The ability of the Osprey tiltrotor to fly farther, as well as faster, than helicopters has been a key factor in its fight for survival for more than a decade. But now, with both CV-22B and MV-22B versions recently pressed into service on longer-range, self-deployed combat and rescue missions in Libya and Afghanistan, the hunt is on for greater unrefueled performance.
Squeezing more range out of the V-22 is not easy, however. Constrained from birth by the need to fit on the restricted decks of U.S. Navy amphibious assault ships, the tiltrotor was necessarily limited to smaller-than-optimal wing and rotor dimensions leading to an inevitable impact on range. Without the options of increasing wingspan or rotor diameter available to them, designers are taking a leaf out of theplaybook and studying nacelle-mounted “sails” that work on the same principle as winglets.
Although Belldeclines to comment on the development, Naval Air Systems Command (Navair) confirms it is preparing to flight test a modified MV-22 with the upgrade which could boost range by almost 5%. The concept, which was studied by Boeing for the Special Operation Forces CV-22 version as far back as 1994, harnesses the energy from the vertical upwash around the wing and nacelle. In the case of the V-22, Bell Boeing's research indicates upwash angles of 10-20 deg. around the nacelles at the nominal cruise pitch attitude of 8 deg. The additional upwash velocity produces a propulsive force by tilting the lift vector forward.
In the run-up to flight tests, set to be performed by Navair's V-22 joint program office, PMA-275, at NAS Patuxent River, Md., starting in November, Bell Boeing has evaluated several configurations for the size, number and positioning of the sails. The locations were initially determined by aerodynamic requirements, before being fine-tuned to reflect places where they could be best attached to the support structure in the nacelle with the least impact on maintainability.
Shipboard “spot” size considerations, the driving factor of the overall aircraft design, also influenced the final choice of 34 in. for the sails' span. Root chord was set at 18 in. long to fit on a 32-in. panel, which worked within structural considerations. The tip chord is 9 in.
The flight tests, which will be conducted on PMA-275's aircraft No. 24, are focused on a configuration with three sails per nacelle. This was selected from a variety of options ranging from a single unit to five sails on each nacelle, all of which were tested in the Boeing vertical-/short-takeoff-and-landing wind tunnel in Philadelphia, during 2008-09. The best combination included a sail mounted forward on the nacelle with a dihedral of 45 deg. and two further aft set at 15-deg. dihedral, and improved the overall lift-drag ratio by 8%. Boeing's analysis indicates the arrangement reduces drag by 10% and will lead to around a 5% increase in the self-deployment range.
However, the modification does not come without potential issues, which Navair expects to explore during flight testing. Wind tunnel evaluation, for instance, showed a drag increase of 10% with the three-sail configuration while in conversion mode with the nacelles angled at 30 and 60 deg. Given the improvements at higher speeds, and the short time the aircraft spends in conversion mode, this is not expected to be a factor.
A potentially greater concern is the slight decrease in longitudinal static stability noted at typical cruise conditions. Although this was anticipated because the sails increase effective wing area while leaving the tail area unchanged, the change in stability will be a focus for flight tests, as will an analysis of a slight reduction in yawing moment versus sideslip angle seen in the tunnel.
Other areas for flight-test investigation will include the potential for tail buffet in conversion mode as well as increased nacelle loads. Although Bell Boeing defines both the loads and conversion-mode performance penalty factors as possible “design risks,” it predicts that the tail buffet boundary and stability and control aspects will not be an issue.