**Introduction**

This theoretical analysis began as an attempt to write a program to simulate the wind gradient model of dynamic soaring including drag losses. It soon became clear that the wind gradient model was not the complete solution because we have to take into account the acceleration of the bird during the windward and leeward turns. The problem here is that any attempt to analyse turning flight would normally involve the whole 360 degree circle whilst it appears the albatrosses use only part of the circle - the crosswind parts. They only gain advantage from two segments of the circle, that is they fly crosswind plus and minus about 20 to 30 degrees. The other 240 odd degrees of the circle are useless to them.

For a more generalised illustration of the effect of the wind on an circling powered aircraft take a look at the Downwind Turn page. The maths is the same.

So the Windward Turn Theory evolved. There are a large number of terms and variables and so I have not attempted to reduce the maths to a single equation. It was a bit like juggling with jellyfish.

Successful dynamic soaring depends on a minimum wind velocity as well as a particular flight technique so, proving a theory of dynamic soaring is probably impossible The best we can achieve is an illustration of what an animal does given a certain wind. The albatross model is known to work, so its data were used as a starting point. The data used is that of an albatross in typical wind conditions as published in Avian Flight by J J Videler 2010. Many of the nominated parameters are estimated by observation of film of albatross in flight.

The object of this exercise is to see whether albatross flight can be modeled using the forces acting on the bird, the triangle of velocities and the equations of motion. An Excel spreadsheet is used with a single set of equations to model the windward and leeward turns. The end result depends on not just the equations but also certain parameters such as the range of wind-angles and the wind speed. By accepting a limited range of wind-angles, typically crosswind plus and minus 20 to 30 degrees, we can calculate for each wind-angle, the effect of angle of bank and angle of pitch. The angle of bank will give a centripetal force which will act with the drag force to give a rate of turn and therefore a rate of change of headwind or tailwind component. The aerodynamic forces will also control the rate of tangential acceleration, which acts opposite to the rate of change of the head/tailwind component to control the airspeed.

**Results**

The motion of the bird is defined by airspeed, lift/drag ratio, load factor and angles of pitch and roll combined with the wind speed and wind-angle. The roll and pitch profile is intended to give a flat windward turn and a leeward wing-over. Using a spread sheet, different angles of bank can be tested to see which produce an airspeed ground-speed and height profile similar to those seen in nature. The important point being to test whether the result gives at least as much airspeed and height at the end of the dynamic soaring manoeuvre as at the beginning.

** The following diagram shows an example of the results of the calculations. See figure 8. The horizontal axis is crosswind distance in tens of metres, an overall distance of 450m and lasting about 20 seconds. The left side is the windward turn and the right side is the leeward turn, with the bird flying from left to right, reversing the direction of turn at about 370m. The model bird has a mass of 10kg, and a lift/drag ratio of 20. The program works with a starting point of airspeed 20m/sec and height 2m. The wind-velocity is a uniform 10m/s with no wind gradient or any vertical motion.**

A profile of angle of bank (10 degrees left in the windward turn and 70 degrees right in the leeward turn) and angle of pitch between 0.5 degrees down and 8 degrees up is then applied at 10m intervals in the crosswind direction giving time intervals of about 0.5 seconds. The program uses the triangle of velocities, the laws of motion and the forces acting on the bird, to calculate the load factor, the centripetal and tangential accelerations, the rate of turn, airspeed and wind-angle (heading relative to the wind), ground speed and track, the rate of climb or descent and height.

The first diagram shows variation of airspeed and ground-speed against distance flown. Starting at 20 m/s the airspeed increases and the ground-speed decreases during the windward turn from 0 to 370m. During the leeward turn from 370m to 450m, the airspeed decreases as the bird gains height in the wing-over and then increases slightly to about 27m/s as the bird descends, leaving __a slight gain of airspeed overall __. The ground-speed decreases slightly as the bird starts to climb and then increases as it turns and descends with a slight gain of ground-speed at the end.

Figure 8

In the middle diagram of height in m against distance in tens of m, the height scale is expanded for clarity. The height at the beginning is 2 metres. The pitch profile gives a slight loss and then a slightly greater gain of height in the windward turn. The leeward turn is then a wing-over rising to about 5 m before descending to 3m, leaving __a slight gain of height overall__. Notice that from about 300m to 370m the model albatross __gains both height and airspeed__ just as seen in the data from actual albatross GPS tracking. (See the Data section) The small kink in the line at 370m is the point at which the model albatross reverses direction.

