Aircraft powered by piston and turboprop engines feature propeller blades that pull or push on the air around the aircraft to provide propulsion. As piston engines have become more powerful, they require more and more propeller blades.
To understand why, we need to understand the working principle of a propeler. The purpose of a propeller is to “absorb” the power produced by the engine and transmit that power to the air passing through the propeller, which generates the thrust force that propels the aircraft through the air. Therefore, if the propeller and engine are not properly matched based on the power of the engine, the system is inefficient.
As engine power increases, the designer has several different options to design an aircraft propeler that can efficiently absorb that power. However, most of these options have severe drawbacks.
- Increasing the blade angle (or pitch) of the propeller blades allows them to impart more energy to the airflow but altering the blade angle damages the aerodynamic efficiency of the blade.
- Increasing propeller length lets the propeller blades impart more energy by affecting a larger volume of air but forces the designer to extend the landing gear as well to keep the prop blades from touching the ground. This in turn forces the landing gear to extend, which causes a domino effect of other structural and weight issues.
- Increasing the revolutions per minute of the propeller is an option, but at a certain speed the propeller blades begin to reach supersonic speeds, causing sonic booms at their tips which drastically increases drag.
- The camber (or curvature) of the blades can be altered to change their airfoil and generate more thrust. However, this alters the aerodynamic efficiency much like changing the blade angle and can also cause structural issues with the blades, negatively affecting the lifespan of a propeler .-
Therefore, there are two viable options for increasing a propeller’s output. Either you can increase the blade’s width, or chord, or increase the number of blades on the propeller. Increasing the blade chord is easier, but once again, changing the chord affects the aircraft’s aerodynamic efficiency. Thus, this leaves us with the last option, increasing the number of aircraft propeller blades.- By doing so, you increase the solidity of the propeller disk, the space that the propeller rotates in. By increasing the solidity, the propeller can transfer more power to the air, thus increasing thrust.