How to 3D Print an Aeroplane - Build Complete + Discussion on Propeller Pitch
After a frantic start where I could achieve a lot in a short space of time the design process slowed to a halt due to running out of PLA. Like Franciscan monks trying to make wine and ending up with champagne, I managed to finish just before my new supply of PLA arrived by accidentally printing the final aileron on vase mode! More on that later....
During the build process I encountered a few problems due to thermal expansion of the filament. It's important when 3D printing planes like this that the layer adhesion is strong so I was printing with a higher extruder temperature than usual. This left many of the holes too small and required them to be drilled out. For the motor mounts this was ok because the settings we used included the 2mm bottom layer so there is plenty of material for the screws to bind into.
Usually I would use a single piece of piano wire for hinges, passing all the way through the control surface but due to the holes being too tight I ended up drilling them out and using a small piece of piano wire at each end which worked great.
The other part where thermal expansion had an effect was the wing mount holes. These would normally have BBQ skeweres passed through for the rubber band to hold on to but because they were too small I ended up using 4 M2 screws, which again worked really well. I am aware that rubber band attachments for the wing don't really add to the 'scale feel' of the aircraft, bu they are secure and simple. It would be easy to design a screw attachment for those who wish.
In terms of strength I have found that the properly glued surfaces are really strong, and not usually the point of failure. This shows me that a carbon spar isn't necessary for this wing.
I have run the motors and it has a great sound to it, interestingly as the trust increases with throttle it seems to top out prior to max throttle, this may partly be due to cavitation. I also suspect that as the pitch of the rear prop is the same as the front but has higher speed airflow, it is potentially reaching its max airspeed, where its angle of attack is zero and thus has no thrust to add? Since I can't fly the aircraft under lockdown I might see if I can do some experimenting.
The real Cessna 337 has constant speed propellers, this means the pitch of the blade is automatically adjusted so that the propeller maintains an RPM set by the pilot in the cockpit. As power on the engine is added (measured by pressure at the inlet manifold) the propeller moves to a higher pitch to absorb the power, and not speed up to a higher RPM as a fixed pitch propeller would. This would mean that with both engines set at the same manifold pressure and RPM the pitch of the rear prop would be greater as it is receiving faster moving air from the front prop. If the air was smooth this would mean it could provide a similar amount of thrust. Unfortunately the air is not smooth so there are losses.
Potentially we could use a higher pitched propeller on the rear prop? I'm happy to take advice on this?
I mentioned my champagne moment. I decided to slice the final aileron and see how much filament I needed so I could go measure my filament. Imaging my suprised when, after measuring, I had enough! I started printing. When I returned I found it was only printing the outside wall in spiralled mode - the mode I had been playing with in Cura previously.
In this mode it doesn't have the strength of a normally printed part, but more importantly it doesn't have a top! I decided to print a thin slice of the top and and see what the part ended up like with it glued on. Rather than waiting for more PLA to arrive.
It worked out great, about 10g lighter as well as half the print time than the other aileron. It was pretty fiddly trying to feed the piano wire hinge through. Had I have thought about it I could have fed it through before gluing to top on.
Thanks for sticking with me, sorry I can't bring you a maiden flight video yet. If anyone is building along I would love to see your efforts, post here or tag me @rc3dprint on instagram.