Posted by PITHOCRATES - January 15th, 2014

Technology 101

Varying the Pitch of the Tail Rotor Blade counters the Twisting Force caused by the Helicopter Engine

If you have a rear-wheel drive car with a longitudinally mounted engine (the crankshaft in the engine runs from front to back) you’ve probably noticed the car twisting along the longitudinal axis when starting or revving the engine.  If you ever operated a hand-drill and had the drill bit get stuck in the material you’re drilling through you’ve probably felt the drill twist in your hands.  These are examples of torque.  As the engine or drill motor spins in one direction they also create a counter torque in the opposite direction.  If a drill bit spins clockwise the motor is trying to spin counterclockwise.  Around the axis of the drill bit.

A helicopter has an engine spinning in one direction.  This spins the rotor that creates lift and directional motion.  But a helicopter is not attached to anything.  Motor mounts hold an engine in place in a car that prevents it from spinning in the opposite direction of the crankshaft.  With the car in contact with the ground.  We hold a drill motor to prevent it from spinning in the opposite direction of the drill bit.  With ourselves in contact with the ground.  A helicopter hovers over the ground, though.  It has no physical attachment to prevent the counter torque from spinning the helicopter in the opposite direction from the rotor.  Which is why there is a tail rotor on a helicopter.

Running out from behind the engine/cockpit of a helicopter is a tail boom.  At the end of that boom is a small rotary wing that spins like a propeller.  We can vary the pitch of this blade to push air through in either direction.  Or move it to a neutral position and move no air through it.  By varying the pitch of the tail rotor blade we can provide a counter force to balance the twisting force caused by the engine.  This is what we do with the foot pedals in the helicopter.  Change the pitch in the tail rotor blade to apply more or less counter-twisting force.  To cancel that created by the engine.  And to turn the helicopter to face a new direction.  Especially when hovering.  Helicopters with two large lift-producing rotors/engines (like a Boeing CH-47 Chinook) don’t need tail rotors.  The two engines just spin in opposite directions.  And cancel each other’s twisting torque.

On a Helicopter they Twist the Rotor Blade to Produce more Lift and Increase the Angle of Attack

Both planes and helicopters produce lift with a wing.  A fixed wing on an airplane.  And a rotary wing on a helicopter.  Air passing over the curved surfaces of a wing produces lift.  The more curved the wing the more lift.  And the greater the angle of attack of the wing the greater the lift.  Which is why planes take off with the nose/wings pitched up to create the maximum amount of lift.

Powerful engines on airplanes produce thrust to move the wing through the air.  Requiring long runways to take off.  And dangerous high speeds on the ground.  About 100 mph or so to get airborne.  Making the take off the most dangerous part of flying.  A helicopter, on the other hand, needs no runway.  For it can take off and land vertically.  Because the engine spins the wing through the air to create lift.  It doesn’t have to accelerate the helicopter to produce lift.

Airplane wings have leading-edge slats and trailing edge flaps to increase the curvature of the wing.  And a tail-mounted elevator that can deflect air up pushing the tail down, pitching the nose and wings up.  Increasing the angle of attack of the wings.  On a helicopter they twist the rotor blade to produce more curvature and increase the angle of attack.  With something called the swash plate assembly.

If you let go of the Controls of a Helicopter it will likely Crash for a Helicopter is Inherently Unstable

The rotor shaft rises up vertically from the engine and terminates in the rotor blade assembly above the helicopter.  And passes through the swash plate assembly.  A fixed lower swash plate that doesn’t spin.  And an upper swash plate that spins with the rotor.  Sandwiched between the swash plates are ball bearings.  Allowing these two plates to be in physical contact with each other.  Yet allows the top plate to spin while the bottom plate remains stationary.

Attached between the upper swash plate and the rotor blades are control rods.  Attached between the lower swash plate and the helicopter control levers in the pilot’s hands are control rods.  It is via the swash plate assembly that the pilot’s control inputs are transferred to the rotor blades.  When the pilot pushes the swash plate assembly up with the collective control in his or her left hand (looks like a parking brake on a car) the control rods on the upper plate push up on one side of each rotor blade equally.  Increasing its angle of attack and curvature of each blade equally.  Creating lift.  And drag.  Causing the engine to slow down from the increased load on it.  So when the pilot lifts the collective he or she also twists the handle to increase engine speed (like the accelerator on a motorcycle).

The pilot’s right hand controls the cyclic.  Or the stick coming up between his or her knees.  This is what gives a helicopter its directional motion.  When the pilot moves the cyclic forward it tips the swash plate assembly forward.  The back side of the swash plate assembly rises up while the front side remains roughly where it was.  So as a rotor blade rotates from the front position (forward of the cockpit) to the back position (behind the cockpit) the control rod begins to push up the leading edge of the blade.  Increasing its angle of attack and the curvature of the blade.  Reaching its highest position at the very back of its rotation.  Producing its maximum lift.  As it travels from the back to the front the control rod begins to lower the leading edge of the blade.  Decreasing its angle of attack and the curvature of the blade.  Reaching its lowest position at the very front of its rotation.  This uneven lifting force of the rotor blade tips the helicopter forward and pulls it forward in directional motion.  If the pilot tips the cyclic to the left lift increases on the right side of the rotor, pulling the helicopter to the left.  If the pilot pulls back on the cyclic the lift increases on the front of the rotor, pulling the helicopter backward.

Planes are inherently stable.  If you let go of the controls it will fly true and straight.  For awhile at least.  If you let go of the controls of a helicopter it will likely crash.  For a helicopter is inherently unstable.  And requires constant inputs to the flight controls from the pilot to maintain stable flight.  As it is a delicate balancing act between the collective, the cyclic and the foot pedals.  For every input of one creates an imbalance that must be corrected by the input of another.  Making the helicopter pilot perhaps the most skilled of all pilots.  Especially those kids just out of high school who flew in Vietnam.  Who flew these complicated flying machines like sports cars as they avoided enemy fire.  Making them without a doubt the finest pilots ever to fly.



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