Rotorcraft Flying Handbook, FAA-H-8083-21

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lift and thrust are greater than weight and drag, the helicopter ascends vertically. [Figure 3-10] Thrust In straight-and-level, unaccelerated forward flight, lift equals weight and thrust equals drag (straight-and-level flight is flight with a constant heading and at a constant altitude). If lift exceeds weight, the helicopter climbs; if lift is less than weight, the helicopter descends. If thrust exceeds drag, the helicopter speeds up; if thrust is less than drag, it slows down. Vertical Ascent Lift As the helicopter moves forward, it begins to lose altitude because of the lift that is lost as thrust is diverted forward. However, as the helicopter begins to accelerate, the rotor system becomes more efficient due to the increased airflow. The result is excess power over that which is required to hover. Continued acceleration causes an even larger increase in airflow through the rotor disc and more excess power. FORWARD FLIGHT In or during forward flight, the tip-path plane is tilted forward, thus tilting the total lift-thrust force forward from the vertical. This resultant lift-thrust force can be resolved into two components—lift acting vertically upward and thrust acting horizontally in the direction of flight. In addition to lift and thrust, there is weight (the downward acting force) and drag (the rearward acting or retarding force of inertia and wind resistance). [Figure 3-11] Resultant Weight Drag Figure 3-10. To ascend vertically, more lift and thrust must be generated to overcome the forces of weight and the drag. Lift TRANSLATIONAL LIFT Translational lift is present with any horizontal flow of air across the rotor. This increased flow is most noticeable when the airspeed reaches approximately 16 to 24 knots. As the helicopter accelerates through this speed, the rotor moves out of its vortices and is in relatively undisturbed air. The airflow is also now more horizontal, which reduces induced flow and drag with a corresponding increase in angle of attack and lift. The additional lift available at this speed is referred to as “effective translational lift” (ETL). [Figure 3-12] No Recirculation of Air Tail Rotor Operates in Relatively Clean Air More Horizontal Flow of Air Reduced Induced Flow Increases Angle of Attack 16 to 24 Knots Thrust Figure 3-12. Effective translational lift is easily recognized in actual flight by a transient induced aerodynamic vibration and increased performance of the helicopter. Helicopter Movement Weight Drag Resultant Figure 3-11. To transition into forward flight, some of the vertical thrust must be vectored horizontally. You initiate this by forward movement of the cyclic control. When a single-rotor helicopter flies through translational lift, the air flowing through the main rotor and over the tail rotor becomes less turbulent and more aerodynamically efficient. As the tail rotor efficiency improves, more thrust is produced causing the aircraft to yaw left in a counterclockwise rotor system. It will be necessary to use right torque pedal to correct for this tendency on takeoff. Also, if no corrections are made, the nose rises or pitches up, and rolls to the right. This is caused by combined effects of dissymmetry of lift and transverse flow effect, and is corrected with cyclic control. 3-5
Translational lift is also present in a stationary hover if the wind speed is approximately 16 to 24 knots. In normal operations, always utilize the benefit of translational lift, especially if maximum performance is needed. INDUCED FLOW As the rotor blades rotate they generate what is called rotational relative wind. This airflow is characterized as flowing parallel and opposite the rotor’s plane of rotation and striking perpendicular to the rotor blade’s leading edge. This rotational relative wind is used to generate lift. As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime a helicopter is producing lift, it moves large masses of air vertically and down through the rotor system. This downwash or induced flow can significantly change the efficiency of the rotor system. Rotational relative wind combines with induced flow to form the resultant relative wind. As induced flow increases, resultant relative wind becomes less horizontal. Since angle of attack is determined by measuring the difference between the chord line and the resultant relative wind, as the resultant relative wind becomes less horizontal, angle of attack decreases. [Figure 3-13] TRANSVERSE FLOW EFFECT As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disc area and increases at the aft disc area. This increases the angle of attack at the front disc area causing the rotor blade to flap up, and reduces angle of attack at the aft disc area causing the rotor blade to flap down. Because the rotor acts like a gyro, maximum displacement occurs 90° in the direction of rotation. The result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots. You can recognize transverse flow effect because of increased vibrations of the helicopter at airspeeds just below effective translational lift on takeoff and after passing through effective translational lift during landing. To counteract transverse flow effect, a cyclic input needs to be made. DISSYMMETRY OF LIFT When the helicopter moves through the air, the relative airflow through the main rotor disc is different on the advancing side than on the retreating side. The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is reduced by the helicopter’s forward airspeed. Therefore, as a result of the relative wind speed, the advancing blade side of the rotor disc produces more lift than the retreating blade side. This situation is defined as dissymmetry of lift. [Figure 3-14] If this condition was allowed to exist, a helicopter with a counterclockwise main rotor blade rotation would roll to the left because of the difference in lift. In reality, the main rotor blades flap and feather automatically to equalize lift across the rotor disc. Articulated rotor systems, usually with three or more blades, incorporate a horizontal hinge (flapping hinge) to allow the individual rotor blades to move, or flap up and down as they rotate. A semirigid rotor system (two blades) utilizes a teetering hinge, which allows the blades to flap as a unit. When one blade flaps up, the other flaps down. A Resultant Relative Wind Rotational Relative Wind Angle of Attack Induced Flow B Resultant Relative Wind Rotational Relative Wind Angle of Attack Induced Flow A B 10 to 20 Knots Figure 3-13. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft portion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disc. 3-6
Page 2: ROTORCRAFT FLYING HANDBOOK 2000 U.S
Page 6 and 7: HELICOPTER Chapter 1—Introduction
Page 8 and 9: Technique..........................
