Aerodynamic Shape Optimisation of a Proprotor and Its Validation by Means of CFD and Experiments

The aerodynamic shape design of a proprotor for a tiltrotor aircraft is a very complex and demanding task because it has to combine good hovering capabilities with high propeller efficiency. The aim of the present work is to describe a two-level procedure and its results for the aerodynamic shape design of a new rotor blade for a high-performance tiltwing tiltrotor aircraft taking into account the most important flight conditions in which the aircraft can operate. Span-wise distributions of twist, chord and aerofoil were chosen making use of a multi-objective genetic optimiser that worked on three objectives simultaneously. A non-linear sweep angle distribution along the blade was designed to reduce the power losses due to compressibility effects during axial flight at high speed. During the optimisation process, the aerodynamic performance of the blade was evaluated with a classical two-dimensional strip theory solver. The optimised blade was than analysed by means of a compressible Navier-Stokes solver and calculations were validated comparing numerical results with experimental data obtained from wind-tunnel tests of a scaled model of the proprotor

Experimental investigation of a helicopter rotor with Gurney flaps

The present work describes an experimental activity carried out to investigate the performance of Gurney flaps on a helicopter rotor model in hovering. The four blades of the articulated rotor model were equipped with Gurney flaps positioned at 95% of the aerofoil chord, spanning 14% of the rotor radius. The global aerodynamic loads and torque were measured for three Gurney flap configurations characterised by different heights. The global measurements showed an apparent benefit produced by Gurney flaps in terms of rotor performance with respect to the clean blade configuration. Particle image velocimetry surveys were also performed on the blade section at 65% of the rotor radius with and without the Gurney flaps. The local velocity data was used to complete the characterisation of the blade aerodynamic performance through the evaluation of the sectional aerodynamic loads using the the control volume approach

Wind-tunnel tests of a heavy-class helicopter optimised for drag reduction

Wind-tunnel tests of a heavy-class helicopter model were carried out to evaluate the effectiveness of several components optimised for drag reduction by computational fluid dynamics analysis. The optimised components included different hub-cap configurations, a fairing for blade attachments and the sponsons. Moreover, the effects of vortex generators positioned on the back ramp were investigated. The optimisation effect was evaluated by comparison of the drag measurements carried out for both the original and the optimised helicopter configurations. The comprehensive experimental campaign involved the use of different measurement techniques. Indeed, pressure measurements and stereo particle image velocimetry surveys were performed to achieve a physical insight about the results of load measurements. The test activity confirms the achievement of an overall reduction of about 6% of the original model drag at cruise attitude

Assessment of a Propeller Model Embedded on a Panel Method Code for Aircraft Aerodynamics

An inviscid actuator disk model is embedded in a three-dimensional low-order panel method code for inviscid incompressible ow in order to study the propeller effects on an arbitrary body. The actuator disk model predicts the time-averaged induced velocities in the slipstream of a propeller with an arbitrary radial distribution of load. The model is constructed by superposition of four vorticity distributions, by neglecting the radial contraction of the vortex tube and assuming a fixed wake for the propeller. Experimental data, available from the licterature, have been used to validate the actuator disk model embedded in the panel method code

Aerodynamic Blade Design with Multi-Objective Optimization for a Tiltrotor Aircraft

Purpose - The purpose of this paper is to present the aerodynamic blade design of a tiltwing aircraft with a multi-objective optimization procedure. The aerodynamic design of tiltrotor blades is a very challenging task in the project of this type of aircraft. Design/methodology/approach - Tiltrotor blades have to give good performance both in helicopter and aeroplane modes. According to the design parameters (the chords, the twists and the airfoils along the blade), as the optimization objectives are different from one operating condition to another, the blade is the result of a multi-objective constrained optimization based on a controlled elitist genetic algorithm founded on the NSGA-II algorithm. The optimization process uses a BEMT solver to compute rotor performance. To avoid negative effects due to compressibility losses in aeroplane mode, the blade shape has been refined following the normal Mach number criterion. Findings - It has been found that the optimized rotor blade gives good performance both in terms of figure of merit and propulsive efficiency if compared with experimental data of existing rotor (ERICA tiltrotor) and propeller (NACA high-speed propeller). Practical implications - The optimization procedure described in this paper for the design of tiltrotor blades can be efficiently used for the aerodynamic design of helicopter rotors and aircraft propellers of all typology. Originality/value - In this work, advanced methodologies have been used for the aerodynamics design of a proprotor optimized for an aircraft which belongs to the innovative typology of high-performance tiltwing tiltrotor aircraft

