Item 0786
OTHER:
Miscellaneous - Thoughtless Idea - Constant Speed RotorOverview:
A means of automatically maintaining a constant rotor speed
[Ω] or [Nr].During normal flight conditions: the pilot has control of the collective pitch
[θ0] and the torque [Q]. A change in one, by the pilot, will automatically result in a change in the other, by the controller. This is done to maintain the constant rotor speed.During an emergency: the pilot can activate a thumb switch, located on the collective lever, and the system's control of rotor speed can be over ridden.
Outside Helicopter
Robinson:
R22B
: Robinson Helicopter Company reported that the governor unit senses main rotor speed through a sending unit on the engine to transmission shaft. An electronic computer maintains the speed within the normal operating range through a servo motor which actuates the throttle on the collective by means of a clutch assembly. The unit can be turned off by a toggle switch on the collective control, and can be overridden by twisting the throttle. The entire governor assembly, including the speed sensing unit, computer, servo, clutch, and collective/throttle were removed from the helicopter and taken to Robinson Helicopter Company by a Safety Board investigator.___________________
The purpose of the governor is to automatically maintain the RPM at its optimum operating range of around 104%. This is achieved by use of electrical sensors situated within the engine's right magneto which detect any variation in RPM. An electrical motor connected to the throttle linkage will automatically increase or decrease the throttle as necessary in order to maintain the RPM at its correct operating range.
A clutch system within the throttle twist grip allows the pilot to manually override the governor by physically twisting the throttle in the desired direction, under normal circumstances this would only be necessary in the event of a governor malfunction.
The governor may be turned on or off as required with a toggle switch located at the end of the collective lever.
Turning it off will not cause any initial change to RPM i.e. the throttle will remain in its current position. Turning it on will cause changes to the RPM only if not already positioned at its optimum setting of 104%. (Note:- the governor only operates above 80% ERPM).
It is necessary to have the governor switched off during start up and shutdown when the RPM should be maintained between 75% and 80% in order to either let the engine warm up or cool down. It should be noted that flight is prohibited with the governor selected off except for in-flight systems malfunction or emergency procedure training.
Bell 206B:
When we increase the collective control, an automatic engine governor/fuel flow controller increases fuel flow to the engine so that engine power is increased so that 100% rotor and engine rpm is maintained. The reverse happens when the collective control is decreased.
Notes:
Assertion:
The only time that the rotor speed should be allowed to vary from 100% is when the pilot has an emergency requirement for the dynamic energy that is stored in the rotating rotor disk. For example; to flare for landing when in autorotation.
Reasons for this device:
Methods of achieving this:
It the mechanical device is located on the engine then it will be turning approximately 9 times the rotor speed. This means the weights can be lighter. The rest of the unit might not be lighter.
If the device is electrical then there must be total redundancy.
Related Information:
See [MDD p.112] for similar idea; using Rotor Governor. Collective pitch [θ0] is linked to rotor speed [Ω].
Related Thoughtless Ideas:
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DESIGN: Control - Power Train - Governor - Rotor Type - Electronic & General |
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DESIGN: Control - Power Train - Governor - Rotor Type - Hydraulic Schematic |
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OTHER: Misc. - Thoughtless Ideas - Auto-Collective Rotor Hub |
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DESIGN: Control - Flight - Synchropter- Hydraulic Schematic |
Postings:
(w/ revisions for clarity):The proposition is that a helicopter would be safer if the rotor RPM was never allowed to drop below 100%, even at the expense of rotor thrust; except in the case of an emergency. Low rotor RPM is the second greatest cause of fatal helicopter accidents; the first being wire strikes.
Emergency being defined as any short-term requirement where the desired rotor thrust exceeds the powerplant's current maximum torque. I.e. flare with inoperative engine(s), hop over an obstacle, etc.
Both the throttle grip and the collective lever appear to serve the same function, in different ways. This function being rotor thrust. It seems that there should be a way to do it with only one control.
Replies:
(w/ revisions for clarity):Author: A.H. on PRA
It sounds like a good idea; maintain rotor rpm automatically. Failure to maintain rotor speed is the cause of a lot of accidents. At cruise pitch you might have a second or two after an engine failure to get that collective down. But what have you gained by fitting a electro-mechanical pitch governing system? When the rare engine failure happens the system saves the day- score one for the system. But if the system itself has a failure at any time it could try to add pitch and stall the rotor or take out pitch and slam you into the ground. You are adding a fair amount of complexity into the control paths. Throttle, collective, engine rpm, rotor rpm, torque are now variables just waiting to be messed up by any software glitch in one of the micro-controllers. If the system actuators had friction overrides like the R-22 throttle- that might be better. The pilot would feel the "suggestions" of the actuator but still have direct control and could override at any time. Eventually automatic flight controls will no doubt replace human pilots altogether. But the systems will be highly redundant and based on fuzzy logic with soft failure modes rather than hard failure
Author: W.H. on rec.aviation.rotorcraft
Heck, I'm in favor of 100% rpm at all times. The question is who maintains it and how. Current governors do fine. Getting more artificial intelligence into the loop invites problems where the pilot wishes desperately to do something the code-writer prohibited. You know, like flying an Airbus into the trees because the pilot couldn't exceed a parameter. Or having the engine shut down when the rotor rpm...
