Assignee - @Andrew Wright

Supervisor - @Megan Spee

Guiding Question

Background

See the Thrust Testing Rig section in https://uwarg-docs.atlassian.net/wiki/spaces/MEC/pages/2151088129

Open

History

WARG’s first ever thrust test rig was designed and fabricated using the teeter totter method (see picture below).

It was determined that the team would need a new thrust test rig due to the old model not having a permanant solution for testing different motors, as the rig requires each new motor to be drilled into the wood, which is not a permanent and lasting solution to the motor mounting problem as the wood would eventually become chewed up and unsafe. Due to the rig being put together using only wood screws and glue, it was deemed to be unsafe when using powerful motors.

What is thrust and how is it measured?

Thrust is the force which propels an object in a typically forward direction. Motors and propellers work together to provide thrust. By spinning a propeller at high speeds a difference in pressure between the forward-facing and rear-facing surfaces of the blades is created. This pressure difference generates a net force in the direction of the low-pressure region, which is the forward direction of the propeller, and it is this force which is measured in a thrust test rig. When using propellers, The amount of thrust produced by a propeller is determined by factors such as the shape and size of the blades, angle of attack, rotation speed, and fluid density and viscosity.

Due to thrust being a measure of force it is measured in newtons, however thrust is often measured in pounds or kg as a convenient way to express the force produced by a propulsion system where mass is important. Kg or pounds of thrust can easily be converted back into it proper SI units by multiplying the given thrust by the gravitational acceleration constant, or 9.81m/s^2.

Current Must Haves

• Accuracy and Efficiency

• Safety - fast moving propellers pose safety threat → how design limits/eliminates these dangers

• Simple and Economic to construct

• Adaptable - Can be used to test diverse iterations of propellers and motors → must be size inclusive

Stages of Design

• Identifying requirements of the testing rig

In order for the thrust test rig project to be successfully completed, the thrust test rig must abide by the following requirements:

1. The thrust test rig must be accurate in measuring the thrust of any motor/propellor configuration

2. The thrust test rig must be safe (limits/eliminates dangers which come with fast moving propelors

3. The thrust test rig must be simple and economic to construct

4. The thrust test rig must be able to be used with varying configurations of propellors and motors

• Outlining any open questions

How accurate must the thrust test rig be? (ex: what percent error from known thrust value)

What is the maximum money constraint for the project? (ex: cannot exceed $50 in cost to build)

• Identifying stakeholders and associated sub-teams

The project stakeholders include the two mechanical leads of Megan and Conal due to needing their approval for purchasing building items and their expertise regarding testing motor thrust. Anny and Sahil may also be considered stakeholders due to their experience utilizing and preparing drone motors and may be assets during the testing phase of the project.

• Design proposals, aiming for multiple. These can be backed by research and/or prototyping

Design Proposal 1. L - Shaped Thrust Test Rig

How it works:

The L-shaped thrust test rig consists of two arms, one of which supports the motor, and the other which presses on a scale which measure the force produced by the motor as it rotates. When power is supplied to the motor, it spins the propellers, generating a thrust force that typically pushes the motor away from the support arm. This force is transmitted to the scale underneath the measurement arm, which records the thrust in kg produced by the motor. When building an L shaped thrust test rig it is important to consider the that the measurement arm must press into the scale at the exact same distance as the motor is applying a perpendicular force to the support arm so that the force felt at each point is the same.

L - Shaped Thrust Test Rig Pro’s and Con’s vs other test rig designs:

Pro’s

• Provides accurate thrust measurements due to the absence of any interfering surfaces which may disrupt airflow

• Provides accurate thrust measurement due to the motor not having to lift its own weight.

Con’s

• Is time intensive to properly design and manufacture do to its complicated shape.

• Expensive to manufacture due to having many building materials and them requiring to be very strong

• Is large and will take up space during storage

• Is more dangerous due to elevating the motor near head level and having a higher likelihood of deformation on the motor supporting arm.

This L- shaped T.T.R. configuration pulls air so that the thrust is directed towards the scale.

Open

Inwards Facing L - Shaped Thrust Test Rig

Pro’s

• The configuration accurately simulates a drone propeller by using forwards facing propeller with clear airflow.

Con’s

• The T.T.R. would need to be larger in order to ensure a clearance between the L- Bracket and the propellers

• The top of the T.T.R. where the motor is located has a higher likely hood of deforming due to the L - Bracket needing to be lowered. Depending on the strength of the material used to hold the propeller, this con could be retracted.

This L- shaped T.T.R. configuration pushes air so that the thrust is directed towards the scale.

Pro’s

• The size of the T.T.R. is minimized and is well supported

Con’s

• The configuration does not completely accurately simulate the motors thrust on a drone with upwards facing motors as this motor is simulating a downwards facing motor.

This L- shaped T.T.R. configuration pulls air so that the thrust is directed away from the scale. Doing this means that there would need to be a heavy counter weight over the scale to ensure that the T.T.R. does not spin forwards and hit the ground. The thrust would be measured here by originally zeroing the scale with the weight resting on top of it and seeing how far the scale reads in the negatives.

Pro’s

• The size of the T.T.R. is minimized and is well supported

• The configuration accurately simulates an upwards facing motor.

Con’s

• The design is complicated due to the addition of weight

• Design is more dangerous as if the thrust is able to lift all the weight the rig will swing forwards

Design Proposal 2. Teeter Totter Thrust Test Rig

How it works:

The teeter totter thrust test rig was the one originally built by WARG and is being redesigned during this project. The teeter totter thrust test rig resembles a teeter totter and uses similar mechanics as the L-shaped T.T.R. to measure thrust. The teeter totter T.T.R. consist of one long arm with a pivot point located in the centre of the arm, and the motor and scale locate equidistant away from each other on alternating side of the arm. When the motor is activated an equal force is pressed down on the scale which measures the thrust.

