Architectural style-spotting (Paul Clements)

I enjoy finding places in the world where concepts I work with daily exist in different forms. In Software Product Lines: Practices and Patterns, one of the sidebars is about the product lines of manufactured goods that I see all around–cars, newspapers, suburban houses, light bulbs, and so on. Recently in this blog I wrote about stylized subway maps and argued that these are architectural representations. (This week I was in Venice, Italy, and saw the same kind of map for its ubiquitous water-based mass transit system, the vaporetti.)

Several years ago I read a wonderful article in the Smithsonian Institute’s Air and Space magazine entitled How the 747 Got Its Hump. Besides answering that question, the article also talked about why, for example, almost all cargo aircraft look alike in a few important ways: Engines in the wings, wings mounted high on the fuselage, a short main landing gear snuggled against the body of the aircraft, and a rear-opening cargo door.

Here are a few examples.  All are in service today, although some of the designs date back to the 1950s.

Lockheed C-130

Lockheed C-141

Boeing C-17

Antonov AN-124

That sounded like an architectural style to me. In Documenting Software Architectures: Views and Beyond, we define an architectural style for software as “a specialization of element and relation types, together with a set of constraints on how they can be used.” We have element types: engine, wing, landing gear.  We have relation types: for example, wings mounted {high, low, middle} compared to the fuselage. And we have constraints on how the design element can be used: wings fixed horizontally on the fuselage, one on each side. (Before you think that constraint is trivial, remember bi-planes.)

The article also talked about another common design–another style, if you like.  If you look at large modern passenger jet transports, you can immediately see that they have a number of design elements in common:

  • Their wings are…

Well, wait. Let me show you a few examples of aircraft in this style, and see if you can put together what they have in common–architecturally, that is.

Airbus A-319

Boeing 777

Embraer ERJ-170

I could have added many more examples, including the Boeing 737, 757, 767, and 787; the Airbus A-300, A-318, A-320, A-321, and A-330; and the Tupolev 204.  Those all have two engines, but four-engined variants of this style go all the way back to the Boeing 707, Convair 880, Douglas DC-8, through the Ilyushin IL-86 and Boeing 747 and Airbus A-340, up to today’s Airbus A-380.

Here is are the characteristics of this style that the article listed:

  • Their wings are swept back and attached at the bottom of the fuselage.
  • They have a tricycle landing gear: one apparatus under the nose and a main pair farther back that carry the bulk of the weight. The main landing gear supports the aircraft under the wings while on the ground, but folds inward into the fuselage for flight.
  • Their engines are housed in external pods hanging underneath the wing.

This style has been around for a long time.  Several years ago, I visited the National Air and Space Museum in Washington and looked at their beautiful Messerschmitt 262 on display.  The placard next to it said that its design–jet engines in pods under the wings, swept-back wings, tricycle landing gear–could be seen in modern jetliners such as the Boeing 737.

Here they are.  Do you see the family resemblance?

Messerschmitt Me-262

Boeing 737

Of course, all aircraft have wings and engines, but the style we’re discussing–the “Me262 style,” if you like–has swept wings and pod engines and tricycle landing gear. We have topological constraints: the wings attached low, the engines are placed under the wings.

These design decisions are not independent. Hanging engines under the wing helps balance counteracting lift forces in flight. But it means that the landing gear has to be longer so the engines don’t scrape the ground, which means the landing gear struts have to fold into the wing.

Like styles for software architectures, styles for aircraft architectures came about because of a need to achieve quality attributes (QAs), and all the aircraft that share a style exhibit the same QAs and achieve them in the same way. Because of this, it makes sense to consider them as a group. The QAs are a bit different than the ones software architects work with. For aircraft, they have to do with serviceability by ground personnel, ability to land on unimproved fields as opposed to paved runways, convenience of loading and unloading cargo, noise abatement, and of course weight and cost, just to name a few.

Pushing the analogy further, we can identify sub-styles–that is, specializations of the style created by making more design decisions or elaborating ones already made.  For example:

  • “Single-aisle” and “twin-aisle” refer to the width of the passenger cabin.
  • “Double deck” is a sub-style that for a long time was synonymous with the Boeing 747, although there were predecessors including the 1940s-era Boeing 377 Stratocruiser. (The upper deck was for economy-class customers, while the lower deck was a VIP lounge and bar.) The Airbus A-380, in which you can fly with 500-800 of your closest friends, is also double-decked.

