14. PRINCIPLES OF FLIGHT – HELICOPTERS

PRINCIPLES OF FLIGHT
PRINCIPLES OF FLIGHT — HELICOPTERS
SUBSONIC AERODYNAMICS
Basic concepts, laws and definitions
International system of units of measurement (SI) and conversion of SI units
List the fundamental quantities and units in SI, such as mass (kg), length (m), time (s).
Be able to convert imperial units to SI units and vice versa.
Definitions and basic concepts of air
Describe air temperature and pressure as functions of height.
Define the International Standard Atmosphere (ISA).
Define air density, and explain the relationship between air density, pressure, and temperature.
Explain the influence of moisture content on air density.
Define pressure altitude and air density altitude.
Newton’s laws
State and interpret Newton’s three laws of motion.
Distinguish between mass and weight, and their units.
Basic concepts of airflow
Describe steady and unsteady airflow.
Define ‘streamline’ and ‘stream tube’.
Explain the principle of the continuity equation or the conservation of mass.
Describe the mass flow rate through a stream tube section.
State Bernoulli’s equation and use it to explain and define the relationship between static, dynamic and total pressure.
Define the stagnation point in the flow around an aerofoil, and explain the pressure obtained at the stagnation point.
Use the pitot system to explain the measurement of airspeed (no compressibility effects).
Define ‘TAS’, ‘IAS’, and ‘CAS’.
Define two-dimensional airflow and its relationship to an aerofoil of infinite span (i.e. no blade tip vortices and, therefore, no induced drag). Explain the difference between two- and three‑dimensional airflows.
Explain that viscosity is a feature of any fluid (gas or liquid).
Explain the tangential friction between air and the surface of an aerofoil, and the development of a boundary layer.
Describe laminar and turbulent boundary layers and the transition from laminar to turbulent. Show the influence of the roughness of the surface on the position of the transition point.
Two-dimensional airflow
Aerofoil section geometry
Define the terms: ‘aerofoil section’, ‘aerofoil element’, ‘chord line’, ‘chord’, ‘thickness’, ‘thickness-to-chord ratio, ‘camber line’, ‘camber’, and ‘leading-edge radius’.
Describe symmetrical and asymmetrical aerofoil sections.
Aerodynamic forces on aerofoil elements
Define the angle of attack (α).
Describe:
— the resultant force from the pressure distribution and the friction at the element;
— the resultant force from the boundary layers and the velocities in the wake; and
— the loss of momentum due to friction forces.
Resolve the aerodynamic force into the components of lift (L) and drag (D).
Define the lift coefficient (CL) and the drag coefficient (CD).
Show that the CL is a function of the α.
Explain how drag is caused by pressure forces on the surfaces of an aerofoil and by friction in the boundary layers. Define the term ‘profile drag’.
Define the L–D ratio.
Use the lift and drag equations to show the influence of speed and density on lift and drag for a given α.
Define the action line of the aerodynamic force and the CP.
Know that symmetrical aerofoils have a CP that is approximately a quarter chord behind the leading edge.
Stall
Explain the boundary layer separation when α increases beyond the onset of stall and the decrease of lift and the increase of drag. Define the ‘separation point’.
Disturbances due to profile contamination
Explain ice contamination, the modification of the section profile and surfaces due to ice and snow, the influence on L and D and the L–D ratio, the influence on α (at stall onset), and the effect of the increase in weight.
Explain the effect of erosion by heavy rain on the blade and subsequent increase in profile drag.
Three-dimensional airflow around a blade
The blade
Describe the various blade planforms.
Define aspect ratio and blade twist.
Airflow pattern and influence on lift (L)
Explain the spanwise flow around a blade and the appearance of blade tip vortices which are a loss of energy.
Show that the strength of the vortices increases as α and L increase.
Show that downwash causes vortices.
Define the relative airflow as the resultant of the undisturbed air velocity and induced velocity, and define α.
Explain the spanwise L distribution and the way in which it can be modified by twist (washout).
Induced drag
Explain induced drag and the influence of α and aspect ratio.
