Printed props?

[Erfolg]

I was a little surprised that Simon has printed a propeller. I guess as a experiment?

 

I do agree with the cautious comments. PLA does not strike me as having a goodish Youngs Modulus, from limited experience. Perhaps my greatest concern is that the processing history, that is the layering of the filament. Again from limited experience, the adhesion of the layers is questionable from a QA perspective, potentially being quite variable. The structure being vaguely similar to carbon graphite layers, being highly directional.

 

It also strikes me that the Tg & Tm is quite limited, compared to other polymers.

 

I would be very suspicious of such a prop.

[Simon Chaddock]

More to complete the experiment than to prove anything a short video of the printed 6x3 (in reality more like a 6x4.5) running with an A2212/6T 2200kV motor on a fully charged 3s.

https://www.youtube.com/watch?v=9e4_RZhtP4o

The Watt meter showed 18.28 A, 12.18 V and 195 W. By taking 85% of 12.18 x 2200 suggests it was spinning at about 24,000 rpm.

This test was run twice and likely represents the sort of power that might be used in flight.

Interesting that there is no visible blade deflection.

By comparison this shows the sort of deflection the blade is capable off without damage.

https://www.youtube.com/watch?v=ST2ddAJ6iyw

It took close to a 1kg push on one blade to create this deflection.?

 

It does not change my view that although a small 6" well prepared printed E prop may be just about structurally adequate in PLA it really cannot be considered as a viable proposition when compared to the modest cost of a better performing commercial one, given lower efficiency and the considerable time and effort required to create a printed version. 

At sizes larger than 6" the structural integrity of a printed prop is very questionable with a severe safety risk from the increase mass of the blades should it fail.     

Edited February 12, 2022 by Simon Chaddock

[Simon Chaddock]

I do intend to fly a printed prop just because.

It will be on a pusher so it will be protected from ground strikes.

It uses a 3s LiPo. The big advantage of this simple but light airframe is it can be hand launched and only needs less than 1/2 power to fly. A full power test can thus be done at a safe altitude should the prop decide to 'let go' although I am pretty sure it won't.

The printed prop is a 9x4.7 modified by 'scaling' to be a 10x3.8 the same as the commercial prop it currently uses.

We shall see if and when the weather calms down! ?

 

 

 

Edited February 16, 2022 by Simon Chaddock

[Simon Chaddock]

Just to complete the picture I wondered if painting could achieve a better surface finish although painting plastic props is really another "Don't"!

The only problem is a light spray highlights all the otherwise 'hidden' surface defects.

Never mind it is now on the Big Dragon test bed.

I spent some time using 'wet and dry' on another printed prop to achieve a better finish.

 

Getting better.

One advantage of spray paint is you can balance a prop very accurately by adding a very light spray on the 'light' blade but it is slow process as you have to let each coat dry thoroughly before checking the balance. ?

Overall it does add to the 'waste of time' aspect of printed electric props!   

 

[Simon Chaddock]

I have now flight tested both the 7x4.5 and the 10x3.8 although neither in particularly good weather conditions.

The 7x4.5

https://www.youtube.com/watch?v=_970mmJfj-Q

The 10x3.8

 Later in both video a slide take off in cluded (despite the rough grass!) suggesting the props have adequate efficiency.

In the second video take off the prop is absorbing about 350W.

Nothing broke! 

[Erfolg]

At present my own believe is that 3d printing is so fraught with quality reservations that it should be discouraged.

 

Compared with almost all of the low cost, products commercially available, 3d printed propeller products are a number of magnitudes poorer, when produced by the average home user.

 

In my case I only intended using my home produced items principally on cosmetic applications.  The most demanding applications will be wheel hubs on small models.

[Andy Stephenson]

There are entire 3d printed models out there, I fly 2 of them, albeit with commercially produced propellers.

Eclipson Pitts 12S and Model R

[Erfolg]

With resect to the majority of home 3d printer users, there are significant differences from their acquired experience and knowledge.

Without resorting to my own text books< i believe a number of issues that can be stated.

• The directional additive variable layer properties, that are very significant even compared to wood.
• That as been stated by some one else, the "Youngs Modulus" is pretty low, that is the ratio of stress to strain.
• We should take seriously the Tg and Tm points, that is the properties at cold(ish) temperatures to  Temp melt. is quite narrow, when compared to what most of us are familiar. 
• Loading rate has a more significant effect than our traditional materials that we all have a feel for. A very extreme example of the phenomena is silly putty. In this case, poke a finger vigorously into it, it is pretty solid, yet push your finger into it slowly, it is soft (very plastic).
• The text book properties such as tensile properties are not good compared to wood, and GRP.

Andy in your case, your models appear ( and I suspect really are) magnificent. Yet compared with my boiler plate modelling, need handling with care. Not taking the brutal, lack of inspection that my models are subjected to.

