Jet Engine Intakes: Why the Squashed, Flat Design?

[Alan G]

I half remember that jet engine intakes were circular a few years ago. Now they have a squashed appearance with the bottom part quite flat.

This isn't new as the first German jet at the end of World War II had the same shape but it seems relatively new on airline jet engines.

So, why are they this shape?

 

[brewnog]

Runway clearance.

Many aircraft built during the 1970s and 1980s, such as the 737, have engines that do not appear perfectly round when seen from the front. This is because the bottom of the engine cowling (or covering) has to have clearance above the runway so that it does not scrape along the ground. When the 737 first appeared in the 1960s, it had fairly small turbojet engines that were suspended high above the ground. As the engines became larger with the introduction of turbofans, the fuselage and wing remained the same height above the ground, so the bottom of the engine cowling was flattened somewhat to still achieve the maximum air intake without scraping the ground

http://www.century-of-flight.freeol... of technology/Noise Reduction Technology.htm



As an aside, when they were testing the Rolls Royce Trent 900 engines for use on the new Airbus A380, the test plane had such little ground clearance beneath the engines that they allegedly put extra air in the tyres to get another couple of inches clearance.