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Chapter 2. AERODYNAMICS OF POWERED FLIGHT (Continued)  Axis of rotation - The axis of rotation of a helicopter rotor is the imaginary line about which the rotor rotates. It is represented by a line drawn through the center of, and perpendicular to, the tip-path plane. It is not to be confused with the rotor mast. The only time the rotor axis of rotation coincides with the rotor mast is when the tip-path plane is perpendicular to the rotor mast (figure 18). Figure 18 - The axis of rotation is the imaginary line about which the rotor rotates and is perpendicular to the tip-path plane.  Coriolis effect - When a rotor blade of a three-bladed
rotor system flaps upward, the center of mass of that blade moves
closer to the axis of rotation and blade acceleration takes place.
Conversely, when that blade flaps downward, its center of mass moves
further from the axis of rotation and blade deceleration takes place
(figure 19). (Keep in mind, that due to coning, the rotor blade will
not flap below a plane passing through the rotor hub and perpendicular
to the axis of rotation.) The acceleration and deceleration actions
(often referred to as leading, lagging, or hunting) of the rotor blades
are absorbed by either dampers or the blade structure itself, depending
upon the design of the rotor system. Two-bladed rotor systems are normally subject to CORIOLIS EFFECT to a much lesser degree than are three-bladed systems since the blades are generally "underslung" with respect to the rotor hub, and the change in the distance of the center of mass from the axis of rotation is small. The hunting action is absorbed by the blades through bending. If a two-bladed rotor system is not "underslung," it will be subject to CORIOLIS EFFECT comparable to that of a fully articulated system. Figure 19 (above) - Coriolis effect is the change in blade velocity to compensate for the change in distance of the center of mass from the axis of rotation as the blades flap. CORIOLIS EFFECT might be compared to spinning skaters. When they extend their arms, their rotation slows down because their center of mass moves farther from their axis of rotation. When their arms are retracted, their rotation speeds up because their center of mass moves closer to their axis of rotation.  The tendency of a rotor blade to increase or decrease
its velocity in its plane of rotation due to mass movement is known as
CORIOLIS EFFECT, named for the mathematician who made studies of forces
generated by radial movements of mass on a rotating disc. Translating tendency or drift - The entire helicopter has a tendency to move in the direction of tail rotor thrust (to the right) when hovering. This movement is often referred to as "drift." To counteract this drift, the rotor mast in some helicopters is rigged slightly to the left side so that the tip-path plane has a built-in tilt to the left, thus producing a small sideward thrust. In other helicopters, drift is overcome by rigging the cyclic pitch system to give the required amount of tilt to the tip-path plane (figure 20). Figure 20 (right) - Drift, cause by tail rotor thrust, is compensated for by rigging the mast or cyclic pitch system to have a built-in tilt of the tip-path plane to the left.  Ground effect - When a helicopter is operated near the
surface, the downwash velocity created by the rotor blades cannot be
fully developed due to the proximity of the surface. This restraint of
rotor downwash occurs as the helicopter reaches a relatively low
altitude - usually less than one rotor diameter above the surface
(figure 21). As the downwash velocity is reduced, the induced angle of attack of each rotor blade is reduced and the lift vector becomes more vertical. Simultaneously, a reduction in induced drag occurs. In addition, as the induced angle of attack is reduced, the angle of attack generating lift is increased. The net result of these actions is a beneficial increase in lift and a lower power requirement to support a given weight. Figure 21 (right) - Ground effect results when the rotor downwash field is altered from its free air state by the presence of the surface. Translational lift - Translational lift is that additional lift obtained when entering horizontal flight, due to the increased efficiency of the rotor system. The rotor system produces more lift in forward flight because the higher inflow velocity supplies the rotor disc with a greater mass of air per unit time upon which to work than it receives while hovering. Translational lift is present with any horizontal movement although the increase will not be noticeable until airspeed reaches approximately 15 miles per hour. The additional lift available at this speed is referred to as "effective translational lift" and is easily recognized in actual flight by the increased performance of the helicopter. Since translational lift depends upon airspeed rather than groundspeed, the helicopter does not have to be in horizontal flight to be affected. Translational lift will be present during hovering flight in a wind - the amount being proportional to the wind velocity - and effective translational lift will be present when hovering in winds of 15 MPH or more. Transverse flow effect - In forward flight, air passing through the rear portion of the rotor disc has a higher downwash velocity than air passing through the forward portion. This is because the air passing through the rear portion has been accelerated for a longer period of time than the air passing through the forward portion. This increased downwash velocity at the rear of the disc decreases the angle of attack and blade lift, hence in combination with gyroscopic precession, causes the rotor disc to tilt to the right (the advancing side). The lift on the forward part of the rotor disc is greater than on the rearward part. According to the principle of gyroscopic precession, maximum deflection of the rotor blades occurs 90° later in the direction of rotation. This means that the rotor blades will reach maximum upward deflection on the left side and maximum downward deflection on the right side. This transverse flow effect is responsible for the major portion of the lateral cyclic stick control required to trim the helicopter at low speed.  Pendular action - Since the fuselage of the helicopter
