In this lesson you will learn Helicopter Fundamentals.
TERMINAL LEARNING OBJECTIVE:
|ACTION:||You will know helicopter fundamentals.|
|CONDITION:||You will be given the information provided in this text, a list of reference manuals, and charts provided in the text.|
|STANDARD:||You will correctly answer all the questions in the practice exercise.|
|REFERENCES:||Listed in appendix I.|
If developing vertical flight has proved as simple as the idea itself, the helicopter undoubtedly would have been the first practical aircraft in the field. In its first form, the helicopter was conceived by Leonardo da Vinci in the early 1500's. Figure 3.1 shows a sketch of his idea. In his notes da Vinci used the Greek work helix, meaning a spiral, and he is believed to have combined this word with the Greek work pteron, meaning wing. It is from this combination of Greek words that our word helicopter is derived.
Figure 3.1. Leonardo da Vinci Sketch of a Helixpteron.
Development proved too difficult and complicated for the early experimenters because they did not have an engine that could ensure flight. When larger, lighter, and more reliable engines were developed, the dream of a helicopter became a reality.
The same laws of force and motion that apply to fixed wing aircraft also apply to the helicopter. Controls for the helicopter are complex, and torque, gyroscopic precession, and dissymmetry of lift must be dealt with. Retreating blade stall limits the helicopter's forward airspeed.
This chapter presents a basic discussion on helicopter controls, velocity, torque, gyroscopic precession, dissymmetry of lift, retreating blade stall, settling with power, pendular action, hovering, ground effect, translational lift, and autorotation.
Weight and drag act on a helicopter as they do on all aircraft. However, lift and thrust for a helicopter are obtained from the main rotor. In a very basic sense, the helicopter's main rotor does what wings and a propeller do for an airplane. Moreover, by tilting the main rotor, the pilot can make the helicopter fly to either side or to the rear. The controls used by the pilot are discussed in the next paragraph.
Figure 3.2. Forces Acting on a Helicopter.
The sketch in figure 3.3 shows the main rotor, cyclic and collective controls, antitorque pedals, and antitorque rotor. Basically, the cyclic control is a mechanical linkage used to change the pitch of the main rotor blades. Pitch change is accomplished at a specific point in the plane of rotation to tilt the main rotor disc. Most of the helicopters now in the Army inventory have hydraulic assistance in addition to the mechanical linkage. The collective pitch is the control that changes the pitch of all the main rotor blades equally and simultaneously. The antitorque pedals are used to adjust the pitch in the antitorque rotor blades to compensate for main rotor torque. Torque is explained in paragraph 3.5.
Figure 3.3. Helicopter Controls.
A helicopter's main rotor blades must move through the air at a relatively high speed to produce enough lift to raise the helicopter and keep it in the air. The main rotor can turn at the required takeoff speed while the antitorque rotor holds the fuselage speed at zero.
The helicopter can fly forward, backward, or sideward as the pilot desires. It can also remain stationary in the air (hover) with the main rotor blades developing the lift to support the helicopter.
The torque problem is related to helicopters of single-main-rotor design. The reason for this is that as the helicopter's main rotor turns in one direction, the fuselage tends to turn in the opposite direction. This effect is based on Newton's third law which states "To every action there is an opposite and equal reaction." The torque problem on single-rotor helicopters is counteracted and controlled by an antitorque (tail) rotor, discussed in paragraph 3.6.
On tandem rotor helicopters the main rotors turn in opposite directions and thereby eliminate the torque effect.
Figure 3.4 shows the direction of travel for a main rotor, the direction of torque for the fuselage, and the location of an antitorque (tail) rotor.
Figure 3.4. Tail Rotor Thrust to Compensate for Torque.
An antitorque rotor located on the end of a tail boom extension provides compensation for torque in the single-main-rotor helicopter. The tail rotor, driven by the engine at a constant speed, produces thrust in a horizontal plane opposite to the torque reaction developed by the main rotor.
The result of applying force against a rotating body occurs 90° in the direction of rotation from where the force is applied. This effect is called gyroscopic precession and it is illustrated in figure 3.5. For example, if a downward force is applied at 9 o'clock, as in the figure, the result appears at 6 o'clock as shown. This will make the 12 o'clock position in the figure tilt up an equal amount in the opposite direction.
Figure 3.5. Gyroscopic Precession.
Figure 3.6 illustrates the offset control linkage needed in a helicopter to enable the pilot to tilt the main rotor disc in the direction he wants to go. If such linkage were not used the pilot would have to move the cyclic stick 90° out of phase or to the right of the direction desired. The offset control linkage is attached to a lever extending 90° in the direction of rotation from the main rotor blade.
Figure 3.6. Offset Control Linkage.
