01.07 Propellers & Rotors

Aim

At the end of this lesson, you should be able to correctly:

Understand the terminology used describe propellers and rotors;
Understand the meaning of:
- blade angle
- helix angle or pitch;
How propellers/rotors generate thrust and torque;
Understand the principles that govern propellers/rotors.

Terminology

Rotor is the generic name for something in rotation.

It can be used for a propeller, the rotating wing of an helicopter, the rotating elements of a compressor or a turbine, etc.

Propeller blades are airfoils (small wings) which accelerate air to create lift and thrust

A propeller is used to propel something, the force at stake being called thrust. Thrust is usually horizontal (but not always, think about rockets).

A rotor of a rotorcraft is used to control anything, including the altitude, the generated force being called lift. Lift is generally vertical (but not always, think about a rudder). Rotors have generally more than two blades.

Blade Angle & Helix Angle (Pitch)

Blade angle: the angle between the chord of a propeller or rotor blade and a plane normal to the axis of rotation, its value varying along the span and decreasing from root to tip because of blade twist
The angle between the resultant direction of the airflow and the plane of rotation is called the angle of advance or helix angle, and it is a different angle at each section of the blade. The sections near the tip move on a helix of much greater diameter, and they also move at a much greater velocity than those near the boss.

The Propeller

The aircraft propeller consists of two or more blades and a central hub to which the blades are attached.

Each blade of an aircraft propeller is essentially a rotating wing.

As a result of their construction, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the aircraft through the air.

The engine furnishes the power needed to rotate the propeller blades through the air at high speeds, and the propeller transforms the rotary power of the engine into forward thrust.

Pitch

Pitch is the displacement a propeller makes in a complete spin of 360° degrees.

This means that if we have a propeller of 40” pitch it will advance 40 inches for every complete spin as long as this is made in a solid surface; in a liquid environment, the propeller will obviously slide with less displacement.

The pitch concept is not exclusive to propellers, other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn of the screwdriver.

Propeller Blade Angle

Fixed Pitch

Fixed-pitch and ground-adjustable propellers are designed for best efficiency at one rotation and forward speed.

They are designed for a given aircraft and engine combination. Since the efficiency of any machine is the ratio of the useful power output to the actual power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. Propeller efficiency varies from 50 to 87 percent, depending on how much the propeller “slips.”

Propeller slip is the difference between the geometric pitch of the propeller and its effective pitch.

Geometric pitch is the theoretical distance a propeller should advance in one revolution; effective pitch is the distance it actually advances.

The Twist on a Propeller

The reason a propeller is “twisted” is that the outer parts of the propeller blades, like all things that turn about a central point, travel faster than the portions near the hub. [Figure 4-38] If the blades had the same geometric pitch throughout their lengths, portions near the hub could have negative AOAs while the propeller tips would be stalled at cruise speed.

Propeller Pitch and Efficiency

The pitch of the propeller is generally chosen to provide the speed characteristic of the aircraft for the purpose required.

Increasing the blade pitch increases the blade drag, and decreasing the blade pitch decreases the blade drag.

A larger (coarser) blade angle, for a given RPM, will adsorb more power and require more torque to turn it at the requested RPM.

Usually 1° to 4° provides the most efficient lift/drag ratio, but in flight the propeller AOA of a fixed-pitch propeller varies—normally from 0° to 15°. This variation is caused by changes in the relative airstream, which in turn results from changes in aircraft speed. Thus, propeller AOA is the product of two motions: propeller rotation about its axis and its forward motion.

Propeller Designation

Propellers are designated by two numbers:

Diameter and Pitch.

A propeller designated as a 12-6 propeller is therefore:

12″ in diameter
6″ of pitch.

…where pitch is the distance a propeller will move forward in one revolution in a perfect fluid (which air is not).

Theoretically a 6″ pitch will move forward 6″ with each complete (360°) revolution of the propeller.

Propeller Selection

Beware the ‘hobby mentality’

The propeller should be chosen to match the aircraft — not the engine. An appropriate engine should then be chosen!

Consider this. Mounting a racing propeller to a scale WWI aircraft will severely limit the model as an early warbird has so much airframe drag that the propeller will never be able to deliver it’s full potential, with the result that the aircraft will be a sluggish flyer at best.

Additionally, using too ‘slow’ a propeller – one with low pitch – on a model intended to go fast may prevent the aircraft from gaining enough speed to fly at all.

A basic mistake is finding a propeller that works great – with a certain engine in a certain aircraft – and then imposing that propeller on that engine regardless of the aircraft!

What is Pitch

The properties of a Propeller with High Pitch:

High speed flight
Poor Acceleration
Poor Climb
Can be difficult to slow down for landing

The properties of a Propeller with Low Pitch:

Low speed flight
Good Acceleration
Good Climb
Finer speed control throughout throttle range — particularly at low throttle settings

An easy way grasp the concept of propeller pitch is to draw a parallel to the gearing in a car.

Low pitch propellers = low gear in your car.

It will get you up hills well but will not take you any where fast.

High pitch propellers = Beginning your drive in fifth gear.

It will take forever to accelerate to speed but the plane is cruising when it gets there.

Number of Blades

With ‘model’ sized aircraft the most efficient propellers are two bladed. Because the diameter of our propellers tend to be small, multiple blade propellers disturb the air that the trailing blade is entering tending to make them less efficient.

Generally, with smaller aircraft, for best overall performance, it is recommended to utilise 2-blade propellers.

Balance

An out of balance propeller can be the cause of a lot of problems. Some of these problems manifest as:

Prevents the engine from developing full power.
Causes excessive vibration through the airframe.
Causes excessive vibration through on-board electronics, leading to premature failure.
Causes fuel foaming which can cause the engine to run ‘lean’, resulting in the engine loosing performance and power, to stall, or just not run smoothly.

This is all amplified in a smaller aircraft.

Before you attempt to balance a propeller, be sure to clean it.

Most propellers are close to being in balance when purchased, so they should only need a small amount of work to bring them into perfect balance.

If the propeller is severely out of balance – return it because too much material would have to be removed which would significantly change the shape of the blade.

If one blade is heavier than the other, then the usual method to bring the propeller into balance is to remove material from the heavy blade using sandpaper.

Thrust Available

The force applied on a surface in a direction perpendicular or normal to the surface is called thrust. Thrust is created by the powerplant / propeller combination:

requires an energy source (electric motor)
aerodynamic force created by increasing the velocity of air through the rotating propeller disc toward the tail
vehicle responds by moving forward

Thrust available decreases as forward speed increases due to air experiencing less velocity change through propeller disc.

Intersection of TR and TA curve determines vehicle’s maximum speed.

Torque

Torque is the rotational equivalent of linear force. It is also referred to as the moment, moment of force, rotational force or turning effect or a force that tends to cause rotation.

Propeller Principles

In summary:

Uses Newton’s Third Law of Motion
Small RPA use fixed pitch prop
Each blade of an airplane propeller is essentially a rotating wing

As a result of their construction, the propeller blades are like airfoils and produce forces that create the thrust to pull, or push, the airplane through the air. The power needed to rotate the propeller blades is furnished by the engine.