Bear in mind that, in straight flight with a lift/drag ratio of 20, at constant speed, the model glider will descend from 2m to the surface in a distance of only 40m whereas in the windward turn the bird is able to fly 300+ m without losing airspeed or height.

The lower diagram is a plan view showing the air-path, the motion of the bird within the air mass, in blue. The heading is the same as the wind-angle with the wind coming from the top. It shows the range of wind-angles, turning left during the windward turn from 120 through 90 to 60 degrees and then turning right during the leeward turn from 60 through 90 to 120 degrees. The ground track, the birds actual path over the ground is shown in orange. The gap between the ends of the lines on the right is the distance lost downwind, about 200m, the effect of the wind. The same bird is is shown at two places on each line which are effectively the same place and time on both lines. The bird is not drawn to scale but does illustrate the angle of drift.

These results are an illustration of how it is possible for an albatross to dynamic soar in a uniform horizontal wind maintaining average airspeed and height. It requires the typical profile of roll and pitch angles which albatrosses are seen to perform. Any other profile results in a loss of airspeed or height.

** Dynamic soaring **depends on there being a single basic mechanism, controlled primarily by the angle of bank, which will enable the bird to maintain airspeed and height throughout the windward and leeward turns. That mechanism is the tendency of the airspeed to change due to two effects. Firstly, the changing headwind component caused by the changing wind-angle, that is the rate of turn. Secondly the opposite acceleration of ground-speed caused by the aerodynamic forces acting on the bird.

The rest of this page is an explanation of the maths used to produce these results. Firstly we will look at how the rate of turn and the wind can cause a change of airspeed.

**Angle of bank and Rate of turn in level flight**

The angle of bank is measured from the vertical. The centripetal acceleration it provides is horizontal. See figure 9 in which a level turn is depicted where the vertical component of lift is equal to the weight.

**Angle of bank x** Figure 9

Tan x = a / g **x** is angle of bank

a = v^{2 }/ r **a** is centripetal acceleration m/s^{2}

** v** is tangential air-speed m/s

** r** is radius of turn m

Tan x = v^{2 }/ g . r

r = v^{2 }/ g .Tan x (1)

**Rate of turn R_{y}**

The angle of bank **x** gives the rate of turn **R _{y}**

R_{y }= 360 . v / C **R**** _{y}** is rate of turn deg/sec

** C** is circumference of circle radius **r**

R_{y }= 360 . v / 2 . pi . r (2)

R_{y }= 360 . v . g . Tan x / 2 . pi .v^{2 (1&2)}

^{ }R_{y }= 180 . g . Tan x / pi . v (3)

So the rate of turn **R _{y}** is a function of tangential speed v and an angle of bank x assuming a level turn in which the vertical component of lift is equal to the weight.

However, we need to nominate the load-factor in the leeward turn which is a wing-over and not level flight.

**Load factor L_{f}**

Load factor is the ratio of lift to aircraft weight. In straight and level flight, lift equals weight and **L**_{f }is one (1G). In a straight pitch-up the load-factor is increased by the increased angle of attack, so that in a loop for example the load factor might be three (3G). In a level turn the vertical component of lift must remain equal to weight and to achieve this the actual lift is increased by increasing the angle of attack, to correspond with the increasing angle of bank, so that the load factor is equal to 1/ cosine of the angle of bank. See figure 9. In a 30 degree banked level turn the load factor is 1.15G

In a wing-over at any given angle of bank, the load-factor can be whatever we want but the vertical component of lift will not necessarily be equal to the weight. In this case we can introduce the load-factor to equation (3) by saying that **tan x = sin x / cos x** and **L_{f} = 1/cos x** therefore

Substituting in equation (3) the Rate of turn is

** ^{}R_{y }= 180 . g . L_{f} . sin x / pi . v (4)**

** R_{w }Rate of change of the headwind component H versus the wind angle y **

** Headwind component H is a function of trigonometry and simply depends on the aircraft heading relative to the wind, the wind-angle. Given wind-speed W there is a headwind component H corresponding to each wind angle y.**

** H = W . cos y**

Taking a given interval between wind-angles, (say 1 degree), then in a turn between each pair of wind angles there is a corresponding change of headwind component. Therefore we get a rate of change of headwind component with respect to the wind-angle.

