Evolution  of  Bridges  and  Other  Things

Back to Bridges     Back to Home Page

Evolution of Bridges

Everyone "knows" that bridges, like every other technology, have evolved from less "advanced" forms to more "advanced" forms.

Some people classify nations or other groups in this way, for example, by counting the number of electric motors, telephones, etc, per person, or the number of bridges per kilometer of road, the average size of a house, or by some other technical quantifier.  The trouble with this is that it doesn't say much about people who don't participate in the activity in question.  For example, nomadic people don't tend to build much at all.  They might be more impressed by people who can get along with belongings that don't weigh very much.  

We don't need to be an expert on bridges to be able to to see differences between bridges of different ages.

This section is far from complete.

. . . . . . . . 

To see the effects of technical evolution more clearly, we need to look at technologies which are often at the limits of the possible, such as flight or communications.  Many bridges are technically superb, but the wealth of different shapes is suggestive of a willingness to experiment and innovate, and a certain freedom in setting the variables.  

But if we look at large airliners, by contrast, they are all beginning to look very similar.  To some extent this has happened because progress in speed has stopped.  Because the speed of sound provides a physical scale factor that is absent in bridge design, there is little incentive to make small gains in speed, because of the costs involved.  Only going to Mach 2 or more has been attractive, and even that has been attempted only twice.  Only one model of supersonic airliner has been in active use for any length of time.

Certainly, there are upper limits to the practical spans of each type of bridge, but they are not created by the existence of an external scale factor in size, though it could be argued that all the limits are the result of the strength and rigidity of the materials.

####

OwlMerlinKSS.jpg (81023 bytes)OwlTakeOffS2S.jpg (369988 bytes)B747xc.jpg (41610 bytes)These pictures show Merlin, an eagle owl, Bubo bubo, and a Boeing 747, both taken shortly after take-off.  In the first picture, Merlin is about to go for a bit of food on another post.  In the second, he is going for it. The pictures were taken at the New Forest Owl Sanctuary (NFOS), an excellent place for a day out for people of all ages.  The staff are exceptionally friendly and helpful, and they allow guide dogs into the sanctuary, except for the flying displays.  There is even a covered car-port so that your dog can be left in your car, out of the sun, while you look around.   Owl Sanctuary   Note the large camber of the owl's wings, more clearly shown by the right wing, to generate lift high lift.

The owl is getting into its level flight mode.  Its fearsome talons are stretched out behind for minimum drag.  The Boeing 747 is cleaning up for the long climb to cruising altitude.  The nose-wheel doors are almost closed, two of the main trucks have been stowed, and the other two are in process of stowing.  The leading edge slats and the trailing edge flaps are still deployed, as the airspeed has not yet reached the point where they can be closed.

OwlPerched.jpg (136944 bytes)OwlLanding.jpg (132420 bytes)Here we see Merlin sitting and landing.  The owl pictures suffered from the dullness of the day, which prevented optimal camera settings being used.  For the second picture, the camera was misaligned; not enough allowance was made for the way that big birds land on a perch, namely upwards.  Looking at the owl, it is hard to escape the feeling that the landing, though practised hundreds of times, is by no means trivial.  Note also the small size of the owl's head and body, especially if we imagine the feathers removed.  The almost monumental effect that we sometimes see in the resting owl is an illusion.

Look also at the leading edges inboard.  The loose feathers hint at a stall.  It is better for the wings to stall inboard before they stall outboard: a tip-stall could provoke instability, or even a spin.  Is it possible that the owl can sense the fluttering of these feathers and take action to avoid the stall.  Some aircraft have small paddles in the leading edge which flip when the stalling angle of incidence is approached.  The stalling speed is slightly higher than it would be in level flight, because the path is curving upwards, increasing the apparent weight.  In the same way, a landing glider must be held at a high enough speed to prevent a stall at the round out.  Fighter planes have sometimes experienced a high-speed stall at air-shows when pulling out of a dive.  The stalling speed is not a constant of the plane: the invariant is the stalling angle of incidence.  If we pull 3g, for example, the stalling speed will be greatly raised.  By what factor?

HawkMK3.jpg (148035 bytes)Returning to the method of landing we realise that a bird flying at 50 kph has momentum and energy.  Hitting a solid perch at that speed would be very risky, and wasteful of energy.  What the bird actually does is to approach too low, and use the final gain in height to convert kinetic energy into potential energy.  At the last moment the wings are deployed into braking mode, generating high drag as well as lift.  The owl does not aim for zero velocity, because a small error in estimating the distance could result in the owl stopping in the air and crashing to the ground, or at least scrabbling on to the perch in a flurry of feathers.  Better to have a little speed left at the end.  All landing aircraft need a reserve of speed to allow for gusts which reduce the airspeed.  Never low and slow, especially in a glider.

