david b.
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Joined: Fri Jun 15, 2001 7:18 pm

Shape Of Engine Nacelle

Wed Jul 03, 2002 11:21 am

Why is the nacelle on this engine shaped like an oval? The one on the lower right hand side.

Something do do with aerodynamics or is the camera?

Teenage-know-it-alls should be shot on sight
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RE: Shape Of Engine Nacelle

Wed Jul 03, 2002 11:30 am

Well thats a 737 engine, and all post 200 series have that shape of engine.
I think its just simply due to the low sitting of the 737, and thus the wing to ground clearance is so small that they need to have the engine nacelle flatter at the bottom to keep clearance from the bottom of the engine to the ground.

Sure, they could of wacked the CFM 56 in a round engine nacelle, but then I doubt it would fit under the wing. So, Im pretty sure thats all it is.
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RE: Shape Of Engine Nacelle

Wed Jul 03, 2002 12:13 pm

Nothing wrong with the picture. That's the shape of the engine on the 737-300 to 500 aircrafts. The 737NG have a more rounded engine nacelles, but is still a little "flattened out" on the bottom.
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RE: Shape Of Engine Nacelle

Wed Jul 03, 2002 12:32 pm

I was told that the flat bottom on those nacelles was due to the placing of the accessory gear box on the side of the engine instead of the bottom. Was this to give more clearance? All NG 737's have round nacelles. The KC-135R also has round nacelles with the same CFM-56 engine. It was a mechanic on the KC-135 that told me that the placement of the gear box was the reason for the flat bottom on the 737 and that it was on the bottom on the KC-135 and that was why it had the round nacelle.
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RE: Shape Of Engine Nacelle

Wed Jul 03, 2002 1:08 pm

The oval shape you speak of is where the accessory gearbox sits. The CFM56 flatened inlet is purely for ground clearance, although I once had a pilot ask me on a preflight, "how are the blades made shorter at the bottom?"

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RE: Shape Of Engine Nacelle

Wed Jul 03, 2002 11:31 pm

Rmm, I hope you are joking!
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RE: Shape Of Engine Nacelle

Thu Jul 04, 2002 4:15 am

Dear Bjones, the CFM56's on the 737 classic and KC-135R are not anywhere near the same engines. The CFM56-2 on the KC-135R has a 68 inch diameter fan and would never fit on a 737. The CFM56-3 on the 737 classic is only 60 inches.

The different versions of the CFM56, the -2, -3, -5 and -7, have four different fan diameters. Only the -2 and the -5A and 5B have identical fan diameters at 68 inches. The -3 is 60, the -5C is 72 and the -7 is 61 inches.

And there is a load of sub-variants with quite substantial differences including varying number of compressor and turbine stages. The largest CFM56, the -5C4, is almost twice as powerful as the smallest CFM56.

CFM56 is rather an engine family than an engine type. Have a look at http://www.cfm56.com
Always keep your number of landings equal to your number of take-offs
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Joined: Sat Feb 16, 2002 11:46 am

RE: Shape Of Engine Nacelle

Thu Jul 04, 2002 8:03 am


Thanks for the info. I knew the engines were somewhat different as they had different rated thrusts. But did not realize that they were that different in fan size.

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RE: Shape Of Engine Nacelle

Thu Jul 04, 2002 12:04 pm

Bjones is mostly correct. I've never worked on Boeings, but went to GE's Line MX Training for the GE CT-7 in Cincinnati. Training for every engine that GE builds is there. Our Instructor explained to us that when Boeing selected that engine there was concern for ground clearance. So GE redesigned the accessory gear case to "flatten' out the bottom.

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