The propeller is a complicated
device. Wilbur Wright said, thinking about it drove him crazy. Yet, the Wrights developed a propeller that was nearly 80% efficient. No one else had ever done that before. A Wing by Any Other NameBy thinking of the propeller as a tiny wing, they developed the idea of twist to keep the same attack angle along the length of the blade. Others still thought of the propeller as a paddle. We will look at props like the Wrights did,
because it is really the simplest way to understand them. The Thrust EquationIn terms of thrust, we write, T = Tc x Density x A x V ^{2}, where
“A” is the surface area of the prop and “V” is velocity of the prop.Propellers have a diameter, a width and a pitch. |
The width and the diameter determine
the area of the prop. The pitch determines the attack angle.The Attack AngleThe first thing to consider is the Thrust Coefficient. As you recall, this is similar to the Lift Coefficient, which was related to the Attack Angle of the wing. So what’s the “attack angle” of a prop? The propeller has a fixed forward velocity called its pitch speed. It moves forward the pitch inches for each rotation.The higher the pitch or the faster the rotation the more it moves. The “attack angle” is simply the difference
between the propeller blade’s advance and the airplane speed.Whoa! These are vectors. We can’t add them directly, so we will just discuss what the Thrust Coefficient means. When the plane is sitting on the ground, it’s prop is cutting the air at a high angle, (the same angle as pitch). As the plane moves forward, this angle
becomes less and less until the plane is moving close to the pitch speed. The actual
attack angle will be somewhere in between. ## Continued on Page 6 |