Fixed Pitch vs. Collective Pitch in RC Helicopters

Lift

The wing of a plane, the wing of a bird and the blade of a helicopter are all airfoils.wing of plane image by jedphoto from Fotolia.com

Lift is the upward force generated by an airfoil. An airfoil is anything that acts to change the flow of air over and under it to create an imbalance in air pressure so one side has more air pressure and one side has less. The area of greater air pressure will push against the airfoil and the area of less air pressure will offer less resistance to the pushing. Fixed-wing aircraft use the shape of the wing to create an airfoil so it is flat on the bottom and curved on the top, causing the air pressure to increase beneath the wing and push up through the decreased air pressure above the wing.

Ceiling fan blades are pitched to push the air down (increasing the air pressure below them).Ceiling Fan image by AJ Sellas from Fotolia.com

Helicopters use the pitch of the blades as they rotate to generate an airfoil. A slight angle to the blades on a helicopter produces a similar effect to the wing shape on an airplane causing the air pressure above to be less and below to be greater, and producing lift. The greater the angle of the pitch, the more lift is generated. However, the extra lift comes at the cost of added turbulence, which increases the power requirements needed to rotate the blades. Pitch can be positive (producing upward thrust) or negative (producing downward thrust) depending on whether the leading edge of the rotor blade is above (positive) the trailing edge or below (negative) the trailing edge.

Fixed-pitch RC helicopters have rotor blades that cannot change pitch. They are set to the maximum lift ratio when the helicopter is built. This allows the helicopter to increase lift by increasing the speed of the main rotor and to decrease lift by decreasing the speed. Fixed-pitch helicopters are more simple to fly, cheaper to purchase, easier to set up and easier to repair than collective pitch helicopters. However they lack the capability to do inverted flying tricks or fly at high speed since the angle of the airfoil cannot be changed.

Collective-pitch RC helicopters mimic their full-size brethren by altering the angle of attack or the pitch of the rotors dynamically. This means the controller can increase the pitch to generate more lift at the same rotor speed, or decrease the pitch and increase the rotor speed to generate more forward speed at the same lift ratio. Collective-pitch helicopters also have the ability to fly inverted since the pitch can be adjusted to the negative region thereby producing lift in the opposite direction. Because of this, collective-pitch helicopters are more expensive and more difficult to learn to fly.