Flight of the dragonflies
The Amateur Entomologists' Societ
Maria Mingallon 2013
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Dragonflies have one of the most astounding flight capabilities in the animal kingdom. With four known distinct flight styles they can fly not just forwards but they can reverse, fly laterally, upside down and hover for extended periods and then there is their amazing agility (Trueman & Rowe, 2009) (Silsby, 2001). They also have incredible range (20km while carrying a mate) and achieve velocities of up to 70km/hr (Silsby, 2001)).
The four distinct flight styles: 1. Counter stroking - is the normal stroke, due to its exceptional power and efficiency, it's used for hovering and slow flight. The actual stroke is where the fore and hind wings move up and down out of phase. When one pair of wings is at the peak of the upstroke the other pair is at the lowest of the downstroke (Wang & Russell, 2007) (Trueman & Rowe, 2009) (Silsby, 2001). 2. Phased stroking - for general flight this stroke is most efficient as it generates more thrust and lowers lift generation. The stroke is lead by the hind wings with the fore wings slightly delayed (by a 1/4 stroke) (Trueman & Rowe, 2009) (Silsby, 2001). 3. Synchronised stroking - used when maximum thrust for agility is needed, usually when chasing prey. As implied by the strokes name this is where the fore and hind wings flap in unison (Trueman & Rowe, 2009) (Silsby, 2001). 4. Gliding - there are three types of gliding:
These types of flight are powered by muscles attached directly to the wing bases. And the wings of dragonflies are broad with the hind wings being slightly more so than the fore wings. As these insects have evolved for high speed flight their centralised and slightly forward mass along their body results in high inertia during flight (also helps with gliding) and with their huge flight muscles (1:2.5 in relation to body weight) are able to beat their long wings at 30 to 50Hz at angles between 70 and 90 degrees for normal flight resulting in 4m/s velocities. When higher velocities are required changing the flight angle and frequency increases the power can result in 7m/s velocities (Trueman & Rowe, 2009) (Silsby, 2001) (Jakeling & Ellington, 1997).
Also as dragonflies can control each wing they can fly with different wings doing different things, even adjusting the flapping style to generate the precise thrust and/or lift for its purpose (Trueman & Rowe, 2009). Asymmetric wing strokes allow wings on one side to propel forwards while the other side propels in the opposite direction causing the insect to spin on its axis in an instant (Trueman & Rowe, 2009). This incredible ability to control their flight path is not as incredible as their ability to coordinate each wings power, shape and angle for their intended purpose such as capturing prey while flying, especially if the prey is another flying insect such as flies and mosquitos (Trueman & Rowe, 2009) (Silsby, 2001) (Wang 2005). The Physics
By flapping the wings, supinating and pronating the ends of the wings using the pterostigma through the air thrust is generated. Dragonfly wings are not simple planar structures, they are highly dynamic. An aerofoil of air is created around the physical wing through the corrugations in the wing. This reduces friction between the surface of the wing and the air itself. And as the wing can flex around several axes due to both muscle action and inertial effects, the pterostigma near the tip of the wing is a weight that causes the wing tip area to flex during a wing stroke, improving aerodynamic efficiency (Pines & Bohorquez, 2006) (Prosser, 2011) (Wang, 2005 & 2008) (Jakeling & Ellington, 1997). The mechanisms behind thrust generation in dragonflies are complex, at least four distinct physical processes are known to be used by these insects: 1. Classical lift - is just like lift on fixed wing aircraft wings 2. Supercritical lift - When the attack angle of the wing increases lift generation increases, however as the attack angle increases, the flow of air above the wing separates from the surface, so if the angle is too high the air will not flow over the leading edge of the wing which results in a stall. That angle between the highest lift generation possible and stalling is the source of supercritical lift. 3. & 4. Vortices & Vortex shedding - Are currently being studied by studying the dragonfly wing replicas in fluids to understand how vortices affect aerodynamics in dragonflies. The current theory based on Hawkmoths is that as the vortex is created on the leading edge of the wing during downstrokes creates a suction force on the top of the wing which helps maintaining lift (Wang 2005) (Ellington et al. 1996) (Trueman & Rowe, 2009). |