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DEPARTMENT OF AERONAUTICAL ENGG

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INTRODUCTION TO HELICOPTERS

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THE MAIN ROTOR SYSTEM

The rotor system found on helicopters can consist of asingle main rotor or dual rotors

Rotor system can be classified as either fullyarticulated, semi rigid, or rigid

The fully articulated rotor system incorporates morethan two blades. Lead/lag is possible by use of verticalhinge pins. Horizontal hinge pins allow for flapping.

The movement of each blade is independent of the

other blades and independent in respect to the rotorhead.

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FULLY ARTICULATED ROTOR SYSTEM

A fully articulated rotor system usually consists

of three or more rotor blades.

The blades are allowed to flap, feather, and lead

or lag independently of each other.

Each rotor blade is attached to the rotor hub by

a horizontal hinge, called the flapping hinge,

which permits the blades to flap up and down. Each blade can move up and down

independently of the others.

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FULLY ARTICULATED ROTOR SYSTEM

Each rotor blade is also attached to the hub by

a vertical hinge, called a drag or lag hinge, that

permits each blade, independently of the

others, to move back and forth in the plane of

the rotor disc

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FULLY ARTICULATED ROTOR SYSTEM

Blade FlapThe upward or downward movement

of the rotor blades during rotation.

Blade Feather or FeatheringThe rotation of the

blade around the span wise (pitch change) axis. Or

Feathering means the changing of the pitch angle of

the rotor blades

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Rigid

The term rigid as applied to rotor systems is

generally misleading due to the considerable

flexibility in the systems.

"Hingeless" may be a better description in

most cases. The hub itself bends and twists in

order to provide for flapping, lead-lag, and

pitch control.

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Semi rigid

The semi-rigid rotor system uses two rotor

blades and incorporates a horizontal hinge pin

only for flapping. Pitch change movement is

also allowed.

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Pitch angle

The rotor blade pitch angle is the acute angle

between the blade chord line and a reference

plane determined by the main rotor hub.

Since the rotor plane of rotation is parallel tothe plane containing the main rotor hub, the

rotor blade pitch angle could also be

described as the acute angle between theblade chord line and the rotor plane of

rotation.

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Airfoil in rotors

the first helicopters designed relatively thickairfoils for their structural characteristics.

Because the rotor blades were very long andslender, it was necessary to incorporate morestructural rigidity into them. This preventedexcessive blade droop when the rotor system wasidle, and minimized blade twisting while in flight.

The airfoils were also designed to be

symmetrical, which means they had the samecamber (curvature) on both the upper and lowersurfaces.

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Airfoil in rotors

Symmetrical blades are very stable, whichhelps keep blade twisting and flight controlloads to a minimum.

Today, designers use thinner airfoils andobtain the required rigidity by usingcomposite materials. In addition, airfoils areasymmetrical in design, meaning the upperand lower surface do not have the samecamber .

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Reflexing

Normally these airfoils would not be as stable,

but this can be corrected by bending the

trailing edge to produce the same

characteristics as symmetrical airfoils.

This is called reflexing. Using this type of

rotor blade allows the rotor system to operate

at higher forward speeds.

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Rotor

the span is the distance from the rotor hub to

the blade tip.

Blade twist refers to a changing chord line

from the blade root to the tip.

Twisting a rotor blade causes it to produce a

more even amount of lift along its span.

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Unit III

IDEAL ROTAR THEORY

Hovering performances - Momentum andsimple blade element theories

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