User Guide

3: GROUND SCHOOL
Although records of experiments in rotary flight date back to the 15th century, it
wasn’t until after World War II that a full-size model of a helicopter capable of
consistently flying more than a few hundred feet was developed. Early attempts at
helicopter design were thwarted by a lack of understanding of the physics of
rotary flight and the special problems it created.
This chapter describes the forces that govern rotary-wing flight. The first section
provides an overview of the basic aerodynamic principles of lift. The next sections
explain the interaction of the forces and velocities inherent to the rotational motion
of the blades and the horizontal motion of the aircraft.
Boxed text provides examples or additional, more detailed information in some sections.
LIFT FORCE
For now, let us ignore forward movement, and look at two basic aerodynamic
forces that allow a helicopter to get off the ground and hover.
Gravitational force. The most fundamental force you encounter is
gravitational
force
, the force that pulls everything toward the center of the earth.
Lift. The
lift force counteracts gravity, and is created as a result of air flowing over
a wing or blade surface.
When lift is greater than the gravitational force, the aircraft rises. When lift is less
than gravitational force the aircraft sinks. When the two forces are equal, the
aircraft hovers.
3.1
When we talk about the weight of an object, we are actually talking about
strength of the gravitational force pulling it toward the earth. Thus, as an object
gets lighter or heavier, the strength of the gravitational force acting on it
decreases or increases.
The heavier an object, the greater the lift force needed to keep it in the air. Thus a
fully loaded helicopter must generate more lift than an empty one. Also, as a heli-
copter flies, it burns fuel, and thus becomes lighter, requiring a smaller lift force.
Lift greater = Rise
Lift
Lift
Gravity
Gravity
Gravity greater = Sink
Equal = Hover
Lift
Gravity