MV-22 Osprey and Disc Loading

USMC MV-22 Osprey producing visible blade tip vortices from condensation of the marine air.

The V-22 Osprey tilt-rotor aircraft has a high disk loading relative to a helicopter in the hover mode, but a relatively low disk loading in fixed-wing mode compared to a turboprop aircraft.

In fluid dynamics, disk loading or disc loading is the average pressure change across an actuator disk, such as an airscrew. Airscrews with a relatively low disk loading are typically called rotors, including helicopter main rotors and tail rotors; propellers typically have a higher disk loading.

Disc loading of a hovering helicopter is the ratio of its weight to the total main rotor disc area. It is determined by dividing the total helicopter weight by the rotor disc area, which is the area swept by the blades of a rotor.

Disc area can be found by using the span of one rotor blade as the radius of a circle and then determining the area the blades encompass during a complete rotation. When a helicopter is being maneuvered, its disc loading changes. The higher the loading, the more power needed to maintain rotor speed.A low disc loading is a direct indicator of high lift thrust efficiency.

Increasing the weight of a helicopter increases disk loading. For a given weight, a helicopter with shorter rotors will have higher disk loading, and will require more engine power to hover. A low disk loading improves auto-rotation performance in rotor-craft.

The helicopter rotor is modeled as an infinitely thin disc with an infinite number of blades that induce a constant pressure jump over the disk area and along the axis of rotation. For a helicopter that is hovering, the aerodynamic force is vertical and exactly balances the helicopter weight, with no lateral force.

The upward action on the helicopter results in a downward reaction on the air flowing through the rotor. The downward reaction produces a downward velocity on the air, increasing its kinetic energy. This energy transfer from the rotor to the air is the induced power loss of the rotary wing, which is analogous to the lift-induced drag of a fixed-wing aircraft.