Helicopter Rotor System and Design

Helicopter Rotor System and Design

INDEX

SR. NO. CONTENTS PAGE NO.

1 INTRODUCTION 31.1 ANATOMY OF A HELICOPTER 32 FORCES ON HELICOPTER 63 MAIN ROTOR SYSTEM 74 CONTROLS 95 IMPORTANT PARTS 105.1 SWASH PLATE 105.2 STABILIZER BAR 126 CLASSIFICATION OF MAIN ROTOR SYSTEM 136.1 RIGID 136.2 SEMI-RIGID 136.3 FULLY ARTICULATED 146.4 ENGINEERED COMBINATION 147 THE ANTI-TORQUE EFFECT 157.1 SINGLE MAIN ROTOR 157.1.1 TAIL ROTOR 157.1.2 DUCTED FAN 167.1.3 NOTAR 167.1.4 TIP JETS 177.2 DUAL ROTORS (COUNTER ROTATING) 177.2.1 TANDEM 187.2.2 COAXIAL 197.2.3 INTERMESHING 197.2.4 TRANSVERSE 207.2.5 QUADROTOR 218 MAIN ROTOR BLADE 218.1 ROTOR BLADE DESIGN 228.1.1 AIRFOIL, LIFT AND DRAG 228.1.2 BLADE TWIST AND TAPPER 228.1.3 BLADE ROOT CUT OUT 238.1.4 TWISTING MOMENTS 238.1.5 BLADE TIP SPEED AND NOISE REDUCTION 248.2 CONSTRUCTION 249 REFERENCES 27

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INDEX OF FIGURE

SR NO. NAME OF FIGURE PAGE NO.

1 Anatomy of helicopter UH-1C 4

2 Helicopter assembly 5

3 Typical helicopter drive train 5

4 Forces on helicopter 6

5 Main rotor system 7

6 Rotor system drawing 8

7 Controls of helicopters 9

8 Swash plate model 10

9 Swash plate working 11

10 Tail rotor helicopter 15

11 Ducted fan helicopter 16

12 Movement of air through NOTAR 16

13 Tandam helicopter ch-47 18

14 Coaxial helicopter 19

15 HH-43 Huskie 19

16 Intermeshing rotor drawing 20

17 Transverse helicopter v-12 20

18 Quadrotor 21

19 Cross-section of rotor blade 21

20 Blade aerofoil shape 22

21 Blade twist and tapter 23

22 Blade 23

23 Force on blade 24

24 Composite design 25

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1. Introduction:

Air resistance behaves the same way as fluid resistance. When you try to swim

through water, the faster you swim, the harder it becomes. For air resistance, a surface

moving through the air will experience drag the faster it moves. What helicopter does

is use the air resistance to create a lifting force. When the leading edge of a surface is

higher and the rear edge is lower, air flow would strike the surface and be pushed

down. The air flow pushing downward is the same as the surface being pushed

upward, which creates lift. For aircraft, the slanted under surface of wing and rotor

blade allows air to create lift.

A helicopter rotor is powered by the engine, through the transmission, to the rotating

mast. The mast is a cylindrical metal shaft which extends upward from helicopter and

is driven by the transmission. At the top of the mast is the attachment point for the

rotor blades called as hub. The rotor blades are then attached to the hub.

1.1. Anatomy of a Helicopter:

Here are some of the component parts that make up a helicopter. {While this is an

example of one specific helicopter (UH-1C), not all helicopters will have all of the

parts listed here, but some of its kind.}

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Fig.1 Anatomy of helicopter UH-1C (helicopterpage.com)

Rotor Blade: The rotary wing that provides lift for the helicopter.

Stabilizer Bar: Dampens control inputs to make smoother changes to the

rotor system.

Swash plate: Transfers non-moving control inputs into the spinning rotor

system.

Cowling: The aerodynamic covering for the engine.

Mast: Connects the transmission to the rotor system.

Engine: Provides power to the rotor systems.

Transmission: Takes power from the engine and drives both rotor systems.

Greenhouse Window: A tinted window above each of the pilot seats.

Fuselage: The body of the helicopter.

Cabin Door: Allows access to the cabin and cockpit.

Skids: Landing gear that usually have no wheels or brakes.

Cross tube: The mounting tubes and connection for the skids.

Motor Mount: A flexible way to attach the engine to the fuselage.

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Tail boom: Also known as an "empenage" is the tail of the helicopter.

Synchronized Elevator: A movable wing that helps stabilize the helicopter in

flight.

