Conceptual aircraft design
by Harvest Zhang
Volans is a highly efficient, high speed VTOL UAV designed to rapidly transport heavy cargo long distances. In the world of emergency medical supplies, whether carrying vital equipment or transfusions or lab samples or life-saving drugs, getting it to where it's needed just a few minutes sooner can make all the difference. Despite the quadratically scaling drag penalty at higher speeds, Volans takes advantage of flight dynamics unique to VTOL designs to be extremely efficient while cruising at a very quick 40 m/s.
This allows Volans to fly a 100 km mission in only 44 minutes from takeoff to landing, where a slower aircraft at 25 m/s would need an hour and nine minutes, arriving a full 25 minutes later. A higher cruise speed pays off even more when wind is a factor: when fighting a strong 10 m/s headwind all the way, Volans still completes the 100 km mission in less than a hour, while the slower aircraft would take almost a full hour longer to fly the same mission.
Volans is capable of flying a huge variety of mission profiles. It supports hub to hub missions like inter-hospital deliveries, where automated loading and unloading requires minimal human intervention and enables extremely fast turnaround time; hub to spoke missions such as disaster relief efforts, where a heavily automated hospital can quickly shuttle vital cargo to responders on the ground; and point to point missions, where humanitarian organizations in developing regions can move cargo efficiently without needing much infrastructure at all.
A modular payload bay design allows for Volans to adapt to missions beyond cargo transport, such as aerial monitoring or mapping. A smaller and lighter sensor would allow Volans to carry even more battery capacity than in the cargo payload bay. Combined with its high cruise speed, Volans can cover a vast amount of ground in a single mission.
Thanks to its simple and functional design, Volans is easily operable by just two people (or even one, if one is willing to move a 25 kg aircraft by oneself!). The payload bay drops straight down from the belly of the aircraft and slides out sideways, eliminating the need for an elevated platform or lifting the aircraft to access the payload. The aircraft disassembles easily into four large sections with standard hand tools and fits in the back of a pickup truck or van. All components are easily accessible for field maintenance. Reliable components, few moving parts, and easy teardown for transport makes Volans equally at home on the roof of a highly automated modern hospital or providing aid in the middle of nowhere.
Volans is designed around its 450x350x200 mm payload bay. Just as high performance sailplane fuselages are essentially streamlined fairings around the pilot, Volans' fuselage is a streamlined fairing around the payload. After many early concepts and iterations, the classic sailplane configuration still provided the best aerodynamic performance: a streamlined fuselage, a high aspect ratio wing, and a relatively small tail with a long lever arm. To minimize fuselage frontal wetted area, the longest 450 mm side of the bay is oriented longitudinally. To minimize the effect of a wide fuselage on the wing's lift and drag, the shortest 200 mm side is oriented laterally to sculpt a narrower but taller fuselage.
The wing is further elevated off the fuselage via an aerodynamically shaped pylon, reducing the loss of lift in the center of the wing due to the fat fuselage and also reducing interference drag. The A-tail configuration was chosen for the empennage due to its many advantages: it minimizes interference drag, provides a certain measure of pitch redundancy without extra surfaces or servos, prevents prop strikes, and has favorable yaw to roll coupling tendencies as well. The NACA 0010 airfoil was chosen for the empennage; it is perhaps slightly thicker than it needs to be, but it must be robust enough for the occasional ground strike and the thickness has minimal impact on efficiency.
Since the flight camera needs an unobstructed view forward and down, the pusher motor and propeller were placed in the tail, allowing for a more aggressive upsweep on the aft portion of the fuselage while still keeping the flow attached. With a motor weight of just 500 g, the weight and balance issues traditionally faced by non-electric pusher setups are easily mitigated here. The nose is lowered to allow the pitot probe to reside above the camera, high enough to be out of its view and far enough forward to be in clean airflow.