For similar reasons to the owl, the Boeing 747 deploys high lift devices before landing, and, after touch-down, air-brakes which help the reverse-thrusting engines to slow the plane.  Carrier-based planes use a wire to absorb their energy.  It is caught by a hook at the tail end of the plane.  Great courage is needed when making a first landing of a fast, heavy jet plane on a carrier, especially if the ship is rolling, yawing and, perhaps worst of all, pitching.  Like the owl, the naval pilot does not aim at zero velocity - he or she bangs it down to make sure.

B52b.jpg (82357 bytes)Here is another way of losing speed.  Fixed wing aircraft are at a tremendous disadvantage when taking off and landing.  The requirements for low speed flying and high speed flying are very different.  Most aircraft have a relatively low range of flying speeds.  Helicopters, autogyros, birds, bats  and insects have the great advantage of mobile wings, enabling them to retain flying speed while the main structure is travelling quite slowly.  A machine with a very high ratio of speeds is the NASA space shuttle.  It loses speed and altitude in a series of wide S-curves, so that it remains in the same general locality instead of travelling far around the earth.  Geese and parachutists often adopt a similar practice, in their case entirely to lose altitude rather than speed.

OwlEyeAug2002A.jpg (153522 bytes)OwlTalonAug2002A.jpg (133745 bytes)Sharp vision, sharp hearing, sharp talons, sharp beak - and soft, downy feathers for silent flying.  When the owl can hear the mouse, the mouse will probably die.  By the time the mouse can hear the owl, it is doomed.  Thanks to owl Guinevere at NFOS, for posing so cooperatively for these two photographs.

Let's take a look at the business of curved flight paths.  To simplify things, imagine a bead, sliding without friction on a thin but rigid wire.  What is the curve of the wire that will minimise the time that the bead requires to get from one place to another?  The problem of the brachistochrone, or minimum time, was solved by Johann Bernouilli.  We can see, without doing the mathematics, that the curve should enable the bead to spend as little time as possible at the lower speeds.  That is why the curve is steepest at the ends.  The curve is called a cycloid, a name which has nothing to do with speed or with time.  It is derived from the fact that the same curve is the path of a point on the rim of a wheel rolling in a straight line on a plane surface, though for the wheel the curve is convex upwards.

With this curve you can glide between two places at almost the same level.  The low road is quicker than the high road.

The world, of course, does not contain many frictionless objects, and so we we would not expect to see many exact cycloids. 

Nevertheless, if you watch jackdaws gliding from one building to another, you may see that the flight path does indeed follow a curve.  To some extent the birds are forced into a steep dive at the start, unless they flap their wings, because at zero speed they get no lift, so they need to drop down to get up to flying speed.  In a sense, a gliding bird is like a bead that creates its own wire.  It can only create the "wire" once it has flying speed.  If you watch large birds at a falconry or owl centre, you may see a much elongated flight-path, not because Bernouilli was wrong, but because for low perches, a single cycloid based on the total distance would intersect the ground.

Furthermore, near the ground, the bird may be able to use a ground effect, enabling it to glide horizontally.  How can it do that?  It does it by using some of its kinetic energy to compensate for energy lost by friction.  So its speed is slowly reduced.  It must, of course, retain enough energy to rise up to the landing spot.  You might see a curve like the one above.

Gliders are towed into the air by external forces (except for the ones at Long Mynd that roll down a ramp, where the external force is gravity), and they land with a long roll.  So their range of flying speeds is not as great as that of a bird, enabling a much simpler construction to be used.  The best gliders have a theoretical glide angle in the region of 1 : 60.  That's pretty amazing - a vehicle that can create its own almost level road as it goes along.  The trick with gliding is to find air that is rising faster than the glider is sinking.  Up you go . . . . .

####

HawkML.jpg (160860 bytes)Buzzards soar in order to scan a wide area for food, or to gain the height they need in order to travel across country, much as glider pilots do.  The wandering albatross travels thousands of miles with little effort, gliding up and down above the waves, using energy derived from gravity and wind shear.

If you see a buzzard gliding effortlessly through a group of gliders that are scratching desperately around near the ground, you will get a sense of just how good these birds are.  Nevertheless, the best gliders, flown perfectly in still air, can achieve a glide angle of around 1 in 60, a loss of only about 17 metres per kilometre.  Some glider pilots will tell you that they feel a great sense of privilege if they are joined in a thermal by a bird such as a buzzard.  Those, the great majority, who have not had this experience, usually hope that they will.  Even the best pilots may envy the soaring vulture, the aerobatic kite, or the whiffling Canada Goose.

To the soaring pilot, flight is merely pleasure, except perhaps during competitions: to the bird, of course, it is a matter of survival.

