Blog How to Test the Thrust of Propellers for Drones/Multicopter

How to Test the Thrust of Propellers for Drones/Multicopter

For most newbies or to-be hobbyists of drone/aircraft model, propellers are prone to break in plane crashes. So, how to pick or choose the right props would be quite important. But, also not that easy for green hands.

Well, actually there are some tips in selecting the most suitable props (yes, there's no definitely perfect props!!) for your aircrafts.

First you need to know, what's your preferred effect? Steady, agile, or crazy flight?

Thrust of the prop depends on: 1) size; 2) shape/surface area; 3) number of blades; 4) materials

Also take into account the diagonal motors distance, motor power, etc. when choosing.

So if you want higher power, you can use longer props like 6-inch ones, but the agility would be sacrificed. For example, to install an FPV camera on, you need the power. How the prop looks like will also determine its thrust - in the shape of bull nose (BN), hybrid BN (HBN), and pointy nose. Thrust: HBN>BN>pointy nose. Also the number of blades can affect the power: 2-blade, 3-blade, 4-blade, etc.; the more blades, the higher power.

After knowing these determinants, you may still be confused. So let the facts speak for themselves. I'll show you how to use a scale to test the thrust of props.


Preparations:

Electronic scale

3-blade CCW prop

Frame arms x 3 (to fix the motor)

SF motor

3S Li-Po battery

Radio transmitter

Receiver

Power distribution board

ESC

3M tape

Wire ties

M3 screws x 4

L-shape screwdriver

Prop removal wrench

proptest01


Step 1: Power on the scale

Usually the weighing tray can be uninstalled easily. Just gently take it away. Below, there are slots and a hollow space underneath. We'll fix the motor by tying through these slots.

proptest02

 

Step 2: Install the motor onto the frame arms

Remove the bullet cap on the motor with the prop removal wrench.

Assemble two frame arms, align their holes to those on the motor, insert the screws into the holes, and screw them tight with the L-shape screwdriver.

 proptest04

 

Step 3: Fix the motor arms

Cut the 3M tape into approx. size to the arm width, stick the tape onto the arm after placing the arm onto the scale to check the sticking position.

proptest05

Then peel off the sheet on the tape, stick the frame arms to the scale - one directly on it, one of the combined part onto the arm already stuck, and the last arm to the other side of the scale.

proptest06

Fix them with wire ties - better across 4 directions centering the joint.

proptest07 


Step 4: Install the prop

Place the prop onto the motor, and fix it with the bullet cap. Spin the cap tight with the wrench.

Pay attention to place the prop correctly - the motor used is a CCW one, so the prop used also rotates counterclockwise.

proptest08


Step 5: Wire devices

First thread the three wires of the ESC (electronic speed control, the other end already soldered to the section board with 2 power wires) through the hollow between the scale body and top frame, connect them with those of the motor.

Note: The connection can be done randomly now; later we'll come back to exchange if necessary. 

proptest09

Then plug the three-pin wire of the ESC to the first channel of the receiver (roll here; any channel is OK except throttle since it won't spring back to center automatically).

proptest10

Power on the radio transmitter now. Calibrate its pairing with the receiver: pull the rudder (roll) slowly to the largest, hear a beep, pull the stick to the minimum quickly, then hear a short piece of music, and the calibration is done. Power off the transmitter.

At last, connect the power - power distribution board wire and battery wire, red to red and black to black.

proptest11

 

Step 6: DONE!

Press the button of peeling off. Now power on the transmitter again.


Note: If the prop spins clockwise which is reversed, try to exchange any two wires of the three of the ESC to those of the motor, till the prop spin counterclockwise as it should do.

Pull the rudder stick to the top. The value measured by the scale should be a negative one. Note down the number when it reaches steady.

Change the prop you'd like to test and mark down another value. In the end, you have the data telling which one meets your needs best. So you got it!