The main challenge of a Separate Lift and Thrust (SLT) VTOL concept is how to incorporate the lift motors and propellers such that they cause the least drag in cruise, while ensuring they have sufficient thrust and moment arm to maneuver the aircraft in hover. As the center of gravity is just about 90mm aft of the leading edge of the wing, the lift motors naturally balance quite well when placed on the ends of twin longitudinal motor booms that attach to the wing via aerodynamically streamlined pylons. These booms have very small frontal area and are also faired around the motor to reduce drag. While in cruise mode, the lift propellers must also be aligned longitudinally to minimize drag. Lateral motor separation is 1200 mm and longitudinal motor separation is 1100 mm. As almost half the mass of the aircraft is battery and payload, which both sit in the middle of the fuselage right between the lift motors, hover control authority and stability are not an issue.
To maintain high cruise efficiency, the motors face downwards and the propellers are dropped down further to reduce airflow disturbance at the wing caused by the motor boom and propeller. And to increase hover efficiency, instead of using a cylindrical section, the motor booms feature a vertically oriented symmetrical airfoil section with a rounded trailing edge, which creates cleaner airflow through the prop while reducing vibration and noise.
The SD-6060 airfoil, which is commonly and very successfully used in competition sailplanes of comparable size, was selected for the main wing due to its favorable performance at low Reynolds numbers; Volans typically operates at a Reynolds number range of roughly 250,000 to 1.5 million. The wing and tail were extensively iterated upon and optimized in XFLR5, with the goal of optimizing for efficiency at 40 m/s cruise. Here the unique advantages of a VTOL platform come into play: instead of having to design for low stall and landing speeds, requiring a broader flight envelope, we can take advantage of the lift motors to allow smaller, narrower wings and relatively high wing loadings, increasing L/D at cruise speed. The final configuration has a wingspan of just under 4.4 m with a MAC of just 0.21 m and an aspect ratio of 22.6. Performing a drag buildup via Raymer's method for the fuselage, landing gear, motor booms, and longitudinally aligned lift propellers and adding these values as extra drag in XFLR5 dropped L/D_cruise to around 22 (see polars). However, as XFLR5 tends to overestimate lift and underestimate viscous drag, my various estimates of Cd may be optimistic, and there are likely to be seams, control surface hinges, antennas, and other protuberances on the aircraft, the actual L/D_cruise is likely a bit lower still. Adding a conservative margin of about 15% and plugging in the resulting drag values into the frame sheet yields an L/D of 16.5 per the frame sheet's estimation, which should be very achievable in the as-built configuration.
This is a fairly high L/D for a fast cruise speed, and is made possible by the high wing loading of ~29.5 kg/m^2; the wing is allowed to fly at a more favorable Cl even when quite fast. A small downside is that low speed performance suffers; in the clean configuration with no flaps, the aircraft stalls at around 22 m/s, although the stall speed can be lowered further by using flaps or flaperon mixing. Fortunately, this can also be mitigated by intelligent VTOL transition logic; when slowing for landing, the lift propellers can mix in as the aircraft slows, and a slight pitch up and climb will bleed off forward airspeed and complete the transition to VTOL flight. The takeoff procedure is similar; the aircraft will pitch down in a slight dive to build up airspeed and spin down the lift motors gradually as it accelerates and transitions into fast forward flight. This configuration gives the flight control computer a more challenging transition period to handle and a narrower optimized flight envelope, but the speed and efficiency gains are worth it.An all-moving ruddervator design was originally considered but discarded due to the need for the tail to withstand the occasional tailstrike without jarring the servo. While the more traditional ruddervator in the final configuration lacks the natural counterbalance of an all-moving surface and requires more actuator torque, the Volz DA-22 servos are oversized and generate significantly more torque than needed.The ailerons are sized using similar methods as typical sailplanes, except that instead of a single 1000 mm aileron per side, there is a 600 mm outboard aileron and a 400 mm inboard aileron, each driven by a Volz DA-15N servo. This is partly due to the wing being too thin to fully enclose a large DA-22 servo, but also provides several other advantages. Redundancy is improved, as the aircraft may be able to continue flying with one failed servo on each wing (provided they are not stuck at an extremely position). There are many possible configurations for flaps or flaperon mixing; to ease transition into and out of hover, the inboard ailerons may act solely as flaps while the outboard ailerons may act solely as ailerons, or both may act as flaperons, or so on -- simulation and flight test will reveal which configuration allows for the smoothest transitions. Additionally, at high dynamic pressure, it is possible to use only the inboard ailerons for roll control, which reduces the risk of control reversal in the thinner, narrower outboard section.All servo linkages are completely internal to reduce drag and improve weatherproofing; for example, the ruddervators may use common bent-Z linkages which connect to the servos inside the fuselage, while the ailerons may use Rotary Drive System (RDS) linkages.
