Prerequisite Knowledge:
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Propellers are just big fans to keep the pilot cool. Don't believe that? Watch the pilot start sweating when they stop spinning.
Propellers are a critical part of a piston engine aircraft, but there's a lot more going on with a propeller than just a spinning piece of metal.
Propeller Shape
The propeller is not just an angled piece of metal, but instead an aerofoil. The propeller functions much like the wings on an aircraft, or a rotor on a helicopter. As the engine spins the propeller, the propeller deflects air (much like a wing does) to pull the aircraft forward.
None
Propellers are just big fans to keep the pilot cool. Don't believe that? Watch the pilot start sweating when they stop spinning.
Propellers are a critical part of a piston engine aircraft, but there's a lot more going on with a propeller than just a spinning piece of metal.
Propeller Shape
The propeller is not just an angled piece of metal, but instead an aerofoil. The propeller functions much like the wings on an aircraft, or a rotor on a helicopter. As the engine spins the propeller, the propeller deflects air (much like a wing does) to pull the aircraft forward.
This diagram shows the cross section of the propeller at different points. The propeller root is the part of the blade closest to the center, with the propeller tip being the farthest from the center. Notice how the thickness of the aerofoil as well as it's general shape changes, as you move from root to tip. Another thing to pay attention to is how the actual angle of the aerofoil changes as you move down the propeller blade, with the propeller root having a much steeper angle than the propeller tip. The angle of the propeller at any given point is known as its pitch.
Propeller Washout
Propeller washout is a design technique, where the propeller is twisted from the root to the tip, instead of being a flat piece of metal. As you move closer to the propeller root, the speed the blade moves at actually decreases, so the blade is twisted to provide a steeper pitch closer to the root. This steeper pitch means the propeller root meets the air at a higher angle of attack, which maximizes the thrust it produces. At the propeller tip, the blade pitch is shallower, since the propeller tips are travelling the fastest. Shallowing out the pitch reduces the thrust the propeller tip generates, helping to keep the propeller forces balanced. In short, propeller washout aims to balance the forces acting on the propeller blade, by increasing the pitch of the blade near the root and decreasing the pitch near the tip.
Propeller Washout
Propeller washout is a design technique, where the propeller is twisted from the root to the tip, instead of being a flat piece of metal. As you move closer to the propeller root, the speed the blade moves at actually decreases, so the blade is twisted to provide a steeper pitch closer to the root. This steeper pitch means the propeller root meets the air at a higher angle of attack, which maximizes the thrust it produces. At the propeller tip, the blade pitch is shallower, since the propeller tips are travelling the fastest. Shallowing out the pitch reduces the thrust the propeller tip generates, helping to keep the propeller forces balanced. In short, propeller washout aims to balance the forces acting on the propeller blade, by increasing the pitch of the blade near the root and decreasing the pitch near the tip.
Fixed Pitch vs. Constant Speed Propellers
Engines come in two main types as well, fixed pitch and constant speed. An engine with a fixed pitch propeller has the propeller mounted at a fixed angle, which means the propeller angle cannot change once it is mounted. With a constant speed/variable pitch propeller, the pilot can vary the angle of the propeller in flight, which has several advantages. Constant speed propellers will be explored in more detail with later articles.
That's it for the propeller basics! The next article will serve as an introduction to piston engines.
Engines come in two main types as well, fixed pitch and constant speed. An engine with a fixed pitch propeller has the propeller mounted at a fixed angle, which means the propeller angle cannot change once it is mounted. With a constant speed/variable pitch propeller, the pilot can vary the angle of the propeller in flight, which has several advantages. Constant speed propellers will be explored in more detail with later articles.
That's it for the propeller basics! The next article will serve as an introduction to piston engines.