Army Aviation
What is Dissymmetry of Lift? - ANSWER: The difference in lift between the advancing
and
retreating blades of the helicopter.
Which side of the rotor has lower relative airspeed? Why? - ANSWER: The retreating
blade. Because the blade velocity is reduced by negative relative airflow.
What are the 3 No Lift Areas? - ANSWER: Reverse Flow, Negative Stall, Negative Lift
How do helicopters compensate for Dissymmetry of Lift? - ANSWER: Blade Flapping
and Cyclic Feathering
Reverse Flow - ANSWER: Airflow flows from trailing edge to leading edge and therefore
produces no lift.
Negative Stall - ANSWER: Airflow impacts above the chord line and outside the critical
angle which produces turbulent air.
Negative Lift - ANSWER: Airflow impacts above the chord line and inside the critical
angle. Same effect as negative pitch in the blade.
Positive Lift - ANSWER: Airflow is inside the critical angle and below the chord line. This
is the good stuff.
Positive Stall - ANSWER: Airflow is below the chord line and outside the critical angle.
Lift is reduced.
Blade Flapping - ANSWER: Occurs when advancing blade flaps up while retreating
blade flaps down. This up flap velocity results in an increased downward flow of air
(induced flow) over the airfoil. This shrinks the angle between the resultant relative wind
and the chord line thereby reducing the angle of attack (AOA). This effect reduces lift
generated from the advancing blade and increases lift for the retreating blade.
Cyclic Feathering - ANSWER: Occurs when the pilot inputs forward cyclic which results
in increasing pitch at the 9 o'clock position and decreasing pitch at the 3 o'clock position
(Gyroscopic Precession). This increases AOA on the retreating side and decreases on
the advancing side of the rotor.
What is Gyroscopic Precession (Phase Lag)? - ANSWER: The phenomenon present in
rotating bodies where an action presents itself 90 degrees later in the plane of rotation.
,How many degrees of delay does Gyroscopic Precession cause? - ANSWER: 90
To move forward, where does the pilot move the cyclic and where does the pitch
increase occur? Backwards? - ANSWER: To move forward, pitch is increased at 9
o'clock.To move backward, pitch is increased at 3 o'clock.
What is Transverse Flow Effect? - ANSWER: The difference in lift, drag, and induced
flow that exists between the forward half and aft half of the rotor system between 10 to
20 knots of airspeed.
At what airspeed range does the Transverse Flow Effect occur? - ANSWER: 10 to 20
knots
What effect does the pilot notice because of the Transverse Flow? - ANSWER: Due to
Gyroscopic Precession, this is manifested 90 degrees later as a right roll when
transitioning through airspeeds between 10 to 20 knots (most prevalent during takeoffs
and landings).
What defines the differences between In Ground Effect (IGE) and Out of Ground Effect
(OGE)? - ANSWER: Airflow at a hover can be broken down into two types: In Ground
Effect (IGE) and Out of Ground Effect (OGE). Ground effect is the increase in lift that
occurs when a helicopter is hovering within one rotor diameter above a surface. (Ex:
Within 50 ft AGL if the rotor is 50 ft in diameter)
Does it require more power to hover IGE or OGE? Why? - ANSWER: There is higher
pitch angle (AOI) and power required for helicopters to hover OGE compared to IGE.
This is due to greater induced flow.
In Ground Effect (IGE) - ANSWER: When helicopters hover, air flows down vertically
and vortices develop around the rotor tips. While IGE, this downwards flow of air is
restricted due to ground friction as the airflow is redirected out and away from the rotor.
Because of this, the high velocity of a downwards flow of air or induce flow is reduced.
Out of Ground Effect (OGE) - ANSWER: While OGE, nothing restricts the downwards
flow of air. Therefore there is a high velocity of induced flow, wing tip vortices are
allowed to develop without restriction, and more power is required compared to IGE.
At what airspeed range does Effective Translational Lift (ETL) occur? - ANSWER:
Between 16-24 knots
What is Effective Translational Lift (ETL)? - ANSWER: Occurs between 16-24 knots of
airspeed (dependent on rotor size, area, and RPM) and is the point at which the rotor
system outruns the recirculation of vortices. Thus, above this speed, the rotor begins
operating in clean, undisturbed air and produces lift more efficiently.
