Key Takeaways Helicopters take advantage of free stream flow along a rotor blade to produce lift and thrust. The blades on a helicopter’s main rotor have an angle of attack, which plays the same role as a wing in an airplane. The tail rotor is responsible for stabilizing the helicopter so that it does not rotate under torque from the rotor. Helicopter aerodynamics The correspondence between helicopter aerodynamics and airplane aerodynamics spans beyond the need for free stream flow across an airfoil. Helicopter aerodynamics involves the same forces that arise in airplane aerodynamics, but these forces arise in different ways due to fluid flow across the aircraft. In this article, we’ll look more at the basics of how a helicopter generates its lift and thrust with only a single main rotor as well as how the design of the rotor influences helicopter aerodynamics. Overview of Helicopter Aerodynamics All helicopters have two rotors that generate the lift and thrust required to steer the aircraft as well as stabilize the helicopter against unwanted rotation. Attached to the engine are the main rotor blades, which rotate against the surrounding air to produce a flow along each rotor blade. Technically, a helicopter’s rotor blades are a set of airfoils, and they can produce lift in the same way as the wing on a fixed-wing aircraft. A helicopter’s main rotor interacts with the surrounding airflow to manipulate the main aerodynamic forces in the following manner: Lift: As the rotor blade spins, airflow across the bottom of the rotor blade produces lift to counteract gravity. Gravity: Obviously a helicopter does not manipulate gravity, but by exerting just enough lift to counteract gravity, the helicopter can hover at a fixed altitude. Thrust: Unlike fixed-wing aircraft or jets, thrust is not produced by the engine directly. Instead, the rotor is tilted, which orients the lift vector away from the vertical direction. Drag: As the helicopter moves, airflow across the body creates drag due to the formation of a boundary layer. It should be clear as to the function of the main rotor: to provide lift and thrust, depending on the relative orientation of the rotor blades and the body. We can now dig a bit deeper into the function provided by each of these elements in helicopter aerodynamics. Angle of Attack and Tilt on the Rotor Blades The role of the rotor is two-fold: it converts lift into thrust and it needs to generate lift. The former is accomplished by tilting the rotor using the cyclic pitch control while the latter is determined by the angle of attack of the rotor blades and the length of each rotor blade. Larger blades, faster rotation, and an appropriate angle of attack can produce maximal lift on the helicopter during flight. During flight, the oncoming free air stream will imbalance the lift provided by the rotor, which will create a rolling motion. This is balanced by designing the rotor to have flapping blades, meaning the blades can natur

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2. ADVANCE HELO AERODYNAMICS
3 & 4
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3. SEQUENCE
• Vortex Ring State / Settling with power
‒ Definition
‒ Conditions
‒ Development
‒ Effects
‒ Indications
‒ Recovery
‒ Precautions
• Loss of tail rotor effectiveness
‒ Definition
‒ Function of anti-torque system
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4. SEQUENCE
• Loss of tail rotor effectiveness
‒ Definition
‒ Conditions
‒ Flight characteristics of LTE
‒ Main rotor vortex interference
‒ Tail rotor vortex ring state
‒ Weathercock stability
‒ AOA Reduction
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5. SETTLING WITH POWER /
VORTEX RING STATE
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6. DEFINITION
• It is a condition of powered flight where the
helicopter settles into its own downwash
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7. CONDITIONS
• Powered flight
• High ROD
• Low indicated airspeed
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8. DEVELOPMENT OF VORTEX RING
STATE
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10. EFFECTS OF VORTEX RING STATE
• Significant vibrations
• Random pitching and rolling
• Fluctuating power demands
• Random yawing
• Slow control response
• Rapid increase in rate of descent
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11. RECOVERY
• Increasing airspeed by a large nose down attitude
change, then applying power
• If sufficient height is available, entering auto-rotation by
reducing power to zero and then gaining airspeed
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12. AVOIDANCE OF VORTEX RING
In order to avoid vortex ring state following stage of flight should be carried out carefully
• Vertical descent
• Steep approach
• Quick stop flares
• Autos
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13. LOSS OF TAIL ROTOR
EFFECTIVENESS
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14. DEFINITION
It is a critical, low-speed aerodynamic flight
characteristic which can result in an uncommanded
rapid yaw rate which does not subside of its own
accord and, if not corrected, can result in the loss of
aircraft control
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15. FUNCTION OF THE ANTI-TORQUE
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16. FUNCTION OF THE ANTI-TORQUE
• On French manufactured single rotor helicopters, the main rotor rotates clockwise as
viewed from above
• The torque produced by the main rotor causes the fuselage of the aircraft to rotate in the
opposite direction (nose left)
• The anti-torque system provides thrust which counteracts this torque and provides
directional control while hovering
• Tail rotor thrust is the result of the application of anti-torque pedal by the pilot
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17. FUNCTION OF THE ANTI-TORQUE
• If the tail rotor generates more thrust than is required to counter the main rotor torque,
the helicopter will yaw or turn to the right about the vertical axis
• If less tail rotor thrust is generated, the helicopter will yaw or turn to the left
• By varying the thrust generated by the tail rotor, the pilot controls the heading when
hovering
• In a no-wind condition, for a given main rotor torque setting, there is an exact amount of
tail rotor thrust required to prevent the helicopter from yawing either left or right
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18. FUNCTION OF THE ANTI-TORQUE
• This is known as tail rotor trim thrust. In order to maintain a constant heading while
hovering, the pilot should maintain tail rotor thrust equal to trim thrust
• The environment in which helicopters fly, however, is not controlled
• Helicopters are subjected to constantly changing wind direction and velocity
• The required tail rotor thrust in actual flight is modified by the effects of the wind
