Co-ordinate system and Angles#

In module 3, four distinct axes sets will be utilised; body, wind, stability, and earth axes. For this module, aircraft body axes is the only one required. Hopefully aircraft body axes have been covered in previous courses but a bit of revision never hurts.

Aircraft Body Axes#

Aircraft body axes is a right-handed Cartesian axis set, centred at the aircraft centre of gravity. \(x\) is defined positive along the aircraft longitudinal axis, positive forward. \(y\) is positive over the starboard wing. \(z\) is positive down, in accordance with the right hand rule.

Along each of the \(x, y, z\) axes the forces, moments, and velocities can be summarised:

Direction

Force

Linear Velocity

Description

Moment

Moment Coefficient

Angular Displacement

Angular Velocity

Nondimensional angular rate

Description

\(x\)

\(X\)

\(U\)

Fore/aft

\(L\)

\(C_\ell\)

\(\phi\)

\(P\)

\(\bar{p}\)

Roll

\(y\)

\(Y\)

\(V\)

Sideward

\(M\)

\(C_m\)

\(\theta\)

\(Q\)

\(\bar{q}\)

Pitch

\(z\)

\(Z\)

\(W\)

Heave

\(N\)

\(C_n\)

\(\psi\)

\(R\)

\(\bar{r}\)

Yaw

The direction of positive rotations is defined in accordance with the right-hand screw rule - see the interactive figure below, which enables you to rotate an aircraft model, and click on the legend to show/hide different components.

Aerodynamic Angles#

The aerodynamic angles, angle of attack - \(\alpha\), and sideslip - \(\beta\), will be familiar to you. You might not be so comfortable however, with their actual definitions.

The aircraft flightspeed is a vector, \(\vec{V}_f\), when defined in body axes is:

\[\begin{split}\vec{V}_f=\begin{bmatrix}U\\V\\W\end{bmatrix}=\begin{bmatrix}\left|V_f\right|\cdot\cos\alpha\cos\beta\\\left|V_f\right|\cdot\sin\beta\\\left|V_f\right|\cdot\sin\alpha\cos\beta\end{bmatrix}\end{split}\]

From the diagram above, you can see the sign convention can be remembered by \(V_f\) being drawn between the \(x-y\) axes, and the projection onto the aircraft \(x-z\) plane, \(V_f\cos\beta\), as drawn between the \(x-z\) axes.

This can be recalled as positive angle of attack is nose up and positive sideslip is nose port, as in the aircraft movement with respect to the flightspeed vector.

Warning

Be careful with the definition of \(V_f\) - \(V_f\) is the aircraft velocity vector, and hence defined as moving away from the aircraft. It is not the vector defining the inclement wind - which is actually colinear to \(V_f\) but opposite in sign.