01 May 2005


Why pitchpull?
Sit down in a kitchen chair. In your mind, visualize that chair is the pilot's seat of a helicopter! Ready to take the controls?

Comfortably extend your feet with your toes pointed skyward. Each of your feet controls one of the "pedals", which are attached to the tail rotor. The pedals control the direction and rate of turn in a hover. To turn left, push left. To turn right, push right. To turn faster, push farther. The linkage is directly attached, so pushing the right pedal results in a corresponding and opposite movement of the left one.
In flight, they are used similarly to an airplane's rudder controls........they keep the nose of the machine straight in front of the tail and move the aircraft through the air most efficiently, thus presenting the smallest area to the passing air.

At your left hand is the "collective pitch control". It's called the "collective" for short and controls the angle of attack, or "pitch" of the main rotor turning above your head. Increasing this lever, or "pulling pitch" increases the angle of attack of both the rotor blades equally and forces more air downward. You lift the helicopter from the ground to a hover by increasing the collective pitch........"pitchpull".

Isaac Newton said "for every action there is an equal and opposite reaction." Pulling pitch increases the amount of torque being applied to the helicopter by the spinning main rotor. This torque attempts to turn the fuselage in the opposite direction of the spinning rotor. To keep the fuselage from turning, (action), you must increase the angle of attack of the tail rotor, (reaction), which moves air to counteract the torque effect. In American helicopters you do this by pushing on the left pedal as you increase the torque with the collective. (The French and Russians build helicopters that have main rotors that turn the opposite direction of American helicopters.) Contrarians!

On the machines I learned to fly years ago, the collective lever also had a motorcycle style twistgrip at the upper end which was used to increase or decrease the RPM of the engine. Your left hand was VERY busy!
In most modern helicopters the RPM is controlled by a governor. With one less thing to worry about, they are easier to fly!

Between your legs is another lever, similar to the "joystick" found in older airplanes. This lever is called the "cyclic pitch" because it increases the pitch in one blade while decreasing it in the other during the 360 degree cycle the rotor blades make in one revolution. The "cyclic" is rigged so as to tilt the rotor in the direction the cyclic is moved, pulling the helicopter in that direction. In a hover, the cyclic controls horizontal movement across the ground.....laterally or fore and aft. In flight, the cyclic controls the relationship of the helicopter to the horizon.......left bank/right bank, or nose up/nose down.

Clear as mud? In a later post, we'll go fly!

No comments: