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What are Reciprocating Engines? Most airplanes use this type of engine to propel their aircraft. It is based on the piston and cylinder model, much like the engine of your typical automobile. However, these engines don't produce power to ultimately turn wheels, they spin propellers. Gasoline and air are mixed in the cylinders to form a fine mist that is exploded. As a matter of fact, this is where engineers really want turbulence, since that helps the mixture become more homogenized and therefore more combustible. Types of Propellers A propeller may have from two to five blades depending on the airplane size. Most single engine planes have the engine and propeller mounted at the nose of the fuselage, where multiple reciprocating engine planes have their engines on the wings. Propellers generally have an engine to themselves, but two propellers sharing the same engine are known as coaxial propellers. Most airplanes have controllable-pitch propellers--the pilot can modify the angle between the plane surface of rotation and the direction of motion. This feature is useful since a different angle can make the flight more efficient at varying speeds. Thrust Producing Propellers The propellers attached to the engine could be considered as modified wings. They have the typical airfoil profile across the whole length of the blade. However, the twist on the propeller is much greater than any twist on a conventional airplane wing. One other important feature to note about propellers is that their camber decreases as you move from the hub, or the axis of rotation, to the blade tip. These propellers also spin at an angle of attack to provide more thrust. Again, both Newton's Third Law of Motion as well as Bernoulli's Principle can be used to explain how a propeller produces thrust. Thrust Explained As a propeller is spinning at its angle, the airflow that it encounters has to move around the blades in the same manner as an airfoil. The airflow going above the curved surface is accelerated faster than the flow on the opposite side. (This doesn't necessarily mean that the flows have to meet at the same time.) According to Bernoulli's Principle, the higher velocity means the air pressure on that side will drop. The air pressure behind the propeller will be around the normal air pressure, which is higher than the pressure in front of the propeller blades. The air that has a higher pressure will push towards an area with low pressure and because of this; a difference in pressure will exert an overall push in the forward direction, thus giving the plane thrust. Bear in mind, this is not the only explanation for thrust, rather its just another way of looking at things. 
Another way to look at this is by focusing on the angle of attack of the blades as well as the direction of the accelerated flow on the curved surface. Both these factors push the airflow behind the propeller, and because of Newton's Action-Reaction Law; there is a reactive force that pushes the propeller forwards, giving a net thrust force on the airplane. Mathematical Thrust By increasing the power of the engine, increases the speed at which the propeller is spinning. If the speed of the blades increase, they produce a greater air pressure difference between the front and back of the propeller. A larger pressure difference increases the force that pushes the propellers forward, and that is what increases thrust in a reciprocating engine. The relationships between some of the variables to the thrust is illustrated on figures 4.1.2. As you can see, thrust increases linearly with increasing diameter. The velocity of the air however does not affect the thrust, only the further acceleration imparted on the air by the propeller. 
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