## What is the Coriolis effect?

The Coriolis effect is when the rotor blades speed up or slow down as the center of gravity moves closer or further away from the axis of rotation.

The Coriolis effect is otherwise known as the law of conservation of angular momentum, which states that an object will have the same rotational momentum unless acted upon by an outside force.  There are two primary factors involved with the Coriolis effect as it relates to the rotor system.  These factors are the distance of the blades center of gravity (CG) from the axis of rotation and the rotational speed of rotor or centrifugal force being applied.

If a blade’s center of gravity moves closer to the axis of rotational axis, that blade’s speed will increase.  The Coriolis effect is significant when the blades cone or lead-lag due to flapping.  When the blades cone, such as due to high G-loading, the rotor disc diameter decreases, and the CG moves inward on all blades.  As the rotor disc diameter becomes smaller, all the blades increase speed.   This can be felt as an increase in rotor RPM, without corrective action by the pilot or governor.

In a fully-articulated rotor system, the CG moves closer to the axis of rotation when blades flap up.  As a blade flaps up it increases speed and leans forward, which reduces stress on the rotor system.  The opposite is true when a blade flaps down.  As it flaps down, the CG moves away from the center of axis and the rotation will decrease resulting in the blade lagging.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-15
Principles of Helicopter Flight, 2nd Edition, pg. 78
FM 3-04.203-2007 Fundamentals of Flight pg. 1-15, 1-53

Other Helicopter Aerodynamic Principles

Lead-lag or leading and lagging is the horizontal movement of the rotor blades forwards and backwards along a vertical hinge.

Leading and lagging is a capability designed into a fully-articulated rotor system to reduce the stress on the rotor system due to blade flapping.  The need to lead-lag is due to the Coriolis effect, otherwise known as the law of conservation of angular momentum.  As a rotor blade flaps up, the blade’s speed increases because the center of mass of that blade moves closer to the axis of rotation.  As the blade flaps downs, the center of mass moves away from the axis of rotation and the speed of that blade slows downs.  The lead-lag hinge allows the forces to equalize, which removes undue stress on the system.  Lead-lag may also be referred to as hunting or dragging.

With a semi-rigid rotor system, such as on the Robinson R22/R44, there is no vertical drag hinge as the design minimizes any impact from the Coriolis force.  Due to the underslung hinge, the blade moves outward when it flaps up, so the center of mass of that blade does not change significantly.  Any remaining lead-lag forces are absorbed through the blades.

A ridged rotor system does not have flapping or lead-lag hinges.  With a ridged rotor system, these forces are absorbed through bending of the blades.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 4-4
Principles of Helicopter Flight, 2nd Edition, pg. 78
FM 3-04.203-2007 Fundamentals of Flight pg. 1-15

Other Helicopter Aerodynamic Principles

## What is cyclic feathering?

Cyclic feathering is the term used to describe the changing of the blade angle separately for individual blades through pilot input via the cyclic.

The pilot changes the angle of the stationary swashplate with movements of the cyclic.  The rotating swashplate imparts this change by moving the attached pitch links up and down as the rotor system turns.  The movement of the pitch links up or down, increases or decreases the angle of an individual blade.  If there is any angle to the swashplate, the blade angles will be changing as the rotor system turns.  However, the total rotor thrust will remain the same.  When one blade angle increases, another decreases.  Although the total rotor thrust remains the same, cyclic feathering creates differential lift across the rotor disc.  The pilot uses this differential lift to control the helicopter’s attitude.  The combination of cyclic feathering and blade flapping compensate for the dissymmetry of lift across the rotor disc due to wind or airspeed.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-12
Principles of Helicopter Flight, 2nd Edition, pg. 89
FM 3-04.203-2007 Fundamentals of Flight pg. 1-41

Other Helicopter Aerodynamic Principles

## How does gyroscopic precession affect helicopter flight?

Due to gyroscopic precession, changes to the rotor system are felt 90 degrees to the right or in the direction of the rotation.

A gyroscope provides rigidity in space, in that a spinning object does not want to change its position.  The spinning rotor is a large gyroscope.  When factors impact the rotor disc, the pilot will experience the effect 90 degrees to the right or in the direction of rotation.  Below are the two primary examples:

Retreating Blade Stall: With retreating blade stall, the blade angle is at its greatest when the blade is directly to the left or 270 degrees.  However, the effect is felt over the tail, as the tail drops, which pitches the nose of the helicopter up.

Transverse Flow (Inflow Roll)Transverse flow causes less lift at the rear of the rotor disc, but the helicopter will roll to the right as the effect is felt 90 degrees to the right.

The helicopter designers take gyroscopic precession into account, so the pilot does not need to worry about it during flight.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-16
Principles of Helicopter Flight, 2nd Edition, pg. 177
FM 3-04.203-2007 Fundamentals of Flight pg. 1-17

Other Helicopter Aerodynamic Principles

## What is center of pressure?

The center of pressure is the point along the chord line of an airfoil where of all aerodynamic forces are considered to act.

With a symmetrical airfoil, the center of pressure remains relatively constant with changes in angles of attack.  With a non-symmetrical airfoil, the center of pressure moves significantly along the chord line with changes in the angle of attack.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-7
Principles of Helicopter Flight, 2nd Edition, pg. 22
FM 3-04.203-2007 Fundamentals of Flight pg. 1-7

Other Helicopter Aerodynamic Principles

## What is effective translational lift?

Effective translational lift (ETL) is when the lift generation from the rotor disc is more efficient due to increased aircraft speed or wind.

