HELICOPTER PERFORMANCE
Assuming that a helicopter engine and all components are operating satisfactorily, the performance of the helicopter is dependent on three major factors:
Density altitude (air density)
Gross weight
Wind velocity during takeoff, hovering, and landing
Air density
Air, like liquids and other gases, is a fluid. Because it is a fluid, it flows and changes shape under pressure. Air is said to be “thin” at high altitudes; that is, there are fewer molecules per cubic foot of air at 10,000 feet than at sea level. The air at sea level is “thin” when compared to air compressed to 30 pounds of pressure in an automobile tire. A cubic inch of air compressed in an automobile tire is denser than a cubic inch of “free” air at sea level.
For example, in a stack of blankets, the bottom blanket is under pressure of all blankets above it. As a result of this pressure, the bottom blanket may be squeezed down until it is only one-tenth as bulky as the fluffy blanket on top. There is still just as much wool in the bottom blanket as there is in the one on top, but the wool in the bottom blanket is 10 times more dense. If the second blanket from the bottom of the stack were removed, a force of 15 pounds might be required to pull it out. The second blanket from the top may require only 1 pound of force. In the same way, air layers near the earth’s surface have much greater density than air layers at higher altitudes. The lower the elevation of the earth’s surface, the greater the density of the air layers. For example, the layer of air at sea level would be denser than the layer of air at the earth’s surface at Denver, Colo., at approximately 1 mile above sea level.
The above principle may be applied in flying aircraft. At lower levels the rotor blade is cutting through more and denser air, which offers more support (lift) and increases air resistance. The same amount of power, applied at higher altitudes where the air is thinner and less dense, propels the helicopter faster.
Altitude
The greater the elevation of an helipad or landing area, the less the atmospheric pressure and, consequently, the less dense the air. The less dense the air, the greater the density altitude. What is the result when operating at a high density altitude? Helicopter performance is decreased.
Atmospheric pressure
The atmospheric pressure at helipads at a given elevation can change from day to day – sometimes a very noticeable amount which, when combined with other factors, could be significant. The lower the pressure at a given elevation, the less dense the air; the less dense the air, the higher the density altitude and, as a result, the less performance the helicopter will have.
Effect of high density altitudes on helicopter performance
High elevations, high temperatures, and high moisture content, all of which contribute to a high density altitude condition, lessen helicopter performance. Because the difference between the power available and the power required is so small for a helicopter, particularly in hovering flight, density altitude is of even greater importance to the helicopter pilot than it is to the airplane pilot. Helicopter performance is reduced because the thinner air at high density altitudes reduces the amount of lift of the rotor blades. Also, the (unsupercharged) engine does not develop as much power because of the thinner air and the decreased atmospheric pressure.
Hovering flight
High density altitudes reduce the hovering capabilities of the helicopter. Under any given load condition, the higher the density altitude, the lower the hovering ceiling; that is, the elevation at which the helicopter will be able to hover will be lowered as the density altitude increases.
Takeoff
For any given gross weight, the higher the density altitude at point of departure, the more power that is required to make a vertical takeoff to a hover. In fact, under certain gross weight and density altitude conditions, a helicopter may not have sufficient power to lift off vertically, in which case, if takeoff is made, it would have to be a running takeoff.
Rate of climb
For any given gross weight, the higher the density altitude, the less the rate of climb for any helicopter. Although a helicopter may be able to take off and clear obstacles close by, higher obstacles farther away may not be cleared because of this reduced rate of climb.
So weight of passengers is most important while flying helicopters at higher altitudes.
