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Manual of Flying: Volume 12: Helicopter

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Manual of Flying: Volume 12: Helicopter AP3456 – 12-1 - Rotor Aerodynamics and Control CHAPTER 1 - ROTOR AERODYNAMICS AND CONTROL (HELICOPTER) ROTOR AERODYNAMICS Introduction 1. The same basic laws govern the flight of both fixed and rotary wing aircraft and, equally, both types of aircraft share the same fundamental problem; namely that the aircraft is heavier than air and must, therefore, produce an aerodynamic lifting force to overcome the weight of the aircraft before it can leave the ground. In both types of aircraft the lifting force is obtained from the aerodynamic reaction resulting from a flow of air over an aerofoil section. The important difference lies in the relationship of the aerofoil to the fuselage. In the fixed-wing aircraft, the aerofoil is fixed to the fuselage as a wing whilst in the helicopter, the aerofoil has been removed from the fuselage and attached to a centre shaft which, by one means or another, is given a rotational velocity. 2. Helicopters have rotating wings, which are engine-driven in normal flight. The rotor provides both lift and horizontal thrust. Rotor Systems 3. Helicopters may be single or multi-rotored, each rotor having several blades, usually varying from two to six in number. The rotor blades are attached by a rotor head to a rotor shaft which extends approximately vertically from the fuselage. They form the rotor, which turns independently through the rotor shaft, see Fig 1. 12-1 Fig 1 The Rotor Head Arrangement Shaft Axis Rotor Blades Plane of Rotation Rotor Head Rotor Shaft The shaft axis is a straight line through the centre of the main drive shaft. The rotor blades are connected to the rotor head, at an angle to the plane of rotation, called the pitch angle, see Fig 2. Revised Jul 12 Page 1 of 8 AP3456 – 12-1 - Rotor Aerodynamics and Control 12-1 Fig 2 Blade Pitch Angle Chordline Pitch Angle Plane of Rotation 4. The axis of rotation is perpendicular to the plane of rotation, and is a line through the rotor head about which the blades rotate. Under ideal conditions the axis of rotation will coincide with the shaft axis. This however is not usually so since the rotor is tilted under most flight conditions, see Fig 3. 12-1 Fig 3 The Rotor Disc Tilted Shaft Axis Axis of Rotation Tip Path Plane (Rotor Disc) n Plane of Rotatio 5. The tip path plane, shown in Fig 3, is the path described by the rotor blades during rotation and is at right angles to the axis of rotation and parallel to the plane of rotation. The area contained within this path is known as the rotor disc. Forces on an Aerofoil 6. The airflow around the aerofoil gives rise to a pressure distribution. The pressure differences produce a force distribution which can be represented by total reaction, see Fig 4. Total reaction may be resolved into a force perpendicular to the relative airflow (RAF) called lift and a force parallel to the RAF called drag. The angle which the chord line makes with the RAF is the angle of attack. 12-1 Fig 4 Total Reaction Lift Total Reaction Angle of Attack Drag Relative Airflow Revised Jul 12 Page 2 of 8 AP3456 – 12-1 - Rotor Aerodynamics and Control The magnitude of lift is given by: C × 1 ρV2S LIFT = L 2 where ρ = Air density V = Velocity of RAF S = Plan area of aerofoil CL = Coefficient of lift The magnitude of drag is given by: C × 1 ρV2S DRAG = D 2 where CD = Coefficient of drag Blade Design 7. The design requirements of a rotor blade are complicated: a. The combined area of the blades is small compared to the wings of an aeroplane of similar weight, so high maximum CL is needed. b. Power to weight ratio problems can be minimized by use of blades having a good lift to drag ratio. c. The pitch angle of a blade is held by a control arm and a large pitching moment caused by movement of the centre of pressure would cause excessive stress in this component. A symmetrical aerofoil has a very small pitching moment and is also suitable for relatively high blade tip speeds. d. Torsional stiffness is required so that pitching moment changes are minimized. A typical blade has an extruded alloy D spar leading edge with a fabricated trailing edge. It is symmetrical, with a thickness ratio of about 1:7, and is rectangular in plan, see Fig 5. Later designs of blade incorporate torsional stiffness, opposing pitching moments, and aerodynamic and planform balancing to allow cambered and high speed sections to be used to improve the overall performance of the blades. Revised Jul 12 Page 3 of 8 AP3456 – 12-1 - Rotor