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## Normal Axis Terminology

• Posts: 3

### Alan50 created the topic: Normal Axis Terminology

Hi, I'm not a pilot and have recently started working my way through Bobs Recreational Aircraft BAK book. Pg54 Aircraft Motion refers to the Yaw axis being called the Normal Axis. It goes on to say that Vertical axis is an old term.

I have searched the site for 'normal axis' and can't find any discussion about it. I was hoping somebody could inform me how the normal axis terminology came about and why it has superseded vertical axis.

TIA, Cheers, Alan

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 882

### John.Heddles replied the topic: Normal Axis Terminology

Alan,

Full marks to you for asking the question. A lot of folks would just roll their eyes up, shake their head a bit in shell shock ..... and keep on reading .... It's far better to find out what a word means when you come across it in a context which is important to you.

The tech term "normal" confuses a lot of non-techo folk.

The dictionary meaning for the geometric application is along the lines of -

(Of a line, ray, or other linear feature) - "intersecting a given line or surface at right angles"

So, for this example, we are talking about an axis (ie the line, or direction, about which rotation is occurring) "normal" (ie at right angles) to the (inferred) nose to tail/wingtip to wingtip plane of symmetry. Rotation about this axis is referred to as a yawing motion which, for the flying side of things, creates a sideslip angle where the wind comes on to the nose of the aircraft somewhat displaced to the side, rather than from straight ahead.

Vertical axis would make a lot more sense but that's the world of techo stuff for you. I don't think that anyone is going to get upset if you use "vertical axis" so long as you are aware of the general usage. Just be aware that we can use "normal" generically. For instance, I can say that the lateral axis is normal to the vertical plane so we need to be a little bit careful sometimes. Basically "normal" just means that a line is at right angles to another line or a 2D surface.

One of the traps for techo folks (like Bob and me) is that, despite our best efforts not to fall into the trap, we tend to assume knowledge on the part of students when we ought not to do so.

I'm not a pilot

Ah, but you are working towards that goal so welcome aboard the merry (sometimes, mad) world of aviation.

Engineering specialist in aircraft performance and weight control.
The following user(s) said Thank You: Alan50

• Posts: 3

### Alan50 replied the topic: Normal Axis Terminology

Thank you for your in-depth reply, John.. It will take me some time to get my head around your explanation, and I appreciate you taking the time... Cheers, Alan

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 882

### John.Heddles replied the topic: Normal Axis Terminology

Engineering specialist in aircraft performance and weight control.

• Posts: 3

### Alan50 replied the topic: Normal Axis Terminology

I have found this explanation from an engineering friend helpful:

""A good way to think about it is that all 3 axes need to relate to a characteristic of the airframe. Lateral & longitudinal axis are easy examples of this. Calling the 3rd axis the “vertical” axis is relating it to an external condition.

An aircraft could roll through 90 degrees, & the “vertical” axis would then be horizontal, which would be confusing. But the terminology of normal, longitudinal and lateral axes makes sense no matter what attitude the aircraft is in. The naming convention doesn’t rely on where “up” is.""

I believe I now understand why Vertical is not an appropriate term, so will accept that the term Normal is valid and it is based at a level of Physics I was never exposed to..

Maybe it could be called the Normvertudinal axis John

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 882

### John.Heddles replied the topic: Normal Axis Terminology

With my engineering hat on, I'm afraid that I disagree with your colleague.

The aircraft axes are (aircraft) body axes and the attitude of the aircraft with respect to any other frame of reference is irrelevant to the naming convention. This is what he/she is saying in the first sentence.

Calling the 3rd axis the “vertical” axis is relating it to an external condition.

Not at all. The vertical or normal axis remains vertical to the aircraft horizontal plane regardless of aircraft orientation to any other frame of reference.

An aircraft could roll through 90 degrees, & the “vertical” axis would then be horizontal, which would be confusing

Not at all. The vertical axis remains vertical with respect to the aircraft horizontal plane.

That is to say, it doesn't matter one iota what you call the axes, it is just a convenient convention. Providing that one remembers the convention details, it doesn't matter at all.

I believe I now understand why Vertical is not an appropriate term

So long as you consider that you have your head around it, all is well.

I think your colleague would benefit from a review of mechanics.

Engineering specialist in aircraft performance and weight control.