Personal Rotorcraft Design and Performance with Electric Hybridization Christopher A. Snyder Aerospace Engineer NASA Glenn Research Center Cleveland, Ohio, USA ABSTRACT Recent and projected improvements for more or all-electric aviation propulsion systems can enable greater personal mobility, while also reducing environmental impact (noise and emissions). However, all-electric energy storage capability is significantly less than present, hydrocarbon-fueled systems. A system study was performed exploring design and performance assuming hybrid propulsion ranging from traditional hydrocarbon-fueled cycles (gasoline Otto and diesel) to all-electric systems using electric motors / generators, with batteries for energy storage and load leveling. Study vehicles were a conventional, single-main rotor (SMR) helicopter and an advanced vertical takeoff and landing (VTOL) aircraft. Vehicle capability was limited to two or three people (including pilot or crew); the design range for the VTOL aircraft was set to 150 miles (about one hour total flight). Search and rescue (SAR), loiter, and cruise-dominated missions were chosen to illustrate each vehicle and degree of hybrid propulsion strengths and weaknesses. The traditional, SMR helicopter is a hover-optimized design; electric hybridization was performed assuming a parallel hybrid approach by varying degree of hybridization. Many of the helicopter hybrid propulsion combinations have some mission capabilities that might be effective for short range or on- demand mobility missions. However, even for 30 year technology electrical components, all hybrid propulsion systems studied result in less available fuel, lower maximum range, and reduced hover and loiter duration than the baseline vehicle. Results for the VTOL aircraft were more encouraging. Series hybrid combinations reflective of near-term systems could improve range and loiter duration by 30%. Advanced, higher performing series hybrid combinations could double or almost triple the VTOL aircraft’s range and loiter duration. Additional details on the study assumptions and work performed are given, as well as suggestions for future study effort. NOTATION achieved parity with energy-dense hydrocarbon fuels, but may be adequate for shorter range missions envisioned for DGW = design gross weight on-demand mobility. The optimum combination of electric GTE = gas turbine engine motors and batteries for short-duration, high power ISA = international standard atmosphere operations, while leveraging hydrocarbon-fueled engines for MCP = maximum continuous power additional range capability needs to be explored and better NDARC = NASA Design and Analysis of Rotorcraft understood. OGE = out of ground effect SAR = Search and Rescue To help explore these potential propulsion and energy SMR = single-main rotor (helicopter) combinations, a system study was performed using two, SOA = state of the art representative vertical lift concepts. One is the more Vbe = best endurance velocity traditional and understood, single-main rotor (SMR) Vbr = best range velocity helicopter; the other, an all-electric VTOL aircraft. As VTOL = vertical takeoff and landing opposed to trying to directly compare such distinct concepts η = efficiency against each other, propulsion system options were explored for each vehicle concept. From such efforts, it is hoped to INTRODUCTION recognize general results that extend to both concepts, while New generations of electric motors / generators are achieving understanding which combinations might have the most high power-to-weight, efficiency, reliability and operational benefits or penalties, based on each concept’s particular flexibility that offer the potential for new, aviation vehicle design and operation. Traditional and advanced and mission opportunities, while mitigating noise and hydrocarbon-fueled engines were modeled as either the main emissions impacts. Concepts that employ vertical takeoff propulsion system or a secondary system to enhance range and landing (VTOL) operations have an additional, unique capability, depending on the vehicle. Electric motors and potential to enhance personal mobility; but VTOL operations generators with battery energy storage that represent state of require significant power. Electrical energy storage has not the art (SOA) systems or be flight ready in 15 and 30 years Presented at the AHS 73rd Annual Forum, May 9-11, 2017, Government and is not subject to copyright protection in the Fort Worth, Texas, USA. This is a work of the U.S. U.S. 1 were also explored. These electric systems could also be the power systems. Notional vehicle representations are shown main vehicle propulsion or short duration, high-power assist in Figure 1 and baseline concept vehicles specifications are to