More Info. for Prop Choosing

 

Also published on Instructables.com:

https://www.instructables.com/id/How-to-Test-the-Thrust-of-Propellers/ 

Comments

Sally _Support
15 July 2017,07:42 am

Exactly! In high speed racing, of course the purpose is to be the first to touch the finishing line. So the requirement on PID is extremely high, when the efficiency is comparatively not that important.
Happy to hear that you have a 12yr experience in multicopter. Can imagine your passion for it and really admire that.
Well, the whole structure of the Silverlit X-Ufo is so cool and I did consider purchasing one many years ago. Besides, the frame of the Bogoframe is unique in protecting the flight controller and battery, and helping with cooling as well.
Would love to see your design if there's a chance.


anonymous
12 July 2017,13:36 pm

Yes, you really Need to know what the copter should be for. A speed racer does not need to hover totally silent and stable - you just hover until the starting gun. Then it is all speed and Performance. Efficiency is also irrelevant - being first at the finish line is the Goal. Battery fried? So be it. High pitch is the right choice.
A stable camera platform is a totally different matter. Stability is of the essence, also maximum efficiency for long flights. Stability means the motors should run at an rpm range that gives the most torque change with the lowest rpm Change - somewhere in the middle of the rpm range of that motor (where the amps/torque curve is mostly linear). Here you want large diameter but low pitch props. sort of like low gear in a car. Also large but flat outrunner motors, low KV.
This is just a rule of thumb. You would actually have to do all the tests for each copter individually.
Besides, I am building multicopters since 2005, starting with modding the Silverlit X-Ufo. I did some frame design (Bogoframe, if you can still find them).
My research phase is mostly done, other than testing a new Motor or prop here and there.


Sally _Support
12 July 2017,02:34 am

For your point of determinants of blade numbers, I can't agree more. Just as you mentioned, the shape of the blade tip or even the structure of the whole propeller has a great influence on the flight efficiency. Therefore, we usually decide whether it's suitable by checking its shape based on personal experience.
Indeed, with high RPM and high pitch, the speed of flight will be greatly improved. However it's difficult to fly such a drone for beginners. As for hover, I may prefer using low RPM and high pitch props since that will make large sounds and get me really excited.
About the balance, maybe changing the any of the prop, hub, and motor each time would give different performance. We can adjust the PID to get the admired effect.
Thank you very much for sharing these with us! Welcome to follow our updates and we'll keep on hard work.


anonymous
10 July 2017,08:52 am

As for the number of blades - it largely depends on the rpm, and what Kind of tip vortex a blade creates. High rpm and a big vortex will affect the next blade, etc.
Back when the only opposing pair props availabe were the EPP1045, I used broken ones to make experimental 3-blade folding props (they break near the hub, so you always get one blade with the hub still attached. File off the stump of the broken blade and arrange 3 on a new hub). Testing with the same motor against a new, 2-blade EPP1045 showed almost no Change in thrust or Efficiency (under 5%).
However, cutting the 10" props to 9" increased efficiency by about 25%! Why? The tips of soft propellers tend to bend out of the wind, or even flutter. The part that flutters does not create thrust, but still causes drag. So, cutting that off really helped.
If you are serous about both power and Efficiency, as in flight time, you either Need to do extensive tests, or find test results from others.

I disagree on the pitch, though.
The faster the prop goes into the wind, like on a fast Propeller airplane, the more pitch you Need to compensate. Otherwise, a hich Speed, the Propeller goes into slip.
But for a hover platform, with a stationary copter, you need low pitch props. Again here it is a Balance between large props with low rpm and a large "sail" area, or smaller props with more rpm (to move the same amount of air over time), but lower efficiency due to turbulence and tip vortex.
Maybe equally important is the issue of balancing. With modern IMUs that are capaple of measuring frequencies in the kHz range, you need to avoid as much Vibration as possible. I say "avoid" since avoiding is better than damping later. Whatever you have avoided you don't need to dampen.
So balance each prop/hub/motor individually, and do it again whenever you replace the prop, or after any damage to the prop. Usually your PC-tool of the flight control board has a Feature to Show Vibration. Find the rpm with the most Amplitude, then ad thin tape to the blade that is oposite of the "bad" one.


Sally _Support
10 July 2017,06:24 am

Thanks for attention and such detailed comment. That's pro.
Well, seems that you're fonder of efficient racing drones. Indeed, good geometry and high RPM with low pitch means high efficiency, high agility, and the drone should be lightweight.
But it's not absolute for all things. Though the largest number of blades does not mean the best performance, but under a circumstance, the more blades will actually provide more power for the plane. Generally the motor power and the plane's weight are also determinants. The larger power the motor gives and the more blades, the larger lift the plane gets (the blades carry nearly all the lift when the drone flies in the air). Therefore, large drones always have many blades. For example, the one that has the most blades is the 8-blade Mi-26 from Russia.
And those with GPS, Sonar, OSD, FPV cam and pan-tilt, or aerial photography drones are often very heavy, so it needs great power (high pitch, large prop with 2 or 3 blades), when it does not require so much for RPM.


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