Compound helicopters, featuring lateral rotors as well as a primary rotor, are increasingly seen as the future of rotorcraft design. Capable of supporting a range of service applications, compound rotorcraft have the potential to deliver increased efficiencies and higher speeds relative to traditional rotorcraft. The Rapid And Cost Effective Rotorcraft (RACER) demonstrator is being developed by Airbus Helicopters (AH) to further validate the compound rotorcraft configuration. AH have chosen to employ a joined-wing design, which increases the stiffness relative to a traditional wing design. The RACER design also includes two lateral rotors, mounted aft of the Nacelles at the outboard extent of the wings, as depicted in Figure 1.

Two assembly methodologies were considered for the RACER wing structures – determinate assembly or traditional assembly set by jigs. Determinate assembly, also known as ‘part-to-part’ or ‘jig-less’ assembly relies on the precision manufacturing of a few critical features, within the definition of each child part, that allow interfacing items to be accurately positioned. This approach significantly reduces the importance of an assembly jig, which may only be required to support the mass of the wing structure. Resultant jig structures may be more cost-effective given requirements on their ability to set part positions are much less stringent. This approach can also reduce the need for assembly processing, deliver a shorter build duration, and reduce the lead-time for assembly tooling. Conversely, the requirement to include precision location features within component definitions can transfer cost into the manufacturing processes. Equally, significant analysis of 3D tolerance stack-ups must be completed to confirm the assembly can meet geometrical requirements.

Employing matched tooling has been proposed to aid the mounting of the joined-wing structure to the fuselage, ensuring the independently built wing structures can be successfully assembled by replicating the interface with the fuselage. The matched tooling configuration was debated and could be designed either as a one-piece tool or a modular version. The decision significantly affects the overall wing assembly processes.The traditional method of assembling wings is to employ jig structures that have been precisely calibrated to ensure they can accurately locate child parts within the wing assembly and allow assembly processing tasks to be completed. Datum features of items within wing assemblies will contact ‘pick-ups’, within the jig structure, to minimize the impact of manufacturing tolerances on the position of parts within assemblies. This approach reduces the requirement for precision manufactured parts but may transfer cost into the assembly process. The traditional method of assembling wings, using precision jigs, was selected for the production of the RACER wings in order to remove complexity and cost from the manufacture of the novel, composite items within the structures. The design of the tooling required to facilitate assembly of the joined-wing structures is also highlighted.

Given the complexity of the wing product, three-dimensional (3D) tolerance analysis methods were needed to perform the analysis and make an informed decision. Several 3D tolerance methods were suggested. Homogeneous Transformation Method (HTM) was selected to perform the variation propagation analysis.

Overview of RACER Compound Helicopter

The compound helicopter RACER was developed, as part of the Clean Sky 2 European research program, in an attempt to fill the mobility gap between conventional helicopters and airplanes. This Vertical Take-Off and Landing (VTOL) aircraft should ensure more efficient emergency and search and rescue services, as well as improving citizen mobility by offering faster gate to gate passenger transport.

Conventional helicopters offer vertical take-off and landing capability but their flight speed, limited by the aerodynamics of the main rotor, is substantially lower than traditional fixed-winged aircraft. In fact, the stall of the retreating blade and the shock wave generation on the advancing blade bound the rotor rotational speed in cruise, and consequently limit the helicopter maximum advancing speed.

The most promising concepts to overcome these limitations are the convertiplane and the compound helicopter. In a convertiplane or tiltrotor, the rotor is tilted perpendicularly to the flight trajectory to provide thrust during cruise flight while a fixed-wing provides the required lift. In a compound helicopter, the main rotor is fixed, providing lift and thrust in cruise flight, but fixed wing (lift compounding) and/or thruster (thrust compounding) are added to off-load the rotor during horizontal flight. The reduction of lift and thrust load required from the rotor allows a decrease of its rotational speed and a consequent increment of the aircraft speed limit.

The RACER is a medium class rotorcraft with a cruise speed exceeding 220 kt. The compound architecture consists of a joined-wing lifting system supporting two pushing propellers providing thrust, yawing control and balancing the torque generated by the main rotor, (Figure 1). A mechanical transmission transfers power from the two turboshaft engines under the main rotor to the lateral propellers. Each upper wing (UW) structure, therefore, houses a drive shaft and relative transmission connections inside.

An H-shape tail was selected to improve lateral maneuverability and reduce the susceptibility to rotor and wing wakes. New technologies were implemented to enhance the aircraft efficiency such as advanced composite structural components, optimized transmission architecture and high voltage DC generation. The 50% increase in cruise speed compared to conventional helicopters enables the RACER to extend the accessible area within an hour flight for rescue and medical emergency missions, as well as for passenger transport.

RACER Joined-Wing Configuration

Figure 2. Racer joined-wing: wing configuration schematic (1. upper wing, 2. lower wing, 3. main landing gear, 4. stub wing, 5. lateral gear box nacelle)

A joined-wing configuration was selected by AH to realize the lift compounding system of the RACER compound helicopter. The joined-wing is defined as a staggered bi-plane configuration with straight UW and lower wing (LW) structures at each side of the helicopter, being connected at their tip as shown in Figure 2.

