Variable Pitch Diesel Quadcopter
Part of the SubT Challenge is to develop a maneuverable underground aerial vehicle with long-term endurance. A variable pitch quadcopter offers the most maneuverability because its blades can change pitch, hence offering greater control of the lift generated. However, current battery-powered quadcopters only have a 20-30 minute battery life; and field experience demonstrates that if sensors are added, the performance time is reduced to 5-8 minutes. For adequate mapping purposes, the aerial vehicle must fly for at least 45 minutes of a three-hour circuit. It was determined that converting a battery-operated drone into a liquid fuel-powered quadcopter, or a hybrid, could significantly increase operation time. Because DARPA rules do not allow gasoline power, something that had never been done before had to be accomplished — the creation of a diesel-fuel powered multi-rotor aircraft.
Research
Current Market
After researching different products on the market, it was determined that existing gas-powered helicopters and quadcopters could not be modified to meet minimum size and endurance capabilities. Additionally, other long-endurance drones, such as hexacopters and octocopters, operate on battery or hybrid-battery/gas power.
Batteries
Several of these models incorporate newer lithium-ion batteries, which have a high C rating (high discharge rate) and longer life. It was hypothesized that the battery could power drone ascents and maneuvers, while a diesel motor could maintain all other functions. In theory, this combination could give maximum functional capabilities and endurance.
Modeling the Stingray
Retrofitting a variable-pitch quadcopter with new batteries, gas tanks, motors, and other accessories required close inspection of the assembly, mechanisms, and design of the machine. In the absence of part measurements and/or CAD models by the manufacturer, individual accurate measurements needed to be obtained (within 0.01 inch) for every part. This action was critical so newly-designed parts could be properly scaled and new weights calculated. Beginning assembly on the actual quadcopter using SolidWorks led to the realization that some measurements were incorrect: the four propellers did not have the same radius if a circle was drawn from the middle of the quadcopter. Several more hours were spent meticulously re-measuring every piece.
The new measurements prompted new successful designs, but the vehicle was not fully developed in time to test in the Tunnel Circuit challenge. This is an ongoing project and the team hopes to use this machine in the Urban and Cave Circuits.