If consumers are going to receive packages by air to their doorsteps or hop onto aircraft to zip across town, engineers must figure out how to make these coming aircraft more resilient to bad weather than today’s early versions. Dennis Bushnell of NASA’s Langley Research Center in Virginia explains.
BY DENNIS M. BUSHNELL
Weather issues caused or contributed to approximately 35% of fatal general aviation accidents in the United States from 1982 to 2013, according to researchers from Northern Illinois University who in 2016 reviewed decades of accident data. Winds, especially updrafts and downdrafts, are the most prevalent cause. The delivery drones and other commercial unmanned aircraft systems under development today could be more prone to weather-related accidents or upsets, because many are smaller in size and weight than GA aircraft and therefore have lower inertia. Some of the coming advanced air mobility aircraft, which will include on-demand passenger aircraft, will be about the same size and weight as some GA aircraft and will be equally vulnerable.
All of them will fly at low altitudes, home to the worst of the weather, including snow, rain, fog, tornadoes, lightning, icing, excessive heat, hail, sleet, microbursts and wind shifts near thunderstorms. In fact, a study by the U.S. Army Research Laboratory concerning weather impacts on its Aerostar tactical unmanned aerial system notes that the manufacturer recommends against flying the aircraft in severe turbulence. Those conditions can amount to 5% to 70% of the time, depending on the time of day, region and season. Delivery drones and small on-demand passenger aircraft will be similarly vulnerable, and they will fly mostly over built-up areas in increasingly large numbers, constituting an increasingly worrisome safety hazard. They could be blown off course, possibly into buildings or other aircraft. All told, weather conditions can affect aircraft visibility, navigation, performance and controllability and reduce flight duration.
Therefore, for reasons of safety, operability, reliability, utilization and econometrics, designers should improve the next AAM and UAS vehicles so that they can fly in challenging weather.
Given that cost is a major metric for producing these vehicles, prospective solution spaces must be both effective and affordable. Also, the eventual replacement of automobiles with on-demand-mobility passenger aircraft will require operability in weather conditions at least to the extent that automobiles have. The usual conventional aeronautics approaches for weather issues include designing for service in all but extreme conditions and detecting and avoiding or flying around the extreme cases.
Attraction of electric vehicles
Increasingly, AAM concepts and UAS are electrically propelled, and the nearly two dozen reasons for this include significant improvements in nearly all of the air vehicle metrics — safety, acoustics, cost, vibration, aero and propulsive performance, maintenance and emissions.
The huge and ongoing decreases in costs of renewable energy generation and storage by some 85% in the last 10 years for photovoltaics and in the past eight years for storage point to the ongoing demise of coal and possibly gas going forward for electricity generation. In fact, nearly two-thirds of the new generation of electricity results from renewables, according to media coverage of IRENA, the International Renewable Energy Agency.
This bodes well for a shift to nearly emissionless electric propulsion for all transportation, whether on land, by sea or by air. Using energy sourced from renewables will have immense favorable implications and positive impacts on the climate. The scalability and relative ease of application of distributed electric propulsion have spawned a plethora of multi-to-many rotor vehicle configurations.
Many of the initial UAS designs are essentially “fair-weather” machines, not waterproofed for rain and moisture. Many are electric, operating off batteries sensitive to cold and heat, and are capable of flying in winds on the order of up to two-thirds of the maximum flight speed. Wind gusts can be up to the order of twice the average wind speed while AAM and UAS flights near buildings and trees or in urban canyons can encounter large-scale organized dynamic vorticity. Waterproofing and de-icing are probably essential for practical, safe operability in many areas and seasons.
The issue of battery sensitivity to heat and cold could be addressed by a combination of insulation and regeneration of battery heat losses and added energy from external photovoltaic films or other energy sources operating a battery pack climate control system. For vehicles carrying passengers, the cabin environmental control system could be designed to include the battery enclosure. Flight in poor weather will, in general, require IFR equipage for visibility and air traffic control. The major remaining weather issues are wind and rain. Therefore, affordable solution spaces for these that are effective for relatively low speeds and inertia are of interest. Rain can affect flight by direct momentum exchange, by decambering wing surfaces and thereby inducing loss of lift and control authority, by increasing the vehicle mass, changing the center of pressure, by creating roughness and drag, and possibly by flow separation. Weather as a whole can affect controllability, speed, angle of attack and sideslip, and reduce range by requiring increased energy expenditure. The current major weather-related efforts with regard to AAM and UAS involve improvements in detailed local flight weather forecasting.