Tail rotor: Provides anti-torque and in-flight trim for the helicopter.

Tail Rotor Driveshaft: Provides power to the tail rotor from the transmission.

45 Degree Gearbox: Transfers power up the vertical fin to the 90 degree

gearbox.

90 Degree Gearbox: Transfers power from the 45 degree gearbox to the tail

rotor.

Vertical Fin: Holds the tail rotor and provides lateral stabilization.

Tail Skid: Protects the tail boom when landing.

Fig. 2 Helicopter assembly (helicopterpage.com)

Fig.3 (helicopterpage.com)

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2. Forces on helicopter:

Fig.4 Forces on helicopter (helicopterpage.com)

When the blades are spinning then the natural reaction to that is for the fuselage of the

helicopter to start spinning in the opposite direction to the rotors. If this torque isn't

controlled, the helicopter would just spin round hopelessly!

So to beat the reaction of the torque, different systems are used and is connected by

rods and gears to the main rotor so that it turns whenever the main rotor is spinning.

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3. Main Rotor System:

Fig. 5(helicopterpage.com)

Root: The inner end of the blade where the rotors connect to the blade grips.

Blade Grips: Large attaching points where the rotor blade connects to the

hub.

Hub: Sits atop the mast, and connects the rotor blades to the control tubes.

Mast: Rotating shaft from the transmission, which connects the rotor blades to

the helicopter.

Control Tubes: Push \ Pull tubes that change the pitch of the rotor blades.

Pitch Change Horn: The armature that converts control tube movement to

blade pitch.

Pitch: Increased or decreased angle of the rotor blades to raise, lower, or

change the direction of the rotors thrust force.

Jesus Nut: Is the singular nut that holds the hub onto the mast. (If it fails, the

next person you see will be Jesus).

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Fig. 6 Rotor system drawing (helicopterpage.com)

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4.

Controls:

Fig. 7 Controls of helicopter (helicopterpage.com)

Swash Plate: Turns non-rotating control movements into rotating control

movements.

Collective: The up and down control. It puts a collective control input into the

rotor system, meaning that it puts either "all up", or "all down" control inputs

in at one time through the swash plate. It is operated by the stick on the left

side of the seat, called the collective pitch control. It is operated by the pilots

left hand.

Cyclic: The left and right, forward and aft control. It puts in one control input

into the rotor system at a time through the swash plate. It is also known as the

"Stick". It comes out of the center of the floor of the cockpit, and sits between

the pilots legs. It is operated by the pilot’s right hand.

Pedals: These are not rudder pedals, although they are in the same place as

rudder pedals on an airplane. A single rotor helicopter has no real rudder. It

has instead, an anti-torque rotor (Also known as a tail rotor), which is

responsible for directional control at a hover, and aircraft trim in forward

flight. The pedals are operated by the pilots feet, just like airplane rudder

pedals are. Tandem rotor helicopters also have these pedals, but they operate

both main rotor systems for directional control at a hover.

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5. Important Parts:

5.1. Swash plate:

Fig. 8 Swash plate model (helicopterpage.com)

The swash plate assembly has two primary roles:

Under the direction of the collective control, the swash plate assembly can

change the angle of both blades simultaneously. Doing this increases or

decreases the lift that the main rotor supplies to the vehicle, allowing the

helicopter to gain or lose altitude.

Under the direction of the cyclic control, the swash plate assembly can change

the angle of the blades individually as they revolve. This allows the helicopter

to move in any direction around a 360-degree circle, including forward,

backward, left and right.

The swash plate assembly consists of two plates -- the fixed and the rotating swash

plates -- shown above in blue and red, respectively. The rotating swash plate rotates

with the drive shaft (green) and the rotor's blades (grey) because of the links (purple)

that connect the rotating plate to the drive shaft. The pitch control rods (orange)

allow the rotating swash plate to change the pitch of the rotor blades. The angle of the

fixed swash plate is changed by the control rods (yellow) attached to the fixed swash

plate. The fixed plate's control rods are affected by the pilot's input to the cyclic and

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collective controls. The fixed and rotating swash plates are connected with a set of

bearings between the two plates. These bearings allow the rotating swash plate to

spin on top of the fixed swash plate.

