By Hank Hogan
All helicopters have to land. And that is when challenges can occur. Some are due to the terrain being landed on or the act of landing itself. Fortunately, there is a new technology that promises a solution.
Traditionally, lower-end helicopters have had landing skids that sit rigidly below the aircraft body. It is what TV or movie heroes or heroines cling to precariously in those do or die scenes. Skid landing gear is simple, inexpensive, and works well — in many situations. However, it requires that the landing surface not be pitched too steeply and the rate of descent not be too great. Otherwise, the helicopter can topple over, sending the spinning rotor into the ground. So, landing on a steep hillside is out. Similarly, safely landing on a ship pitching back and forth due to wave action also can be a big no-no. Even coming down hard on a flat, unmoving surface can be problematic. For the military, that means a lot of missions are out of reach for widely available helicopters.
“The landing gear for helicopters is essentially the same as it was when the vehicles were first invented,” says Mark Costello, a professor who investigates autonomous systems at the Georgia Institute of Technology. Costello and a team at Georgia Tech, another name for the Atlanta-based school, have come up with a viable solution: replacing the rigid skids with four, articulated “legs.” When deployed on the ground, these make a helicopter look like a really big insect, resembling something out of a sci-fi movie. The impression is enhanced when the helicopter is airborne with the legs pulled in close; that configuration cuts down on drag, Costello says.
The researchers used a clever approach to create landing gear that can handle uneven surfaces, doing so without resorting to a complicated control scheme. Each of the legs has an electric motor at its hip and knee joints. The bottom of each foot has a force sensor. As the helicopter lands, the sensor picks up on the force of landing, and the motors pull the foot back a bit. When all four legs report having touched down, they lock in place, much like a parking brake on a car. “That very simple method has worked very well in simulation and also in flight tests,” Costello says.
He adds that there is a failsafe mechanism, something that the researchers spent some time developing. If power to the landing system fails, the legs move and lock in a fixed position. When they do so, they form a four-point array that functions much like skid landing gear. This ensures there will not be a case of a helicopter in the air being unable to land because its legs are flopping around. The increased capability to land on uneven surfaces does come at a cost: the articulated legs weigh more than skids. Calculations on an optimized design for a 3,000-pound MH-6 Little Bird helicopter, used by U.S. Army Special Operations forces, put the penalty at about 7 percent of payload. Hence, there will be some reduction in the helicopter’s payload, range, or flight time.
On the other hand, there also could be an increase in speed, because when the legs are fully retracted, they offer less drag than skids. Costello does point out that most helicopters with skid gear have a mission profile where speeds are low and there is a lot of hovering. Consequently, drag is not usually much of a concern.
The Georgia Tech researchers contacted the Defense Advanced Research Projects Agency (DARPA) a few years ago with a proposal about this innovative landing gear. Costello reports that DARPA was enthusiastic about the idea and provided funding. After extensive simulation, the Georgia Tech team built a prototype by modifying an unmanned Rotor Buzz helicopter. Built by UAV Research Lab and originally intended for agricultural uses, the helicopter weighs about 250 pounds when empty and has a 10.5 foot main rotor. The researchers removed the skid landing gear and attached a set of articulated legs. Then, they conducted flight tests. The altered helicopter performed well, eventually starring in movies put out by DARPA. It did well enough to attract the attention of a major helicopter manufacturer, which is having the design work done to put the new technology to use in a deployed system. Such a system could be in flight testing soon, Costello says.
An important point is that the new landing gear is not an all-or-nothing solution. Skids can be removed and legs bolted on as needed. A version also could be applied to helicopters that use something other than skids. Thus, once an effective design is ironed out, the legs could be deployed when and where they make sense.
In talking about this, Costello says, “There’s a lot of potential with this technology for retrofit.”