Minerva A Spanloader Concept

Minerva A Spanloader Concept by D. Felix Finger M.Sc. in Aerospace Engineering In Response to the Airbus Cargo Drone Challenge

Contents 1 Requirements... 3 2 Design Inspiration... 4 2.1 Three-view... 4 3 Explanation of design details... 5 3.1 Wings... 5 3.2 Booms and Hover Lift System... 6 3.3 Empennage... 7 3.4 Fuselage... 8 3.5 Landing Gear... 9 4 Aerodynamics and Flight Performance... 10 4.1 Aerodynamic Analysis... 10 4.2 Flight Performance... 12 5 Loads and Structural Concept... 13 5.1 Loads... 13 5.2 Structural Concept... 14 5.3 Modularity and Ease of Handling... 15 5.4 Waterproofness and Environmental Control Systems... 15 5.5 Fail Safe Components and Safety Provisions... 16 6 Systems... 17 6.1 Weight and Systems... 17 6.2 Payload and Cargo Concept... 18 2

Named after the ancient Roman goddess of medicine and wisdom, this UAV shall bring supplies to everyone in need. 1 Requirements Top Requirement Means of Compliance Reference on Page General Design 1 Design Capable of vertical Lifting System 4 take-off and landing 2 At least one fixed wing One spanloaded, high aspect 4 3 10 >= Total rotors >= 4 Hover rotors >=4 At least 1 pusher propeller ratio wing 4 + 1 design 4 4 No tilting wings/ rotors Fixed systems 4 Specifics 5 Wingspan < 5m Wingspan: 4,34 m 5 6 Aircraft length < 4m Max. Length: 2,29 m 8 7 Aircraft is modular / Wings and booms detachable 15 Can be disassembled 8 Part length < 2m Longest Part: 2,00 m 15 9 Transportable by two avg. Storage in slender box 15 size people when disassembled 10 Failure planned for Parachute System 16 11 Power purely electric E-Motors 17 12 Off the shelf rechargeable battery MaxAmps.com LiPo 17 13 Swappable Batteries Stored in Booms 6 14 Reserved weight, space and Fits inside the fuselage 8 power for items of Ignition Kit 15 Fixed-pitch propellers Fixed Props 17 16 Propulsion directly driven No gearbox 17 17 Single, fixed payload bay (PLB) One PLB 8 18 PLB located near COG Exactly at COG 8 19 PLB dimensions Payload bay: 8 > 450x350x200mm 450 x 350 x 200 mm 20 PLB accessible from lower Lower side only 8 side of the aircraft 21 PLB Interchangeable/ modular Simple clip system 18 22 Payload Restrained Compartments 18 23 Payload impossible to jettison PLB is part of the fuselage 14 structure Performance 24 MTOM < 25kg MTOM: 25,00 kg 17 25 5 kg payload: > 60 km range Range: 119 km 12 26 3 kg payload: > 100 km range Range: 158 km 12 27 Cruise speed >80km/h Cruise speed: 80 km/h 12 28 Max speed <194km/h Max speed: 160 km/h 12 29 Capable of flight with 10m/s ahead and cross wind v-n diagram, structural design 13 3

2 Design Inspiration The twin boom layout is often used for UAVs, but it is applied in new context here: Because heavy parts as the lifting motors and the batteries are placed inside the boom structures, a spanloading effect is achieved. This gives a lower wing root bending moment and consequently a lighter wing structure. The wings are long and slender, inspired by those of long distance migratory birds. To keep the landing gear as short as possible, an inverse gull wing layout is used. Inspiration for this feature came from the F4U Corsair. For minimum interference drag and a high wing root thickness, the wing blends into the fuselage. The lifting rotors (low disk loading for minimal power and weight) on the booms arrest their propellers in flight direction during cruise flight and thus minimize the drag impact. For a maximum lift-to-drag ratio (L/D) much effort was spend on reducing the wetted area, and obtaining a high wetted aspect ratio. 2.1 Three-view Figure 2-1 Three view and isometric view 4

