Simon Chaddock Posted February 9, 2022 Share Posted February 9, 2022 Over the various forums the question is regularly raised about 'printing' a prop. The usual advice, and I have given it myself, is "Don't", Advising there is a real risk of structural failure as well as likely being less efficient than a commercial one. Leading to the conclusion that not only is it a waste of time but there is a risk of personal injury from a flying blade. There are good 'Strength of Materials' reason why printing a bead filament is significantly inferior to an injection moulding process but on the basis that you should practise what you preach and being in mind the saying "Don't do this at home' I decided to actually see what a printed prop was really like. The STL file I found on Thingyverse was for a 9x6 prop that looks like this. Obviously for electric and maybe a slightly unusual blade outline but the twist certainly looks something like a true helix. The first problem concerned the printing time, well over 3 hours. Given that much of the initial testing would concern some trial and error over how it was actually printed I really wanted a smaller prop that would be quicker to print. The other advantage of using a small prop is it would be unlikely to absorb more than 150W. A 9x6 could easily absorb 300W and each blade would be 4 times the weight ? CURA has a powerful scaling function so it occurred to me that reducing the X & Y axis by 66% and the Z by 50% would in theory produce a 6x3! I have several 6" commercial props I can use for a diect performance comparison. The considerable reduction in print volume meant it would print in a more manageable 1 hour. The shape of a prop means it is bound to need some print support, shown in blue. This of course has to be removed once the print is complete, not easy on such a thin structure. By selecting a concentric fill pattern the filament would be laid down around the hub and into the blade root giving the best strength to this critical area. With a fine setting to give as good surface fish as possible the print went ahead without difficulty however removing the support structure from the surface of the blade proved as difficult as I feared. It requiring quite a bit of careful sanding to achieve both a reasonable blade profile, a fine trailing edge and a smooth surface. It all worked well enough but rather negated the inherent accuracy and finish of the printing process. The use of a translucent PLA helped to judge where and where not to sand away material. The completed 6x3 mounted on an Emax 2822 1200 kV on my test stand. One advantage of plenty of sanding meant it was possible to accurately balance the prop using my magnetic bearing prop balancer. It was only as I tightened up the prop nut I realised the prop was CW rather than the conventional CCW. Never mind do it up tight! ? It certainly looks the part and felt plenty strong enough but how would it perform. The best part of a whole day t produce one 6x3. I must be daft. Just as well I am retired. To be continued. 1 Quote Link to comment Share on other sites More sharing options...
leccyflyer Posted February 9, 2022 Share Posted February 9, 2022 I think the advice that you've given out yourself in that second sentence is sound. With just the one pair of eyes, which are quite vital to the mission and with props available at such modest cost, I'm struggling to see the possible benefits of 3D printing one, especially with PLA. 5 Quote Link to comment Share on other sites More sharing options...
Mike T Posted February 9, 2022 Share Posted February 9, 2022 Talking of retired, before you fire that thing up, you should advise anyone in the vicinity to retire to the next county ? 1 Quote Link to comment Share on other sites More sharing options...
alan p Posted February 10, 2022 Share Posted February 10, 2022 I think if you are determined to test the prop, it may be best to have it within a enclosure so that in the possibility of material failure the resulting debris is contained and no danger to you or others nearby. Definitly pushing the boundries. Quote Link to comment Share on other sites More sharing options...
Don Fry Posted February 10, 2022 Share Posted February 10, 2022 And if it holds up on the bench, well done, and consign it to be your letter opener, as stress fatigue is completely unknown quantity. Quote Link to comment Share on other sites More sharing options...
MattyB Posted February 10, 2022 Share Posted February 10, 2022 ....all I'm going to add is please make sure you capturea video of the first test, it may be interesting...! ? 2 Quote Link to comment Share on other sites More sharing options...
Don Fry Posted February 10, 2022 Share Posted February 10, 2022 I get the impression, lots of people on here used to mess with energetic chemical reactions, and the curiosity never abated. Video please. Quote Link to comment Share on other sites More sharing options...
PatMc Posted February 10, 2022 Share Posted February 10, 2022 Printed fireguard next project ? 1 Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted February 10, 2022 Share Posted February 10, 2022 7 minutes ago, PatMc said: Printed fireguard next project ? Maybe a Helmet is more wise,,,,,? Quote Link to comment Share on other sites More sharing options...
Don Fry Posted February 10, 2022 Share Posted February 10, 2022 4 Quote Link to comment Share on other sites More sharing options...
Zflyer Posted February 10, 2022 Share Posted February 10, 2022 Don's audition for the masked singer! 1 Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted February 10, 2022 Share Posted February 10, 2022 Lol. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 10, 2022 Author Share Posted February 10, 2022 Video as requested. Only a modest run but note I was alone in the room and I stayed well behind the prop. The camera was on a tripod. https://www.youtube.com/watch?v=gk0F_T-lnjw Absolutely nothing out of the ordinary and no visible blade deflection either! The Watt meter figures were 6.1A, 11.2 V & 67W indicating around 12000 rpm. (85% of 1200kV at 11.2V) For comparison the figures for a commercial 6x3 and 6x4.5 were: 3.8A, 41 W and 6.1A, 67W respectively. This picture rather suggests the actual pitch of the printed 6x3 is indeed closer to the 6x4.5 A measured thrust test will follow but my judgement was the printed prop thrust was closer to the 6x4.5 commercial prop than to the 6x3. Don't get me wrong I am not in any way trying to prove a printed prop is 'safe' but rather trying to find what its structural and performance limits actually are to justify the negative advice. Quote Link to comment Share on other sites More sharing options...
