The Boeing 737 is among the most popular families of commercial aircraft ever produced. On your
travels or in photographs, you may or may not have noticed that this popular aircraft’s
engines have a certain visual quirk to their design. Specifically, the engine casing’s
bottom flattens out, rather than continuing around with a perfect circular shape. What’s
the reason for this oddity?
Let’s first examine the 737‘s initial design principle. Originally, the aircraft
was built for airports in a different age, first flying in the late-1960s. This was a
time when few airports had stair cars, and even fewer had jet bridges.
Back then, aircraft needed doors (especially cargo doors) that an operator or cargo handler
could climb into without specialized equipment. With that in mind, Boeing designed early 737
variants, such as the -100 and -200, to operate as low to the ground as possible. Compared
to today’s aircraft, its belly almost seemed to scrape along the surface of the runway.
However, as engine technology improved, engineers discovered that the more air an engine could
suck in, the more efficient its operation will be. For an increased air intake, engines
need larger turbines powered by larger fans, thus requiring larger casings. 20 years after
the 737, Airbus built the rival A320 family- but this airframe had bigger engines in mind.
Early generations of 737s were too low to the ground to accommodate larger engines.
Boeing could have moved the engines to a different area of the plane, redesigning it like its
trijet 727. However, this would have significantly changed its aerodynamic profile, forcing pilot
recertification and retraining. As such, Boeing was out of luck, and faced an expensive redesign
of the 737. However, engineers looking at the aircraft
realized that the engines’ casing, rather than the size of the powerplants as a whole,
was the problem. As such, they needed to find a shape that was not purely circular to ensure
sufficient ground clearance for larger engines. Subsequently, the engines on future 737s were
flattened along the bottom. This allowed them to contain a larger fan and turbine without
compromising on ground clearance. The solution was not perfect, as it did have certain aerodynamic
penalties, causing more drag than a round engine. However, the increased amount of air
entering the engine caused the bypass ratio to expand so much that it made any drawbacks
negligible. The Boeing 737 classic series (-300, -400,
and -500 variants) entered service in 1984 and were the first examples to feature CFM56
engines. These are well-known for their non-circular ‘hamster pouch’ design.
The newer aircraft had similar enough aerodynamics that its pilots did not have to retrain nor
be recertified. Instead, the redesigned engine casings meant that pilots could work interchangeably
between older and newer 737s at the time. The flat bottom is a visually conspicuous
design feature that can certainly look a little odd at first glance. However, it ultimately
helped the 737 advance towards the commercially successful iterations we are familiar with
today. The 737 NextGen series, in particular, was wildly popular with airlines around the
world. Did you notice the flat bottom of 737 engines?
Let us know
in the comments.