3 Explanation of design details 3.1 Wings The high aspect ratio wings are smoothly blended into the fuselage for minimal interference. OpenVSP is unable to handle curved wings. Because of that, unfortunately, the leading edge looks a little bit jagged at the moment. A better impression can be found in figure 6-1 on the last page. The wings incorporate the inverse gull-wing shape to keep the landing gear as short as possible. I stole that idea for this feature from Chance-Vought s F4U Corsair. This will help to keep the impact of the landing gear on weight minimal. The packaging requirements are satisfied by splitting the wings. See section 5.3 for details on packaging. NACA 6-series sections are used to lower the friction drag by keeping the boundary layer laminar as long as possible. Therefore, aggressive 66 sections are used inboard. To get an acceptable stall behavior the more docile 64- and 63- sections are used outboard with both aerodynamic and geometric twist. Aerodynamic efficiency is further optimized by winglets. With the current aerodynamic configuration (managed by careful twist + airfoil adaption see figure 3-1) the inner wing will stall first, still allowing roll control with the ailerons. This is very important, especially during transition from hover to cruise flight. The ailerons on this design are split into outboard and inboard ailerons (see figure 2-1). The main goal is the ability to press on the mission, even if any one of the control surfaces is malfunctioning. This additional redundancy also allows for the optimization of the spanwise lift distribution with respect to the actual airspeed and trim conditions. Table 3-1 - Wing Data Specifications - Wing Wingspan (ref.) 4,300 m Reference Area 1,100 m² Aspect Ratio 16,8 Taper Ratio 0,37 Leading Edge Sweep 0,0 Total Aileron Area (L+R) 0,090 m² Aileron Chord / Chord 0,15 Lever Arm (Aileron) to CG 1,3 m Airfoils See Figure 3-1 5

Figure 3-1 - Airfoil Concept (The flat fuselage top is used as reference for the angle of incidence i) 3.2 Booms and Hover Lift System The twin booms house the lift propulsion system. In addition, the entire battery space is provided in the booms. This is necessary to achieve the spanloading effect and to keep the structural weight down. The booms were sized by the maximum length requirement of 2 meters. Of course this influenced the tail volumes. Located evenly about the center of gravity, the hover lift system provides the VTOL capability to the airframe. Large 30 propellers offer a low disk loading. This greatly reduces the necessary power and also the weight impact. Because the chosen motors have a high diameter, small bulges are added to the booms to fit them. CFD analysis prove that no undue flow separation appears, due to appropriate shaping of said bulges. 6

3.3 Empennage The inverted-u tail is chosen for its compatibility to the twin boom layout. It places the horizontal tail outside the propwash, reducing its drag. Safety is improved further by having redundant control surfaces. Even the elevator is split to mitigate the risk of failure. Airfoils on the vertical stabilizers are chosen for their minimum drag characteristics, while the elevator airfoil is selected to give a minimum deadband at low Reynolds numbers. Detailed data concerning the stabilizing surfaces is summarized in tables 3-2 and 3-3. Table 3-2 - HT Data Specifications Horizontal Stabilizer Span 1,25 m Area 0,139 m² Aspect Ratio 11,3 Taper Ratio 1,0 Leading Edge Sweep 20,3 Elevator Area 0,048 m² Elevator Chord / Chord 0,35 Airfoil Selig S8035 (14% Symmetrical) Lever Arm to CG 1,228 m Table 3-3 - VT Data Specifications Vertical Stabilizer Span 0,400 m Total Area 0,16 m² Aspect Ratio 2,0 Taper Ratio 0,45 Leading Edge Sweep 0,0 Rudder Area 0,048 m² Airfoils NACA 64A011 (Root) NACA 0009 (Tip) Lever Arm to CG 1,218 m 7

3.4 Fuselage The fuselage is kept as small as possible. It is sized by the cargo compartment and the avionics and flight termination equipment. A transparent bulge is added for the camera system. The system layout is shown in figure 3-2. A pusher cruise propulsion layout is chosen, so that the propeller will ingest/re-energize the boundary layer. This will decreases the fuselage drag and improve flight performance. Additionally, the pusher propeller will cure any flow separation due to the fuselage upsweep. The fineness ratio is chosen for minimum drag in subsonic conditions. This concept was also very successfully applied by the Questair Venture. Table 3-4 - Fuselage Data Specifications Fuselage Length 1,200 m Frontal Area 0,096 m² Equivalent Diameter 0,35 Fineness Ratio 3,43 Figure 3-2 - System Layout 8