Boeing 377 Stratocruiser

  • If you like, you can count the number of main landing gear “trucks” (and find examples at “2,” “3,” and “4”) or wheels (from 2 to over 20) to define more sub-styles or style specializations.
  • Swept wings, yes, but with inboard and outboard ailerons or just outboard?  Spoilers, and leading-edge slats, or no?  Simple flaps, or articulated?

Each specialization results from an additional design decision made once the more general style is adopted. And each puts the aircraft into a smaller corner of the design space, resulting in the achievement of very specific quality attribute values.

Is the “Me262” style the only style possible for large jet airliners? No. A significant alternative style emerged in the 1960s and 1970s. This style put the engines at the rear of the aircraft. Aircraft such as the Boeing 727, Vickers VC-10, Fokker F-28, British Aerospace BAC-111, the Tupolev 154, the Yakovlev Yak-42, and the Douglas DC-9 all were members of this style family.

British Aerospace BAC-111

Douglas DC-9

Tupolev Tu-154

Why did this alternative style arise? The answer, of course, lies in quality attributes–in this case, noise. Jet engines in that period were thunderous, and designers thought that by putting the engines in the rear it would decrease cabin noise, especially in the first-class cabin at the front of the aircraft, where the highest-paying passengers sat sipping drinks. Eastern Airlines was the first airline to fly the 727, which at full throttle could wake the dead. Eastern’s marketing department invented a new name to draw attention to the new engines-in-back style. With a straight face Eastern called the 727 the “Whisperjet.”

Boeing-727 "Whisperjet"

Why did that style not survive? Because next-generation jet engines were much, much quieter. And so the other quality attributes that made the 707 style popular in the first place trumped low noise, which was no longer as difficult to achieve. (The first version of the 727’s successor, the Boeing 737, flew with old-style screaming engines. But my guess is that Boeing knew that quieter powerplants were on the way, and the early models were soon replaced.)

But the engines-in-back style still can be found in smaller short-range passenger jets. The Embraer 145 and the Bombardier CRJ regional jets are exemplars, as is every single small business jet you can think of. Why? Again, quality attributes supply the answer. First, the wings of these aircraft are smaller so the balancing effect of engines is not as important. These aircraft also need to be close to the ground for easy boarding and deplaning (small airports might not have jet bridges).

Embraer ERJ-145

Bombardier CRJ-700

We can also observe two specimens that are heterogeneous with respect to the Me262 style (engines under the wings) and the B-727 style (engines in the rear)–that is, they belong to both. If you’re an airplane enthusiast, you’ve probably already thought of them: The McDonnell Douglas DC-10 (reincarnated as the Boeing MD-11) and the Lockheed L-1011. Given their design parameters and the engine power available in their time, both required three engines. Where to put the third engine? The L-1011 put the engine in the tailcone, with an S-shaped duct going up to an air intake in front of the vertical fin, which is exactly what the Boeing 727 did with its middle engine. The DC-10 has the entire engine up on top, with the vertical fin mounted atop the nacelle.

Lockheed L-1011

McDonnell Douglas MD-11

Authors who write about architectural styles almost inevitably appeal to housing styles for analogy. But I find I like analogies like this much better. For one thing, house styles are often distinguished by superficial adornments like latticework trim (Victorian) or non-load-bearing for-looks-only columns (modern American Colonial) that to me don’t seem to be essential parts of the design that help a house achieve its most important quality attributes. By contrast, aircraft styles have a clear relation to the achievement of measurable quality attributes. And the number of design elements that act as discriminators among these major style families is small and easily understood, and each one is absolutely critical to the function of the overall construction.

We could play this game in other domains, I’m sure. For example, someone more knowledgeable than I am about sailing ships (and that would be just about everyone) could write about sloops, yachts, barques, frigates, corvettes, cutters, brigs, schooners, and the architectural differences and similarities among them, and the quality attributes each was designed to achieve.

(To illustrate this article I used public domain photos from http://www.vectorsite.net/gfxpx.html#m2, Wikipedia, or www.af.mil.  There is one exception:  The photo of the Tu-154 is mine.)

- Paul Clements, SEI

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