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Aeronautical Electrical & Instrument Technology

                                                         BASIC AERODYNAMICS

           GENERAL INTRODUCTION OF AIR CRAFT

v    The idea of human flight has engaged the thoughts' of many men since the beginning of history. The achievement of mechanical flight, unlike so many pursuits of science, was not brought about by any pre­ssure of need, but envy. There are always men, who look at the birds and envy them as they ride the winds. This dream of mechanical flight needed courage, a science of aerodynamics, experience of construction and control and the achievement of a light and powerful engine to drive the propellers. All this was conceived by Sir George Clayey who is often called as 'the father of aerial navigation', at the beginning of the 19th century. Thus the foundations for aerial navigations were laid, in a true sense, between 1799 and 1809.

v    On the foundations laid down by Clayey, many people tried to plan devise and design and to construct models and even full size vehi­cles in pursuit of mechanical flight The essentials of wing form stability and propulsion began to emerge and by the 1880's were accompanied by the concrete achievements in automobile engineering, the sphere which aviation used later on very successfully

v    Then, in 1890's Lilienthal in Germany started riding the air in gliders and it was his example which fired the imagination of Wright brothers in America and turned their attention to solving the practical problems of aviation. Taking into their hands the varied threads of aerodynamics, construction, pilot age engine technology and propeller design, the Wright brothers wove them into a fabric, the aero plane.

v    Ballooning after its first period of excitement was over, settled down to become the sphere of showmen, sportsmen and the scientists who were interested in high altitude research. The balloon was joined by the parachute in 1797 when the Frenchman, Garner in made the first human drop at Paris. In 1852 the steam driven 'airship' became feasible, and also the 'light pressure airship' of Santos and Dumont.

v    The powered aero plane took ten years (1895-1905) to emerge from the glider that was perfected by the Wright brothers. In 1905 Wright Flier III emerged which could be banked, turned, circled and flown with ease and which could comfortably stay in the air for more than half an hour at a time. In 1910 Roe's tractor biplane appeared.

v    With the advent of War the British, the French and Germans proceeded methodically to develop various types of aero planes and the quality gradually improved with competitions. In 1919 the Rolls-Royce Eagle Engine was the outstanding achievement. In 1928 the Hele-Shaw Beacham Propeller was designed. The Bristol Centaurs Engine was de­veloped in 1947. The Whittle W-1 Turbojet was developed in 1941. In 1954 the Rolls Royce Vertical Lift Test Rig was developed. The German V-2 Rocket Engine took its shape in 1942.

                        TERMS AND DEFINITION

                        AERO FOIL

             A surface design to produce when driven through the air a reaction at right angle to the direction of motion.

v    BERNOULLI’S PRINCIPLE

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Bernoulli’s principle states that when a fluid (air) flowing through a tube reaches a constriction, or narrowing of the tube, the speed of the fluid flowing through that constriction is increased and its pressure is decreased. The cambered (curved) surface of an airfoil (wing) affects the airflow ex­actly as a constriction in tube affects airflow. This resemblance is illustrated in figure 3.1 bellow.

             LIFT

            The components of the resultant aerodynamic force at right angle to air flow.

DRAG

            The component of the aerodynamic force parallel to the air flow.

            

             THRUST

          

            The component of the resultant air flow on air screw parallel to the air screw axis.

             CENTRE OF PRESSURE

           

            The point of intersection of the resultant aerodynamic force and the chord line of an aerofoil.

    CHORD        

The length of the part of the chord line which is intercep­ted by aerofoil section.

   CHORD LINE

 

The straight line through the centre of curvature at the leading and the trailing edges of an aerofoil section.

                    CAMBER

            

             Curvature of a surface of an aerofoil.

                      ANGLE OF ATTACK

           

Is the acute angle formed between the chord line of the Aerofoil and the relative wind direction.

