“pitch,” and the motion about its vertical axis is “yaw.” Yaw  is the left and right movement of the aircraft’s nose.  The three motions of the conventional airplane (roll, pitch,  and yaw) are controlled by three control surfaces. Roll is  controlled by the ailerons; pitch is controlled by the elevators;  yaw is controlled by the rudder. The use of these controls  is explained in Chapter 6, Flight Controls. Other types of  aircraft may utilize different methods of controlling the  movements about the various axes.  For example, weight-shift control aircraft control two axes  (roll and pitch) using an “A” frame suspended from the  flexible wing attached to a three-wheeled carriage. These  aircraft are controlled by moving a horizontal bar (called a  control bar) in roughly the same way hang glider pilots fly.  [Figure 5-19]  They are termed weight-shift control aircraft  because the pilot controls the aircraft by shifting the CG.  For more information on weight-shift control aircraft, see  the Federal Aviation Administration (FAA) Weight-Shift  Control Flying Handbook, FAA-H-8083-5. In the case of  powered parachutes, aircraft control is accomplished by  altering the airfoil via steering lines.  A powered parachute wing is a parachute that has a cambered  upper surface and a flatter under surface. The two surfaces are  separated by ribs that act as cells, which open to the airflow  at the leading edge and have internal ports to allow lateral  airflow. The principle at work holds that the cell pressure is  greater than the outside pressure, thereby forming a wing that  maintains its airfoil shape in flight. The pilot and passenger  sit in tandem in front of the engine, which is located at the  rear of a vehicle. The airframe is attached to the parachute  via two attachment points and lines. Control is accomplished  by both power and the changing of the airfoil via the control  lines.  [Figure 5-20]  A study of physics shows that a body that is free to rotate  will always turn about its CG. In aerodynamic terms, the  mathematical measure of an aircraft’s tendency to rotate  about its CG is called a “moment.” A moment is said to be  equal to the product of the force applied and the distance at  which the force is applied. (A moment arm is the distance  from a datum [reference point or line] to the applied force.)  For aircraft weight and balance computations, “moments”  are expressed in terms of the distance of the arm times the  aircraft’s weight, or simply, inch-pounds.  Aircraft designers locate the fore and aft position of the  aircraft’s CG as nearly as possible to the 20 percent point  of the mean aerodynamic chord (MAC). If the thrust line  is designed to pass horizontally through the CG, it will not  cause the aircraft to pitch when power is changed, and there  will be no difference in moment due to thrust for a power-on  or power-off condition of flight. Although designers have  some control over the location of the drag forces, they are not  always able to make the resultant drag forces pass through the  CG of the aircraft. However, the one item over which they  have the greatest control is the size and location of the tail.  The objective is to make the moments (due to thrust, drag, and  lift) as small as possible and, by proper location of the tail,  to provide the means of balancing an aircraft longitudinally  for any condition of flight.  The pilot has no direct control over the location of forces  acting on the aircraft in flight, except for controlling the  center of lift by changing the AOA. The pilot can control  the magnitude of the forces. Such a change, however,  immediately involves changes in other forces. Therefore,  the pilot cannot independently change the location of one  force without changing the effect of others. For example,  a change in airspeed involves a change in lift, as well as a  change in drag and a change in the up or down force on the  Figure 5-19.  A weight-shift control aircraft.  5-13  Figure 5-20.  A powered parachute.  Moment and Moment Arm 