Helicopter Flight Theory

 Helicopter flight theory

SYNOPSIS

Helicopter is one of the earliest ideas for achieving flight. The advances in helicopter technology made it an extreme flying machine. In this seminar I discuss the historical backgrounds and early concepts of helicopter, then introducing introducing the aerodynamic theory and principle behind helicopters, discussing the most potent mechanisms that are used in it. Also different configurations of helicopters have been described. Then discussing the first analytical theory to consider for a helicopter in forward flight (The momentum theory.Also discussed the strength and design re!uirements of the helicopter. "riefing the seminar with the describing some typical applications of helicopters.

HISTORICAL BACKGROUND The helicopter is arguably one of the earliest ideas for achieving flight. #ver two thousan thousand d years years ago, ago, the $hines $hinesee constr construct ucted ed what what are known known as $hines $hinesee Tops, Tops, illustrated below. These simple toys consisted of a propeller attached to a stick that would be spun rapidly through ones hands to spin the propeller and achieve lift.

%ig-1 $hinese top

%ig&' eonardo da )inci*s +Helicopter+

ater, in the -th $entury, famed inventor and artist eonardo da )inci designed one of the more aesthetically pleasing concepts for a helicopter, but such a craft was never actually constructed. In nglan ngland d in /01, /01, 2ir 3eorge 3eorge $ayley $ayley constru constructe cted d the first power powered ed model modelss of helicopters that were driven by elastic devices which attained an altitude of 04ft. In 56', fellow nglishman 7. H. 8hillips constructed a model helicopter that weighed '4 pounds (0 kg and was driven by steam. In 5/5, nrico %orlanini, an Italian civil engineer, also constructed a steam driven model helicopter that only weighed 9.-kg.

' Helicopter flight theory

%ig&9 2ir 3eorge $ayley*s helicopter The first manned helicopter to rise vertically completely unrestrained was constructed by 8aul $ornu, a %rench mechanic, in 04/. $ornu*s helicopter had two propellers that were rotated at 04 rpm by a 5 k7 engine. $ornu was most probably the first helicopter experimenter who was concerned with control. 7hile cornu:s helicopter was historically significant, its performance and control was rather marginal and it was never a practical machine.

%ig&6 $ornu*s helicopter The next influential development in the field of helicopters was brought about by a man man who who neve neverr actua actuall lly y buil builtt a heli helico copt pter er hims himself elf.. In 0'9 0'9,, ;uan ;uan de la $ier $ierva va successfully successfully flew his $.6 autogiro, autogiro, an aircraft that has two propellers, propellers, a powered powered one to provide thrust, and an un powered rotor to provide lift. $ierva*s autogiro was noteworthy because it was the first to use an +articulated+ rotor that allowed its blades to flap up and down in response to aerodynamic aerodynamic forces on the blades during forward forward flight. The first recogni
9 Helicopter flight theory

%ig&- =*Ascanio*s helicopter ;ust before and during 7orld 7ar II, 3ermany made several large, significant steps in helicopter development. The %A&1 helicopter, designed by Heinrich %ocke, first flew in ;une 091, and was later used in publicity stunts by the >aavy, and over 444 of them were produced. This helicopter utili
%ig&1 2ikorsky*s )2&944 The first American helicopter was the )2&944, designed by Igor 2ikorsky of the )ought&2ikorsky $ompany. The )2&944 was the first helicopter to use a tail rotor to counteract the tor!ue produced by the main rotor, and it was this innovation that solved the last ma@or hurdle in making helicopters practical flying vehicles. This design is now the most common in today*s helicopters.


AEROFOIL THEORY AND PROPELLOR ACTION AEROFOIL THEORY :

An aerofoil is a streamlined body, which is designed to produce lift or thrust when passed through air. Airplane wings, propeller blades and helicopter main and tail rotor blades are all aerofoil.

%ig&/ Aerofoil features

Helicopter flight theory

$hord is the distance or imaginary line between the leading and trailing edge of an airfoil. The amount of curve or departure of the airfoil surface from the chord line is known as camber. pper camber refers to the upper surfaceB lower camber refers to the lower surface. If the surface is flat, the camber is ot all of the air met by an airfoil is used in lift. 2ome of it creates resistance, or drag, that hinders forward motion. ift and drag increase and decrease together. The airfoil:s angle of attack into the air, the speed of airflow, the air density, and the shape of the airfoil or wing therefore affect them.

