could carry patients inside the aircraft, allowing a medical corpsman to administer in-flight treatment.
In almost all other respects it was less than perfect. One of its major problems was the comparatively low power of the engine. The critical factor in planning all helicopter flights with heavy cargoes is what pilots know as "density altitude"-the effective height above sea level computed on the basis of the actual altitude and the air temperature. The warmer the air, the less its resistance to the rotor blades and the less lift they produce. Because of its lack of fixed wings, which permit a powerless glide, a helicopter whose engine quits or fails to produce adequate power at a high density altitude can easily crash. Given enough forward airspeed and height, most helicopters, including all the Huey models, can drop to the earth and still land if the power falls, using the limited lift produced by the freely-spinning rotor blades. But this maneuver, called an autorotation, is virtually impossible to execute in a low-level, hovering helicopter. A writer for the Marine Corps suggests that this explains "...why, in generality, airplane pilots are open, clear-eyed, buoyant extroverts and helicopter pilots are brooders, introspective anticipators of trouble."
Although the A- and B-model Huey engine often lacked enough power to work in the heat and high altitudes of South Vietnam, it was much stronger than earlier Army helicopter engines. A great advance in helicopter propulsion had come in the 1950s with the adaptation of the gas turbine engine to helicopter flight. The piston-drive engines used in Korea and on the Army's UH-34 utility helicopters in the 1950s and early 1960s had produced only one horsepower for each three pounds of engine weight. The gas turbine engines installed on the UH-1 Hueys, which the Army first accepted in 1961, had a much more favorable efficiency ratio. This permitted the construction of small, low-profile aircraft that was still large enough to carry a crew of four and three litter patients against the back wall of the cabin. But the high density altitudes encountered in II Corps Zone in Vietnam meant that the UH-1A and UH-1B with a full crew-pilot, aircraft commander, crew chief, and medical corpsman-often could carry no more than one or two patients at a time.
In the early 1960s, shortly after the first U.S. Army helicopters were sent to South Vietnam, the Army began to use an improved ver-sion of the UH-1B: the UH-1D, which had a longer body with a cabin that could hold six litter patients or nine ambulatory patients. The longer rotor blade on the UH-1D gave it more lifting power, but high density altitudes in the northern two corps zones, where U.S. troops did most of their fighting, still prevented Dust Off pilots from making full use of the aircraft's carrying capacity. Finally in 1967 the commander of the 4th Infantry Division registered a complaint about his aeromedical evacuation support.
