Unmanned Aerial Vehicles

Multirotor Drones

Multirotor drones (quad/hex/octacopters), are generally thought of as tolerant of lateral offset center of gravity (CG) because one motor needs only to work a bit harder to compensate for any offset CG.  Vertical CG is deemed similarly less critical because the fly-by-wire stability algorithms easily handle a wide range of vertical CG.  However, we have seen that in some high-performance applications sensitivity to CG location increases to a level of importance, especially where high-stability under difficult environmental conditions are present.  

Although we state above that CG location for drones is generally not critical, this statement only applies when viewed within the spotlight of tight constraints.  Two such constraints are: that the aircraft is a pure multirotor where it exhibits vertical takeoff, remains in the same orientation for flight, and vertically lands; or when the multirotor type aircraft is not used for transportation of humans.  

If a multirotor craft is intended to transport people, a segment called advanced air mobility, the level of safety and safety-related regulation increases greatly.  This people-moving application will likely fall under the FAR Part 135 (or similar) scrutiny where safe CG bounds must be demonstrated either through analytical means or from CG verification measurements.

Drone Size Considerations

The size of the drone leads us to observe different viewpoints.  The smaller the drone, the greater the mass fraction of inexact wiring runs and other ancillary components are.  The high mass fraction of these rogue components increases the gap between the CAD design and the as-built mass properties.  On the flipside, the larger the drone, the less uncertainty there might be in construction variation but the greater the consequences are of getting it wrong; large objects falling out of the sky into population centers are worse than small ones.

Size, weight, and power (SWAP) along with operational cost savings are driving forces spearheading the development and use of drones.  To satisfy the need for weight reduction most manufacturers have turned to composite construction techniques.  Composite construction consists of layers of carbon fiber cloth impregnated with a thermosetting epoxy forming the skin and control surfaces of the aircraft.  Our experience at Raptor Scientific has shown that the variation between as-designed and as-built can be up to 20% of total mass.  Once again, the smaller the aircraft, the higher the likelihood of missing the design targets; the larger the aircraft, the greater the consequences of error.

In the current regulatory environment, smaller drones, are subject to FAR part 107 and are not required to have a “Type Certificate” instead having operating limitations in altitude and location. Waivers are required to use the smaller drones where the restrictions are valid.  For the purposes of this conversation, it means that no mass properties control is mandated by FAR.  Although not mandated, small, high-performance drones would benefit from precision mass properties management.

Larger drones do require validation of operating capabilities and safety requirements that are found in the long-established FAR parts 91 and 135. These requirements are similar to the certification of small aircraft and commercial aircraft operations respectively. Those requirements include creating an operator’s handbook that identifies the weigh and CG limitations and safety standards. The larger drones are currently in the “gestation” phase of operation and manufacturers and users of these drones have been required to request exemptions from the FAA and the Secretary of Transportation to operate as a commercial entity.  As the evolution of this industry unfolds, exemptions and waivers are certain to become less commonplace and replaced by updated FAR clauses.