Fig. 9 Swash plate working (helicopterpage.com)

The pitch of main rotor blades can be varied cyclically throughout its rotation in order

to control the direction of rotor thrust vector (the part of the rotor disc where the

maximum thrust will be developed, front, rear, right side, etc.). Collective pitch is

used to vary the magnitude of rotor thrust (increasing or decreasing thrust over the

whole rotor disc at the same time). These blade pitch variations are controlled by

tilting and/or raising or lowering the swash plate with the flight controls. The vast

majority of helicopters maintain a constant rotor speed during flight, leaving only the

angle of attack of the blades as the sole means of adjusting thrust from the rotor.

The swash plate is two concentric disks or plates; one plate rotates with the mast,

connected by idle links, while the other does not rotate. The rotating plate is also

connected to the individual blades through pitch links and pitch horns. The non-

rotating plate is connected to links which are manipulated by pilot controls,

specifically, the collective and cyclic controls.

The swash plate can shift vertically and tilt. Through shifting and tilting, the non-

rotating plate controls the rotating plate, which in turn controls the individual blade

pitch.

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http://en.wikipedia.org/wiki/File:Sikorsky_S-92_rotor_P1230176.jpg


5.2. Stabilizer bar:

Arthur M. Young found that stability could be increased significantly with the

addition of a stabilizer bar (also called a flybar) perpendicular to the two blades. The

stabilizer bar has weighted ends which cause the bar to stay relatively stable in the

plane of rotation. The stabilizer bar is linked with the swash plate in such a manner as

to reduce the effect of external forces on the rotor. The result is a much more stable

rotor system which eases the workload of the pilot to maintain control of the aircraft.

Stanley Hiller also arrived at a method to improve stability by adding a bar

perpendicular to the rotor, but he added short, stubby airfoils, or flaps, at each end.

Hiller's "Rotormatic" system was used to deliver cyclic control inputs to the main

rotor as a sort of control rotor, the flaps providing added stability by also dampening

the effects of external forces on the rotor.

6. Classification of main rotor system:

Main rotor systems are classified according to how the main rotor blades are attached

and move relative to the main rotor hub. There are three basic classifications:

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Rigid

Semi rigid

Fully articulated

Engineered combination of the above

6.1. Rigid:

The term "rigid rotor" usually refers to a hingeless rotor system with blades flexibly

attached to the hub. The two basic types of rigid rotor include the Reiseler-Kreiser

feathering system and the Lockheed flapping system. In a flapping rigid rotor system,

each blade flaps, drags, and feathers about flexible sections of the root. The flapping

rigid rotor system is mechanically simpler than the fully articulated rotor system.

Loads from flapping and lead/lag forces are accommodated by bending rather than

through hinges. The rigid rotor can also be called a hingeless rotor.

6.2. Semi-rigid:

The confusingly termed "semi-rigid" rotor is more accurately known as a "teetering"

or "seesaw" rotor. This rotor system allows for flapping and feathering motions with

two per revolution inplane motions accommodated by the blade roots and rotor shaft.

This system is normally composed of two blades which meet at a common flapping

hinge at the rotor shaft. This allows the blades to see-saw or flap together. This

teetering hinge combined with an adequate coning angle and undersling minimizes

variations in the radius of each blade's centre of mass from the axis of rotation as the

rotor turns. Secondary flapping hinges may also be provided to provide sufficient

flexibility to minimize bouncing. Feathering is accomplished by the feathering hinge,

which changes the pitch angle of the blade.

6.3. Fully articulated:

In a fully articulated rotor system, each rotor blade is attached to the rotor hub through

a series of hinges which allow the blade to move independently of the others. These

rotor systems usually have three or more blades. The blades are allowed to flap,

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feather, and lead or lag independently of each other. The horizontal hinge, called the

flapping hinge, allows the blade to move up and down. This movement is called

flapping and is designed to compensate for dissymmetry of lift. The flapping hinge

may be located at varying distances from the rotor hub, and there may be more than

one hinge. The vertical hinge, called the lead-lag or drag hinge, allows the blade to

move back and forth. This movement is called lead-lag, dragging, or hunting. Each

blade can also be feathered, that is, rotated around its spanwise axis. Feathering the

blade means changing the pitch angle of the blade. By changing the pitch angle of the

blades the thrust and direction of the main rotor disc can be controlled.

6.4. Engineered combination:

Modern rotor systems may use the combined principles of the rotor systems

mentioned above. Some rotor hubs incorporate a flexible hub, which allows for blade

bending without the need for bearings or hinges.

7. The anti-torque effect:

Most helicopters have a single main rotor but require a separate rotor to overcome

torque. This is accomplished through a various ways.

