Coverts inspire adaptive wing design

Earlier this summer, when we were walking the South West Coastal Path [see ‘The Salt Path‘ on August 14th, 2019], we frequently saw kestrels hovering above the path ahead of us.  It is an enthralling sight watching them use the air currents around the cliffs to soar, hang and dive for prey.  Their mastery of the air looks effortless.  What you cannot see from the ground is the complex motion of their wing feathers changing the shape and texture of their wing to optimise lift and drag.  The base of their flight feathers are covered by small flexible feathers called ‘coverts’ or ‘tectrix’, which in flight reduce drag by providing a smooth surface for airflow.  However, at low speed, such as when hovering or landing, the coverts lift up and the change the shape and texture of the wing to prevent aerodynamic stalling.  In other words, the coverts help the airflow to follow the contour of the wing, or to remain attached to the wing, and thus to generate lift.  Aircraft use wing flaps on their trailing edges to achieve the same effect, i.e. to generate sufficient lift at slow speeds, but birds use a more elegant and lighter solution: coverts.  Coverts are deployed passively to mitigate stalls in lower speed flight, as in the picture.  When I was in the US last month [see ‘When upgrading is downgrading‘ on August 21st, 2019], one of the research reports was by Professor Aimy Wissa of the Department of Mechanical Science & Engineering at the University of Illinois Urbana-Champaign, who is working on ‘Spatially distributed passively deployable structures for stall mitigation‘ in her Bio-inspired Adaptive Morphology laboratory.  She is exploring how flaps could be placed over the surface of aircraft wings to deploy in a similar way to a bird’s covert feathers and provide enhanced lift at low speeds.  This would be useful for drones and other unmanned air vehicles (UAVs) that need to manoeuvre in confined spaces, for instance in cityscapes.

I must admit that I had occasionally noticed the waves of fluttering small feathers across the back of a bird’s wing but, until I listened to Aimy’s presentation, I had not realised their purpose; perhaps that lack of insight is why I specialised in structural mechanics rather than fluid mechanics with the result that I was worrying about the fatigue life of the wing flaps during her talk.

 

The picture is from a video available at Kestrel Hovering and Hunting in Cornwall by Paul Dinning.

 

Taking a sketch instead of snapping a photo

We are lucky to live in a house with a great view of Liverpool cathedral [see picture in ‘Two for one‘ on January 2nd, 2019].  Hundreds of tourists visit every day and take pictures of the cathedral with their smart phones.  A few even turn around and take a picture of our house!  It is a modern disease: capturing pictures of a spectacle without actually looking at it and then probably never looking at the photograph.  There is some small level of fulfilment in having taken the photograph; however, 120 years ago there were fewer tourists and they had no cameras.  Instead, when Charles Rennie MackIntosh visited Naples on April 8th, 1891, he admired the tower of the Church of Santa Maria del Carmine and ‘took a sketch’.  It must have taken him some time and concentrated effort.  The level of pleasure and fulfilment from taking a sketch must have been much greater than from our modern experience of snapping a photo.

Of course, there was no Liverpool Cathedral in 1891 and ten years later, Rennie Mackintosh was disappointed that his proposals for it were not selected from the 103 submitted.

Photograph taken on 17th August 2019 by the author at the Rennie Mackintosh Exhibition at the Walker Art Gallery, Liverpool.

Spokesperson for society

There is an excellent exhibition of Keith Haring’s work at the Tate Liverpool until November 2019.  Keith Haring and I were born a couple of years apart but that’s where the similarity ends.  He was an American artist who collaborated with the likes of Andy Warhol and Jean-Michel Basquiat and was influenced by Pablo Picasso, Walt Disney and Dr Seuss.  He was part of the New York street culture of the 1980s and many of his early works were forms of graffiti painted in subways and on the sides of buildings.  Some people think that art should challenge the way you think about the things; however, “Haring felt that the artist is ‘spokesman for a society at any given point in history’ whose visual vocabulary is determined by their perception of the world”.  His work about racism, the excesses of capitalism and the misuse of religion for oppressive purposes seem as relevant today as thirty years ago.

Sources:

Quotation from the one of exhibition displays with apologies to curators of the Tate exhibition, Darren Pih and Tamar Hemmes.

Comment on art challenging the way we think based on an article by Orla Ryan in the Financial Times Magazine on June 29th & 30th, 2019.

When an upgrade is downgrading

I had slightly surreal time last week.  I visited the USA to attend a review of a research programme sponsored by the US Government and reported on two of our research projects.  When I arrived in the USA on Monday evening, I went to collect my rental car and was told that I had been upgraded to a pick-up truck because the rental company did not have left any of the compact cars that had been booked for me.  I gingerly manoeuvred the massive vehicle, a Toyota Tacoma, out of the parking garage and on to the freeway.  I should admit to having owned a large SUV when we lived in the USA and so driving along the freeway was not a totally new experience, except that the white bonnet in front of me seemed huge.

The following morning, I drove to the location of the review and strategically selected a parking space with empty spaces all around it so that I could drive through into the space and avoid needing to reverse the behemoth.  As I was walking across the parking lot, someone accosted me and said: ‘Nice truck, how do you like it?’  Embarrassed at driving such an environmental-unfriendly vehicle, I responded that it was a rental car that I just picked up.  To which he replied that the best protection against my Tacoma, was his Tacoma. And, that it was his dream car.  Then, I noticed that he had parked his black one alongside mine.

Our children learnt to drive in our ancient Ford Explorer and loved it.  We all knew that it was wrong to drive something that consumed fuel so voraciously even if it did get us effortlessly through the most horrendous winter storms.  However, we have left all that behind and now either use public transport or drive cars that achieve 60 mpg or more on good days. But here I was being admitted to a club that worshipped their pick-up trucks.

We walked together into the review which was held in a small lecture theatre equipped with comfortable armchairs, which was just as well because we sat there from 8.30 to 4.30 for two days listening to half-hour presentations with only short breaks.  We were presented with some stunning research based on brilliant innovative thinking, such as materials that can undergo 90% deformation and fully recover their shape and how the rippling motion of covert feathers on a bird’s wings could help us design more efficient aeroplanes.  More on that in later posts.  Of course, there were some less good presentations that had many us reaching for our mobile phones to catch up on the endless flow of email [see: ‘Compelling Presentations‘ on March 21st, 2018).  At the end of each day, we dispersed to different hotels scattered across town in our rental cars.  On Thursday, I drove back to the airport and topped up the fuel tank before returning my truck.  I worked out that it had achieved only 19 mpg (US) or 23 mpg (UK), despite my gentle driving – that’s almost three times the consumption of my own car!  On the plane home I started reading ‘Overstory‘ by Richard Powers, a novel about our relationship to trees and the damage we are doing to the environment on which trees, and us, are dependent.