Learning from whales
June 27, 2012
Humpback whales can grow up to 15 meters in length and weigh up to 30 tons. Over time, they have evolved to easily glide through water. Their enormous bodies are almost perfectly streamlined.
But their pectoral fins - with their unusual array of bumps - seem out of place.
The fins have captured the imagination of a group of researchers at the German Aerospace Center (DLR) in Göttingen, who have been investigating air flow disruption in helicopter rotor blades.
It's a problem that both helicopters and humpbacks have in common.
Although humpback whales swim slowly through water and helicopters fly fast in the air, both experience flow disruption. It is also referred to as vortex breakdown or stall.
Strong turbulence
Flow disruption occurs when a whale's pectoral fin or a helicopter's rotor blade is pitched too steeply.
The flow separates from the back of the fin, or the blade, causing strong turbulence and reducing boost from the fin in water, and the rotor blade in flight.
Helicopters encounter flow disruption - or "dynamic separation" - when a rotor blade rotates against the direction of flight. The retreating blade moves through air much slower than an advancing blade and as a result creates less lift.
Retreating rotor blades need to be more steeply pitched than advancing ones to keep helicopters from slanting and to balance the amount of lift.
To enable this, rotor blades are constructed so that their angle changes constantly - two times every rotor revolution, and at six revolutions per second that is a total of 12 times.
This constant back and forth creates powerful forces that grow stronger the faster the helicopter flies and that can cause a possible stall.
"The control rods could tire and break, making a rotor blade uncontrollable and the helicopter impossible to navigate," says Kai Richter, a helicopter developer at the DLR's Institute for Aerodynamics and Airstream Technology.
But the humpback whale seems to have solved the problem for scientists.
It has a bump-like deformation on the outer edge of its pectoral fins.
The bumps cause tiny vortices that suck the airflow along the fins and reduce flow disruption. It means the whale can raise its fins more steeply and move better.
New invention
A few years ago, US researchers demonstrated the process with a whale fin model in a wind tunnel. The test prompted Holger Mai, the DLR's director of aeroelastic experiments, to apply the principle to a helicopter.
He wanted to reproduce the bumps on the back of a humpback whale.
"We used small rubber nubs taped over with plastic film to create an even, smooth leading edge," says Mai.
But Mai's approach didn't produce the desired result.
"So we tried to find out what the bumps actually do," Mai says. "And we discovered that small cylinders, which are sharper than what you find on humpback whales, perform much better, and we saw that in tests, too."
The experiment led to a new invention: Leading Edge Vortex Generators (LEVoGs).
LEVoGs are tiny - only six millimeters in diameter and just two millimeters high.
The artificial bumps can be punched out of a rubber mat with adhesive film and then stuck to the front edge of wings, creating tiny vortices in the air that hold the flow on the rotor blade.
Last fall, an experimental DLR helicopter equipped with LEVoGs took off for the first time. The pilot was able to gain some initial, if not exactly scientific, insight into the technology's performance.
Special test blade
From his seat, the pilot noticed a slight change in the behavior of the helicopter and it was thought to be because of the LEVoGs.
To make a more scientific assessment, the researchers will now need to measure pressure conditions on rotor blades with and without LEVoGs.
For this, they have developed a special test rotor blade, covered with 100 tiny holes, just 0.3 diameters in size.
A sensor is mounted under each hole to measure pressure.
With the data that comes from the sensors, researchers will be able to measure actual air flow changes and whether undesired forces on the rotor blade decrease.
But it could take several years for the researchers to reach a result because all work on aerodynamics has safety implications and is subject to a complex approval process. It took two years for Germany's Federal Aviation Authority to allow LEVoGs for test purposes. And the test rotor blade will undergo an equally lengthy approval process.
Technological challenges
While they wait, the researchers are considering how to apply the principle of the humpback whale to areas where safety regulations are less stringent. It could, for instance, be used to improve ship rudders or wind turbines.
Helicopter developer Kai Richter says he will continue to look to nature for solutions to technological challenges.
"When we talk about stall, we look to see how it's done [in nature]," says Richter. "But the fact that we found an answer in whales is almost a coincidence."
Their work is part of a long tradition.
Aircraft builders have taken inspiration from water and the knowledge that rough shark skin reduces flow resistance. Beyond the seas, birds and bats have also served as an inspiration.
Nature, it seems, just keeps giving.
Author: Fabian Schmidt / jrb
Editor: Zulfikar Abbany