Aerodynamics and Control 12-1 Fig 5 Typical Rotor Blade Section Blade Tip Lift Area Attachment Area A A a Plan View Balance Spar Polyurethane Weight Carbon Band Filling Skin Fillet Tab b Section AA Relative Airflow 8. If a rotor blade is moved horizontally through a column of air, the effect will be to displace some of the air downwards. If a number of rotor blades are travelling along the same path in rapid succession then the column of air will eventually become a column of descending air. This downward motion of air is known as induced flow (IF), see Fig 6. The direction of the airflow relative to the blade (RAF) is the resultant of the blade’s horizontal travel through the air and the induced flow, see Fig 7. The angle between the Relative air Flow and the Chord line is the angle of attack. 12-1 Fig 6 Induced Airflow Column of Still Descending Air Air (Induced) Direction of Rotation Revised Jul 12 Page 4 of 8 AP3456 – 12-1 - Rotor Aerodynamics and Control 12-1 Fig 7 Forces Acting on a Rotor Blade Axis of Rotation Drag Lift Rotor Total Induced Reaction Flow Angle of Thrust Attack α R elative Airflow Plane of Rotational Pitch Rotor Rotation Airflow Drag Lift and Drag 9. The Total Reaction is the vector resultant of lift, which is produced by the relative air flow passing over the blade at an angle of attack, and drag, which is perpendicular to the lift, or parallel to the RAF. The Total Reaction may be split into components; the Rotor Thrust acting along the axis of rotation, and the Rotor Drag acting parallel to the plane of rotation. Total Rotor Thrust 10. The rotor thrusts of each blade are added together and make up the total rotor thrust. The total rotor thrust is defined as the sum of all the blade rotor thrusts and acts along the axis of rotation through the rotor head, see Fig 8. 12-1 Fig 8 Total Rotor Thrust Total Rotor Thrust Rotor Rotor Thrust Thrust Equalising Lift 11. The rotational velocity of each part of a rotor blade varies with its radius from the rotor head; the blade tip will always experience a greater velocity of airflow than the root. Lift, and hence rotor thrust, is proportional to V2 and will be much greater at the blade tip than at the root - an unequal distribution of lift which would cause large bending stresses in the rotor blade. There are various methods used by blade manufacturers to equalise lift as follows: a. Washout. Washout is a designed twist in the blade which reduces blade pitch angle from root to tip giving a more uniform distribution of lift (see Fig 9). The angle of attack, and hence rotor thrust, is decreased with the pitch angle at the tip. Revised Jul 12 Page 5 of 8 AP3456 – 12-1 - Rotor Aerodynamics and Control 12-1 Fig 9 Lift Distribution with Washout Potential Lift Realistic Lift Ideal Lift V Low V High Nil or very little wash-out Excessive wash-out Correct shaping and wash-out b. Varying Aerofoil Section and Tapering. Varying the aerofoil section, in particular the flattening of the aerofoil section on the outboard, high speed, portion of the blade will reduce the lift produced. Additionally, tapering the outboard section of the rotor thereby reducing the chord and therefore the lifting section can be used to aid equalisation of lift. CONTROL Introduction 12. For various stages of flight, the total rotor thrust requirements will change. Although rotor rpm (Nr), and hence rotational velocity, can be changed, the reaction time is slow and the range of values is small. The other controllable variable is pitch angle; a change in pitch angle will cause a change in angle of attack and, therefore, total rotor thrust. Collective Pitch Changes 13. The pitch angle of a rotor blade is changed by turning it about a sleeve and spindle bearing on its feathering hinge by means of a pitch operating arm connected to a rotating swash plate. The rotating plate may be raised and lowered or have its angle changed by a non rotating swash plate below, which is connected to the collective pitch lever and cyclic control stick in the cockpit by control rods which are usually hydraulically assisted, see Fig 10. Revised Jul 12 Page 6 of 8 AP3456 – 12-1 - Rotor Aerodynamics and Control 12-1 Fig 10 Rotor-Head Detail Pitch Operating Arm Sleeve and Spindle Feathering Hinge Rotating Swash Plate Non-rotating Swash Plate Control Rods The pitch angle is thus increased or decreased collectively by the pilot raising or lowering the collective pitch lever or changed cyclically by movement of the cyclic control stick. Control of Rotor RPM (Nr) 14. Changes in total rotor thrust will produce corresponding changes in rotor drag.
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