Low-G/Mast Bumping

Robinson Safety Notice SN-11

Low-G condition is a phase of aerodynamic flight where the airframe is temporarily unloaded. The pilot—and the airframe—feel temporarily “weightless”. This “weightless” feeling is the same feeling as starting down in an elevator.

In helicopters with underslung teetering rotor systems, the rotor must be loaded with 1G at all times.  If the rotor is not loaded, as with low-G, the helicopter body is free to move.  The tail rotor creates thrust to the right and the helicopter will experience an uncommanded right roll.

The natural tendency of a pilot is to correct a roll to the right with left cyclic.  But, nothing could be more wrong. The flapping stop hits the mast and the mast could be deformed or fatally separated, and then everyone has a bad day.

What to do instead?

  • Never experiment with low-G and stay out of situations that are likely to lead to low-G and excessive flapping
  • If you do get into a low-G situation, recognize it for the weightless feeling that it is.  If then the helicopter starts to roll right, reload the rotor by gently applying aft cyclic.  Then and only then correct the roll.

Use these precautions to avoid Low-G

  • Avoid Cyclic pushovers
  • Avoid using the cyclic to avoid birds and other craft- If you must descend rapidly, lower the collective
  • Slow down in turbulent air
  • Avoid abrupt flight controls

Robinson Safety Notice on LOW-G

Source: https://en.wikipedia.org/wiki/Low-g_condition