Greetings fellow drone enthusiasts…
Drones are offering unprecedented aerial perspectives to amateurs, but their high price and often steep learning curve make them inaccessible to may hobbyist that could be aided by them.
This project is intended to develop a drone to compliment people's current interests and hobbies, while not replacing them. That requires a drone which is reliable, easy to use and can be thrown in an equipment bag, the back seat of a car or quickly tethered to a backpack without a second thought.
About 5 years ago my brother and I moved from Massachusetts to Arizona for an aerospace job in The Valley. We had always enjoyed New England's beautiful forests and top notch climbing destinations, so the idea of being surrounded by craggy mountains and winding canyons was too good to pass up. One particular destination that caught our attention was Granite Mountain in Prescott AZ, a 500 ft cliff of Proterozoic granite surround by low laying shrubs, conifers and junipers.
Excited to scale this crag, we packed our climbing gear and set out on the 2 and a half hour drive from Phoenix to Prescott. Before long we were hiking along the well marked trails of Prescott National Forest.
As many climbers know, when approaching a cliff there comes a time when you must leave the well marked and maintained hiking trails and pick your way along the often confusing and poorly marked climbing access trails that lead to the base of your climb. Granite Mt was no difference. After a few hours of hiking we knew we should be close to the access trail, but in the high desert, surround by low shrubs, towering boulders, washes and game trails we realized that identifying and following the trail would be no small feat. Time and time again, we started up what we thought was the correct path only to back-track when the trail went cool. Eventually, the afternoon monsoons began gathering in the distance and it became obvious that this trip would end like many others, a lengthy, frustrating hike with 40+ pounds of gear that would not be used.
There would be 2 more attempts at Granite Mountain before we found the right path and completed our climb, but once at the top there was a realization.... Looking back down over the approach from 500 ft above, the access trail was obvious. The bird's eye view clearly showed a long winding path unobstructed by boulders or spiny shrubs leading right from the hiking trail to the base of the climb. We knew what we needed the next time our climbing mission was threatened by a confusion approach; a compact, autonomous drone capable of providing aerial snap shots of our surroundings.
Leveraging our aerospace backgrounds, our skills with embedded electronics and 3D modeling/printing we started on a drone which would be free from what we saw as their current limitations and created a drone which would be rugged, inexpensive, compact and intuitively work without any prior experience in R/C hobbies.
From the beginning we knew that the traditional quad or multi-copters would be challenging to pack due to the many arms and propellers protruding from their bodies. Also, each arm requires a separate motor and speed controller adding to the total weight, expense and complexity of the drone.
To overcome these issues, the configuration of our drone is based on a co-axial, fixed pitch helicopter. This configuration allows for a tighter profile and requires only two motors and speed controllers which can be mounted safely inside the main body. To further reduce the overall size and allow the optimum positioning of sensors we wanted to utilize the space not just below the plane of the rotors, but between and above them as well. This was realized by mounting the rotors on a fixed, hollow, carbon fiber shaft (which we refer to as the “spine”). The spine runs the length of the drone and not only allows routing of electrical cables past the planes of the spinning rotors but gives the drone wonderful stiffness and strength.
Propeller blades offer another challenge to packing a drone. They tend to be fragile when bent and will catch and snag on other equipment. To overcome this, our drone utilizes torsion springs in the blade’s flapping hinges to keep them tightly folded along the profile of the drone when not spinning. In flight, however, the centripetal force of the spinning rotors overcomes the springs and allows the blades to extend outward. Testing has shown that these torsion springs have negligible effect on the dynamics of the blades even at low RPM.
The Drone is designed to be a fully autonomous and capable of flying a variety of missions which are pre-programmed by the user, using a mobile app and uploaded via Bluetooth. Once the mission is selected/defined and uploaded, the drone is hand launched to commence its mission: flying a circuit, capture images or sending a text message from a better vantage point than the user on the ground.
To increase the packageability of the drone, there are no landing features which protrude from its body. Instead, power is cut to the motors and the torsion springs mounted in the blade’s flapping hinges fold the blades along body allowing it to dive towards the ground. While falling, the drone tracks its altitude and descend rate and when certain conditions are met, power is restored to the rotors extending the blades and braking the fall. Finally, just prior to impact with the ground, power is again cut, the rotor blades fold into their retracted position and the drone makes a soft impact with the ground. Mission details and images are then downloaded to the mobile app.
Current Status of the Project...
The first cut at this drone was intended to prove out several fundamental concepts such as the electrical routing, gear ratios, 3D printed design, flybarless stability and the sensor’s resistance to vibrations. However, because it still lacked several key features it was difficult to call a prototype and instead was referred to as the “Flying Test Stand”. Eventually more features such as GPS/barometer, ultrasonic range finder, torsion springs, aeroshells, etc. where added. This allowed us to begin outdoor testing to investigate ascent rates, GPS tracking and hand launching all of which were successfully demonstrated. Shortly, testing will begin on giving the drone a greater level of autonomy and stringing together multiple mission events while also increasing the durability and robustness of the drone.
Hope to update everyone with videos the more progress soon!!!