Aeronautical Science Perspective Paper
Marcel Melo
ASCI 202
Embry Riddle Aeronautical University

Rotary-wing aircraft are designed and built to fulfill a multitude of purposes and missions. Cargo, attack, transport, observation, etc. and with these different mission types come a plethora of design features. Such features include, but are certainly not limited to fully-articulated or semi-rigid rotor systems; two, three, four, or even five main rotor blades; skids, wheels; one or two engines. Yet, despite all the differences to the structure and therefore the aerodynamic properties of the helicopter, the one thing that remains constant in every aircraft are the flight controls and how they control pitch, roll, and yaw. All helicopters have three basic sets of controls. The cyclic controls the pitch of the rotor system as a whole, and therefore affects the pitch and roll of the fuselage. The collective changes the pitch of the blades, which changes the angle of attack and therefore lifts to change, which in turn causes the aircraft to pitch up or down. Additionally, adjusting the collective also has causes a change in torque of the main rotor and so the fuselage yaws left or right. And finally, to offset the torque of the main rotor the pedals change the pitch angle of the tail rotor which provides control in the yaw axis.
The cyclic can affect changes in the roll and yaw of the fuselage by changing the lift vector of the rotor system. Aviator inputs to collective and cyclic pitch controls are transmitted to the rotor blades through a complex system. This system consists of levers, mixing units, input servos, stationary and rotating swashplates, and pitch-change arms. In its simplest form, movement of collective pitch control causes stationary and rotating swashplates mounted centrally on the rotor shaft to rise and descend. The movement of cyclic pitch control causes the swashplates to tilt; the direction of tilt is controlled by the direction in which the aviator moves the cyclic. ("Fundamentals of Flight," 2007) The resultant tilt of the swashplates cause the rotor to tilt, the tilt of the rotor changes the lift vector of the rotor and therefore, the aircraft moves in the direction of lift. In helicopters, the controls are rigged is such a way that when forward cyclic is applied, the helicopter moves forward, likewise for aft, etc. To accomplish this, the pitch horn is offset 90º to the rotor blade. The controls still tilt the swash plate in the same direction as the control input is made, but due to the pitch horn placement, the input to the blade occurs 90º earlier in the plane of rotation. ("Helicopter flight information: understanding," 2012) The rotor response to cyclic control input on a single-rotor helicopter has no lag. Rotor blades respond instantly to the slightest touch of cyclic control. The fuselage response to lateral cyclic is noticeably different from the response to fore and aft cyclic applications. Normally, considerably more fore and aft cyclic movement is required to achieve the same fuselage response as achieved from an equal amount of lateral cyclic. This is not a lag in rotor response; rather, it is due to more fuselage inertia around the lateral axis than around the longitudinal axis. For single-rotor helicopters, the normal corrective device for the lateral axis is the addition of a synchronized elevator or stabilator attached to the tail boom. This device produces lift forces keeping the fuselage of the helicopter in proper alignment with the rotor at normal flight airspeed. This alignment helps reduce blade flapping and extends the allowable CG range of the helicopter; however, it is ineffective at slow airspeeds. ("Fundamentals of Flight," 2007)
Newton’s third law of motion states: when a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction. ("Newton's laws of," 2012) According to Newton’s law of action/reaction, action created by the turning rotor system will cause the fuselage to react by turning in the opposite direction. The fuselage reaction to torque turning the main rotor is torque effect. Torque must be counteracted to maintain control of the aircraft; the anti-torque rotor does this (figure 1-43). In the tandem rotor or coaxial helicopters, the two rotor systems turn in opposite directions, effectively canceling the torque effect. Most rotary-wing aircraft have a single main rotor and require a tail rotor or other means to counter the torque effect. As the initial action is generated by engine power (torque) turning the main rotor system, this torque will necessarily vary with power applied or the maneuver performed. The tail rotor is designed as a variable-pitch, anti-torque rotor to accommodate the varying effects of such a system. The tail rotor is usually driven by the main transmission through a drive shaft arrangement leading to its position at the end of the tail boom. The engine power required to motor and control the tail rotor can be significant. The aviator must consider this during performance planning for varying conditions and situations. It is easy to understand why various emergency procedures have been written to compensate for problems such as loss of engine power, insufficient engine power, and tail rotor malfunction. Most American-built single-rotor helicopters turn the main rotor in a counterclockwise direction; therefore, the application of right pedal decreases pitch in the tail rotor and creates less thrust, allowing the nose of the aircraft to turn right. The opposite is true for application of left pedal. So, when the main rotor spins in order to keep the body of the aircraft from spinning in the opposite direction a tail rotor is built on most helicopters to provide an anti-torque. ("Fundamentals of Flight," 2007) The tail rotor provides thrust in the direction opposite the main rotor, the amount of this thrust is controlled by the pitch of the blades, which in the cockpit are controlled by the pedals. Using the pedals to control the yaw of the helicopter allows the pilot to pivot about the main rotor system at a hover and steepen or shallow out turns in directional flight.
When used together the three main flight controls of a helicopter, the cyclic, collective, and pedals, are able to affect the three axises of movement, pitch, roll, and yaw. Despite the many different ways helicopters are designed, all the components must still provide the pilot to manipulate these basic functions.

REFERENCES

Department of the Army, (2007). Fundamentals of Flight (FM 3-04.230). Washington, D.C.: Headquarters Department of the Army.

Helicopter flight information: understanding gyroscopic precession and the helicopter rotor as a gyroscope. (2012, March 30). Retrieved from http://helicopterflight.net/gyroscopic P.htm

Newton's laws of motion. (2012, September 24). Retrieved from http://en.wikipedia.org/wiki/Newton's_laws_of_motion