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Flying Knights Newsletter

2000 First Quarter

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       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 Name
       By 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.
       If the blade is a tiny wing, then it should develop a lift force perpendicular to its upper surface, just like a big wing.
       But since the blade is moving vertically, the resultant force is in the forward direction as shown in Fig. 3.
      Voila! Just what we need to balance the drag force pulling us back. And we already know how to calculate the lift of a wing, do we not?
       Let’s look at the Lift Equation again. There were four parts. The Lift force was equal to a ‘Coefficient of Lift’ times the ‘mass density of air’ times the ‘wing area’ times the square of the ‘velocity of the air’ over said wing.
       That is, L= Cl x Density x A x V2. Same equation, different flies.
The Thrust Equation
        In terms of thrust, we write,
        T = Tc x Density x A x V2, 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 Angle
       The 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.
        The optimum attack angle of a wing was about 5 to 10 degrees which
gave a coefficient of about 0.8 to 1.2. For a prop that is 80% efficient, the angle is small just like a wing in level flight.
        The “thrust coefficient” therefore is very close to “one”.

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