Both Boeing and Airbus have trumpeted the efficiency of their newest aircraft, the 787 and A350 respectively. Their clever designs and lightweight composites certainly make a difference. But a group of researchers at Stanford University, led by Ilan Kroo, has suggested that airlines could take a more naturalistic approach to cutting jet-fuel use, and it would not require them to buy new aircraft.
The answer, says Dr. Kroo, lies with birds. Since 1914, and a seminal paper by a German researcher called Carl Wieselsberger, scientists have known that birds flying in formation—a V-shape, echelon or otherwise—expend less energy. The air flowing over a bird’s wings curls upwards behind the wingtips, a phenomenon known as upwash. Other birds flying in the upwash experience reduced drag, and spend less energy propelling themselves. Peter Lissaman, an aeronautics expert who was formerly at Caltech and the University of Southern California, has suggested that a formation of 25 birds might enjoy a range increase of 71%.
When applied to aircraft, the principles are not substantially different. Dr. Kroo and his team modeled what would happen if three passenger jets departing from Los Angeles, San Francisco and Las Vegas were to rendezvous over Utah, assume an inverted V-formation, occasionally swap places so all could have a turn in the most favourable positions, and proceed to London. They found that the aircraft consumed as much as 15% less fuel (with a concomitant reduction in carbon-dioxide output). Nitrogen-oxide emissions during the cruising portions of the flight fell by around a quarter.
There are, of course, kinks to be worked out. One consideration is safety, or at least the perception of it. Would passengers feel comfortable travelling in convoy’ Dr. Kroo points out that the aircraft could he separated by several nautical miles, and would not be in the unnervingly easy groupings favoured by display teams like the Red Arrows. A passenger peering out of the window might not even see the other planes. Whether the separation distances involved would satisfy air-traffic-control regulations is another matter, although a working group at the International Civil Aviation Organization has included the possibility of formation flying in a blueprint for new operational guidelines.
It remains to be seen how weather conditions affect the air flows that make formation flight more efficient. In zones of increased turbulence, the planes’ wakes will decay more quickly and the effect will diminish. Dr. Kmo says this is one of the areas his team will investigate further. It might also he hard for airlines to coordinate the departure times and destinations of passenger aircraft in a way that would allow them to gain from formation flight. Cargo aircraft, in contrast, might be easier to reschedule, as might routine military flights.
As it happens, American’s armed forces are on the ease already. Earlier this year the country’s Defense Advanced Research Projects Agency announced plans to pay Boeing to investigate formation flight, though the programme has yet to begin. There are reports that some military aircraft flew in formation when they were low on fuel during the Second World War, hut Dr. Lissaman says they are apocryphal. "My father was an RAF pilot and my cousin the skipper of a Lancaster lost over Berlin," he adds. So he should know.
Which of the following can support "the principles are not substantially different"

A:Three passengers were in the most favourable positions during the experiment. B:15% less fuel reduction in carbon-dioxide output and nitrogen-oxide emissions during flying. C:Kroo and his team conducted their experiments in Los Angeles, San Francisco, Las Vegas, Utah and London. D:Air crafts can occasionally swap places to turn in the most favourable positions as well as birds.

Sulphur has occasionally been found in the earth in an almost pure state.

A:regularly B:accidentally C:sometimes D:successfully

Sulphur has {{U}}occasionally{{/U}} been found in the earth in an almost pure state.

A:regularly B:accidentally C:sometimes D:successfully

I go to the movies occasionally at the weekends.

