Mast bumping occurs when main rotor hub of heliopter contacts and damages the main rotor mast.
The hub making contact with the mast requires excessive flapping. Even under high speed, high gross weight and high altitude conditions, the degree of blade flapping is well within maximum allowable values and the risk of mast bumping is zero under all normal flight conditions when correct piloting techniques are maintained. However, incorrect techniques that causes excessive flapping set the stage for disaster.
A blade’s flapping amplitude is increased by:
- Gusty wind conditions
- Sudden attitude changes caused by abrupt cyclic inputs
- Sideways flight at or near the helicopter’s maximum allowable speed
- Flight under low, zero or negative ‘g’ conditions
Low-g condition is a phase of aerodynamic flight where the air-frame is temporarily unloaded.
In such a situation, main rotor blades may flap beyond normal use limits.
The excessive flapping can cause the root of the blades to exceed the limit of their hinges and this condition, known as mast bumping, can cause the separation of the blades from the hub or for the mast to shear, and hence detach the whole system from the helicopter, falling from the sky
Tail Rotor Drift
Tail rotor drift or translating tendency, is typically balanced by a slight tilt of the main rotor disc in a direction opposite tail rotor thrust. thrust.
Pilot Induced Low-G Condition
Sometimes Low-G conditions during flight can be induced by poor pilot teqnuiqe. When cyclic is moved forward rather firmly, such as during a push-over at the end of a zoom climb, especially if the pilot lowers collective at the same time.
The tail rotor thrust also forms a right-hand turning moment around the aircraft’s center of gravity, which is now much lower down than the tail rotor. As the aircraft continues to roll right ,while cyclic is forward but not to the side, the clearance between the rotor head and the mast is reduced, figure 1.
The unwary pilot may instinctively use left cyclic to oppose the increasing right roll.
Total rotor thrust must be restored. To do this the pilot must reload the disc by either moving the cyclic aft or increasing collective.
- Apply gentle rearward cyclic to reload the rotor (reintroducing a positive g situation), then
- Use left cyclic to roll the aircraft level
Avoiding Mast Bumping
Helicopter pilots have to avoid low-g conditions in all situations, because these situations are very dangerous to flight safety.
- Principles of helicopter flight. Second edition
- VID 241036 - Creation
DATE OF SUBMISSION
- 12:31, 23 February 2021
- This documentation is copyrighted as part of the intellectual property of the International Virtual Aviation Organisation.
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