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.

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Karl

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.