A:now and again B:frequently C:once in a while D:here and there

I close my eyes and can still hear her—the little girl with a 1 ． SO strong and powerful we could hear her halfway down the block． She was a(n) 2 peasant who asked for money and 3 gave the。nly thing she had--her V。ice．I paused。utside a small shop and listened She brought to my mind the 4 of Little Orphan Annie．I could not understand the words she， but her voice begged for 6 .It stood out from the noises of Arbat Street，pure and impressive， like the chime of a bell．She sang 7 an old—style lamp post in the shadow of a building，her anns extended and 8 thrown back． She was small and of unremarkable looks．Her brown hair 9 the bun(发髻)it had been pulled into，and she occasionally reached up to 10 a stray Diece fmm her face．Her clothing I call’t recall．Her voice，on the other hand，is 11 imprinted in my mind. I asked one of the translators about the gin．Elaina told me that she and hundreds of others like her throughout the 12 soviet union add to their families’income by working on the streets. The children are unable to 13 school，and their parents work fulltime．These children know that the consequence of all 14 day is no food for the table．Similar situations occurred during the De’pression(萧条)in the United States，but those Amefican children were 15 shoeshine boys of me l6 .This girl was real to me． When we walked past her I gave her money．It was not out of pity l7 rather admiration Her smile of 18 did not interrupt her singin9．The girl watched US as we walked down the street．I know this because when I looked back she smiled again． We 19 that smile，and I knew I could never forget her courage and 20 strength．

A:never B:permanently C:occasionally D:sometimes

I close my eyes and can still hear her—the little girl with a 1 ． SO strong and powerful we could hear her halfway down the block． She was a(n) 2 peasant who asked for money and 3 gave the。nly thing she had--her V。ice．I paused。utside a small shop and listened She brought to my mind the 4 of Little Orphan Annie．I could not understand the words she， but her voice begged for 6 .It stood out from the noises of Arbat Street，pure and impressive， like the chime of a bell．She sang 7 an old—style lamp post in the shadow of a building，her anns extended and 8 thrown back． She was small and of unremarkable looks．Her brown hair 9 the bun(发髻)it had been pulled into，and she occasionally reached up to 10 a stray Diece fmm her face．Her clothing I call’t recall．Her voice，on the other hand，is 11 imprinted in my mind. I asked one of the translators about the gin．Elaina told me that she and hundreds of others like her throughout the 12 soviet union add to their families’income by working on the streets. The children are unable to 13 school，and their parents work fulltime．These children know that the consequence of all 14 day is no food for the table．Similar situations occurred during the De’pression(萧条)in the United States，but those Amefican children were 15 shoeshine boys of me l6 .This girl was real to me． When we walked past her I gave her money．It was not out of pity l7 rather admiration Her smile of 18 did not interrupt her singin9．The girl watched US as we walked down the street．I know this because when I looked back she smiled again． We 19 that smile，and I knew I could never forget her courage and 20 strength．

A:never B:permanently C:occasionally D:sometimes

I close my eyes and can still hear her—the little girl with a 1 ． SO strong and powerful we could hear her halfway down the block． She was a(n) 2 peasant who asked for money and 3 gave the。nly thing she had--her V。ice．I paused。utside a small shop and listened She brought to my mind the 4 of Little Orphan Annie．I could not understand the words she， but her voice begged for 6 .It stood out from the noises of Arbat Street，pure and impressive， like the chime of a bell．She sang 7 an old—style lamp post in the shadow of a building，her anns extended and 8 thrown back． She was small and of unremarkable looks．Her brown hair 9 the bun(发髻)it had been pulled into，and she occasionally reached up to 10 a stray Diece fmm her face．Her clothing I call’t recall．Her voice，on the other hand，is 11 imprinted in my mind. I asked one of the translators about the gin．Elaina told me that she and hundreds of others like her throughout the 12 soviet union add to their families’income by working on the streets. The children are unable to 13 school，and their parents work fulltime．These children know that the consequence of all 14 day is no food for the table．Similar situations occurred during the De’pression(萧条)in the United States，but those Amefican children were 15 shoeshine boys of me l6 .This girl was real to me． When we walked past her I gave her money．It was not out of pity l7 rather admiration Her smile of 18 did not interrupt her singin9．The girl watched US as we walked down the street．I know this because when I looked back she smiled again． We 19 that smile，and I knew I could never forget her courage and 20 strength．

A:never B:permanently C:occasionally D:sometimes