rev. 11/20/12
A Propeller is an Airscrew...

       The British still occasionally refer to the propeller as the "airscrew", a term that accurately reflects the basic design and function of the propeller, which dates back as far as 1493, when Leonardo daVinci proposed the concept of a "helical screw" to power a machine vertically into the air.  While it was not implemented until the 19th century, the propeller uses that principle to provide propulsion through the air, much like a threaded screw advances through a solid medium, but with some notable exceptions, primarily related to the loss of forward movement because the medium is not solid.  Nonetheless, the propeller is similar to a screw in some common features.
       First, the pitch of a propeller is the theoretical distance the propeller would move forward in one revolution (similar to a screw) and conceptually is the same as the pitch of a screw, namely the distance between threads if the propeller were a continuous helix.  For this reason, propellers will frequently be stamped with a designation such as "D 2550/P2610" (example.)  This means that the diameter (in this case length of propeller or thickness of a screw)  is 2.550 meters, and the pitch is 2.610 meters, so that in a mathematical sense, one revolution of this propeller would move it forward a distance of 2.610 meters.  (Technically, the propeller is more of a double helix, in order to combine the two blades into one, but the principle is still that of a screw.) 
      The second feature that relates to its screw design is that the angle of the blade changes along the radius, so that close to the hub, the angle is very steep and at the tip of the blade it is much more shallow (see diagram).  From a practical standpoint, this means that unless the pitch for a given propeller is known, it requires a trigonometric calculation to determine the pitch empirically.
      Thirdly, just as screws come in left hand and right hand threads, propellers have the same designation.  When facing the airflow if the top of the propeller moves to the right, it is designated "Right Hand" and if to the left it is "Left Hand".   (As viewed from the front a right hand propeller turns counterclockwise and a left hand propeller turns clockwise.)  Propellers will frequently be stamped as "RH" or "LH" to reflect this design feature..

A Propeller is Also a Wing...

     The cross section of any propeller will demonstrate that the forward travelling surface is convex, while the trailing surface is either flat or slightly concave (see diagram).  This is similar to the basic design of most aircraft wings.  The propeller gains efficiency by using this same airfoil concept, and it is important to recognize that virtually all propellers have a "front", curved surface, and a "back" flat surface.  Whether the propeller is designed as a "pusher" application or a "tractor" application, the features will be the same, and it is not possible to determine which of those were intended by the features of the propeller airfoil alone.   One feature which does allow at least a likely determination of pusher or tractor application is in the situation  where decals are still present.  In the pusher prop, decals are likely to be mounted on the rear (flat) surface, whereas in tractor applications the opposite is usually true.  This is simply to allow the decal to be visible to someone standing away from the aircraft.  The Airco DH 1 propeller is an example of this.

A Propeller is a Mechanical Device...

        As a consequence, propellers will have blueprints which are used to manufacture them to exact specifications determined by the designer.  Also for this reason, older propellers are identified by their "drawing number", referring to the blueprint that defines their dimensions and shape.  Newer propellers usually have a "model number", but this number simply refers back to a blueprint drawing as well.  The schematic diagram of a propeller layout, used in the sections above, resembles a typical propeller blueprint, although with much less detail.  The blade outline, the cross sectional detail, and the pitch are all specific elements on a numbered drawing.

Basic Propeller Design
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