The airflow around the fuselage
Describe the fuselage and the external components that cause (parasite) drag, the airflow around the fuselage, and the influence of the pitch angle of the fuselage.
Describe fuselage shapes that minimise drag.
Define profile drag as the sum of pressure (form) drag and skin friction drag.
Define ‘interference drag’.
Know the drag formula.
TRANSONIC AERODYNAMICS and COMPRESSIBILITY EFFECTS
Airflow speeds and velocities
Speeds and Mach number
Define the speed of sound in air.
State that the speed of sound is proportional to the square root of the absolute temperature (in Kelvins).
Explain the variation in the speed of sound with altitude.
Define Mach number.
Explain the meaning of incompressibility and compressibility of air; relate this to the value of the Mach number.
Define high subsonic, transonic and supersonic flows in relation to the value of the Mach number.
Shock waves
Describe shock waves in a supersonic flow and the changes in pressure and speed.
Describe the appearance of local supersonic flows on the surfaces of a blade.
Influence of aerofoil section and blade planform
Explain the different shapes that allow higher Mach numbers without generating a shock wave on the upper surface, such as:
 reducing the section thickness-to-chord ratio;
 a planform with a sweep angle.
ROTORCRAFT TYPES
Rotorcraft
Rotorcraft types
Explain the difference between an autogyro and a helicopter.
Helicopters
Helicopter configurations
Describe (briefly) the single-main-rotor helicopter and other configurations: tandem, coaxial, side-by-side, synchrocopter (with intermeshing blades), the compound helicopter and tilt rotor.
The helicopter, characteristics and associated terminology
Mention the tail rotor, the Fenestron, and the no tail rotor (NOTAR).
Define the rotor disc area and the blade area.
Describe the teetering rotor with its hinge axis on the shaft axis, and rotors with more than two blades with offset hinge axes.
Define the fuselage centre line and the three axes: roll, pitch, and normal (yaw).
Define gross weight and gross mass (and the units involved), disc and blade loading.
MAIN-ROTOR AERODYNAMICS
Hover flight outside ground effect
Airflow through the rotor disc and around the blades
Based on Newton’s second law (momentum), explain that the upward vertical force from the disc, i.e. the rotor thrust, is the result of vertical downward velocities inside the rotor disc.
Explain why the production of the induced flow requires power applied to the shaft, i.e. induced power. Induced power is least if the induced velocities have the same value on the whole disc (i.e. there is uniformity of flow over the disc).
Explain why vertical rotor thrust must be higher than the weight of the helicopter because of the vertical drag on the fuselage.
Define the pitch angle and the α of a blade element.
Explain L and D relating to a blade element (including induced and profile drag).
Explain the necessity for collective pitch angle changes, the influence on the α and rotor thrust, and the need for blade feathering.
Describe the different blade shapes (as viewed from above).
Explain how profile drag on the blade elements generates a torque on the main shaft, and define the resulting rotor profile power.
Explain the influence of air density on the required powers.
Anti-torque force and tail rotor
Using Newton’s third law (motion), explain the need for tail-rotor thrust, the required value being proportional to main-rotor torque.
Show that tail-rotor power is proportional to tail‑rotor thrust.
Explain the necessity for feathering of the tail-rotor blades and their control by the yaw pedals, and the maximum and minimum values of the pitch angles of the blades.
Total power required and hover outside ground effect (HOGE)
Define ancillary equipment and its power requirement.
Define the total power required.
Describe the influence of ambient pressure, temperature and moisture on the required power.
Vertical climb
Relative airflow and angles of attack (α)
Describe the dependence of the vertical climb speed on the opposite vertical air velocity relative to the rotor disk.
Explain how α is controlled by the collective pitch angle control.
Power and vertical speed
Define total main-rotor power as the sum of parasite power, induced power, climb power, and rotor profile power.
Explain why the total main-rotor power required increases when the rate of climb increases.
Forward flight
Airflow and forces in uniform inflow distribution
Explain the assumption of a uniform inflow distribution on the rotor disc.
