The ongoing and nuanced discussion about drone regulation is rightly centered on operators’ responsibilities – addressed through measures such as licensing and operating restrictions. But there are multiple aspects that must be considered. It is critical that the discussion around drone regulation is measured and thoughtful, and part of it is looking beyond just that of the operators’ responsibilities because, without question, drones are a growing part of the future.
Operational safety requirements
Aspects that must be considered when developing drone regulation for operators are extensive and varied. Considerations such as altitude, number of operators present, airspace classification, and precautions such as issuing NOTAMs prior to execution, all play a role. Rules addressing such operational aspects seek to ensure robust processes are in place to maintain control and avoid collisions with other airspace users.
Operators have a responsibility to take their missions seriously, and consider the consequence of human error. This is why procedures for ‘startup’ and mission planning require the disciplined and methodical approach typical of aviation. Just like full-sized aircraft, it is vital to methodically check that all electronic and mechanical systems are online and operational, and that control surfaces respond and move fully and freely. And all this must be scripted in a way designed to minimise the risk of skipping a step and missing something – whether an operator is flying in controlled airspace or in isolated areas, on a regular basis as a commercial activity or only sporadically. The responsibility is serious, and it must be taken seriously, with an awareness of the possible consequences of a mishap.
Another aspect of regulation currently being developed has to do with size bands. One approach is to rate UAVs in terms of their momentum (some weighted combination of mass and flight speed). The idea is to restrict (and demand higher standards of reliability for) machines heavy enough to do serious damage were they to collide with manned aircraft. Typically the limiting cases are contact with a windshield or ingestion into an engine. Below a certain size and flying speed, very small drones may pose no more of a hazard than bird strikes (drone flying speed comes into it assuming the worst case scenario of drone velocity being opposite to manned aircraft flight path). But larger airframes would cause more damage, so need to have higher standards of operational safety.
While these aspects of safety largely lie with operators, there is another extremely important aspect of drone standards evolution that may not be getting enough attention. It lies with the manufacturers. There are critical aspects of manufacturing that affect airframe reliability from the standpoint of structural and flight characteristics, and these safety aspects are often overlooked as manufacturers rush to get drones into the mainstream market.
Design for manufacture
From the beginning of the design phase, it is important to keep in mind how the final product will be physically built and assembled. Brainstorming new wonderful shapes is fine. But part of the evaluation process when selecting candidates for advancement to the next stages of design refinement must be considerations of ‘buildability’.
Some aspects of suitability for manufacture are universal and others are material-specific. Knowledge of material properties is important in selecting the appropriate one for each job. Balancing strength, weight, cost, operating temperature range, and processing characteristics is key to developing successful products.
Universal considerations boil down to accessibility. When putting together two halves of an assembly, can the fabricator reach all areas that need to be accessed for applying glue, clamps and fasteners? In some cases specialised tools such as drivers with extended handles may be justified. But access does not just mean physical. Can the assembled part be visually inspected? Can the fabricator see what he/she is doing? ‘Blind’ joints and bond surfaces are not to be avoided at all costs, but they must be a last resort. And when inevitable they must be accompanied by well thought-out inspection procedures to make sure the specified properties have been achieved on each unit.
Specific considerations include requirements such as being able to extract a part from the former/mould/tooling it was made in. Metal stampings, and mouldings in advanced composites, must be designed in such a way that they are not ‘locked in’ by the shape of their tooling. In the case of composites, there are few practical limitations on the shapes that can be achieved. But expertise is required to split the tooling correctly, eliminating returns or ‘negative draft angles’. For example, it is possible to mould a closed cylinder in carbon by splitting the mould into two semi-cylindrical halves that can be connected for lamination, and then pulled apart to release the cured product. Mould design ties into access considerations too. The fabricator must be able to reach and inspect all parts of the laminate and place vacuum bags correctly to achieve optimum debulking (compacting and void exclusion).
When designing machined parts, access for the cutting tool (and mounting arm) must be considered. 3D printing technology in some cases eliminates such access considerations: Any part of a vat of pellets or liquid can be solidified remotely. However, other considerations then come into play. For example, when 3D printing metals, steep temperature gradients exist between the newly formed part ‘front’ and previously hardened layers. So distortions from unbalanced thermal stresses must be accounted for.
The additive nature of building up a composite laminate stack means material properties can be tailored with great precision. Think of how the fibres in a tree trunk or branch are aligned to give great strength in the long axis to resist bending loads from the wind. In a similar way we can add fibres to advanced carbon composites. For example, running along the length of a wing spar to resist bending due to lift. Instead of adding isotropic doubler sheets as was necessary with aluminium fabrications, we can build up the fibre stack at the root, and gradually taper it out toward the tip, adding strength in the desired direction and smoothly reducing thickness along the span. We can combine these unidirectional reinforcements with other fibre layers at different orientations to give favourable behavior in twist. And we can add other fibre types specifically to give impact toughness or limit damage propagation. All in a mould with complex curves and twists, smoothly creating sophisticated aerodynamic shapes at cost no higher than making a simple box.
Unidirectional reinforcement fibres visible along the shaft line of this Carbonix rudder. They can be arranged to exactly match anticipated loads. Note also the non-linear taper of the blade.
To take advantage of the wonderful versatility of advanced composites, our processes must address the particular demands of the material. For example, we must have checks in place to make sure the various fibre layers smoothly consolidate to exclude voids and achieve full contact throughout the part.
Another great characteristic of advanced composites is that we can use bonding to permanently connect parts or sub-assemblies. A glue joint spreads loads smoothly and, when done properly, can be stronger and tougher than fasteners. Again it is vital to have checks in place to ensure the bonding process gives a product free of voids, with all mating faces properly in contact.
Once the part has been designed, and the manufacturing process has been proven, being able to repeat the process becomes key. Just like a preflight checklist, we must put in place processes to account for each step. We have checklists where every fibre layer is ticked as it goes into the mould. For particularly complex stacks it makes sense to account for each sheet of backing paper peeled off, to double-check that each layer has gone into the part, and all pieces of discarded consumables are present.
Keeping samples of every batch of cured glue, and noting ambient and cure temperature for every batch of parts, are other examples of how problems can be caught immediately. If a part were to present with a fault during service life, knowing which other components were made in the same lot can help determine the extent of any issue, and inform about which other parts may need to be flagged.
In-house process quality control also has to dovetail with supply chain documentation. A properly set up system will demand accountability from material suppliers, but also double-check that actual material properties match claimed specifications for each batch delivered.
In the case of ‘prepreg’ (pre-impregnated) composites, variables such as accumulated out-time (time away from the cold chain), and age for each roll should be known and noted.
What this all really means.
Discipline in the process, good record keeping, and an understanding of how to track important variables, are all part of building reliability into each component.
From the concept design stage of a part, to the layout of the working space (for example confining processes to different areas to minimise the risk of cross-contamination), experience, knowledge, and ethics, all combine to make reliable products.
Any critical machine, whether floating or flying, where failure could result in loss of the vehicle or injury to the operator or third parties, should be manufactured to exacting standards. The machine must be inherently reliable before operating checks and regulations are applied to make sure it is used responsibly.
This must be a consideration for all drone operators, who take on the responsibility of upholding the regulations. Are they absolutely sure their aircraft is built for the job?
Airframe with QA sheet, ready to undergo checks before final finishing.