Not especially pretty, but efficient and easy to manufacture : ALFA Design team proudly presents its Cargo Drone.
The philosophy that drove the design of this drone was to keep it as simple and light as possible. This lead to the following conceptual choices:
1. An unusual configuration featuring :
- A streamlined fuselage that minimizes drag for a given volume (set up by payload bay and avionics). Albeit inspired from Burnelli’s approach, the fuselage is not intended to generate lift here (its very low aspect ratio would indeed lead to excessive induced drag).
- An inverted V-tail, which provides increased yaw and pitch authority during transition phase (<=> at high AOA). This tail arrangement reduces drag both by minimizing interferences and deleting the need for specific rear landing gear.
2. A strut-braced wing structural layou
t, which keeps weight low (in combination with the 16% thickness airfoil) in spite of its high aspect ratio (14). Induced drag varies with the square root of lift coefficient. Since wing loading is not driven by stall and landing speeds usual requirements, it has been set up at 29kg/m² in combination with a 85km/h cruise speed so that the airfoil works at lift coefficient that maximizes L/D (thus range). For such a configuration, a high aspect ratio is the most efficient way to limit induced drag. The impact of wires on drag is deemed negligible in view of the weight reduction they allow in comparison with cantilever layout. It must be noted that the high wing loading also make the airplane less sensitive to gust during cruise phase.
3. A low Reynold high performances airfoil
for the wing, which has been carefully chosen to maximize lift to drag ratio at Re = 4.105.The iterative selection process, which has led to the choice of SG6040 airfoil, is described in details in “Airfoil selection.pdf”.
4. An unsymmetrical quadcopter configuration
: vertical thrust is primarily produced by 2 direct drive propellers located at each wing tip. The large propellers (34’ diameter) enhance efficiency by comparison with usual multicopters designs. Pitch balance and authority during hover & vertical climb is ensured by 2 additional motors located close to fuselage trailing edge. Their power can remain very low (55W each), since the 2 main propellers produce thrust very close to the cog. Differential RPM of the 4 motors allows for roll control, while yaw authority is ensured by differential use of cruise motors. In case of windy conditions, they turn the airplane heading into the wing. The forward propellers then direct air flow over the tail stabilizers, whose additional action helps controlling the airplane. During cruise phase, the vertical lift props are swept backward in order to minimize both drag and wing pressure distribution on the wing, while a movable hatch prevent airflow from entering in rear rotor ducts.
4. Untwisted straight constant chord (“Hershey-bar”) wing
. Main pros for such a planform are its easier manufacturing, efficiency for subsonic flight and favorable stall characteristic (flow separation begins at the root and moves to the tips). The latter is very desirable during transition phase, allowing roll authority at high AOA. No leading/trailing edge high lift device is used, since there is need to minimize stall speed (motor / propeller help compensating loss of wing lift during transition phases).
5. Easily interchangeable payload box
thanks to “clip” fixation system based on nylon straps + clips . This box can be unloaded both from upper and lower sides of fuselage. Direct access to batteries is ensured by removing the upper panel.
6. Light-weight materials
- Extruded low density (30 kg/m3) foam core both for the fuselage and wing ribs.
- High strength CFRP tubes for wing spars and fuselage box frame
- Easy to form 0,3mm sheets of magnesium (which ensures water resistance to water) both for fuselage, wings and tail skins. An alternative consists of a skin made of one layer of bidiagonal CFRP with a mass of 100g/m² and a carbon fiber fraction of 35%. It features a thickness of 0,162mm, which is the minimum manufacturable thickness for this kind of CFRP.
The airplane therefore entirely meets technical requirements:
Conservative hypotheses have been adopted for performances calculation:
- Oswald efficiency factor: although many formula are available to assess this factor for usual light airplanes, it has been demonstrated (see in particular “Span Efficiencies of Wings at Low Reynolds Numbers » paper) that their domain of validity doesn’t extend up to low Reynolds numbers encountered here. Consequently, a conservative value of 0,75 has been adopted for flight performances assessment.
- Drag : parasite (zero-lift) drag has been estimated by components buildup method from data provided by “General aviation aircraft design” book. The philosophy has consisted in calculating Cd0 for the main components (fuselage, wings and tail), then increasing the sum by 50% in order to take into account the effect of miscelleous parasite drags (in particular the one generated by the lift propellers during cruise, which cannot be estimated analytically with sufficient accuracy). The drag analysis is detailed in enclosed “drag_analysis.pdf”.
- FOM : 0,6
- Aspect ratio : assuming that fuselage lift is not negligible, the usual formula includes it in the wing area and spans considered for aspect ratio calculation. Such an approach would lead to underestimating lift-induced drag, since the fuselage is intended not to generated lift here. Aspect ratio is consequently assessed considering wings alone.
In spite of this, required performances are exceeded :
- 100 km : 4,7 kg payload (> 3kg)
- 60km : 6,7 kg payload (> > 5kg)
- Cruise speed : 24m/s (85km/h) > 80km/h
- The total capacity of the battery deliberately exceeds the needs, which allows for extra range or capacity to fulfill missions with a constant headwind during the whole flight
-100km mission : +14 km <=> 3m/s headwind => by replacing the 2 x (6P8S) configuration by 2x(7P8S), 100km range is ensured with 7,5m/s headwind all flight long and a payload capacity of 3,8kg.
-60km mission:+ 28km <=> 10m/s headwind
General design :
- Hover motors = 2 x Pro-tronik 5330/Kv200 (thatensure 95% of lift) + 2 x Pro-tronik 2204/Kv 1750 (dedicated to pitch balance)
- Cruise motors = 2 Pro-tronik 2825 / Kv 650
- Total motors = 6 < 10
- Fligth control actuators: 2 Volz DA-15 for each aileron, 2 Volz DA-15 for tail, 2 DA-20 for elevator.
