Cutting down on drag

By R. David Whitby, Contributing Editor | TLT Worldwide November 2023

Examples in nature and competitive sports show the energy preservation benefits of reducing drag.

---

Elite cyclists often ride one behind another, to take advantage of the slipstream.

---

I have written on two occasions about the resistance to movement, of sharks traveling through water or of tiny insects flying through air. 

There are several other examples of how animals and people can reduce drag (friction) when moving. Large migrating birds, including geese, swans and pelicans, fly in a V formation. Several groups of researchers have studied the flight of migrating birds. It has been found that, in a flock of pelicans fitted with heart-rate monitors, those at the back of the V had slower heart rates than those at the front and flapped their wings less often. It also was observed that birds took turns in flying at the front of the V. Flocks of geese and ibis fitted with tiny data-loggers sensitive enough to record a bird’s position, speed and heading, several times a second, confirmed that the positions of birds in the V rotated regularly, presumably to even out the effort of flying long distances. 

Other studies used aeronautical theory to investigate the flapping of bird’s wings. When a bird flaps its wings, each wing tip produces a rotating vortex of air. As a result, the air immediately behind the bird gets constantly pushed downward (down-wash) while the air off to the sides and further behind gets pushed upward (up-wash). Another bird flying in either of these upwash zones gets a free lift. Attempting to calculate the resulting energy savings is difficult because the necessary physiological measurements would be too invasive for most birds. Earlier studies estimated that birds can use between 20% and 30% less energy while flying in a V. 

In a competitive bicycle race, whether on-road or in velodromes, elite cyclists often ride one behind another, to take advantage of the slipstream. This is the partial vacuum created in the wake of a moving object. Riders can save energy by riding close to or be-hind other riders. This is known as drafting or slipstreaming. The main group or pack of riders is known as a peloton. By riding in the middle of a well-developed peloton, drag can be reduced to as little as 5%-10%.1Using this saving in potential energy leads to complex cooperative and competitive interactions be-tween riders and teams in race tactics. 

Pelotons form as a cohesive unit in which each rider makes positional adjustments in response to the movements of adjacent riders and those in front. Riders at the front are fully exposed to wind resistance, so they experience higher energy demands than riders further back. A continuous rotation of riders pushes from the back through to the front and then fall away, so the peloton is in constant motion during a race. 

It now seems that swimmers participating in events in open waters, particularly for example triathlons, might benefit from using similar methods to reduce drag. 

A group of researchers at École des Ponts Paris Tech, France,2have experiment-ed with two plastic mannequins that resembled swimmers with their arms held tight to their bodies, in an 80 meter channel of water. They were fitted with sensors to measure the drag forces as they were pulled through the water. Different configurations of the mannequins were tried, including side by side and one behind the other. 

The measurements and subsequent computer simulations revealed that a swimmer can reduce the drag on their body up to 40% by swimming behind someone else. When swimming side by side with a competitor at the level of their rival’s hips, it reduces drag by about 30%.2It also was confirmed that swimming exactly level with a competitor is tiring, and trying to pass them is difficult. Swimming slightly behind is similar to the behavior of birds, in that they are surfing the waves created by the other competitor. 

Presumably, if someone wants to win a swimming race, at some point they will have to overtake the person in front. They may have been able to save sufficient energy to do this in the last several meters. If everyone decided to swim behind someone, at least one person would have to be at the front. In a triathlon, since the third leg is running (following swimming and cycling), it might be best to leave the overtaking to near the end.