Author: M. on rec.aviation.rotorcraft
The above page has some good ideas, but the electronically controlled system has been built in many forms, and the mechanical system raises a few concerns. Some points I had:
A mechanical system has already been built and has proven to be very effective. It's called the constant speed propellor. I imagine the reason it is not used in helicopters is that with a prop you don't need very fine control of blade pitch with the engine out or with most engine control problems.
Since collective pitch gives direct control of lift I'm certain it follows that the simplest controls would utilize this. To primarily control throttle, but in some emergencies revert to controlling collective pitch means that you're increasing the responsibilities of the weakest link in flight...the pilot. (I'm not an engineer, just honest).
"...never allowed to drop below 100%". Does this mean at high power settings a sudden demand for power will happen slowly or is this where the exception can come in to play? For example, in the governed system I'm flying, this situation will droop Nr by ~2% but then the engines will work to restore Nr. The reason it's only ~2% is because there are potentiometers connected to the collective that look for sudden movement (called Droop Stop Pots).
"On loss of power, the blade pitch
Some positive points:
Governing systems of all types experience failures, but constant speed props seems relatively reliable. Could you incorporate a "fail-safe" mechanical governor into this plan and then 'tweak' it with either pilot input or the computer stuff?
Preserving Nr in an engine failure is a good idea. But considering my point above, a system that wouldn't take over collective pitch, but would let the pilot know what's best would be excellent. For example, a mechanical force trim that tries to hold the controls centered wherever the pilot desires could be used. The pilot sets the "zero force" position of the collective to cruise power and then hands off it will stay in that position. In an engine failure, the "zero force" position immediately becomes the ideal autorotative pitch setting, accomplishing what you described. If the helicopter is in a bad position, as I indicated, then the pilot would only feel an increased force in the collective, but would still have positive control.
Author: S.G. on rec.aviation.rotorcraft
100% Nr is not always desirable. In the Bell 412, Nr is reduced to 97% for cruise, for both fuel economy & vibration reduction.
Author: N.L. on rec.aviation.rotorcraft
The 100% Nr is only a reference, and has little meaning as to Best or Optimal. It is exactly like the prop rpm of an airplane. The range of acceptable Nr's is made by looking at all the operating conditions, and finding out how stable the rotor is (structurally) across that range. Then we determine if adequate controllability is available. There is no best, except for a narrow band of conditions, typically determined by the following possible variables:
Power on or autorotation?
Gross weight?
Density altitude?
Is control more important, or is performance?
Airspeed?
Is the condition dynamic or steady state?
Can you recover from a power failure while set there?
The Nr for a steady slow climb with limited power can be quite different from that which favors a high G turn near Vne, and both might be different than the Nr that is best during the flare in a touchdown autorotation.
General rules:
High Nr is better where stall is a problem, near high Gross weight and high speed.
Low Nr is best where hover performance or slow speed max. rate climb is needed.
Low Nr is good for hover performance (weight carrying capacity) but low Nr is bad for control margins, like LTE (loss of tail rotor effectiveness). The old joke about Huey rpm "6200 plus 100 for each dependent" was actually backwards, there is more hover lift at lower rpm!
Fixed Nr is often used because the manufacturer decides to simplify the whole thing, and pick one Nr that is OK for everything, and let it go. Multiple Nr's are used to milk then last bit of capability out of the aircraft, if the workload is found to be OK. Governed turbines are beepable, but sometimes we don't let you beep because the fixed Nr is best for vibration, especially where absorbers are used that only quell the vibes at one Nr. The aircraft would shake miserably at any other Nr, so we set the Nr to only one place.
No Nr is better for structural loads. Low Nr is better for centrifugal forces, but much worse for stall loads (which pound on the push rods and swashplates).
Any device that automatically lifts up the collective to control the Nr in autorotation would possibly also lift it up, or drop it at the wrong time. To make that system reliable, it must be fly-by-wire, a pretty expensive proposition, and not necessarily justified by the one job of Nr maintenance. Of course once you have FBW (fly-by-wire), why not make the Nr maintenance one of your sub-tasks?
Response by R.W.
>Of course once you have FBW, why not make the Nr maintenance one of your sub-tasks?
I think the EC135 does exactly this. Nr is commanded to drop a few per cent at low forward speed, to improve the certified approach noise level. High-density altitude or large pedal inputs in one direction (suggesting a need for tail rotor thrust) raises the Nr reference speed. Approach noise measurements are not done with large pedal inputs, or at high altitudes:).
You're right Nick, having FADEC and FBW opens lots of opportunities for tweaks like this.
Author: F.D. on rec.aviation.rotorcraft
Pitch-cone coupling is a mechanical method of maintaining rotor rpm used on some designs
Professional pilots above have commented on the need to used slightly different rotor speeds for different flight regimes. My thinking is that for the low-time pilot safety is much more important than having a 2-3% change in rotor speed and therefor a fixed speed will be okay for all situations the possible exception of just before a flare while in autorotation.
Additions:
Patent US 6,158,960, December 12, 2000, Propeller hub with self-adjusting pitch mechanism. This is by Torque-Pitch Coupling. Have hard copy.
http://www.notplanejane.com/images/AeroMatic/AeroMatic%20Propeller%20Info%205-05.doc. This is by RPM-Pitch Coupling. Have hard copy.
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Last Revised: September 28, 2008