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Rotor RPM (NR) hunting
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In modern turbine helicopters there is a governor which keeps rotor rpm (NR) at 100% and for the most part it does a very good job at doing so. While there are situations which could cause NR to drop below 100%, under most normal circumstances it will be at and remain at 100%. Early turbines suffered from compressor stalls which could led to surging or flameout when rapid changes in fuel flow were made. Modern governors have the benefit of working more quickly and smoothly then a pilot ever could and hardly ever make a mistake. Most of the default aircraft don't maintain their NR very well however I was content to ignore this under the assumption BIS had bigger fish to fry until I saw SPOTREP #00076 and referenced T126770. Needless to say I was delighted as it appeared BIS was attempting to address the problem which has been bothering me for some time. Unfortunately after installing the update and doing some flight testing it appears the underlying issue still exists.

While N1, N2 and NR are now matched up better while at flight idle on the ground NR is still all over the place in flight as the rotor is loaded and unload and if you would like to fix it I would like to help. I believe the correction involves raising the parameter(s) that control rotor inertia slightly so the blades will hold more energy and will not tend to droop as readily. If anything they should have a tendency to overspeed which can be corrected by raising collective slightly. Now if there isn't a single value that handles the blade inertia it would be worth while experimenting with making the blades slightly longer or heavier or a combination of both to help increase the inertia hopefully without adversely affecting anything else.


Operating System
Windows 10 x64
Operating System Version
1703 - 15063.726
Advanced Flight Model
Steps To Reproduce

In a default MH-6 or AH-6 pull about 80% Torque, lower the nose and let the aircraft accelerate. Let the airspeed stabilize and once it does roll the aircraft to the right or left and start pulling aft cyclic to tighten the turn. You notice you can hear the NR start to droop and if you persist you'll lose enough NR that the low Rotor RPM horn goes off and the aircraft begins to wallow. This is in fact the opposite of what should happen. Upon entering a high G turn in a helicopter the NR will if anything start to climb as the force of the tightening turn will cause the rotor RPM to increase. In a real helicopter one way to recover rotor RPM is to lower collective and either flare the aircraft slightly or enter a high g turn which will increase rotor RPM. In addition if the throttle is rolled off and the rotor is free wheeling you actually may have to add some collective to keep the rotor from overspeeding. However in Arma 3 the opposite occurs and even if you lower collective fully before rolling the aircraft and pulling aft cyclic the RPM still droops which is incorrect and should really be addressed.

In addition if you lower collective sharply in the Mohawk the NR overspeeds and if you raise it sharply it often droops. The aircraft simply doesn't react like this fortunately with a few tweaks to the blade inertia I expect this too can be corrected.

Additional Information

Conditions where the RPM will droop include but are not limited to when power required is greater than power available as in that case the RPM will drop as the rotor starts to cone and unless the collective is lowered the low rotor horn will come on and the aircraft will start to wallow, sink and if sufficient altitude isn't available crash. One example of this would be approaching a high altitude LZ or one downwind or at or near max gross weight. Often in these situations the power available is not sufficient to maintain a hover out of ground effect or even sometimes in ground effect. Often the pilot doesn't realize until he's out of translational lift. In very powerful aircraft like a chinook this is less of a problem where as in a R22 or a heavily loaded 206 it will undoubtedly result in a crash. Another example would be operating a max gross take off weight and trying to climb to high altitude eventually the amount of power required will exceed the power available and the blade RPM will start to droop. Again the only solution is to lower the power. If you require any additional info or clarification please let me know. In addition I'd be happy to discuss the parameters to adjust and flight test should you require it.

Also should you wish to confirm what I'm saying is correct you can check Transport Canada TP 2552 Helicopter Theory of Flight from April of 1980. As well as "The Art and Science of flying helicopters" by Shawn Coyle or "Principles of Helicopter Flight" by W.J. Wagtendonk.

Event Timeline

Gazoo created this task.Dec 2 2017, 3:34 AM
Gazoo updated the task description. (Show Details)Dec 2 2017, 3:40 AM
Gazoo edited Steps To Reproduce. (Show Details)
Gazoo edited Steps To Reproduce. (Show Details)Dec 2 2017, 4:05 AM

OMG @Gazoo sorry I haven't seen this post sooner.

I fully agree with the issue raised here. It's also bothered me whenever I fly with the AFM.

Rotor RPM drooping during a high G maneuver is the opposite of what should happen in a real helicopter.

Here is a nice explanation from FAA's Helicopter Flying Handbook:

The rotor blade rotating about the rotor hub possesses angular momentum. As the rotor begins to cone due to G-loading maneuvers, the diameter of the rotor disk shrinks. Due to conservation of angular momentum, the blades increase speed even though the blade tips have a shorter distance to travel due to reduced disk diameter. The action results in an increase in rotor rpm which causes a slight increase in lift. Most pilots arrest this increase of rpm with an increase in collective pitch. This increase in blade rpm lift is somewhat negated by the slightly smaller disk area as the blades cone upward.