Boeing — Why they use 7x7 as a designator for commercial aircraft?
There’re a lot of misconceptions about how Boeing chose the number 7 and the 7x7 pattern as a designation for Boeing’s commercial jetliners. In this article, I’ll reveal the truth about it.
Many people around the world wonder why the number 7 come to mind if the people is talking about Boeing jet-powered airliners (except if is about the MD-10, which is derived from McDonnell Douglas’s DC-10 — McDonnell Douglas merged with Boeing in 1997). This question, about why Boeing come up with the 7x7 designator is frequently asked, even as today.
One myth around this question is that the name originated from the sine of the swept angle of Boeing 707’s wings (the sine of 45 is 0.707). However, that’s not true at all, since the Boeing 707 has a 35-degree swept wing angle, not 45. Other myth around this is that the 7 was chosen by Boeing because is a lucky number and has a lot of positive connotation around.
However, the real reason why Boeing chose to name and designate their commercial jetliners in the form of 7x7 is more mundane, practical and is probabily one of the biggest marketing ideas ever made. Is a fact that Boeing didn’t always used the form of 7x7 for designating commercial airliners. The propliners Model 40, Model 247, 307 Stratoliner, the seaplane 314 Clipper (used by Pan Am and BOAC) and the 377 Stratocruiser were such examples. Military airplanes built by Boeing were known by their military designator, such as the B-17 (Model 299), the B-52 (Model 464) and the KC-97 (Model 367).
After the World War II, Boeing was a company which most of the products were focused and used on the military. The Boeing’s president at the time, William McPherson Allen, decided that Boeing should expand into the commercial airplane market, alongside other fields in spacecraft & missile technology. So, to support that strategy, Boeing’s engineering department decided to divide the model numbers in blocks of 100 for each of the new product areas: 300 and 400 were kept as a number for representing aircraft, 500 would be used for turbine engines, 600 for missiles and rockets, and 700 were destined for transport, jet-powered aircraft.
The first jet-powered swept-wing aircraft built by Boeing was the B-47 military bomber, which with its maximum speed of 977 km/h (607 mph / 528 knots) sparked the interest of the airlines, which, with the introduction of jet-powered aircraft, saw the opportunity to make air travel faster than it was at the time. Pan American World Airways asked Boeing for studies to determine the feasibility of the B-47 as a commercial jet transporter. At the same time, Boeing was studying the conversion of the aforementioned KC-97 tanker (model 367) into a jet-powered refuelling tanker, which would be capable to keep the pace with the jet-powered B-52 bombers while doing refuelling missions. Boeing, for that purpose, developed various versions of the 367, until a final version, the 367–80 was selected. The model was nicknamed “Dash 80”.
With the 367–80, Boeing took a serious risk of constructing the prototype with its own funds. The airplane prototype was worth more even than the company at the time (the prototype was built in 1954). Since both the commercial airliner and the refuelling tanker were stated to be based on the prototype, thus both being jet-powered transport aircraft, a number in the 700 range as a model designator for both was imposed by the Boeing’s numbering system. Boeing engineers considered that “Model 700” wasn’t too appealling for a commercial jetliner, so they decided to skip towards Model 707, which they considered to be attractive and catchier enough to both passengers and airlines which will order and operate the aircraft.
The other offspring of the Dash 80, which retained the fuselage width of the prototype (unlike the 707, which fuselage width was increased in order to incorporate 6-abreast seating in economy class) was named Model 717, with the US Air Force designation of KC-135 Stratotanker. After that, Boeing engineers decided that any Boeing aircraft which designation started and/or ended in 7 must be an commercial aircraft. The 717 was the only exception (since the KC-135 was a military transport tanker), and maintained as a exception until the late 1990s, when Boeing, merged with its archrival McDonnell Douglas and decided to re-use the 717 number to identify the MD-95 as a part of the Boeing family, thus making the KC-135 jet-powered military transport tanker officially named as the 717–100, and the DC-9-based jet-powered airliner (since both MD-80, MD-90 & 717 (MD-95) were based on the DC-9), the 717–200.
And since the 707, every Boeing jetliner has given a designation which follows the pattern of 7x7 (except the 707-derived medium-haul jetliner 720 (former 707–020), which United Airlines was the launch customer), including the trijet 727, the small, reliable and still widely-used 737, the jumbo-jet 747, the extraordinarily capable 757, the 767, and the 777, which is the world’s largest twin-engined airliner. The 7x7 designation still is used nowadays, with the most recent 787 Dreamliner, which is one of the most advanced and one of the most fuel efficient airliners ever produced.
In another post, I’ll explain why Airbus aircraft have the designation form of A3x0, also explaing a bit the background of both Airbus and the A300, their first aircraft model. This has the objective of giving a explanation of how both aircraft manufacturers designate their airliner models. Please follow me on Twitter (@O530CarrisPT is my username — I have 700 followers!)
UPDATE as of 2017–04–10: Some parts of this text were altered in order to give a more historically correct situation of the Boeing after World War II.
UPDATE as of 2017–04–11: The correct model number for the B-52 bomber is 464, not 454.
UPDATE as of 2017–04–12: The correct maximum (not cruising) speed in miles per hour & knots for the B-47 is 607 mph, not 557 (is 528 knots, not 484)
Sources:
- Boeing Frontiers Online, “ Why 7’s been a lucky number”, February 2004 (http://www.boeing.com/news/frontiers/archive/2004/february/i_history.html)
- Aviões e Musicas, “De onde a Boeing tirou a numeração 7x7 pra seus aviões?”, September 2010 (in Portuguese) (http://www.avioesemusicas.com/de-onde-a-boeing-tirou-a-numeracao-7x7-pra-seus-avioes.html)