%ig&5 "ernoulli:s principle The amount of lift that an aerofoil develop depends on . Area (si

The production of thrust in helicopters is based on the propeller action. The rotation of propeller causes the air to accelerate from one side to the other side of it, which results in the development of thrust in the opposite direction of the flow. A propeller does the conversion of tor!ue into axial thrust by changing the momentum of the fluid in which it is submerged. 7hen a propeller submerged in an undisturbed fluid rotates, it exerts a force on the fluid and pushes the fluid backwards. The reaction to this force on the fluid provides a forward thrust, which is used for propulsion. Although the complete design of a propeller cannot be done according to the momentum theory, yet the application of this theory leads to some useful results s indicated by simple analysis of problem below.

%ig&0 8ropeller et  be the upstream velocity and u be the downstream velocity. et A be the propeller disc area and C the mass flow rate of air. "y "ernoulli:s principle we get the velocity through the propeller e!ual to average of upstream and far down stream velocities. Therefore the induced velocity u through the propeller e!uals, u

U =

+u

'

− U =

u

− U

'

8r opulsive 8ower = Thrust × 8ropulsive velocity

=   ρ A 

 + u  '

 ( u −U )U 

 + u   u ' −U '    8ower Input =  ρ A    '     ' 

8ropulsive .fficiency =

=

8ropulsive 8ower 8ower Input

, ,+

u

'U

If 8 is the power supplied and T the thrust developed then from momentum theory we have

/ Helicopter flight theory P T

=

, '

T A ρ

This formula is applied for hovering condition of the helicopter where tor!ue T e!uals weight to be supported. The actual flow through the propeller differs considerably from the model depicted above since the propeller works in an Dinfinite sea of air DB there is no well&defined boundary between the fluid at rest and fluid motionB therefore the actual thrust will differ considerably from the values in the above expressions.

CONFIGURATION OF HELICOPTERS SINGLE ROTOR HELICOPTER

The most popular helicopter arrangement is that of single rotor using a tail rotor. The single rotor helicopter is relatively lightweight and is fairly simple in design with one rotor one main transmission and one set of controls. The disadvantage of single rotor machine are its limited lifting and speed capabilities and a severe safety ha
%ig&4 2ingle rotor helicopter TANDEM ROTOR HELICOPTER

This helicopter uses two synchroni

%ig& Tandem rotor helicopter SIDE-BY-SIDE HELICOPTER

It has two main rotors mounted on pylons or wings positioned out from the sides of the fuselage. The side by side has rotors turning in opposite direction, which eliminates the need for a tail rotor. The advantages are its excellent stability and disadvantage is having high drag and structural weight both resulting from structure necessary to support the main rotor.

%ig&' 2ide by side helicopter COAXIAL HELICOPTER

In this fuselage tor!ue is eliminated by two counter rotating rigid main rotors mounted one above the other on common shaft.

%ig&9 $oaxial helicopter TILT ROTOR AIRCRAFT

The tilt rotor has the ability to combine the vertical take off low speed capabilities of the helicopter with high&speed performance of a turboprop airplane.


DESIGN CONSIDERATIONS DISSEMETRY OF LIFT

All rotor systems are sub@ect to of =issymmetry ift in forward flight. At a hover, the lift is e!ual across the entire rotor disk. As the helicopter gain air speed, the advancing blade develops greater lift because of the increased airspeed and the retreating blade will produce less lift, this will cause the helicopter to roll. A non&articulated rotor in forward flight is shown in figure.

%ig&6 )elocities of rotor in forward flight If the blades were to rotate at a fixed incidence, then this velocity differential would cause four&fifths of the total lift of the rotor to be created on the advancing side. The calculated pressure contours for a fixed incidence rotor with an advance ratio of E4.9 are shown below

4 Helicopter flight theory

%ig&-$alculated pressure contours for fixed blade incidence #bviously, this large imbalance of force on the rotor would lead to large oscillatory stresses at the blade roots, along with a large rolling moment. This would make the helicopter very unflyable, both from a dynamics and structural viewpoint.

