April 8, 2006 One of the greatest difficulties with vertical take-off and landing (VTOL) aircraft is that they invariably don’t have high speed as one of their attributes. The principles are well explained in our article about the Cartercopter, and it’s one of the prime reasons the US military has persisted with the V-22 Osprey. There has been much emphasis on the development of new unmanned air vehicles (UAVs) in recent times, and development imperatives have often been torn between the various needs for UAVs that can quickly move from target to target yet loiter as a fixed and stable platform, all the while operating with no launch and retrieval infrastructure. One of the planet's most innovative companies, Aerovironment, has proposed an innovative configuration known as the Skytote to meet all of these needs. The SkyTote is a novel UAV using dual counter rotating propellers that will take off and land vertically like a helicopter, but also transition into horizontal flight like a conventional aircraft for efficient point-to-point operation. This complex vehicle uses an intricate drive system to allow helicopter operations with cyclic and collective control, as well as blade pitch control, combined with normal aircraft control surfaces in conventional flight operations
United States Air Force Research Laboratory scientists are working on the novel SkyTote whose primary mission is to deliver a payload to a specific point within a tactically relevant range and time. It is a Small Business Innovative Research Phase II effort orchestrated by AeroVironment for AFRL's Air Vehicles Directorate.
According to AFRL's 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 said.
Researchers are hopeful that after careful analysis and testing, SkyTote will become a safe, inexpensive and reliable choice for assessing bomb damage and conducting resupply missions or helping with emergency troop evacuations. The SkyTote combines the vertical takeoff, and landing and hover capabilities of helicopters with the high-speed cruise capability of a fixed-wing aircraft.
Counter-rotating rotors with individual cyclic control provide propulsion. Propulsion and transition from wing to propeller flight are some of the major technical challenges in this effort.
Because of the cyclic control, SkyTote looks like a helicopter when it is flying in helicopter mode. When it is flying like an airplane, the helicopter propeller-rotor system functions more like a propeller.
"It's not a great rotor or 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 is that this system will work well for this type of airplane,” Mr. Cord said. “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 is one of the big areas we have addressed during the past few years."
Researchers have been working on various versions of the SkyTote since 1998. It was smaller then with a design meant to deliver a 400-pound payload to a point within a 300-mile range in less than two hours. During the initial design phase, the vehicle was redesigned from the original 2-foot to an 8-foot vehicle to make it more realistic and usable for customers.
The new test vehicle was altered to carry a 50-pound payload within 150 nautical miles because researchers believed a medium-sized vehicle would be a more representative test of technology and would be directly useful to customers. Mr. Cord said these characteristics cannot be met by conventional helicopters and fixed-wing vehicles.
Conventional helicopters with the same payload characteristics can only attain speeds between 100 and 105 knots, while the SkyTote, equipped with hover capabilities, can attain speeds of 200 knots.
The change in payload capabilities created a more realistic, mechanical system, rather than a simplified concept demonstrator. It also created more challenges for developers.
"We had to choose a different type of engine, a 52-horsepower engine from UAV Engines, Ltd., a real engine that could be seen in a car almost," Mr. Cord said. "Our vehicle is 208 pounds, 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 less thrust-to-weight, 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 said.
During testing, SkyTote is expected to operate in hover and conventional wing-borne flight while also transitioning from hover to wing-borne and back. A pilot will be standing by to take control of the plane, if needed.
However, most of the testing will be controlled autonomously. Developers say these transitions have to be done at least five times to demonstrate that SkyTote and its characteristics are a good concept and should be considered for future applications.