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We normally check the aircraft CG along the longitudinal axis and the lateral axis (sometimes referred to as the transverse axis) to ensure it is within limits. Until recently I was unaware of any aircraft that required a check and control of the CG along the vertical axis. None of the aircraft (B727, B747, DC-8, DC-9, L1011) that I have been involved in weight & balance detail for have required a vertical CG check. Recently, however, I have found that some B767 freighters do.

My question is: Are there aircraft other than some 767s that require a vertical CG check?

Edit in response to a comment: My purpose in asking the question was to learn how others might have handled a vertical CG check. You can see the method I used by going to http://terryliittschwager.com/WB/B767/load.html and searching the page for 'vertical'.

Additional edit in response to additional comments: Some comments have expressed the opinion that there is little or nothing that can be done about the vertical CG. In practice, the vertical CG of cargo (and thus the airplane) is easily affected by specifying which ULDs (unit load devices—the so-called "igloos") will be used for loading. ULDs come in various standard heights. 96", 64", and 45" are oft used heights but there are others, and an array of custom heights. A usually safe assumption is that the mean vertical CG across ULDs is no higher than half their height. In the case of the 767, the published weight & balance documentation gives tables for average CG heights of 42" and 36".

Terry
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  • I would expect this more on low-wing cargo aircraft like the Boeing 747 Dreamlifter, Aero Spacelines Super Guppy, or A300-600ST Beluga. I can't find any SOP's for any of those aircraft to verify though. – Ron Beyer Oct 14 '16 at 01:57
  • Helicopters are sensitive to vertical CofG and in general, should have the CofG under the rotor mast and as low as possible. This is to reduce the likelihood of dynamic rollover. A higher CofG also reduces roll authority. That said, I am not aware of any helicopter that includes it in the W&B calculations. – Simon Oct 14 '16 at 07:00
  • @Simon That's probably due to there being little you can do to adjust the vertical CG with cargo/fuel. – KorvinStarmast Oct 14 '16 at 13:12
  • @KorvinStarmast There is actually a lot you can do, mainly not taking off. The lack of the ability to adjust it doesn't necessarily mean its not important to verify within limits. The reason it is probably omitted is because, most likely, in any loading configuration that is within the aircraft maximum weight limits would be inside the vertical CG limits given the cabin configuration. – Ron Beyer Oct 14 '16 at 15:17
  • @RonBeyer You can't do anything substantial about the vertical CG location. What you can do is not drag your skid or main mounts, laterally while taking off. – KorvinStarmast Oct 14 '16 at 15:24
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    @KorvinStarmast You can't modify the vertical CG, but you can elect to not take-off in an unstable aircraft, and you need a way to verify that if it is applicable. – Ron Beyer Oct 14 '16 at 15:31
  • @RonBeyer I realize we are separating the fly turds from the pepper here, but if that aircraft is certified to fly by FAA or CAA or EASA, then the "it's an unstable aircraft" has already been addressed via the development and cert process, has it not? – KorvinStarmast Oct 14 '16 at 15:33
  • It has a published/certified design envelope, yes, but that doesn't mean the operator hasn't loaded it incorrectly causing it to be unstable... For aircraft that have vertical CG limits that can be exceeded within MTOW, you need to be able to verify you haven't exceeded them, otherwise the aircraft will have controllability issues, like the 767 Freighter aircraft Terry mentions. – Ron Beyer Oct 14 '16 at 15:36
  • I found examples in the first few entries in google – user6035379 Oct 15 '16 at 19:01
  • @user6035379 What search string did you use? – Terry Oct 15 '16 at 19:33
  • @RonBeyer military aircraft (particularly carrier aircraft and those with external stores) are sensitive to vertical Cg on landing. That is one of the reasons that they have certification limits for bring back of ordnance. While you can't "modify" it, it is not uncommon to jettison heavy ordnance prior to landing and part of that gets the Cg in the right basket (but the bigger driver is landing load on the pylons...usually). – tillmas Dec 23 '16 at 12:01
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    @RonBeyer I wrote the weight & balance software used by the Dreamlifter while Evergreen International Airlines was operating them for Boeing. During that time, at least, there was no vertical CG check required. – Terry Mar 10 '17 at 22:17

4 Answers4

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Since no one mention it I would propose Shuttle Carrier Aircraft

It is mentioned in page 4 of the Space Transportation System Cargo Abort and Recovery Operations: (pdf)

This requires not only removal of the payload but removal of Orbiter main engines, tires, landing gear and other components as well. In addition, the Orbiter Z axis (vertical) and X axis (longitudinal) center of gravity location must be within a limited envelope, as shown in Figure 1.

enter image description here

Related : https://aviation.stackexchange.com/a/23808/679

vasin1987
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In many aircraft the vertical (Z axis) placement is of little impact on weight distribution. However, in some aircraft it affects maneuverability. So for an aircraft like a C-5 it is noticeable, yet a C-5 is not considered an aircraft where vertical CG would be a factor, as the nature of the operations are, well, cargo hauling. In a helicopter, which tends to have a higher (Z-axis) CG with the top placement of the engine and transmission, the considerations are greater. It doesn't impact loading in a normal sense, but there is a noticeable impact in handling particularly when roll moments are involved.

Stated perhaps more simply, static stability is normally bounded by longitudinal and to a lesser degree latitudinal forces. However, a higher CG will impact dynamic stability where the consideration is not primarily limited to wing (rotor) and airfoil loading.

So the simplest answer is that it does make a difference, which is more than just measurable in some applications, but normally the difference is of small consequence and only a tertiary consideration.

mongo
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    This does not answer the question, and provides nothing other than opinion. – Ralph J Apr 02 '17 at 20:35
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    A common consideration of high vertical CGs is the effect of thrust changes on pitch in a fixed wing aircraft. – mongo Apr 17 '17 at 17:50
  • Why no one has mentioned the biplanes and other non-planar wing arrangements for which the z-coordinate of CoG may be more sensitive and thus affect handling and stability characteristics? – ares Jul 31 '17 at 21:50
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As Mongo mentioned, it is generally more important in taller aircraft. However, it must be noted that small aerobatic aircraft also have the operator check the vertical CG. A good example of this is the Extra 300. Look up a POH and check out the way CG is handled in this aircraft - it's really interesting and different than any other I have seen.

Pugz
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Seaplane designers need to take vertical CG into account when designing landing gear. The CG needs to be 10 degrees forward of the step in the hull, so a higher CG needs weight more forward on the landing gear. It's not adjustable as part of operation, but it's necessary for design.

user1493362
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  • In that sense all airplanes with tricycle gear must take vertical CG into account. And they do during design, though the angle is usually taken about 15°. Otherwise, like seaplanes, they would have difficulty rotating on takeoff. – Zeus Jun 10 '21 at 01:00