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March 19, 2006

End of Evolution: Airliners

Donald Pittenger writes:

Dear Blowhards --

Ever notice that some kinds of objects don't change significantly over time?

Examples include straight pins, buckles, coffee cups and drinking tumblers.

Yes, buckles, cups and tumblers vary in detail, but they each embody a fundamental or Platonic shape that underlies the variations.

Why is this so?

It's largely a matter of function and technology. Once a function has been "elegantly" embodied at a given level of technology, the essential form will cease to evolve and changes will be cosmetic.

Consider the buckle. Its basic form hasn't changed in centuries. Its function is to fasten together ends of one strap (usually made of leather) or connect separate straps (also usually of leather) in a way such that the amount of overlap of the connection can be varied. The buckle is normally made of metal or some other hard material. Attached to it is a "tongue," also usually of metal, that can be inserted in holes punched through the strap in order to secure the fastening and set the overlap.

Nowadays buckles are being replaced on shoes and other objects by Velcro. The fastening function continues, but new technology has added the advantage of allowing the fastening overlap to take place over much smaller increments than is possible using a buckle. On the other hand, buckles allow a stronger binding than Velcro.

The same sort of thing can be seen in more complex objects, especially those whose functionality is tightly constrained. Early versions tend to exhibit varied shapes. Over time, through trial and error, less practical shapes are discarded and technology advances to enhance configurations that are proving successful. Eventually, barring a major technological advance or other disruption, the object will evolve toward its fundamental form.

Here I deal with commercial passenger aircraft -- airliners. My contention is that airliners first attained their fundamental shape in the mid-1930s. The advent of turbine (jet) engines allowed greater speeds and the need for adding back-sweep to wings and empennage, thus changing the fundamental shape a little. This happened in the mid-1950s and the basic shape of airliners has remained essentially unchanged.

Since our main concern is appearance, it seems best to simply show you how airliners have evolved using pictures backed by captions.

Historical Gallery

Armstrong Whitworth Argosy over London.jpg
1925 -- Armstrong Whitworth Argosy.
The Argosy was one of the first transports able to carry more than a few passengers. It has a long fuselage with windows for the passengers, features common to nearly all future airliners. On the other hand it's a fabric-covered biplane with fixed landing gear and has a open cockpit for the pilots, not to mention a motor mounted on its nose. Nevertheless, it's a great advance over early, kite-like, airplanes.

Curtiss Condor - 1930.jpg
1930 -- Curtiss Condor.
Although it was one of the last biplane airliners, the Condor has a fairly streamlined fuselage and retractable landing gear.

Douglas DC-3 - 1935.jpg
1935 -- Douglas DC-3.
This is the classical piston-engine airliner -- a nicely-streamlined all-metal monoplane. The similar Boeing 247 and Douglas DC-2 entered service earlier, but the DC-3 dominated the airline industry in the years leading up to World War 2. And note that the DC-3 appeared just over 30 years after the Wright Brothers' first flight.

Boeing 307 - color pic.jpg
1940 -- Boeing 307 Stratoliner.
This Boeing B-17-based airliner has four motors and a pressurized fuselage for high-altitude flight. But its appearance is that of an elaborated DC-3: nothing much new here.

Douglas DC-6.jpg
1950 -- Douglas DC-6.
First flown in 1946, it's an enlarged and pressurized version of the four-motor DC-4 that appeared a few years earlier. It, the Lockheed Constellation and the later DC-7 were the leading long-distance airliners around mid-century. Again, its appearance is generally similar to that of the DC-3.

Boeing 707.jpg
1960 -- Boeing 707.
Not the first jetliner, but the first commercially successful one, the 707 entered service in 1958. Its fuselage follows the pattern set in the 1930s but the wing and tail surfaces are swept back to allow the faster speed potential of powerful jet engines. Because jet engines lack conventional propellers, designers have more freedom when placing the motors. Some jetliners have some or all engines mounted near the tail. But most have their engines mounted on short pylons under the wings as on the 707. Thus the 707, whose prototype (Model 367-80) flew in 1954, established the shape for future airliners. Even so, its appearance is not that far removed from that of the DC-3.

McDonnell-Douglas DC-10.jpg
1970 -- McDonnell-Douglas DC-10.
First flown in 1971, the DC-10 is an example of an airliner with one of its engines mounted near its tail. The contemporaneous Lockheed L-1011 was almost identical.

Airbus 340.jpg
1990 -- Airbus 340.
This four-engine wide-body airliner first flew in 1991, yet looks little different from the 707 prototype of 1954, more than 35 years earlier. The turned-up ends of the wings (winglets) reduce air turbulance and thereby improve fuel economy.

Boeing 787.jpg
2005 -- Boeing 787.
This is the latest in jetliner thinking. Its design was frozen in 2005, its first flight is expected in 2007 and entry to commercial service should be in 2008. It too follows the DC-3/707 pattern although its engineering details are far advanced.

