AFRL scientists are working on SkyTote, a novel unmanned air vehicle (UAV) that will take off and land vertically like a helicopter (see figure) but also transition into horizontal flight like a conventional aircraft. SkyTote's primary mission is to deliver a payload to a specific point within a tactically relevant range and time. AeroVironment, Inc., of Monrovia, California, developed the vehicle under a Small Business Innovation Research effort for AFRL.
According to AFRL's Mr. Tom Cord, SkyTote program manager, the aircraft is a concept demonstrator and not a working system. "We are not trying to reach a certain performance and capability; we are trying to show that a hovering UAV with a fast, forward speed is a likelihood. It's something we can do in a simple way," he explains.
Researchers are hopeful that after careful analysis and testing, SkyTote will become a safe, inexpensive, and reliable choice for activities such as assessing bomb damage, conducting resupply missions, or helping with emergency troop evacuations. SkyTote combines the vertical takeoff, landing, and hover characteristics of helicopters with the high-speed cruise capability of fixed-wing aircraft. Counter-rotating rotors with individual cyclic control provide its propulsion. Issues related to propulsion and the vehicle's transition from rotor- to propeller-based flight pose some of the major technical challenges of this effort. Because of its cyclic control, SkyTote looks like a helicopter when it is flying in helicopter mode. When the vehicle is flying like an airplane, however, the propellerrotor system functions more like a propeller."It's not a great rotor or [a great] propeller; it's a good compromise between a helicopter rotor system and an airplane propeller, and that's part of what we are trying to show—that this system will work well for this type of airplane," Mr. Cord asserts. "When you look at the design parameters, you either go one way or the other. When you start to blend the two systems together, it becomes challenging. That's one of the big areas we have addressed during the past few years."
In fact, researchers have been working on various versions of the SkyTote since 1998.1,2 It was smaller then, with an initial design meant to deliver a 400 lb payload to a point within a 300 mi range in less than 2 hrs. During this initial design phase, however, researchers determined that redesigning the vehicle—increasing its original size from 2 ft to 8 ft—would make it more realistic and usable for customers.
Researchers have since altered SkyTote even further; the new test vehicle is designed to carry a 50 lb payload within 150 nmi. This latest change occurred because researchers believed a medium-sized vehicle would not only allow a more representative test of the technology, but would be more directly useful to customers. Mr. Cord emphasizes that neither conventional helicopters nor fixed-wing vehicles are capable of achieving SkyTote's unique characteristics. Conventional helicopters with the same payload characteristics are limited to speeds between 100 and 105 kt, while SkyTote, equipped with the same hover capabilities as conventional helicopters, can attain speeds of 200 kt.
The evolution of SkyTote's payload capabilities has created a more realistic mechanical system, rather than a simplified concept demonstrator. Yet this same progress has also created more challenges for developers: "We had to choose a different type of engine—a 52 hp engine from UAV Engines, Ltd., a 'real' engine [that you might see] in a car," Mr. Cord states. "Our vehicle is 208 lbs, so we've grown significantly. We added a more complex transmission, too."
A more complex, heavier mechanical system also meant that developers had to look closely at the control aspects of the vehicle. "The reason for this is that excess power reduces the need for a carefully conceived flight control system and helps avoid several problem areas, such as loss of control. With a lesser thrust-to-weight [ratio], we have to rely on the flight control system and the pilot to keep us out of dangerous flight conditions. Autonomous control gives us an advantage during testing because it provides safer, more efficient tests," Mr. Cord explains.
The expectation is that during testing, SkyTote will demonstrate its capacity to take off, hover, and transition from hover to wing-borne flight and back. As a precaution, a pilot will stand by to take control of the vehicle, but researchers will use autonomous controls to test most of SkyTote's functionality. This current phase of the development program will conclude after SkyTote successfully achieves five complete transitions from hover to wing-borne flight and back.
Ms. Karen Jackson (Anteon Corporation), of the Air Force Research Laboratory's Public Affairs Office, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document HQ-H-06-06.
1 Cord, T. J. "The SkyTote Unmanned Air Vehicle." AFRL Technology Horizons®, vol 3, no 2 (Jun 02): 35-36. http://www . afrlhorizons.com/Briefs/Jun02/VA0201.html.
2 Withrow, M. "The Future Unmanned Air Vehicle." AFRL Technology Horizons®, vol 5, no 6 (Dec 04): 48-49. http://www . afrlhorizons.com/Briefs/Dec04/VA0308.html.