The area within the circle made by the rotating blade tips of a helicopter is known as the disc area or rotor disc. When hovering in still air, lift created by the rotor blades at all parts of the disc area is equal. Dissymmetry of lift is the difference in lift that exists between the advancing half of the disc area and the retreating half, and it is created by horizontal flight or wind.
When a helicopter is hovering in still air, the tip speed of the advancing blade is about 600 feet per second. The tip speed of the retreating blade is the same. Dissymmetry of lift is created by the horizontal movement of the helicopter in forward flight, and the advancing blade has the combined speed of blade velocity plus speed of the helicopter. The retreating blade loses speed in proportion to the forward speed of the helicopter.
Figure 3.7 illustrates the dissymmetry of lift and shows the arithmetic involved in calculating the differences between the velocities of the advancing and retreating blades. In the figure, the helicopter is moving forward at a speed of 100 knots, the velocity of the rotor disc is equal to approximately 355 knots, and the advancing rotor speed is 455 knots. The speed of the retreating blade is 255 knots. This speed is obtained by subtracting the speed of the helicopter (100 knots) from the tip speed of 355 knots. As can be seen from the difference between the advancing and retreating blade velocities, a large speed and lift variation exists.
(ROTATIONAL VELOCITY) ± (HEL FORWARD SPEED) = (AIRSPEED OF BLADE)
Figure 3.7. Dissymmetry of Lift.
Cyclic pitch control, a design feature that permits changes in the angle of attack during each revolution of the rotor, compensates for the dissymmetry of lift. As the forward speed of the helicopter is increased, the aviator must apply more and more cyclic to hold a given rotor disc attitude. The mechanical addition of more pitch to the retreating blade and less to the advancing blade is continued throughout the helicopter's range.
Figure 3.8 illustrates the tendency for the helicopter's retreating blade to stall in forward flight. It is a major factor in limiting their forward speed. Just as the stall of an airplane wing limits the low-airspeed possibilities of the airplane, the stall of a rotor blade limits the high-speed potential of a helicopter. The airspeed of the retreating blade slows down as forward airspeed is increased. The retreating blade must produce an amount of lift equal to that of the advancing blade, as shown in figure 3.8B. As the airspeed of the retreating blade is decreased with forward airspeed, the blade angle of attack must be increased to equalize lift throughout the rotor disc area. As this angle increase is continued, the blade will stall at some high forward airspeed as shown in figure 3.8C.
Figure 3.8. Retreating Blade Stall.
Upon entry into blade stall, the first effect is generally a noticeable vibration of the helicopter. This vibration is followed by the helicopter's nose lifting and a rolling tendency. If the cyclic stick is held forward and the collective pitch is not reduced, or is raised, the stall becomes aggravated, and the vibration increases greatly. Control of the helicopter may then be lost.
An aviator may experience settling with power accidentally. Causes of settling with power are typified by a helicopter in a vertical or nearly vertical powered descent of at least 300 feet per minute with a relatively low airspeed. The rate depends upon the load, rotor rpm, and density altitude. The rotor system must be using from 20 to 100 percent of the available engine power and the horizontal velocity must not exceed 10 knots. The helicopter descends in turbulent air that has just been accelerated downward by the rotor. This turbulence reacts upon the main rotor blades at increased angles of attack and stalls the blades at the main rotor hub. The stall then progresses outward along the blades as the rate of descent increases. To recover from settling with power, reduce collective pitch, lower the helicopter's nose into the wind, and, as airspeed increases, start a slow right turn.
In general, the fuselage of the helicopter is suspended from a single point and has considerable mass. Because of this it has some freedom to move laterally or longitudinally, a pendular action. However, correct aviator cyclic control movements, free of overcontrol, cause the rotor tip path and the fuselage to move in unison.
For semirigid helicopters, the normal corrective device is the addition of a synchronized elevator attached to the tail boom and operated by the cyclic stick.
A helicopter hovers when it maintains a constant position at a selected point in the air, usually a few feet above the ground. To hover, a helicopter main rotor must supply lift to equal the total weight of the helicopter, including crew; fuel; passengers and cargo, if carried; and armament, if armed. The necessary lift is created by rotating the blades at high velocity and increasing the blade angle of attack.
At a hover, the rotor system requires a great volume of air upon which to work. This air must be pulled from the surrounding air mass; this is an expensive maneuver that takes a great deal of horsepower. The air delivered to the rotating blades is pulled from above at a relatively high velocity, forcing the rotor system to work in a descending column of air.
The main rotor vortex and the recirculation of turbulent air add resistance to the helicopter while hovering. Such an undesirable air supply requires higher blade angles of attack and an expenditure of more engine power and fuel. In addition, the main rotor is operating in air filled with abrasive materials that cause heavy wear on helicopter parts while hovering in ground effect.
