Module 4.4
How can the separation of unmanned aircraft be monitored and maintained (among other unmanned aircraft and manned aircraft) in the National Airspace System (NAS)? What considerations need to be made for varying sizes (i.e., Group 1 to 5) and airframes of UAS (e.g., fixed-wing, rotary-wing, and lighter than air)? What technology is currently employed by manned aircraft and is it adaptable for use with unmanned?

In general, upcoming regulations for UAS should be constructed by taking into account the size, weight, and mission of the UAS being used.  The regulations should not be overly burdensome on the nascent UAS commercial industry while meeting the intent of safety regulations.  To some extent it makes sense for small UAS operated within line of sight to be self-separated by the operators.  This means that each small UAS operator would be responsible for overseeing the flight of the UAS and for avoiding obstacles as well as other UAS and manned aircraft.  Granted some training would be required so that the operator can understand how far they must operate from objects, UAS, other manned aircraft.

In the past, self-separation of aircraft has been done by the pilot ensuring that they stay “well clear” of other aircraft.  When an automated system needs to provide self-separation, and ambiguous term such as “well clear” is not helpful.  Weibel’s work shows a probabilistic approach using a predefined risk threshold to define the boundary of “well clear” numerically so it can be acted upon by an automated system. 

Well clear performance would be assessed through safety assessment, as part of an overall target level of safety analysis for midair collision. An unambiguous boundary provides a clear measure of when execution of well clear separation has failed.

An analytical definition of well clear, such as the one proposed above would be implemented in a UAS collision avoidance system as a desired avoidance boundary. Avoidance actions would be designed to be executed by automated systems or a human in the loop who would seek to maintain well clear through horizontal or vertical maneuvers.

The horizontal contours of 5% collision risk suggest that a well clear boundary based on this threshold would extend approximately 8,000 ft ahead of an aircraft, 3,000 ft. laterally, and 3,000 ft behind.

By framing the threshold as a separation standard, boundaries can be derived based on an acceptable threshold of risk that is incurred from violating the threshold. This has the additional advantage of being comparable to the expected decision-making of pilots in executing self-separation. In the discussion below, a potential well clear boundary is proposed, and additional considerations and future work are outlined (Weibel, 2011).

Automated self-separation will occur most likely with a UAS that is operated out of line of sight range of the operator.  Video cameras may not be sufficient for the operator to provide self-separation of the UAS.  In this case automated systems operated by cell tower signals may be appropriate.  The automated system would need to actively “sense and avoid”. 

ADS-B (Automatic Dependent Surveillance Broadcast) is one way that UAS could provide their location to other UAS, manned aircraft, and ATC.  ADS-B is already being rolled out and will be required by all manned aircraft by 2020.  Smaller ADS-B systems are being developed that could be carried on a UAS.  By using this technology UAS could broadcast their location, while seeing others’ location.  Traffic Information System technology could be used to alert the UAS operator to nearby aircraft.

 

 

 

References

Weibel, R. E. (2011). Establishing a Risk-Based Separation Standard for Unmanned Aircraft Self Separation. Ninth USA/Europe Air Traffic Management Research & Developement Seminar, (pp. 14-17). Berlin. Retrieved from http://www.atmseminar.org/seminarContent/seminar9/papers/64-Weibel-Final-Paper-4-12-11.pdf

The Quadcopter, Then and Now

The quadcopter was actually an early design for vertical lift.  The design showed promise because the short rotors were easy to manufacture, the counter rotating sets of blades cancel the torque so the craft does not spin in a circle, mechanical linkages aren’t needed to vary rotor pitch, and the load is less per rotor since there are 4 instead of just one.  Also, as opposed to the single rotor copter, the quadcopter balances torque by using motors that are creating lift.  In a single rotor copter, there is waste in having a tail rotor that doesn’t create lift or thrust.  Other factors that lead to a prominent single rotor design was that a payload naturally hangs below a single rotor in a stable manner and there could be one engine, tied to only one rotor (reduced complexity).  The disadvantages of the early quadcopters were the heavy weight associated with 4 propulsion systems and the overwhelming workload on the pilot to independently control 4 rotors simultaneously.  Early flying designs included the Oemichen No2 (1920), De Bothezat (1923), Covertawings (1956), Curtis Wright VZ-7 (1958)

Figure 1: Oemichen No. 2, 1922 [12]

Figure 4:Curtiss-Wright VZ-7, 1958 [9]

Probably the significant recent developments that are allowing quadcopters to make a comeback and be viable in the UAV field is the material weight reduction and computing power.  Advances in material science allow for very strong, but very lightweight materials to be manufactured.  Lighter weight puts less strain on the rotors.  High power density lightweight motors enable very small and inexpensive, but powerful motors to be used.  Also, the drastic improvement in computing power enables miniature flight controllers and electronic speed controllers that automatically adjust the speed of each of the 4 rotors to keep the torque balanced, ascend and descend, forward/backward/side to side movement, and balancing for disturbances such as drift and wind currents.  Everything is handled by the computing system so that the pilot does minimal compensation/correction control and can focus on flying where he wants to go.  Quadcopters and other multi-rotors will increasingly become prevalent for Unmanned applications because of their mechanical simplicity and ease of control.  For safety reasons, I think that a manned quadcopter may not happen.  If a conventional single rotor helicopter looses its engine, the helicopter can perform an autorotation landing and have a chance of the pilot surviving.  With a quadcopter, if one rotor/motor is lost then the craft will be unstable and crash.

 

References

History of Quadcopters and Other Multi-Rotors. (n.d.). Retrieved from Krossblade Aerospace: http://www.krossblade.com/history-of-quadcopters-and-multirotors/

Ottaviano, N. (n.d.). History. Retrieved from Quadrotor Drone: https://sites.google.com/site/quadrotordrone/home/history

What Makes the Quadcopter Design so Great for Small Drones? (n.d.). Retrieved from Forbes: http://www.forbes.com/sites/quora/2013/12/23/what-makes-the-quadcopter-design-so-great-for-small-drones/