MattyB Posted February 10, 2022 Share Posted February 10, 2022 Not quite sure why your video didn't embed directly above, but here it is for ease of viewing. I agree the lack of deflection is impressive, though I'm not sure I'd have been brave enought to put the camera there on the first run-up! Having seen this some of my fears on safety have been allayed, but I still wouldn't trust this on a model with others in the immediate vicinity - the big unknown is how it will stand up over time. 1 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 10, 2022 Author Share Posted February 10, 2022 The thrust test on the printed prop. https://www.youtube.com/watch?v=OP9jXhHeakM A bit dark as I wanted to show the LED display on the balance. The Wattmeter showed 73.9W. The balance reading is taken as 305g thrust thus giving an efficiency of 4.13 g/W. A second run gave 76.6W and 313g giving an efficiency of 4.14 g/w A commercial 6x4.5 absorbed the nearest power level and gave readings of 77.2W and 373g trust resulting in a rather better 4.83g/W. An improvement of 16.9%. This does rather confirm the advice that a printed prop is unlikely to match the efficiency of a commercial one and by quite a substantial margin. This result was despite quite some finishing effort to give the printed prop a smooth surface although still well short of the gloss finish of a commercial injection moulded one. Just a note but a crude test showed the commercial 6x4.5 could easily withstand a combined load of 1kg applied at just the prop tips. The same test on the printed prop showed considerably more deflection at just 800 g. 1 Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted February 10, 2022 Share Posted February 10, 2022 What surprised me even more than the lack of an explosion in the video was the lack of vibration. To me, this demonstrates just how accurate 3D printing has become. Quote Link to comment Share on other sites More sharing options...
Don Fry Posted February 10, 2022 Share Posted February 10, 2022 Or someone balancing it with sandpaper, a balance, and some skill and patience. Quote Link to comment Share on other sites More sharing options...
Paul De Tourtoulon Posted February 10, 2022 Share Posted February 10, 2022 6 minutes ago, Don Fry said: Or someone balancing it with sandpaper, a balance, and some skill and patience. And a couple of glasses of Whisky would also help,, Don, is the helmet an own design ?, I couldn't find it here,https://www.thingiverse.com/? Quote Link to comment Share on other sites More sharing options...
David perry 1 Posted February 10, 2022 Share Posted February 10, 2022 Call me sceptical but I would like to see that prop flown because I cant see why it should self destruct. Ive used a lot of 3d stuff we have made here and when you TRY to break it its flipping tough. D Quote Link to comment Share on other sites More sharing options...
Don Fry Posted February 10, 2022 Share Posted February 10, 2022 Arrrrrrr, but have you got a bending machine, or any other method of measuring Young’s Modulus. Strong and tough are different measurements. Glass is strong, but not tough. And so far, it’s good to go, exhibits stiff behavior, but that is not a measure of tough. cos you fly it, and do a low pass, best you work out how to put a window in my bucket. And not poison the mussels it’s a home to till tomorrow teatime. Quote Link to comment Share on other sites More sharing options...
alan p Posted February 10, 2022 Share Posted February 10, 2022 Suitably impressed, as an experiment it does show a capability. Well done. Whether practicle a different question. 1 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 11, 2022 Author Share Posted February 11, 2022 Gary The lack of vibration is purely down to the fact I had to substantially sand the prop to remove the marks left by the support structure as well create a smooth radius leading edge and a fine trailing one. It was then balanced using my magnetic bearing prop balancer with further very careful sanding. Tedious and time consuming. It took a lot longer to prepare the prop than it did to print and even that took over an hour! Quote Link to comment Share on other sites More sharing options...
kevin b Posted February 11, 2022 Share Posted February 11, 2022 This thread is obviously the result of Long Covid Lockdown !!! 2 Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted February 11, 2022 Author Share Posted February 11, 2022 Perhaps more of interest than practicality but my version of CURA has a 'flip' facility which allow any axis to be reversed. This means that from the same STL file you can print a prop CW or CWW and with scaling any diameter and pitch, subject only to the printer bed dimensions. For example the original 9x6 'expanded' to a rubber type 10x10 prop. As it is not intended to run fast it can be printed hollow with a single wall and a 10% infill. It would weigh just 10 g. Quote Link to comment Share on other sites More sharing options...
Dad_flyer Posted February 11, 2022 Share Posted February 11, 2022 This is all really thought provoking, like all of @Simon Chaddock's threads. I think there is a real use case for this, if I am correctly remembering that there was an article about casting a prop from RCM&E a couple of years ago. If casting props works, then you need a blank. If you can 3D print a blank which is strong enough for even a short test in a fully guarded environment, then you have a quick way of checking the shape before committing to moulds. There is still not much point for a 6×3 2-blade. But to achieve your longed for scale outline, or for 3 or more blade or other unusual requirement maybe there is a safe and useful purpose. As I say, that is if it is considered safe and sensible to mould or glass or carbon your own props. Quote Link to comment Share on other sites More sharing options...
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