Propeller

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Overview

GWS 9x4.7 Slow Flyer propeller

A propeller is the most common propulsion device used in UAVs. A propeller is composed of a number of blades that rotate around an axis to produce thrust by pushing air much like a fan does. A propeller's performance is determined by its geometry, and different propeller geometries are optimized to produce thrust efficiently at given rotational and forward flight speeds.


Types

Different propeller manufacturers use varying geometry styles for their propellers. Although there are many styles available, some have become more commonly used and known. For example, a popular propeller style for multicopters is the Slow Flyer style by GWS. This style is designed for low rotational speeds and low forward flight speeds, and is ideal for multicopters. APC style propellers are designed mainly for high-speed electric motor applications. Master Airscrew styles are more popular in gas motor applications.

Propellers also come in clockwise and counter clockwise designs. In some multi-propeller UAVs it is desirable to use counter-rotating propellers so that the net torque generated by the propellers as they rotate is canceled to zero. This is the case for a quadcopter, where two propellers spin clockwise and two spin counter-clockwise, cancelling the net torque on the aircraft.

Another possible variation is the number of blades. Although two blades is commonly used in UAVs, more blades allow you to provide similar performance with a smaller blade diameter, at the cost of lower propeller efficiency.

Some commonly used materials for propellers include plastic, wood, fiberglass, and carbon fiber. The most important characteristic of the material is its rigidity. Rigid materials make for more efficient propellers, since deflections can cause a reduction in efficiency. Rigidity also helps to reduce vibrations and propeller noise.

Other types of propellers include folding propellers, where the propeller blades are placed on hinges so they can fold back. This is useful for gliders, where the blades fold back to reduce resistance to air when the motor is turned off for gliding, or to prevent damage to the propeller when landing directly on grass. Coaxial propeller configurations place two counter-rotating propellers in the same axis, one in front of another. This provides the power of two propellers at the size of roughly one, but at the cost of lower performance. Coaxial propellers are typically 20% less efficient than single propellers. Variable pitch propellers are able to selectively control the pitch of the propeller blades, and thus produce reversed thrust. This is useful for making multicopters that can fly upside down, or airplanes with improved aerobatic capabilities.


Geometry

Sample propeller geometry. Reference: UIUC Database

While one typically selects a propeller based on just its diameter and pitch, a propeller's geometry is fully defined by both its diameter and its twist, chord, and airfoil distributions. The twist is the angle of the blade relative to the rotation plane, the chord is the width of the blade, and the airfoil is the cross-sectional shape of a section of the blade at any point along its length.

The plot on the right shows the twist and chord distribution for the GWS Slow Flyer 9x4.7 propeller, where the horizontal axis is the location along the length of the blade (0 being the root, and 1 the tip), c/R is the chord of the blade divided by the diameter of the propeller, and \beta is the twist angle of the blade relative to the rotation plane in degrees. As can be observed in the plot, a propeller's twist is highest near the root and decreases towards the tip. For this particular propeller, the chord is largest near the middle of the blade, and lowest near the tip.


Propeller Pitch

A propeller's twist curve is always close to the 'perfect pitch curve', which is the curve that would allow the propeller to move forward as it rotates through the air without slipping, in a manner similar to a screw that rotates into a solid without having the threads slip. Although a propeller's actual pitch curve lies close to this geometrically-defined 'perfect pitch curve', designs usually deviate slightly in order to achieve desired performance characteristics. Having defined this, a propeller's pitch is simply the distance a propeller would travel forward in air if it is rotated once, without any slipping. Thus, a propeller's pitch specifies how much twist a propeller has; a higher pitched propeller has more blade twist than a lower pitched propeller.

Aerodynamics

Propeller thrust vs speed characteristics

As a propeller rotates, its blades push the air in one direction in a manner similar to a fan. By Newton's third law, if the propeller is pushing air one way, the air is also pushing the propeller the opposite way with equal force. This force is known as thrust, and is used to propel the aircraft. Besides thrust, using a motor to rotate a propeller will also produce a torque force which will want to rotate the UAV in the direction opposite of the rotation of the propeller. This torque force can have an important effect on the dynamics of the UAV. For example, multicopters often rely on varying the speed of their motors in order to produce a net torque that rotates the aircraft in a desired direction.

As shown on the diagrams to the right, the thrust a given propeller produces depends on the speed the propeller is rotating, and the speed at which the propeller is moving forward. The thrust is proportional to the square of the rotational velocity, meaning that if you double the rotational speed of the propeller, the thrust will increase by a factor of four. The thrust also decreases linearly proportional to the forward speed. This means that the faster the aircraft is moving forward, the less thrust the propeller will produce. At a certain forward speed the propeller will produce no thrust, and beyond that speed the propeller will actually work as an airbrake, resisting the forward movement. We can get a ballpark estimation of this zero-thrust speed using the following equation:

V_{0}\approx p{\frac  {\omega }{60}}

Where V_{0} is the zero-thrust speed in meters per second, p is the propeller pitch in meters, and \omega is the rotational speed in revolutions per minute (RPM). If the UAV will fly at a given forward speed V, the propeller pitch and rotational speed must be selected so that the resulting V_{0} is greater than the desired V.


