Sir Isaac Newton’s third law of motion: If an object A exerts a force on object B, object B must exert a force of equal magnitude and opposite direction back on object A. This law is not always evident to the naked eye, but it is the crux of anything that moves us through the world. This “action and reaction” principle even applies to how an airplane propeller works.
Your feet push against the ground propelling your body forward when you walk. Tires kick back against the road as the wheels on your car turn and move you down the road. But what about propeller-powered planes? These do, too!
A propeller is essentially a machine that moves you forward through the air as it turns, “lifting” you in the intended direction. Though it works much the same as a screw, it looks slightly different. Generally, a propeller has two, three, or four twisted blades (sometimes more) poking out at angles from a central hub spun around an engine or motor. The twists and angles are really important. A propeller is shaped like a wing, producing higher air pressure on one surface and lowering the air pressure on another surface.
We acknowledge that not everyone is as passionate about how propellers work as we are. And it’s okay if you’re not! If you are a private licensed pilot in California or Nevada, looking for “propeller repair near me,” contact Stockton Propeller.
How An Airplane Propeller Works
When the Wright brothers learned how to combine engine-powered propellers with the other parts of their flying machine design so they could go forward and upward simultaneously, the flight was born. Planes took to the skies!
The propellers’ inventors designed them to look somewhat like screws — and it’s easy to see why this basic design was their starting point.
To “push” a screw into a wall, you apply a clockwise turning force to the screw with your screwdriver. The screw’s spiral groove (sometimes called a helical thread) converts the turning energy into a push that forces the screw into the wall and secures it there.
Propellers are similar to screws, but they are not exactly twins. They are, of course, doing a completely different job. An airplane propeller’s purpose is to make more or less thrust (driving force) to varying points of a flight (during takeoff, during landing, or at a steady cruising speed). The propeller blade’s angle and its overall size and shape affect the thrust, and so too does the engine’s speed.
Another difference is that while a screw moves into a simple, solid material and meets a (more or less) constant oppositional force, a propeller is moving in a fluid airstream, and there are all kinds of extra factors to consider. For example, although a propeller produces enough thrust to move you forward, it also has enough drag to hold you back and slow you down.
Another difference between screws and propellers is that propellers have both twists AND angles. A screw has a constant pitch, while the slope of a propeller blade varies along its length. The rise is steepest at the hub (in the center) and shallowest at the tip.
The propeller’s parts move at different speeds: the propeller blades’ tips move faster than the hub’s positions. The propeller blade’s angle should be greater near the hub, where the propeller is moving slowest. Then, shallower near the tips where the propeller is moving fastest. This reasoning is why propeller blades are slightly twisted. Without this twist, the propeller would be producing different amounts of thrust at the hub and the ends, which would put it under great stress.
How A Propeller Generates Thrust
Propellers generate thrust, but how exactly does that happen?
A spinning propeller sets up an air pressure lower in front of the propeller and higher behind it. Downstream, the pressure eventually returns to normal conditions. As air passes through the propeller, the velocity is greater than the free stream because the propeller works on the airflow.
What About Acceleration?
For airplane acceleration, the thrust must be greater than the drag. By increasing both the engine power and the propeller revolutions (RPM), the air is increasingly accelerated across the propeller blades, creating a stronger pressure differential, pulling the airplane forward. This pressure differential accelerates the aircraft but limits the available thrust. As you accelerate, the drag load increases. Because of this, higher airspeeds require more power to accelerate.
A propeller’s efficiency also plays a large part in acceleration. At approximately the 80% efficiency point, any increase in forward airspeed results in a loss of propeller efficiency. This lack of efficiency at higher airspeeds also decreases the thrust and power available.
Propeller Diameter
A variable-diameter propeller would be most efficient in an ideal world, allowing for a large diameter for low airspeeds as well as a smaller diameter for high airspeeds. Due to structural, control, and weight issues, variable diameter propellers aren’t practical. Instead, the diameter of most propellers allows for a “happy medium” between varying airspeed operations.
Putting All Of This Together & Making It Work
Propellers convert engine horsepower into thrust by accelerating air and creating a low-pressure differential in front of the propeller. Since air naturally moves from high to low-pressure, you are pulled forward when your prop is spinning.
But if the propeller isn’t spinning correctly, isn’t precisely balanced, or needs other maintenance or repairs, you won’t be pulled forward. And this is where it gets dangerous!
We realize that everyone is not as passionate about how propellers work as we are. And it’s okay if you’re not! If you’re in Northern California or Nevada and you’re looking for “propeller repair near me,” contact Stockton Propeller.