Guest Post: The Jack Sprat Effect

Today it’s the turn of Colin Palmer, an aero engineer turned pterosaur researcher in Bristol who takes us through his new paper on their flight mechanics. And inevitably has some issues with how his work was portrayed in the media. Colin of course comes into this problem from the opposite end as do most pterosaur workers, but as he explains, that’s no bad thing…

As you may have seen in the media coverage of my recent Proceedings B paper, I am an engineer of a certain age who has come late to the study of pterosaur flight. The media like stories, stories with a human angle, something unusual about someone who does not conform to their ideas of the normal patterns of life, which may be why this story was picked up to widely. Yes I am an engineer and many years ago, in a different life, I ran experiments in wind tunnels and water tanks to investigate the performance of sailing vessels. Through this I came to understand about the dynamics of flexible wings and how to go about testing them, knowledge that is helping me in my pterosaur flight research.

If the media are to be believed, I came up with a simple idea in my paper that had eluded more eminent palaeontologists. Well not really. What I did was to do what I know best, which is to take a practical experimental approach to analysing a problem. I was simply doing what is more or less second nature to an engineer though maybe challenging for a palaeontologist. In the same way, systematics is second nature to palaeontologists but a struggle for me. In engineering terms, there was nothing unusual about what I did – it was the standard approach that has been used in aerodynamics since the days of the Wright brothers. But this the great strength of cross disciplinary work, the Jack Spratt effect. What is easy for me may be difficult for others and vice versa, but if we work together we can bring our combined strengths to solving problems. That’s why I was rather saddened by the media’s idea that somehow I’ve got one over on palaeontologists. Wrong. All I’d done was to take the approach that I know best, relying on information produced by palaeontologists who have a far far deeper understanding of the morphology of these animals than I ever will.

So what was it that seemed to be so simple and logical to do? I was trying to understand the aerodynamic performance of the pterosaur wing using aerodynamic analysis and one way to do this is to start with wind tunnel tests of 2-dimensional wing sections. These are constant section ‘extruded’ models that extend right across the wind tunnel from wall-to-wall so as to avoid the end effects and the effects of different planform shapes. Armed with the data from these tests there are a well-established methods for using it to produce the estimated performance of complete three-dimensional wings. None of this is unconventional or revolutionary and has formed the basis of aircraft design for many years. People also test models of complete aircraft in wind tunnels and make use of sophisticated computer modelling techniques such as computational fluid dynamics, but these methods are very expensive and only really worthwhile once you have a very deep and detailed understanding of a specific geometry. We don’t have that for pterosaurs. What we do know in considerable detail is the bone morphology, particularly for the larger specimens that I’m interested in, because there is very good 3-D preservation from locations such as Brazil and the Cambridge Greensand. This means that we can reliably reconstruct the wing bones of a 5 to 6 m wingspan pterosaur, but sadly we know much less about the actual shape and extent of the wing membrane. Recent work suggests that it was attached proximally at the ankle and presumably also extended to the extreme distal tip of the wing finger, but we can only make a range of bounded assumptions about the shape between these points.

What I did was to take four locations along the wing and to produce a range of possible cross sections at those locations. I was not trying to reproduce one particular specimen or even species in detail, but wanted to get information of a more generic nature that could be applied to a range of possible reconstructions and remain useful in the future as our knowledge of the wing bones and wing morphology develops. This again is a classic engineering approach – to use a parametric methodolgy to provide a range of data that can be interpolated for specific solutions.

The figure shows a selection of the different sections which I tested. One of the advantages of two-dimensional testing, particularly in a small tunnel such as the one I used at UCD in Dublin, is that you can test a lot of different models and configurations very quickly, rapidly building up a wide database of information. It may not be the most accurate data in the world, but at this stage of understanding the accuracy is more than good enough to discern the relative differences between the configurations and comparison with published data for similar shapes such as yacht sails confirmed the absolute magnitude of the results.

I represented the wing membrane in two different ways – with rigid, thin plate sections made from a carbon fibre composite and with flexible, reinforced latex rubber material. You may wonder how a rigid shape can be representative of the flexible wing of a pterosaur. Well, in an overall sense it is not, but it is representative of one particular configuration of that wing, so by fixing the shape it is possible to minimise the number of variables. I made my tests with curved plates having different amounts of curvature (camber in aerodynamic parlance). Then depending on what camber is thought appropriate, the results can be interpolated to get the results for any specific reconstruction geometry.

In order to understand the specific effects of flexibility I also tested some sections made with flexible membrane. They covered the same range of camber as the rigid structures, and so allowed me to understand where the flexibility itself, rather than the simply the geometry had particular effects.

