October 2020 – Stockton Propeller

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.

The science and art behind the design of airplane propellers started simply. The basis for the airplane propeller design’s original concept was the same as the concept behind the screw. That’s right, the simple screw.

Of course, this screw design was well before different repair shops could handle repairing and replacing various parts for all aircraft sizes. For today’s aircraft propeller repairs, contact Stockton Propeller.

Back In The BCs

According to most aerospace history experts, Archytas of Tarentum is responsible for the invention of the screw. He lived from 428 BC to 350 BC.

This invention was adopted quickly by the ancient Greek scientist Archimedes in 200 BC.

The first screws were used to extract oils from olives and juice and move water up from wells with less effort. We know for sure that this was a commonplace technology used from Egypt to Greece and beyond.

A “Little” Later On

Leonardo DaVinci, the great artist and inventor, sketched his first “flying machine,” or helicopter design in the mid-1400s.

He never built this first helicopter or flying machine. But sketches of the design included an upward-facing “airscrew” that he believed would lift the machine off the ground with enough rotation.

The 1700s and 1800s Starred Major Developments

Yet it wasn’t until the mid-1700s that inventors began discussing how to use this technology to power boats by creating rotating screws, or boat propellers, to power vehicles through the water.

By the 1800s, boat propellers had become the standard technology for a wide variety of marine vessels.

In the 1840s, Sir George Cayley designed a flying machine. His design included twin propellers.

Another early pioneer was Alberto Santos Dumont, who designed his own propellers for his airships. He used aluminum for his propeller designs.

The 1900s Took Over

However, the Wright Brothers would take the airplane propeller’s idea from paper to practicality in 1903. They introduced their twisted airfoil propeller design. The Wright Brothers threw out the old notion that airplane propellers’ design should be based on screws. They hypothesized that the design of an airplane propeller should look more like a wing than a screw. After all, wings create the lift that buoys the airplane. Airplane propellers, they reasoned, should be able to displace air backward to produce forward thrust. This reasoning led them to add a twist along the blade’s length. The twist ensured a more consistent angle of attack for the blade. The twist allowed for pulling a consistent amount of air toward the plane with each rotation.

Until the mid-1920s, propellers were made from wood and were fixed pitch, which significantly limited the aircraft’s performance capabilities. Wood propellers turned the aircraft engine’s power into thrust to propel the plane forward. They featured a permanently set blade angle, called fixed-pitch, and consisted of layers of wood. They were cheap, easy to manufacture, and light, and were the dominant propeller type for high-performance aircraft until the mid-1920s. After that, small general aviation aircraft relied upon them for thrust. The Vin Fiz, T-2, Douglas World Cruiser Chicago, and the Piper Cub feature wood propellers.

In 1929, Wallace Turnbull patented his original design for a variable pitch propeller. This new design allowed the pilot to manually adjust the blades’ pitch and maintain better control over the aircraft’s performance and operational efficiency.

In the 1940s, wide rectangular blades came into use as engine power increased, as they absorbed more energy than traditional round-tip blades.

Later, engineers developed constant-speed propellers. Constant-speed propellers are variable-pitch propellers. They adjust pitch automatically to maintain a constant rotational speed easily. Many of today’s high-performance propeller airplanes use constant-speed props because they offer better performance and fuel efficiency.

Fast Forward To Today’s Aircraft Propellers & Repairs

Aviation has come an incredibly long way since the Wright Brothers first introduced their propeller design. Just as airplane design has progressed since the beginning of powered flight, aircraft propellers have transformed, too. The Wright Brothers’ newly-designed propellers were about 82% efficient compared to today’s 90% efficiency rate. To achieve those gains in efficiency, engineers have modified airplane propeller designs over the years.

Today’s aircraft propellers are made from wood, aluminum, or composites. Designers may also reinforce the leading edge with nickel for strength and durability.

Today, we see anywhere from two blades to six or more blades for propellers in operation. The blade count for any particular aircraft depends on many factors. These factors include:

the engine power,
the operating RPM for the propeller,
the propeller’s diameter limitations,
that aircraft’s performance requirements (including high-speed cruise, takeoff, loiter, etc.),
any noise requirements,
and various others.

As an aircraft’s power increases, additional blades are generally required to utilize the increased power efficiently. High efficiency in modern airplane propellers comes from running the blade tip speed close to the speed of sound.

The perfect propeller design aims to convert the airplane’s engine’s energy into the thrust that propels the aircraft forward. Looking at an airplane propeller, you can see that its blade angle varies as you move from the base to the blade’s tip. This variance has to do with the fact that the blade’s speed is lower inboard and higher at the tip. The blade’s varying angle ensures that all of the thrust generated is about equal across the blade’s whole.

Stockton Propeller is a full-service governor, metal, and composite propeller overhaul and maintenance facility. We provide service to individuals, FBOs, and Air Carriers. For today’s aircraft propeller repairs, contact Stockton Propeller.

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