Propeller Calculations
Besides just handling the Inventor drawings, concept designs and landing gear designs & calculations, I also decided to find out more about propellers and how they work. The problem with the commercial drone industry is usually the thrust rating given by the motor manufacturer. This thrust rating normally has to be accepted by the consumer, without much thought or theory given behind it. Regarding this, I decided to derive a thrust equation to accurately calculate & compare the actual thrust rating given off by the motor.
But first, what defines a propeller?
Introduction
A propeller is a mechanical device that converts rotational motion (torque of motor) into thrust, i.e, angular speed into linear speed. A pressure difference is induced between the front & back of the airfoil shaped blades, and the air is accelerated behind it. This is the means by which the drone is able to fly. While doing the project, I sought to understand how a propeller really works, the calculations and the understanding behind it. The basic knowledge of propellers and how they work were then collected and compiled for our further understanding.
Fundamentals of Propeller Design
One of the few things that determine the efficiency of the propeller are pitch, diameter & number of blades of the propeller. Fluid dynamics, induced flow & Reynolds Number ensure a propeller will not be 100% efficient.
What is Pitch?
A pitch of a propeller is similar to the pitch of a screw in mechanical terms. In other words, if you pictured air as a medium like wood, the pitch of the propeller is the amount of distance the propeller moves forward in one rotation. Of course, in the real world due to a number of factors the propellers may not move the exact pitch forward in one revolution, but it is used as an indicator of how steep the propeller angle will be. More pitch generally equals more thrust.
Why is a propeller more twisted at the hub than it is at the tip?
The reason is quite simple. This is because the tip has to go through a larger radius, & it must screw through the air at the same rate as the blade section that is closer to the hub. The blade section that is nearer to the hub has a larger angle because it travels a smaller radius, but it must advance at the same rate if the pitch is to be constant. Linear speed is the same, as the whole propeller is advancing forward at the same rate, however, at the blades, the blade tip has a higher angular speed than the blade at the hub.
What is Thrust?
Thrust is a mechanical force generated by engines or motors to move an aircraft through air. It has a magnitude & direction. Knowing the thrust is important, as this enables proper selection of motors for the drone to be able to carry a given weight or load. It should be noted, however, that there is no clear given thrust equation, and thrust data varies from manufacturer to manufacturer, with slight marginal errors.
General Thrust Equation
As of now, there are no clear & direct thrust equations for RC drones available online. The purpose of doing this was to be able to derive an equation such that I may get a clear estimate of how much thrust my propellers & motors are actually producing, as some manufacturers did not provide a thrust table. Also, it would be good to be able to compare the calculated thrust against the given thrust, so that any corrective action may be taken to improve the drone’s flying performance.