3.1.2.10                      ANGLE OF INCIDENCE 

           

Angle of incidence is the angle at which the wing is permanently inclined to the longitudinal axis of the Air plane. `

3.1.2.11           WING SPAN

            

             The overall distance from the wing tip

3.1.2.12           FUSELAGE

            

             The main structural body of an aerodyne except in the case of a flying boat.

3.1.2.13           UNDER CARRIAGE

              

             That part of the alighting gear which embodies the main wheels, skids or floats. This does not include tail skid.

3.1.2.14                      AXES OF AN AIRCRAFT

           

An airplane is free to revolve or move around, three axes while being supported in-flight, by LIFT and propelled through the air by thrust. These axes pass through the air planes centre of gravity.

v    LATERAL AXIS

The straight line through the center of gravity normal to the plane of symmetry.

v    LONGITUDINAL AXIS

The axis which extend length wise through fuselage from nose to tail.

v    VERTICAL AXIS

The axis which pass vertically through the fuselage at the center of gravity.

 

           

                                   

Figure - 3.4 Axis of an Air Craft

3.2                                      AIR CRAFT STABILITY AND CONTROL

           

Stability of an aircraft means its ability to return to some particular condition of flight (after having been slightly disturbed from that condition) without any efforts on the part of the pilot.

3.2.1                                STATIC STABILITY

An aircraft is in a state of equilibrium when the sum of all the forces acting on the aircraft and all the moments is equal to zero. An aircraft in equilibrium experiences no accelerations, and the aircraft con­tinues in a steady condition of flight. A gust of wind or a deflection of the controls disturbs the equilibrium, and the aircraft experiences accelera­tion due to the unbalance of moment or force.

3.2.2                                DYNAMIC STABILITY

           

While static stability deals with the tendency of a displaced body to return to equilibrium, dynamic stability deals with the resulting motion with time. If an object is disturbed from equilibrium, the time history of the resulting motion defines the dynamic stability of the object. In general, an object demon­strates positive dynamic stability if the amplitude of motion decreases with time. If the amplitude of motion increases with time, the object is said to possess dynamic instability.

3.2.3                                LONGITUDINAL STABILITY

        

If the aircraft is stable about the lateral axis, (along the longitudinal axis) it is said to be longitudinally stable.

3.2.4                LATERAL STABILITY

           

Stability is the aircraft about its longitudinal axis (from rolling) is known as lateral stability.

3.3                   AIRCRAFT CONTROL SURFACES

3.3.1                                INTRODUCTION

           

During flight an air plane is rotated about the, three axes by means of the three primary flight controls.

3.3.2                MOVEABLE CONTROL SURFACE

v    AILERONS

Ailerons are primary control surfaces, control aircraft movement about the longitudinal axis used to provide lateral control of aircraft.

v    ELEVATORS

     

Elevators are the control surfaces. Which govern the movement of the aircraft around the lateral axis (Pitch).

 

v    RUDDERS

         Rudder is vertical surface control that is usually hinged to the tail post after of the vertical stabilizer and designed to apply yawing moments to the air plane.

                                               

Figure - 3.5 Moveable Control of an Aircraft

3.3.3                UNUSUAL CONTROLS

v    “T”-TAIL

“T”-Tail arrangement positions the stabilizer and elevator at the top of the vertical fin.

Figure - 3.6 “T”-Tail Aircraft

This design makes the fin and rudder more effective because of the end plate action of the stabilizer location and avoiding the wing turbulence.

v  RUDDERVATORS

Ruddervator are used on V - Tail and the surfaces serve both as rudders and elevators.

