Dynamic Soaring Upwind

Can an albatross use dynamic soaring to travel against the wind?

 The Windward Turn Theory does not allow upwind dynamic soaring because there is always a large downwind drift angle. However, there is anecdotal and data-logged evidence of upwind soaring but with a suggestion of higher metabolic rates on upwind legs compared with downwind or crosswind-soaring. This suggests that a different technique is being used to achieve upwind soaring and that if the bird is not actually flapping then one reason for this greater effort may be higher-G manoeuvring.

 Gliding upwind at constant airspeed must involve a loss of height. Therefore, in order to do this, the bird must first gain airspeed greater than the wind and/or excess height which it can sacrifice. Before the upwind glide commences there must be a force, other than gravity, in the direction of flight to provide the acceleration of airspeed.   This can occur if the albatross catches a horizontal gust or penetrates a shear boundary whilst in a steep angle of bank, during the leeward wing-over turn.  This could happen in the lee of a breaking wave or in the wind shadow between two swells. See figure 18 & 19.

Soaring upwind1

 How does a gust accelerate a gliders airspeed? In normal straight 1G flight at the best lift/drag ratio angle of attack (3-4 degrees), the total aerodynamic force resolves into lift and drag respectively normal to and opposite to the direction of flight. If a vertical gust causes the angle of attack to increase close to the stalling angle of attack (about 15 degrees), the aerodynamic force is increased and tilted forward of the aircraft vertical and then resolves into lift and thrust. The aircraft accelerates forward and upward (the surge experienced by glider pilots flying into rising air). This motion reduces the angle of attack and the aircraft returns to a state of equilibrium so that the effect can only be brief.

 Consider the albatross making a regular leeward turn as a steeply banked wing-over but close to the lee side of a steep swell or a breaking wave (figure 18). In the lee of the wave the wind may break away leaving a wind-shadow of relatively still air in the trough and a marked horizontal shear boundary between the strong wind above and the still air below.  As the bird penetrates the shear boundary at a steep angle of bank, it encounters a sudden increase in wind-speed and a sudden increase in airspeed and angle of attack. The lift and drag forces increase and tilt forward (Figure 19). This will cause a component of aerodynamic force T to act momentarily as a pulse of thrust and the other component C to act as the centripetal force, maintaining the turn. Because of the increased angle of attack, this will be inherently high-G and therefore will require more effort on the part of the bird. Having gained an increment of airspeed, the bird then reverts to normal angles of attack. The bird can use the excess airspeed in any of three ways:

1 - It can gain height, or

2 - It can continue the turn and drop below the shear boundary in the downwind trough and gain distance downwind or crosswind, or

3 - It can reverse the direction of turn and drop below the shear boundary into the upwind trough and gain distance upwind, gliding as far as the next wave crest. If the swell is deep enough to create an air-flow separation in the troughs between wave crests, it may be possible to find still air, although the swells may be moving downwind.

 The increased load-factor due to the gust and the reversal of turn for upwind progress will take more effort than rolling out of the turn into the downwind trough, hence the greater effort and metabolic rate during upwind dynamic soaring.

 This gain of airspeed when penetrating a shear boundary is not the same as the Raylegh cycle of wind-gradient dynamic soaring. However, it is similar to the lee-soaring model of dynamic soaring practised by RC model glider pilots. The pulse of propulsive force is similar to the process of auto-rotation which drives windmills and spins helicopter rotors in power-off glides.

Soaring upwind 3

 Figure 19 is a plan view showing a bird in a steeply banked turn to the right, the wind coming from the left. It shows how the aerodynamic forces change as the bird penetrates a shear boundary in a steeply banked turn - a wing-over. The left-hand diagram shows the bird in a steeply banked right turn in the lee of a swell and therefore in a light wind. Lift L and Drag D combine to give a Resultant. In the right-hand diagram, there is a sudden increase in wind Vw from the left as the bird penetrates a shear boundary. The sudden increase in angle of attack and load factor (G) changes the lift L and drag D components briefly into thrust T and centripetal force C ,which maintains the turn. The bird thus gains a pulse of acceleration of ground-speed (and air-speed) and then drops back down below the shear boundary into light winds and resumes a normal angle of attack.

 This model depends upon there being a gust or a wind shear which will most likely occur in the lee of a breaking wave but cannot be guaranteed especially in light winds or in the absence of steep waves. Therefore paradoxically it appears that upwind dynamic soaring is only possible in strong winds.

Optical illusions

 When it appears that albatrosses are dynamic soaring upwind in light winds, there is an optical effect which can mislead the observer. There is an explanation for this optical illusion and it is down to those pesky triangles of velocity again!

appears to soar upwind

 See Figure 20. Here is a diagram showing how the bird appears to fly upwind. The ship velocity is CB, the wind velocity is AB and the wind velocity relative to the ship is AC . A glance at the flag at position C suggests the ship is steaming approximately into the wind. The bird is seen dynamic soaring along path DH relative to the ship (average direction DG) and is actually keeping up with the ship. The observer only sees the bird following path DH relative to the ship. He cannot see the motion described by triangle DEF. The observer on the ship concludes that the bird is making distance upwind.

 In reality the bird is flying the dynamic soaring pattern along a mean air-velocity DE (that is the path relative to the air). Applying the wind velocity EF (same as AB) to that air-velocity DE, the birds track-made-good (path relative to the ‘ground’ ) is DF. The track-made-good resolves into components DG, lateral to, and GF parallel to the motion of the ship. Velocity component GF means the bird keeps up with the ship, (same as velocity CB). Angle d is the birds actual drift angle while angle tmg is the track-made-good relative to the wind. Angle tmg is greater than 90 degrees and therefore the bird is actually losing distance downwind.

 

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