Airplanes were only able to fly when Orville and Wilbur Wright discovered how to combine airframe wings and engine-powered propellers so that it was possible for airplanes to go forward and upward at once. All propellers – including for ships and airplanes, obey Newton’s third law of motion that says “to every action, there is an equal and opposite reaction”. The aircraft propeller essentially moves the aircraft forward through the air. However, when you look closely at a propeller, the question that immediately comes to mind is “why is it twisted?”.

Propellers are twisted in order to generate the most thrust and least drag for the aircraft, and to decrease wear on the blade itself. This is because the tip of the blade cuts through the air faster than the part closer to its hub. As such, the pitch needs to be lower at the tip, in order to generate the same amount of thrust throughout the length of the propeller blade. 

In this article, we will look at how propellers produce thrust, the number of blades a propeller should have for optimum efficiency, the materials used in making a propeller, and also briefly discuss the variable-pitch propeller.

How Propellers Produce Thrust

Propellers are like airfoils, often referred to as “props”. They are sometimes called screws because they work in a similar manner.

A propeller is simply a machine that when turned moves an object forward through liquid or gas (fluid). The aircraft propeller converts rotational energy into a propelling force called thrust. 

An aircraft propeller can be powered by a jet engine or attached directly to the crankshaft of a piston engine. It usually has two or more twisted blades spaced evenly with a fixed or variable pitch from a central hub rotated by a motor or an engine.

This rotary motion generated by the engine, in combination with the airplane’s forward motion, changes the effective direction of the incoming air by creating a differential air pressure at different points along the propeller blade. This acts to drive the airplane forward.

The Parts of a Propeller

The propeller is made of several features. These include:

• The leading and trailing edges, the blade face, the blade back or thrust face, 
• The blade tip, the blade shank/root, the blade hub. Propellers also have camber and chord lines just like wings. 

The Twisted Feature of Propeller Blades

On a closer look, the blade angle of a propeller varies from the root to the tip. As the propeller spins, the speed of the blade is fastest at the tip and slowest at the root.

In the same period, the tip of the blade travels a farther distance than the root of the blade in one turn.

However, by twisting the blade, you get a relatively uniform angle of attack (angle where the chord of the wing meets with the relative wind) across the entire length of the propeller blade.

Because thrust varies a lot from the root to the tip of the blade, because the tip will move faster than the root, a propeller that had the same pitch throughout the length of the blade would generate the most thrust on the tips, inducing a lot of strain on the blade. 

Hence, it is important to vary the blade angle by twisting the blade to make the thrust the same throughout the length of the blade.

The lowering of the angle of attack at the tip subsequently reduces the lift created at the tips to balance the lift generated at the root. This results in a uniform lift along the length of the blades, thereby decreasing the likelihood of an in-flight breakup and making the blades more durable.

Twisted Propellers Create Pressure Difference

Propellers change engine horsepower into thrust by generating a pressure difference between the low air pressure in front of the propeller and the high static pressure behind the rotating propeller.

As the propeller spins, air naturally moves from high to low pressure pulling the airfoil forward and creating a forward thrust as a result.

Twisted Propellers Generate Acceleration

The acceleration of air is increased across the blades by increasing both the engine power and propeller speed (RPM), thereby creating a pressure differential that increases the thrust on the airplane.

As you may know, thrust must be greater than drag for an airplane to accelerate. As acceleration increases, the drag load also increases. Hence, more power is required for increased acceleration.

In a propeller, the speed of each blade’s part and the aircraft’s speed through the air are primarily considered. Regardless of the aircraft speed, the twist of the propeller allows each part to generate closely equal thrusts through the blade’s length.

The pitch of the propeller of a high-speed aircraft flying at a high altitude needs to be made more coarse for it to be efficient. This is because the propeller needs to “bite” more air as the air gets thinner and the angle of attack decreases at a high altitude with increasing aircraft speed.

How Propeller Size Affects Thrust Generation

If only we lived in an ideal world, a “variable-diameter propeller” would have been the most efficient kind of propeller. This is because it will have a smaller diameter for high airspeeds and a large diameter for low airspeeds. Unfortunately, “variable-diameter propellers” are not practical for control, structural, and weight concerns.

Rather, the diameter of most propellers is designed for medium (between slow and fast airspeed) operations. The diameter of the propeller is determined by engine requirements and aircraft mission.

The stresses acting on a propeller in flight include tensile stress, torsional stress, and bending stress. And the forces acting on a propeller in flight are thrust, twisting forces, and centrifugal force. 

Are More Propeller Blades Better?

