3D Printed RC Plane Design
Categories: radio control
I decided to build a 3D printed model airplane.
The RQ-3 DarkStar was a prototype autonomous stealth UAV from the early 1990s. Typical for prototype aircraft, the RQ-3 is an aircraft with a very strange design; the fuselage looks like a flying saucer, the wings are quite long and very subtly forward-swept, and the plane lacks the tail or vertical stabilizers seen on most planes. Upon first glance, it might not even be clear which direction the RQ-3 should fly. As a result of this unusual design, the RQ-3 is a fairly unstable aircraft, and heavily relied on computer control to maintain flight. This was a huge challenge back in the 1990s and flight stability problems contributed to the eventual end of the program, but modern microcontrollers, software, and electronics are much more capable and make this possible.
Design Outline
First of all, note that I didn't finish the CAD design before I started building the plane, so this is just a summary of the results of the design process. This helped me both confirm that certain parts of the design made sense and helped keep me motivated about the design. This post focuses on the CAD design, and subsequent posts will describe the process of actually building the model.
I ambitiously decided to design the plane to have a 2 meter wingspan, and because of that, the wings must be detachable in order to make transportation simpler. To fit on my 3D printer, the model is divided into smaller pieces that will be glued together, for the most part. The wings are reinforced by a pair of carbon fiber tubes. The RQ-3 uses an internal jet engine with a intake on the front and a "stealth" exhaust nozzle on the top back of the fuselage; so I will be using an EDF inside the fuselage to remain faithful to the real aircraft.
For the wings, I used a MH60 airfoil, which is specifically designed for tailless airplanes and flying wings.
In order to maintain stable flight without a tail, the normal control surfaces on the wings are not enough. The RQ-3 has multiple control surfaces on each wing that the flight controller can set independently. I implemented this with split ailerons, where a pair of control surfaces overlap but can be moved independently. If they move together they act like a normal aileron, but if they split apart, they cause extra drag that will pitch the aircraft towards one side or the other.
This also allows me to place the servos close to the root of the wing, so the wires are shorter and the weight is more centralized.
I want the option to add sensors such as cameras for FPV, so there is a modular compartment on the front where a module with a camera could be placed.
The fuselage has an internal structure that houses the EDF, and provides mounting points for electronics and batteries. Because the air duct takes up most of the center, the batteries will be placed on the side, and I will shift them back and forth to adjust the center of mass.
Covering the internal structure are some skin pieces that screw on in a few places and provide the aircrafts unique profile. They have to be removeable to change the batteries and do any other internal maintenance, so they can't be too hard to remove; a few screws might be a bit of a pain but should be secure.
Designing for 3D Printing
Nowadays, there are 3D printing filament materials that are well suited to RC planes; specifically "LWPLA", or lightweight PLA, which is normal PLA with a foam structure to reduce weight. The weight of parts can be reduced further by printing the parts in "vase mode", which only prints the outside perimeter in one continuous extrusion (to the extent possible, depending on geometry). However, just the outside shell does not provide enough rigidity to make useful parts.
The solution is to model in thin cuts from one surface to just before the other surface to add internal structure. The process I use in Fusion 360, my CAD program of choice at the moment, is as follows. As an example, I will add internal spars to a small section of faux-airfoil with a hole for an additional carbon fiber spar. This is very similar to what I did for the wings in my RQ-3 design.
First I create a sketch from a top view and draw lines along where I want the spars. I chose one to follow along the center line of the internal hole, and one spaced a bit further back. Note that the spars don't have to be parallel to the wing, and you can get better rigidity with crossing or angled spars. I use the thin extrude tool to extrude a 0.1mm thick wall all the way through the model.
I then use the combine tool to intersect the airfoil with the thin walls. Note that the thin walls are the target body and the airfoil is the tool body (which we keep with the "Keep Tools" option).
The result (with the airfoil hidden) should be these small pieces left over. The spar that goes through the internal hole is divided into two parts.
Next, I offset the top face of the spars by -0.5mm so that it doesn't quite meet the top surface of the airfoil. Note that I don't offset the internal faces created by the internal hole.
Next, I once again use the combine tool to cut away the spar bodies from the airfoil body (without keeping the tool bodies this time).
From the side view, the thin cuts into the airfoil are apparent. The cut of 0.1mm is thin enough that the extrusion lines will bond together when the model is actually printed, as well as connect to the top of the airfoil with the gap of 0.5mm.
Slicing the model in spiral vase mode reveals how the spars are incorporated into the continuous print. Its also nice that the seam can be hidden on the spars internally, if you are printing more complex geometry that can't be printing in one continuous extrusion.
