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I understand the definition of aircraft 'Stability-Axis', but I am unsure as to the circumstances in which forces or moments would be expressed in this axis system. Is there a convention?

What I do know:

  • It is useful for wind tunnel analysis, where forces and moments are often calculated from a force balance fixed to the aircraft that is invariant with model Alpha and Yaws with the aircraft for variations in Beta. (Obviously depending on windtunnel/model set-up).
  • It can be used as an approximation for Wind-Axis is Alpha is known and Beta is assumed to be small.

What I don't know:

  • Many things
  • Why Stability-Axis is used instead of Body-Axis or Wind-Axis?
William
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1 Answers1

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Body-axis is used to construct the equations of motion and express the aircraft angular velocity and Euler attitudes. By conventional, it is defined such that the x-axis is aligned with the nose and the x-z plane is the plane of symmetry.

Stability-axis, whose x-axis is aligned with the longitudinal incidence of the flow against the body-axis, is used to express aerodynamic data, which are derived from semi-analytical methods (e.g. DATCOM, ESDU), computational methods (e.g. vortex-lattice methods, Navier-Stokes), wind tunnel testing, or full flight data.

Stability-axis is used for aerodynamics because lift and drag are conveniently aligned with the axes. And as the OP alluded to, it is much easier to rotate the aircraft against the wind tunnel balances, than to rotate the flow against the test model.

Wind-axis has its x-axis aligned exactly with the flow velocity vector. It's not really used anywhere. We don't use wind axis because it breaks the x-z symmetry and makes things more complicated than need be. It does not offer any additional benefit than stability-axis.

enter image description here

Image from: https://www.mathematica-journal.com/issue/v8i2/features/codegeneration/contents/html/Links/index_lnk_3.html

JZYL
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  • Thank you for your answer! That has helped to explain; the symmetry argument definitely makes sense. A further query about the image you posted - lift and drag would conventionally be given in stability or wind axis system. Why is the wind/stability axis indicated to be positive nose-forwards, when positive drag is aft (and also lift, down)? – William Oct 10 '19 at 19:31
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    @William Drag is negative of the "forward" motion, and lift is negative of the "downward" motion, in the stability axis. That's just the convention agreed upon by the industry. – JZYL Oct 10 '19 at 21:46