V-22 Osprey Project - Design Changes for v3
It's been a busy end of 2021, and as we start 2022 I'm getting closer to building version 3 of the Osprey wing. The single biggest problem I've faced is the amount of backlash in the direct spur gear drive of the tilt mechanism. It introduces some nasty instability that, left unchecked, would likely destroy the model in flight.
I've investigated other tilt mechanisms, but the one I keep coming back to is the rack and pinion design that Tom Mast uses in his model at Rotormast. His design uses a high-torque digital servo with a small pinion gear driving a rack attached to the rotor nacelle. The servo has been modified with an external potentiometer that's driven by a gear on the translation tube to give positional feedback to the servo. I like this design a lot because 1) It works, and his model is proof, and 2) it requires less torque from the servo and has less backlash.
I ordered the pair of modified servos, potentiometer, and rack from Tom's site and started modeling the new system in CAD. I quickly found a problem, though. The large rotor drive gear would interfere with any rack and pinion system because it's located above the rotation axis. Tom solved this problem by running a 2-stage gear reduction on his model. Instead of directly copying this design, I opted to extend the main rotor shaft and position the main gear below the translation axis. I was able to purchase some solid stainless 5mm shafts to machine my own. I don't have a mill or a lathe, but I do have my grandfather's
Emco Unimat Mk.4 mini-lathe/mill that with some work was able to make some accurate holes for the rotor and gear bolts.
In the previous design, I didn't like the external servo linkages for the flaperons. The servo was inaccessible without cutting the wing open, and it didn't look very nice. The V-22 has quite a thick airfoil, so I thought that a 9-gram servo could fit in the wing and drive the flaperons directly. I quickly modeled this up in CAD and found that I could fit a servo in the wing and directly attach it to the flaperon. I designed a servo mount hatch that would bolt into the wing and provide maintenance access without damaging the wing.
A minor change that will likely have larger implications as I move forward is that in the previous model I tried for the scale look with the wing swept forward a few degrees and a few degrees of dihedral. That model also had zero degrees of wing incidence built into the center mounting block. To simplify, I went with a straight wing this time and added 3 degrees of incidence in the mounting block. I'm hoping that in forward flight it should generate a bit more lift from the symmetric airfoil.
In the 3D printing world, there is a new PLA formulation on the market that relies on temperature-activated foaming agents to create a lightweight, but pretty durable structure. Available from Polymaker and eSun, this new PLA-LW is being adopted quickly by 3D printed airplane manufacturers as a replacement for regular PLA and PETG for parts that don't need to be super strong. I purchased a roll of the eSun LW-PLA and started to rethink certain parts in the wing design. Could I use this material as a replacement for shaping balsa stock for leading and trailing edges? Could I use it to print the bulk of the aileron itself? This redesign is giving me a chance to try these out.
From the test prints with ribs and other components, the LW-PLA is 50%+ lighter than the stock PLA. It's certainly not as strong and feels like a plastic-y foam. With a minimal infill, the structures can be quite rigid and light.
As of this writing, I've completed the left wing and rotor assembly and have started printing the right side. I'm waiting on some carbon fiber rod from China that acts as the spar and once I have that I'll start assembly of the right wing. Here's hoping the new design works!
https://aggiehobbies.blogspot.com/2022/01/v-22-osprey-project-design-changes-for.html