The rate of change of head-wind component per degree of wind-angle

** R _{w }= (W . cos(y+1)) - (W . cos y)**

** R _{w }= W . (cos(y+1) - cos y)** (5)

Where **W** is the wind and **y** is the wind angle.

Note that **R _{w}** , in units

**R _{H} rate of change of headwind component with time**

The rate of change of the headwind component **R_{H}** in units

** R _{H} =R_{w }. R_{y}** (6)

Looking at the units here we get:

m / sec^{2} = (m / sec . deg) x (deg / sec)

**The Drag Force F_{1}**

The drag force depends approximately on the lift divided by the lift/drag ratio **L _{d}**. The lift is equal to the weight (m.g) at 1G. In a turn it is then multiplied by the load factor

** F _{1} = m . g . L_{f} / L_{d}** (7)

**The Centripetal Force F_{2}**

The centripetal force **F _{2}** in

** F _{2 }= m . g . L_{f }. sin x** (8)

**The minimum angle of bank**

Now we can calculate the minimum angle of bank x, which will give constant airspeed while turning through any particular wind-angle.

For simplicity we will use **F _{1}** for the tangential load and

In figure 12 it can be seen that airspeed is the sum of the headwind component **H** and the ground-speed component **K**. The acceleration of airspeed is the sum of the acceleration **R _{H}** of component

**R_{H}** can therefore be substituted as the acceleration term in F = m . a

R_{H} = F_{1} / m (9)

R_{H} = ~~m~~ . g . L_{f} / (L_{d} . ~~m~~) = R_{w} . R_{y} (6,7)

R_{H} = g . L_{f} / L_{d} = R_{w} . R_{y}

R_{H} = ~~g~~ . ~~L~~_{f} / L_{d} = R_{w} . 180 . ~~g~~ . ~~L~~_{f} . sin x / (pi . v) (4)

1 / L_{d} = R_{w} . 180 . sin x / (pi . v)

sin x = pi . v / (L_{d} . R_{w} . 180 ) (10)

R_{w }= W . (cos(y+1) - cos y) ** (**5)

So, for each wind-angle **y**, at say one degree increments, there is a value of **R _{w}** (m/s per deg) combined with a rate of turn represented by the angle of bank x, which will give a rate of change of tail/headwind component which will make the airspeed increase. The load factor and mass have cancelled-out and there remains a drag load, represented by the lift/drag ratio

The result gives small angles of bank which correlates quite well with the small angles of bank observed in film of albatross. The nominated values of airspeed and wind-speed can then be modified to see, for example, what the minimum wind speed needs to be and what effect the airspeed /windspeed ratio has on the useable range of wind-angles.

**For example:**

Given wind-angle 80 to 79 deg, bird airspeed 20m/s, bird L/D 20, wind 8m/s

R_{w }= W . (cos(y+1) - cos y)

Rw = 8 .cos 81- cos 80)

Rw = 0.13 m/s per deg

Angle of bank x = 7.7 deg

This is the minimum angle of bank needed to maintain height and airspeed at one point in the windward turn, at a wind angle of 80 degrees. Remember that the drag load is represented by the L/D ratio. In practice, the GPS data suggest that the airspeed increases during the windward turn with a gain of height before rolling into the leeward turn. To achieve this, the angle of bank is made slightly greater than the minimum.

This is somewhat counter-intuitive because it means that airspeed can be constant or even increase slightly during a windward turn, even though there is an unbalanced drag load. Fortunately it demonstrates the viability of the Windward Turn Theory. In the leeward turn this effect is reversed and there will inevitably be a reduction of airspeed. The job of the albatross is to ensure that it gains more airspeed in the windward turn than it loses in the leeward turn.

In the windward turn it is easy to understand how the drag load causes the ground-speed to reduce even as the airspeed increases. However, in the leeward turn the ground-speed increases and it is less easy to see how this is possible given an unbalanced drag load. The aerodynamic mechanism is the same in both the windward and leeward turns but the effect reverses as the angle of bank flips. Next we will see how the forces work in the windward turn.