Wind Tunnel Data

GWS 9x4.7 Slow Flyer propeller thrust coefficient vs advance ratio. Reference: UIUC Database
GWS 9x4.7 Slow Flyer propeller power coefficient vs advance ratio. Reference: UIUC Database

Due to the rotational motion of propellers, they produce complex aerodynamic behaviors that are not easily predicted. Thrust and torque prediction for a propeller often relies on wind-tunnel or other test data. Wind tunnel data is often provided in 'nondimensionalized' values, where C_{T} is the thrust coefficient, C_{P} is the power coefficient, and J is the advance ratio. The advance ratio specifies how fast the propeller is moving forward through the air; an advance ratio of 0 is a propeller that is rotating without moving forward (like the propeller in a hovering multicopter), and an advance ratio greater than 0 is a propeller that is moving forward through the air as it rotates (like in an airplane). The advance ratio value combines both the forward speed and the rotational speed of the propeller into a single value, such that an advance ratio of 1 means the propeller is moving forward a distance equal to one propeller diameter in the same time it takes the propeller to make one revolution.

The plots to the right show the thrust and power coefficients for a GWS 9x4.7 propeller, as obtained from the UIUC propeller wind tunnel study data site. Similar data can be obtained for APC propellers from the APC site. This coefficient data can be used to accurately calculate the propeller thrust and torque performance at any flight conditions using the following equations:

T=C_{T}\rho ({\frac  {\omega }{60}})^{{2}}d^{{4}} \tau =C_{P}\rho ({\frac  {\omega }{60}})^{{2}}{\frac  {d^{{5}}}{2\pi }}

Where T is the thrust in Newtons, \tau is the torque in Newton-meters, \rho is the air density (approximately 1.18 kg/m3), \omega is the rotational speed in revolutions per minute (RPM), d is the propeller's diameter in meters, and C_{T} and C_{P} are the thrust and power coefficients obtained from the database for a given advance ratio. The advance ratio is calculated as a function of the forward speed and rotational speed of the propeller using the following equation:

J={\frac  {60V}{\omega d}}

Where V is the forward flight speed in meters per second.


Other Experimental Data

Propeller experimental data is also given in terms of thrust produced at a given rotational speed. This type of data can be obtained from online databases such as the Fly Brushless website. These will provide accurate predictions of thrust for a given propeller rotational speed. However, it is important to keep in mind that this data was obtained for zero airspeed. At flight conditions, if the propeller is moving forward it will receive an airspeed and the thrust produced by the motor will be significantly less. Read the beginning of the Aerodynamics section to see how to calculate this effect.

Selection

Propeller selection is one of the most critical design decisions for a UAV. Often times it is the first component to be selected based on required thrust, and the rest of the UAV is scaled around the selected propeller.

Propellers are selected so they provide the required thrust while using the minimum power at a given operating condition. Due to the complexity of the propeller dynamics, and the many variables involved, there is no simple way of doing this. Although propeller selection often relies on experience, brushless motor and propeller performance prediction tools can also be used to find good propeller-motor combinations by trial and error.

A few general rules of thumb can help in the propeller selection. For slow-moving UAVs or for hovering UAVs such as multicopters, large propellers always give you more efficient performance. The larger the propeller area is (either by having multiple propellers or just a few large ones), the less power will be required to fly. However, if the UAV is meant to fly fast, smaller propellers with larger pitch will be required.

Setup

A propeller can be attached to a brushless motor in various different ways. If a motor has its main rotating shaft extending out from the motor, a propeller can be attached to this shaft using a 'propeller adapter' device, which must be bought separately. Another way of connecting a propeller to a motor is by using a 'prop saver', which is a component that attaches the propeller to the main shaft of the motor using rubber bands. If the propeller hits something, the propeller detaches from the motor quickly and the chance of being damaged is reduced. Outrunner motors usually provide an alternate way of connecting the propeller, which consist of attaching another shaft directly to the outer rotating frame of the outrunner motor using screws. This shaft has threads and is of larger diameter than the main shaft, and therefore the propeller can be directly secured to it using an included nut known as a prop nut. This attachable shaft is sometimes included with the motor, but often times it must be bought separately (search for the motor's accessories).

Fuel engines usually have a threaded main shaft, so the propeller can be attached to it directly and held in place using a nut and washer.

Propeller 'spinners' are sometimes used for aesthetic reasons. A spinner is a cone-shaped piece that covers the center area of the propeller. Spinners also help electric starter motors get a better grip on the propeller when trying to turn on a gas engine.

The hole of a propeller must sometimes be enlarged for it to fit into a given shaft. A 'prop reamer' tool can be used to do this. Some propellers come with a set of interchangeable rings that provide various hole sizes to the propeller so reaming is not necessary.

It is sometimes desirable to balance a propeller in order to minimize vibrations during flight. This is especially important when the UAV carries cameras, as vibrations can cause bad video quality. The easiest way to do this is using a propeller balancer device, however there are various other ways of balancing a propeller.

External Resources

Propeller and Brushless Motor Prediction Tools