A propeller is the rotating device on most R/C, control line and free-flight aircraft used to provide the thrust necessary to move the plane at sufficient speed for air to flow over the wing, thereby creating lift. On powered boats, the propeller or screw is used to push the vessel through the water. In aircraft applications, a propeller is sometimes referred to as an airscrew, although this term is not always correct. In fact, the blades of most aircraft propellers are designed more as airfoils than as blades of a screw.
As an aircraft or airboat propeller turns, its angled blades "bite" into the air and actually create lift in much the same manner as a wing, drawing the aircraft or vessel forward in much the same way as a hydraulic screw draws water per Newton's Third Law of physics. A submerged boat propeller, more similar to a screw, works along these same principles as well.
R/C aircraft propellers are identified with a two-part number. The first number is the diameter in inches, regardless of the prop's country of origin or use (although Gunther props, made in Germany, are often labeled in millimeters). The second number denotes the pitch of the propeller, or the angle of relationship between blade and air. Under ideal conditions, the second number would be the distance an aircraft would travel with a single revolution of the propeller, again in inches. Therefore, a 10x6 prop would be a propeller with a diameter of ten inches which would draw the aircraft forward six inches per revolution.
The most basic R/C propellers are those found on most park flyer aircraft and boats, regardless of size. Generally, these are made of cheap, flexible molded plastic designed to withstand abuse as much for aerodynamic efficency. Better quality propellers are made of nylon/carbon composite or even precision-machined wood. Hand carved propellers are very rare today, although they were common in the early days of aeromodeling.
A two-piece machined aluminum "prop adapter" or collet may be used to attach propellers of these types to the aircraft's gearbox, motor or engine. Many larger aircraft engines may only require the use of a machined, dome-shaped nut which attaches the propeller directly to the crankshaft while aiding with aerodynamics.
Some electric gliders utilize a folding propeller to assist in bringing the glider to sufficient altitude to fly on its own. Centrifugal force pushes the blades outward while the propeller is under power and fold back when powered down in order to reduce drag.