Unmanned Aviation Systems: Studying Aircraft Behavior

Ben Coughlan

Ben Coughlan prepares for the Outback Challenge

There are two aircraft involved with my research. Both are gliders, but have drastically different characteristics. As my research is focused mainly on the behaviour of the aircraft at this point, the design of the airframe is out of scope. For this reason, my supervisor and I selected commercial off-the-shelf airframes and their power trains to provide the most flexibility.

The quicker of the two is the Alex F5B. This is a popular competition ‘hotliner’ and one of the most exciting aircraft in our hangar. The fuselage is moulded with Kevlar and fibre glass with carbon reinforcement. The wing is 1.8 meters of moulded carbon fibre with a carbon spar. It has a 1.5 kilowatt Neu motor and a 16×10 inch folding prop which can deliver up to 4.9 kilograms of static thrust. It weighs just over 1.7 kilograms and can reach speeds well over 300 kilometres per hour.

At the other end of the scale is the Pulsar 4E. This is a much larger, much slower, and much lighter aircraft than the Alex F5B. This was selected for its potential carrying capacity and endurance. The Pulsar has balsa ribbed, carbon reinforced wings spanning 4 metres and a fuselage of fibre glass and carbon sheet.

Pulsar 4E

The Pulsar 4E is still under construction. We intend to fit it with a modest 550 watt Neu brushless motor, 3 cell LiPo battery, and a prop between 14×6 and 15×8 inches depending on results and load. All of it is expected to weigh up to 2.5 kilograms without a payload, and cruise at around 50-60 kilometres per hour.

The reason the Pulsar 4E was chosen for its carrying capacity is the Outback Rescue Challenge coming up in October 2012. This competition requires an aircraft to deliver 500 millilitres of water to a dummy stranded in the Australian Outback. The extra weight is a significant increase to the wing loading on most gliders. The Pulsar 4E is hopefully big enough and built to a high enough quality to accommodate it.

The aircraft pictured is my own plane, the Paprika. It has a wingspan of 2 metres and weighs about 1.5 kilograms. It is a similar but less extreme set up to the Alex F5B featuring an 850 watt Hyperion motor and a 12×6 inch folding prop with an estimated static thrust of 2.8 kilograms.

You may be wondering why these ‘gliders’ have such massively powerful motors onboard. A DC motor is most efficient when it’s off, and only slightly less efficient when it’s running at its maximum RPM. Anything between these two extremes is wasteful. The aircraft themselves are most efficient when they are cruising straight and level at an optimal cruise speed. The idea behind overpowering these gliders is to maintain cruise speed while climbing as quickly as possible (i.e., vertical), and run the motor for as short a time as possible at maximum throttle. This leads to long periods of gliding with short, aggressive climbs when needed.

One of the biggest challenges when working with these kinds of aircraft is space in the fuselage. Even the 4 metre wide Pulsar only has a 60 mm wide fuselage. This impacts the size of the avionics onboard as well as the selection and placement of antennas.

For the Outback Challenge, the Pulsar will have three separate radios on board with at least five antennas between them. For manual control I’m using standard 2.4GHz RC equipment. Telemetry, command and control, and data are sent over a 900MHz, 250kbit/s link. Finally, I have a 151MHz, 10kbit/s uplink for failsafe termination. Given the amount of carbon in the airframe and the lengths of the longer wavelength antennas, their placement takes a lot of planning.