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7.1. Single main rotor:

These types are as follows:

7.1.1. Tail rotor:

Fig. 10 Tail rotor helicopter (helis.com)

The tail rotor is a smaller rotor mounted so that it rotates vertically or near-vertically

at the end of the tail of a traditional single-rotor helicopter. The tail rotor in normally

linked to the main rotor via a system of drive shafts and gearboxes, so both are

usually connected to the same transmission. Most helicopters have between 3:1 to 6:1

ratio. (In the first case, every time the main rotor turns one rotation, the tail rotor

makes three revolutions)

For straight flight, the pitch of the tail rotor is set to prevent the helicopter from

turning to the right as the main rotor turns to the left. The pilot pushes the left pedal to

increase the pitch of the tail rotor and turn to the left. Pushing the right pedal

decreases the pitch of the tail rotor and turns the helicopter to the right.

7.1.2. Ducted fan:

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Fig. 11 Ducted fan helicopter (by, Taylor Cox)

Fenestron and FANTAIL are trademarks for a ducted fan mounted at the end of the

tail boom of the helicopter and used in place of a tail rotor. Ducted fans have between

eight and 18 blades arranged with irregular spacing, so that the noise is distributed

over different frequencies. The housing is integral with the aircraft skin and allows a

high rotational speed, therefore a ducted fan can have a smaller size than a

conventional tail rotor.

7.1.3. NOTAR:

Fig. 12 Movement of air through the NOTAR system (helistart.com)

NOTAR, an acronym for NO TAil Rotor, is a helicopter anti-torque system that

eliminates the use of the tail rotor on a helicopter. Although the concept took some

time to refine, the NOTAR system is simple in theory and works to provide anti-

torque the same way a wing develops lift. A variable pitch fan is enclosed in the aft

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http://en.wikipedia.org/wiki/File:Heli.fenestron.750pix.jpg

http://en.wikipedia.org/wiki/File:Notar_helicopter.png


fuselage section immediately forward of the tail boom and driven by the main rotor

transmission. This fan forces low pressure air through two slots on the right side of

the tail boom, causing the downwash from the main rotor to hug the tail boom,

producing lift, and thus a measure of anti-torque proportional to the amount of airflow

from the rotor wash. This is augmented by a direct jet thrusters (which also provides

directional yaw control) and vertical stabilizers.

There are currently three production helicopters that incorporate the NOTAR design,

all produced by MD Helicopters. This anti-torque design also improves safety by

eliminating the possibility of personnel walking into the tail rotor.

7.1.4. Tip jets:

Another single main rotor configuration without a tail rotor is the tip jet rotor, where

the main rotor is not driven by the mast, but from nozzles on the rotor blade tips;

which are either pressurized from a fuselage-mounted gas turbine or have their own

turbojet, ramjet or rocket thrusters. Although this method is simple and eliminates

torque, the prototypes that have been built are less fuel efficient than conventional

helicopters and produced more noise. Other aircraft relied on supplemental thrust so

that the tip jets could be shut down and the rotor could auto-rotate after the fashion of

an autogyro. Perhaps the most unusual design of this type was the Rotary Rocket

Roton ATV, which was originally envisioned to take off utilizing a rocket-tipped

rotor. No tip jet rotorcrafts have ever entered into production.

7.2. Dual rotors (counter rotating):

Counter-rotating rotors are rotorcraft configurations with a pair or more of large

horizontal rotors turning in opposite directions to counteract the effects of torque on

the aircraft without relying on an anti-torque tail rotor. This allows the power

normally required to drive the tail rotor to be applied to the main rotors, increasing the

aircraft's lifting capacity.

7.2.1. Tandem:

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Fig. 13 Tandem helicopter ch-47 (helis.com)

Tandem rotors are two horizontal main rotor assemblies mounted one behind the

other. Tandem rotors achieve pitch attitude changes to accelerate and decelerate the

helicopter through a process called differential collective pitch. To pitch forward and

accelerate, the rear rotor increases collective pitch, raising the tail and the front rotor

decreases collective pitch, simultaneously dipping the nose. To pitch upward while

decelerating (or moving rearwards), the front rotor increases collective pitch to raise

the nose and the rear rotor decreases collective pitch to lower the tail. Yaw control is

developed through opposing cyclic pitch in each rotor; to pivot right, the front rotor

tilts right and the rear rotor tilts left, and to pivot left, the front rotor tilts left and the

rear rotor tilts right.

