Why Are The Bottom Of Boeing 737 Engines Flat?

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If you have ever looked at a Boeing 737 engine you might have noticed something odd. That the bottom circle of the engine casing flattens out instead of continuing the same circumference. There is a very special reason for this and we will explain why.

Boeing 737 engine
The Boeing 737 engine. Photo: Davidilit via Wikipedia

The history of the Boeing 737

To answer this question, we first need to examine the initial design principles of the Boeing 737. Originally the aircraft was built for airports in a different age, one in which only a few airports had stair cars and even less had jet bridges. An aircraft back then had to have doors (especially cargo doors) that an operator or cargo handler could climb into without any ladder or truck.

With that in mind, the Boeing 737 series was made to operate as low as possible, with a belly that almost scraped along the surface of the runway (overdramatic but you get the idea).

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Lufthansa 737
A look at Lufthansa’s 737-100 on the ground. The first Boeing 737s didn’t need big engines or lots of room. Photo: Lars Söderström via Wikimedia Commons

However, as engine technology improved, engineers discovered the more air that you can put into an engine, the more efficient its fuel will be. This is called the bypass ratio.

To get more air into the engine you need a bigger turbine which is powered by a bigger fan. Bigger fans require a bigger intake and an engine casing to fit. Problem is, unlike the Airbus A320 (the Boeing 737 rival) that was built with bigger engines in mind, the Boeing 737 was actually too low to the ground to have any bigger engines.

Boeing could have moved the engine to a different area of the plane (such as a trijet model or overwing) but this would have significantly changed the aerodynamic profile, leading to pilots having to retrain and be recertified (this story might seem familiar for long time readers, but don’t worry we will explain at the end).

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Thus Boeing was out of luck and faced an expensive redesign of the 737.

Southwest 737 MAX
You can see the flat design principles still on the Boeing 737 MAX. Photo: Southwest Airlines

Why is the engine bottom flat?

However, engineers looking at the aircraft realized that the casing of the engine was the problem. Could they simply have a shape that wasn’t round and thus would not hit the ground as the plane rolled along the runway?

Thus the new Boeing 737 engine was flattened along the bottom, despite still containing a larger fan and turbine. This did have bad aerodynamic properties and caused more drag than a round engine, but the increased amount of air entering the engine caused the bypass ratio to expand so much that is made any problems negligible.

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The Boeing 737 classic series (-300 -400 -500) was the first to feature the CFM56 engine, well known for its ‘hamster pouch’ non-round design.

Because the aircraft still flew the same and had the same aerodynamics, pilots did not have to retrain nor be recertified. In retrospect, perhaps not improving the Boeing 737 design to take bigger engines back in the day resulted in the situation we have with Boeing 737 MAX, however that is beyond the scope of this question and best left for the comment section.

What do you think? Do you like the look of the Boeing 737 flat engines? Let us know in the comments.

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