DRAG Drag is the force that tends to hold an aircraft back. Drag is caused by the disruption of the airflow about the wings,  fuselage  (body),  and  all  protruding  objects  on the aircraft. Drag resists motion as it acts parallel and in the opposite direction in relation to the relative wind. Figure 3-6 shows the direction in which each of these forces acts in relation to an aircraft. Up to this point, you have learned the physical laws of   aerodynamics,   airfoils,   and   the   forces   affecting flight. To fully understand flight, you must learn about the rotational axes of an aircraft. Q3-7. What are the four forces that affect flight? ROTATIONAL AXES LEARNING OBJECTIVE: Identify the three axes of rotation and the terms relative to the aircraft's rotation about these axes. Any vehicle, such as a ship, a car, or an aircraft, is capable   of   making   three   primary   movements   (roll, pitch, and yaw). The vehicle has three rotational axes that are perpendicular (90 degrees) to each other. These axes  are  referred  to  by  their  direction—longitudinal, lateral,   and   vertical.   Perhaps   the   most   descriptive reference  is  by  what  action  takes  place  about  a  given axis or pivot point—roll, pitch, and yaw. LONGITUDINAL AXIS The longitudinal axis is the pivot point about which an aircraft rolls. The movement associated with roll is best described as the movement of the wing tips (one up and the other down). Figure 3-7 shows this movement. This axis runs fore and aft through the length (nose to tail) of the aircraft. This axis is parallel to the primary direction  of  the  aircraft.  The  primary  direction  of  a fixed-wing aircraft is always forward. Figure 3-8 shows the longitudinal axis. LATERAL AXIS The lateral axis is the pivot point about which the aircraft pitches. Pitch can best be described as the up and down motion of the nose of the aircraft. Figure 3-7 shows this movement. The pitch axis runs from the left to the right of the aircraft (wing tip to wing tip). It is perpendicular to and intersects the roll axis. Figure 3-8 shows the pitch axis and its relationship to the roll axis. VERTICAL AXIS The vertical axis runs from the top to the bottom of an  aircraft.  It  runs  perpendicular  to  both  the  roll  and pitch axes. The movement associated with this axis is yaw.  Yaw  is  best  described  as  the  change  in  aircraft heading to the right or left of the primary direction of an aircraft. Figure 3-7 shows this movement. Assume you are walking from your work space to an aircraft located 100 feet away. You are trying to walk there in a straight line  but  are  unable  to  do  so  because  there  is  a  strong wind   blowing   you   off   course   to   your   right.   This movement to the right is yaw. The yaw axis is shown in figure 3-8. Q3-8. Any vehicle (ship, car, or aircraft) is capable of making what three primary movements? FIXED-WING AND ROTARY-WING AIRCRAFT LEARNING  OBJECTIVE: Recognize  the difference in aerodynamic principles that apply to fixed- and rotary-wing aircraft. A fixed-wing aircraft depends on forward motion for   lift.   A   rotary-wing   aircraft   depends   on   rotating airfoils  for  lift.  The  airfoil  sections  of  a  fixed-wing aircraft   aren't   symmetrical.   The   rotor   blades   of   a helicopter    are    symmetrical.    These    differences    are important to you if you're to understand aerodynamic principles. 3-4 Figure 3-6.—Forces affecting flight.