The results showed a number of interesting features. In the proximal region of the wing the propatagium is anterior to the wing bone and so the wing membrane is split into two sections by the wing bones. It had previously been hypothesised that this propatagium had the effect of enhancing the wing’s ability to generate lift. This may well be true to the extent that the camber can be varied, but what I found was that it also reduces drag in a way that had not previously been identified. Because the proximal wing bones are necessarily the largest in diameter in order to take the loads from the wing, they are the ones that will interfere most with the airflow, and a propatagium in this region reduces the otherwise high drag they would create. Thin membrane wings in which the membrane forms a sharp leading edge are very sensitive to the angle of attack – if they’re not correctly orientated to the incident flow, separation can create damaging aerodynamic characteristics. To avoid this is necessary to vary the angle of the wing in the leading edge region and perhaps this is what the pteroid was for – it enabled the animal to manipulate the membrane in that region to optimise the angle to the incident flow.

Moving more distally, I tested sections representative of the first and second wing phalanxes, which in some specimens have quite different cross sections. The first wing phalanx tends to have an oval cross section whereas the second phalanx is more triangular. No one has explained why these differences exist but one possibility is that they are for aerodynamic reasons, which is something I wanted to investigate. The results showed that there was no difference between these two cross-section shapes in terms of the aerodynamic performance, so presumably the differences are for other reasons, perhaps structural.

The reason for this is that the flow separates from the ventral side of the wing bones before it reaches the region where the shape is different, so in a sense the flow doesn’t know anything about the different wing shapes and this is why they don’t have any material effect on the aerodynamic performance.

The other thing that people had hypothesised was that a small fairing wedge posterior to the wing bone, perhaps comprising pneumatised tissue, might also improve the aerodynamic performance. I tested that, but like the different wing bone sections, it made no difference whatsoever and for the same reason – flow separation. In order to understand the degree of faring necessary to change the aerodynamic performance significantly I ran a computer analysis (using a two-dimensional analysis program called XFOIL) of different sections to come up with one that had the minimum amount of faring needed to remove the separation. A model of this shape confirmed that the drag was indeed much reduced. The minimum drag was about half that of the section which simply had a wing bone attached to the membrane. Whether or not this was a practical possibility is open an open question. The potential extent of pneumaticity in the wing is very difficult to determine since it is rarely, and poorly preserved in the fossil record, but now at least we got some idea of how much pneumaticity would be required in order to achieve a significant improvement in aerodynamic performance.

Once I had the results of the two-dimensional tests it was possible to combine them using a aerodynamic theory to calculate the performance of a full three-dimensional wing. There are a number of ways it can be done and I used a vortex lattice model, based on theory that goes back to the 1930s but which has only become practical relatively recently with the availability of computers. This methodology has its limitations but they are not particularly significant for analysing the soaring flight of the ornithocheirid pterosaurs that I’m interested in.

By putting my data into this model and using it to produce a polar flight performance curve, a curve defining the gliding performance in terms of the horizontal flight speed and the vertical sink (descent) speed I was able to see how the characteristics I was predicting compared to those of previous estimates. What I found was that my results indicated a much less efficient flight performance than previous workers such as Bramwell and Whitfield. The reason for this was that they had not done tests on sections but had used the best available extant aerodynamic data, which was derived from tests on a curved plate airfoil used many years ago on aircraft. This was a sensible enough approach in the absence of specific data, but it gave very optimistic results compared to sections where there is a wing bone on the leading edge affecting the airflow over the whole section. I found that the aerodynamic efficiency of the wing (the lift:drag ratio) with the the sections I tested was about 10, half that predicted by Bramwell and Whitfield. In fact this typical of many birds and the predicted sink rate rate of a little less than 1 m/s is again comparable with soaring birds.

The polar curve also allowed me to predict the optimum flight speed of the animals and this came out to be not dissimilar to the estimates of previous workers – around 10 m/s at cruise speed, though my results showed inferior high speed performance. The big difference however was in the low speed flight capability. I found that the flexible sections were able to distort to a very large camber and this enabled the wing to generate high lift, which extended the low-speed flight envelope beyond what had previously been estimated. This is potentially important because it would allow the animals to make controlled, very low speed landing approaches, so minimising the potential for impact damage on wing bones. These bones were stiff and efficient structures for resisting the overall bending loads experienced in flight, but like an eggshell their thin walls (sometimes less than 1 mm) could be easily fractured by local impact.

In summary, my key conclusions were that the anterior wing bone had an adverse effect on the aerodynamics of the wing sections but that the ability of pterosaurs to vary the camber of their wings, something the birds can’t do and bats can only do to a limited extent, was important in enabling them to fly very slowly on landing approach and thus minimise the possibility of damaging they’re necessarily thin-walled wing bones. I also think that the relatively low aerodynamic efficiency, especially as flight speed increases, and the flexibility of the membrane makes it extremely unlikely that pterosaurs were able to exploit the dynamic soaring flight style used by seabirds such as Albatrosses. They were much better suited to slow speed flight in thermal and or slope lift.