7.2.2. Coaxial:

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Fig. 14 Coaxial helicopter (Eduard PETROSYAN, Deputy Chief Designer of Kamov Company )

Coaxial rotors are a pair of rotors mounted one above the other on the same shaft and

turning in opposite directions. The advantage of the coaxial rotor is that, in forward

flight, the lift provided by the advancing halves of each rotor compensates for the

retreating half of the other, eliminating one of the key effects of dissymmetry of lift:

retreating blade stall. There is an increased mechanical complexity of the rotor system

because it requires linkages and swash plates for two rotor systems. Add that each

rotor system needs to be turned in opposite directions means that the mast itself is

more complex, and provisions for making pitch changes to the upper rotor system

must pass through the lower rotor system.

7.2.3. Intermeshing:

Fig. 15 HH-43 Huskie (helis.com)

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http://en.wikipedia.org/wiki/File:DF-SN-82-00891.JPEG


Fig. 16 Intermeshing rotor drawing

Intermeshing rotors on a helicopter are a set of two rotors turning in opposite

directions, with each rotor mast mounted on the helicopter with a slight angle to the

other so that the blades intermesh without colliding. This configuration is sometimes

referred to as a synchropter. Intermeshing rotors have high stability and powerful

lifting capability.

7.2.4. Transverse:

Fig. 17 Transverse helicopter v-12 (helis.com)

Transverse rotors are mounted on the end of wings or outriggers, perpendicular to the

body of the aircraft. Similar to tandem rotors and intermeshing rotors, the transverse

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http://en.wikipedia.org/wiki/File:MI-12.JPG


rotor also uses differential collective pitch. But like the intermeshing rotors, the

transverse rotors use the concept for changes in the roll attitude of the rotorcraft

7.2.5. Quadrotor:

Fig. 18 Quadrotor (Bothezat Quadrotor, 1923)

A quadrotor helicopter has four rotors. An "X" configuration quadrotor has a front-

left rotor, a front-right rotor, a rear-left rotor, and a rear-right rotor. Rotors to the left

and right of the body of the aircraft—like the transverse configuration. Rotors in the

front and to the rear of the aircraft—like the tandem configuration. The main

attraction of quadrotor is their mechanical simplicity—a quadrotor helicopter using

electric motors and fixed-pitch rotors uses only four moving parts.

8. Main Rotor Blade:

Fig. 19 (helicopterpage.com)

Leading Edge: The forward facing edge of the rotor blade.

Trailing Edge: The back facing edge of the rotor blade.

Chord: The distance from the Leading Edge to the Trailing Edge of the rotor

blade.

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http://en.wikipedia.org/wiki/File:De_Bothezat_Quadrotor.jpg


8.1. Rotor Blade Design:

8.1.1. Airfoil, lift and drag:

Probably the single most important rotor design parameter is its Lift/Drag ratio, which

should be as high as possible.

This ratio depends on the design of the aerofoil, and before we go on to discuss a

number of types, we will first introduce the fineness ratio. This is the thickness of the

airfoil as a percentage of the chord length. A blade with a good L/D performance has

a fineness ratio of about 15%, with its maximum chamber being a quarter of the way

back from the leading edge. A typical L/D value for a helicopter blade is 30:1.

The types of aero foils used with a rotor blade differ (figure below).

Fig. 20 Blade aerofoil shape (helistart.com)

8.1.2. Blade twist and tapper:

When a blade rotates, each point on it travels at a different speed. The further away

from the root, higher the velocity. This means that the contribution to lift and drag of

every point on the blade differs, with each aspect getting larger when moving closer

to the rotor tip. Clearly, the lift distribution over the blade is not constant. This is not a

desirable situation, because the contribution diminishes when getting closer to the

root. Tapering the blade also contributes to achieving a more evenly spaced lift

distribution.

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Fig. 21 (helistart.com)

8.1.3. Blade root cut out:

Blade twist and taper leads to large angles of attack and large blade surfaces at the

root. However, close to the root, the blade is travelling over the hull, so the generated

downwash does not contribute to helicopter thrust. For this reason, rotor blades are

often cut out near the root. Another reason for rotor blade cut out is to reduce the

effects of potential reverse flow (on the retreating rotor blade) when flying at high

speeds.

Fig.22 Blade (helistart.com)

8.1.4. Twisting moments:

Rotor blades are constantly strained by moments that try to twist them. This twisting

has its origins in the moments which exist between the centre of pressure (due to the

aerodynamic forces) and the mass centroid over the chord line. The blade designer

must take these twisting moments into account by designing a blade with high

torsional stiffness. He must also ensure that the mass centroid is located ahead of the

centre of pressure for all blade angles (in its operational range). In this way, lift tends

to lower the angle of attack: a stable condition.

