Stall fence Aileron gap seal Tab gap seal 1-35 Figure 1-81.  Gap seals promote the smooth flow of air over gaps between fixed and movable surfaces.  Figure 1-79.  The Symphony SA-160 has two unique vortex  generators on its wing to ensure aileron effectiveness through the  stall.  Figure 1-78.  Vortex generators.  Figure 1-80.  A stall fence aids in maintaining chordwise airflow  over the wing.  and disrupt the upper wing surface airflow, which in turn  reduces lift and control surface responsiveness. The use of  gap seals is common to promote smooth airflow in these gap  areas. Gap seals can be made of a wide variety of materials  ranging from aluminum and impregnated fabric to foam  and plastic.  Figure 1-81  shows some gap seals installed on  various aircraft. The landing gear supports the aircraft during landing and  while it is on the ground. Simple aircraft that fly at low speeds  generally have fixed gear. This means the gear is stationary  and does not retract for flight. Faster, more complex aircraft  have retractable landing gear. After takeoff, the landing  gear is retracted into the fuselage or wings and out of the  airstream. This is important because extended gear create  significant parasite drag which reduces performance. Parasite  drag is caused by the friction of the air flowing over the  Often, a gap can exist between the stationary trailing edge  of a wing or stabilizer and the movable control surface(s).  At high angles of attack, high pressure air from the lower  wing surface can be disrupted at this gap. The result can  be turbulent airflow, which increases drag. There is also a  tendency for some lower wing boundary air to enter the gap  Landing Gear