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Check the MD80's empennage below.

enter image description here

Can easily tell it's not built on exactly centerline by naked eyes.

Is that because aluminum joint or consider aerodynamics?

I don't think it is manufacturing defects or imprecision.

Some of the small aircraft have the same construction like this.

Couldn't find the similar discussion online though.

Vikki
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ReinFore
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  • Welcome to av.se :) –  Mar 16 '17 at 12:15
  • Very nice website, thx :) – ReinFore Mar 16 '17 at 12:41
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    Looking at the picture, I see that the forward portion of the empennage fairing does not line up with what appears to be a joint in the skin structure. However, the joint may be off-center compared to the rivet placement. Looking at the empennage in person may reveal it to be off centerline, but this is not clear to me from the picture. Could you further explain or document how the empennage is not on centerline? – J W Mar 16 '17 at 13:04
  • @Jonathan Walters Good point! I don't figure out that I should compare to the rivet placement. Joint line is not centerline clearly. – ReinFore Mar 16 '17 at 13:11
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    This is a good question, because some aircraft do have off-center (or divergent or angled from the longitudinal axis) empennage. The C172 is an example of such an aircraft; I'm not sure that the DC-9/MD-80 series is. – J W Mar 16 '17 at 13:13
  • @Jonathan Walters Off-center empennage? For better aerodynamics or compensate the other problems? Could you give me more information about this? – ReinFore Mar 16 '17 at 13:19
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    I haven't had my hands on a C172 in a few months, but if I remember right, the built in angle is to counteract some of the left turning tendencies and provide built in trim. The C172 doesn't have cockpit-adjustable rudder trim. – J W Mar 16 '17 at 13:25
  • @Jonathan Walters Oh, I thought manual control the rudder to counteract that turning tendencies on a C172 is the only way. Thx, that help a lot. – ReinFore Mar 16 '17 at 13:29
  • @ReinFore Rudder control is the primary way, but the built in angle and the ground adjustable trim tab also provide trim. – J W Mar 16 '17 at 13:34
  • Another possible reason is the stability of yaw control. Some plane designs had the curious (and unintended) feature that perfectly symmetrical flight was unstable in yaw, but there were two stable attitudes yawing slightly (a fraction of a degree) left and right. Early autopilots sometimes "hunted" between those two stable states while trying to maintain the unstable symmetrical state, and the side-to-side oscillations could make passengers in the rear rows of seats feel a bit airsick. Intentionally breaking the "perfect" symmetry cured the problem. – alephzero Mar 16 '17 at 13:40
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    You said "aircraft", not "planes", so you should look at helicopters because almost all of them are asymmetric by definition : ) – Agent_L Mar 16 '17 at 15:45

2 Answers2

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The vertical stabilizer is on the centerline.

The line you see is not the centerline, it's the overlap of the aluminum sheets that form the circular fuselage.

enter image description here
(Source)

enter image description here
(Source) Non-aviation example of what I mean by overlap and centerline.

On the other hand, propeller aircraft may employ different methods to counter the left turning tendencies.

Tail

The tail group consists of a vertical stabilizer, a horizontal stabilizer, two elevators, and a rudder. The vertical stabilizer is mounted on the aft fuselage, and the horizontal stabilizer is mounted on the top of the vertical stabilizer. The rudder and elevators are mounted on the vertical and horizontal stabilizers, respectively.

The vertical stabilizer is fully cantilevered and sweptback. A scoop for cooling the air-conditioning systems is located on the lower leading edge section and is heated when the Airfoil Anti-ice is operating.

– MD-80 Flight Manual

Back to the MD-80 in the photo, from its manual there is no mention of an offset. I searched for multiple terms such as offset, angled, and off center.

Finally, from an official CAD drawing from Boeing's website:

enter image description here

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Its my first time seeing this kind of defects. For sure its not a defect caused by manufactures. It could be that this aircraft had gone through a skin change procedures. Its the overlapping of two skins that appear to be not center or could be the dorsal fin's anchor nuts not rivets properly i guess so (refer to my experience working with b737-family).

Young soul
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  • this does not add anything to the already existing answer – Federico Mar 21 '17 at 17:06
  • Are you suggesting that this is a defect? Are you familiar with the construction of the MD-80 type? Is this, in fact, an example of defective manufacture? I have seen nothing else that suggests the photo in question depicts a defect; my understanding is that the photo shows proper construction that differs from the expectations of the OP. – J W Mar 21 '17 at 17:09
  • I'm sure this section haven't been modified and other MD82 are the same. Have repairing experience on MD82/83. – ReinFore Mar 21 '17 at 23:36