Page 10 and 11: Collision Avoidance at Night.......
Page 12 and 13: Helicopters come in many sizes and
Page 14 and 15: Main Rotor Wake Air Jet Downwash Li
Page 16 and 17: There are four forces acting on a h
Page 18 and 19: Increased Local Velocity (Decreased
Page 20 and 21: Load Factor - "G's" 9 8 7 6 5 4 3 2
Page 22 and 23: Once a helicopter leaves the ground
Page 24 and 25: otation and blade deceleration take
Page 28 and 29: Relative Wind Retreating Side Blade
Page 30 and 31: Bank Angle Vertical Component of Li
Page 32 and 33: percent of the radius. In the drive
Page 34 and 35: Note: In this chapter, it is assume
Page 36 and 37: The rotor disc tilts in the directi
Page 38 and 39: By knowing the various systems on a
Page 40 and 41: If the first and second stage turbi
Page 42 and 43: The horizontal hinge, called the fl
Page 44 and 45: RECIPROCATING ENGINES Fuel is deliv
Page 46 and 47: incorporated to prevent excessive v
Page 48 and 49: depends on the temperature of the o
Page 50 and 51: Title 14 of the Code of Federal Reg
Page 52 and 53: l0 5 l5 20 25 30 35 MANIFOLD PRESSU
Page 54 and 55: It is vital to comply with weight a
Page 56 and 57: sive forward displacement of cyclic
Page 58 and 59: Once you are satisfied that the tot
Page 60 and 61: Basic Empty Weight Pilot Fwd Passen
Page 62 and 63: Your ability to predict the perform
Page 64 and 65: Approximate Density Altitude - Thou
Page 66 and 67: the entire flight operation. Being
Page 68 and 69: From the previous chapters, it shou
Page 70 and 71: 2. brief passengers on the best way
Page 72 and 73: VERTICAL TAKEOFF TO A HOVER A verti
Page 74 and 75: Control pressures and direction of
Page 76 and 77:
HOVER TAXI A "hover taxi" is used w
Page 78 and 79:
3. Assuming an extreme nose-down at
Page 80 and 81:
tain altitude and r.p.m. As the tor
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Start Turn At Boundary Turn More Th
Page 84 and 85:
the helicopter is being crabbed int
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APPROACHES An approach is the trans
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The maneuvers presented in this cha
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TECHNIQUE Refer to figure 10-2. To
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COMMON ERRORS 1. Failing to maintai
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Figure 10-7. Slope takeoff. be cons
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made in the forward portion of the
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Today helicopters are quite reliabl
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After touchdown and after the helic
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HEIGHT ABOVE SURFACE - FEET 500 450
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helicopter, while the relative airf
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When performing slope takeoff and l
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the main rotor blades having an ang
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WEATHERCOCK STABILITY (120-240°) I
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LOW FREQUENCY VIBRATIONS Low freque
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Attitude instrument flying in helic
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ways. If the ram air inlet is clogg
Page 118 and 119:
to true north. Because the aircraft
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AIRCRAFT CONTROL Controlling the he
Page 122 and 123:
vertical speed indicator immediatel
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POWER CONTROL DURING STRAIGHT-AND-
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50 60 70 80 40 TORQUE 90 30 100 20
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to the approximate setting for the
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used, the rate of cross-check must
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AUTOROTATIONS Both straight-ahead a
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{ { Flying at night can be a very p
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White Red Red White Green Red Green
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you are operating off-airport, you
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Aeronautical decision making (ADM)
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THE DECISION-MAKING PROCESS An unde
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Night VFR Accident Rate Per 100,000
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STRESSORS Physical Stress—Conditi
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OPERATIONAL PITFALLS Peer Pressure
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January 9th, 1923, marked the first
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LANDING GEAR The landing gear provi
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Helicopters and gyroplanes both ach
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elative wind striking the advancing
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subject to pendular action in the s
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Due to rudimentary flight control s
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Gyroplanes are available in a wide
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Figure 18-4. This prerotator uses b
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inability to use differential braki
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As with most certificated aircraft
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8 TOTAL TAKEOFF DISTANCE TO CLEAR 5
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The diversity of gyroplane designs
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minimize the length of the ground r
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adjust the fuel/air mixture to achi
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maximum performance is desired, two
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Start Turn At Boundary Turn More Th
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Shallowest Bank UPPER HALF OF CIRCL
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of failing to monitor and maintain
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for the gyroplane. When using the s
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Gyroplanes are quite reliable, howe
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BUNTOVER (POWER PUSHOVER) As you le
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As with any aircraft, the ability t
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leaves few places to safely land in
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GLOSSARY ABSOLUTE ALTITUDE—The ac
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FREE TURBINE—A turboshaft engine
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TRANSLATIONAL LIFT—The additional
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A ABORTED TAKEOFF, GYROPLANE 21-1 A
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turn indicators, 12-4 GYROSCOPIC PR
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R RAPID DECELERATION 10-3 RECIPROCA
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Rotorcraft Flying Handbook, FAA-H-8083-21

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