The Economics of Flight, 1 - Shoreline Formations

 

Razorbills

Last winter I received an interesting response to Notes from Halibut Point # 333, "Alcids and Evolution" from train engineer Alan MacMillan regarding seabirds flying close to the water.

I can't remember if I told you this or not, but I'm a pilot (single engine land, single engine sea ratings)....ALL seabirds that you see flying close to the water are flying in what is known in aeronautical terms as "ground effect," flying at an altitude above the surface (water, here) of 1/2 the length of their wingspan (for example: if a bird's wingspan is 2 feet, flying at 1 foot above the water.) Doing so requires HALF the energy of flying at altitude.

Alan went on to relate the story of a B-29 bomber experiencing loss of power to three of its four engines en route from Hawaii to San Francisco, yet conserving aerial buoyancy for the final 300 miles of the flight by descending to an altitude of 70 feet above the Pacific.

Surf Scoters and Red-breasted Mergansers skim the sea

In this marvelous era of the Internet I pursued Alan's insight to an article * published from Loyola College of New Orleans, where people marvel at Brown Pelicans soaring within the troughs of waves. The same ground effect, or compression gliding, comes into play when the bird is within its full wingspan of the surface. Efficiency increases as it nears the water and has everything to do with the relative length of its wings. Birds with long, narrow wings like skimmers, petrels, albatrosses, and shearwaters, which spend their lives far at sea, have not surprisingly evolved the most beneficial designs. 

As these birds glide over the water the air is funneled between the lower surfaces of the wings and the upper surface of the water. The compressed air functions as a cushion that supports the bird aloft, in addition to the normal aerodynamic forces at work. The phenomenon is most efficient over calm (flat) water. 

Pilots coming in for a landing are suddenly buoyed upward by the ground effect when they get within half a wingspan of the runway. As a passenger you may notice a buoyant sensation just before touchdown. Watch the glide of a landing duck. It often extends improbably long before feet finally meet water, in part due to help from the ground effect that softens splashdown into a gently controlled crash.

A male and two female Common Eiders
flying with  Black Scoters

Energy-saving factors sometimes bring birds of different species to combine in formations for mutual benefit. We will look at some of these in the next posting. 

* Bob Thomas, "Bird Flight Over Water," Loyola (New Orleans) University Center for Environmental Communication, June 2, 2011.