, Does power required to fly increase or decrease after passing through Effective
Translational Lift (ETL)? Why? - ANSWER: During this transition when the main rotor
becomes more efficient less torque is required for lift. The tail rotor gains efficiency as
well and less left pedal is required.
Stationary Hover - ANSWER: The rotor system has zero airspeed is operating in its own
wing tip vortices. The rotor system is less efficient and requires more power to sustain
lift.
Transition - ANSWER: As the helicopter moves forward, the rotor is affected by
Dissymmetry of Lift where the advancing side creates more lift than the retreating side.
Gyroscopic Precession (GP) causes this to manifest as a pitch up tendency. Also during
the transition phase, Transverse Flow affect takes places between 10-20 knots which
causes a right roll.
Forward Flight - ANSWER: After passing ETL the efficient rotor creates more lift.
Dissymmetry of Lift is still present and causes a nose up tendency that is counteracted
by forward cyclic input.
What is Translating Tendency? - ANSWER: Also known as the "torque effect" this is the
tendency for helicopters to drift laterally in the direction of tail rotor thrust while at a
hover.
What is the Torque Effect and how is it compensated for? - ANSWER: As the main rotor
turns CCW, it pushes the aircraft fuselage in the opposite direction. If this were left
uncompensated, the fuselage would continue to rotate leaving the aircraft
uncontrollable. This is fixed by adding an a tail rotor. This counteracts main rotor torque
by pushing the fuselage CCW. The tail rotor does this by pushing the tail to the right.
Because it pushes the tail right and the main rotor torque pushes the nose right, the
helicopter develops a lateral drift to the right. To offset the drift, control rigging, flight
management computers, or pilot inputs can compensate.
Which side does the helicopter tend to drift towards? - ANSWER: CCW helicopters drift
to the right.
When translating tendency is compensated, which landing gear hangs lower in a CCW
rotating system? - ANSWER: The left wheel/skid tends to hang low while hovering.
What is a Vortex Ring State? - ANSWER: A condition where a rotor system has a
secondary set of vortices in addition to the wing tip vortices. This second set of vortices
when combined with wing tip vortices creates highly disturbed/turbulent airflow through
the rotor disk causing massive rotor inefficiency.
What is Settling with Power? - ANSWER: The helicopter "settles" or descends despite
having the necessary power applied for flight.
What is Dissymmetry of Lift? - ANSWER: The difference in lift between the advancing
and
retreating blades of the helicopter.
Which side of the rotor has lower relative airspeed? Why? - ANSWER: The retreating
blade. Because the blade velocity is reduced by negative relative airflow.
What are the 3 No Lift Areas? - ANSWER: Reverse Flow, Negative Stall, Negative Lift
How do helicopters compensate for Dissymmetry of Lift? - ANSWER: Blade Flapping
and Cyclic Feathering
Reverse Flow - ANSWER: Airflow flows from trailing edge to leading edge and therefore
produces no lift.
Negative Stall - ANSWER: Airflow impacts above the chord line and outside the critical
angle which produces turbulent air.
Negative Lift - ANSWER: Airflow impacts above the chord line and inside the critical
angle. Same effect as negative pitch in the blade.
Positive Lift - ANSWER: Airflow is inside the critical angle and below the chord line. This
is the good stuff.
Positive Stall - ANSWER: Airflow is below the chord line and outside the critical angle.
Lift is reduced.
Blade Flapping - ANSWER: Occurs when advancing blade flaps up while retreating
blade flaps down. This up flap velocity results in an increased downward flow of air
(induced flow) over the airfoil. This shrinks the angle between the resultant relative wind
and the chord line thereby reducing the angle of attack (AOA). This effect reduces lift
generated from the advancing blade and increases lift for the retreating blade.
Cyclic Feathering - ANSWER: Occurs when the pilot inputs forward cyclic which results
in increasing pitch at the 9 o'clock position and decreasing pitch at the 3 o'clock position
(Gyroscopic Precession). This increases AOA on the retreating side and decreases on
the advancing side of the rotor.
What is Gyroscopic Precession (Phase Lag)? - ANSWER: The phenomenon present in
rotating bodies where an action presents itself 90 degrees later in the plane of rotation.
,How many degrees of delay does Gyroscopic Precession cause? - ANSWER: 90
To move forward, where does the pilot move the cyclic and where does the pitch
increase occur? Backwards? - ANSWER: To move forward, pitch is increased at 9
o'clock.To move backward, pitch is increased at 3 o'clock.
