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

What is lead-lag?

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

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

How does the pilot handle low rotor RPM?

Lower the collective to reduce drag and increase throttle for more power, if available.  If in forward flight, gently apply aft cyclic.

Low rotor RPM is a significant concern for pilots.  The slower blade speed reduces the amount of lift as velocity is a key factor in lift production.  If the pilot tries to retain the same altitude, additional lift will be needed.  Should the pilot attempt to raise the collective for more lift, the increased drag will further reduce the rotor speed.  As the rotor RPM decays, the centrifugal force reduces and results in increased conning of the blades.  The increased coning compounds the problem as the rotor thrust is directed inward, reducing its vertical lift capacity.  In addition, the size of the overall disc becomes smaller, further increasing the needed for lift is the altitude is to remain the same.

Lowering the collective reduces the blade angle, which reduces the angle of attack.  With a reduced angle of attack, the induced drag will be less, and the rotor RPM should increase, assuming the same power requirements.  In addition, aft cyclic will tilt the rotor disc aft which in turn lowers the inflow angle on the rotor, which reduces the angle of attack, and reduces the induced drag.  Due to the aft cyclic application, some of the airflow is now coming from underneath the disc, which reduces the induced flow, which in turn reduces the angle of attack, induced drag, etc.  In addition, the coning angle will increase with aft cyclic.  The increased coning angle reduces the size of the disc, and because of the Coriolis effect, the disc will increase rotation.  Should the rotor RPM be allowed to decay beyond the lower limits, the lack of adequate centrifugal force may allow the blades to collapse.  Remember, lower the collective and increase power to maintain rotor RPM within limits.  Do not let RPM decay.

Reference(s):

FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-15, 11-15
Principles of Helicopter Flight, 2nd Edition, pg. 171

Other Helicopter Flight Conditions

What are the primary forces that affect wind?

The three primary forces that affect wind are pressure gradient force, Coriolis force, and friction.

The atmosphere has different areas of pressure.  Wind is created as the pressure systems try to equalize.  The airflow moves from high pressure to low pressure.

The Coriolis force causes the wind to deflect to the right in the Northern hemisphere.  he Coriolis force is directly proportional to the speed of the wind.  The greater the wind speed, the greater the Coriolis force at a given latitude.  The Coriolis force is zero at the equator, and more pronounced at middle and higher latitudes.  The Coriolis force affects all moving objects.  This is the same Coriolis force that affects the rotor system, such as when coning.

Wind is slowed down near the surface due to friction with the earth.  The rougher the terrain, the greater the frictional effect.  Also, the stronger the wind speed, the greater the friction.  The frictional drag of the ground normally decreases with height and becomes insignificant above a few thousand feet.

Local winds are small-scale wind systems driven by heating or cooling of the ground.  Air temperature differences develop over adjacent surfaces.  Air in contact with the ground heats during the day and cools at night.  Local winds are a significant factor for helicopters because their flight is generally local and often close to the ground.

Reference(s):

FAA AC 00-6B Aviation Weather pg. 7-1, 9-1

Other Weather and Atmosphere Topics