ARTICULATED ROTORS To reduce this large force differential, a cyclical variation of the blade incidence is needed. The most common way of reducing the blade incidence is with flapping hinges. 7hen using flapping hinges, the blade is hinged as close as possible to its root, allowing the entire blade to +flap+ up and down as it rotates.

%ig&1 An articulated rotor hub 7hen a blade is on the advancing side, its increased lift causes the blade to flap upwards, which effectively reduces its incidence. The opposite occurs on the

 Helicopter flight theory

retreating side. =ue to the presence of the flapping hinges, none of the bending forces or rolling moments is transferred to the helicopter body. $entrifugal force is typically enough to prevent the blades from flapping to a large degree, but many helicopters also employ stops as an added preventative measure. The use of flapping hinges also creates a better force balance on the rotor, distributing the lift more evenly. $alculated pressure contours for a variable incidence rotor can be seen below.

%ig&/$alculated pressure contours for variable incidence This diagram also denotes a region of reversed flow on the rotor. As the forward speed of the helicopter increases, a region near the blade roots on the retreating side actually experiences a reversed flow. $ombined with the large blade incidence on the retreating side, as forward speed increases, the blades approach a stalled condition. At the same time, regions near the tips on the advancing side experience a very high velocity flow, approaching the point where shock waves form, leading to shock induced flow separation. =ue to these limiting factors, the maximum forward speed of a helicopter is limited to about 64'kph. TORQUE

Helicopter lift is obtained by means of one or more power driven hori
' Helicopter flight theory

the rotor. This is called Tor!ue reaction. The tail rotor is used to compensate for this tor!ue and hold the helicopter straight. #n twin&rotors helicopter, the rotors spin in opposite directions, so their reactions cancel each other.

%ig&5 The tail rotor in normally linked to the main rotor via a system of drive shafts and gearboxes that means if you turn the main rotor, the tail rotor is also turns. Gost helicopters have a ratio of 9 to 1. That is, if main rotor turns one rotation, the tail rotor will turn 9 revolutions (for 9 or 1 revolutions (for 1. In most helicopters the engine turns a shaft that connected to an input !uill in the transmission gearbox. The main rotor mast out to the top and tail rotor drive shafts out to the tail from the transmission gearbox.

%ig&0 GYROSCOPIC PRECESSION

The term gyroscopic precession describes an inherent !uality of rotating bodies in which an applied force is manifested 04 4 in the direction of rotation from the point where the force is applied. 2ince the rotor of a helicopter has a relatively large diameter and turns at several hundred revolutions per minute precession is a prime factor in controlling the rotor operation.


The cyclic pitch control causes variation in the pitch of the rotor blades as they rotate about the circle of the tip path plane. The purpose of this pitch change is in part to cause the rotor disc to tilt in the direction in which it is desired to make the helicopter move. 7hen only the aerodynamic effects of blades are considered it would seem that when the pitch of the blades is high the lift would be high and the blade would rise. Thus if the blades had high pitch as they passed through one side of the rotor disc the side of the disc having low pitch should rise and the side having low pitch should fall. This would be true except for gyroscopic precession. 3yroscopic precession is caused by a combination of a spinning force and an applied acceleration force perpendicular to the spinning force. Thus if force is applied perpendicular to the plane of rotation the precession will cause the force to take effect 044 from the applied force in the direction of rotation. As a result of the fore going principle, if a pilot wants the main rotor of a helicopter to tilt in a particular direction, the applied force must be at a angular displacement 04 4 ahead of the desired direction of tilt. The re!uired force is applied aerodynamically by changing the pitch of the rotor blades through the cyclic pitch control. 7hen the cyclic control is pushed forward the blade at left increases its pitch as the blade on right decreases pitch. This applies an up force to the left hand side of the rotor disc, but the up movement is therefore at rear of the rotor plane and the rotor tilts forward. This applies a forward thrust and causes the helicopter to move forward. VIBRATION

