Electric VTOL Aircraft Subscale Prototyping Overview Francesco Giannini fgiannini@aurora.aero 1 08 June 8 th, 2017
Contents Intro to Aurora Motivation & approach for the full-scale vehicle Technical challenges Design and building the subscale models Lessons learned 2
Aurora has been developing autonomous aircraft systems for 28 years AUTONOMY ELECTRIC PROPULSION AIRFRAME DESIGN AURORA BY THE NUMBERS Heritage Locations 28 8 Years in Business Worldwide UAS Design 30+ Hours 80 Built and Flown Endurance Record Patents 100+ Feet 103K Issued & Pending Highest Altitude Flown 3
Three principles used in concept design LARGE NUMBER OF REQUIREMENTS TO CONSIDER USE THREE DESIGN PRINCIPLES 1 2 3 Keep it simple Design for efficient cruise Base design on existing technology 4
Detailed Concept Modelling and Selection Traditional and novel promising configurations were modeled and analyzed before selecting a lift + cruise configuration 5 https://vertipedia.vtol.org/vstol/wheel.htm
Aurora BCG DV concept: All-Electric VTOL Aircraft Design Optionally piloted can be flown fully autonomously or piloted Eliminates mechanical complexity Direct drive, fixed pitch rotors/prop Takeoff noise comparable to road traffic Inaudible in cruise flight No safety critical single points of failure 6 Light-weight, rigid structure avoids induced vibrations >3x as efficient as a multi-copter in cruise
Video at: https://www.youtube.com/watch?v=bsfvvhqp7ws&t=6s 7
Design challenges: Need to look out for negative wingrotor interactions during transition HOVER TRANSITION Rotor lift (reduced due to wing interaction) Wing lift Rotor lift v Download from rotor wash Rotors placed so that rotor wash does not impinge on the wing As aircraft gains forward speed, rotor wash impinges on the wing to create a download Rotor-wing interaction reduces lift generated by the rotor, requiring rotor to increase power output 8
Three main test objectives for the sub-scale model Limited experience in industry with low disc loading vs. high wing loading design. Need to verify rotor-wing interactions are manageable. 1 Prove that configuration is able to perform the full mission 2 Verify power consumption and controllability throughout mission is as expected, particularly transition 3 Determine suitable sequence of reducing rotor thrust to support low-power transitions Furthermore, flight testing helps us identify any unknown unknowns early 9
Sizing the Subscale Vehicle For data obtained from scale-model to be representative of full-scale aircraft, need to observe similitude requirements and scaling relationships for both the aircraft and test conditions: Scale Factor Linear dimension n 1/4 Relative density (m/ρl3 ) 1 Froude number (V2 /lg) 1 Angle of attack 1 Linear acceleration 1 Weight, mass n^3 /σ (1/4)^3 = 1/64 Moment of inertia n^5 /σ Linear velocity n^1/2 (1/4)^(1/2) = 1/2 Angular velocity 1/ (n^1/2) Time n^1/2 Reynolds number (Vl/ν) n^1.5 * (ν/ν0) Full-scale span = 8 m M = 800 kg v = 180 km/h Sub-scale span = 2 m M = 12.5 kg v = 90 km/h Note: For rigid, free-flight models where compressibility and Mach number effects not of concern Chambers, J. Modeling Flight: The Role of Dynamically Scaled Free-Flight Models in Support of NASA s Aerospace Programs NASA SP 2009-575 10
Subscale Vehicle Design 2.0 m 2.0 m Mass, kg 12.5 Span, m 2.0 Rotor diameter, cm 35.5 Cruise prop diameter, cm 30.5 Cruise motor, max, kw 2.0 Lift motors (max/cont), W 600/300 Max Speed, km/h 90 Hover time, min. 7.0 11
10% subscale design The aerodynamics of the threelifting surface configuration were analyzed using a Vortex-lattice method. However, much uncertainty remained regarding the extent of the de-stabilizing effect of the fuselage, as well as the desired cg location A 10% scale model was built and flown with and without the fuselage, providing useful information on cg position and desired engine thrust line 12
Subscale Materials and Construction Flying surfaces : CNC Foam core/balsa skin/ultracote Electronics: COTS RC batteries, servos, motors, propellers, ESCs, autopilot boards and software, computer for telemetry and logging 100+ 3D printed parts (High Temperature PLA with carbon fiber fill) COTS RC landing gear COTS carbon tube 13
Lessons Learned A properly-sized subscale model is a very good development tool Brings to light unknown unknowns Even very small subscale models can represent substantial bang for the buck value Adjustable features on the model prevent timeconsuming rebuilds Have high-def video of all flights for post-flight review Size controllers, motors, servos, with ample margin Next-day shipping and in-house AM capability compressed our schedule 14
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