Show the upstream air velocities relative to the blade elements and the different effects on the advancing and retreating blades.
Define the area of reverse flow.
Explain the influence of forward speed on the circumferential speed of the blade tip.
Assuming constant pitch angles and rigid blade attachments, explain the roll moment from the asymmetric distribution of L.
Show that through cyclic feathering this imbalance could be eliminated by a low α (accomplished by a low pitch angle) on the advancing blade, and a high α (accomplished by a high pitch angle) on the retreating blade.
Describe the high air velocity at the advancing blade tip and the compressibility effects which limit maximum speed.
Describe the low air velocity on the retreating blade tip resulting from the difference between the circumferential speed and forward speed, the need for high α, and the onset of stall.
Define the blade tip speed ratio.
Explain the total rotor thrust that is perpendicular to the rotor disc and the need for tilting the thrust vector forward.
Explain the conditions of equilibrium in steady straight and level flight.
The flare (powered flight)
Explain the flare in powered flight, the rearward tilt of the rotor disc and the thrust vector. Show the horizontal thrust component that is in the opposite direction to forward velocity.
State the increase in thrust due to the upward inflow, and show the modifications in the α.
Explain the increase in rotor rpm for a non‑governed rotor.
Non-uniform inflow distribution in relation to inflow roll
Describe the inflow distribution which modifies α and L especially on the advancing and retreating blades.
Power and maximum speed
Explain that the induced velocities and power values decrease as the speed of the helicopter increases.
Define profile drag and profile power, and the increase in their values with the speed of the helicopter.
Define parasite drag and parasite power, and the increase in their values with the speed of the helicopter.
Define total drag and its increase with the speed of the helicopter.
Describe the power required for the tail rotor and the power required by ancillary equipment.
Define the total power requirement as a sum of the above partial powers, and explain how it varies with the speed of the helicopter.
Explain the influence of helicopter mass, air density, and additional external equipment on the partial powers and the total power required.
Describe translational lift and show the decrease in required total power as the helicopter increases its speed from the hover.
Hover and forward flight in ground effect
Airflow in ground effect, downwash
Explain how the vicinity of the ground changes the downward flow pattern and the consequences on lift (thrust) at constant rotor power. Show that ground effect depends on the height of the rotor above the ground and the rotor diameter. Show the required rotor power at constant all-up mass (AUM) as a function of height above the ground. Describe the influence of forward speed.
Vertical descent
Vertical descent, power on
Describe the airflow around the rotor disc in a trouble-free vertical descent, power on, the airflow opposing the helicopter’s velocity, the relative airflow, and α.
Explain the vortex-ring state, also known as settling with power. State the approximate vertical descent speeds that allow the formation of vortex ring, related to the values of the induced velocities.
Describe the airflow relative to the blades, the root stall, the loss of lift at the blade tip, and the turbulence. Show the effect of raising the lever and describe the effects on the controls.
Autorotation
State the need for early recognition and for a quick initiation of recovery. Describe the recovery actions.
Explain that the collective lever must be lowered quickly enough to avoid a rapid decay of rotor rpm due to drag on the blades, and explain the influence of rotational inertia of the rotor on the rate of decay.
Show the induced flow through the rotor disc, the rotational velocity and relative airflow, the inflow and inflow angles.
Show how the aerodynamic forces on the blade elements vary from root to tip and distinguish three zones: the inner stalled region, the middle driving region, and the driven region.
Explain the control of the rotor rpm with collective pitch.
Show the need for negative tail-rotor thrust with yaw control.
Explain the final increase in rotor thrust caused by raising the collective pitch to decrease the vertical descent speed and the decay in rotor rpm.
Forward flight — autorotation
Airflow at the rotor disc
Explain the factors that affect inflow angle and α, the autorotative power distribution, and the dissymmetry over the rotor disc in forward flight.
Flight and landing
Show the effect of forward speed on the vertical descent speed.
Explain the effects of gross weight, rotor rpm, and altitude (density) on endurance and range.
Explain the manoeuvres for turning and touchdown.
Explain the height–velocity curves.