Sometimes a glider pilot may execute a diving approach to a landing.  The reason here is different.  He or she does it because they have started an approach that is too low for the strength of the headwind.  By reducing the height, the glider gets into a region of lower headwind, because of the wind-shear.  This is not, of course, done using the air-brakes, but by pushing the stick forward to gain airspeed and kinetic energy at the expense of height and potential energy.

Birds have many ways of hunting.  The peregrine soars and stoops.  The kestrel hovers and pounces.  The swift flies continuously all day, dawn to dusk, never resting, like a hawker dragonfly.  The gannet, kingfisher and pelican dive from the air.  The goosander swims.  The dipper dives to the bottom and walks around.

It is tempting to think of something like a bird as approaching perfection.  But no bird is a perfect glider or a perfect flyer.  Birds have to eat and reproduce, and avoid predators, as well as fly.  Some have to walk, run, swim or dive, or even all of these.  A perfect flyer would possess no organs that it did not need for flying.  So a real bird is a compromise, just like an aircraft.  If Concorde could be launched at 1000 kph, it could manage with much smaller wings, and it might look more like a large air-launched cruise missile. Experimental air-launched aircraft have been produced, and the Shorts Maia-Mercury comprised a large flying boat which carried a smaller seaplane on top.  Since the seaplane had to land on the sea, and not on to another carrier plane, it still required full sized wings.

Some migrating birds can shrink some of their organs to reduce weight, and then regenerate them afterwards.

#######

OwlEyeBigS2.jpg (296745 bytes)"The eye" has been used as a complex example which suggests that evolution cannot take place.  Well, if evolution has taken place, it has produced eyes by over fifty of completely independent routes.  It does seem difficult, perhaps, to believe that something so complex as an eye can evolve, or that en route to the "perfect eye", the intermediate organs can be of use.  Complexity is irrelevant, because if a simple organ can evolve, so can a complex one.  And the current eyes are not necessarily "perfect": they are quite possibly still evolving.  In some cases they may be getting worse, if their possessors are moving in directions in which other organs are becoming increasingly important, and eyes are of decreasing utility.

Even where an eye is not evolving, it is not necessarily perfect.  It may, for example, have enough retinal cells for the required function, or it may be that increasing the complexity of the eye would require more processing power from the brain, leading to increased use of energy and generation of heat.  It would be useful if the brain were in the middle of the body, in cold places, so that it could help to keep the animal warm.  But the nerves from the eyes would have to be much longer, and the signals would take a longer time to travel.  Reaction times would be longer.  Putting the eyes in the middle of the belly would help, but would greatly reduce their utility.  Imagine designing a stadium for people with eyes in their bellies.

Why don't we have eyes right at the tops of our heads, like crocodiles and hippopotamuses?  We could see just that little bit better, and a little bit is enough for evolution to work.

Airbus2.jpg (103883 bytes)People have used the idea of complexity and fitness for purpose to argue for the existence of a designer.  If we went to another planet with no apparent life, but we did find the remains of complex machines, such as the aircraft illustrated earlier, we would rightly guess that someone had been there to design them.

What do we mean by "design"?  On earth, we don't imagine that one day, someone suddenly decided to invent all the types of aircraft that we see today.  No, what actually happened was that over a long period of time, numerous people made contributions to the field of aviation, and eventually, Orville and Wilbur Wright, after a long period of careful experiments, produced a controllable powered man-carrying aircraft.  After that, progress was almost explosive, compared with the many years of hesitant crawling.  

As new uses were found for flying, such as pleasure, record seeking, reconnaissance, bombing, and transport, aircraft types began to proliferate.  This drove technologies such as alloy design, stress calculation, engine design, aerodynamics, and so on.  Airports came into being, and they too, gradually became bigger and more complex.  A 1990s airport would have been useless to a 1920s aircraft, and a 1920s airport would have been useless for a 1990s airliner.  Like beetles and flowers, many things evolved together.

Sometimes people design artefacts that are "ahead of their time".  However "good" they are, they are seldom successful.

What drove the evolution of airliners, for example?  The wish of sufficient numbers of people to fly for reasons of business, pleasure or emergency must be taken as a large contributor.

So there is a sense in which nobody actually decided to invent the Airbus A340 - it just emerged as a next step in the chain of airliner evolution.  This is not to denigrate the ingenuity and persistence with which designers have made travel so easy, but it does illustrate the idea that a context is needed for a technology to be successful.  Most of the types of aircraft that have existed are now extinct, though a few examples are kept alive by enthusiasts and museums.  This does not mean that the earlier designs were all failures: on the contrary, many were great successes, but they have been overtaken by developments over time.  The DC-3 has been flying since around 1934 (then as the DC-2), but the uses to which it has been put over the years have probably varied greatly as time passed.  The DC-3 is certainly not a front line airliner now.

