Why do Boeing 737 engines have flat bottom?

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Why do Boeing 737 engines have flat bottom?

Why do Boeing 737 engines have flat bottom? - wikimedia

In the beginning of 1965 Boeing launched their 737-100 and 737-200 aircraft model. Boeing 737-200 was extended version of Boeing 737-100. Those were aircraft models of Boeing 737 original series. Both of the aircraft in Boeing original series used Pratt & Whitney JT8D engines. These engines were rounded in the shape.

Boeing 737-200 - wikimedia

Then Boeing decided produce stretched and more efficient variety of their existing 737 models. Hence, After 737 original series in 1984, Boeing started production of 737 Classic series. Which consists of Boeing 737-300, 737-400 & 737-500.

Changes in 737 variant


Boeing 737 - 300 - wikimedia

Boeing 737 Classic series aircraft are extended version of Boeing 737-200. So due to extended length it was necessary to maintain location of centre of gravity of the aircraft. As location​ of center of gravity of the aircraft plays an important role in Aircraft stability. Hence, engines of Boeing 737 Classic series Aircraft were located at some distance ahead of aircraft wing to maintain Center of Gravity(CG).

To make aircraft more efficient Boeing 737 Classic series replaced Pratt & Whitney JT8D engines with CFM International CFM56 engines.

Boeing 737-500 wikimedia

CFM56 engines has high bypass ratio than JT8D engines. High bypass ratio results in higher efficiency. But there was one problem to replace JT8D engines with CFM56. CFM56 engines were much larger than JT8D engines. Replacing JT8D engines without modification in CFM56 engines resulting aircraft engines were hitting the ground. So to overcome this problem without any major changes in Aircraft design, Aircraft engines bottom were made flat. This increased the distance of aircraft engines from the ground.

It was possible to increase the length of landing gear but this will increase distance of aircraft fuselage from the ground which could increase difficulties in Cargo loading in the Aircraft.

So simplest way was to make bottom of the Aircraft engine flat.

Then in 1993 Boeing introduced Boeing 737 next generation series consisting of 737-600, 737-700, 747-800, and 737-900 even this series has CFM56 engines with flat bottom!

Do you know!
Even Newest Boeing 737 Max engine has flat bottom!

Thanks for reading!


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