Definition of aircraft longitudinal axis?

1. I know that longitudinal axis connect nose and tail, but at what exactly point? What definition says?

I have a glider and I want to draw with pen longitudinal axis to measure angle of incidence for wing and stabilizer, but I don't know where exactly I need to draw longitudinal axis.

2. Is airflow parallel with longitudinal axis during cruise, for well design real aircraft(to reduce fuselage pressure drag)?

• Principally, the axis is set by the manufacturer. But there are some choices which are preferable over others. For flight mechanics, it is good practice to use the main inertial axes for the coordinate axes. Commented May 16, 2021 at 17:23
• That sounds like an answer-- Commented May 16, 2021 at 17:45
• @quietflyer … but is too short. Since I have enough reputation, I leave it to others. Why did I write it then? I was not satisfied with the existing answer Commented May 17, 2021 at 5:15

Aircraft designers use many different axis frames, depending on what is the most convenient under the circumstances.

In the context of the question, I need to mention only two of them, or rather even - two classes of them: body axes and wind axes.

The body frame is tied, well, to the body. It is rigidly fixed to the body and moves with it. By convention, the X axis points "forward", to the nose.

The wind frame is aligned with the streamflow "at the infinity", and its X axis points against the free stream. If you rotate the wind frame by the wind angles - angle of sideslip and angle of attack - you get to the body frame.

The term "longitudinal axis" refers specifically to the body X axis.

But where is the origin? It depends!

For most calculations, especially in flight mechanics, it is best to put the origin at the centre of mass. Furthermore, the direction of the longitudinal axis, over which the designer has some liberty (that arbitrary "forward"), should be such that it formed (one of) the principal axis of rotation.

This is not always easy, nor necessarily convenient. Centre of mass moves, the aircraft may not be symmetric, and the airframe designer will not like the reference axes moving about.

To avoid ambiguity, it is often necessary to state in which system the calculation is done, and how exactly it is defined. Oftentimes, engineers in a certain sub-field (e.g. flight mechanics) assume a certain frame and understand each other well, but this assumed knowledge is not carried to other fields, leading to errors and misunderstandings.

The situation is exacerbated by the fact that in many practical cases, the difference between the frames seems to be small, and the result may be "approximately correct" in most of them.1

So, as a designer, you decide how you define the longitudinal axis. If the aircraft is relatively symmetric, draw it as the axis of symmetry.

Your choice will affect how you read angle of attack (AoA). There is nothing strange about that: AoA of the aircraft is also "conventional": it is with respect to an essentially arbitrary axis. It will likely be different to the AoA of the wing, which is of the most concern for the aerodynamicist. (One of the conventions in pure aerodynamics is to choose the X direction such that it resulted in zero lift at zero AoA).

Whether the airflow will align with the axis at cruise depends on your choice. You will need to choose the angle of incidence of the wing to minimise drag, but the AoA may not be zero if you chose the axis direction with other considerations in mind.

Note that the choice of origin will not affect the wind angles. Essentially, you can leave the choice of origin until you need to decide on the other two axes.

1 A common example: Lift is defined in the wind axes. It is normally linearly dependent (~proportional) on AoA, but this only holds perpendicular to wind, and not with respect to "up" of the aircraft. But the difference is small for small angles.

The longitudinal axis of an airframe on the drawing board is distinct from the longitudinal axis of the plane in flight.

On the drawing, it is an arbitrary reference line drawn by the designer. Typically it connects the most forward part of the nose with the most rearward part of the fuselage tailcone. Any angle of incidence of the wing and/or tailplane is referenced to this line.

In flight, it passes through the centre of mass but remains parallel to the design reference axis. It may or may not align with the forward direction in normal flight.

The axis in flight may be close to the line drawn by the designer, or it may not. There is often some displacement of the designer's axis from the center of mass, for example if the engine is mounted above a high wing as with many flying boats.

Usually the designer tries to get the direction as close as possible in order to minimise fuselage drag, which answers your second question.

• This "aligns with the forward direction in normal flight" is not quite correct. It would be for the wind or trajectory axis frames, but these "X" axes are not actually called "longitudinal". Longitudinal is necessarily a body axis, and this means its direction does not necessarily align with the direction of flight.
– Zeus
Commented May 17, 2021 at 7:35
• Thanks, I have updated my answer accordingly. The relation to the CG position on the design drawings vs. in flight is still a significant one, especially for pitch trim. Commented May 17, 2021 at 11:00

Per the Pilot's Handbook of Aeronautical Knowledge: Longitudinal axis. An imaginary line through an aircraft from nose to tail, passing through its center of gravity. The longitudinal axis is also called the roll axis of the aircraft. Movement of the ailerons rotates an airplane about its longitudinal axis.

1. The aircraft's coordinate axes originate from its centre of mass (gravity).
2. The longitudinal axis is not necessarily parallel to airflow, as its angle with airflow is angle of attack (AoA).
3. The roll axis is perpendicular to the other two axes with its origin at the center of gravity, and is directed towards the nose of the aircraft. A rolling motion is an up and down movement of the wing tips of the aircraft.

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