Any type of machine vibrates. However greater than normal vibration usually means that there is a malfunction. Galfunctions can be caused by worn bearings, out&of& balance conditions, or loose hardware. If allowed to continue unchecked, vibrations can cause material failure or machine destruction. Aircraft && particularly helicopters && have a high vibration level due to their many moving parts. =esigners have been forced to use many different dampening and counteracting methods to keep vibrations at acceptable levels. 2ome examples are . =riving secondary parts at different speeds to reduce harmonic vibrationsB this method removes much of the vibration buildup. '. Gounting high&level vibration parts such as drive shafting on shock&absorbent mounts. 9. Installing vibration absorbers in high&level vibration areas of the airframe. LATERAL

ateral vibrations are evident in side&to&side swinging rhythms. An out&of&balance rotor blade causes this type of vibration. ateral vibrations in helicopter rotor systems are !uite common. VERTICAL


)ertical vibrations are evident in up&and&down movement that produces a thumping effect. An out&of&track rotor blade causes this type vibration.

HIGH-FREQUENCY

High&fre!uency vibrations are evident in bu<
3round resonance is the most dangerous and destructive of the vibrations discussed here. 3round resonance can destroy a helicopter in a matter of seconds. It is present in helicopters with articulated rotor heads. 3round resonance occurs while the helicopter is on the ground with rotors turning it will not happen in flight. 3round resonance results when unbalanced forces in the rotor system cause the helicopter to rock on the landing gear at or near its natural fre!uency. $orrecting this problem is difficult because the natural fre!uency of the helicopter changes as lift is applied to the rotors. 7ith all parts working properly, the design of the helicopter landing gear, shock struts, and rotor blade lag dampeners will prevent the resonance building up to dangerous levels. Improper ad@ustment of the landing gear shock struts, incorrect tire pressure, and defective rotor blade lag dampeners may cause ground resonance. The !uickest way to remove ground resonance is to hover the helicopter clear of the ground.

Helicopter flight theory

CYCLIC CONTROL The tip path plane, or T88, is the plane connecting the rotor blade tips as they rotate. 7hile hovering, the thrust vector of a helicopter is oriented upward, perpendicular to the tip path plane. In order for the helicopter to travel forward, this thrust vector needs to be rotated slightly in the forward direction. To rotate the thrust vector, it is in turn necessary to rotate the T88 by the same amount, as illustrated below.

%ig&'4 Tip path planes and thrust vectors for hovering and forward flight 2ince tilting the rotor hub or rotor shaft is impractical, an alternative means of rotating the T88 is needed. Gost modern helicopters use a system of swash plates. 2een in the following diagram, the swash plate system is composed of upper and lower swash plates.


%ig&'$yclic control and swash plates The lower swash plate remains stationary relative to the helicopter. The upper swash plate rotates with the rotor, while remaining parallel to the lower swash plate. "y utili
/ Helicopter flight theory

MOMENTUM THEORY The first analytical theory to consider for a helicopter in forward (no axial flight is the momentum theory. The analysis for vertical (axial flight is very similar to that of a simple propeller, and will not be discussed here. #ne notable result of that analysis, however, is the induced velocity of the rotor in hover.

7here w is the disc loading, given by

In the terms of basic momentum theory, the thrust of a rotor in no axial flight is very difficult to derive. In the context of this discussion, a relationship for the thrust that was proposed by 3lauert in 0'5 will be used. A simple diagram of an actuator disk in no axial flow is depicted below.


%ig&'' Actuator disk in no axial flow The thrust of the actuator disk can be given by

%ar downstream from the disk, the downwash v f is doubled. Also, the term becomes the mass flow through the stream tube that is defined by the actuator disk. 2ome validity for these relationships can be inferred by comparing them to the formula for the lift of a wing having 'F span with a uniform downwash. The lift of such a wing is expressed by an e!uation similar to that shown above. After assuming that this e!uation is valid, determining the thrust re!uires that the induced velocity in forward flight be determined.

These two e!uations allow the determination of thrust and induced velocity of a helicopter in forward flight.


STRENGTH AND DESIGN REQUIREMENTS The helicopter structure must be strong enough to with stand all the loads expected to be experienced in service life. This comprises large loads, which are experienced rarely, and repetitive small to medium loads which are experienced in a normal flight. 7here as large loads are important in designing the non&rotating parts of helicopter like the fuselage, the tail boom, the landing gear etc. The repetitive loads are important in designing the rotating parts such as the main rotor, the tail rotor, the shafts, the main rotor gearbox, the tail rotor gearbox etc. ROTOR STRUCTURE

The rotor blade structure must possess sufficient strength to with stand not only the aerodynamic loads generated on the blade surface but also the inertial loads arising from the centrifugal, the coriolis, the gyroscopic and the vibratory effects produced by the blade movement .the blade must also possess sufficient stiffness and rigidity to prevent excessive deformation and to assure that the blades will maintain the desired aerodynamic characteristics.