                       

Figure - 3.7 Operation of Ruddervator

when the pilot wants to increase the angle of attack, he pulls back the control wheel and both ruddevaters move upward and inward and pushed forward to decrease the angle of attack, the ruddervator move down ward and out wards to turn the air plane to right = right pedal is applied right ruddevator moves downward and out ward, while the left ruddervator upward and inward.

v    ELEVONS

Elevons are combination of Elevators and Ailerons used on outer tips of some wings. When used as elevators, they both move in the same directions. When used as ailerons they move in the opposite direction, used on all wing air planes or flying wings.

v  FLAPERONS

These are ailerons rigged to serve as ailerons or flaps. When use as ailerons, flap Ron moves in opposite directions. When used as flap, flaperons on opposite wings move upward or downward together. ­

Figure - 3.8 Control of an Aircraft

3.3.4                                  TRIM TABS

            Trim tabs are small secondary flight control surfaces, set in to trailing edges of the primary control surfaces.

           

The Purpose of trim tabs is to permit the pilot to fly the airplane in a desired attitude, under various load and airspeed conditions without the need to apply constant pressure in any particular direction on the flight controls, this is done by “loading” the control surfaces to a neutral or timed centre position tabs can be fixed or variable.

Figure - 3.9 Trim Tabs

3.3.4.1             FIXED TRIM TABS

           

            This is normally a piece of sheet metal attached to the Trailing edge of the control surface? This fixed TAB is adjusted on the ground by bending it in an appropriate direction to eliminate flight control forces for a specific flight condition. These are used to adjust Rudders and ailerons of light Aircraft. Fixed tab is normally adjusted for zero control forces in cruise flight adjustment of the tab is trial and error method based on the pilots report..

 

Figure - 3.10 A Fixed Trim Tab may be Adjusted on the Ground

3.3.4.2                            CONTROLLABLE TRIM TABS

           

            Controllable trim tabs are adjusted by means of control wheels, knobs, or cranks in the cock-pit, and an indicator is provided to denote the position of the tab. These may be operated mechanical electrically, and hydraulically. When the trim  control system is activated the trim tab is deflected in the direction opposite to the desired movement of the control.

Figure - 3.11 Control Trim Tab

3.3.4.3                            SERVO TABS

           

            Servo tabs referred to as the flight tabs, mainly used on large control surfaces. This is directly operated by the primary controls of the air plane. In response to movement of the cock pit control 4 only the servo tab moves. The force of the air flow on the servo tab then moves the primary control surface.

Figure - 3.12 Servo tab

3.3.4.4             BALANCE TABS

           

            Balance tab is linked to the air plane in such a manner. That a movement of the main control surface will give an opposite movement to the tab. Thus balance tab will assist in moving the main control surface. Balance tabs are useful in reducing the effort required to move the control surface of a large plane.

Figure - 3.13 Balance Tab

3.3.4.5             SPRING TABS

           

            Spring tabs are used on aircraft that require considerable force to move a control surface. This provides a boost thereby aiding the movement of a control surface. The control horn is connected to the control surface by springs.

Figure - 3.14 Spring Tab

3.3.5                                  FORCE IN ACTION IN FLIGHT           

           

            Without taking in to account the force on the tail unit an aero plane, when flying straight and at constant level, it will be under the influence of four main forces.

3.3.51                              LIFT

           

The lift (L) of the main planes acting vertically upwards through the centre of pressure.

3.3.5.2                            GRAVITY

           

The weight (W) of the aero plane acting vertically downwards through the centre of gravity.

3.3.5.3                            THRUST

           

The thrust (T) of the propeller is pulling force horizontally forward along the propeller shaft.

3.3.5.4             DRAG          

The drag (D) is acts horizontally backwards. This is the total drag on the aircraft consisting of the drag on the aero ­foils and also of the remaining parts of the aero plane.

Figure - 3.15 Force in Action in Flight

3.4                   Flight control

3.4.1                LONGITUDINAL AXIS AND ROLLING

           

            Longitudinal axis is an imaginary line running through the center of gravity (CG), and is horizontal, when the aero plane is in the attitude of normal horizontal flight. Any rotary motion about this axis is called rolling.

Figure - 3.16 Longitudinal Axis and Rolling

3.4.2                                  NORMAL AXIS AND YAWING

            Normal axis is straight line through the CG and it is vertical when the aero plane is in the attitude of normal horizontal fight any rotary motion about this axis is called yawing.