The energy and mass budget analysis sets a high bar for motor and propeller efficiency in both hover and cruise. This leads to an interesting optimization problem: propeller efficiency increases significantly with a larger propeller spinning slower, but the increased torque required to spin a larger prop requires a larger, lower kv motor that is much heavier.
The T-Motor U10 Plus 80kv proved to be the right combination of high torque and relatively low mass, at only about 500 g each but with enough torque to spin large 31"x11" propellers on a 12s battery pack, giving a very low disc loading of only 12.4 kg/m^2. Using the T-Motor thrust charts, each motor and propeller should generate about 10 kg of thrust at full throttle for a maximum vertical thrust of 40 kg. This gives a 1.6:1 thrust to weight ratio for the lift propellers, which is more than sufficient for takeoff, light maneuvering, and landing.
For cruise flight, the same motor is used, while the propeller design was optimized using Javaprop for high efficiency at 40 m/s cruise. Surprisingly, a 3-bladed 21"x33" propeller was the result, achieving around 82% propeller efficiency at 40 m/s. At the design point, it generates around 1.6 kg of thrust at around 900 W and 3100 RPM. While this very highly pitched, oversquare propeller is mostly stalled at lower airspeeds, the efficiency gains at cruise speed are significant. I also ran optimization studies using the faster spinning U10 Plus 100kv or 170kv motors, which allowed for smaller, lower pitch propellers like a 16"x18" at closer to 6000 RPM, but propeller efficiency dropped down to the mid 70% range. As with the wing sizing, I made the tradeoff to optimize for cruise efficiency; transition into cruise may be rather inefficient momentarily or even require a slight dive, but once at cruise speed, the high pitch / low RPM propeller proves most efficient. Of course, using the same motor across the entire aircraft allows complete interchangeability and reduces the chance that the wrong motor is accidentally installed during maintenance.
With a 20% SOC reserve, a 12s7p Lithium-ion battery pack (nominal voltage 44.4V) composed of Panasonic NCR18650GA cells will fulfill the 60 km mission under a constant 10 m/s headwind, for an aerial distance of 80 km, and a 12s10p pack will fulfill the 100 km mission under the same wind conditions, for an aerial distance of 133 km. Being quite conservative, and taking into account integrated resistive heating elements in each battery pack, 25% overhead from 240 Wh/kg cell energy density yields 190 Wh/kg pack energy density. The integrated heaters also mean that in cold weather operations, capacity does not degrade nearly as badly.
As the payload bay must support any mass payload up to 5 kg, it must sit very near the CG in order for the CG to remain in an acceptable location. The payload bay and the batteries are by far the densest components, accounting for almost half the mass of the entire aircraft. To maintain proper weight and balance and to allow for different payload weights and mission ranges, the battery packs are split into fore and aft 6s packs (nominal 22.2V) connected in series to achieve 44.4V; they directly drop into the payload bay and lock into place via mechanisms similar to that used by DJI multirotor batteries or pedelec batteries. It is highly convenient to have the battery slots located in the payload bay, as the batteries may be swapped while cargo is being loaded or unloaded.
