3D printed props

Paul
At the moment it is a lightweight 'hollow' e prop. To run it on IC it would have to be printed solid, as was the 6" prop, both for strength and inertia.
About the only diesel I have that would suit such a prop would be my vintage Mills 1.3 but I have no fuel!

I do intend to fly a printed prop. It will be a 10x3.8 in a pusher to avoid the risk of ground contact.

This is the 10x3.8 prop.

It has been scaled from a Thingyverse 9x4.7 which is about the best design of printed prop I have seen so far.
Using Jon's pictures I have actually managed to convert the original Thingverse stl file into a FreeCad mesh file so a suitable 'insert' has been permanently added. Many thanks Jon.
A video of the prop on the test stand running on a 3s. It is absorbing 155W.

This power level will be plenty to hand launch & fly the Big Dragon.
The Big Dragon actually has a more powerful 250W motor so I will only try it at full power when it is at an altitude sufficient to be safe from a possible 'flying' prop blade! :woohoo:
Of course only when the weather has decided to calm down a bit!

It is interesting just how many surface defects there are on a printed prop even after much sanding when you spray on a really shiny paint.
The printed and commercial 10x3.8.

This was with a second coat after the first was cut back with wet and dry. I recon with a primer coat and proper wet and dry top coats(s) it would be possible to get a true gloss finish.
The printed prop is lighter at 7.7g against 12.4g for the commercial.
The 10x8.3 on the Big Dragon fuselage.

It certainly gives plenty of thrust but no full power test as it is not much fun holding a pusher by the nose if the prop does break. :woohoo:

When storm Eunice has passed (red warnings on the Devon north coast, yellow for almost all of England) it might be calm enough to actually fly it.

"My" prop arrived today Simon, many thanks, I'm going to give it a coat of acrylic varnish and I need to make a driver for it, take it epoxy is OK?.
May be a while until I test fly it, I'm proposing to time a flight using 200 turns with the standard prop then try the printed job with the same amount of turns.

No problem with epoxy. I have used it myself on PLA
Varnish no problem either but I fear it might highlight all the surface defects. Wet and dry with plenty of water and repeat coats until it is matt all over then a final coat is the answer. I am in progress doing this on a prop at the moment so I will let you know.

Flight testing is the only real test. My guess is be prepared for some disappointment.

All purely experimental Simon, I'm only going to varnish it inorder to protect it from damp, not worried about a standard of finish....I'll report back ( try to get wifey to record the test flights).

I will try soaking a printed prop in water (the one I did the bend test on) to see if it becomes any more brittle.
I understand the humidity problem is only for the filament before it is used. Once is has been melted and laid down I don't believe it is anything like as susceptible to moisture.

Just a confirmation.
One blade of a printed 6" prop in a cup of water all night (8 hours).

Still flexes the same in the morning.

And recovers the same too.

The only danger would be with a printed hollow blade prop as any water that leaked through the skin would do nothing for the balance. :woohoo:

Spent a bit of time to get a reasonable finish on a printed prop.

A nice metallic bronze.
It would be interesting to see what difference it made particularly to the g/W figure but that would assume I had tested it before I painted it. I didn't. Doh! :woohoo:

Apart from the above prop I have printed quite a few more in the course of this development.

And that does not include those that failed to print properly and went straight in to the bin!
ps
Paul
The third one in from the right is a direct replacement for the one you have, just in case you break it.

I now have a pair of 7x6 props.

They suit the Blue Dragon pusher.

That will be my second plane to fly a printed prop, if the weather ever settles down.

The 9x4.7 STL file - do you have a specific reference for this? e.g. a thingaverse number? I find a
-https://www.thingiverse.com/thing:2605230 which is a 9x4.5 from 2015. -Jon

jon
I was wrong. The file I am using on the latest props is a 9x4.5. It number is thing:2605230
Sorry for the confusion.

I see on the thingaverse page < www.thingiverse.com/thing:2605230/files > there is also a 3 blade version. It would require more support to print axis-vertical. I could live with that and having established a method in freeCAD to adapt hubs as needed I should be able to adapt to the various motor shafts I have. I fear that it will be problematic to design a 'prop saver' for a 3 blade - so it would probably be most useful as a pusher.

I know a 3 blade will potentially deliver more power within a constrained diameter with less efficiency. The shape and performance of the 2 blade has been commented on. Should we expect the 3 blade variant to also perform fairly well? -Jon

jon
You are quite correct that a 3 blade will absorb more power for a given diameter but at a slightly reduced efficiency.
The "9x4.5" blade shape is more efficient than some on Thingyverse but still does not truly match that of the commercial types.
I would suggest its blades are simply too thick likely due to the need to maintain stiffness resulting from the lower mechanical properties of the 3D printing process.
As an example the blade thickness at half span of the the 3D printed 9x4.5 is 3.9 mm whereas on a commercial 9x6 is only 1.6 mm.
At small aerofoil sizes, like a model prop blade, the thickness of the section has much greater effect of the power to drive it through the air than the loss of lift (thrust) resulting from its less than ideal section.
In other words a thinner blade spins faster for the same thrust but uses less power to do it.

I would be interested to know how is the balance of your 'edge' printed props. In my case is was very poor. The blade printed trailing edge down was quite a bit heavier than the other blade and rather beyond what I was happy to sand off to achieve balance.
My current printed props are now printed 'flat' on the bed to ensure both blades come out exactly the same and just put up with the time & effort to remove the effects of the support structure. In fact I print them up side down as it is easier to remove and sand the support structure from the upper blade surface.
One benefit of the amount of sanding required is it allows the prop to be very accurately balanced.
A well balanced and true running prop can make a measurable improvement to its performance.

Jon, I suspect you don't know that a single blade prop is even more efficient than a 2 blade, the free flight guys use them.
All the 3 bladed props I've seen don't look the be a very good airfoil shape to me.
As for bending in flight, I have a number of folding props that unfold to point almost directly forward. I thought this was a problem, no, under power the centrifugal force holds them at 90^o to the motor shaft.
I've spoken to Mike, he's going to organise a way to upload stl files, They will only be available to registered members of the site, not guests.