For a number of years, the armed services have been experimenting with the use of unmanned aerial vehicles (UA Vs) for battlefield surveillance and reconnaissance. Used by military forces in many countries, UA Vs offer a cost-effective, low-risk alternative to manned aircraft. Though they are necessarily placed in vulnerable positions and are therefore subject to high attrition, their unit cost is low, and, most importantly, they do not place personnel at risk. They can also be smaller and less observable than conventional aircraft, making them a good alternative for covert surveillance.
As with all aircraft engines, size and weight are important, as is the ability to perform well at altitude. In addition, a UA V engine should be simple and reliable. Unmanned aerial vehicle propulsion systems must be smail, lightweight, and have a high power-to-weight ratio - on the order of 0.34 kW /kg for smaller UAVs. A rated power output of about 20-50 kW is desired, according to the size of the aircraft. A key feature of the powerplant is that it must use heavy distillate fuel, such as jet fuels JP-5 and JP-8, to be consistent with current and future military supply strategy, which stresses improved logistics and reduced fire hazard. Existing UAVs operate on gasoline. To meet the new powerplant requirements, SwRI engineers recently designed a unique UAV engine and built and successfully demonstrated a working prototype.
Many researchers, including those at SwRI, have experimented extensively with combustion systems designed to burn heavy fuels in a low-compression engine. Several difficulties have limited the success of these systems, primarily the low volatility of heavy fuels, which restricts cold startability, and the tendency to knock, which limits compression ratio, in turn limiting thermal efficiency and fuel economy. Because overall aircraft propulsion system weight includes considerations for mission fuel weight as well as system hardware weight, fuel economy is a significant factor in the selection of a UA V powerplant. Based on their experience with spark-assisted combustion systems, Institute engineers concluded that the best combustion system for a UA V engine was a conventional two-stroke cycle diesel with two power cylinders. The combustion system is indirect injection, fueled by two single-plunger injection pumps driven from the crankshaft. Such a configuration affords reliable combustion, even under cold starting conditions, and good fuel economy.
Beginning in 1993, Stanley K. Widener, manager of design and analysis in the Department of Engine Design, Engine and Vehicle Research Division, led an internally funded project to design and demonstrate an engine to meet the military's UA V requirements of high power density, power output compatible with small aircraft, heavy-fuel combustion, reliability, durable construction, and producible design. The effort has culminated in the successful construction and operation of a demonstrator engine, with characteristics suitable for other military or commercial applications requiring lightweight construction coupled with compact heavyfueled powerplants, such as unmanned drones for aerial mapping, hybrid vehicles, and portable power equipment.
The Institute-designed UAV engine delivers 22.4 kW at 4,500 rpm, using diesel fuel, JP-5, or JP-8. Expected fuel consumption for a production engine with JP-8 is 0.24 kg/kW-hr, and engine weight (dry) is 15.9 kg. The prototype is air-cooled and fits in a box measuring 31.75 cm long, 14.2 cm high, and 23.1 cm wide. This size engine would fit a UA V of about 90 kg gross weight. A preliminary design has been developed for a higher horsepower version of the engine for larger aircraft.
Published in the Summer 1995 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.