Lower the collective to reduce drag and increase throttle for more power, if available. If in forward flight, gently apply aft cyclic.
Low rotor RPM is a significant concern for pilots. The slower blade speed reduces the amount of lift as velocity is a key factor in lift production. If the pilot tries to retain the same altitude, additional lift will be needed. Should the pilot attempt to raise the collective for more lift, the increased drag will further reduce the rotor speed. As the rotor RPM decays, the centrifugal force reduces and results in increased conning of the blades. The increased coning compounds the problem as the rotor thrust is directed inward, reducing its vertical lift capacity. In addition, the size of the overall disc becomes smaller, further increasing the needed for lift is the altitude is to remain the same.
Lowering the collective reduces the blade angle, which reduces the angle of attack. With a reduced angle of attack, the induced drag will be less, and the rotor RPM should increase, assuming the same power requirements. In addition, aft cyclic will tilt the rotor disc aft which in turn lowers the inflow angle on the rotor, which reduces the angle of attack, and reduces the induced drag. Due to the aft cyclic application, some of the airflow is now coming from underneath the disc, which reduces the induced flow, which in turn reduces the angle of attack, induced drag, etc. In addition, the coning angle will increase with aft cyclic. The increased coning angle reduces the size of the disc, and because of the Coriolis effect, the disc will increase rotation. Should the rotor RPM be allowed to decay beyond the lower limits, the lack of adequate centrifugal force may allow the blades to collapse. Remember, lower the collective and increase power to maintain rotor RPM within limits. Do not let RPM decay.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-15, 11-15
Principles of Helicopter Flight, 2nd Edition, pg. 171