3D printed props

As a novice flyer I find that props are consumables. An APC prop is a beautiful thing but a) beauty costs, b) they can take a while to get assuming there is stock. I have a Prusa Mini and although I would not design my own prop I found some likely candidates. I test printed various STL files and tested them on my shielded thrust rig to see how they performed. I settled down on two variants - both derived from an 8x4.5 - one was described as a 'slow prop' and it is noticeably beefier and wider. The other is seen in the montage below.

Hints: rotate the prop for printing. If you print with the shaft vertical you have a lot of cleaning to do. The Prusa slicer software allows one to paint on support locations (see the blue dots on the upper left). This results in just enough support. If you ask for supports 'from the build plate' you get lots more and they are difficult to remove. I increase the support Z gap by 0.05mm and the supports tend to pull off easily.

Light sanding with P600 is usually all that is required to tidy the edges. I paint a bit of correction fluid on the front for ease of identification in the field. As printed these beasties are not quite balanced. I have a simple rig for balancing and I find that adding a bit of electrical tape is faster and works as well for me as sanding down for getting a balance.

I only had one pop on me at an early stage of learning how to print and balance. I have a habit of testing every other prop I print - it goes on the model and gets ~2/3 throttle. I use prop-savers - some motors come with them, otherwise I have a prop-saver that I print as well (adapting as needed for the motor shaft and the prop). I keep a 2-3 spares in my field box - especially if it is a maiden session or I have not quite got the feel of the model. Sometimes I will swop the one shown in the photos with the 'slow' variant to see if that has a better feel on the day.

I am happy to share the STL file and or the Prusa .3mf file but those are rather large for the upload facility. Suggestions? -Jon

In my early testing on the thrust checker.... I went one 'notch' at a time on the Futaba throttle. Half throttle is about 10 notches out of 18-20 on the stick. I don't have a Watt meter so can only report the thrust. I used the same

APC 7x4 (178mm) 2cell ~half throttle = 86g top = 190g thrust,
3cell half throttle = 122g top = 350g thrust

175mm 'slow-fat' printed prop 2cell ~half 84g top = 190g thrust,
3cell ~half 134 top = 272g thrust
186mm 'slow-fat' printed prop 2cell ~half 82g top = 176g thrust,
3cell ~half 130 top = 300g thrust

180mm normal 8x4.5 printed prop 2cell ~half 48g. top = 134g
3cell ~half 80g top = 244g

195mm normal 8x4.5 printed prop 2cell ~half 68g. top = 174g
3cell ~half 126g top = 290g

The 180 normal printed 8x4.5 does not do so well at upper revs. The 'slow-fat' wins on thrust if I am running on a 2cell (saves some weight. I like to start with 2cell and then move to 3cell after I am a bit more comfortable with flight characteristics). With printed props I rarely go much over half throttle - briefly on launch. The exception are pushers as the prop diameter is less and they seem underpowered.
The APC is quieter and provides more thrust.


-Jon

Nice rig Jon! And a very interesting post, there was a article in aeromodeller recently concerning printing props, equally very interesting, being a predominantly diesel user and hence hand start everything I hate apc props, to bloody sharp!

jon
Interesting but the really important figure for most electric flying is the thrust/W figure (thrust efficiency) rather than simply the maximum thrust obtainable. You do need a Watt meter to determine it.
It would be most interesting to compare the efficiency of a printed prop against a moulded one.

With electric you can't really afford to 'squander' power, the battery is just too heavy, whereas with IC you can and in practise have to simply to make the prop strong enough to withstand the cyclic loads the engine generates.

I had a quick look at DC meters. Goodness so many have complaints about wild inaccuracies. I picked one that have fewer complaints and we shall see what happens. I will solder on some T30 connectors.... I am assuming black-to-black, red-to-red and it should go between the battery and the ESC. If anyone has a photo of a proper setup I would be interested.... -Jon

Jon
My Watt meter set up and its actually running!

Note the use of a servo tester to work the ESC to run the motor. Very simple. No radio is required. I use Deans connectors.
A close up of the Watt meter.

Running at low power, 0.42A, 8.17V (a charged 2s) using 3.4W.
This rig is for testing the amps and the balance of props. It does measure thrust.

Brilliant. Thanks for the clear images and descriptions. I might try using my servo tester rather than the Futaba for my next session. There has been a run of very windy days here that has kept me from heading out to the field so that suggests that I use the time for these background tasks.

jon
If you are using that type of servo tester note its servo signal output is connected to both the input and output centre pins so although the ESC is connected to the "input" side, so it powers the servo tester, the tester also provides a 'signal' that will control the ESC.
Just be careful not to select "auto" as it will then cycle the motor repeatedly from zero to full throttle.