** **Figure 10

**Force and acceleration components**

Now things get complicated. On the face of it there are two horizontal force components: the drag force **F _{1}** and the centripetal force

**Horizontal resultant F _{3 }and components**

Figure 10 is a plan view of a windward turn with the wind coming from the top of the diagram and the bird is flying left to right in a banked left turn. The horizontal component of lift **F _{2}** is normal to the direction of the air velocity and combines with the drag

We can calculate **F _{3}** using

**F _{3} =sqrt (F_{1}^{2} + F_{22})**

**Forces and angles**

Force **F _{3}** is not exactly in line with the ground velocity and can be resolved into components

Forces are positive in the direction of flight, that is the direction of the air-velocity, therefore force **F _{1}**, the drag force, has a negative sign.

**Force components relative to the ground-velocity**

In the windward turn (figure 10) a tangential force component **F**_{gt } acts opposite to the direction of the ground-velocity.

**F_{gt} = F_{3} cos e (11)**

Note that angle **e** is not the internal angle between **F _{3}** and

The centripetal component **F _{gc}** provides the centripetal acceleration which creates the curved path relative to the ground.

**F _{gc} = F_{3} sin e**

Once again **Fgc** has a negative sign because **e** is negative.

**Acceleration or equilibrium**

The logical inference is that under acceleration, the aerodynamic forces cause acceleration of the ground-velocity and then that acceleration causes acceleration of the air-velocity. This is somewhat different to the situation when the aircraft is under equilibrium, when the ground-velocity is the vector sum of air-velocity and wind-velocity. In fact, the only difference is that under equilibrium the accelerations are all zero.

(It should be noted that this process is the same as that which causes ground speed to change during turns in a wind by a normal powered aircraft where thrust is equal to drag. **F _{1}** is then effectively zero and

** Figure 11**

**Angles b, d & e**

** **Angle **b** is the angle between **F _{3}** and the glider heading

Angle **e** is the angle between **F_{3}** and the ground track (the direction of the tangential acceleration)

The tangential acceleration of the ground-velocity is : **A _{gt} = F_{3} cos e / m (12)**

The centripetal acceleration of the ground-velocity is **A _{gc} = F_{3} sin e / m**

**Sorting out the positives and negatives**

** Angles b, d** and **e** are illustrated in figure 10 for the windward turn and figure 11 for the leeward turn:

__By inspection__ in the windward turn, angle e = b + d Angle** d** is the angle of drift and has a positive sign. Angles **b** and **e** are on the opposite side and are therefore negative.

**(-e) = (-b) + (+d)**

**e = b - d**

In the leeward turn in figure 11, __by inspection__, **e = b - d but all the angles are on the same side and have the same sign (positive) therefore**

**e = (+b) - (+d)**

**e = b - d**

** So that in the calculations, we use the same formula e = b - d** in both the windward and leeward turns. Angle

The sign of the drag force **F _{1}** is negative, opposite to the direction of flight. The value of centripetal force

Note that in a straight glide, when the angle of bank is zero, then **F _{2} is zero, F_{3} is the same as F_{1} and angle b is 180 degrees._{}**

This seems rather complicated but it is necessary treat all the forces equally __and__ account for the acceleration of ground speed. If the albatrosses did not exist you might think that this is all a bit far-fetched but this treatment of the force components is the only way to produces a realistic model of albatross flight as illustrated in the diagrams of the dynamic soaring manoeuvre at the top of the page.

So far we have seen how the aerodynamic forces cause the ground-speed to decrease in the windward turn and increase in the leeward turn. Does this have an efect on the airspeed?

**Components of airspeed and ground-speed**

There is a component of the ground speed **K = G . cos d** and a component of the wind-speed **H = W . cos y** Both are parallel with the air-velocity which is the sum of the two components. See figure 12

Airspeed **V= K + H**.

