Fan blades and propellers are used to convey power from a source (such as a jet engine) for propulsion or the movement of air. Propellers are an array of fan blades that transmit power by converting rotational motion into thrust. They are sometimes referred to as props. Typically, fan blades and propellers are used in axial fans, industrial blowers, boats and airplanes. Types include controller pitch propellers, skewback propellers, and modular propellers. Flow rate, air pressure, and air temperature are important parameters to consider when selecting fan blades and propellers. Industrial fan blades are usually made of cast or fabricated aluminum, fabricated steel or stainless steel, or exotic metals such as nickel alloys. These materials are suitable for use in axial fans as well as in many industrial blowers. Aerospace blades and propellers are usually made of nickel alloys, titanium alloys, superalloys, or proprietary metals such as Inconel, a registered trademark of Special Metals Corporation. Typically, aerospace blade designs use special coatings, metallurgical techniques, and casting processes. Because they are relatively expensive, single-crystal titanium fan blades are reserved for specific sections of jet engines. Fan blades and propellers made of superalloys provide high yield strengths, but require cooling vanes because of the high temperatures from jet exhaust. Gas turbine blades are often made of nickel and chromium alloys. In addition to materials of construction, specifications for fan blades and propellers include the number of blades, blade shape, and blade pitch angle. As a rule, the size of the fan or prop determines the number of blades. Although most designs have either 4 or 6 blades, some have as few as 2 blades or as may as 16. Configurations with an odd number or blades are also available. Typically, these props use 3, 5, or 7 blades. With regard to blade shape, choices range from rounded to rectangular. Square blades are also available. Typically, however, the corners of fan blades and propellers are rounded to minimize noise and maximize efficiency. With regard to blade pitch angle, products may be set by the manufacturer or adjustable by the end-user. In aerospace applications, this latter type of product is referred to as in-fight adjustable.