3.5 Landing Gear Minerva s wing is laid out in the inverse gull wing design, because a short landing gear is always the best option for a light landing gear. VTOL aircraft are very weight critical, but because the range criterial for this mission are very challenging also great aerodynamics are a must-have. For this reason it was decided to offer a retractable skid landing gear. It employs a very safe and simple swing mechanism. A skid gear is favorable for pure VTOL missions also in rugged terrain. The gear is only needed for ground clearance reasons to remove the payload successfully from below the aircraft. Otherwise, a landing on the booms would be completely feasible. The retraction of the gear is inspired by the Douglas DC-3 s gear, which would not completely retract, but rather still allow belly landings on the wheels. For Minerva the gear swivels aft along the side of the booms and still allows emergency landings. Figure 3-3 - Landing Gear Concept The same simple mechanism could also allow for conventional rolling take-offs and landings to improve range if wheels are attached. This could also be useful for flight testing, but comes at a small drag penalty. 9

4 Aerodynamics and Flight Performance 4.1 Aerodynamic Analysis Minerva really excels in the flight performance department. Due to the sophisticated aerodynamic design, very high L/D ratios are reached. The aero-analysis is based on a RANS-CFD method with the following settings: Constant Density Model k- Turbulence Model, fully turbulent 12 million cells 18 prism layers y+ < 1 The results are shown in figure 4.1 below. Figure 4-1 - Results of CFD Analysis 10

Analysis of the landing gear showed, that a drag penalty of +13 drag counts must be accepted for flying with the gear extended. Consequently, the retractable gear solution really pays off. Static stability is about 12,8 % and no pitch-up tendencies are observed. The modified drag polar, fitted to the RANS results, is as follows: CD,min = 0,0340 CL,min Drag = 0,22 AR = 16,8 e = 0,55,,. ² The low value for the Oswald factor e is caused by the impact of the stopped rotors. Because no extend of laminar flow is modeled by these calculations, and because the effect of boundary layer ingestion by the cruise motor is also neglected, the drag values are conservative. Two explary pictures are shown below for the cruise condition: 0 Fuselage-AoA, C L = 0,65 Figure 4-2 - Pressure Coefficient Distribution - 0 AoA The slightly separated flow on the back of the fuselage will disappear, as soon as the cruise engine provides some suction in this area. 11

Figure 4-3 - Flow Visualization - 0 AoA 4.2 Flight Performance Due to low weight and even lower drag, the payload-range figures are exceptional. The performance is given in Table 4-1 below: Table 4-1 - Flight Performance Performance V max 44,4 m/s 160 km/h V Stall 17,4 m/s 63 km/h Climb Rate 3,9 m/s 780 fpm Maximum Range (Vcruise = 23 m/s) Range with 3 kg payload Range with 5 kg payload 158 km 119 km Maximum Speed for given distance Max. Speed with 3 kg payload over 100 km 34,9 m/s 126 km/h Max. Speed with 5 kg payload over 60 km 38,6 m/s 139 km/h Minerva is a very strong performer, surpassing flight-distance by almost 100%. Twice the required range is offered for a given payload range. This is mission performance worthy of a Goddess name! Further performance information can be obtained from the provided frame sheet. 12

5 Loads and Structural Concept 5.1 Loads Minerva complies with the certification requirements of STANAG 4703 amended by the CS-VLA. The corresponding v-n diagram is shown below in figure 5-1. Because STANAG 4703 only applies to unmanned aircraft heavier than 25 kg, it is possible to deviate from the regulations. For this challenge, the official requirements only call for gust loads of 10 m/s, this is the standard the UAV is designed to. Minerva s structure is not sized by gust loads, but it was selected to increase the maneuvering load factors to +4,1 and -1,8. These limit loads give a ultimate structural limit of n max = +6,15 and n min = -2,7 with a factor of safety of SF = 1,5. Figure 5-1 - v-n Plot 13

5.2 Structural Concept The plan is to use the cargo box mount as part of the primary structure. The wings will attach to that structure and the bending moment will be taken up by the two spars at the central box. By using this layout a very stiff but lightweight fuselage can be realized. The key word here is structural synergism. Because the payload is kept within a very strong framework, maximal protection is ensured. All the systems attach to this central box, making additional frames unnecessary. Figure 5-2 - Structural Concept 14