The rate of change of airspeed **R _{V}** is the sum of the rate of change

**R _{V} = R_{H} + R_{K}**

Figure 12

__In order for the airspeed to be constant__, the ground-speed component **K** must reduce and the wind component **H** must increase at the same rate **R _{H}**

**R _{H }= - R_{K}**

But force **F _{1} does not directly affect component H, it only affects component K by causing a tangential acceleration of the aircraft.**

To understand this, compare with straight and level flight in a uniform wind where the headwind component **H **is constant because the aircraft is not turning. Therefore, an __unbalanced__ drag force **F _{1}** would cause the airspeed

**Acceleration of component K **

The tangential acceleration **R _{K}** of the component

**R_{K} = A_{gt} . cos d**

** = Fgt . cos d / m**

** = F _{3} . cos e . cos d/ m**.

** As mentioned before, the calculation gives e a large negative value in the windward turn which makes R_{K} a retarding tangential acceleration.**

**Rate of Turn**

** The rate of turn depends on the centripetal force F_{2}. This gives a good result in the spreadsheet because F_{2} is the centripetal component of F_{3} in the air frame of reference. Any other treatment of the centripetal forces will reduce the rate of turn and reduce the rate of change of the headwind component and the rate of increase of airspeed in the windward turn. **

** Referring to equation (4) the rate of turn is R _{y }= 180 . g . L_{f} . sin x / pi . v The term g . Lf sin x is the centripetal acceleration caused by the angle of bank in units m/s^{2} . In the spreadsheet this is substituted with centripetal acceleration F_{2 }/ m which depends on angle of bank x. The velocity term v is substituted with component K**

**R _{y }= F_{2 }. 180 / m. pi . K **

_{}The story so far

The aerodynamic forces **F _{1}** and

**The effect of angle of bank**

Figure 13 shows the effect of different angles of bank on acceleration of airspeed at one point in the windward turn, using a modified version of the spreadsheet that produced the diagrams at the top of the page. It consists of **R _{K}** , the rate of change of component

** Rv is **negative at small angles of bank but becomes positive at about 6 degrees angle of bank. This shows that 6 degrees is the minimum angle of bank needed to maintain airspeed (and height) at this particular point in the windward turn (with a certain combination of airspeed and wind-speed). Also, it shows that by increasing the angle of bank, a slightly greater rate of increase of airspeed is achieved, which will enable the albatross to control its airspeed by varying its angle of bank. This will enable the albatross to climb and descend to follow the rising and falling surface of the sea without much change of airspeed and to gain some excess airspeed prior to commencing the leeward wing-over.

**The Leeward Turn**

Having lost ground-speed in the windward turn, but having maintained airspeed and height, the albatross now has to recover that ground speed in the same range of wind angles and end up with at least the original airspeed and height. Figures 11 and 14 show plan views of the leeward turn with the wind coming from the top. Drag force **F _{1}**

**R_{K}**

The centripetal component of acceleration giving the rate of turn is

** F_{2} /m**

In the calculations, angle **e** has a small positive value, **R _{K}** is positive and therefore propulsive. Centripetal acceleration

However, whereas in the windward turn the increasing headwind component tends to increase airspeed, in the leeward turn the reducing headwind component tends to reduce airspeed. See figure 14. Therefore the propulsive effect of **F _{3}**

**Airspeed components in the Leeward turn**

In figure 14 can be seen a plan view of the leeward turn with the wind coming from the top. The airspeed **V **consists of a ground-speed component **K** and a wind-speed component **H (which could be a headwind or a tailwind). As the wind angle y increases, the headwind component reduces and then becomes an increasing tailwind with the same effect. Acceleration of airspeed is the sum of the acceleration of the ground speed component K** and the acceleration of the wind-speed component

** The maths are the same as the windward turn** except that in the leeward turn, the bank angle and the drift angle are on the same side and have the same sign, so that the horizontal resultant has a propulsive component in the direction of ground-velocity.

Figure 14

Although it seems as if drag should always reduce airspeed, in fact the drag force only exists as a part of the horizontal resultant **F _{3}**. When the angle of bank and the angle of drift are big enough and on the same side, the propulsive component makes the ground speed increase which in turn makes component

Component **K** is part of the airspeed. Therefore tangential acceleration component **R _{K}** causes tangential acceleration of the air-speed, while the other component of the airspeed, the wind component

*This boils down to the fact that we can use the same set of equations to describe the bird motion in both the windward and leeward turns. The only difference is that the angle of bank has a negative sign in the windward turn and a positive sign in the leeward turn.*