So what next? Well now I think I have a pretty good idea about the basic aerodynamics of the wing sections so I’m turning my attention to the structural aspects of the wing. I want to understand more about the the strength and stiffness of the wing bones and how they deflect under the various loads that would be experienced during flight – both the load due to the lift and also that due to the tension in the membrane. With this information it will be possible to go back into the vortex lattice model and create a more refined three-dimensional wing shape and rework the overall flight prediction programme. And maybe one day I can use all this information to create a free flying model.……

Palmer, C. Flight in slow motion: aerodynamics of the pterosaur wing. Proc. R. Soc. B published online before print November 24, 2010, doi:10.1098/rspb.2010.2179

12 Responses to “Guest Post: The Jack Sprat Effect”


  1. 1 Mike Habib 13/12/2010 at 7:18 pm

    Very pleased to see Colin Palmer pitching in on Archosaur Musings. I do think that Colin might be selling himself a bit short, though. While it is true that he has not “got one over on palaeontologists” in a general sense, my opinion is that his work is much better than most of the pterosaur flight work that has come previously – not just in the sense of having more tools and a new outlook, but simply better work flat out. Bramwell and Whitfield had some constraints, yes, but they also made some really whacky assumptions. So did Chatterjee and Templin, etc. So great stuff, all around.

    I do wonder (and will probably chat with Colin directly on this at some point) whether the local impact problem was likely to have been a driver of slow flight in pterosaurs. It seems reasonable, but there are plenty of pterosaurs without thin-walled bones that would have had a similar speed regime, given Colin’s results, and modern birds with similar bone walls do not land especially slowly or avoid impacts (some even plunge-dive). Colin and I have talked about this a bit already, but generally speaking, I wonder if perhaps there are other issues of interest here such as foraging strategy and prevalent soaring conditions that might have been more important than protected landings. Cheers everyone!

    • 2 David Hone 13/12/2010 at 7:40 pm

      Well I do know where he is coming from wrt the media. I saw several places announce things along the line of ‘he was the first person to have even thought of using a wind tunnel with pterosaurs’ which is stupid. I thought of it years ago, but had not the money, time, serious interest, or access to the expertise or wind tunnel to do the work. I can’t imagine I have been the only pterosaur worker in the last 70 odd years to have had the same thought, and of course there is already an Elgin et al paper featuring both myself and Colin who were working on pterosaurs in wind tunnels (if not on wings, though Ross is now doing this in part for his PhD). Certainly this was painted by some as ‘why didn’t they think of this before?’.

      Ah the joys of the media….

      Great work though!

      • 3 Mike Habib 13/12/2010 at 9:27 pm

        I see your point – the media release did play up the drama, as usual, and made some pretty silly statements in the process (as usual). Still, I am happy that the paper received as much attention as it did. I was interviewed for responses to Colin’s paper for several sources, and it was generally apparent that they struggled a bit with the significance of the work. When that happens, popular writers do, I think, often default to the “human element”.

      • 4 David Hone 13/12/2010 at 9:43 pm

        Yeah, because popular writer does not equal science writer. In fact, as noted, irritatingly often, science writer does not equal science writer….

  2. 5 Mark Robinson 14/12/2010 at 2:26 am

    Great post! Thanks Colin (and Dave for hosting). This is a good example of the benefits that can come from scientists from one discipline reaching out and embracing experts from another (or at least allowing them to come and play in the same sandpit).

    To someone whose training and work solely involves identifying, analysing, and comparing old bones, a brachiosaur standing in 10m of water might seem like a good idea. A physicist could tell you the pressure at that depth but wouldn’t necessarily see any problem with it either. But get a dialogue going between the two, or involve a marine mammologist, and the idea starts to look less likely.

    You mention that modern birds are unable to vary the camber of their wings. Doesn’t the alula (for those birds that possess it) perform essentially the same function of providing greater control at lower air-speeds?

  3. 6 Colin Palmer 15/12/2010 at 3:44 pm

    Mark,

    It seems that there is some controversy (yet more!) about the role of the alula. The most common view is that it acts like a leading edge slat on an aircraft wing, so helping to maintain attached flow at high angles of attack (delaying stall.) The less common, but in my view more likely explanation is that it acts as a flow destabiliser and helps to establish a different (vortex) flow over the distal wing when a bird slows down to land. This also delays stall and allows slower, controlled flight.