'4 Helicopter flight theory

VIBRATION

The vibration, its causes and reduction are as discussed previously. SERVICE LIFE

7hile considering the expected service life of the helicopter or its components all types of expected loads must be considered. Three basic factors, which govern the service life, are . $orrosion '. $reep and 9. %atigue STRUCTURAL MATERIALS

2ome of the important factors, which govern the selection of material for airframe and the primary load selection of material for airframe and the primary load bearing members of the helicopter, are . A high strength to weight ratio '. 2tiffness 9. 2pecific gravity 6. Fesistance to impact loads -. Temperature effects 1. $orrosion resistance /. %atigue strength 5. Fate of crack propagation

CONCLUSION ven though the concept of the helicopter is arguably older than that of the airplane, there is still a great amount of research and advancement yet to occur. As the political climate of our world continues to change and military conflicts approach the small&scale urban warfare of recent years, the importance of the helicopter will continue to grow. It is rather ironic that an idea first conceived long before the $ommon ra will be key to winning military conflicts in the 'st century. Fotary wing research and development is a complex interrelated challenge. The advanced tools used are $omputational fluid dynamics ($%=, %inite element method (%G, and $omputational structural dynamics ($2= for physical understanding of complex aerodynamics and structural phenomena. Integration of these will enable us to design rotorcraft, which will have superior productivity, enlarged mission capabilities and improved environmental acceptance.

' Helicopter flight theory

The future of helicopter is bright with its ability to land in any small clear areaB the helicopter finds use in air taxi service, police work, Inter city mail, and rescue work, power line patrolling and other areas. The development is still to continue.

REFERENCES . "hagwat, Gahendra ;., et al. Flow Visualization and Measue!ents in t"e #a$e o% a Roto wit" a Su&win' Ti() Annual Fou! Po*eedin's+A!ei*an ,eli*o(te So*iet-) vol. , 000, pp. 0-&01'. '. "ramwell, A. F. 2. ,eli*o(te D-na!i*s. >ew ork Halsted 8ress, 0/1. 9. 3essow, Alfred and Gyers, 3arry $., ;r. Aeod-na!i*s o% t"e ,eli*o(te . >ew ork Gacmillan $ompany, 0-'. 6. 3hee, Terence A. and lliot, ;oe 7. T"e #a$e o% a S!all+S*ale Roto in Fowad Fli'"t Usin' Flow Visualization, .ounal o% t"e A!ei*an ,eli*o(te So*iet-) vol. 64, no. 9, ;uly 00-, pp. -'&1-. -. 3unston, "ill and 2pick, Gike. Moden Fi'"tin' ,eli*o(tes. >ew ork $rescent "ooks, 051. 1. ;ohnson, 7ayne. ,eli*o(te T"eo-. Gineola, > =over 8ublications, 054. /. Gc$ormick, "arnes 7. Aeod-na!i*s) Aeonauti*s and Fli'"t Me*"ani*s. >ew ork ;ohn 7iley J 2ons, 00-. 5. >ewman, 2imon. T"e Foundations o% ,eli*o(te Fli'"t . >ew ork Halsted 8ress, 006.

'' Helicopter flight theory

0. 2eddon, ;/ Basi* ,eli*o(te Aeod-na!i*s. Feston, )A American Institute of Aeronautics and Astronautics, 004. 4. 2tepniewski, 7. K. and Leys, $. >. Rota-+#in' Aeod-na!i*s. Gineola, > =over 8ublications, 0/0. . oung, Faymond A. ,eli*o(te En'ineein' . >ew ork Fonald 8ress $ompany, 060. '. ;ournal of Aeronautical 2ociety of India&Gay 00/ 9. ;ournal of American institute of Aeronautics and Astronautics. WEBSITE INFORMATION

www.copters.com www.helis.com www.aerospaceweb.org

Helicopter Flight Theory

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