The basic nomenclature of the parts of a propeller is shown in  Figure 7-1  for a simple fixed-pitch, two-bladed wood propeller.  The aerodynamic cross-section of a blade in  Figure 7-2  includes  terminology to describe certain areas shown. The many different types of propeller systems have been  developed for specific aircraft installation, speed, and mission.  Propeller development has encouraged many changes as  propulsion systems have evolved. The first propellers were  fabric-covered sticks made to force air in a rearward direction.  Propellers started as simple two-bladed wood propellers  and have advanced to the complex propulsion systems of  turboprop aircraft that involve more than just the propeller.  As an outgrowth of operating large, more complex propellers,  a variable-pitch, constant-speed feathering and reversing  propeller system was developed. This system allows the  engine rpm to be varied only slightly during different flight  conditions and, therefore, increases flying efficiency. A basic  constant-speed system consists of a counterweight-equipped  governor unit that controls the pitch angle of the blades so  that the engine speed remains constant. The governor can be  regulated by controls in the cockpit so that any desired blade  angle setting and engine operating speed can be obtained.  A low-pitch, high-rpm setting, for example, can be utilized  for takeoff. Then, after the aircraft is airborne, a higher pitch  and lower rpm setting can be used.  Figure 7-3  shows normal  propeller movement with the positions of low pitch, high pitch,  feather (used if the engine quits to reduce drag), and zero pitch  into negative pitch, or reverse pitch.  The aircraft propeller consists of two or more blades and a  central hub to which the blades are attached. Each blade of  an aircraft propeller is essentially a rotating wing. As a result  of their construction, the propeller blades produce forces  that create thrust to pull or push the airplane through the air. The power needed to rotate the propeller blades is furnished  by the engine. The propeller is mounted on a shaft, which may  be an extension of the crankshaft on low-horsepower engines;  on high-horsepower engines, it is mounted on a propeller  shaft that is geared to the engine crankshaft. In either case,  the engine rotates the airfoils of the blades through the air at  The engine supplies brake horsepower through a rotating  shaft, and the propeller converts it into thrust horsepower.  In this conversion, some power is wasted. For maximum  efficiency, the propeller must be designed to keep this waste  as small as possible. Since the efficiency of any machine  is the ratio of the useful power output to the power input,  propeller efficiency is the ratio of thrust horsepower to brake  horsepower. The usual symbol for propeller efficiency is the  Greek letter  η  (eta). Propeller efficiency varies from 50 percent  to 87 percent, depending on how much the propeller slips. If the power is measured in horsepower units, the power  expended by the thrust is termed thrust horsepower.  Work = Thrust × Distance Power = Thrust × Velocity  high speeds, and the propeller transforms the rotary power  of the engine into thrust. An airplane moving through the air creates a drag force  opposing its forward motion. If an airplane is to fly on a  level path, there must be a force applied to it that is equal to  the drag but acting forward. This force is called thrust. The  work done by thrust is equal to the thrust times the distance  it moves the airplane. The power expended by thrust is equal to the thrust times the  velocity at which it moves the airplane. Blade back Blade face Angle of attack Relative wind Chord line Figure 7-2.  Cross-sectional area of a propeller blade airfoil.  Figure 7-1.  Basic nomenclature of propellers.  Leading edge Trailing edge Blade tip Hub Propeller information Back of blade 7-2 Basic Propeller Principles Propeller Aerodynamic Process