MAIN-ROTOR MECHANICS
Flapping of the blade in hover
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Centrifugal turning moment (CTM)
Describe the centrifugal forces on the mass elements of a blade with pitch applied and the components of those forces. Show how the forces generate a moment that tries to reduce the blade pitch angle.
Explain the methods of counteracting CTM with hydraulics, bias springs, and balance masses.
Coning angle in the hover
Define the tip path plane and the coning angle.
Show how the equilibrium of the moments about the flapping hinge of lift (thrust) and of the centrifugal force determine the coning angle of the blade (the blade mass being negligible).
Justify the lower limit of rotor rpm.
Explain the effect of the mass of a blade on the tip path and the tracking.
Flapping angles of the blade in forward flight
Forces on the blade in forward flight without cyclic feathering
Assume rigid attachments of the blade to the hub and show the periodic lift, moment and stresses on the attachment, the ensuing metal fatigue, the roll moment on the helicopter, and justify the necessity for a flapping hinge.
Assume no cyclic pitch and describe the lift on the advancing and retreating blades.
State the azimuthal phase lag (90° or less) between the input (applied pitch) and the output (flapping angle). Explain flapback (the rearward tilting of the tip path plane and total rotor thrust).
Cyclic pitch (feathering) in forward flight
Show that in order to assume and maintain forward flight, the total rotor thrust vector must obtain a forward component by tilting the tip path plane.
Show how the applied cyclic pitch modifies the lift on the advancing and retreating blades and produces the required forward tilting of the tip path plane and the total rotor thrust.
Show the cone described by the blades and define the virtual axis of rotation. Define the plane of rotation.
Define the reference system in which the movements are defined: the shaft axis and the hub plane.
Describe the swash plates, the pitch links and horns. Explain how the collective lever moves the non-rotating swash plate up or down the shaft axis.
Describe the mechanism by which the desired cyclic blade pitch can be produced by tilting the swash plate with the cyclic stick.
Explain the translational lift effect when the speed increases.
Justify the increase of the tilt angle of the thrust vector and of the disc in order to increase the speed.
Blade-lag motion in forward flight
Forces on the blade in the disc plane (tip path plane) in forward flight
Explain the Coriolis force due to flapping, the resulting periodic moments in the hub plane, and the resulting periodic stresses which make lead-lag hinges necessary to avoid material fatigue.
Describe the profile drag forces on the blade elements and the periodic variation of these forces.
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Ground resonance
Explain the movement of the CG of the blades due to lead-lag movements in the multi-bladed rotor.
Show the effect on the fuselage and the danger of resonance between this force and the fuselage and undercarriage when the gear touches the ground.
Rotor systems
See-saw or teetering rotor
Explain that a teetering rotor is prone to mast bumping in low-G situations, and that it is difficult to counteract because there is no lift force to provide sideways movement.
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Hingeless rotor, bearingless rotor
Show the forces on the flapping hinges with a large offset (virtual hinge) and the resulting moments, and compare them with other rotor systems.
Blade sailing
Blade sailing and causes
Define blade sailing, the influence of low rotor rpm and of a headwind.
Minimising the danger
Describe actions that minimise danger and the demonstrated wind envelope for engaging and disengaging rotors.
Droop stops
Explain the purpose of droop stops, and their retraction.
Vibrations due to main rotor
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TAIL ROTORS
Conventional tail rotor
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Tail-rotor aerodynamics
Explain the airflow around the blades in the hover and in forward flight, and the effects of the tip speeds on noise production and compressibility.
Explain the effect of wind on tail-rotor aerodynamics and thrust in the hover, and any problems.
Explain tail-rotor thrust and the control through pitch alterations (feathering).
Explain tail-rotor flapback, and the effects of Delta 3.
Describe the roll moment and drift as side effects of the tail rotor.
Explain the effects of tail-rotor failure.
Explain the loss of tail-rotor effectiveness (LTE), tail-rotor vortex-ring state, causes, crosswind, and yaw speed.
Strakes on the tail boom
Describe the strake and explain its function.