The standard flight pack is a 6s7p pack, two of which are connected in series to achieve a 12s7p pack that can power a 60 km / 5 kg mission. To fly the 100 km / 3 kg mission, two extended range packs, each a 6s3p pack, must be attached to the standard flight packs, all together forming a 12s10p pack. The flight control computer interrogates the battery management system to ensure the correct battery capacity is installed for the intended mission. The position of the payload bay, balanced just slightly ahead of the CG, helps to maintain a positive static margin at all times -- as long as the cargo in the payload enclosure is not shoved all the way to the front or rear, the static margin will be acceptable for flight. Without a payload and with standard flight packs, the static margin is about 10%. Static margin increases to around 20% with a full 5 kg of payload biased slightly forward, which still has minimal impact on elevator trim and efficiency.
Structural concept and mass budget
Once the mass of all the motors, ESCs, servos, propellers, avionics, and payload is accounted for, as well as half a kilogram for miscellaneous wiring, installation, and connectors (which always turn out heavier than planned), about 6.5 kg can be allocated for the structure. To minimize weight while achieving high structural strength, carbon fiber (CFRP) sandwich structures are used throughout the aircraft. The choice of sandwich material, thickness, and layup schedule depends on the structural loads requirements for each component. Extremely lightweight sections that need minimal impact resistance may do well with a very light 5 mm foam core such as Airex and single ply carbon fiber (~0.6 kg/m^2), while certain parts may increase the carbon layup slightly (~0.8 kg/m^2) in order to prevent impacts from causing local delamination and gradual weakening of the part. Finally, components that need to be extremely strong may use an aramid honeycomb core instead of foam, with an even thicker layup schedule (~1.5-2 kg/m^2 for 10mm core).
A rigorous structural and loads analysis must be carried out in order to optimize the structural design. However, we may estimate the mass of the structure to ensure that it is in the same general ballpark as the target mass. The center wing section must be extremely rigid in torsion as well as in bending, as the motor booms must not be allowed to twist in flight. The outer wing sections must also be extremely strong, as they are quite thin and have the potential to twist and cause aileron reversal or flutter at high speed if not sufficiently stiff. Assuming that the wing is composed of aramid honeycomb sandwich with channels cut out for wiring passthrough, the mass of the main wing will be about 1700 g. The A-tail, similarly constructed, will weigh around 200 g. Each of the motor booms, with solid molded motor mounts, should be around 400 g. The fuselage has around 4 m^2 of surface area, including internal avionics trays and the tail section sleeve. assuming a monocoque structure using mainly single-ply / 5 mm foam core layup, with two-ply / 10 mm layup in structurally critical locations, a mass of 2800 g should be achievable. The landing gear is about 300 g (as described below) and the locking payload enclosure is approximately 300 g as well. This leaves approximately 400 g for miscellaneous waterproof gaskets, hardware, motor mounting plates, and so on. However, it is worth keeping in mind that these are only rough estimates to suggest feasibility of manufacture; optimization pending FEA and loads analysis may be able to reduce the necessary structural mass further or redistribute it as necessary.
Volans features helicopter-style landing skids with flattened, aerodynamic struts to minimize landing gear drag during cruise. Tricycle, taildragger, and retractable gear were also considered in a trade study; retracts were ruled out early out of concerns around weight and reliability despite their obvious drag advantage in cruise, while taildragger configurations generally faced payload bay clearance issues. Tricycle gear has some advantages, including better rough-terrain operation, but wheels -- even with wheel pants -- would still be significantly heavier and draggier than skids. Nevertheless, it would not be difficult to make the landing gear modular, allowing for a tricycle configuration where rough terrain operations are common and maximum range or payload capacity are not needed.
The skids do require that the landing patch not be too rocky or uneven for the aircraft to sit flat, although if upon landing it does tilt onto a wingtip or the tail, no damage is expected; the A-tail uses a beefy NACA 0010 section and the ruddervators end slightly before the tip of the empennage, allowing the tail to better absorb any tailstrike events without damaging the pusher propeller or putting large shock loads on the ruddervator servos.
