Gurney Flap Could be the Key for Better Helicopter Rotors

RACING INSPIRED by Graham Warwick, Managing Editor-Technology at AviationWeek.com

It worked for Indy cars, now the Gurney flap could be the key for better helicopter rotors.

Dan Gurney, American racing car driver and constructor, is providing inspiration to European helicopter manufacturers, with Agusta Westland planning in 2015 to fly an active rotors incorporating the aerodynamic device that carries his name.

The Gurney flap (see photo) is a small tab set perpendicular to the flow at the trailing edge of a wing. It has the effect of increasing lift with minimal impact on drag. In the early 1970s, he first used the eponymous device on the rear wing of a racing car to increase downforce.

Fixed Gurney flaps are used extensively on helicopters to increase the effectiveness of horizontal and vertical stabilizers over a wide angle-of-attack range. Now, with funding from Europe’s Clean Sky research program, Agusta Westland is to use active Gurney flaps to increase the performance of helicopter rotor blades.

Rotor design is a compromise between hover and forward-flight requirements, and the ability to squeeze more performance from conventional blades is reaching its limits. “In the 1980s and 90s we saw big gains”, says Simon Spurway, Agusta Westland principal engineer. “The next step is active rotors.”

Under Clean Sky’s Green Rotorcraft program, Airbus Helicopters is leading work to see how much further a conventional blade can be passively optimized. The manufacturer also is heading a project to develop active blade twist, which Spurway says poses fail-safe design challenges. Agusta Westland, meanwhile is in charge of the active Gurney flap project.

Projecting from the lower surface close to the trailing edge, and just 1-2% of blade chord in height, the flap produces counter-rotating vortices that increase pressure on the lower, pressure side of the airfoil and decrease pressure on the upper, suction side. The vortices help the boundary stay attached to trailing edge and increase the maximum lift coefficient for only a small penalty in drag coefficient.

In forward flight, rotor blades experience different conditions as they rotate. On the advancing side, forward speed adds to the rotational speed and increases lift. On the retreating side, forward speed subtracts from rotational speed and blade pitch must be increased to maintain left. As airspeed rises, the retreating blade begins to stall and the pilot must add power to overcome the rising drag.

Retracted on the advancing side, the active Gurney flap is deployed on the retreating side to delay the stall. Covering the middle section of the blade, the flap locally improves lift and allows the outer section of the retreating blade to be offloading. This reduces the power required to maintain airspeed and lowers fuel consumption and emissions, and overall goal of Clean Sky.

The system is being developed in stages, beginning with wind-tunnel tests of a two-dimensional airfoil, completed in January at the University of Twente in the Netherlands. The representative blade section was held at a fixed but adjustable angle of attack, and the flap was deployed to pre-set heights to determine its aerodynamic effect.

Next will come 2-4 dynamic tests, planned for year-end in a wind-tunnel at Italian aerospace research center CIRA. This scaled airfoil section model will oscillate in pitch at rates representative of a rotor blade in normal operation. The active Gurney flap will be deployed using the schedule and rates anticipated for the full-scale flight system.

Tests of a subscale four-blade rotor model are planned for the first quarter of 2015, in a wind tunnel at Politechnico di Milano. Less than 1 meter (3.2 ft.) in diameter, with blades just 95 mm (3.7 in.) in chord, this model will allow the active flaps to be tested on rotating blades in controlled conditions. “We will look at different deployment schedules - rapid, sinusoidal, multi-harmonic - and assess performance as progressive blade stall is encountered,” Spurways says.

A full-size main rotor with active Gurney flaps is to be tested on a whirl tower in March 2015, using flight-rated electrical actuation and control systems supplied by Microtecnia. Flight tests on an AW139 (see photo) are planned for June 2015. The fourth AW139 built will be modified with an active rotor controller installed on the hub under an enlarged “beanie” fairing. This will provide electrical power distribution and power electronics for the actuation systems in the blades.

The program is picking up pace. “The blades are in manufacture. Structural test specimens are being configured, to prove it is safe to fly. The rotor head controller has been prototyped,” Spruway says.

Gurney’s modest bent-metal flap is taking on a new complexity and capability.