* *

**Flat turns and wing-overs**

In the windward turn the vertical component of lift is equal to the weight of the bird and therefore the actual lift is increased proportionate to the load factor. The angle of bank is quite small and therefore the load-factor is small and the drag increase is minimal. However, in the leeward turn, because of the steep angle of bank, the vertical component of lift is less than the weight of the bird and we can assume (guess!) that the load factor is one and therefore the actual lift is equal to the bird weight. The rate of change of the headwind component **R_{w}** versus the wind angle

Rate of turn is proportional to load-factor but increasing the load-factor will increase the drag load. To avoid increasing the drag load, we keep the load-factor at one (1G) and the rate of turn is then only proportional to the sine of the angle of bank. Figure 15

However at a steep angle of bank and only 1G, the vertical component of lift will not be enough to sustain level flight. So, in the leeward turn the rate of turn can be increased at a load-factor of one without increasing the drag loading __but__ only by doing a wing-over and not a level turn. (A __level turn __at 48 deg would require a load factor of 1.5G, which is relatively high-stress. 70 degrees angle bank would require 3G which is jet-fighter territory!)

**How does a wing-over differ from a level turn?**

In level flight, when an aircraft banks, the lift force is tilted off the vertical and the horizontal component provides the centripetal acceleration causing the aircraft to turn. (See figure 15) The total lift force must be increased, normally by increasing the angle of attack, in order that the vertical component can balance the weight of the aircraft and prevent a loss of height. This increase in load-factor will increase the drag loading.

In dynamic soaring the leeward turn is a wing-over, at a load-factor of about one, in which total lift is not increased and remains approximately equal to weight. The vertical component of lift partly supports the weight whilst the horizontal component provides the centripetal acceleration which makes the bird turn. Drag is not shown. The flight path is partly ballistic as the bird climbs and descends and with a large centripetal force and a quick rate of turn the bird has enough time to complete the turn before running out of height. Figure 15 shows, in the middle, the forces during a level 45 degree banked turn at a load-factor of 1.4 (1.4G) with the vertical component equal to the weight. On the right is an 80 degree wing-over at a load-factor of one and therefore without an increase of drag but with the vertical component of lift less than the weight.

**Effect of Load Factor**

Figure 16

Figure 16 shows the acceleration of speed at different angles of bank from 0 to 80 in a__ level leeward turn__ with, a load-factor of 1/cos angle of bank as for a level turn.

Up to about 60 degrees angle of bank, both **R _{K}** and

In figure 17 is seen acceleration of speed versus angle of bank __in a wing-over__ with the load-factor equal to 1G at all angles of bank. __Note the different vertical scale__. The negative acceleration of airspeed in the 1G wing-over is about -2.8 m/s^{2} at 70 degrees angle of bank compared to -8.2 m/s^{2} in the level turn.

Thus in a wing-over, large angles of bank and therefore a quick rate of turn, can be achieved without a big increase in deceleration.

Figure 17

**Exchanging speed and height**

During the leeward turn the bird climbs and descends in a wing-over. An angle of climb will give an additional retarding force equal to the weight times the sine of the climb angle.

In a climb the bird gains potential energy as it loses airspeed. However, the propulsive acceleration will reduce the airspeed losses and the height energy then converts to speed in the descent.

**Vertical Acceleration**

Whenever the vertical component of lift is less than the weight, the bird will accelerate downwards. Its rate of descent will then be the sum of the effect of gravity and the effect of its angle of climb or descent. This limits the time and length of the leeward wing-over.

**Roll reversals **

Between each turn there is a roll reversal with associated drag losses. The gain of height in the leeward turn will have to compensate for this. A little height can be gained by briefly increasing the angle of bank and rate of turn at the end of the windward turn. The penalty is a small loss of the distance flown in the windward turn.

**Limits on the energy gained**

Doing a wing-over is simply a way of gaining time to gain ground-speed before running out of height. The amount of energy gained in this way will be limited by the amount of time the bird can spend in the wing-over in a steeply banked attitude (about 2 to 5 seconds) and this will depend on how much excess height or air-speed can be gained first.

The birds cross-country course has to be fairly straight to maintain a suitable range of wind-angles relative to the wind direction. The effects described so far diminish when the heading gets within about 50 degrees of an upwind or downwind heading. Therefore the amount of turn in the leeward turn has to be the same as that in the windward turn in order to maintain an approximately straight average course. A suitable range of wind-angles is found to be not less than about 50deg and not greater than about 130 deg (probably less than this in nature). In effect, the bird is flying a crosswind heading +/- 20 to 30 deg. Even with this reduced range of wind-angles, the bird can still gain sufficient momentum because the drift angle and the propulsive force, is greatest in the middle part of the leeward turn.