The skids themselves are commonly available carbon fiber tubes with an O.D. of 16mm and an I.D. of 14mm, with custom molded tips on both ends. The struts are wrapped hollow carbon fiber tubes with similar wall thickness; these are similar to existing carbon fiber tubing used in multirotor booms and ski poles. There are about 1.8 meters of tubing in the skids at 60 g/m, and about 1.6 meters of wider, flatter tubing in the struts at 100 g/m, giving about 270 g; with assorted hardware for assembly, the landing gear is expected to weigh around 300 g in total.
To remove the payload bay, the retaining latches must be disengaged, either manually via latches on the bay itself, or via the ground control station in the case of an automated system. The payload bay can then be dropped straight down from the bottom of the fuselage, automatically disconnecting all power and comms wiring from the aircraft via integrated connectors. There is enough landing gear clearance for the bay to be removed sideways from under the fuselage without having to lift the aircraft or use an elevated platform. The payload bay for cargo transport consists simply of a lockable cargo enclosure and the battery packs; the payload enclosure is a lightweight carbon fiber tub with the specified internal dimensions of 450x350x200mm, with a watertight gasket on the lid that prevents any liquids in the cargo enclosure (e.g. condensation from ice packs) from spilling out into the fuselage and damaging electronics.
Packing the provided enclosure uses simple, familiar methods to prevent contents from shifting and damage: any packing technique used for standard cardboard shipping boxes may be used, including packing peanuts, bubble wrap, crumpled paper, and so on. While perhaps not as elegant as a dedicated restraint system with partitions or straps, it is flexible, lightweight, and instantly familiar; it offers the greatest versatility for packing arbitrary cargo.
After reattaching the payload bay to the aircraft, as part of the aircraft's self-checks before takeoff, the flight control computer reads load values from a pair of strain gauges on the front and rear landing gear struts, which allows it to derive the aircraft's center of gravity and all up mass. If the CG is not in an acceptable location or the AUM is too high, the ground station warns the operator, who will then adjust the payload to ensure that it is under the weight limit and well secured in the payload bay. Together with the flight control computer's battery capacity check to make sure it has adequate capacity for the intended mission, Volans will only take off when it knows that it has an acceptable payload and battery configuration to make it all the way to its destination.
Compatible cargo enclosures are supported; the supplied cargo enclosure attaches to the payload bay via a simple latch locking mechanism, and any other enclosure that fits into the payload bay and uses the same locking interface may be used. For example, insulated containers for transporting items that need to be chilled or kept warm can be used as long as they have the same locking interface. Customers transporting items with known form factors may easily design custom enclosures with various features as needed.
Adapting Volans for uses beyond cargo transport, such as mapping or monitoring, only requires designing a custom payload bay suited for the job and tightly integrated with the specific payload. Making the entire payload bay modular instead of just the cargo enclosure ensures ultimate flexibility and aerodynamic efficiency. For example, if a downwards looking sensor is to be used, it may be integrated with a faired radome in the bottom of the payload bay, minimizing additional drag. Smaller or lighter payloads may also allow for increased battery slots; up to four standard battery packs may be used if the payload sensor is only a few hundred grams in total, allowing Volans to loiter for several hours if necessary (cruising at a slower, more efficient speed in the 30 m/s range) or to cover much more ground rapidly (well over 250 km is achievable).
Wind and gust
Even in the worst case steady 10 m/s headwind throughout the flight, Volans is capable of achieving the mission range requirements of 60 km with a 5 kg payload and 100 km with a 3 kg payload. At 40 m/s cruise speed, a 60 km ground track becomes 80 km through the air, and a 100 km ground track becomes 133 km through the air. In most real world missions, the wind will be considerably more favorable, allowing for significantly longer ground track missions.
