3D printed props

jon.in.glasgow wrote: 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.

Can I suggest your first test run is without a propeller?
Also Jon, my sister is coming over in March, can you send me a picture of the battery pack for the 6EX, I've probably got one here I can send with her. No wonder you didn't want to replace it, nearly 50quid is a bit steep!

I have a similar tester. I only use it for servos - that auto-cycle anywhere near a motor is super-scary.
One thing that sometimes catches me out is when I switch off the Futaba with power on to a model is a brief full throttle blast - about once every dozen flights. Lost quite a few props to that kind of noise. Usual habit now is to disconnect model battery prior to shutting down the transmitter.

It just uses AA batteries - I have some 1.6V recharagables that I use for this. Lasts a long time. -Jon

jon
The absolute rule is to disconnect the model from its battery before you switch off the transmitter.

If you don't do it this way round you are relying on the Speed Controller to go to a "motor off" condition on a loss of signal.
It is possible for the transmitter to radiate something from the actual process of switching off, particularly if it is close by to the plane. This can fool the ESC to go full throttle before it recognises the true lack of a transmitter signal and shuts the motor down.

Always - but always - transmitter ON before receiver - and receiver or motor power OFF before transmitter. (In case you use a separate motor power to receiver power arrangement!)
Train yourself to always check that the throttle stick is FULLY down, and there is nothing in the way or loose in front of the prop, before plugging in a flight pack. The speed controller 'should' refuse to 'boot up' if the throttle is not closed, but don't rely on it!
An electric motor will not stop if something gets in the way! Treat the prop as you would a circular saw blade!

Thanks for the advise - I suppose most of the time when it happens is when the transmitter is close to the model. In the case of a crash 100m away I do tend to switch off the transmitter while walking to the model - I should probably modify this to keeping the transmitter on until I can unplug the battery. Thanks for the advise.

I have gotten the Watt metre wired up and am doing a new series of thrust-power tests so will have something to chat about in the near future. I will look to see if there is an existing chat about that and post there if I find it.... -Jon

So what are the pros and cons of printed props. There is an acoustic risk - they can be noisy. They are often a bit rougher on the surface than one that arrives via an Amazon van. Various STL sources - well they might look nice, but often there is only a minimum of provenance to the source of the STL. You might get '8x4.5' as a descriptor. But it might be a slim profile that is quite a challenge to print.

One way to find out the good/bad/indifferent is to test them vs similar commercial props. I found some possible thrust testers on-line that could be printed. But most left the motor and prop totally exposed. Not such a good idea with rapidly spinning plastic that is hot off a 3D printer, perhaps with a new filament. So I designed a ring and support for a thrust tester that reduces the risk somewhat.

I had posted a couple of images of my test rig. I recently added a Watt metre to the setup and I uploaded to YouTube a video of how the beastie came about and recorded a few test sessions - including one that looks at the performance hit of a reversed prop and those early signs from an unbalanced prop as well as the differences between two 8x4.5 props - one a so-called 'slow' prop.

I include a table of V/A/W/thrust at 3 speeds x two battery types x three props. I don't pretend to know how to interpret this data other than the printed non-slow prop fits with 3 cell flights while the slow prop seems to be preferable if I am doing a 2 cell flight and have run out of commercial props.

Here is a link:

It would be great to hear about other folks testing of props or indeed about their experience of printing props.

One thing I noticed - the display on the Watt metre dimmed as I cranked up the Amps when driving it with a 2 cell battery. Eventually I could not see any feedback - so the small print about needing a separate power supply if working at lower voltages certainly seems to be true. No issues with 3 cells.

jon
The fact that the watt meter dims suggests you are significantly drawing down the voltage as the amps rise.
Of course the volts will drop a bit but maybe you over loading the battery? What capacity is it?
I presume your Watt meter displays the volts as well as the amps & watts in real time.

Interesting video but without knowing the pitch of the printed props any comparison does not tell you much. A 7x6 will produce more thrust than a 7x4 at the same volts and will take more amps to do it.

The pitch of a prop can be vital to give the performance of a plane. A plane with a higher wing loading (a heavier battery has been added) will have to fly faster and the motor may have to use a bigger pitch prop to achieve the required speed.

Both the printed props source STL claimed 8x4.5. I scaled them to get different diameters but the pitch should still be consistent. The slow prop was beefier and had more surface area while maintaining the same pitch. The APC was a 7x4.

The batteries I was using a 650mAh 2 cell and a 650mAh 3 cell. I have others I could use. The meter gave the usual V A W. I noticed V drop a bit during the tests but only on the order of a half volt. I did see that one of the solder joints was a bit tenuous and should be re-done. Soldering a T30 connector always brings up the risk of melting the housing in order to get brass hot enough for a good joint.

See the comments on another thread about soldering plugs and connectors.
A big beefy iron transfers heat to the metal a lot faster, so the housing doesn't have time to melt or distort.
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I'm not surprised you are dropping volts from that tiny battery, swinging a prop that big! If you are not careful you can overload the battery, causing it to burst into flame!
It should say on the battery casing, or at least in the paper spec's - just what the max output load should be! There are usually tw sets of figures, max continuous - and burst. (Burst doesn't mean it goes bang - just that it can deliver a lot of amps for a very short period)

Hi Jon, Eric is referring to the 'C' rating which should be printed on the pack. You should only ever charge at 1C, in your case, .65amps or 650mA. Discharging at 1C means you can draw .65amps for 1 hour, discharging at 10C means you can draw 6.5amps for 6 minutes (1/10 hour) Exceeding the battery C rating will seriously shorten its life.

The 2cell 650mAh is rated at 50C and the 3cell 650mAh is rated at 80C. In my testing I only briefly got to 11A on the metre.

And I did find the discussion on soldering. I tried with another T30 plugged into what I was soldering and that improved it a lot.

The note about the limitations of small batteries (I wanted to save weight) are noted. The 2cell 650mAh is rated at 50C and the 3cell 650mAh is rated at 80C. In my testing I only briefly got to 11A on the metre. In my flying I tend to run at half throttle much of the time and bursts tend to be short duration.

I have found an STL for a 9x6 prop and am doing a test print scaled to 180mm as a comparison with the other props....

Hmm, in theory the 2s would give you 32.5amps, if you got 11amps that's only 16C. Not that it really matters since you're able to draw enough to fly.

I pushed the Futaba throttle until the prop revs peaked. The motor was only a 2206. I had a look at the chats about soldering and have re-soldered only of the connections on the meter, that might help - we shall see when I next setup the test rig.