• If an uncommanded right yaw occurs in flight, it may be because the wind increased the
tail rotor effective thrust
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19. FUNCTION OF THE ANTI-TORQUE
• The wind can also reduce the anti-torque system thrust. In this case, the
helicopter will react with an uncommanded left yaw
• The wind can and will cause anti-torque system thrust variations to occur
• Certain relative wind directions are more likely to cause tail rotor thrust
variations than others
• These relative wind directions or regions form an LTE conducive environment
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20. CONDITIONS
• Any maneuver which requires the pilot to operate in a high-power,
low-airspeed environment with a right crosswind or tailwind
creates an environment where unanticipated left yaw may occur
• Late response to uncommanded left yaw
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21. FLIGHT CHARACTERISTICS OF
LTE
• Extensive flight and wind tunnel tests have been conducted by aircraft manufacturers
• Four regions
‒ Main Rotor Disc Vortex Interference (1-2 o’ clock)
‒ Weathercock Stability (4-8 o’clock)
‒ Tail Rotor Vortex Ring State (1-5 o’clock)
‒ AOA Reduction (8-10 o’clock)
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22. MAIN ROTOR DISC VORTEX
INTERFERENCE (030° to 060°)
• Winds at velocities of about 10 to 30 knots from the right front will cause the main rotor vortex to be
blown into the tail rotor by the relative wind
• The effect of this main rotor disc vortex is to cause the tail rotor to operate in an extremely turbulent
environment
• During a right turn, the effect of this main rotor disc vortex is to increase the angle of attack of the tail
rotor blades (increased thrust)
• The increase in the angle of attack requires the pilot to apply left pedal (reduce thrust) to maintain the
same rate of turn
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23. MAIN ROTOR DISC VORTEX
INTERFERENCE (1-2 O’CLOCK)
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24. MAIN ROTOR DISC VORTEX
INTERFERENCE (030° to 060°)
• As the main rotor vortex passes the tail rotor, the tail rotor angle of attack is reduced
• The reduction in the angle of attack causes a reduction in thrust and a left yaw acceleration begins
• This acceleration can be surprising, since the pilot was previously adding left pedal to maintain the
right turn rate
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25. MAIN ROTOR DISC VORTEX
INTERFERENCE (030° to 060°)
Precaution
• Reduction in tail rotor thrust can happen quite suddenly
• React quickly and counter that reduction with additional right pedal
input
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26. WEATHERCOCK STABILITY (120°
TO 240°)
• Tailwinds from 120° to 240°, like left crosswinds, will cause a high pilot workload
• The most significant characteristic of tailwinds is that they are a yaw rate accelerator
• Winds within this region will attempt to weathervane the nose of the aircraft into the
relative wind
• This characteristic comes from the fuselage and vertical fin
• The helicopter will make a slow uncommanded turn either to the right or left depending
upon the exact wind direction unless a resisting pedal input is made
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27. WEATHERCOCK STABILITY (4-8
O’CLOCK)
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28. WEATHERCOCK STABILITY (120°
TO 240°)
Precaution
• It is imperative that the pilot maintain positive control of the yaw rate and devote full
attention to flying the aircraft when operating in a downwind condition
• The helicopter can be operated safely in the above relative wind regions if proper
attention is given to maintaining control
• If the pilot is inattentive for some reason and a right yaw rate is initiated in one of the
above relative wind regions, the yaw rate may increase
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29. TAIL ROTOR VORTEX RING STATE
(030° TO 150°)
• Winds within this region will result in the development of the vortex ring state of the tail rotor
• As the inflow passes through the tail rotor, it creates a tail rotor thrust to the left
• A right crosswind will oppose this tail rotor thrust
• This causes the vortex ring state to form, which causes a non uniform, unsteady flow into the tail rotor
• The vortex ring state causes tail rotor thrust variations which result in yaw deviations
• The net effect of the unsteady flow is an oscillation of tail rotor thrust
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30. TAIL ROTOR VORTEX RING STATE
(1-5 O’CLOCK)
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31. TAIL ROTOR VORTEX RING STATE
(030° TO 150°)
Precaution
• Timely corrective action
• Avoid over correction
• When hovering in right crosswinds, the pilot must concentrate on smooth pedal coordination and not
allow an uncontrolled left yaw to develop
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32. ANGLE OF ATTACK REDUCTION (8-
10 O’CLOCK)
• Winds within this region will result in the increase in IF
• As the IF increases, angle of attack reduces resulting in
decrease in tail rotor thrust
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33. RECOVERY
If a sudden unanticipated left yaw occurs, the pilot should perform the following:
• Apply full right pedal. Simultaneously, move cyclic forward to increase speed If altitude permits,
reduce power
• As recovery is effected, adjust controls for normal forward flight
• Collective pitch reduction will aid in arresting the yaw rate but may cause an increase in the rate of
descent
• Any large, rapid increase in collective to prevent ground or obstacle contact may further increase the
yaw rate and decrease rotor rpm
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34. RECOVERY
• The amount of collective reduction should be based on the height above obstructions or
surface, gross weight of the aircraft, and the existing atmospheric conditions
• If the rotation cannot be stopped and ground contact is imminent, an autorotation may
be the best course of action
• The pilot should maintain full right pedal until rotation stops, then adjust to maintain
heading
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35. POINTS TO PONDER
• The various wind directions can cause significantly differing rates of turn for a given
pedal position
• Tail rotor is not stalled
• Corrective action is to apply pedal opposite to the direction of the turn
• Avoiding LTE may best be accomplished by pilots being knowledgeable and avoiding
conditions which are conducive to LTE
• Appropriate and timely response is essential and critical
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36. POINTS TO PONDER
• By maintaining an acute awareness of wind and its
effect upon the aircraft, the pilot can significantly
reduce LTE exposure
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