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.