is suspended from a single point and has considerable mass, it is free
to oscillate either longitudinally or laterally in the same way as a
pendulum (figure 3-3). This pendular action can be exaggerated by
overcontrolling; therefore, control stick movements should be
moderate. Figure 3-3 - Since the helicopter is suspended from the
rotor mast head, it acts much like a pendulum. AUTOROTATION Autorotation is the term used for the flight condition during which no engine power is supplied and the main rotor is driven only by the action of the relative wind. It is the means of safely landing a helicopter after engine failure or certain other emergencies. The helicopter transmission or power train is designed so that the engine, when it stops, is automatically disengaged from the main rotor system to allow the main rotor to rotate freely in its original direction. For obvious reasons, this autorotational capability is not only a most desirable characteristic but is indeed a capability required of all helicopters before FAA certification is granted. When engine power is being supplied to the main rotor, the flow of air is downward through the rotor. When engine power is not being supplied to the main rotor, that is, when the helicopter is in autorotation, the flow of air is upward through the rotor. It is this upward flow of air that causes the rotor to continue turning after engine failure.  The portion of the rotor blade that produces the forces
that cause the rotor to turn when the engine is no longer supplying
power to the rotor is that portion between approximately 25 percent and
70 percent of the radius outward from the center. This portion is often
referred to as the "autorotative or driving region" (figure 23).
Aerodynamic forces along this portion of the blade tend to speed up the
blade rotation. The inner 25 percent of the rotor blade, referred to as the "stall region," operates above its maximum angle of attack (stall angle), thereby contributing little lift but considerable drag which tends to slow the blade rotation. The outer 30 percent of the rotor blade is known as the "propeller or driven region." Aerodynamic forces here result in a small drag force which tends to slow the tip portion of the blade. The aerodynamic regions as described above are for vertical autorotations. During forward flight autorotations, these regions are displaced across the rotor disc to the left (figure 23). Figure 23 (above) - Contribution of various portions of the rotor disc to the maintenance of RPM during an autorotation - vertical autorotation (left); forward flight autorotation (right). Rotor RPM during autorotation Rotor RPM stabilizes when the autorotative forces (thrust) of the "driving region" and the antiautorotative forces (drag) of the "driven region" and "stall region" are equal. Assume that rotor RPM has been increased by entering an updraft; a general lessening in angle of attack will follow along the entire blade. This produces a change in aerodynamic force vectors which results in an overall decrease in the autorotative forces and the rotor tends to slow down. If rotor RPM has been decreased by entering a downdraft, autorotative forces will tend to accelerate the rotor back to its equilibrium RPM. Assuming a constant collective pitch setting, that is, a constant rotor blade pitch angle, an overall greater angle of attack of the rotor disc (as in a flare) increases rotor RPM; a lessening in overall angle of attack (such as "pushing over" into a descent) decreases rotor RPM. Flares during autorotation Forward speed during autorotative descent permits a pilot to incline the rotor disc rearward, thus causing a flare. The additional induced lift created by the greater volume of air momentarily checks forward speed as well as descent. The greater volume of air acting on the rotor disc will normally increase rotor RPM during the flare. As the forward speed and descent rate near zero, the upward flow of air has practically ceased and rotor RPM again decreases; the helicopter settles at a slightly increased rate but with reduced forward speed. The flare enables the pilot to make an emergency landing on a definite spot with little or no landing roll or skid. What You Said...Date = Monday, 6 February, 2012 13:11 Name = Dave SComments = FYI: Notwithstanding the ancient factual error in Army helicopter training manuals, replicated in the RFH, what the manuals and most CFI's call "Coriolis" effect/force is actually Conservation of Angular Momentum. Coriolis is the tendency of a moving mass to move in a straight line while the observer's frame of reference rotates; notably in the case of winds aloft, which appear to be diverted by the rotation of the earth. I will probably never succeed in getting the aviation community to stop referring to Coriolis in rotor systems, but I beg of you: at least give lip service to Conservation of Angular Momentum. (For verification: any competent physicist.) Your Thoughts... |
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