Figure 7-29.  Engine net thrust versus aircraft speed and drag.  Points  a through f are explained in the text below. purposes only and is not for specific models of engines. The  following are the four types of engines:  By plotting the performance curve for each engine, a  comparison can be made of maximum aircraft speed variation  with the type of engine used. Since the graph is only a means  of comparison, numerical values for net thrust, aircraft speed,  and drag are not included.  Comparison of the four powerplants on the basis of net thrust  makes certain performance capabilities evident. In the speed  range shown to the left of line A, the reciprocating powerplant  outperforms the other three types. The turboprop outperforms  the turbofan in the range to the left of line C. The turbofan  engine outperforms the turbojet in the range to the left of  line F. The turbofan engine outperforms the reciprocating  powerplant to the right of line B and the turboprop to the  right of line C. The turbojet outperforms the reciprocating  powerplant to the right of line D, the turboprop to the right  of line E, and the turbofan to the right of line F.  The points where the aircraft drag curve intersects the net  thrust curves are the maximum aircraft speeds. The vertical  lines from each of the points to the baseline of the graph  indicate that the turbojet aircraft can attain a higher maximum  speed than aircraft equipped with the other types of engines.  Aircraft equipped with the turbofan engine attains a higher  The fuel tanks, normally located inside the wings of an  airplane, have a filler opening on top of the wing through  which they can be filled. A filler cap covers this opening.  Both gravity-feed and fuel-pump systems may incorporate a  fuel primer into the system. The fuel primer is used to draw  fuel from the tanks to vaporize fuel directly into the cylinders  prior to starting the engine. During cold weather, when  engines are difficult to start, the fuel primer helps because  there is not enough heat available to vaporize the fuel in the  carburetor. It is important to lock the primer in place when  it is not in use. If the knob is free to move, it may vibrate  out of position during flight which may cause an excessively  rich fuel-air mixture. To avoid overpriming, read the priming  instructions for the aircraft.  Aircraft with fuel-pump systems have two fuel pumps. The  main pump system is engine driven with an electrically- driven auxiliary pump provided for use in engine starting  and in the event the engine pump fails. The auxiliary pump,  also known as a boost pump, provides added reliability to  the fuel system. The electrically-driven auxiliary pump is  controlled by a switch in the flight deck.  The gravity-feed system utilizes the force of gravity to  transfer the fuel from the tanks to the engine. For example, on  high-wing airplanes, the fuel tanks are installed in the wings.  This places the fuel tanks above the carburetor, and the fuel  is gravity fed through the system and into the carburetor. If  the design of the aircraft is such that gravity cannot be used  to transfer fuel, fuel pumps are installed. For example, on  low-wing airplanes, the fuel tanks in the wings are located  below the carburetor.  [Figure 7-30]  The fuel system is designed to provide an uninterrupted  flow of clean fuel from the fuel tanks to the engine. The  fuel must be available to the engine under all conditions  of engine power, altitude, attitude, and during all approved  flight maneuvers. Two common classifications apply to fuel  systems in small aircraft: gravity-feed and fuel-pump systems.  Fuel, electrical, hydraulic, and oxygen systems make up the  airframe systems.  maximum speed than aircraft equipped with a turboprop or  reciprocating powerplant.  Airframe Systems  Fuel Systems  Gravity-Feed System  Fuel-Pump System  Fuel Primer  Fuel Tanks  • Reciprocating powerplant • Turbine, propeller combination (turboprop) • Turbine engine incorporating a fan (turbofan) • Turbojet (pure jet) Net thrust  Airspeed  A i r c a f t d r a g A B C D E F Reciprocating Turboprop Turbofan Turbojet 7-25 