The right answer to this question depends on a number of factors, including the aircraft type, aircraft’s engine power, the propeller’s RPM, and your definition of efficiency, performance, and comfort. The propeller blades attached to an engine must equalize the power output of the engine in order to generate thrust efficiently. 

The balance between thrust and power generated by the engine is critical to the take-off weight of the aircraft as well as the maximum operating altitude of the airframe. Therefore, the number of propeller blades is dependent on engine power and it controls the take-off weight.

Are More Propeller Blades More Efficient?

If all these parameters highlighted above are unchanged, the addition of more blades minimizes the efficiency of a propeller.

However, as engine power increases, more blades would be required to efficiently maximize the increased power and generate thrust. Hence, the most efficient number of propeller blades for an aircraft is determined by the combination of these factors which varies on the type and mission of the aircraft. 

In comparison, a 2-blade propeller is slightly more efficient, a 3-blade propeller usually offers high-speed performance while a 4-blade propeller provides maximum thrust and smooth cruising operation. However, thrust propels an aircraft, efficiency does not.

In the case of turboprops, there is a need to add more propeller blades so as to efficiently maximize their higher power output.

Are More Propeller Blades Noisier?

Consider this scenario: In a single-engine aircraft, the propeller blade beats against the windshield and produces cabin noise. A double-bladed propeller produces two pressure pulses in one turn, while a 3-blade propeller with a smaller diameter than the double-blade propeller will generate three smaller pulses in one turn for the same amount of total thrust.

This means that the 3-blade propeller will run more smoothly and quietly than the single and double blade propeller. Moreso, in a twin-engine aircraft, the reduced diameter of the 3-blade propeller also allows less tip-generated noise as a result of the larger clearance between the fuselage and blade tip.

The propeller is one of the sources of noise in the cockpit. Increasing the number of propeller blades can help to lower cabin noise. This is important because high levels of cabin noise can cause stress, hearing difficulties, and fatigue among pilots. Therefore, the addition of more propeller blades helps to reduce vibration and noise, thereby, enhancing overall cabin comfort for passengers and pilots aboard.

On the other hand, props with fewer blades weigh less and are less expensive. Also, smaller engines do not need more blades because engine power would be lost trying to overcome the extra drag of the additional blades.

Variable-Pitch Propellers: What Are They?

A variable-pitch propeller can take a bigger bite of air whenever it is necessary by changing the pitch of its propeller blades.

The variable-pitch allows the pilot to manually manipulate the propeller pitch so that it can maintain the most efficient flow of air during any of the flight phases; either take-off, climb, or cruise phase. This means variable-pitch greatly improves fuel efficiency and engine performance.

Many modern large aircraft are equipped with variable-pitch propellers. They come in three basic kinds. They are the adjustable-pitch propellers, controllable-pitch propellers, and constant-speed propellers.

1. Adjustable-pitch propellers can have their pitch adjusted when the plane is on the ground, but not during flight. They are sometimes called ground-adjustable propellers.
2. Controllable-pitch propellers can be adjusted by the pilot during flight through a hydraulic mechanism as opposed to the adjustable-pitch propeller.
3. Constant-speed propellers have hydraulic mechanisms that are automated to change the blade pitch when necessary, allowing the propeller to always rotate at the same unchanging speed. This helps the engine to generate power efficiently regardless of the action and speed of the aircraft.

The overwhelming benefits of variable-pitch propellers over the fixed-pitch propellers have made them remain the preferred choice over the century with most commercial aircraft manufacturers. In addition, variable-pitch propellers also have the ability to feather if an engine fails, which is a great safety feature. 

Feathering means turning the propeller blades edge on so that they minimize drag by making a very shallow angle to the oncoming air. As such, it allows the plane to continue flying on the other engine(s) or safely glide towards an emergency landing.

Besides, on some aircraft, the variable-pitch propeller can come handy for extra braking in case the main wheel brakes suddenly fail by reversing the pitch of the blades to make a forward draft of air.

Materials Used in Making Aircraft Propeller 

Old propellers were carved from wood. They have now been replaced with more predictable lightweight materials like aluminum, magnesium alloys, hollow steel, composites, or wooden laminates.

Final Words

Historically, aircraft propellers, sometimes referred to as “airscrews”, have narrow and thick blades. These blades are twisted so that they can rotate at great speed and efficiently cut through air. 

Interestingly, as crucial as propellers are to the operation of an aircraft, many aircraft owners believe the propeller is a no-maintenance part. This may be due to the fact that accidents and fatalities from propellers are not common. Generally, propellers are recommended for overhauls after 1,500 – 2,000 flight hours, or after five years whichever occurs first. So, in essence, they are a very sturdy part of the plane.