By Ben Simpson, Fluent Europe

Questions were recently raised in the UK regarding the firing of Hellfire antitank missiles from Apache helicopters. Concerns were that debris ejected during the launch of a missile might cause damage to the main or tail rotor, resulting in a possible crash. Engineers at Fluent Europe performed an independent investigation into the potential dangers through simulations using FLUENT.

A fully loaded Apache helicopter in hovering flight mode was modeled using over four million cells. Two Hellfire anti-tank missiles, one on either side of the fuselage, were simulated during fireup in their prelaunch positions. The rotation of the main rotor was represented using the sliding mesh model and that of the tail rotor was represented using the multiple reference frames model. This approach allowed for the complex interaction between the main rotor downwash, the forced convection from the helicopter engine exhausts, and the hot plume from two missiles being launched.

Pathlines released from the missile exhausts (colored by release position)

The results illustrated several interesting flow features. In particular, the flows from the engine exhausts and missile plumes were both found to be significantly affected by the rotor downwash, which redirects them toward the left side of the helicopter. When the left missile is fired, this flow pattern helps keep debris released with the missile plumes from colliding with the helicopter and rear rotor (which is on the left side of the aircraft). The diverted streams also cause asymmetric heating on the tail of the helicopter. When the right missile is fired, some interference with the rear of the aircraft may occur for certain flight conditions, and more studies are needed to further study this possibility. The project as a whole illustrates how CFD can be a viable source of information for investigations into safety issues for a wide range of flight conditions. For example, debris ejected when the missiles are launched could be tracked using the discrete particle model in FLUENT. The likelihood of the debris hitting the tail rotor or the main rotor could then be assessed using a series of runs to investigate the trajectories and possible impact points of particles of various sizes and weights.

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