chemistry of the air must be considered when  determining forces on the object. When the space  shuttle re-enters the atmosphere at high hypersonic  speeds, close to Mach 25, the heated air becomes an  ionized plasma of gas, and the spacecraft must be  insulated from the extremely high temperatures.  Additional technical information pertaining to high-speed  aerodynamics can be found at bookstores, libraries, and  numerous sources on the Internet. As the design of aircraft  evolves and the speeds of aircraft continue to increase into  the hypersonic range, new materials and propulsion systems  will need to be developed. This is the challenge for engineers,  physicists, and designers of aircraft in the future.  Rigging the helicopter coordinates the movements of the  flight controls and establishes the relationship between the  main rotor and its controls, and between the tail rotor and its  controls. Rigging is not a difficult job, but it requires great  precision and attention to detail. Strict adherence to rigging  procedures described in the manufacturer’s maintenance  manuals and service instructions is a must. Adjustments,  clearances, and tolerances must be exact.  Rigging of the various flight control systems can be broken  down into the following three major steps:  The flight control units located in the flight deck of all  helicopters are very nearly the same. All helicopters have  either one or two of each of the following: collective pitch  control, throttle grip, cyclic pitch control, and directional  control pedals.  [Figure 2-22]  Basically, these units do the  same things, regardless of the type of helicopter on which  they are installed; however, the operation of the control  system varies greatly by helicopter model.  Rotary-Wing Aircraft Assembly and Rigging 1.  Placing the control system in a specific position— holding it in position with pins, clamps, or jigs, then  adjusting the various linkages to fit the immobilized  control component.  2.  Placing the control surfaces in a specific reference  position—using a rigging jig, a precision bubble  protractor, or a spirit level to check the angular  difference between the control surface and some fixed  surface on the aircraft.  [Figure 2-23] 3.  Setting the maximum range of travel of the various  components—this adjustment limits the physical  movement of the control system. Pedals Maintain heading  Cyclic control stick Controls attitude and direction of flight Collective pitch stick Controls altitude Throttle Controls rpm Figure 2-22.  Controls of a helicopter and the principal function of each.  2-16