Aerodynamics of Helicopter / Ship Interface

Fully unsteady CFD simulation of the flow over a generic frigate for a 45° wind off the starboard bow. The CFD captures several important flow features of concern to maritme aviation pilots including vortices, shear layers and regions of separated flow. (Click on image to activate video). (click on image to view video)

Launch and recovery of helicopters from the decks of ships such as frigates is a demanding challenge for the pilot. The envelope for flight operations is always smaller than for the same helicopter operating from the ground because of the ship airwake, a region of separated unsteady flow through which the helicopter must pass.

This airwake contains significant gradients in mean wind speed and direction, as well as increased turbulence intensities. These gradients require pilots to initiate control inputs to keep the helicopter trimmed and positioned correctly over the flight deck in advance of touchdown. Increased pilot workload and reduced control margins eventually lead to the operating limit.

Video sequence illustrating a montage of a typical test series. Initial rotor power up is followed by a change in ship orientation (wind direction), a lateral translation of the fuselage, and a second change in wind direction. Geometric scale is 1:50 (click image to activate video). (click on image to view video)

The prediction of these operating limits has been an area of active research at the NRC Institute for Aerospace Research (NRC Aerospace) for the past decade. In addition to standard airwake flow survey capabilities, NRC Aerospace researchers have developed an unparalleled high-fidelity wind tunnel simulation of the helicopter/ship dynamic interface. The combination of a scaled powered rotor and a fuselage model mounted around a stiff, lightweight balance has led to a unique capability.

NRC Aerospace can now provide reliable predictions of operating envelopes for a given ship/helicopter combination in advance of first-of-class flight trials. These predictions will reduce the amount of expensive, risky and time-consuming sea trials and provide advance warnings to pilots of dangerous wind directions. Additionally the technology provides rapid and reliable assessment of potential changes to operating envelopes when ship modifications have been made.

NRC Aerospace has also developed the technology to measure on-deck rotor loads to assist with the analysis of securing and traversing helicopters once they are on the flight deck. In beam winds, the on-deck rotor loads generated by upwash over the deck can be destabilizing and contribute to blade sailing during rotor engage-disengage operations.

Numerical prediction of airwakes using sophisticated CFD tools is also an expert capability at NRC Aerospace. CFD-airwake predictions can be coupled with wind tunnel data to provide validated data for input to flight simulators used today to train pilots, and in the future, to assist with flight envelope development.

NRC Aerospace provides this technology and expert analysis to Canadian and foreign navies.

CFD prediction of frigate airwake for 0° wind

Related Information

Institutes:

• NRC Institute for Aerospace Research