Unmanned Aerial Vehicles

Drone Delivers Payload to USS Henry M. JacksonUnmanned aerial vehicles (UAV) more commonly referred to has drones, represent a fast-growing aviation segment featuring quickly evolving technologies and design philosophies.  Today, drones are used extensively in military and commercial applications ranging from commercial drone deliveries of goods, to intelligence, surveillance, and reconnaissance (ISR) missions, to sophisticated weaponized military assets.  This ever-changing landscape of drone configurations and applications drives different needs from a mass properties perspective.    

Raptor Scientific has a 50-year history of supporting aviation mass properties verification testing.  We have measured mass, center of gravity, moment of inertia (MOI), and product of inertia (POI).  The following are brief examples of our instrument sales and measurement services provided to military aviation, UAVs, and commercial drone applications.

  • Mass, CG and MOI of canopies
  • Mass, CG and MOI of control surfaces to reduce flutter
  • Mass, CG and MOI of landing gear
  • Mass, CG and MOI of fixed wing drones
  • Mass, CG, MOI, POI of quadcopters
  • Mass, MOI and balance of props
  • Mass, CG, MOI, and POI of surrogate airframes

Our decades of expertise combined with the world’s most comprehensive collection of mass properties instruments available, allow us to quickly match-up and meet your verification needs.  Our class-leading solutions offer unparalleled capability in high-accuracy measurement. 

 

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.

Hybrid-Multirotor Drones – Vertical Take-Off and Landing (VTOL)

Differently from their multirotor brethren, hybrid-multirotor drones that transition from vertical takeoff to horizontal flight (akin to an airplane), have flight dynamics requirements much more stringent than the multirotor examples above.   Hybrid-multirotors can take on various configurations.  They can be lift-plus-cruise airplanes with supplemental helicopter-like liftoff rotors, or thrust-vectored airplanes with rotor-nacelles that pivot from vertical thrust to horizontal thrust, or aircraft where the whole fuselage and rotor pitch over inflight – effectively transitioning from vertical lift to horizontal flight.  These configurations are intolerant of variation in CG location, meaning the relationship between CG and mean aerodynamic chord (MAC) are as important to flight worthiness as it is for the flight stability of manned aircraft.  For these configurations of drones, mass properties are important both for the sakes of flight dynamics as well as to satisfy regulatory requirements.

As with the hybrid-multirotors, fixed wing drones have all the same critical physics and aerodynamic relationships as fixed wing manned aircraft.  The same importance is placed on the relationship of CG to MAC to ensure flight stability.

Drone Size Considerations

Global Hawk flying environmental mapping missions in Latin America, CaribbeanThe 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.

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