Droning Around

By By Frank Lombardi | February 11, 2015


Plenty of chatter this past year was devoted to the clear uprising of unmanned aerial vehicle (UAV) technology. This tech is now within the purchasable grasp of the general public. It has spread, almost virally, and as a result has outpaced much of the development of rules for use in the National Airspace System.

The FAA is scrambling to implement some sort of legal control over the use of “drones,” as they now have been irrevocably called. While this is no doubt important, being a science guy, I would rather talk about another aspect of their control, as in their stability and control, and handling qualities characteristics.

At first you may not see the importance or even relevance of assessing handling qualities of these autonomous or semi-autonomous vehicles. Perhaps it’s because at this point, the most popular retail models exist somewhere between a hobbyist’s toy and a professional-grade remotely piloted vehicle (RPV). But as drone users invent new “missions” and developers invent new models, the characteristics that are desired for adequate mission performance and safety will begin to take shape.

The terms “handling qualities” and “stability and control” are subsets of the more-encompassing term of “flying qualities.” How an aircraft acts aerodynamically, along with how it handles in the hands of a pilot while accomplishing a task, make up its overall flying qualities. The fact that drones are not “physically” in the hands of its pilot creates a new area of study.

The military developed flying qualities standards to assure adequate mission performance and flight safety “regardless of the design implementation, flight control system augmentation, or impact of other related subsystems.” Loosely translated and made relative to RPVs, this means that no matter how technologically advanced you think your drone may be, having a design guideline to define and ensure safe operation is certainly a good idea and may become a regulatory requirement of the future.

Being more mission-oriented around complex tasks, UAVs have different design requirements than most civil aircraft. Many of these drones are of the “powered lift” variety and are much smaller and lighter than full-scale aircraft. As such, they react to gusts and turbulence differently due to their low inertia and high susceptibility to air viscosity. Their low cost and light weight can lend to aeroelastic effects, such as blade or fuselage flex, negatively affecting stability. Yet even the simplest of them have sophisticated servo-based flight control systems for navigation and stability augmentation. This can put a premium on power requirements, making them especially sensitive to changes in payload weight, or when flying in turbulent air. Flight control systems and displays are software-based, and so they have complex failure modes that must be evaluated. Without a pilot aboard, redundant systems tend to be avoided for weight savings.

Due to the lack of natural “seat of the pants” cues that normally exist with a pilot in the cockpit, a big area of concern is the need for more pilot feedback. As the number of drones increases, so does the need to increase pilot situational awareness. Since all primary flight data will be transmitted to the ground-based pilot, of greatest importance is a robust data link, which essentially becomes the aircraft’s “control cables.”

Latency, or time delay in data transmission, can cause adverse handling qualities, so the need for a minimum data refresh rate may be required. Also, anyone who has had some flight training in a simulator that lacks audio will understand the value of audible engine and aerodynamic noise cues during emergencies or system failures. Handling emergencies without such acoustics is a handicap, and is what can be expected when the pilot is quite a distance away from the actual aircraft.

This may still seem like minutia to you, if all you want to do is take aerial video of your backyard pool party to post on the Internet. But as regulations evolve, the concerns over autonomous UAVs sharing airspace with aircraft carrying passengers will effectively keep a human pilot in the loop for some time to come, and flying qualities of remotely piloted vehicles must necessarily evolve in order to meet a minimum standard if safety of flight is to be ensured.

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