EQUILIBRIUM, STABILITY AND CONTROL
Equilibrium and helicopter attitudes
Hover
Explain why the vector sum of forces and moments must be zero in any acceleration-free situation.
Indicate the forces and the moments about the lateral axis in a steady hover.
Indicate the forces and the moments about the longitudinal axis in a steady hover.
Deduce how the roll angle in a steady hover without wind results from the moments about the longitudinal axis.
Explain how the cyclic is used to equalise moments about the lateral axis in a steady hover.
Explain the consequence of the cyclic stick reaching its forward or aft limit during an attempt to take off to the hover.
Explain the influence of density altitude on the equilibrium of forces and moments in a steady hover.
Forward flight
Explain why the vector sum of forces and of moments must be zero in unaccelerated flight.
Indicate the forces and the moments about the lateral axis in steady straight and level flight.
Explain the influence of AUM on the forces and moments about the lateral axis in forward flight.
Explain the influence of the CG position on the forces and moments about the lateral axis in forward flight.
Explain the role of the cyclic stick position in creating equilibrium of forces and moments about the lateral axis in forward flight.
Explain how forward speed influences the fuselage attitude.
Describe and explain the inflow roll effect.
Stability
Static longitudinal, roll and directional stability
Define static stability; give an example of static stability and of static instability.
Explain the contribution of the main rotor to speed stability.
Describe the influence of the horizontal stabiliser on static longitudinal stability.
Explain the effect of hinge offset on static stability.
Describe the influence of the tail rotor on static directional stability.
Describe the influence of the vertical stabiliser on static directional stability.
Explain the influence of the main rotor on static roll stability.
Describe the influence of the longitudinal position of the CG on static longitudinal stability.
Static stability in the hover
Describe the initial movements of a hovering helicopter after the occurrence of a horizontal gust.
Dynamic stability
Define dynamic stability; give an example of dynamic stability and of dynamic instability.
Explain why static stability is a precondition for dynamic stability.
Longitudinal stability
Explain the individual contributions of α and speed stability together with the stabiliser and fuselage to dynamic longitudinal stability.
Roll stability and directional stability
Know that a large static roll stability together with a small directional stability may lead to a Dutch roll.
Control
Manoeuvre stability
Explain how helicopter control can be limited because of available stick travel.
Explain how the CG position influences the remaining stick travel.
Control power
Explain the meaning of the control moment.
Explain the importance of the CG position on the control moment.
Explain the influence of hinge offset on controllability.
Static and dynamic rollover
Explain the mechanism which causes dynamic rollover.
Explain the required pilot action when dynamic rollover is starting to develop.
HELICOPTER FLIGHT MECHANICS
Flight limits
Hover and vertical flight
Show the power required for HOGE and HIGE, and the power available.
Explain the effects of AUM, ambient temperature and pressure, density altitude, and moisture.
Describe the rate of climb in a vertical flight.
Forward flight
Compare the power required and the power available as a function of speed in straight and level flight.
Define the maximum speed limited by power and the value relative to VNE and VNO.
Use the power graph to determine the speeds of maximum rate of climb and the maximum angle of climb.
Use the power graph to define true airspeed (TAS) for maximum range and maximum endurance, and consider the case of piston engine and turbine engine. Explain the effects of tailwind or headwind on the speed for maximum range.
Explain the effects of AUM, pressure and temperature, density altitude, and humidity.
Manoeuvring
Define the load factor, the radius, and the rate of turn.
Explain the relationship between the angle of bank, the airspeed and the radius of turn, and between the angle of bank and the load factor.
Explain the influence of AUM, pressure and temperature, density altitude, and humidity.
Special conditions
Operating with limited power
Explain operations with limited power, use the power graph to show the limitations on vertical and level flight, and describe power checks and procedures for take-off and landing.
Describe manoeuvres with limited power.
Overpitch, overtorque
Describe overpitching and show the consequences.
Describe situations likely to lead to overpitching.
Describe overtorquing and show the consequences.
Describe situations likely to lead to overtorquing.

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