Commentary

Seventy years after the DC-3 and 50 years from the 707 the basic shape of airliners has remained surprisingly consistent. Probably the main reason for this has to do with the nature of flight itself, especially at speeds where aerodynamic efficiency is essential. In other words, airplanes have to be able to fly safely and fly efficiently at speeds that make flying a viable alternative to ground-based transportation. There requirements pose strong limitations on the aircraft designer; car stylists, for instance, are not nearly so constrained.

As for the DC-3, it's "fundamental" appearance was due to technological advances, the main ones being:

  • Aerodynamics had become more scienticially-based thanks to development of better wind-tunnels and establishment of aeronautical engineering departments in universities such as MIT, Michigan and Washington.

  • Improved metallurgy along with years of experimentation -- especially in Germany and the USA -- led to the development of all-metal aircraft. Previously, airplanes were made of a wood structure covered by canvas, materials not as well suited for making streamlined shapes. Metal aircraft have the important advantage that the metal skinning could add structural strength, impossible with fabric. The disadvantage of metal was its weight. But eventually that was offset by ...

  • Development of motors powerful enough to power metal airplanes. Initial DC-3 engines were rated at 1,000 horsepower each.

Future airliners are likely to be refinements of the long-established pattern provided (1) there is no major technological breakthrough as in the case of the jet engine, and (2) cruising speeds stay 9/10ths or less of the speed of sound.

Trans-sonic and supersonic speeds require, among other things, differently-shaped wing and tail configurations whose departure from the norm can be made easier thanks to computer-aided control systems. (Some military aircraft are aerodynamically unstable, but can fly thanks to computers monitoring performance and almost instantaneously making appropriate adjustments to control surface positioning. In principal, such systems can be applied to commercial aircraft. But the implemenation of such systems has been slow due to safety concerns.)

I'll be writing more on the subject of the evolution to fundamental shapes in future posts.

Later,

Donald

posted by Donald at March 19, 2006




Comments

You seem to have left out that most majestic of airliners, the 747. Not to mention the Concorde and the Tu-144.

Posted by: Frankenstein on March 19, 2006 8:23 PM



The 747 isn't really different from the basic airliner design, except for the nose hump. And that exists solely because in the early design phases there were plans to make a cargo version of the 747 with a upward-swinging nose door.
As for the Concorde and TU-144, they seem to represent dead ends more than anything else.

Posted by: Peter on March 19, 2006 8:53 PM



What about the B-52. That plane design is almost 50 years old and still seems to do the job.

Posted by: Larry Levin on March 19, 2006 11:08 PM



Carbon fibre, more expensive (carbon taxed?) aviation fuel and increased safety requirements are likely to lead to lifting body designs being adopted.

May take a while, though.

Posted by: Nigel on March 20, 2006 4:12 AM



The shape has stayed the same, but less obvious things like construction materials and engine efficiency have made the later aircraft significantly superior to their predecessors. Evolution as to external shape stopped, but other things continue to evolve.

Posted by: Lexington Green on March 20, 2006 10:07 AM



What I've found fascinating since I was a boy is the accelerated evolution in airplane technology caused by the Second World War (and the runup to it). The basic ideas behind, say, the Brewster Buffalo and the P-51 Mustang are similar: monoplane, single-engine, enclosed cockpit. But the way that these ideas were executed, and the materials that were available to do it, had changed dramatically.

An equally interesting phenomenon was the experimentation that went in during the same period. All sorts of out-there designs (pusher-puller propellers, plywood-framed turbojets, rocket-fueled fighters) were rushed into development, mostly by the losing sides.

Posted by: Derek Lowe on March 20, 2006 11:01 AM



Granted, there was a lot of capital put into aviation from very early on, but it is striking how rapidly it evolved over a very short time span. One example: at Orville Wright's funeral in 1948, there was a fly-over by Air Force jets. I suppose this is the general fate of militarily-useful technology: government contracts (motivated by fear)provide a hot-house environment.

Donald, one question: how about the long-term evolution of the flying wing?

Posted by: Friedrich von Blowhard on March 20, 2006 11:12 AM



"Some military aircraft are aerodynamically unstable, but can fly thanks to computers monitoring performance and almost instantaneously making appropriate adjustments to control surface positioning. In principal, such systems can be applied to commercial aircraft. But the implemenation of such systems has been slow due to safety concerns."

Fighter aircraft benefit from dynamic instability by being able to turn more sharply, and sharp turns are life in air-to-air combat (and when avoiding missiles). There's not much reason for a commercial aircraft to be unstable.

It's almost certainly technically feasible to build a 40-passenger bus with only two wheels. While this would be less safe than the current versions with more wheels, it's not the reduced safety that is primarily responsible for choosing not to build such a vehicle.