Teeter Totter Thrust Test Rig’s Pro’s and Con’s vs other test rig designs:

Pro’s

• Is less complicated then L - shaped T.T.R and will take less time to manufacture

• Will not require as many materials as L - shaped T.T.R. → reduces cost

Con’s

• Due to the thrust being directed vertically, the thrust must also compensate for the arm’s and motors weight which will slightly skew the thrust reading (although proper math can be done afterwards to account for these issues)

• Is large and will take up space during storage

Design Proposal 3. Half Teeter Totter Thrust Test Rig

How it works:

This T.T.R. Design uses a motor placed directly over a scale to acquire a reading of thrust.

This static thrust test rig works by placing the motor directly on top of the scale, and configuring it so that the thrust is directed down. In order to achieve this the motor has to be placed on the motor upside down.

Pro’s

• The size of the T.T.R. is small

• Design of T.T.R. is uncomplicated

• T.T.R. is safe and unlikely to break due to no stress points

• Is cheap to manufacture → 3D printed parts

Con’s

• T.T.R. is likely to be slightly inaccurate. This is because of two reasons, one, the propeller needs is placed on the motor upside down which will never occur during flight and two, due to the propellers being so close to the ground they will likely be starved of pulling some of the air which is directly under them. It should also be noted that the degree of inaccuracy with this design increases as the propeller gets larger, as small motors and propellers have extremely similar results using this method, as seen in video - https://www.youtube.com/watch?v=iENb2xf1fyQ&ab_channel=EngineerX

Design Decision

Design proposal 3 was chosen for this thrust test rig because it can be made and fabricated using cheap and replaceable items, has an uncomplicated design, is unlikely to break due to having no stress points, and can be easily stored due to its small size. After researching and comparing data of other similar thrust test rigs online, they had extremely similar results to more accurate thrust test rig designs such as the L - shaped thrust test rig, (as seen in video linked above). However, some of WARG’s motors are much larger and more powerful then the motors used in these videos so there is a potential for it to work with less accuracy when testing large motors and propellers.

CAD Design:

The CAD model for the thrust test rig was made using SolidWorks and is located on the WARG GrabCAD in the General section under the folder name, “Thrust Test Rig”. The CAD design consists of the full T.T.R. setup including the WARG table, scale, and clamp.

Materials and Construction:

Due to choosing a relatively safe and uncomplicated design for this thrust test rig, the large parts of the rig can be made out of high infill PLA. These parts consist of the clamping block and the arm which holds the motor. The bearings which will be used can consist of the four bearings used on the last thrust test rig. The mounting plate for the motor can be water jet out of 2mm aluminum sheet metal and can be fastened to the motor arm using M4 bolts. The rotating rod which attaches the clamping block and motor arm can be fabricates using an aluminum rod.

Final Build

After sourcing and fabricating all the necessary components, the T.T.R. was assembled as shown below.

Instructions For Use: - How to Assemble

To set up and test different motor configurations on the T.T.R. is very strait-forward. Firstly, un-bolt the aluminum plate from the motor arm and then fasten a motor to the top of the plate using the appropriate M3 screws. Using washers may make the motor more secure on the sheet. Secondly, bolt the aluminum sheet with the motor attached to the motor arm. To make this easier, flip the T.T.R. onto its back and bolt the sheet in using its appropriate M4 bolts. After this is done, ensure that the aluminum rod goes through both the clamping block and motor arm and has a screw on each side to prevent it from coming out. Then position the T.T.R. near the edge of the table and clamp the end of the clamping block down. Finally, place the scale underneath the motor and now the T.T.R. is ready to test the motor.

Instructions For Use: - Thrust Test Rig Procedure:

To use the T.T.R. you must first acquire permission from one of the mech leads. Before starting to test the motor you must ensure that there is no debris on the table within a 3 foot radius of the T.T.R. to prevent any debris from being sucked into the propellers. When utilizing the T.T.R. everyone in the bay must be wearing proper PPE including safety glasses and must be at least 2 meters away from the T.T.R. when it is being tested. Also when testing, the user must use some sort of safety barrier between themselves and the T.T.R. to protect against any shrapnel in the case of a catastrophic failure.

Testing:

The T.T.R. has been tested on three separate occasions using three separate motors. During the first test a small motor was used and the outputted thrust was extremely close to the expected value. The second test was used to check a malfunctioning motor and was successful in determining what was wrong with the motor. The third test was used to test a swamped competition motor. The test measured a thrust value which was approximately 30% less than its expected value. Whether this is due to the motor being faulty or the T.T.R. not giving an accurate reading will be determined by testing more of the swamped motors and checking if they are all around 30% under their expected thrust value, or if the first one tested is just faulty.

Lessons Learned + Future Steps:

Assuming that the T.T.R. is indeed giving inaccurate readings for larger motors due to its close proximity with the ground, two improvements can be made to the T.T.R.'s design to compensate for these issues. Firstly, the motor arm can be thinned so that it does not take up as much space below the motor as seen below:

Secondly, the T.T.R.'s accuracy can be improved by raising the motors higher off the ground as seen below:

By reducing the interference below the motor, by slimming the width of the motor arm and elevating the motor arm father off the ground, the T.T.R. will provide more precise results, especially for larger motor/propeller configurations.