Aerodynamic Interaction Between Rotor and Tilting Wing in Hovering Flight Condition

The hovering performance and the lifting capability of tiltrotor aircraft are strongly affected by the aerodynamic interaction between wing and rotors. The tiltwing concept represents an interesting technology to increase the hover performance by reducing the wing–rotor interference. The present work investigates the aerodynamic interaction between wing and rotor in hover for a scaled tiltwing aircraft half-span model. A comprehensive experimental campaign, including force measurements and particle image velocimetry surveys, was performed together with computational fluid dynamics simulations. Numerical predictions were validated using experimental data and were used to describe the flow field

Robustness and limits of vortex generator effectiveness in helicopter drag reduction

The effect of vortex generators on helicopter drag was investigated in a wind tunnel test. An array of counterrotating vortex generators was mounted on the rear ramp of a heavy-class helicopter model, slightly downstream of the fuselage upsweep. The wind tunnel campaign also included tests with a radome positioned upstream of the vortex generators to evaluate the robustness of the vortex generators. At cruise angle of attack, a drag reduction of about 2% with respect to the complete helicopter configuration was measured for the baseline fuselage configuration with the vortex generator array installed. The range of angles of attack and sideslip where the vortex generators were effective for drag reduction was established. The addition of a radome mounted upstream on the fuselage lower side renders the vortex generators ineffective. However, for the model attitudes where the vortex generators were ineffective, the vortex generators did not increase helicopter drag. Steady and unsteady pressures measured on the fuselage rear ramp revealed the flow behavior due to the presence of the vortex generators

Proprotor–wing aerodynamic interaction in the first stages of conversion from helicopter to aeroplane mode

Tiltrotors have the potential to revolutionise air transportation since they combine the flight performance of aeroplanes with the versatility of helicopters. One of the most crucial features characterising tiltrotors is represented by the aerodynamic interaction between wing and rotor. Although this phenomenon has been largely studied using both experiments and calculation, analyses were mainly focused on helicopter operative mode and hovering condition. The present work is aimed at describing the effects due to the aerodynamic interference between wing and rotor of a tiltrotor aircraft employing the tiltwing concept solution during the first stage of the conversion manoeuvre from helicopter to aeroplane configuration. Experimental tests carried out in the open test section of the Politecnico di Milano Large Wind Tunnel with a half-span scaled powered model are reported in detail and results are illustrated in terms of rotor performance and wing aerodynamic loads. Both isolated rotor and half-span model test results are presented as function of several parameters. Overall aircraft performance during the first part of the conversion manoeuvre is finally discussed

Numerical Investigation of Perpendicular Blade-Vortex-interaction on a Pitching Airfoil in Light Dynamic Stall

Computational Fluid Dynamics was used to investigate the effects of perpendicular blade vortex interactions on the aerodynamic performance of an oscillating airfoil. In particular, the test case studied was a stream-wise vortex impacting on the leading edge of a NACA 23012 airfoil oscillating in light dynamic stall regime, representing a typical condition of the retreating blade of a helicopter in forward flight. The results of time-accurate simulations were validated by comparison with particle image velocimctry survcys. Thc analysis of thc numerical results enabled to achieve a detailed insight about the overall effects on the interacting flow field and on aerodynamic loads acting on the oscillating airfoil due to the vortex interaction. Indeed, the comparison with the clean airfoil case shows a severe loss of performance produced by vortex interaction during downstroke motion of the airfoil, as the vortex impact triggers the local stall of the blade section