Purpose
To learn to recognize and recover from low rotor RPM.
Description
- Discuss indications of a low rotor RPM condition.
- Demonstrate the change in noise and vibration that indicates a change in rotor RPMs.
- Practice rotor RPM recovery during cruising flight.
- Practice rotor RPM recovery during hover/hover taxi.
Instructional aids and pre-requisites
- Lessons BM-1 through BM-8 are pre-requisites
- Ground Lesson pre-requisite: Emergency Procedures, Robinson R22 Safety Notices, Aerodynamics
- This lesson can be combined with Lesson PS-2 (Governor-off Procedures)
Content
- Discuss the signs of low rotor RPM (increase in vibration, decrease in engine noise, cyclic shake, and activation of the low rotor RPM system) and the proper recovery (lower collective, increase throttle, and add aft cyclic in forward flight)
- Prior to take-off and with collective full-down, have the student close his eyes while the instructor increases and decreases throttle until the student perceives a change in noise
- Allow the governor to restore RPMs to the normal range and conduct a normal take-off
- During cruise flight, the instructor will repeat the demonstration so the student can focus on the signs of the low rotor RPM condition
- Once the student can recognize a loss of RPMs, the instructor will demonstrate the recovery
- Simultaneously and smoothly lower the collective and add throttle to restore RPMs to 104%
- Aft cyclic in forward flight can be used to assist with the recovery, but the collective and throttle inputs should be the primary means for recovering RPMs
- Forward cyclic will cause a further decay in rotor RPMs
- Collective can be increased once RPMs have been recovered
- Allow student to practice recognition and recovery while the instructor initiates the maneuver by reducing throttle
- In a hover or while hover taxiing, perform the same demonstration for the recognition and recovery
- As the throttle is reduced, allow the student to raise the collective in an attempt to maintain hover altitude
- Initiate the recovery by adding throttle and momentarily reducing the collective
- If ground contact cannot be avoided, ensure that the aircraft touches down in a level attitude
- Allow the student to practice recognition and recovery while the instructor initiates the manuever by reducing throttle
Common errors
- Overreaction
- Conduct demonstrations in a controlled and secure way to reduce student anxiety
- Emphasize that the recovery can be accomplished with smooth and deliberate inputs
- Guard against overspeeds from an abrupt decrease in collective, excessive aft cyclic, or aggressive throttle inputs
Completion standards
- The student should be able to recognize increases and decreases in engine/rotor RPM
- In cruise flight, the student should be able to initiate the recovery before rotor RPM reaches 90% or the instructor’s predetermined minimum
- In a hover or hover taxi, the student should be able to initiate the recovery without making ground contact
Teaching considerations
- Be a model for how your student should react: smooth and deliberate
- Consider teaching before autorotations
Additional practice
- Demonstrate low rotor RPM recovery during a normal take-off (may be conducted during a later lesson or during commercial training)
- Demonstrate low rotor RPM recovery during a maximum performance take-off after completion of Lesson PM-1 (Maximum Performance Take-off)
Additional resources
- None specified
#1 by bob kuriger on November 18, 2009 - 10:21
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The R22 POH does not provide any clue as to what would cause a low rpm condition in a hover. I can only think of 2 reasons, equipment malfunction, and instructor overriding my throttle control, as governor has been disabled for this exercise. Obviously, there may be COMBINATIONS of other things like density altitude, abrupt collective and carb icing. Taken one at a time, however, they do not seem likely to cause the same effect as when the instructor turns the throttle down. Can you provide what Robinson obviously cannot ? Thanks.
#2 by Chris on November 18, 2009 - 11:07
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Hi Bob,
Hopefully somebody with more experience than me can offer a more satisfying answer, but I’ll give it a try.
This is one of those areas where the training scenario departs from the real-world, but the idea is to teach a response, even though set-up is fake. My thought is that the instructor should manipulate the throttle during training–the way I learned had me (the student) reducing throttle and counter-intuitively trying to maintain the hover at the same time.
I think you’ve identified the real-world scenarios where you’d find yourself in a low-RPM scenario in a hover. The mechanical issues are pretty obvious (carb ice, belt failure, fouled plug, or other loss of power). Some will be relatively sudden (=hover auto, and nothing like this exercise), and some will be gradual enough that you end up nursing it to the ground (which seems to be the real world objective of this manuever).
The pilot-induced situations I think are worth noting: a student with a death grip on the throttle could put you in a LR RPM situation. Or, as you said, overly aggressive collective inputs, possibly.
For the environmental conditions, take a look at the HIGE chart for the HP/Alpha/Beta. There are (or at least, were) guys flying these aircraft at >5000 MSL and +30C. I could see being right there on the margins of available performance. Certainly a situation where you’d get the horn in hover. Maria Langer posted here about that type of scenario. This would seem to be the scenario that would be closest to replicating the conditions in this maneuver: pick up into a low hover at your MAP limit, then end up getting the horn as you transition to forward flight.
One last thing: the value in the ground/hover work on this lesson is, in part, to get the student to recognize how the noise and vibration of the aircraft changes. This is a less threatening environment to let the student experiment, and translates to a benefit during autos from altitude (eg, correlating subtle changes in noise with an inappropriate attitude, rather than relying on the tach).
Hope this was helpful.
#3 by Karl Gudmunsson on February 12, 2010 - 03:32
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Hi guys.
Bob
There are a few places in the real world where you can, and sometimes will get into low RPM situation if you forget yourself enjoying the view.
Think about the Hot High & Heavy conditons. You can get into one or all of the 3 H’s at any height/altitude.
What you need for that to happen, given the helicopter is working properly, is beeing heavy and not having enough power for the conditions at hand. That is with full tanks of fuel, yourself beeing above standard Army issue weight, your passenger/customer beeing 120kg an you are flying him around gathering his livestock (at sea level) or flying him to his mountain hut. (We often try to do things in underpowered machinest to cut costs) This means you can easily overpitch your rotor system.
In the Schweizer 300C,you don’t have a throttle governor so you have to control the throttle yourself. If your technique is good, you should not have to get into low RPM but sometimes you miscalculate or plainly fail to increase your RPM to the setting required upon landing and you find yourself in this unfavorable situation.
In my opinion, low RPM happens when one puts himself into situations that his machine does not handle in terms of power required and one’s reactions are to slow to get out of the situation.
Of course this can happen in all helicopters, all of them can be overpitched.
The reason why we teach this is to get the right reactions into our students minds. They need to have the skills to get out of low RPM in the least amount of time and the correct technique should be ingrained in their minds.
For safety reasons, we only teach this in a safe environment.
Chris.
I like your blog. Please have a look at my sites and contact me if you have any comments. We might want to share information and experience and work together with our information websites?
Karl