Barnacle Geese have been observed to complete their 2500 km journey from Scotland to Svalbard in no more than 59 hours. V-formation flight is believed to be a technique used by these birds to save energy. In my master’s thesis I examined if this really is the case…
Bird’s formation flight has puzzled scientists for over a century. Most studys are in favor of a significant drag decrease for birds that fly in a V-formation.
The effect is clearly detectable in aircraft formations: the wing tip vortex of any finite length wing creates an upwards flow component just outside of the wing tip. Another aircraft flying into this region experiences a lift increase and therefore is able to reduce its angle of attack and thereby its drag.
While that might be true for fixed wing aircraft, the situation looks different for flapping wing birds. In my masters thesis I was able to show that it’s not all that easy for a bird to gain an advantage from it’s wingman’s aerodynamic footprint during flapping flight.
Flapping robots
To do this I build two flapping robots and visualised their wakes using a self-made 3-dimensional 2-component flow visualisation system in order to examine the size and path of the upwash region. In addition, I conducted lift force measurements on the trailing bird to track possible lift increases during formation flight.
The robots had a wing span of 30 cm and were completely hand-made using Delrin for the main frame and Depron for the wings. Electronics are based on an Atmel AtTiny2313 and are able to monitor and control the flapping frequency and phase relation between both robots as well as the wings geometric angle of attack.
The flapping mechanism is based on a technique devised by Italian mathematician Girolamo Cardano as described by Horst Räbiger. The wing’s change of geometric angle of attack is induced by a custom made coil actuator sitting on the wing spar. The whole mechanism was manufactured on a watchmakers lathe using brass and carbon fibre rods.
For lift force measurements on the trailing bird a simple force measurement device was built using strain gauges.
Results
The flow visualisation of the robots wake turned out quite as expected. The wing tip vortex did show up nicely behind the wing tip. This means, that it would undulate up and down along with the tip.
What does this mean for the aerodynamic advantage during V-formation flight? It means that the trailing bird has to synchronise its wing beat to the leading bird in order to position its wing inside of the upwash region all the time. As the upwash region is quite small, the synchronisation must be very exact to gain an advantage.
Is there an advantage?
The question is: is it possible for a bird to synchronise its wing beat to the one of another bird? Studies on morphological parameters of Geese have collected data that show significant variations in weight and wing span (i.e. wing inertia) throughout flocks. These have a high impact on the individual birds flapping frequencies — which in consequence must be varying throughout the flock as well.
So, to gain an energetic advantage from V-formation flight, the trailing birds energetic penalty induced by driving the wings at a frequency other than the one dictated by its morphology in order to synchronise to the leading bird must be smaller than the energy savings from aerodynamic advantage when synchronised to the leading bird. Is this the case? We don’t know yet…
However, studies on wing beat synchronisation were not able to show a relation between individual bird’s wing beats when flying in formation. It is therefore unlikely that a serious energetic advantage is achieved by flying in V-formation with flapping wings. If there is, it should be significantly lower than the 60–70% savings widely proposed in the media.
Why V-formation?
So why should birds use a formation if the energetic advantage is small? There are other reasons: V-formation is said to easy communication between birds by placing the leading bird on the spot of sharpest view of the trailing birds eyes. It might also reflect the flocks social relations or hierarchies. This makes sense as the older, higher ranking and more experienced individuals might also be the ones to guide the way.
Also, there still is an aerodynamic advantage in gliding flight where no wing beat synchronisation is needed. This can nicely be seen in Pelicans which show increased glide times when flying in formation.
After all, there is still much research to do. This study gives an insight into the unsteady aerodynamics that have to be considered for the formation flight of birds. It will be continued with new robots and better visualisation, so stay tuned…