Whether or not we think we can demonstrate that arguments for a creator are wrong, such demonstrations do not prove anything at all about the existence of such a being.  They only show, if valid, that the arguments are invalid.

#######

How the owl gets along

You only have to watch a bird for a short time to see that it is doing some pretty complicated things, especially during take off and landing.  Various birds execute many kinds of manoeuvres, such as take-off, landing, straight level flight, gliding (including thermal soaring, ridge soaring and ocean gliding), turning, diving, aerobatics, whiffling, accelerating, braking, hovering, and even flying backwards.

OwlTakeOffS2S.jpg (369988 bytes)Here is Merlin again, taking off.  His wings are pointing down at a steep angle, and still going down.  To understand what is going on, we will separate the motion into its vertical component and its horizontal component.  It is dangerous to simplify with aerodynamics, but we have a three dimensional system, and it is rather complicated.  Our conclusions must only be qualitative.

The diagram below shows an idealised cross-section through one wing, seen from the side.

It looks rather complicated, so we take it step by step.  Looking first at the right hand side, we have the owl's wing travelling downwards.  The wing has camber, that is, it is curved along the direction of the air flow.  This is one means of getting the vertical asymmetry that is needed to generate lift.  The wing is also travelling forwards, because the bird is going forwards.  The resultant of these two speeds is the velocity of the wing relative to the air.  A second means of generating lift now becomes apparent - the wing is not parallel to the flow - it has an angle of attack - another asymmetry.

It is customary in aerodynamics to resolve the resultant force generated by the air on the wing into lift (perpendicular to the velocity) and drag (parallel).  As we may be going off your screen we show the diagram again, which won't cost you any download time.

In this example it is more useful to look at the total force on the wing, shown in black on the left side.  In level flight, what matters is the forward force, or thrust, and the vertical force, or lift.  So the wing is generating lift to balance weight, and thrust to balance drag.  In steady level flight, the total force on the owl is zero.

At the time when the wing is roughly horizontal, these are the only diagrams we need, though we should remember that the air-flow is not exactly parallel to the flight; there is a small flow along the wing.  This is outwards on the top surface and inwards underneath.  At the tip, the flow actually rolls over, and when your airliner lands in fog or moist air, you can often see the path of the resulting vortex, which is a source of drag.  Many planes now have winglets to reduce the size and energy of the vortices.  You can often see vortices behind a car if there is fog or if the exhaust is visible.

The Spitfire had elliptical wings, which tended to spread the wingtip effects along the wing, reducing the problems.  This was one reason that the Spitfire was expensive.  Sometimes a much cheaper machine is only a little less efficient than an expensive one.   Here is one way in which engineering may differ from evolution.  If you can make five planes instead of four, and if each one has its range reduced by three percent, or its passenger load reduced by four percent, you might still win, though you have to remember that you have a greater salary budget.

Evolution doesn't necessarily work like that.  It does sometimes use vast numbers, as in the case of the eggs of many animals, but what also matters is the efficiency of an individual in terms of achieving successful reproduction.

#######

OwlTakeOffS2S.jpg (369988 bytes)Let's get back to Merlin.  The next diagram is a plan view, showing only one wing section, since in straight flight, the system is completely symmetrical.  The direction of flight is up the screen.  The centre line of the bird is shown by CL.

Using the same line of argument as before, we see that there is forward thrust, and an outward force that is not useful to the bird.

Summing up what we have so far, we see that the bird generates thrust, lift, and unwanted outward forces.

Now comes the question.  What happens when the wings go upwards?  Why doesn't this cancel out all the good work done on the down stroke? 

The answer lies in the word asymmetry.  We already saw two asymmetries.  Perhaps there is also a difference between the down stroke and the up stroke.  Rowers, for example, achieve the asymmetry by removing the oars from the water during the forward stroke.  But flying is not at all like rowing.  it is much more like the method used by someone standing at the back of a boat, wiggling a single oar in a peculiar way.  The blade oar never leaves the water, and in fact it generates thrust on both strokes.

The up stroke of a wing is also not a passive stroke.  The next diagram shows how it differs from the downstroke.  Again, the owl is flying from left to right.

The red resultant force shows that lift is generated, but instead of a forward thrust, we now see a large backward force.  So we see that on the upward stroke, the wing must be moved quite differently from the downward motion, to reduce this drag.  It could be moved more slowly, and it could be held at a lower angle of incidence.  Now try watching some big birds.  Small ones flap too quickly for us to see what happens. Why not go and see Merlin and his colleagues.

Don't forget - all these explanations are grossly over-simplified.

See also The Restless Kingdom, by John Cooke, Blandford, ISBN 0-7137-2292-4.

 

Back to Bridges     Back to Home Page