Helicopters Radio-controlled helicopters, although often grouped with RC aircraft, are in a class of their own due to the vast differences in construction, aerodynamics and flight training. Hobbyists will often venture from planes, to jets and to helicopters as they enjoy the challenges, excitement and satisfaction of flying different types of aircraft. Some radio-controlled helicopters have photo or video cameras installed and are used for aerial imaging or surveillance. Newer "3d" radio-control helicopters can fly inverted with the advent of advanced swash heads, and servo linkage that enables the pilot to immediately reverse the pitch of the blades, creating a reverse in thrust. Flying bird models, or ornithopters Some RC models take their inspiration from nature. These may be gliders made to look like a real bird, but more often they actually fly by flapping wings. Spectators are often surprised to see that such a model can really fly. These factors as well as the added building challenge add to the enjoyment of flying bird models, though some ARF (almost-ready-to-fly) models are available. Flapping-wing models are also known as ornithopters, the technical name for an aircraft whose driving airfoils oscillate instead of rotate. Toy-class RC Since about 2004, new, more sophisticated toy RC airplanes, helicopters, and ornithopters have been appearing on toy store shelves. This new category of toy RC distinguishes itself by: Proportional (vs. "on-off") throttle control which is critical for preventing the excitation of phugoid oscillation ("porpoising") whenever a throttle change is made. It also allows for manageable and steady altitude control and reduction of altitude loss in turns. LiPo batteries for light weight and long flight time. EPP (Expanded Polypropylene) foam construction making them virtually indestructible in normal use. Low flying speed and typically rear-mounted propeller(s) make them less harmful when crashing into people and property. Stable spiral mode resulting in simple turning control where "rudder" input results in a steady bank angle rather than a steady roll rate. As of 2013, the toy class RC airplane typically has no elevator control. This is to manage costs, but it also allows for simplicity of control by unsophisticated users of all ages. The downside of lack of elevator control is a tendency for the airplane to phugoid. To damp the phugoid oscillation naturally, the planes are designed with high drag which reduces flight performance and flying time. The lack of elevator control also prevents the ability to "pull