 

I do not yet have much experience of 3d printing. In my case I have experienced some variable properties. I use PLA, I perhaps do not store it as well as ideal. I have experienced very variable results, unexpectantly. I may live with 3d printing for cosmetic covering, as FF models make do with a bit of tissue in many cases. For actual load carrying, such as Spars, Structural Longerons, NO. I have also been surprised at the weight build up, when trying to increase structural properties, relative to functual duties.

 

To finish, yep I like 3d printing as a home tool. I have made a few things from bits foe a 1/32 Fokker Tri plane, ballast boxes for a flying model, a cover for a model yacht and a mock BMW for a 50" Fokker V29. I have a lot to learn, particularly with respect to Fusion 360, and other things. For me horses for courses. 

 

My motor

 

 

Edited March 1, 2022 by Erfolg

[kc]

Surely hand launching a new prop is the danger time when a person is only inches away with the motor revving up!   Hard hat and safety glasses needed?

[Andy Stephenson]

It seems to me that 3D printed props would be better suited for a low stress application like rubber powered free-flight models.

[Simon Chaddock]

it was the lack of factual knowledge that spurred me to find out what they are actually like and to be honest it took quite a bit of searching before I found one that even began to look correctly proportioned.

I would also point out that the props were tested both structurally and at very high power, around 23,000 rpm, on the bench in a secure environment before being used in flight where the rpm used at launch was no more than half that figure.

Although these tests seemed to show they can be 'adequate' they do require quite a bit of finishing to get a performance anywhere near that of a commercial one and whilst they can produce a similar thrust they consume more Watts doing it. The working life of the prop was not investigated whereas I do have some commercial props that have been used regularly for many years. 

I entirely agree with erfolg that printing "flight" props is not a practical option and has greater risks associated with it than using a commercial one.

 

If asked my advice about using printed props was and still is "Don't".   

Edited March 2, 2022 by Simon Chaddock

[Erfolg]

Simon and myself both worked in the same Chemical Engineering Company, although I do not think we ever met. We probably have very similar experiences with emphasis on materials of construction and operation. In my case having retired some 20 years now, I admit to forgetting a whole lot of stuff, compounded with my later working (and some post retirement) years mainly centred on financial aspects of projects and long term issues.

 

Additive processes, are nothing new, I remember some 55 years ago, seeing a journal having an additive metal coating being added to a turbine journal, prior to being ground. Why or what were the benefits and losses, never even entered my mind, as a young person, knowing nothing about the "strength of materials", not "Process History". To some extent I find myself with same issues with 3d printing. My only experience to date is with one material PLA, one machine, one modeling package (Fusion 360)and the one splicer (Cura).

 

It does strike me that Cra has so many options for slicing the variables are numerous. It seems common to have a partial infill. Again nothing new, aluminium sheeting with honeycomb infill was common in the 60s. 

 

Which brings me to although Engineers only tend to calculate the structural requirements of critical areas and components, much of the remainder being reliant on experience, common practice and sometimes cosmetics. In the case of 3d printed items, the calculation of anything process a problem. The manufacturing process is so unpredictable that anything approaching a ball park value is not practical. Although not taking credit for the infill areas, has to be normal.

 

Again empirical methods are not unique often being used for complex assemblies. In the case of 3d printing, I suspect that a whole bunch of test specimens would be required, produced at various  times etc., would be required. I am not convinced that the results would be anything but emphasising the limitations of the process for structural purpose.

 

That is without considering strength to weight analysis or cost per application analysis.

 

All the same I believe there is a major place for the home/modeller 3d printer user

[Erfolg]

I am interested in the surface finish issue. This is because my surface finish is not quite what I like for many uses.

I have been reluctant to sand etc. Mainly because when attempted, hmm, takes longer, not quite as good as I would desire. 

Although relevant for the propeller, i see it as a common issue.

[Simon Chaddock]

Erfolg

Almost by definition building up a 3 dimensional object using layers is not going to produce a mirror finish. You can only reduce the individual layer thickness by so much.

CURA does have an 'ironing' feature where the tip of the hot nozzle is passed over the surface to locally 'smear' the surface smooth.

Not tried it myself yet but the warning is it takes quite a long time.

Normally the only smooth surface is that created by the surface of the bed. 

[Erfolg]

I cannot but note that some others and one of first prints from a Thingverse file, (of a approx. 1/4 scale radial cylinder, to big for my use) are much better than my own design and slicings. 

 

It seems I am lacking in my selections in setting up Cura.

 

I have tried sanding etc, though the polymer does not finish well, with my skills. I have thought of perhaps filling, which seems messy in concept. Solvents were a thought, but

the issue as a generic concept of ESC (environmental stress cracking) does not encourage me.

 

I am still learning, and I have issues of the tutorial options in Fusion do not always appear, yet the options seem to be almost limitless, if you get your sequencing and operations are right. I do not always get my operations correct?

[Ace]

Loooooong time ago in my military days with new boots we smoothed the bobbles down with a warm spoon. Would something work on a 3d printed object? 

[Erfolg]

 

i think that is essentially what Simon is suggesting with the "smear" setting. The question is who has tried it?

 

Although I have noticed that a print I have produced from some one else's files has produced a better surface result than my design and print. The question is why?