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Source: The Conversation – UK – By David Hone, Senior Lecturer in Zoology, Queen Mary University of London

Pterosaurs were an amazing group of flying reptiles that occupied the skies around the same time that dinosaurs roamed on land. Appearing in the fossil record around 230 million years ago, pterosaurs survived until 66 million years ago, when an asteroid impact helped wipe them, and many other life forms, out.

The pterosaurs are often the animals in the background, while the dinosaurs occupy the foreground. However, they are worthy of much more recognition than they are commonly given, not just as interesting ancient animals, but because they could also inspire aircraft designs.

Pterosaurs were the first vertebrates to evolve powered flight. They were in the air 80 million years before birds and around 180 million years before bats. However, their flight apparatus was rather different to either. The wings of bats are supported by multiple digits (like our fingers). Birds use feathers as structural units in the wings.

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But pterosaurs primarily had one finger to support their wings. Their main wing was composed of a single giant “spar” – a structural unit – made of up of the bones of the arm and the greatly elongated fourth finger, with a membrane that stretched from the tip of the finger down to the ankle. This membrane acted as a flight surface.

As a group, pterosaurs were diverse – some were specialist fishers, filter feeders, terrestrial predators, insect hunters, seed crackers, and more. Some could climb well and many species were highly mobile on the ground.

They also got very large. The biggest pterosaurs had wingspans of over 10m and could weigh over 250kg. Even the smallest pterosaurs could fly: juveniles with 10cm wingspans were probably capable of flight within days or even hours of hatching.

The bones of pterosaurs, like those of birds and many dinosaurs, were filled by extensions of the lungs called air-sacs, and they were extremely thin walled. This made the skeletons of the animals very stiff for their weight (rather important when flying). It also made their skeletons very fragile after death, and so pterosaur fossils are rare.

However, in a handful of sites around the world – most notably in Germany, Brazil and China – where the preservation of fossils is exceptionally good, we have huge numbers of pterosaur fossils with both complete skeletons and a lot of soft tissue. This gives us an incredible insight into the shape and structure of their wings and how they flew.

In addition to the main wing surface, pterosaurs had two other smaller subsidiary surfaces that would have given them extra control. At the front of the main wing sitting in the crux of the elbow was a small membrane between the wrist and the base of the neck, supported by a unique long wrist bone called the pteroid.

At the back of the body, earlier pterosaurs had a single large sheet of membrane between the legs, supported in the middle by a long tail and on each side by long fifth toes on the feet. Later pterosaurs split this rear membrane and had only a small piece of membrane running from the ankle on each leg to the base of a short tail.

As well as the outer skin-like layers, the wings had at least three major layers, comprising blood vessels, a layer of muscles, and a layer of stiffening fibres. Some might well have had extensions of the airsacs in the main wing membranes too, which could presumably be inflated and deflated to a degree. The wing as a whole was therefore extremely elastic and flexible.

This would have given pterosaurs extraordinary control over their wings. All of this makes them an intriguing model for future aircraft design.

Flight challenge

Aircraft wings are not (and cannot) be perfectly stiff. Adding flexibility, or better still, actual shape changing potential, could give them substantial performance benefits. But stiffness and flexibility need to be balanced. Problems with aeroelasticity – the tendency of a soft wing to vibrate in ways that greatly reduce performance (or even cause flight to fail outright) – limit how pliable the wings can be.

Pterosaurs had multiple mechanisms to address this challenge, from passive mechanisms, such as fibres within the wing, to active mechanisms, such as the muscles that ran throughout the wing and could tighten on demand. This wing tensioning anatomy is*is?* among the most sophisticated aeroelastic control systems known to science.

The key to applying our knowledge of pterosaurs to future aircraft design comes not in closely mimicking the exact shape and form of pterosaurs, but instead, in understanding and extracting core principles from their anatomy.

The membranous wings of pterosaurs were great at changing shape. The leading
edge could lie flat or depress to a sharp angle, thanks to the small anterior membrane. The main wing surface could change its curvature, or camber. There is even evidence that the wing could manage what is called reflex camber – a shape in which the trailing edge of the wing curves upwards.

Even the stiff portion of the wing (the spar) made of bone and surrounding muscles, was mobile – through motions of the shoulder, elbow, and wrist and flexibility within the bone itself near the wingtip. This soft, shape changing structure gave pterosaurs exceptional control over their moment-to-moment wing performance, optimising for lower speed or higher speed within fractions of a wingbeat. This would have made them particularly adept at slow speed flight – good for tight turns and precise, soft landings.

Greater manoeuvrability and pinpoint landings are a premium for autonomous vehicles working in busy environments – such as cities or natural disaster zones full of debris. So future survey and rescue drones could take lessons from pterosaur wing control systems.

The jointed, flexible wing anatomy of pterosaurs also meant that the wings could fold tightly, and unlike the wings of birds, the folded wings of pterosaurs doubled as powerful walking limbs. Because the hands contacted the ground while walking, the forelimbs were available to help push the animals into the air during take-off leaps. Mathematical models predict half-second launch times, from a standing start, in even the largest pterosaurs.

The exceptional mechanical loads associated with these launches were handled
by one of the highest stiffness-to-weight skeletons to ever evolve. This folded-wing, rapid-launch system has great potential for applications to future technologies.

So much so, in fact, that a prototype folding wing system modelled on pterosaurs has already undergone some testing (through a Nasa-funded university project on which one of the authors, Michael Habib, consulted). A folding, flapping wing that doubles as a launch system could allow future drones to take off with limited space – perhaps while on ships at sea. It could also be used to allow small flying drones to land and launch again out of craters on Mars.

The red planet has just enough atmosphere to make flapping wing and rotor wing systems work. But it’s energetically costly and hovering is tough – better to land, measure and launch again. Similarly, rapid take offs from uneven terrain, precise landings, tight turns, and on demand tweaks to improve performance are all features that could be applied to the drones of the future, in wingsuits, and more.

As the control systems for drones become increasingly driven by intelligent software, we will need a new generation of hardware to match. Pterosaurs may hold the keys to unlocking a future of highly manoeuvrable autonomous aerial vehicles that are competent in harsh conditions and urban environments. These would be ideal for search and rescue or surveys in locations that are too dangerous for humans.

So despite having been extinct for 66 million years, the pterosaurs have huge potential as the inspiration for aircraft design. Sometimes looking back can be the best way to look forward.

Michael Habib has worked on a prototype folding wing system based on pterosaur flight through a Nasa-funded university project.

David Hone and Liz Martin do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

– ref. How pterosaurs can inspire aircraft design – https://theconversation.com/how-pterosaurs-can-inspire-aircraft-design-256823