What is Transverse Flow Effect? - ANSWER: The difference in lift, drag, and induced
flow that exists between the forward half and aft half of the rotor system between 10 to
20 knots of airspeed.
At what airspeed range does the Transverse Flow Effect occur? - ANSWER: 10 to 20
knots
What effect does the pilot notice because of the Transverse Flow? - ANSWER: Due to
Gyroscopic Precession, this is manifested 90 degrees later as a right roll when
transitioning through airspeeds between 10 to 20 knots (most prevalent during takeoffs
and landings).
What defines the differences between In Ground Effect (IGE) and Out of Ground Effect
(OGE)? - ANSWER: Airflow at a hover can be broken down into two types: In Ground
Effect (IGE) and Out of Ground Effect (OGE). Ground effect is the increase in lift that
occurs when a helicopter is hovering within one rotor diameter above a surface. (Ex:
Within 50 ft AGL if the rotor is 50 ft in diameter)
Does it require more power to hover IGE or OGE? Why? - ANSWER: There is higher
pitch angle (AOI) and power required for helicopters to hover OGE compared to IGE.
This is due to greater induced flow.
In Ground Effect (IGE) - ANSWER: When helicopters hover, air flows down vertically
and vortices develop around the rotor tips. While IGE, this downwards flow of air is
restricted due to ground friction as the airflow is redirected out and away from the rotor.
Because of this, the high velocity of a downwards flow of air or induce flow is reduced.
Out of Ground Effect (OGE) - ANSWER: While OGE, nothing restricts the downwards
flow of air. Therefore there is a high velocity of induced flow, wing tip vortices are
allowed to develop without restriction, and more power is required compared to IGE.
At what airspeed range does Effective Translational Lift (ETL) occur? - ANSWER:
Between 16-24 knots
What is Effective Translational Lift (ETL)? - ANSWER: Occurs between 16-24 knots of
airspeed (dependent on rotor size, area, and RPM) and is the point at which the rotor
system outruns the recirculation of vortices. Thus, above this speed, the rotor begins
operating in clean, undisturbed air and produces lift more efficiently.
, Does power required to fly increase or decrease after passing through Effective
Translational Lift (ETL)? Why? - ANSWER: During this transition when the main rotor
becomes more efficient less torque is required for lift. The tail rotor gains efficiency as
well and less left pedal is required.
Stationary Hover - ANSWER: The rotor system has zero airspeed is operating in its own
wing tip vortices. The rotor system is less efficient and requires more power to sustain
lift.
Transition - ANSWER: As the helicopter moves forward, the rotor is affected by
Dissymmetry of Lift where the advancing side creates more lift than the retreating side.
Gyroscopic Precession (GP) causes this to manifest as a pitch up tendency. Also during
the transition phase, Transverse Flow affect takes places between 10-20 knots which
causes a right roll.
Forward Flight - ANSWER: After passing ETL the efficient rotor creates more lift.
Dissymmetry of Lift is still present and causes a nose up tendency that is counteracted
by forward cyclic input.
What is Translating Tendency? - ANSWER: Also known as the "torque effect" this is the
tendency for helicopters to drift laterally in the direction of tail rotor thrust while at a
hover.
What is the Torque Effect and how is it compensated for? - ANSWER: As the main rotor
turns CCW, it pushes the aircraft fuselage in the opposite direction. If this were left
uncompensated, the fuselage would continue to rotate leaving the aircraft
uncontrollable. This is fixed by adding an a tail rotor. This counteracts main rotor torque
by pushing the fuselage CCW. The tail rotor does this by pushing the tail to the right.
Because it pushes the tail right and the main rotor torque pushes the nose right, the
helicopter develops a lateral drift to the right. To offset the drift, control rigging, flight
management computers, or pilot inputs can compensate.
Which side does the helicopter tend to drift towards? - ANSWER: CCW helicopters drift
to the right.
When translating tendency is compensated, which landing gear hangs lower in a CCW
rotating system? - ANSWER: The left wheel/skid tends to hang low while hovering.
What is a Vortex Ring State? - ANSWER: A condition where a rotor system has a
secondary set of vortices in addition to the wing tip vortices. This second set of vortices
when combined with wing tip vortices creates highly disturbed/turbulent airflow through
the rotor disk causing massive rotor inefficiency.
What is Settling with Power? - ANSWER: The helicopter "settles" or descends despite
having the necessary power applied for flight.