Posted by: Doug Sundseth on March 20, 2006 12:24 PM



Frankenstein and Peter -- I left out the Concorde and "Concordski" because they represent another functional class of airliner such as the flying boat liners of the 20s and 30s. I also tried to peg the illustrations and comments to nice round (divisible-by-five) years. This meant I could have used the 747 to represent 1970, but I elected to use the DC-10 so that I could show an alternative engine configuration.

Larry -- Well, I was talking airliners and generations thereof, not bombers. BTW when I was a kid I saw the YB-52 prototype for the first time when it cruised over my house at low altitude with both landing gear and flaps down on its way to land at Boeing Field (this was not its first flight, but a fairly early one). It was hugely impressive to a 12-year-old boy.

Nigel -- Most lifting-body craft I recall have rather stubby shapes. This might mean fewer windows and window seats (which I love). Perhaps the TV/videogame generation won't care about looking outside, being content to interact with the little screen on the seatback. Then the lifting-body-as-airliner concept might actually fly!

Derek -- The U.S. had some oddies too. I'm not handy to my reference material so I can't give the model numbers, but here are a few: (1) the "flying pancake" that Chance-Vought developed for the Navy [XF5U ?], (2) the XP-67 twin-engine fighter by McDonnell ... their first airplane, (3) two other XPs by Curtiss [the "Ascender"] and maybe Northrop, all 3 being from a "get wild" commission from the Army, and (4) the Douglas XB-43 "Mixmaster" light bomber with engines in the fuselage and contra-rotating props at its tail.

This exciting binge of prototypes probably got started in the mid-Thirties and continued till around 1960. By the Sixties it became evident that aerial combat was almost entirely a sub-sonic matter and planes were getting awfully expensive to develop. Since then, we see few new military planes: the old ones keep soldiering on for years (I see where the F-14 Tomcat is standing down after more then 30 years of service life). The change and excitement are now on combat electronics which keep being updated in those old airframes.

Friedrich -- I've noted that it seems to take about 30 years +/- 10 for a new device to move from its primitive to approaching its "fundamental" form (well, for the period 1825-1975 perhaps). I once considered doing a book about this and gathered some material. Then I decided it wasn't a hot enough topic -- hence this and some future blog posts.

As for flying wings, well, a wing is just a wing and they all look pretty much the same. Besides, almost every flying-wing aircraft came from Northrop (it being Jack Northrop's hobby-horse). I'll grant that the B-2's shape is more distinctive than that of the XB-49's, however. Flying wings have always had control issues, so my comments about computer-linked controls might apply here. Then there is the matter of lack of windows for an airliner flying wing; if passengers resist, airlines won't buy.

Doug -- The most likely need for computer-linked controls would be if airliner speeds moved into the trans- or supersonic zones. In such regimes the optimal wing-shapes become a lot different from shapes needed at lower speeds (for take-off and landing, for instance). At supersonic speeds the wing's center of pressure migrates to the rear (if I recall correctly) and this creates an imbalance with the center of gravity. The Concorde dealt with this problem be pumping fuel fore and aft as needed. Computerized controls might be helpful here too, smoothing out the compromises required for high and low speed flight. Boeing mooted a trans-sonic liner (the "Sonic Cruiser") before dropping back to the 787. It had a "canard" pattern of horizontal stabilizers to the front and wings more to the rear. I don't know if computerized controls were part of the plan, but fly-by-wire was.

Posted by: Donald Pittenger on March 20, 2006 4:08 PM



Friedrich: I suppose this is the general fate of militarily-useful technology: government contracts (motivated by fear) provide a hot-house environment.

Best exemplified by the technologies developed for ICBMs and adapted for the space race 1957-1969. The DoD spent about 2.5x times the cost of Apollo on those in 1953-1963 (not just big rocketry, but guidance, re-entry, telemetry, exotic materials etc). That pushed them towards limits set by physics, and provided -- as a side effect -- the ability to put things in orbit, albeit very expensively.

By the mid-1960s, our fundamental ability to blow things up fast from far away was "good enough," and DoD spending shifted to accuracy, more convenient (though less powerful) solid fuels, and surveillance satellite technologies. Since then there's been only marginal reduction in $/lb to orbit. Space enthusiasts lament that, but you've nailed the underlying reasons: first, that there's not a lot of headroom left in core chemical rocket technology; second, that for forty years there has not been hot-house pressure or funding to break through to a new one.

The alt.space entrepreneurs (Branson, Elon Musk, et al) argue that there are big savings to be had in more "airline-like" design, manufacturing and lean operations. In principle, yes -- if there's enough private demand to drive that virtuous circle. We shall see...

Posted by: Monte Davis on March 22, 2006 8:28 AM






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