CUTLAS MK1 was a technology demonstrator for wing folding and control surface actuation. The initial design uses a canard wing setup, with the main wings at the back, and smaller canards at the front. The reason for this choice was to help 'lock' the wings into position as the fold out. Since the wings swing out backward, once they are fully extended they can simply reach a hard limit and the drag forces in flight will tend to hold them in place, so no lock is required.
Because the design was tube shaped, the intial choice for propulsion was an electric ducted fan (EDF). This seemed to fit the design as EDFs need to fly fast to be efficient. Unfortunately, there are two considerable problems with this design.
To fit inside a tube, the wing chord must be no larger than the diameter of the tube, and the half wingspan must be no longer than the tube itself. This ends up creating relatively small wings, which mean that the plane would need to fly very fast to stay in the air. While not necessarily a problem, I wanted to be able to test the plane on small fields, which is difficult with a very fast plane.
Canard planes are notorius for being hard to control, and this configuration also suffered from a difficult CG, as many of the required electronics were very far in the back of the plane. In addition, the total mass was far too high and resulted in incredibly high wing loading.
After MK1 proved unflyable, I wanted to test the fuselage with a simple foamboard wing design to better understand characteristics at this scale and how well the electronics performed. I swapped the folding wings for a set of simple foamboard wings with carbon fiber spars running through. It might not be pretty, but at least it looked like it might fly!
Unfortunately the design was still too heavy and underpowered with the EDF. I decided that moving forward, I would abandon the EDF and move to conventional propellers.
It is worth noting at this point that I am using two 6mm carbon booms that go the length of the aircraft and act as hardpoints to mount everything to. These ended up being quite flimsy and I broke several throughout testing of MK1.5.
At this point, the only was to create a small, relatively slow, and low weight tube launch RC plane seemed to be by adding flexible wings. I chose coated ripstop nylon (commonly used on kites) because of the very light weight and price. This would allow me to create relatively large wings that could just fold up without worrying about tube dimensions.
The flexible wing design solved a lot of problems. High wing area was possible for very little added mass. However, several new problems were introduced. Moving away from a canard design to a conventional v-tail meant that the wings would have to be held forward entirely by the elastic. Mantaining strong enough elasic proved to be difficult and many flight tests resulted in the wings folding mid flight or very large vibrations. Control surfaces also proved challenging. The way I set up ailerons and v-tail surfaces was by having a servo at the root trailing edge that kept the surface in tension and could pivot up or down. This appeared fine on the ground but seemed to have no effect in the air, potentially due to the tension not being high enough or the wing deforming in unexpected ways.
I decided to switch the two thing carbon spars for a single square cross-section carbon fiber rod. Although slightly heavier, this was so much more rigid, and I have yet to be able to break one (and not for lack of trying!). The square cross section means that any parts that are mounted are at very little risk of twisting compared to a circular tube.
MK2 was able to fly several times for a few seconds! However it was uncontrollable in the air and the wings would fold back at high speed.
MK3 has drastically more wing area than MK2. This should help a flexible wing perform well, since it doesn't have a natural airfoil that can be made with a solid wing. The other large change to mk3 is that the entire wing and tail surfaces pivot up and down to act as control surfaces. This is important for a couple of reasons.
The lack of control authority in MK2 would hopefully be fixed by making the tail a v-tail stabilator design. Although the v-tail has limied throw because the two surfaces can intersect, the ~20 degree throw should be plenty for control. Since the main wing also fully pivot on separate servos, I can get aileron control but also (crucially) angle of attack can be varied. MK2 may well have had problems with AoA as the flexible wings would not create lift in a neutral AoA. Just by throwing both like gliders, the difference in lift force was like night and day.
To allow the wings to pivot up and down on a servo in addition to folding back, the folding mechanism was miniaturized and put on a pivot which could then be controlled by the servo through a pushrod. Unfotunately, this miniaturization meant that the elastic keeping the wings out had very little moment arm. The wings would easily fold back and the plane would drop from the sky. I tried using thicker elastic but ultimately the mechanism needs a redesign to give the elastic a larger moment arm.
MK3 is still a work in progress and shows promise of flying soon! While I was hoping to redesign and test by the end of the semester, I recently got covid and will have another update by January!
"This isn't flying, it's falling with style"