 

Figure - 3.17 Normal Axis and Yawing

3.4.3                                  LATERAL AXIS AND PITCHING

           

            Lateral axis is a straight line passing through the CG, and right angles to the other axis. It is horizontal when the aero plane is in the attitude of normal horizontal flight and is parallel to the straight line joining the wing tips. Any rotary motion about axis is called pitching.

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Figure - 3.18 Lateral Axis and Pitching

3.5                   Helicopters

3.5.1                INTRODUCTION               

The main difference between the Helicopter and an Airplane is the main source of lift. Helicopter derives its lift from a rotating aerofoil called rotor. The rotating wing.  Main rotor of a Helicopter has two or more blades depending on the design and size of the Helicopter. Each blade is an aerofoil in design.

3.5.2                FORCES ACTING ON A HELICOPTER IN FLIGHT           

There are four forces acting on the Helicopter. Lift is the force required to support the weight of the Helicopter.

Figure - 3.19 Forces Acting on a Helicopter in Flight

3.5.3                TAIL ROTOR OR ANTI-TORQUE ROTOR     

According to Newton third law of Motion the torque force applied to the rotor shaft of a helicopter to turn the rotor causes an equal and opposite force which would turn the fuselage of the helicopter in the opposite direction unless measures were taken to prevent it. Tail rotor prevents the helicopter fuselage from turning.

3.5.4                HELICOPTER CONTROLS

3.5.4.1             COLLECTIVE PITCH CONTROL         

Increases or decreases the pitch of all main rotor blades simultaneously. This control is located at the pilots left. To raise the helicopter from the ground, collective pitch is increased, thereby increasing lift to all blades evenly.

                                   

Figure - 3.20 Collective Pitch Control Mechanism

3.5.4.2             CYCLIC - PITCH CONTROL

           

            Purpose of the cyclic pitch control is to cause tip path plane of the main rotor to tilt, as required to provide for movement of the helicopter in a desired direction.

                                   

Figure - 3.21 Cyclic Pitch Control Mechanism

3.5.4.3             AUTOROTATION

          

           Curve’s contribution to rotating wing machine, paved the way for practical helicopters. He discovered the principle of “autorotation”. This enabled the design of a safe flying machine, which would not stall and crash when its speed through the air was reduced to ZERO.

           The principle of autorotation is that there is a small positive angle of pitch at which the blades could be set to ensure that the rotor would continue to rotate automatically in the airstreams, without an engine to drive it, and still develop enough lift for sustained in flight.

3.6                   HELICOPTER CONFIGURATIONS

3.6.1                SINGLE ROTOR ARRANGEMENT

           

            In this arrangement single rotor and a tail rotor is used. Single rotor helicopter is light in weight, But single  rotor machines have limited lifting and speed capabilities and severe safety hazard during ground operations due to the position of the Tail rotor.

                                   

Figure - 3.22 Signal Rotor Arrangement Air Craft

3.6.2                TANDEM ROTOR HELICOPTER         

This helicopter uses two synchronized rotors turning in opposite directions. This design eliminates the need for an anti-torque rotor, since one rotor cancel the torque of other. Each rotor has three blades. This helicopter is capable of lifting large loads.

Figure-3.23 Tandem-rotor Helicopter

3.6.3                SIDE BY SIDE ROTOR HELICOPTER

Side by side helicopter has two main rotors, positioned out from sides of the fuselage. Rotors turning in opposite direction which eliminate the need for a tail rotor. It has excellent stability and efficient in forward flight.

 

                                                Figure - 3.24 Side by Side Helicopter.

3.6.4                COAXIAL ROTOR HELICOPTER       

           

In coaxial rotor helicopter, fuselage torque is eliminated by using too counter rotating rigid main rotors, mounted one above the other on a common shaft. This has the forward speed more then 250 KN. [129 m/s] Lift load at high forward speed is carried by the advancing blades.

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