Initial Conditions <ul><li>Tactical flight, following the terrain’s profile (N.O.E.) </li></ul><ul><li>Push Over Maneuver Performed by Cyclic’s application together with a </li></ul><ul><li>sudden lowering of the collective </li></ul>
Events <ul><li>UNCONTROLLED RIGHT ROLL </li></ul><ul><li>MAST BUMPING </li></ul>
… .Gravitational acceleration…G-force …Apparent Weight… - All objects on earth are subject to the gravitational force. - In normal conditions this gravitational force is equal (by convention) to 1 g positive. - During manouvered flight, depending on the type of manouver performed, the g- force which we are subject to,changes (both positive and negative) APPARENT WEIGHT. - If we perform a 60 ° turn, we will be subject to a force equal to 2 g’s positive (the APPARENT WEIGHT will be equal to twice the normal).
-Mast Bumping- Mast Bumping is generally caused by pilot cyclic overcontrol in a condition where the helicopter is subject to NEGATIVE or LOW g’s. It might also occur when flying in turbulent air or when landing on a slope.
-Mast Bumping- The Mast Bumping occurs when the rotor head impacts the main rotor shaft. It affects a particular rotor type 2 Bladed Semi-Rigid R22/R44, Bell47, Bell 206, and Bell205/212
-Mast Bumping- The rotor head literally strikes the mast causing damage or even detachment. This is caused by excessive flapping generated when flying outside the helicopter‘s operating envelope.
<ul><li>Normal Flight conditions </li></ul>The movement of the cyclic involves a contextual rotor displacement. The TRT (Total Rotor Thrust) shifted from the vertical is now made up of 2 forces: one vertical denominated Vertical Rotor Thrust and one horizontal called Horizontal Rotor Thrust (HRT). The moment created between the HRT and the center of gravity of the fuselage, which acts as arm, will cause the helicopter as a whole (Rotor/Fuselage) to Roll in the direction desired by the pilot.
<ul><li>Low/Negative Gs CONDITIONS </li></ul>Hypothesis Sudden lowering of the collective in combination abrupt application of forward cyclic. Fact The total thrust generated by the rotor will be very low or close to zero.
Fact The only force in play at this point will remain the one generated by the tail rotor thrust Fact The pilot will still have rotor disc control by the use of cyclic, but not fuselage control, because the moment between HRT and CG will be zero, there will be no force capable to induce roll COMMANDED by the pilot. Low/Negative Gs CONDITIONS
The lack of aerodynamic forces capable of balancing the tail rotor thrust will result in: - Left Yaw - Right Roll - Lack of authority of the Lateral Cyclic command Low/Negative Gs CONDITIONS
<ul><ul><li>The unaware pilot would, to counteract the roll to the right, use a increasing </li></ul></ul><ul><ul><li>amount of left cyclic. </li></ul></ul><ul><ul><li>The rotor disc will tilt progressively to the left in accordance with cyclic inputs </li></ul></ul><ul><ul><li>but without any aerodynamic authority. The lack of moment between the HRT and CG (as result of the zero apparent </li></ul></ul><ul><ul><li>weight) will determine that the fuselage will not follow the rotor disc but will </li></ul></ul><ul><ul><li>continue its rolling motion to the right. </li></ul></ul>Low/Negative Gs CONDITIONS
At some point, the rotor head will eventually hit the rotor mast, the shaft’s torque load in addiction of the rotor’s movement, will lead to damage or total detachment of the rotor shaft itself. Low/Negative Gs CONDITIONS
<ul><li>Low Gs Recovery </li></ul>Positive load must be restored in order to achieve rotor thrust by: - Applying back cyclic - Applying collective pitch NOTE: The application of the collective pitch command ONLY is the least desirable action, because under low-g conditions, the power increase is associated with possible controllability problems around the vertical axis ONLY ONCE POSITIVE LOAD ON THE ROTOR IS RESTORED: Apply cyclic left to resume level flight
<ul><li>Flight Conditions that might lead to MAST BUMPING: </li></ul><ul><li>Low or Negative g Flight conditions due to intentional maneuvers or turbulence - Abrupt and sudden cyclic excursions, especially along the pitch axis. - Abrupt and sudden lowering of the collective (intentionally or not). - Strong wind gusts (associated with updrafts during mountain flight </li></ul><ul><li>operations, especially when hovering or landing). - Sidewards flight exceeding machine’s limitations. - Slope Landings. </li></ul>
REFERENCES <ul><li>ADVANCED ROTARY WING AERODYNAMICS –US NAVY- </li></ul><ul><li>MAST BUMPING –CAUSES AND PREVENTION- US ARMY VIDEO, 1980 </li></ul><ul><li>PRINCIPLES of HELICOPTER FLIGHT –W.J. Wagendonk- </li></ul><ul><li>CYCLIC and COLLECTIVE –Shawn Coyle- </li></ul>