When the designer lays out the basic helicopter configuration, he has several options of what landing gear to use and how it should be arranged.
Almost all early helicopter designers copied airplanes and installed wheels, which were mostly used to roll the machine in and out of the hangar. Engineers at Bell Helicopter were perhaps the first to seriously consider skids.
Its first Model 47 had four un-braked wheels with the front two castering. This made ground handling as easy as maneuvering a big market basket. But two experiences led the engineers to rethink the configuration. The first was a series of forced landings during flight tests near the Bell factory in upstate New York. Usually, the only good emergency landing site was a plowed field where, unfortunately, the front wheels tended to dig in and upset the forward-moving helicopter upon touchdown.
The second experience was reported by operators in the field who often had to operate from sloping areas. After shutting down and walking away, they would sometimes turn around to find their helicopters rolling downhill. The solution was skids.
Skids have now become standard on almost all small helicopters. By giving up some convenience in ground handling, these helicopters gained the ability to operate from sloping or soft surfaces. Skids can also be easily equipped with floats or "bear feet" for operations on very soft sites, and are convenient places to strap external loads.
A helicopter equipped with skids moves from one spot to another on the ground by air taxiing, using special ground handling wheels, or by being carried on a dolly.
But then there are wheels
The larger the helicopter, the more difficult it is to handle on skids. For this reason, mos large helicopters use wheels. This makes it possible to ground taxi like an airplane using some collective pitch to generate rotor thrust and some cyclic pitch to tilt it forward (or even backward). Ground taxiing in this manner produces much less rotor downwash than air taxiing and thus less danger of lowing over other aircraft parked alongside the taxiway, less debris blown around and better visibility if there is dust or snow on the ground. Wheels also have obvious advantages in rolling takeoffs and landings. It is significant that builders of large helicopters, Sikorsky and Boeing Vertol, have used wheels from the start. Bell has recently given buyers the option on their larger machines. The Model 222 was designed with retracting wheels but is now available in a utility version with fixed skids. On the other hand, their Model 214ST, which originally used skids, can now be delivered with wheels.
To retract or not to retract
If the designer wants his helicopter to fly fast, he will consider retracting the landing gear. Wheels lend themselves to this easier than skids, although skids can be retracted as was demonstrated by Bell on its prototype Huey Cobra and by Lockheed on the XH-51.
Retractable gear reduces drag but at a price. Non-retractable gear is lighter, simpler, costs less and is more foolproof. It provides more energy absorption for unexpected touchdowns where retracting gear might not be extended Consequently, helicopter engineers still find difficulty in deciding between the advantages of each type.
Most skid gears look about the same but wheels can be arranged in many ways. A survey of helicopters shows that there is no true consensus on wheel placement, even among aircraft designed by each major company. This indicates the decision is being dictated by design requirements. Heavy-lift helicopters, for example, use four wheels to simplify loading. But most others use only three, the single being either a trail wheel or a nose wheel.
Sikorsky had equipped the U.S. Army's Black Hawk with a far-aft tail wheel to protect the back end from accidental ground strikes in nap-of-the-earth flight. But when the manufacturer marinized the design for the U.S. Navy's Seahawk program, it had to move the tail wheel forward so the tailboom could be folded, allowing the aircraft to fit in cramped shipboard hangars.
Incidentally, another change requested for the Seahawk was aluminum wheels in place of the lighter magnesium wheels on the Black Hawk. Magnesium has a bad reputation for corrosion in a salt-spray environment.
Preventing slip on the slope
Regardless of the landing-gear configuration, engineers must still consider the helicopter's stability while parked on a hillside or rolling deck. For example, the U.S. Army required that McDonnell Douglas Helicopters design the AH-64 Apache so it can rest on a 15-deg side slope, or face up or down while resting on a 12-deg slope.
Such requirements dictate the location of the wheels with respect to the aircraft's center of gravity and to do it right, designers must make their calculations while allowing for the compression of the downhill oleo strut and tire.