Volans will be capable of withstanding horizontal gusts of up to 10 m/s in VTOL and cruise flight, and most vertical gusts except for severe vertical (downward) gusts in VTOL mode, which at 10 m/s will almost certainly overwhelm the maximum VTOL climb rate. The high wing loading reduces the dynamic effect of vertical gusts during cruise, while even the worst case horizontal gust, an instantaneous tailwind gust of 10 m/s during 40 m/s cruise, does not reduce airspeed below stall. A full turbulence model-based analysis will be required to confirm that gust response dynamics are acceptable, as well as to generate the gust load cases for structural analysis.
The specified operating temperature range of -30°C to 50°C is fairly wide; assuming that the specified avionics components are already rated for that range, the main components deserving thermal attention are the servos, motors, ESCs, and batteries.
The Volz DA-15N and DA-22 servos are both rated for -30°C to 70°C, so they are not likely to have any thermal issues.
BLDC motors are not generally bothered by low temperatures, but at temperatures significantly higher than ~80°C, efficiency decreases and issues such as demagnetization start to occur. For our purposes, we will consider 80°C to be T_max. By analyzing the T-Motor U10 Plus thrust tests, we can see that at hover thrust, the motor reaches a steady state (10 min.) temperature of ~53°C. Ambient temperature during the test was not specified, but assuming 20°C ambient, delta_T = 33°C. This indicates that the U10 Plus can sustain hover throttle for almost 10 minutes at 50°C ambient before hitting T_max, so the one minute takeoff and landing periods in the mission profile should not cause any thermal issues in the motors.
At cruise throttle, consuming about a kilowatt of power, the U10 Plus reaches ~60°C steady state, giving a delta_T of 40°C. We can thus cruise along at a full 40 m/s at ambient temperatures up to around 40°C while still keeping the cruise motor under 80°C; ambient temperatures from 40°C to 50°C will require a slightly lower cruise speed to reduce the chance of overheating. If extremely hot weather operations are routine for a certain fleet of aircraft, lower pitch pusher propellers optimized for slower cruise may be swapped in for higher efficiency.
The thermal properties of ESCs are more of an unknown quantity; operating temperatures are rarely supplied for hobby ESCs, and little test data is available to indicate what their thermal characteristics under load may be. From experience, high voltage / high current ESCs available (T-Motor, Castle Creations, Jeti) can provide significantly more power without overheating than the hover or pusher motors can draw. The VTOL ESCs are less of an issue as they only have to run for a minute at a time; the pusher ESC must supply about a kilowatt continuously, so custom heatsink fins may be designed that protrude out of the fuselage into the airstream. In any case, due to the need to stop hover propellers longitudinally for cruise flight, custom motor controllers may eventually be needed, which shall be designed to meet the thermal requirements.
The battery packs, which use NCR18650GA lithium-ion cells, need to be at least around 0-10°C to be relatively efficient, even though the datasheet indicates that the discharge temperature range is -20°C to 60°C. The standard battery pack thus contains a built-in resistive heating element, controlled by its Battery Management System and consuming a small amount of battery power if needed to maintain the battery temperature above a preset minimum. Fortunately, the initial minute of hover/climb has the highest discharge rate of the entire flight, which serves to heat the battery as well; the built in heater is only needed to initially bring the pack up to operating temperature, or supplement heat if the battery is not supplying enough current during cruise to self-heat. For missions in warm or hot regions of the world, it would make sense to manufacture battery packs without the built in heater, which would boost energy density up from 190 Wh/kg to over 210 Wh/kg.
The datasheet for the NCR18650GA cell indicates that at ~1C discharge, which is fairly close to Volans' cruise discharge rate without using the extended range pack, the maximum delta_T across the discharge curve is less than 15°C, and is below 10°C for the majority of the discharge curve. Thus, with adequate heatsinking, Volans can still cruise along at the full 40 m/s at 50°C ambient and remain under the 60°C maximum for much of the flight without any active cooling. In any case, the FCC monitors the battery temperature as reported by the BMS, so in case the battery starts to touch 60°C, the FCC can reduce cruise speed to allow the battery to cool down.