Figure 3.9 illustrates air flow in and out of ground effect. Notice the information in A and B that is printed close to the sketches. Ground effect is a condition of improved performance found when hovering near the ground. The best height is approximately one-half the main rotor diameter.
Figure 3.9. Airflow In and Out of Ground Effect.
The improved lift and airfoil efficiency while operating in ground effect are due to the following effects. First and most important, the main rotor-tip vortex is reduced. When operating in ground effect, the downward and outward airflow reduces the vortex. A vortex is a flow involving rotation about an axis or center. This makes the outward portion of the main rotor blade more efficient. Reducing the vortex also reduces the turbulence caused by recirculating the vortex swirl, as shown in figure 3.9A. Second the angle of the air is reduced as it leaves the airfoil, as illustrated in figure 3.9B. When the airfoil angle is reduced the resultant lift is rotated slightly forward, making it more vertical. Reduction of induced drag permits lower angles of attack for the same amount of lift and reduces the power required to drive the blades.
The efficiency of the hovering rotor system is improved by each knot of incoming wind gained by forward motion of the helicopter or by a surface headwind. As the helicopter moves forward, as shown in figure 3.10, fresh air enters in an amount sufficient to relieve the hovering air-supply problem and improve performance. At approximately 18 knots, the rotor system receives enough free, undisturbed air to eliminate the air supply problem. At this time, lift noticeably improves; this distinct change is referred to as translational lift. At the instant of translational lift, and as the hovering air supply pattern is broken, dissymmetry of lift is created. As airspeed increases, translational lift continues to improve up to the speed that is used for best climb.
Figure 3.10. Airflow with Translational Lift in Forward Flight.
In forward flight, air passing through the rear portion of the rotor disc has a higher downwash velocity than the air passing through the forward portion. This is known as transverse flow effect and is illustrated in figure 3.11. This effect, in combination with gyroscopic precession, causes the rotor to tilt sideward and results in vibration that is most noticeable on entry into effective translation.
Figure 3.11. Transverse Flow Effect.
If engine power fails, or certain other emergencies occur, autorotation is a means of safely landing a helicopter. The transmission in a helicopter is designed to allow the main rotor to turn freely in its original direction when the engine stops. Figure 3.12 illustrates how the helicopter is allowed to glide to earth and by using the main rotor rpm, make a soft landing.
Figure 3.12. Approach to Landing, Power Off.
The rotor blade autorotative driving region is the portion of the blade between 25 to 70 percent radius, as shown in figure 3.13, blade element A. Because this region operates at a comparatively high angle of attack, the result is a slight but important forward inclination of aerodynamic force. This inclination supplies thrust slightly ahead of the rotating axis and tends to speed up this portion of the blade during autorotation.
Figure 3.13. Autorotation Blade Forces.
The blade area outboard of the 70 percent circle is known as the propeller or driven region. Analysis of blade element B in figure 3.13 shows that the aerodynamic force inclines slightly behind the rotating axis. This inclination causes a small drag force that tends to slow the tip portion of the blade. Rotor rpm stabilizes, or achieves equilibrium, when autorotative force and antiautorotative force are equal.
The blade area inboard of the 25 percent circle is known as the stall region because it operates above its maximum angle of attack. This region contributes considerable drag that tends to slow the blade.
All helicopters carry an operators manual (-10) that has an air speed versus altitude chart similar to the one shown in figure 3.14. The shaded areas on this chart must be avoided. The proper maneuvers for a safe landing during engine failure cannot be accomplished in these areas.
Figure 3.14. High Velocity Diagram.
Weight, lift, thrust, and drag are the four forces acting on a helicopter. The cyclic for directional control, the collective pitch for altitude control, and the antitorque pedals to compensate for torque are the three main controls used in a helicopter.
Torque is an inherent problem with single-main-rotor helicopters. Gyroscopic precession occurs approximately 90° in the direction of rotation from the point where the force is applied. Dissymmetry of lift is the difference in lift that exists between the advancing and retreating half of the rotor disc.
Settling with power can occur when the main rotor system is using from 20 to 100 percent of the available engine power, and the horizontal velocity is under 10 knots. At a hover, the rotor system requires a great volume of air upon which to work. This air must be pulled from the surrounding air mass. This is a costly maneuver that takes a great amount of power.
Ground effect is improved performance when hovering near the ground at a height of no more than approximately one-half the main rotor diameter. Translational lift is achieved at approximately 18 knots, and the rotor system receives enough free, undisturbed air to improve performance. At the instant translational lift is in effect and the hovering air-supply pattern is broken, dissymmetry of lift is created. Autorotation is a means of safely landing a helicopter after engine failure or other emergencies. A helicopter transmission is designed to allow the main rotor to turn freely in its original direction if the engine fails.