3D printed props

Having been a bit critical of printed props I thought I had better print some.
Made a start with the 9x6 from Thingyverse.
Printed solid with a 0.15 layer it would take 3.5 hours.
As I just wanted to see what the shape looked like I printed a 'test' with a 10% gyroid fill in 1 hr 25 min. It needs support over about 1/3 of the blade area.
This is how it is printed on the bed.

Of course the underside needs quite a bit of 'finishing' to remove the support structure and to give a nose profile and a fine trailing edge.

Obviously it is much easier to improve a prop than to carve from scratch but it still take some time and would require it to be very carefully balanced.
Quite a reasonable blade shape & twist and surprisingly stiff even when 'hollow' but it is quite a thick section and has a very deep root.

Not quite IC standard but overall more 'massive' than a true electric slow fly.

Wanting to try some other print settings so I have a mathematical question.
If you scale the X and Y coordinates by 1/3 and the Z by 1/2 do you end up with a 6 x3?
I think it should mathematically but how well it might print is another matter.
Being smaller (its only 1/4 the volume) should allow even a solid print in a reasonable time.
We shall see.

The 9x8 was reduced to a 6x3 and printed solid.

Took quite a bit of finishing but it ends up as quite a reasonably thin prop.
Carefully balanced with my magnetic balancer and mounted on an Emax 2822 1200 kV. The 9x6 would be more suitable but it was the only motor I could find.
Video of it running tomorrow.

I also found this on Thingyverse called "a "racing propeller".

Interesting as it is in two halves split along the length of the blades. This allows each half can be printed 'flat' on the bed without needing any support structure.

It is 6" diameter but the pitch is not given. It looks about a 4.
I think it is based on a full size prop as the downloaded STL file gives a 1.78 m (70") diameter!

A bit too big for the printer so it had to be scaled down to just 8%.

Very clever indeed!

A video as promised.

An Emax 2822 1200 kV on a 1500 mAh 3s LiPo.
Not highly stressed. 6.1A, 67W at 11.2V giving a nominal 13,400 rpm.
However in a 'conventional' application for a 6" prop it would be expected to be running at 20,000+ rpm.

No thrust measurement yet but I did run both a 6x3 and 6x4.5 commercial props giving:
3.7A, 41W and 6.1A, 67W respectively.
Suggesting the printed prop is a nearer equivalent to a 6x4.5.
This is rather confirmed by looking at the blades side on.

Printed at the top, commercial 6x3 at the bottom.

So far, apart from the degree of time and effort required to create it, I am quite impressed by just how good the printed prop is.

I have found a better way to print the prop so the support structure has very little impact on the prop blade surface thus reducing the amount of finishing required to get a smooth surface.

It takes 59 minutes to print a 6x4.5 but well over 3 hours to print a 9x6.

My version of CURA has a MIRROR facility which with one click allows you can create either a CW or CCW prop from the same STL file.
I have also ordered a 2200 kV motor which should allow testing of the 6x4.5 up 25,000 rpm.

I note the 3x3.5" 4 blade on my V-2 rocket spins at over 40.000 rpm on a VTO take off.

Very very impressive Simon!

I have now printed out both CW and CCW versions of the 6x3.

They are indeed identical but correctly mirrored.
Thus in theory it is possible to print any diameter (up to the bed limit) or pitch both CW and CCW from a single STL file.

However unless a thrust test shows a distinct g/W advantage (unlikely?) whether it is worth the trouble or the risk, particularly on the bigger sizes, is a different matter altogether.

My thrust test rig.

The centre line of the motor is exactly the same distance from the pivot as the rubber button over the scale is so it reads the thrust directly.
A short video of the first test.

A bit dark as I wanted to show the scale's LED.
The Watt meter readings were 6.35A, 10.79V & 73.9W. The thrust is taken as 305 g
This gives an efficiency of 4.13 g/W
A second run gave 6.47A, 10.83V, 75.6W and 4.14 g/W
A commercial 6x4.5 on the same rig gave 6.65A, 10.82V, 77.2W and 373 g thrust with an efficiency of 4.83 g/w.

This does rather confirm the statement that a printed prop will not match the efficiency of an injection moulded prop and by quite a margin, 16.9% in this case. This result is despite the time and effort to hand finish the printed prop a smooth fine surface but it is nevertheless still well short of the gloss surface of a commercial one.
Whilst it may be possible to improve the performance of the printed prop it is hard to see it ever exceeding that of a commercial one.

A further test using the rig showed that the commercial prop could easily withstand a total force of 1kg applied to just the tips of the blades.
The same test on the printed prop showed an alarming deflection at a total load of only 800g.

So the conclusion does confirm the normal advice given on the use of printed props for even electric RC, Don't!
They are likely to be significantly less efficient and are certainly neither as stiff nor as strong as a similar commercial injection moulded one.

Interesting stuff! Not knowing much about printed "things" would a printed prop withstand a ground strike? Is the material brittle? If you ever fancy printing a huge pitched 10" prop I'd buy it..be interesting to see the performance on my rubber jobs.

Paul
PLA is indeed a bit more brittle than the plastics used in basic injection moulding. The real weakness would show if it was underpower when it hit the ground as its lower ultimate strength would likely be the limiting factor.

Just out of curiosity it would be just possible to print a 10x10. It would look like this.

I would suggest printing it 'hollow' but with a complex internal structure. This would save a great deal of weight. At 10g it would be less than 1/3 of solid but with only a small penalty in stiffness. It would still take over 4 hours to print as nearly 50% is taken up printing the required support structure.
You would unfortunately be stuck with the power type hub and a 6 mm centre hole but it should be possible to print a separate insert with a hole diameter to suit shaft diameter you use.

If you think it might be useful I will certainly have a go.

A very kind offer Simon, could it not be flipped so the hubs vertical? Don't think it would need the support structure then.

Paul
The reason it is printed like that is to minimise the support structure that impinges on the blade surface. The originator of the Thingyverse file intended it to be printed like that.
However with the much greater pitch more of the the blade is at a steep angle so significantly reducing the area that needs support.
In general a surface that is at 45 degrees or greater can be printed without support so it looks like it can done like this.

It still needs a small amount of support but it is mainly confined to the blade leading edge area which is far easier to sand.
By fiddling with the print parameters it now prints in under 2 hours and still weighs 10g (0.4 oz). How much does you current plastic prop weigh?
What I can't tell is what the blade surface will look like or its strength come to that. The only solution is to actually print one.
When its finished I will let you know.

Just for interest this is the blade deflection of the initial 9x6 printed prop with a 500g load on the tip.

It represents a total thrust of 1kg and is a worst case scenario as far as bending is concerned.
That degree of deflection would likely result from nearly 2kg thrust if the blade was correctly loaded.

Paul
Here is the 10x10 running on my test stand.

More of a desk fan than a prop!
The blades are quite 'thick'. A whopping 6mm max at half span but as a result it is really stiff.
The shaft hole is 5 mm diameter so I expect it will need some sort of insert. If you tell me the shaft diameter you intend to use I could print one, either to be glued in or it could be built in as part of another prop print.
It is running at about 5000 rpm and is absorbing 65 W. Probably a bit more than you will ever use!
It has been balanced and weighs 10.0 g exactly.
It is CW rotation, does that matter? If it does it would be simple enough to 'flip' the X&Y axis and print another.
If you PM me your address I will post it too you.