The Effect of Tailboom Strakes and Vertical Fin Modifications on the Performance and Handling Qualities of OH-58 Helicopters

The Bell OH-58D helicopter is used in the armed reconnaissance role by the U.S. Army worldwide. Operations in support of this mission require the aircraft to be operated at a hover for extended periods of time at high altitude and in hot conditions. This places large demands on a power limited aircraft and increases pilot workload, especially in the pointing task so important to weapons delivery. The addition of aerodynamic modifications provides an inexpensive and effective method of reducing power required and pilot workload.

An investigation was conducted to determine the effects of adding tailboom strakes and a vertical fin modification (FastFin^®) to a Bell OH-58D. The effect of the combined devices on helicopter performance, vibration and handling qualities was evaluated to determine any decrease in power required vs. power available (increased power margin). The strakes and modified fin were attached to the tailboom in such a manner as to alter the flow pattern of air passing over the tailboom. This change in flow pattern reduced the anti-torque requirements of the tail rotor. This in turn reduced the overall power requirement. The modified fin and strake combination reduced the vortices and turbulence at the tailboom and resulted in lower pilot workload and vibration in an out-of-ground-effect hover. This study also evaluated the effects on performance and handling characteristics of the helicopter. Testing included hover, low airspeed forward, rearward, sideward flight and level forward flight power measurements and handling qualities using modified Cooper-Harper and Vibration Assessment Rating (VAR) scales. Results showed an overall improvement in handling qualities and vibration levels at critical wind azimuth angles in low airspeed flight. For out-of-ground effect (OGE) hover, there was also a 2-3% reduction in power required due to the improved aerodynamic efficiency of the modified fin and strake combination.