5.3 Modularity and Ease of Handling Minerva can be easily disassembled and handled by two average-size people. The fuselage and the central wingbox remain as one part, and therefore the wing length is reduced to below two meters. The booms are sized by the packaging requirements, and after the forward rotors are detached, they, too, fit into a two meter box. This leaves only the horizontal stabilizer as a different part (stored between the booms in figure 5-3), because the vertical stabilizers are an integral part of the booms. Therefore Minerva is composed of only five separate parts (not counting the propellers) and can be quickly assembled and launched. The transport box inside measurements are shown below in figure 5-3. Such a box fits on the bed of any pick-up truck or small transport vehicle, thereby maximizing options for all kinds of transport. Figure 5-3 - Transport Box Concept 5.4 Waterproofness and Environmental Control Systems Because a lot of effort went into a sophisticated aerodynamic design, a high quality surface is needed to obtain the maximum benefit. Molded composites lend themselves very well to waterproofing a design. Even without special precautions regarding sealing, operation in moderate rain is possible. All vital avionics are placed in the fuselage where drainage provisions can easily be incorporated. Because all electronics (apart from the motors) are place tightly together, it is very simple to facilitate an effective cooling system. The easiest solution would be air cooling with ram air recovery. A ram air scoop could be added for operations with high ambient temperatures, or the inlet could be closed off in cold weather ops. The simplest solution 15

would be to make the cooling system ground-adjustable only, but even an in-flight adjustable option would only add very little weight. 5.5 Fail Safe Components and Safety Provisions The heavily loaded center section is constructed in a two spar layout, a feature offers multiple load paths and increases redundancy. The landing gear mechanism is not required to function for safe operation of the UAV, as landings on the retracted skids are completely safe. All control surfaces are fully redundant. Each aileron is split with separate actuators, as is the elevator. Because of the twin boom layout, the rudders offer redundancy anyways. Catastrophic failures are avoided by having the emergency parachute system aboard. A basic weight-on-wheel sensor (weight on skid, respectively) is used to determine a safe landing. The rotors are then shut down. Additionally, an electric switch can be built into both winglets. The user must activate this switch before working on the UAV, to forbid any rotor movements. After the switch is released from the wingtip (a safe distance away from the props), a new takeoff sequence can be initiated. 16

6 Systems 6.1 Weight and Systems Table 6-1 - Chosen Equipment and Weights Mass [kg] System: Structure 5,200 Wing 1,038 Horizontal Tail 0,140 Vertical Tail 0,119 Fuselage 1,748 Booms 1,177 Landing Gear 0,567 Payloadbox 0,400 System: Avionics and Electronics 3,400 System: Flight Control Actuation 0,260 Actuators Left Aileron 2 x 25gr 0,050 Actuators Right Aileron 2 x 25gr 0,050 Actuators Elevator 2 x 25 gr 0,050 Actuators Rudder 2 x 25 gr 0,050 Actuators Landing Gear 2 x 30 gr 0,060 System: Propulsion 12,725 Subsystem: VTOL 2,644 4x T Motor U10Plus + ESC 2,200 4x CF Propeller 30x10.5" 0,444 Subsystem: Cruise Propulsion 0,630 Scorpion SII 4035 250KV + ESC 0,584 CF Propeller 15x10" 0,046 Subsystem: Energy Storage (MaxAmps.com) 9,451 12 S Lipo, 180 Wh/kg 9,451 Additional Mass for Installations 0,400 Empty Mass 21,985 Payload 3,000 17

6.2 Payload and Cargo Concept The original idea was to have a Payload box with segregated compartments to fixate the payload and restrict CG movements. But I will not elaborate on this idea and here is why: Martin S. Franken, B.Eng, developed an ingenious payload bay / payload box concept for the entry PhoenAIX by the ACDesign Team. It is effective, easy to use even by untrained personal and very reliable. In my opinion, this very design feature alone deserves the cargo experts price! PhoenAIX concept could be used on any aircraft that uses the minimal payload bay dimensions and it should definitely be used by in Minerva s design. Figure 6-1- Design Sketch 18