Professional Pilot, October 2018

Enabling electric aircraft C limate concerns have instigated serious research for weight critical batteries to enable electric vehicles EVs initially intended for ground transportation Research has advanced to where in less than 10 years EVs could reach parity with gas turbine engines GTE via weight and drag reduction to minimize battery requirements Renewable electric generation could allow essentially emission less aircraft This tech now produces 25 of current electrical generation and 62 of new generation capability Continued rapid cost reductions is driving faster adoption of renewable electric generation Technologies suggested here could reduce battery size and weight via airframe performance improvements allowing OEMs to employ electric propulsion This enables a massive new aeronautics market for affordable safe personal air vehicles Battery propulsion could solve most emissions issues NOX CO2 and water All types of aircraft could go electric Electric aircraft propulsion can be envisaged for all classes of aircraft from small drones and personal air vehicles to long haul and supersonic transports The long standing issue for electric aircraft has been what to do about the long extension cord size weight and functionality of the electricity source The extensive battery research and development instigated by ground EVs has direct application to and created a renaissance in electric aircraft 110 PROFESSIONAL PILOT October 2018 The nominal goal for widespread electrification of aircraft using batteries resides in matching the energy density of current transportation fuels Were now approaching system parity via the extensive research on lithium air batteries coupled with highly efficient electric motors along with vehicle weight and drag reductions Research has resulted in Li Air batteries achieving 15 times the energy density of lithium ion batteries and other research has achieved 750 recharge cycles Also a lithium metal battery at 500 kWh Kg is entering the market This will open the entire speed range for electric aircraft up to supersonic speeds What follows is a synopsis of technology approaches which could improve electric aircraft performance and reduce required battery capacity to approach system level parity Electric propulsion technologies Distributed scalable propulsion technologies that could reduce electrical power requirements via lighter weight and or lower drag include Flow control or designer fluid mechanics Designer fluid mechanics subsumes a large number of flow control approaches and applications including Laminar Flow Control LFC separated flow control for high lift vortex control turbulence control and favorable wave interference for drag reduction Battery weight issues for electric vehicles puts LFC again under consideration to reduce requisite battery capacity For turbulent drag reduction options include relaminarization and riblets Electric propulsion proffers the possibility of straight forward distributed energy for flow separation control Aeropropulsion Synergies Conventional design practice in civilian Vahana the all electric self piloted aircraft from A by Airbus aeronautics is to essentially separate the aerodynamics and the propulsion systems Examples of aeropropulsive synergies include circulation control wings for up to a factor of 4 increase in CL coefficient of lift boundary layer inlet ingesting lower momentum air for up to 10 to 15 propulsion efficiency increase wingtip engines to reduce drag due to lift wing strut and truss bracing are conducive to wing tip engine placement and thrust vectoring that places the engines at the rear of the fuselage to use them for aero controls in lieu of the weight and drag of the empennage hybrid laminar flow with leading edge suction utilized for high lift separation control Wave Drag Reduction Approaches include wing sweep area ruling and reduced thickness as well as wing twist camber and warp Other techniques include nose spikes either physical or forward projection of energy gases liquids or particulates to extend effective body length Favorable shock interference is an approach that uses shockwaves via reflection or interaction to create a favorable interference either for body thrust or lift or both Parasol wings can provide a 20 improvement in overall lift to drag ratio at cruise Reduction of drag due to lift Elliptical loading increased aspect ratio and span lower CL and reduced weight are the primary approaches This has been addressed in many cases via creative overall aircraft configuration designs like truss braced wings The use of non planar lifting surfaces such as distributing the lift vertically through up swept tips and multiple vertically spaced wings can provide sizable reductions In addition devices can be inserted into the tip flow to produce or recover thrust and or energy from local flow EV FLIGHT With changes in propulsion will also come changes in design By Dennis Bushnell Chief Scientist NASA Langley Research Center Robert Moses Research Engineer NASA Langley Research Center Photo courtesy Airbus