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Fig. 23 Forces on blade (helistart.com)

8.1.5. Blade tip speed and noise reduction:

When the blades are very long or the helicopter is designed with a high rotor RPM,

the blade tip speed can become extremely high. When the tip speed reaches the sound

of speed, pressure waves come into existence, which causes rotor drag. A high tip

speed is also the single most important design parameter influencing generated noise

levels. It is, therefore, logical to expect more designs with lower RPM and very

efficient (larger L/D ratio) performance blades. In this way, blade efficiency is traded

off for noise reduction instead of better flight performance.

8.2. Construction:

Some important design requirements for blades are high torsional stiffness and a good

L/D ratio. Note that the weight of the rotor also has important consequences for both

the necessary engine power and stored kinethic energy (important for good auto-

rotation performance).

The early designs of rotor blades, which resemble early classic wing design, consisted

of long steel tube spars, wooden ribs and some light surface material attached to them.

From the 1960s onwards, all metal aluminium alloy blades were introduced. These

were constructed from long hollow leading edge D-spar extrusions, allied with some

light (probably aluminium) trailing edge constructions. The use of extrusions made

blade taper difficult to produce. Honeycomb constructions were added to achieve a

stiff and light construction.

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These days, composite materials like fibreglass and carbon fibre are used for the

fabrication of rotor blades. Stainless steel leading edge spars are also used, and all

composite spar designs exist too. The fatigue life properties of composite materials

are far better than those of metals. Fibreglass is used for its strength and chemical

inertness. Carbon fibre layers, sandwiched at right angles, are used to add stiffness. A

sample design might look like the figure below. Generally, composite blades also

have some extra added weight (for example, at the blade's tip) in order to achieve

desirable inertial characteristics. At the leading edge, an (often metal) erosion shield is

used.

Fig. 24(Helistart.com)

When using modern composite materials, lightning strikes have to be considered

because these are more dangerous to composite constructions. This is because of the

much greater electrical resistance of composite materials compared to all metal

blades. A lightning strike on composite materials produces a lot of heat along the

current’s path, which can damage the blade significantly. In order to provide a low

resistance electrical path, the solution is to have an outside skin that possesses low

electrical resistance and connects all of the rotor segments.

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"You don't fly a helicopter; you just stop it from crashing!"

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9. References:

Chiles, James R. The God Machine: From Boomerangs to Black Hawks: The

Story of the Helicopter. New York: Bantam Books, 2007

Flight Standards Service. Rotorcraft Flying Handbook: FAA Manual H-8083-

21. Washington, D.C.: Federal Aviation Administration, U.S. Dept. of

Transportation, 2001

Frawley, Gerard. The International Directory of Civil Aircraft, 2003-2004.

Fyshwick, Canberra, Act, Australia: Aerospace Publications Pty Ltd., 2003

Watkinson, John. Art of the Helicopter. Oxford: Elsevier Butterworth-

Heinemann, 2004

Connor, R. Lockheed CL-475. Smithsonian National Air & Space Museum

Cox, Taylor. "Blades and Lift". Helis.com

Landis, Tony and Jenkins, Dennis R. Lockheed AH-56A Cheyenne -

WarbirdTech Volume 27, p.5. Specialty Press, 2000

FAA Flight Standards Service 2001

Alpman, Emre and Long, Lyle N. "Understanding Ducted-Rotor Anti-torque

and Directional Control: Characteristics Part II: Unsteady Simulations."

Journal of Aircraft Vol. 41, No. 6, November–December 2004

"NOTAR Fleet Marks 500,000 Flight Hours". American Helicopter Society

Boeing.com

"Quadcopter, Hexacopter, Octocopter ... UAVs" by Markus Waibel

Jim Bowne, Public Affairs Office, U.S. Army Aviation and Missile Command

(February 2004). "These boots are made for flying: Rotor blades get new

protective shields". RDECOM Magazine. U.S. Army Research, Development

and Engineering Command (Provisional)

Warren (Andy) Thomas; Shek C. Hong;, Chin-Jye (Mike) Yu, Edwin L.

Rosenzweig. "Enhanced Erosion Protection for Rotor Blades: Paper presented

at the American Helicopter Society 65th Annual Forum, Grapevine, Texas,

May 27 – 29, 2009." American Helicopter Society

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Helicopter Rotor System and Design

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