by Allan Flowers
Early CAD models, Etc.
There are multiblade propellers available in the size needed (e-Calc) but the hub would need drilled out for a nearly 2” diameter mechanical system. Therefore the propellers and hub would have to be hand fabricated. Not having the ability to NC mill the unit, it would have to be made largely of laser cut wood and aluminum sheet. The thing would have to hold together at up to 7000rpm without flying apart – so accurate balancing and mechanical strength were critical.
The early CAD designs were eventually married to the available components from McMasters-Car and B&B Manufacturing (maker of sprockets and belts).
The idea was to make and test the drive system before embarking on the rest of a two year’s long build.
Eventually an APC propeller was selected (22x10E) and two were purchased. These were fixed in a mold where resin collars could be cast around them. This element was cut off and finished. In the meantime, my laser cut metal and wood parts had arrived and the hub unit was assembled.
Using a crude wood test fixture, I determined the inter shaft distance between the sprocket centers. Later I was informed about a website calculator which verified my empirical test results to within .05mm.
As the hub was being created and balanced, it was necessary to design and build the forward fuselage portion, to complete the drive system. This would enable a test of the drive unit – a critical step to determine its feasibility. The system would have to produce sufficient thrust and not fly apart.
The hub unit needed to act as a fan to bring cooling air past the motor which, being located rather far back in the airplane, could be prone to overheating. The final part is designed as a centrifugal pump and should keep the motor fairly cool.
As the CAD design progressed, other components were being finalized for fabrication. One item was the nosecone which was a challenging part to make. Ultimately a 3D printed part was made by Thomas White, a prominent scale modeler on the East coast who has large scale 3D printing capability.
The fuselage was built around the aluminum tube upon which the propeller would run. The forward part was completed and the tube, motor and drive system installed for testing. I needed a “test sled” to hold everything and allow thrust measurements to be made.
The laser cutting included the floats and rear fuselage parts so I moved ahead with those things while finishing the front fuse. Used extensively throughout was a .01” resin composite sheet (brand name – Fliteskin). This is a waterproof, very strong sheet material assembled with expanding Gorilla Glue. It is light enough and nearly bulletproof. I tried to economize on the wood structure but the weights are still a bit more than desired.
One necessary study involved how to transport a finished RC airplane in my little car. After making a rough model of its interior, I could digitally maneuver the plane model into the “car”.