Spin doctors

Researchers are looking to boost helicopter thrust by manipulating airflow over the rotor blades.

The performance and reliability of helicopters could be improved without significantly increasing their power requirements using technologies to control the flow of air around rotor blades.

Researchers at City and Glasgow universities are investigating the use of flow control technologies to expand the flight envelope of rotorcraft by delaying the onset of dynamic stall in a project with Westland Helicopters.

The project, being funded by the DTI as part of a long-term Defence Aerospace Research Partnership (Darp) programme into rotorcraft, will examine the use of technologies such as air jet vortex generators and synthetic air jets to control the airflow around the blades, said Frank Coton, professor of aerospace engineering at Glasgow University.

A helicopter’s speed is limited by the fact that if its blades rotate too fast they simultaneously produce stall effects on the rearward-moving blade and compressibility effects – or shocks – on the blade moving forward.

Controlling the flow of air around blades can help to delay dynamic stall and limit its effects, said Coton.

‘Helicopters can’t suffer a full stall, because they are built to perform within certain parameters; we are looking at technologies to expand those parameters.

Basically, we are trying to improve the reliability and generalperformance of helicopters,’ said Coton.

By introducing the technologies, the researchers hope to increase the maximum take-off weight and maximum speed of helicopters in forward flight, without paying an undue penalty in the amount of power required, according to Prof Anastasios Kokkalis, director of the Centre for Aeronautics at City University. ‘We are looking to increase thrust by 20 per cent; the maximum speed will depend on how much of this increase in thrust loading is used for lift, but we are looking for an increase in speed of around 15-20 knots,’ he said.

Airjet vortex generators, originally developed at City, consist of a series of rectangular slots, evenly distributed over the upper surface of the rotor blade. These are supplied with air which is pushed out in small jets at a skewed angle relative to the oncoming free flow of air around the blade.

These jets mix with the free-flowing air and create a vortex with sufficient momentum to overcome the adverse pressure that would otherwise cause blade stall.

Airjet vortex generators have previously been applied to wind turbine blades, and the researchers will now conduct studies with a view to optimising them for use in rotorcraft.

‘By 2007, Westland will be in a position to go ahead with themanufacture and flight testing of such a blade,’ said Kokkalis.

Also being investigated are synthetic jets, or zero mass flux jets, which are small cavities in the surface of a rotor blade with a membrane at their base. When an electrical potential is applied to the membrane, it flexes and contracts. The flexing motion draws air into the hole and the contraction pushes air out. In this way, relatively strong vertical jets of air are produced, which re-energise the airflow at the surface of the blade, again allowing it to overcome adverse pressure and expand the helicopter’s range before the onset of dynamic stall.

As a useful by-product of the research, the technology has been shown to lessen the strength of the compressibility shocks on the advancing rotor blade thatare caused by an increase in speed.

The researchers are also looking at materials that can be manipulated by the application of an electrical potential to move a trailing edge flap on the blade. This again could be used to mitigate the effects of dynamic stall.

‘We are looking at developing a prototype blade which will involve the use of a trailing edge flap in association with either an airjet vortex generator or a synthetic jet. We will optimise them to work together and see if that enhances the benefits of each,’ said Kokkalis.

This second blade is likely to be ready to be built and flight tested in around 2008/9, although the exact timetable for the project has yet to be finalised.

The same materials could be used to twist the blade itself, allowing designers to tailor the distribution of load across the blade and so reduce drag.

The team will soon begin wind-tunnel testing the vortex generators on a blade that is continually changing in incidence.

The rotorcraft Darp, which also involves Qinetiq, is one of 12 programmes funded by the DTI, EPSRC, MoD, and industry.

Kokkalis is also working with 14 European partners including Westland, Eurocopter, and Spanish aerospace firm Gamesa, to put a proposal to the European Commission for a complementary research project. This is due next March and could result in a e3.5m grant.

Eurocopter announced in June that it will begin tests of the first full-scale rotor blade equipped with an active servo flap next year.

The flap, fitted to the trailing edge of the blade, is designed to counter the vibration caused by the aerodynamic loading of the moving blades which gives rise to noise and fatigue and can reduce the life of rotor aircraft.