Helicopter Pilot's Manual Vol 1

1 THE PRINCIPLES OF
HELICOPTER FLIGHT

Helicopters and other related rotary-wing aircraft are widely varied in their concept and configuration. This book concerns primarily the single-rotor helicopter, of the type that employs a compensating tail rotor.

Although the aerodynamics of the helicopter are based on the same laws that govern the flight of a fixed-wing aircraft, the significance of some considerations is somewhat different.

Both rely on lift produced from air flowing around an aerofoil, but whereas the aeroplane must move bodily forward through the air, the helicopter's rotors ('wings') move independently of the fuselage and can produce lift with the aircraft remaining stationary (hovering).

Both autogyros and helicopters have rotating wings (rotor blades), but those of the autogyro are not driven. Instead, they rotate freely in flight under the single influence of the airflow. The helicopter's rotor blades are engine driven in powered flight, giving it the ability to hover.

Before considering the principles of helicopter flight, it is necessary to explain some terms and definitions.

The principles of helicopter flight.

Aerofoil (Airfoil in USA) An aerofoil is any surface designed to produce lift when air passes over it. On a helicopter, the rotor blades are the aerofoils and normally are classed as symmetrical, because the blade's upper and lower surfaces have the same curvature.

Aerofoil section.

Chord line This is an imaginary line joining a rotor blade's leading and trailing edges.

The chord line.

Axis of rotation An actual or imaginary line about which a body rotates.

Plane of rotation This is normal to the axis of rotation and parallel to the rotor tip-path plane. It is at right angles to the axis of rotation.

Tip-path plane The path described by the tips of the rotor blades as they rotate.

The tip-path plane.

The rotor disc The area contained by the tips of the rotor blades.

The rotor disc.

Pitch angle The angle between the chord line and the plane of rotation.

The pitch angle.

Coning angle The angle between the spanwise length of a rotor blade and its tip-path plane.

Coning angle.

Coning Movement of the rotor blades aligning them along the resultant of centrifugal force and lift. An increase in lift would increase the coning angle; conversely, an increase in rotor rpm would decrease the coning angle.

Feathering The angular movement of a rotor blade about its longitudinal axis.

Feathering.

Flapping The angular movement of a rotor blade about a horizontal axis. In fully articulated rotors, the individual blades are free to flap about their flapping hinge.

Flapping.

Dragging The angular movement of a rotor blade about an axis vertical to that blade. The dragging hinge is only incorporated in fully articulated rotor systems.

Dragging.

Angle of attack The angle between the chord line and the relative airflow.

Angle of attack.

Total rotor thrust The sum of lift of all the rotor blades.

Disc loading The ratio of weight to the total main rotor-disc area.

Solidity ratio The ratio of the total blade area to the total disc area.

THE LIFTING FORCE OF THE ROTOR

Lift

To understand how lift is created, first we must review the basic principle of pressure differential. This was discovered by a Swiss physicist, Daniel Bernoulli. Simply put, Bernoulli's Principle states that as the velocity of a fluid (air) increases, its internal pressure decreases. When a relative wind blows across a rotor blade, the air divides, passing over the top of the blade and underneath it. Essentially, the air blowing across the top moves at a greater speed than that passing below, thereby creating a pressure differential, which results in lift.

The pressure differential.

The lift produced from the wing of an aeroplane results from a combination of many things and commonly is expressed in the formula: L = CL ½ pV2S, where L = lift; CL = coefficient of lift; p = air density; V = velocity; and S = surface area of the blade.

Lift from a helicopter rotor blade can generally be expressed in the same terms, but because the rotor blade moves independently of the fuselage, the velocity (V2) when hovering in still-air conditions is purely the result of the rotation of the blade (rotor rpm).

Blade Pitch

The wing of an aeroplane is fitted to the fuselage at an angle, the datums being the chord line and a line running longitudinally down the fuselage. The angle between the two is known as the angle of incidence.

Blade pitch.

A rotor blade, when attached to the main rotor head, will also have a basic setting. The datums are the chord line of the rotor blade and the plane in which the rotor blade is free to rotate. This angle between the two datums is the pitch angle.

If the rotor blade had a constant value of pitch throughout its length, problems would arise in relation to blade loading, because each section of the blade would have a different rotational velocity and, therefore, a different value of lift. As lift is proportional to V2, if the speed were doubled, the lift would increase fourfold.

Blade lift.

To avoid this considerable variation of lift, it is necessary to increase lift at the root and decrease it at the tip. This can be achieved by tapering the blade, twisting the blade (washout), or a combination of the two. Even then, lift from the blade will have its greatest value near the tip, but its distribution along the blade will be more uniform.

Relative Airflow

Consider a column of still air through which a rotor blade is moving horizontally. The effect will be to displace some of the air downward. If a number of rotor blades are travelling along the same path in rapid succession (with a three-bladed rotor system operating at 240 rpm, a blade will be passing a given point every twelfth of a second), the column of still air will become a column of descending air.

Induced flow.

This column of descending air is known as the induced flow. Therefore, the direction of the air relative to the rotor blade will be the resultant of the blade's horizontal travel through the air and the induced flow.

Relative airflow.

Total Reaction

This force acting on an aerofoil can be understood more easily if split into two components: lift and drag. Lift acts at a right angle to the relative airflow, but, as a result, does not provide a force in direct opposition to weight. Therefore, the lifting component of the total reaction must be the part that is acting along the axis of rotation. This component is known as rotor thrust. The other component of total reaction will be in the rotor blade's plane of rotation and is known as rotor drag.

Total reaction.

Total Rotor Thrust

If the rotor blades are perfectly balanced and each blade is producing the same amount of rotor thrust, the total rotor thrust can be said to be acting through the rotor head at a right angle to the plane of rotation.

Total rotor thrust.

Coning Angle

The effect of rotor thrust will cause the rotor blades to rise until they reach a position where their upward movement is balanced by the outward pull of the centrifugal force generated by the rotation of the blades.

Coning angle.

At high rotor speeds, the blades produce a great deal of centrifugal force, keeping the coning angle low. When rotor speed is decreased, there is less centrifugal force, so the coning angle will increase. As this centrifugal action through rotor rpm gives a measure of control of the coning angle, provided the rotor speed is kept within the specified limits for a particular helicopter, the coning angle will remain within safe operating limits.

There will also be upper limits to the rotor rpm, due to engine and transmission considerations as well as end loading stresses where the blade is attached to the rotor head.

HELICOPTER SYSTEMS

There are many variations in the design of a modern helicopter. Even though helicopters come in all shapes and sizes, however, they share many of the same major components.

Flight Control Systems

Main Rotor Systems

Main rotor systems are classified according to how the rotor blades move relative to the main rotor hub. The main categories are fully articulated, semirigid and rigid.

Fully articulated Each main rotor blade is free to move up and down (flapping), to move back and forth (dragging), and to twist about the spanwise axis (feathering). This type of system normally has three or more blades.

Fully articulated rotor system.

Semi-rigid system Normally, two main rotor blades are rigidly attached to the main rotor hub, which is free to tilt and rock independently of the main rotor mast on what is known as a teetering hinge - as one blade flaps up, the other flaps down. There is no vertical drag hinge.

Semi-rigid rotor system.

Rigid rotor system...