The Windward Turn Theory described in this third revision is a new explanation of dynamic soaring as practised by the albatross and other oceanic birds. To understand it you only need an open mind but high school maths will help. This page is a brief introduction to the theory. The next page, Windward Turn Theory is a more detailed, verbal explanation and the Analysis page has the scary maths. But if you are familiar with the classic wind-gradient theory you might like to check out the Wind Gradient Theory page first - to purge your mind.
Dynamic soaring on YouTube
Search on YouTube for Ďalbatross dynamic soaringí and you will find several films on this subject
For a 15 minute animated version of this website use this link Dynamic Soaring The flight of the albatross
Since mankind began exploring the great oceans we have been fascinated by the flight of the albatross. These huge birds are seen flying at low level over the sea with a characteristic, apparently effortless, undulating flight pattern mostly without flapping their wings. Research has shown they are capable of flying great distances at high average speeds with very little effort. Their flight manoeuvres are different to those used by land birds on soaring flights, so what are they doing?
What does an albatross do all day?
Any theory of dynamic soaring has to explain what the albatross and the giant petrel actually do, whilst soaring over the oceans. It has to explain their swooping, turning flight path, the direction of their large scale flight patterns (mostly downwind), their high-aspect ratio wings and their tube-nostrils.
When you look at film of albatross in flight, what you will see is the bird the ocean and the sky. What you will not see is the air. This is where things get complicated because although you can see the motion of the bird, you cannot see the birdís airspeed, drift angle or its acceleration because everything you can see, the waves, the swell the water and you are moving relative to everything else.
Wind Gradient Theory or Windward Turn Theory?
In the ornithology literature albatross dynamic soaring is explained by the Wind Gradient Theory, which has been repeated without question since Lord Rayleighís article in the journal Nature in 1883. Later in this article it will be shown that, although the wind-gradient does have a role to play in dynamic soaring, it is not the whole story. So, I will propose a new description of dynamic soaring, the Windward Turn Theory, which requires only a sufficient wind and the kind of manoeuvres which birds have been observed to perform. It will explain the true role of the wind gradient but does not depend upon it.
So, we are trying to explain this: The albatross glides in a fairly flat windward turn, flown with a small angle of bank, at approximately constant height above the surface and lasting about 5 to 15 seconds. The bird then reverses the direction of turn and flies the leeward turn as a steeply banked wing-over taking about 2 to 4 seconds before reversing the direction of turn again and commencing the next windward turn.
The Windward Turn Theory
In dynamic soaring, the albatross flies approximately on a cross-wind heading, turning alternately left and right by plus and minus about 20 to 30 degrees. Thus there is a windward turn flown close to the surface and a leeward turn in which height is gained and lost, plus rolling manoeuvres linking them. The windward turn starts with a tailwind component and ends with a headwind component. There is a reversal of the direction of turn and the leeward turn starts with a headwind and ends with a tailwind.
The Windward turn
In the windward turn, the albatross loses momentum due to the unbalanced drag force. It maintains height by expending kinetic energy; instead of maintaining momentum by losing height and expending potential energy. The loss of momentum is seen as a loss of ground-speed rather than a loss of airspeed. Airspeed is constant because the tendency to lose airspeed due to drag is balanced by the tendency to gain airspeed from the increasing headwind component, whilst turning relative to the wind.
Ground-effect improves the efficiency of the turn by reducing drag. The albatross flies the windward turn at the least angle of bank which will give it a rate of turn sufficient to maintain airspeed and height and thereby maximise the distance flown.
As the albatross loses momentum, so the wind gains momentum. The lift and drag forces produced by the wing are the equal and opposite reactions to momentum given to the air. When an aircraft banks, the horizontal component of lift acts as a centripetal force. Horizontal momentum is given to the air, opposite to the centripetal direction. In the windward turn, a component of this momentum is parallel to the wind and gives momentum and energy back to the wind.
The Leeward turn
At the end of the windward turn, the albatross pitches-up and reverses the direction of turn, making the leeward turn as an arched turn or wing-over. A wing-over, has a partially ballistic trajectory with the weight of the bird partly supported by a a small vertical component of lift. This enables a steep angle of bank to give a large horizontal component of lift without a large increase in actual lift and without a large increase in load-factor. Despite the large bank angle the load factor is only about 1G.
(The alternative would be a turn at constant height and reducing airspeed. This would require an increase of load factor with the corresponding increased effort, to ensure the vertical component of lift is equal to the weight of the bird).
The bird gains momentum in the leeward turn using a component of aerodynamic force to act as a propulsive force. This propulsive force provides the acceleration which is seen as an increase in ground-speed rather than airspeed. A large bank angle and a large drift angle mean that the horizontal component of lift added vectorially to the drag creates a horizontal resultant. This force then provides the propulsive and centripetal force components which cause an increase in ground speed and the curved path over the ground.
The large drift angle is due to the wind being a large proportion of the birdís airspeed. When the bird is gaining height a part of the propulsive force enables the bird to gain extra height which means a gain of potential energy. This extra potential energy balances drag losses anytime the bird is losing height.
The birdís change of direction is again caused by a centripetal force. This is a horizontal component of the lift force, the result of the wing imparting horizontal momentum to the air, opposite to the centripetal direction. In the leeward turn, a component of that momentum is parallel to and opposite to the wind direction, therefore the momentum of the wind is reduced whilst the horizontal and vertical momentum of the bird is increased.
The bird maintains average speed and height during successive turns and the change of momentum is the same in each turn. However the leeward turn is shorter than the windward turn and less air is given more acceleration compared with the windward turn in which more air is given less acceleration. So the air loses more energy in the leeward turn than it gets back in the windward turn because kinetic energy is a square law and momentum is a direct law. The difference in energy is equivalent to the birds drag losses. In effect wind speed has been converted into air turbulence in the wake of the bird.
The wind-gradient may improve the efficiency of the leeward turn during the climb and descent by reducing the loss of airspeed. The wind gradient will not increase the airspeed.
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