**The effect of the wind gradient**

As the bird turns, it climbs and descends and airspeed converts to height and back to airspeed again. If there is a wind gradient, the wind will change with height and the effect of this will be to reduce the change of airspeed. During the climb the reducing airspeed will be offset by the increasing headwind. During the descent the increasing airspeed will be assisted by the decreasing tailwind. Thus the wind-gradient, by adding a little airspeed, may improve the efficiency of the wing-over by decreasing the loss of airspeed. This, combined with the reduction of drag due to the ground-effect, may be the reasons why the albatross stays close to the surface

*However, it should be noted that the overall effect of the wind gradient is to reduce the average wind near the surface which will reduce the birds ability to dynamic soar.* Also bear in mind that according to GPS data logging, dynamic soaring does not appear to involve 180 degree turns and the effect of any wind gradient will be diminished by the angle off the wind achieved at the end of each turn. Therefore, wind-gradient effects will be relatively small.

(For simplicity, and to make a point, I have left the wind gradient effect out of the calculations as the results seem to work without it.)

**Downwind Drift**

The bird can maintain height using dynamic soaring but can only do so at the expense of drifting downwind due to the consistently large drift angles. This is probably why albatross circumnavigate the Antarctic continent with the prevailing wind and fly downwind around the high pressure patterns of the North Pacific

It is not possible to soar upwind using the Windward Turn Theory. Although the mechanism allows the bird to use wind energy to maintain average airspeed and height, the geometry of the windward and leeward turns means that there will always be a fairly large downwind drift angle and it only works when the wind-speed is a large proportion of the birds airspeed. On the other hand, the manoeuvre is inherently low-G and therefore requires minimum effort from the bird.

If the bird needs a wind speed equal to half its airspeed to maintain height while dynamic soaring, then its maximum angle of drift will be 30 degrees at the cross wind position and it never gets within about 60 degrees of an upwind heading. If the bird flies an air-distance of 290 m at 20m/s in 14 sec with a 10m/s wind, the distance lost downwind will be 140m. If the bird tries to recover that distance by flying upwind at a ground speed of 10m/s that distance would take 14 secs. Thats a lot of flapping!

It is not possible to do the dynamic soaring manoeuvre with 180 deg turns. The bird cannot extend the windward turn because the reducing wind-angle means that it cannot maintain airspeed and height. It cannot extend the leeward turn because again the reducing drift-angle means it loses the propulsive effect and it cannot gain any more ground momentum. The turns have to join up in terms of speed, height and wind-angle. Although it appears the bird could gain height slowly by trading airspeed for height, it would lose the advantage of ground effect and also the ability to judge whether it is climbing or descending. So, again the bird remains close to the surface.

Alternatively, if the bird can find some vertical component in the wind, for example on the upwind side of a swell or a ship, then it might be possible to make distance against the wind but this would not be dynamic soaring and of course a swell will normally be moving with the wind.

Upwind dynamic soaring is possible using a different technique and with greater effort on the part of the bird. This is discussed on the Upwind Dynamic Soaring page

**Frames of Reference**

Some people have objected to this hypothesis because velocity, momentum and kinetic energy are frame-specific. In other words these parameters can be measured relative to any Inertial Frame of Reference (IFR) and one IFR is as good as another. An IFR is a frame of reference which is moving with uniform velocity that is, not subject to acceleration or rotation.

However, while the ground can be treated as an *approximate* IFR for the purposes of these calculations, the fact is that the ground (the surface of planet Earth) is a rotating, curved surface and any object moving across the surface is subject to gravity and is also moving in a curve. This means that velocity is really angular velocity and momentum is really angular momentum.

Any change of velocity of the bird is caused by forces acting on the bird, modified by the wind. It is the bird which is accelerating and not the ground, therefore the ground can be regarded as being stationary. Energy in the wind is derived from pressure and temperature gradients in the atmosphere caused by solar heating. Energy in the wind is gained because it is the wind and not the ground, which gains the momentum. Therefore the ground is a perfectly valid frame of reference against which to measure the velocity, momentum and energy of the wind and the glider

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