positive rate of climb (Continued takeoff after engine failure) was that the rotor system was more efficient in a climb at 96%. That being said, perhaps lower is actually better? On the other hand, that 96% is more efficient in a climb may be based on the assumption that by THAT point in the procedure there is a significant amount of translational lift helping the situation?

In the lift equation, Lift becomes a function of Velocity squared, right? So, theoretically, higher revs = more lift. I do understand that there is a point of diminishing returns which plays into what Mast Bumper is saying. Maybe it IS pointless to discuss unless we know the airfoil. Mr. Bumper, can you divulge your source on the main rotor aerodynamic properties of the S76? Does anyone know why the recommended Nr of an S76A++ is 107%, yet the A model is lower?

Thoughts?

[BlenderPilot]

I don't speak because of my knowledge, but from experience.

In my job, I operate hot and high, (10-12 K DA's) and the helicopter I fly is usually limited either by Torque on colder days and TOT on warmer days.

The helicopter has a NR 100 to 102% switch, you are supposed to use 102% for Takeoff and Landing, always.

But on warm days, we will not use 102% because it will raise the TOT to the point we wouldn't be able to take off without reaching the TOT limit. Some argue that that extra 2% helps with the take off, it doesn't, and it just gives you less TOT margin on Hot and High conditions.

Now what that extra 2% will give you, is a little more pedal authority, a little, but every bit helps when you are short of it.

The aircraft I am talking about is a Agusta 109E.

[the coyote]

I believe the 107% Nr was due to the increased gross weight of the A++ and subsequent models - to counter coning angles and disc loadings and stuff....

[Gomer Pylot]

The maximum gross weight of the A++ is identical to the A and A+, it does not increase.

[212man]

The maximum gross weight of the A++ is identical to the A and A+, it does not increase.

Correct, but I think for expediency the A++ upgrade was certified on the back of the C - as they both have the Arriel 1S1 - therefore the performance data is all predicated on 107% and 100%. Could be wrong.....

[John Eacott]

Does anyone know why the recommended Nr of an S76A++ is 107%, yet the A model is lower?

The original 76A had a Tech Bulletin (back in about 1981) which recommended 107%Nr for cruise. We certainly used this setting, and obtained about 5kias greater cruise speeds at 8-9000ft to the Bass Strait compared to 100%Nr. We were always temp limited with those early C30's, regardless ;)

[unstable load]

Chat to some of the logging guys about NR/Max lift ratios and you may be surprised as to where a rotor is most efficient.

[Gomer Pylot]

Yes, the data is predicated on 107%Nr, and that's where it is supposed to be operated, but that Nr is not because of increased gross weight.

[the coyote]

Wasn't the A model 10,500lbs MAUW and the A+ and A++ 10,800lbs? Stretching my memory. I think with 11,700 lbs hanging from the rotor head on the later models you don't want to go beeping the Nr back too much from 107%.

[Matthew Parsons]

If you are temperature limited, then you have only so much power to apply to the helicopter. It is correct that with the same power, increasing Nr reduces torque, and doesn't provide any additional power to the main rotor.

However, reducing torque on the main rotor also means you have to produce less anti-torque. With a tail rotor, you produce the anti-torque by applying some of the engine power to the tail rotor, so if you can reduce that, then you should have more engine power available to apply to the main rotor.

Net effect is that with higher RPM you can lift more.


That will probably not stand the practical experiences of everyone, because there are many other factors involved. For example, increasing the rotor speed can increase Cd for the same Cl. However, I'm currently doing low speed performance testing on a Bell 412 and have found many cases where higher Nr at ITT limits provides better performance than lower Nr at the same limits. I won't say that is always the case, but for what I was doing this am, it definitely was true.

[spinwing]

Mmmm....

Matthew,

higher Nr at ITT limits provides better performance than lower Nr at the same limits.

Definately the case when doing max weight sling work with the 412EP!

:E

[Shawn Coyle]

If the higher NR means you can use a lower blade pitch setting, then there will probably be benefits - lower blade pitch setting means lower CL, but also probably much lower CD (in comparison to CL).