BELL BOEING AND the US Marine Corps are playing down concerns over downwash which emerged from testing of the V-22 Osprey tilt-rotor. A report rated downwash as a "moderate risk" after a test in which a soldier rappelled 18m (60ft) from the rear ramp of a hovering V-22 and was blown 3m aft of his aim point by proprotor downwash.

Despite this concern, the V-22 has passed the phase 2B operational-assessment test (OT2B), Bell Boeing says, clearing the way for continued development and low-rate production. The V-22 received the highest rating possible from the OT2B, the team says, being declared "potentially operationally suitable and potentially operationally effective".

The USMC says that the Sikorsky CH-53E heavy-lift helicopter was rated as having a serious downwash problem, which was mitigated by developing special tactics and procedures. Bell Boeing expects similar "work-around" to be developed for the V-22, involving optimisation of the hover entry and height, exit choice and rapelling technique.

The rappelling test is more alarming for the US Air Force, which will receive 50 special-operations CV-22s, than it is for the USMC, which is to get 425 MV-22 tactical transports. USAF Special Forces use rappelling to insert troops without landing, whereas the Marines use "fast-roping", a quicker technique using heavier ropes. The USMC says that fast-roping tests with the V-22 have been successful.

Compared with downwash produced by the tandem-rotor Boeing CH-46, worst at right angles to the aircraft, that generated by the V-22 aligns with the fuselage. While the CH-46 is loaded from the rear, it is likely that, the V-22 will be loaded from the side, the USMC says. It is opposed to fitting downwash deflectors, which would add 1,400kg to the V-22's weight. See feature, P28.

Source: Flight International