improve vehicle capability. The combination of various given in Table 1. propulsion and hybrid systems will be discussed in a The vehicle payload mission capability was selected as one subsequent section. to two passengers (450 lb., 205 kg maximum total payload) with a 200 pound (91 kg) pilot. The design range capability Vehicle concepts will be covered first, highlighting varied between these two concepts. The SMR helicopter similarities and differences among the chosen vehicles and model is representative of present, operational vehicles in their respective design philosophy. Next, present and future that size class. This particular vehicle class has almost 200 motive propulsion and energy systems will be examined, nautical mile range and significant loiter capability, although including performance levels expected in the near and farther at typical helicopter speeds (generally best range velocity, term. Then, the analysis methodology section will explain V , is around 100 knots, with maximum endurance velocity, the various study assumptions, the specific tools and vehicle br V , closer to 60 knots). Approximately one hour flight models. Finally, results will be presented, potential future be duration seemed reasonable for the VTOL aircraft. The efforts will be proposed, and some final conclusions given. notional design had a Vbr approximately equal to 170 knots. This combination of design choices led to the design mission VEHICLE CONCEPT range being set to 150 nautical miles. A few, additional A SMR helicopter and two, all-electric VTOL aircraft design considerations are mentioned for each concept in the enabled by distributed propulsion were modeled to estimate next section; a more thorough discussion for each concept their performance with a combination of propulsion and can be found in many textbooks and is unnecessary here. Figure 1. Notional vehicle representations: left) Single Main Rotor (SMR) Helicopter, right) All-Electric VTOL Aircraft. Table 1. Baseline Concept Vehicles Specifications. Single Main All-Electric All-Electric Vehicle → Rotor (SMR) VTOL Aircraft, 15 VTOL Aircraft, 30 Parameter ↓ Helicopter year technology year technology Design gross weight (DGW), lb. (kg) 2,050 (930) 2,840 (1,291) 2,172 (987) Empty weight, lb. (kg) 1,100 (500) 2,185 (993) 1,517 (689) Disk loading / wing loading, lb./ft^2 3.6 / N.A. 15 / 50 15 / 50 Nominal fuel weight, lb. (kg), % DGW 588 (267), 21% 250 (113), 11% 160 (73), 8% * (552 MJ battery) (421 MJ battery) Sea level maximum rated power, hp 190 (142) 465 (347) 336 (251) (kW) Reciprocating All-electric, 15 year All-electric, 30 year Engine type (Otto cycle) technology technology Engine weight, lb. (kg), % DGW 270 (123), 13% 136 (62), 5% 69 (31), 3% Engine power / weight, hp/lb. (kW/kg) 0.71 (1.2) 3.4 (5.6) 4.9 (8.0) Sea level power specific fuel 0.500 (0.305) N.A. N.A. consumption, lb./hp-h (kg/kw-h) Power / DGW, hp/lb. (kW/kg) 0.09 (0.15) 0.16 (0.27) 0.15 (0.25) Cruise velocity (Vbr), knots (km/h) * 100 (185) 170 (315) 170 (315) Range, nautical mile (km) * 195 (360) 150 (280) † 150 (280) † * from mission analysis † Design value 2 All-Electric VTOL Aircraft 186 kW), the spark-ignited, Otto cycle is dominant. Such legacy engines best operate using leaded, aviation gasoline, The all-electric, VTOL aircraft is a hybrid helicopter / but the adverse effects from the lead additives have resulted airplane design, enabled by advances in electric propulsion in legislation to eliminate the leaded versions of this fuel and technologies. Advanced electric motors, with their high the engines that use it. Fuel and engine research have efficiency and power-to-weight, also have the potential to developed non-leaded fuel alternatives and it appears these scale with reduced or no performance penalties. Instead of engines will remain in operation for the foreseeable future. one or only a few, vertical lift rotors; many, distributed, Overall efficiency is rather poor (≈ 27%) and power-to- smaller electric motor / rotor combinations can be used to weight also tends to be low, partially a result of conservative enhance performance, propulsion redundancy and safety. design margins to achieve safety. Since there seems to be Using distributed propulsion adds the potential for additional insufficient interest for significant investment for design freedom to optimize for one or multiple missions improvements, only one technology level was assumed for (including bias toward various cruise, hover or other desired this cycle. requirements). Many of the vehicle and multiple-rotor propulsion design interactions are still being explored as the Table 2. Motive engine and energy storage requisite technologies develop. A recent work by Young characteristics (100 hp / 75 kW class). (Reference 1) noted