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Sokes
Topic Author
Posts: 2775
Joined: Sat Mar 09, 2019 4:48 pm

Wingboxes

Thu Feb 18, 2021 9:30 am

Why do wingboxes often project the fuselage so much?

Image
https://en.wikipedia.org/wiki/Airbus_A320_family

Image
https://en.wikipedia.org/wiki/Airbus_A330

Image
https://en.wikipedia.org/wiki/Airbus_A380

But it's not always so extreme.
Image
https://en.wikipedia.org/wiki/Boeing_737

From the area rule I would expect that a wingbox should project the fuselage as little as possible. Actually the fuselage should project the wingbox. But I assume that makes the joint complicated. Was it ever done for a pressurised aircraft?
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tommy1808
Posts: 14563
Joined: Thu Nov 21, 2013 3:24 pm

Re: Wingboxes

Thu Feb 18, 2021 9:53 am

Sokes wrote:
From the area rule I would expect that a wingbox should project the fuselage as little as possible.


Wave drag is lowest when the cross section area is the lowest, but to avoid creating shock waves causing drag you want those area changes to be smooth as well. It would appear it is just worth hauling the extra structure around, and the cost to build it, since the 737 would otherwise look bad drag wise. Much, much improved mathematical models and orders of magnitude more computing power to drop on those also makes a lot easier to find the optimum shape. If you compare those over time it would also seem that the involved shapes get a lot more complicated the newer the air-frame is. The A350 for example as a step in the change that almost seems like someone extracted some extra lift from it.

best regards
Thomas
Well, there is prophecy in the bible after all: 2 Timothy 3:1-6
 
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Starlionblue
Posts: 20698
Joined: Fri Feb 27, 2004 9:54 pm

Re: Wingboxes

Thu Feb 18, 2021 10:23 am

- Wingbox: The structure that joins the wings and the fuselage. This is not visible from the outside.
- Wing-body fairing: The aerodynamic fairing that covers the wing/fuselage join.

The area rule does not imply that the wing-body fairing should not be proud of the fuselage ("project from"). The area rule states that the same cross-sectional area distribution gives the same wave drag, and that cross-sectional changes should be smooth.

Effecting said smooth cross-sectional change is done mostly by fine-tuning the shape of the wing-body fairing, the nacelles, or the flap-track fairings.

As tommy1808 says, you want changes to be smooth. This is in order to decrease interference drag.
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