Excellence is our middle name. Our mothers were onto something.
Our Structure
Mechanical Structure Overview
1) Aluminum frame is heavier than it needs to be. In-flight forces on the airframe will be very small, and the quadrotor is never expected to fall more than eight feet (worst case) if it is going to crash. We can eliminate much of the excess weight introduced by the aluminum airframe.
2) The set screws that are used to mount the carbon-fiber rods eventually wear out the rods, making them loose.
3) Set screws on the motor mounts suffer from the same problem.
4) Motor mounts are difficult to align well.
5) There is no easy way to mount shrouds around the rotors.
Our goal was to improve upon the old design and eliminate the aforementioned flaws. In addition, we hoped to construct the frame in a manner that would provide easy access to device electronics while placing sensors directly at the center of mass of the aircraft.
Overall Design
The airframe consists of a central hub constructed using carbon fiber sheets and nylon blocks as shown. Motor controllers were sandwiched between two layers of carbon fiber to keep them out of view and give a clean appearance. Control electronics was mounted on the top of the airframe using adhesive Velcro.
Carbon fiber rods extend from each side of the central hub and support the four motors. Wiring was placed inside the carbon-fiber tubing, again for the sake of appearance.
Dihedral
The carbon-fiber rods extend from the main body of the airframe at ten degrees to lower the center of mass with respect to the motors. Our choice of dihedral placed the center of mass of the quadrotor at roughly the same location as the sensors of the autopilot. In addition, there were stability advantages to lowering the quadrotor center of mass.
Shrouds
Shrouds were constructed using thermo-formed plastic; specifically, we cut the tops off of popcorn bowls and attached them to the quadrotor. Carbon-fiber rods spanning the motor mounts held the shrouds, which were held in place using rubber bands. These shrouds were light, inexpensive, and easy to manufacture. However, they were a little too small, which made it difficult to align them so that the props wouldn't hit the sides of the shrouds.
Motor Mounts
Motor mounts were machined out of nylon blocks. Carbon-fiber rods were held rigidly using four 6-32 nylon machine screws, which compressed the nylon around the tubes.
Legs
Legs were constructed using carbon-fiber rods and nylon blocks as shown.
Summary/Conclusions
Aside from making the quadrotor "look cool," there weren't very many advantages to adding the dihedral. Stability gains weren't really that significant, and the relocation of the center of mass to the location of the autopilot sensors could have been accomplished in other ways. Adding the dihedral increased the complexity of the design significantly. If we were to do it again, we probably wouldn't have implemented dihedral.
The props had a tendency to rub against the sides of the quadrotor; the shrouds had to be adjusted after every flight. In practice, our design for shrouds wasn't as efficient as it may have been. Other teams actually surrounded the shrouds with a square carbon-fiber airframe; this appears to be a more suitable option.
The motor mounts worked well, but could have been manufactured better. Tolerances were off, holes were drilled in incorrect places, and the stock wasn't squared before being machined (meaning, measurements were off from the beginning). In addition, nylon screws were light, but their threads weren't strong enough to tighten the mounts sufficiently. The motor mounts were therefore able to slide back and forth more than desired. The design is valid, but could use adjustments if it were to be used in the future.
While we were able to "hide" the motor controllers to give the quadrotor a clean appearance, we didn't do enough with the other wiring. There was a "rat's nest" of loose wires on one side of the aircraft that greatly detracted from its appearance. While it definitely looked better than the quadrotor developed in the preceding semester, it could have been improved a great deal.
Batteries were mounted on the base of the quadrotor using adhesive Velcro. The Velcro was strong enough to do the job, but it was difficult to place the batteries in the same place every time they were mounted. This created a need to re-tune servo biases every time the batteries were replaced. Future designs should provide a means to place batteries exactly to reduce the number of pre-flight adjustments that have to be made.