Lighweight is of prime importance for this project, since every additional kilogram on the MTOW theoretically increases battery mass by about half a kilogram. This made it necessary to refine calculations early in the design process. Basing on detailed CAD and preliminary analysis (for instance for the wing spar, see enclosed excel file), the following reliable weights are obtained:
- Fuselage and tail: 3,6kg
- Wings : 2,2kg
- Motors + propellers: 2,34kg
- Flight control actuators : 0,33kg
Such a limited structure mass allows us to meet performance requirements with a maximum take-off weigth of only 23kg.
It must be noted that an in-depth analysis has been completed to accurately assess battery mass. It leads to a 1,3kg reduction (for the 100km mission) compared to the mass automatically calculated by the provided Excel (thus leading to an equivalent increase of payload mass)
- Span end to end (including lift propellers) = 4995mm < 5000mm
- Wing span : 3260mm
- Aircraft length = 1569mm < 4000mm
- Max part length = 1689 mm < 2000mm
Once the airplane has landed, a ground operator will be able to easily access from the outside to payload bay, internal equipments (flight computer, camera systems…) and battery packs thanks to 2 large hatches on the upper and lower surfaces of fuselage. Batteries are switched by simply removing the three or four Velcro straps holding everything in place. Although electric propulsion is virtually free from maintenance, lift and cruise motors can be easily removed. A close inspection of main lift motors is possible by removing their fairing.
Static pitch stability :
There are 2 reasons that justify a careful analysis of static longitudinal stability for this airplane :
1. It has been conceived so that during ascent phase, thrust of the 2 main motors/propellers (1 at each wing tip) is applied very close to its center of gravity (from longitudinal point of view), thus maximizing efficiency thanks to larger propellers. However, such an approach reduces the static margin with respect to pitch stability, since it forces the wing spars (which bear the motor and its propeller) to be very close to center of gravity. This obviously reduces the distance between the latter and the aerodynamic center of the wing.
2. Although it does not participate to lift during cruise phase, the wing-shaped fuselage obviously has a direct effect on stability of the whole airplane.
Payload has been located and oriented so that weight variation barely affects stability. Several iterations have been carried out to calculate tail surface and dihedral angle (≈38°), thus leading to a static margin of 12%.
Modularity & shipping :
ALFA Cargo Drone can be disassembled a few minutes with standard hand tools by removing only the wings and tail sections. For these 2 components, all wiring interfaces are integrated in a single positive locking connector to reduce the number of separate connections.
The transport configuration therefore has 5 large sections : the fuselage, the 2 wings (including the lift motors and propellers, whose blades are folded along the span to limit the total size) and the 2 tails. Fuselage is the largest component with 1,5m long and 0.75m wide. The longest part is the wing spar, which reaches 1,7m (in compliance with requirements). Since their weight doesn’t exceed 3,1kg, these parts can therefore be easily handled by a single ground operator and and transported by foot or with a regular Mercedes Sprinter.
Operational safety :
Both during take-off and landing, low frequency sound from propellers warns surrounding crowd of its presence. As soon as the airplane touches the ground, motors are shut down. This job is ensured by a sensor fixed on the landing gear and calibrated to a certain compression limit. When propellers stop spinning, all motors get into the unarmed state. For maximum safety, the propeller tips shall be marked with na orange paint so that untrained personnel can visualize the rotors.
Consequences of failure :
Flight control feature high level of redundancy to mitigate consequences of an actuator failure:
- Roll axis: each aileron is controlled by 2 actuators
- Pitch axis: elevator (hinged trailing edge of fuselage) is actuated by 2 servos. Should both fail (a highly unlikely event!), the inverted V-tail could be used to ensure control.
Thanks to its fixed wing and wheels, ALFA Cargo Drone is capable of conventional landing in case of emergency (for instance motor failure). Its high visibility yellow paint then makes it very easy to recover. In case of catastrophic failure in flight, the airplane 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
Landing gear :
Quadricycle arrangement made of 2 front wheels fixed to fuselage via leaf-springs (main landing gear) and 2 rear crutches fixed at the end of the tail (tail gear). Skis are optionnaly mounted on the first in order to allow for take-off and landing from any type of unprepared airstrip, in particular gravel runway or mud. In addition, the lift propellers are mounted on top of high wing, thus allowing for landing on long grass.
In compliance with technical specifications, off the shelf rechargeable batteries cells are used . The chosen ones (Panasonic NCR18650GA) feature the following characteristics (per cell):
- Capacity : 3,3Ah
- Voltage : 3,6V
- Weight : 48g
- Max continuous discharge rate : 10C
The battery is split in 2 distinct packs (located on both sides of the payload), which can be tuned following the mission:
- 100km/3kg mission : 6P/8S (for each pack) giving a total capacity of 2x20Ah (=> 4Ah additionnal capacity)
- 60km/5kg mission : 5P/8S (for each pack) giving a total capacity of 2x16,5Ah (=> 8Ah additionnal capacity)
Total capacity for the 60km mission deliberately exceeds the need, which allows for mission fulfillment with 10m/s constant headwind.
The analysis is detailed in the Excel sheet and in “Battery dimensioning.pdf”. In addition, it must be noted that cooling of both battery and motors has been carefully studied. The results have been summed up in “Battery cooling” document (*.png and *.pps formats)
All components (FCC, Comm Systems, Power Modules, etc.) are located inside the fuselage, which is water proofed thanks to its metal skin. Rubber seals shall be added between main hatch and fuselage skin so that water cannot get into the payload box. Servo actuators and motors are water resistant and there are no other exposed electronics which can be damaged by nature.
ALFA Design team : Alexandre Fichant & Arnaud Landuré