radius. The rounded edges of a component that forms a smooth curved intersection between the bottom and sides of a groove.

radius bar (sheet-metal brake component). A bar, having an accurately ground radius on its edge, used with a sheet-metal brake. The metal bent in the brake is formed around the radius bar to give the bend the correct radius.

radius dimpling (sheet-metal forming). A method of cold-forming the edges of a hole in a piece of sheet metal so it can be riveted with flush rivets. A cone-shaped male die fits into the rivet hole and forces the metal down into a depression in a female die. The edges of a radius-dimpled hole are rounded, rather than sharp, as they are in a coin-dimpled hole.
radius gage. A precision measuring tool consisting of a number of thin steel blades used to find the radius of a curved surface. Each blade is shaped with an inside and an outside radius of a specific size with the radius of the curve marked on the gage. Different gages are tried, until the gage is found that exactly fits the curvature of the surface.
radome. A strong, electrically transparent housing, used to enclose a radar antenna and protect it from the wind and weather.
rag wing. A term commonly used to describe an airplane with fabric covered wings.
rain. A form of liquid water which falls from the atmosphere. The drops of water forming rain are usually more than a half-millimeter in diameter. Precipitation having smaller droplets than that of rain is called drizzle.
rain gage. An instrument used to collect and measure the amount of rain which has fallen at a given location.
ram air pressure. Pressure produced when moving air is stopped. Ram air pressure is the same as pitot pressure.
ram drag. The loss of thrust produced by a turbojet or turbofan engine that is caused by increasing the velocity of the air entering the engine. The amount of ram drag is the difference between gross thrust and net thrust.
ramjet engine. The simplest type of air-breathing reaction engine. The air enters the front of the engine at a high velocity and fuel is sprayed into it and ignited. The air entering the front of the engine forms a barrier that forces the expanding gases to leave through the nozzle at the rear. The energy added by the burning fuel accelerates the air and produces a forward thrust.
Ramjet engines are used in some military unmanned aircraft that are initially boosted to a speed high enough for the engine to function.
ramp (aircraft service area). A paved area on an airport, usually around the hangars where aircraft can be parked. Most ramps have provision for securing aircraft so they will not be blown away by strong winds. Ramps are also called aprons and tarmacs.