UW and LW structures feature opposed dihedral and sweep angles with a positive stagger arrangement at their roots, forming a triangular framework in both front and top views. The upper wing is connected to the upper fuselage whereas the lower one is connected to the stub wing part of the lower fuselage. Lateral pushing propellers are located at the wing joint region, behind the trailing edges, offering improved characteristics in terms of passenger safety and crashworthiness.

Joined-wings are generally characterized by higher aerodynamic efficiencies than traditional wing configurations due to the reduced induced drag, along with an increased structural efficiency owing to the global deformation behavior of the double wing structure. The joined-wing shows considerably larger stiffness on the vertical plane (flapping plane), but lower stiffness in the horizontal plane (feathering plane), in comparison with a traditional cantilever wing of equal total lift. This is consistent with the higher inertial and aerodynamic loads acting on the wing longitudinal plane of the compound helicopter and the stiffness requirement on the longitudinal plane imposed by the driveshaft and drive system deflection limits.

Torsional stiffness is enhanced by the wing connection and the staggered wing arrangement, which converts a portion of the global torsion into single wing bending. The joined-wing architecture reacts to bending loads on the longitudinal plane as a triangular beam system, with the wings mainly subjected to tensile and compressive loads. Additional superimposed local bending, shear and torsion stress characteristics are introduced by the aerodynamic lift distribution on the wing skin, flap actuations and propeller thrust.

The axial nature of the upper and lower wing loadings is an indication of structural efficiency and promotes weight reduction, as the entire wing section contributes to load carrying. Owing to the reduced portion of bending load carried, the suggested joined-wing configuration does not require structural continuity inside the fuselage, as conventional cantilever wings do, and the wing to fuselage attachment can be realized by simple hinges that transfer transverse and longitudinal loads only. This minimizes the space allocation of the wing structure in the upper fuselage deck where the engines and the main gearbox are housed. Moreover, the joined-wing distributes the lifting and propulsion loads between the upper fuselage deck and the subfloor structure, reducing local structural stresses and mass of such fuselage components.

The upper wing is essential for housing the propeller drive shaft and its sweep and anhedral angles are a consequence of the respective positions of the main gearbox and the lateral propeller (Figure 2). The geometric characteristics of the lower wing are mainly defined by the propeller location and the position of the stub wing, in turn determined by the position of the main landing gear, which is housed in the stub wing when retracted. Those constraints result in the peculiar staggered configuration with a lower wing positioned aft of the upper wing at their roots.

In addition, the LW provides a physical barrier between the passenger area and the rotating lateral propellers, acts as additional buoyancy in case of ditching, and as a safety measure preventing the upper wing from breaking and obstructing the cabin door in case of a crash. From an aerodynamic perspective, no efficiency increase is achieved in this configuration due to the non-slender wing geometries and the airflow disruption caused by the main rotor and the lateral propellers. However, wing staggering causes a favorable aerodynamic interaction between the wings and reduces downwash during hovering.

Figure 3. RACER joined-wing assembly overview

The joined-wing assembly consists of UW, LW and Nacelle structures. Each UW or LW structure includes a flap for trimming and stability purposes. The UW and LW structures interface with the fuselage separately via hinges, identified in Figure 3. The two wings are joined at their outboard tips through a connecting structure, known as a ‘cradle’, which also supports the nacelle. The cradle is part of the UW structure and features two lugs to realize the connection with the LW.

Spherical bearings and sliding bushes are utilized at various locations to mitigate translations and deformations of the wing structures relative to each other, and to the fuselage during flight operations. A schematic of the RACER wing architecture is shown in Figure 3. Connecting the wings using spherical bearings ensures an isostatic behavior on the wing system vertical plane and the hinge position was optimized to maximize the stiffness of the joined structure. Loads along the helicopter longitudinal axis direction are transferred from the wings to the fuselage using dedicated X-trusses, allowing the hinges to carry only lateral and vertical shear. The gearbox located within the Nacelle, driving the propeller, is mechanically connected to the UW component of the wing to wing joint by an isostatic arrangement. Axial load is reacted by a dedicated X-truss between the gearbox and the cradle within the UW structure.

Preliminary FE and CFD analyses confirmed the joined-wing as an efficient solution for a high-speed helicopter based on the compound formula with lateral propellers such as the RACER. However, the design and assembly of a statically indeterminate structure as a joined-wing presents some additional challenges compared to conventional, cantilever wings. In this scenario, innovative building solutions and analyses have to be implemented to develop an efficient and robust assembly process for the non-conventional, RACER joined-wing structure.

This article was adapted from SAE Technical Paper 2019-01-1884. To obtain the full technical paper and access more than 200,000 resources for the aerospace, automotive, and commercial vehicle industries, visit the SAE MOBILUS site here .

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This article first appeared in the June, 2020 issue of Aerospace & Defense Technology Magazine.

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