Optons for enhancing AAM/UAS for safe maneuvering in wind and rain include:
- Increased onboard energy for thrust vectoring for controllability beyond what is available from the usual controls and to maintain speed/range. These increased energy and thrust vectoring capabilities could, in fair weather in the absence of significant weather effects, be utilized to extend the range and increase controllability for vertical takeoff and landing, VTOL, operations.
- Aero controls with morphing surfaces, flow control. Of the preceding, thrust vectoring is probably the more effective overall approach. Thrust vectoring for lift can be achieved by tilting wings or just the nacelles, or by vectoring jet engine nozzles. These designs have been flown over the years on military aircraft and missiles, as well as on some lighter-than-air applications.
Options to enable thrust vectoring for enhanced AAM/UAS weather operability include:
- Gimbaled engines, nozzles and propulsors.
- Fluid injection into the exhaust stream to redirect momentum.
- Auxiliary thrusters.
- Exhaust/jet vanes for directing exhaust.
In particular, electric propulsion handily enables thrust vectoring for enhanced operations in weather. This could be done either by vectoring extant distributed propulsion units or added units that could, when not needed for additional controllability in weather, be used for additional propulsion, for takeoff or for cruising. Overall, a system, capabilities and vehicle configuration tradeoff study should be conducted that includes the option of thrust vectoring for weather. Thanks to electric propulsion, thrust vectoring for enhanced operations in difficult weather is a viable approach. The design and operability of such wind and gust control systems should ensure suitable ride quality for AAM carrying passengers.
To maintain fair-weather cruise range when flying in difficult weather, additional vehicle energy capacity and requisite thrust vectoring for control are needed due to energy losses associated with flying in challenging conditions such as high winds. Electric propulsion can be enabled by two options: fuel cells and batteries. Compared to conventional internal combustion and gas turbine engines, propulsion fuel cells are more efficient but heavier. Batteries are more efficient than fuel cells, but much heavier, due to their much lower specific energy-density, expressed as power per kilogram. Currently, batteries power most electric vehicles, but hydrogen-powered fuel cells are available commercially and proffer some three-times greater range than the battery standard, lithium-ion. Some of that additional range capability could be traded for weather operability via thrust vectoring of distributed electric propulsors.
Advanced batteries are a work in progress, enabled and driven by massive markets for all transportation modes and much else. Going forward, batteries will be increasingly recharged by renewable energy generation, providing nearly emissionless propulsion. The near ultimate advanced battery would utilize atmospheric oxygen as part of the battery operation providing a theoretical energy-density nearly equal to hydrocarbon fuels and an expected initial nominal energy density approximately five times that of lithium-ion. This battery energy-density would be a considerable improvement over hydrogen fuel cells. The U.S. Department of Energy has tested Li-Air batteries with some 700 recharges. Perhaps nearer-term are a plethora of other advanced batteries: lithium-metal, solid-state, glass and others proffering twice to three times the range of lithium-ion designs.
These are closing in on the range of hydrogen fuel cells. Therefore, compared to lithium-ion batteries, the increased energy-density of advanced batteries and hydrogen fuel cells would enable maintaining the Li-ion battery design range and employing distributed electric thrust vectoring, enhancing flight in poor weather. Additional opportunities to improve energy-density include utilization of the increasingly efficient solar photovoltaic films as a vehicle covering to recharge the batteries and provide electricity during cruise. These films could going forward double the efficiency values of photovoltaics via advanced materials and designs by producing two electrons per photon and exploiting much more of the solar spectrum. Another energy-density enhancing possibility would be to store fuel or electrical energy in the vehicle structure.
The potential ahead is enormous. The Vertical Flight Society lists about 200 AAM and UAS variants in development now. Also, the nascent financial aero markets for AAM and UAS would result in a doubling of the current civilian aero markets. It is early days yet for these vehicles and markets. Engineers and researchers are improving vehicle characteristics and capabilities. The major metrics for safety, operational capability, affordability, emissions and acoustics are becoming clearer.
Improved weather capability must feature prominently if the market is to progress as planned.