When at a hover in calm, no-wind conditions, the induced flow is a significant factor affecting the resultant relative wind.  As a result, the blade angles are significant and it takes more power for flight.  As the aircraft increases speed, approximately 16-24 knots indicated airspeed, the impact of induced flow is reduced.  Because of forward movement or wind, there is undisturbed air* meeting the front of the rotor disc.  As speed increases, the portion of the disc receiving undisturbed air increases.  As a result, the inflow angle is decreasing as more air is received horizontally versus vertically.  As such, a lower angle of attack will produce the same lift, resulting in less power needed for flight.

The tail rotor also becomes more efficient with an increase in forward speed.  There are two factors involved.  First, like the main rotor, the tail rotor becomes more efficient when it operates in undisturbed air.  Second, the forward movement of the aircraft reduces the amount of anti-torque thrust needed as the horizontal stabilizer or similar component, becomes more effective.  As the need for anti-torque lessens, there is more power available for the main rotor.

It is a common mistake for pilots to refer to flying through ETL.  An aircraft achieves ETL.  Once in ETL, the aircraft is receiving the benefit of transitional lift until the relative wind is changed so the ETL is no longer achieved, such as by slowing down, flying downwind or a change in wind velocity.  In general, the increase in horizontal speed or wind, the more efficient the rotor system.

* Some people will make reference to clean/dirty air instead of undisturbed/disturbed air.  The use of the term clean air and dirty air should be avoided as the level of particulate matter in the air is not a factor, it is the turbulence of the air that is relevant.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-20
Principles of Helicopter Flight, 2nd Edition, pg. 64, 99
FM 3-04.203-2007 Fundamentals of Flight pg. 1-41, 1-43

Other Helicopter Flight Conditions

## What is translating tendency?

Translating tendency is the movement of the helicopter to the right due to the combination of main rotor torque and tail-rotor anti-torque.

If uncompensated, power applied to the main rotor would turn or yaw the helicopter to the right.  To prevent turning, anti-torque is applied via left pedal.  As anti-torque is applied, the tail rotor produces thrust which pushes the tail to the right, stopping the potential turn.  There are now two forces moving the helicopter to the right, and one force to the left.  The forces to the right are greater and the helicopter tends to drift to the right, assuming no pilot inputs.  Translating tendency is often called rotor drift.

Some helicopter designs compensate for translating tendency by various methods, such as tilting the mast or designing a bias into the cyclic.  Most training helicopters, such as the Robinson R22/R44 and the Sikorsky 300CB, do not have any of these compensating characteristics.  In these helicopters, the pilot must counteract the translating tendency, or rotor drift, through pilot inputs to tilt the rotor disc to the left.

Not all helicopters experience translating tendency, specifically dual rotor helicopters such as the CH47.  In these helicopters, the torque produced by one rotor is counteracted by the other, which rotates in the opposite direction.  As such, forces are equal in each direction.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-14
Principles of Helicopter Flight, 2nd Edition, pg. 70
FM 3-04.203-2007 Fundamentals of Flight pg. 1-36

Other Helicopter Flight Conditions

## What is transverse flow?

Transverse flow is the decreased lift at the rear of the rotor disc due to an increase in induced flow as the disc moves through the air, producing a roll to the right.

Transverse flow occurs as a result of forward flight or a significant wind.  As the helicopter moves forward, the airflow at the front of the disc has not started its downward flow.  At the back of the disc, the induced flow or downwash is more significant, which reduces the angle of attack on the blades.  As a result, the front of the rotor disc is more efficient and produces more lift.  The rear of the disc wants to descend, but because of gyroscopic precession, the result is that the helicopter wants to roll to the right.  The transverse flow effect is also referred to as an Inflow Roll.

The transverse flow effect is felt as a vibration when just below effective translational lift (ETL) on takeoff and after losing ETL on landing.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-22
Principles of Helicopter Flight, 2nd Edition, pg. 101
FM 3-04.203-2007 Fundamentals of Flight pg. 1-42

Other Helicopter Flight Conditions

## What is blow-back (flap-back)?

Blow-back is the rearward tilt of the rotor disc during the transition to forward flight.

Blow-back is due to the combination of flapping and the transverse flow affect.  In forward flight, the advancing blade produces more lift than the retreating blade.  Flapping is used to correct for the dissymmetry of lift across the rotor disc.  As the advancing blade flaps up, it causes the front of the helicopter to rise.  In addition, the induced flow will be greater at the rear of the rotor disc during forward flight, increasing the lift on the front of the disc, thus increasing blow-back.  As forward movement occurs, the pilot will have to continually increase forward cyclic to compensate for flap-back/blow-back.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-19
Principles of Helicopter Flight, 2nd Edition, pg. 94
FM 3-04.203-2007 Fundamentals of Flight pg. 1-40

Other Helicopter Flight Conditions

## What is reverse flow?

At high airspeed, the airflow from the helicopter’s forward movement may exceed the rotational speed of the retreating blade.

The rotor blade is slower at the root than at the tip.  As the helicopter’s speed increases, the forward speed may exceed the rotor speed near the root.  In this portion of the disc, the airflow will be reversed.

Although the blade is not stalled, it is not generating lift.  As a result, the other sections of the blade need to provide more lift to equalize the lift on the rotor disc.  The angle of attack (AOA) must be increased on the retreating blade, but there is only so much increase in AOA before a blade will stall.

Reference(s):

Principles of Helicopter Flight, 2nd Edition, pg. 98

Other Helicopter Flight Conditions