During flight, the downtubes are similar in compression to the  crossbar and must be stiff and straight to maintain structural  integrity. The base tube/control bar is under tension during  fl ight. Front and rear flying wires hold the control frame in place  fore and aft. Side flying wires hold the control frame in place  side to side and provide structure to hold the wings in place  while fl ying.  [Figures 3-2, 3-5,  and  3-6]  Strutted wings use  struts in place of the side flying wires, which is discussed  later in this chapter.  Training bars are added for dual controls so the person in back  can fly the aircraft. These are typically used by an instructor  for training but can be used by a passenger in the back also.  [Figure 3-7]  Similar to the lower control frame holding the wing in  position during flight, the king post is attached to the keel  and supports the upper ground wires which hold the wing  in position on the ground and negative loads during fl ight.  [Figure 3-2]  It also provides a structure for reflex lines which  is discussed later in wing systems.  Similar to airplanes with struts to support the wings, some  WSC aircraft replace side flying wires with struts, eliminating  the king post and ground wires on top of the wing. This  provides a number of benefits, but primarily, no king post is  needed because the struts can take a compression load and  hold the wings up on the ground and also take the negative  loads during flight. With struts, a WSC aircraft is much shorter  in height allowing it to fit into hangars with lower doors and  ceilings. This can make a big difference in finding a suitable  storage for the aircraft if leaving it set up.  [Figure 3-8]  Topless Wings With Struts   King Post With Wires-on-Top Wing Design Training bars used  for steering from  the back seat Figure 3-7.  Passenger using training bars which are also used by the instructor during training. Figure 3-8. Strutted wing on WSC aircraft carriage.  3-5