A Variable Diameter Short Haul Civil Tiltrotor

The Short-Haul-Civil-tiltrotor (SHCT) component of the NASA Aviation System Capacity Program is an effort to develop the technologies needed for a potential 40-passenger civil tiltrotor. The variable diameter tiltrotor (VDTR) is a Sikorsky concept aimed at improving tiltrotor hover and cruise performance currently limited by disk loading that is much higher in hover than conventional helicopter, and much lower in cruise than turbo-prop systems. This paper describes the technical merits of using a VDTR on a SHCT aircraft. The focus will be the rotor design. Nomenclature p Wing torsion mode c Wing chord mode b Wing up/down (beam) bending mode Gimbal-1 Coupled rotor regressing flap/gimbal-1 mode Gimbal+1 Coupled rotor progressing flap/gimbal+1 mode 1Fc Collective flap mode 1Fp Progressing flap mode 1Fr Regressing flap mode 1Lp Progressing lag mode 1Lr Regressing lag mode Introduction1 Tiltrotor aircraft have the capability to vertically take off and land like a helicopter and have the high speed cruise performance approaching that of a conventional propeller airplane. The Short-HaulCivil-Tiltrotor (SHCT) component of the NASA Aviation System Capacity Program is an effort to develop the technologies needed for a potential 40passenger civil tiltrotor [1]. The variable diameter tiltrotor (VDTR) is a Sikorsky concept aimed at improving tiltrotors’ hover and cruise performance. With a VDTR, the rotor blades can be fully extended during hover, and then retracted to about 70% of their full length during cruise flight. Figures 1 and 2 illustrate this concept. Presented at the 55th Annual Forum of the American Helicopter Society, Montreal, Canada, May 25-27, 1999. Copyright © 1999 by the American Helicopter Society, Inc. All rights reserved. Benefits of a VDTR There are numerous advantages of a larger diameter rotor in hover. The lower disk loading reduces the magnitude of rotor downwash, improves the autorotation index, and enhances load-carrying capability. The benefits of having a reduced rotor diameter for cruise are also significant. Our analysis indicates that a smaller rotor diameter reduces the potential for proprotor-whirl flutter, and lessens the wing stiffness and weight requirements. Smaller propellers ameliorate the sensitivity to head-on longitudinal gusts and roll/yaw coupling [2]. This reduces the size requirement of horizontal and vertical fins, and significantly improves passenger comfort. A VDTR, unlike conventional tiltrotors, does not require a rotor speed reduction for efficient cruise flight. The necessary tip speed reduction is obtained instead by reducing the rotor diameter. This allows operation at 100% rpm that optimizes engine performance. Additional discussion of the performance gains associated with a VDTR can be found in Refs. [1-5]. For a SHCT that has to operate in urban helipads, the rotor outwash velocity becomes an important issue. Figure 3 compares the rotor outwash velocity at 50 ft away from the rotor shaft and 3 ft above the ground, for a SHCT VDTR at 50,000 lb gross weight, and a single-rotor helicopter with 25,000 lb gross weight and 56 ft diameter, and a single-rotor helicopter with 50,000 lb gross weight and 70 ft diameter. The diameters for the helicopter rotors are chosen to represent typical single-rotor helicopters of that gross weight. For a tiltrotor, the outwash velocities at the front and back of the aircraft are larger than at the sides. The larger hover diameter of the VDTR helps keep the outwash velocities reasonable as compared to conventional single-rotor helicopters. This attribute is especially beneficial for a civil tiltrotor that will be loading and unloading passengers and cargo with the rotor turning. The flight simulation study of Ref. 3 shows that a conventional tiltrotor aircraft and a VDTR aircraft both exhibit Level One handling qualities during normal maneuvers with both engines operating. However, the VDTR has a greater power margin and performance in helicopter mode resulting from a lower disk loading and requires 20 to 25% less power at similar thrust levels. This is an advantage during one-engine and all-engine inoperative procedures. Reference 3 quantifies that a VDTR can enhance safety near the terminal area. Figure 4 shows that the autorotation index for the VDTR SHCT is better than conventional tiltrotors and is almost as good as single-rotor helicopters. This index is defined as (rotor inertia)Ω/[(gross weight/no. of rotors)(disk loading)] and a value of approximately 20 or higher is needed for safe autorotations. Description of the SHCT VDTR Rotor Table 1 shows the properties of a Sikorsky SHCT VDTR design. The VDTR selected for this SHCT study is a 4-bladed gimbaled rotor design. The four major components are the blade, the torque tube, the jackscrew and the tension strap (Fig. 5). The blade is the main lifting section. It slides in and out on the torque tube to effect radius changes. When the electric motor rotates the jackscrew, the jackscrew exerts a corkscrew action on the nut sitting inside the torque tube, which then extends or retracts the blade. The tension straps are responsible for holding the blade against the centrifugal force. The tension straps connect the blade tip to the nut on the jackscrew. During flight, the main portion of the blade is in compression, rather than extension, which is typical for conventional rotors. A Froude-scale and a Mach-scale model of this VDTR design have been built and successfully tested at the UTRC wind tunnel, at the Sikorsky hover stand, and on the BART stand at NASA Langley. The flight envelope tested includes hover, transition flight, and airplane cruise mode [5-8]. The retraction mechanism has been tested on the Froude-scale rotor over a wide range of conditions. Table 1: Gross Weight 50,000 lbs Cruise Speed 350 knots