29 March 2014

Here is a Bit of Neat Tiny Aviation Tech

AugustaWestland is looking at variable geometry rotors using a miniscule (1-2% of chord) trailing edge flap which can be extended of retracted under different conditions:
Dan Gurney, American racing car driver and constructor, is providing inspiration to European helicopter manufacturers, with AgustaWestland planning in 2015 to fly an active rotor incorporating the aerodynamic device that carries his name.

The Gurney flap (see diagram) 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.
In the interest of accuracy, the Gurney flap, also known as a wickerbill, was actually independently invented by a number of people as far back as 1931
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, AgustaWestland 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. Now they are smaller. We have more powerful computational tools, but are only getting incremental gains,” says Simon Spurway, AgustaWestland 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. AgustaWestland, 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 the 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 rotational speed and increases lift. On the retreating side, forward speed subtracts from rotational speed, and blade pitch must be increased to maintain lift. 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 offloaded. This reduces the power required to maintain airspeed and lowers fuel consumption and emissions, an overall goal of Clean Sky.
Flaps on rotors are not new. Kaman's helos have been using servo flaps as alternative to hub based actuators for years, but the application of Gurney flaps, along with their use to handle issues of the different lift modes and retreating blade stall, is new.


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