Volans is designed to fly through moderate rain showers, although significant rain will have an impact on range due to increased drag and heavier weight. The fuselage shall be rated for either IP64 or IP65, depending on what is achievable without adding too much mass in the form of gaskets. There are relatively few access panels and joints between components, but all of these require rubber gaskets to maintain a watertight seal. All five motors are fairly well isolated from rain, but in any case brushless motors do not mind operating in wet conditions; their wiring leading back to the ESCs goes through a small waterproof gasket so that critical electronics behind the motor are not affected. Both the Volz DA-15N and DA-22 servos are rated at IP67, which is not even particularly needed here as all servos are internal.
There are no provisions made for flying in freezing and rainy or snowy conditions where icing may occur. This would require changes to the avionics (e.g. using a heated pitot probe) and additional SWAP allocated for wing de-icers, which for this application is likely overkill. However, if a dedicated polar operations variant is required, these cold weather changes could certainly be applied with a corresponding impact to range or payload capacity.
Operational safety and failure modes
There are two main safety considerations to consider for the mission profile of this aircraft: ground operations safety, and in-flight failures. Here, ground ops safety mainly concerns the proximity of operators or bystanders to the exposed hover and pusher propellers when spinning down or up, and in-flight failure safety mainly concerns mitigating catastrophic failures in flight such that no one on the ground is injured if the aircraft must come down (if the aircraft is mostly unharmed as well, so much the better).
Ensuring that the hover propellers do not spin while operators are near the aircraft is primarily achieved by careful human factors design coupled with clear operating procedures, which are quite similar to those for existing multirotor platforms. During the VTOL phases, the pusher propeller will not be spinning; as soon as the aircraft detects that it has landed, it shall command the hover propellers to actively brake to a stop. These lightweight, low inertia propellers should take no more than a couple seconds to spin down. A loud audible tone shall be emitted and the ground control station should display that the aircraft is now disarmed and safe to approach.
After exchanging batteries and payloads and reinstalling the payload bay, the operator will back away from the aircraft, power on the avionics, perform the preflight checklist, and allow the aircraft to perform its preflight self checks. Immediately before takeoff, the operator must ensure that all personnel are well clear of the aircraft before rearming, at which point the aircraft emits a different loud tone to indicate that it is armed; it should then pulse the hover propellers at very low speeds a few times as a warning, just in case anyone is somehow still near the aircraft. At this point it will spin up and take off within seconds.
Optionally, a physical motor kill switch placed near the wingtip would increase operator safety margin further; before approaching the payload bay, the operator could flip the kill switch, and after the payload bay is reinstalled, the operator must physically walk to the wingtip before toggling the kill switch, which puts her outside the hover propeller area and reduces the chances of accidental arming.
To mitigate catastrophic failure in flight, Volans implements the SkyCat FTS launcher and parachute as specified in the requirements. FTS deployment is commanded either by the ground crew manually or triggered automatically by the flight control computer if it detects a critical failure or that the aircraft is not responding to commands. A breakaway hatch is built into the tail section so that firing the launcher will blow open the hatch and allow the parachute to deploy properly.
During the VTOL phases of takeoff and landing, any failures of a single motor will trigger the FTS. During cruise flight, failure mitigation can be a little more sophisticated: having redundant aileron surfaces potentially allows for one aileron servo per side to fail and still have sufficient roll authority to fly to the destination or an emergency divert waypoint before transitioning to VTOL for landing. Similarly, the A-tail design may allow for a single tail servo failure to be mitigated; the ailerons may be able to counter the yaw and roll moments generated by a single ruddervator controlling pitch, such that the aircraft may be able to fly to the destination or the divert waypoint before landing. Despite the higher failure rate of servos compared to motors in these applications, the use of highly reliable, aerospace-proven Volz servos also reduces the likelihood of a servo failure. Failure of the pusher motor will result in immediate transition to VTOL mode. Whether a VTOL landing or FTS deployment is safer for those on the ground will be mission and regulation dependent.