    So yes, the alula helps to provide low speed control by delaying stall, but is arguably less effective than being able to vary the camber.

    Mike may have other views…..

    Colin

    • 7 Mike Habib 16/12/2010 at 3:29 pm

      My thoughts on the alula are exactly the same as Colin’s. I have spoken to Colin Pennycuick, Jeremy Rayner, and Jim Cunningham about it, and they all seem to agree, as well. That is awfully good company for Colin and I, so I suspect the vortex generation model is going to shift to the standard. For those that actually work on quantitative flight mechanics in animals (as in all eight of us or so…) I think it may already be the standard, despite all the old slat-based models in the literature.

  4. 8 Colin Palmer 15/12/2010 at 4:07 pm

    Thanks Mike and Dave for your positive comments.

    Re the wing bone thickness and impact damage, I am very open to wider interpretations – after all so many things we see in nature are not single purpose adaptations – indeed I suspect very few are that narrow.

    As I understand things (which in this case is sketchy) the pterosaurs with thicker wing bones tend to be the smaller species, no?

    Since wing bone bending moment increases approx as L^4 but section modulus (Z) which determines the surface stress (which is a constant) varies as L^3, larger animals will require deeper wing bone sections (or thicker walls). The deeper the wing bone (relative to section chord) the higher the drag and the relatively slower the optimum speed. So maybe getting bigger required deeper bones, which resulted in slower flight, which made thin walls less risky?

    Re the plunge diving, I think that is a distributed stress so while no doubt onerous it is not an impact in the same sense as hitting a rock or other sharp bit of terra firma. From film I have seen of fast flying (high wing loading) birds such as albatrosses and big swans landing it looks like a traumatic event. Do we know if that suffer damage sometimes?

    Colin

    • 9 Mike Habib 16/12/2010 at 3:36 pm

      Those all seem like reasonable thoughts. The only caveat is that while the very largest pterosaurs all have thick-walled bones, there are a fair number of mid-sized species with thick-walled bones. Dsungaripterids, for example, were built like tanks by pterosaur standards, and some of them were relatively large.

      Plunge-diving aside (and I think the distributed stress argument is a good one on that front), animals such as pelicans do not seem to land any differently from other birds. The outliers are, as you suggest, the landings of highly loaded seabirds. The trick there is that the hind limbs of semi-aquatic birds are shifted caudally on the body, relative to other avian taxa, and so they naturally pitch forward heavily on landing (unfavorable moment arm). In loons, for example, the legs are so far back that they basically hit the water with the chest first on landing.

      The other question I have about the impact constraint for pterosaurs is if we are overestimating their sensitivity to local impacts. That should be one thing that the trabecular braces can greatly reinforce against, for example (by comparison, their effect on bending is modest, despite some anecdotal claims to the contrary), and the transmission of energy to a local area of bone is noticeably affected by surrounding soft tissue (again, unlike bending where it takes a really hefty tendon or ligament to make a difference).

      Still, I’m happy that Colin brought up the issue of impact loads in his paper and press release because it’s something I think a lot of workers ignore.

  5. 10 Mark Robinson 16/12/2010 at 5:25 am

    Thanks for the response, Colin. Interesting what you say about vortex flow. I presume that this would increase drag whilst maintaining some semblance of stability since the airflow hasn’t become detached from the wing?

    It’s nice to get a peek at the leading edge (hur hur) of research into animal flight dynamics. With regard to how the alula works, I say it’s time to throw some ducks into a wind tunnel!

  6. 11 Colin Palmer 16/12/2010 at 12:35 pm

    Mark,

    Yes, big increase in drag – which is good ‘cos it slows you down, and also lift. Birds appear to flex their outer wings when landing, which gives a swept, ‘delta wing’ shape to the distal regions, so the whole flow regime probably changes, helped along by the alula.

    Good images of this in: Aerodynamics of aerofoil sections measured
    on a free-flying bird
    A C Carruthers∗ , S M Walker, A L R Thomas, and G K Taylor Proc. IMechE Vol. 224 Part G: J. Aerospace Engineering 2009

    and
    Use and Function of a Leading Edge Flap on the Wings of
    Eagles
    Anna C. Carruthers*, Graham K. Taylor†, Simon M. Walker‡,and Adrian L.R. Thomas§ 45th AIAA Aerospace Sciences Meeting and Exhibit
    8 – 11 January 2007, Reno, Nevada AIAA 2007-43


  1. 1 Student guest blog post: pterosaur wings and flight capabilities :: Lorena A. Barba Group Trackback on 30/11/2013 at 8:05 pm
Comments are currently closed.



@Dave_Hone on Twitter

Enter your email address to follow this blog and receive notifications of new posts by email.

Join 456 other followers


%d bloggers like this: