Adding ridges to rotor blades could improve helicopter agility

Researchers at the German Aerospace Centre (DLR) took inspiration from similar ridges found on the pectoral fins of humpback whales — the fastest and most acrobatic of the order. 

Helicopters owe their unique ability to vertically take off and land to their main rotor, but this also contributes to aerodynamic instabilities, as Dr Kai Richter of DLR explained to The Engineer.

‘The main problem is that on the advancing blade the rotational speed and the flight velocity of the craft itself combine, so locally on the blade there are very high flow velocities — you even get supersonic flow on the outer part of the blade. But on the retreating blade it’s the other way round — the rotational speed and the flight speed of the helicopter subtract and you have low flow velocities.’

The result of this asymmetry is that the rotor blade’s inclination changes, so that the advancing side has a low angle of attack and the retreating side a high angle of attack.

‘Typically, if you increase the angle of attack of the rotor blade then the lift you generate increases as well; but at some point when your angle of attack is too high, the flow separates and your lift drops — this is called dynamic stall. This phenomenon occurs especially when you fly at very high speed and when you’re doing narrow turns,’ said Richter.

In an effort to delay this stalling, the researchers came across work modelling humpback whales, which, due to their unusually large pectoral fins with characteristic bumps along the front edge, are quite agile in the water.

Because flow phenomena in water are similar to those in air, they were able to adapt these bumps for the latter, taking into account the greater density of the water. The result was a series of leading-edge vortex generators (LEVoGs) with a diameter of 6mm and a weight of just 0.04g.

After performing wind-tunnel testing to get the right configuration of LEVoGs, they are now essentially gluing them to real helicopter blades.

‘The flights we did were certification flights, because we interfered with the aerodynamics of the rotor and that is the most dangerous thing you can do to a helicopter, so we had to make sure that when we use the LEVoGs on the blades, the helicopter is still safe to fly the entire flight envelope.’