Shipping and transport
Volans can be disassembled for transport in just a few minutes, requiring only the outer wing sections, the tail section, and the lift propellers to be removed. For most removable components, all wiring interfaces going across the component break will be integrated in a single positive locking connector to reduce the number of separate connections; for example, single-side bulkhead connections may use circular connectors commonly used in aerospace applications, such as the lightweight composite Amphenol MIL-DTL-38999 series.
The outer wing section plugs into the center wing section via a tongue and groove mechanism, locked in place with a single countersunk bolt; an integrated push-pull connector in the tongue automatically connects the wiring harness for the aileron servos whenever the outer wing is plugged in. Detaching the outer wings thus consists of simply undoing one bolt and pulling the wing off.
The tail section attaches to the main fuselage via a sleeve and is locked in place by countersunk bolts on the top and bottom. A single positive locking circular connector carries the wiring harness for the FTS, pusher motor, and ruddervator servos. Detaching the tail section thus consists of undoing two bolts, pulling the tail section free, and disconnecting the wiring connector. Finally, the lift propellers are removed to reduce the size of the main fuselage section and to protect the propellers from damage.
The transport configuration thus only has four large sections: the main fuselage, the two outer wings, and the tail. The main fuselage section is the largest component in all dimensions, at just about 1.8 meters long, 1.3 meters wide, and 0.6 meters tall. Both outer wings and the tail tuck in neatly alongside the fuselage. In this form, Volans easily fits in the bed of a standard pickup truck like the F-150, or cargo vans like the Sprinter with a bit of careful positioning through the narrower door opening. In a pinch, it will even fit in the bed of a compact pickup truck, though the motor booms will just be slightly hanging off the side. For short or urgent movements in remote areas without much infrastructure, a small one or two person crew can disassemble the aircraft in a few minutes, throw some padding around each section, strap them down in the back of a pickup, and go.
For longer range shipping and better protection against rough handling, Volans can be securely packed in a padded shipping case with internal dimensions of 1.9 x 1.5 x 0.7 m. The hard case can then be transported via ISO container or any other common cargo transport method without fear of damaging the aircraft.
Volans allows convenient access to all aircraft components for maintenance while minimizing the use of access panels. The camera assembly is removable and disconnects from the fuselage just aft of the clear camera dome. The avionics tray, which holds the FCC, uADC, comms modems, and ADS-B transponder, slides and locks into slots in the fuselage and features a single positive locking circular connector to interface with the fuselage wiring harness. This allows maintenance crews to easily remove and replace the avionics tray:
1) Remove the payload bay to gain access to the forward fuselage.
2) Loosen the pitot probe boom and slide it back into the fuselage; as the tubing that connects the pitot ports to the uADC should not be disconnected or reconnected often, this allows the pitot probe to come out with the avionics tray.
3) Disconnect the avionics wiring connector as well as any external antenna cabling (e.g. transponder, comms, video antenna connectors), disengage the avionics tray lock, and slide the avionics tray out of the fuselage.
4) Reinstall the avionics by sliding the avionics tray back in until it locks into place. Reconnect the avionics wiring connector and all antennas. Reinsert the pitot probe boom and secure. Reattach the payload bay.
Due to its position near the CoG, the IMU is mounted on the underside of the wing pylon instead of on the avionics tray. Removing the payload bay allows easy access to the IMU as well as the front and rear landing gear strut mounts, which may need replacing after an exceptionally hard landing.
The flight termination system is easily accessed for maintenance when the tail section is disconnected. The dual tail servos and pusher motor ESC can be accessed via a small panel directly above the servo mounting plate, which also allows tool access to detach the pusher motor from the rear firewall. Aileron servos are accessed via their servo covers on the bottom side of the wing, and loosening the bolts on the upper side of the motor booms allows the lift motors to be removed. All five motors on the aircraft are identical, conveniently allowing any spare motor to be used in any position.