Low Rotor RPM From Live Tuesday

Nov 15, 2017

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Low Rotor RPM from Live Tuesday

 Low Rotor RPM From Live Tuesday

 Here is what members are saying: (and if you send me something and do not specify, I always get your permission before posting it for others to see.)

"Hi Gary,  I wanted to let you know how much I enjoyed yesterday’s program.  I had to watch it after the fact but I think its one of the best you have done and the amount of good information it provided was great!   The informational section on low RPM was well put together but I also enjoyed hearing about the mishaps that you presented.  Presenting and discussing situations like this is extremely helpful and drives the points home far better than in a lecture format. I think it would be helpful to share things like this on a more regular basis as it adds tremendous additional education to your already great site. Thanks again from a newcomer here on the east coast.  Van Geraniotis"

Low Rotor RPM From Live Tuesday

Experienced CFIs talk about personal experiences with Low Rotor Rpm and roll a couple crash videos from YouTube, where the pilot flew into the ground with the low rotor RPM horn going off. No recovery of RPM, no auto rotation initiation, no flare..... just fly into the ground. We here at Helicopter Online Ground School try an keep up with what is killing helicopter pilots and adjust our training emphasis on those issues, while at the same time maintaining content that will help you pass your check ride. 

When you are training with your CFI, whether it is a BFR or you are working on a rating, do low RPM recovery exercises often enough to make it muscle memory to lower collective and roll on throttle. Especially if you are flying a Robinson with a governor, this is important.  As a CFI in the Robinson, I turn the governor off on the student periodically and guard the collective. I want to see him recognize the governor light illuminated and also manage the throttle without the governor.

On my R44 instructor endorsement ride with the examiner, he switched the governor off a dozen times at least and rolled the throttle down. He even did it in the hover taxi twice. This emphasis on low RPM recovery by the examiner, shows the priority that he gives it. He ranks RPM recovery ability pretty high in his decision to sign you off for instructing in the aircraft.

So many bad things come from Low rotor RPM. Retreating blade stall, LTE (loss of tail rotor effectiveness), excessive blade coning, and catastrophic stall of the main rotor blades. I can't emphasise enough, how important it is for the pilot to react promptly and appropriately to low rotor Rpm.

Low Rotor RPM From Live Tuesday

Chris discusses an Enstrom crash that started with a student allowing the RPM to drop below operating range. Chris took over and tried to recover and decided to do a run on landing when the RPM could not be restored. Due to the terrain, the helicopter rolled over.

Gary discusses a transmission failure in a Hiller that had no warning light or horn. He caught the engine and rotor rpm dropping and lowered collective to keep them in the green. He continued to lower until the needles eventually split and he landed safely with an auto rotation. The engine did not have enough power to keep power to the rotor system through a transmission that had bearings falling apart. By lowering collective to keep the rotor RPM in the operating range, the sprag clutch allowed the rotors to keep spinning despite the transmission falling apart and eventually bogging the motor to  a complete stall moments before landing.

Low Rotor RPM From Live Tuesday

Gary also has had a cog belt (rotor way transmission) break in flight. This caused an immediate split in the tach needles as the engine RPM went to red line and the rotor RPM began dropping. If the collective would not have been lowered completely within a couple seconds, the blades would have stalled at altitude. Gary also had a second belt failure in the same helicopter a couple years later, only this time the failure was gradual as the cogs stripped off the belt and the tach needles started separating gradually. As the belt lost more cogs, the collective had to be lowered more and more to keep rotor RPM in the operating range. Eventually the collective was all the way down and the throttle had to be reduced to save the engine from overspeed (full auto rotation). 

We do not know when these things are going to happen, but when they do, main rotor rpm is your priority. Our stored energy in the helicopter is altitude, airspeed, and RPM. We can deal with a lack of altitude and airspeed possibly, but a lack of RPM will most always end badly.

 Low Rotor RPM From Live Tuesday

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Gary Cleveland, Chief Pilot

Helicopter Online Ground School

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