Chapter 8
Measurement of Aerodynamic Forces
and Moments
8.1 The Aerodynamic Forces and Moments
The presence of the flow around a vehicle exerts aerodynamic forces and moments,
which act at the centre of pressure. The problem posed to the aerodynamicist is to
determine these forces and their point of action relative to the centre of gravity, G, of
the aircraft. The equilibrium of the aerodynamic forces is thus reduced to a force and
a moment applied at G (see Fig. 8.1). These are the necessary information required
to calculate the aircraft’s trajectory and its motion around the centre of gravity G.
In the wind tunnel, the loads on a model can be measured by multi-components
balances, with respect to a reference position, the centre of reduction of the forces
whose position must be provided to transport the moment to any other point (frequently the mean aerodynamics centre which is at around the quarter chord position
in the case of standard aerofoil). This allows the calculation of the aerodynamic forces
and moment coefficients which can be therefore extrapolated to the real vehicle.
The aerodynamic coefficients can be decomposed into the Euclidean space, resulting into three forces and three moments as shown in see Fig. 8.2. The decomposition
being done according to various coordinate systems:
– the aerodynamic axis, linked to direction of the headwind,
– the aircraft axis, linked to the aircraft or the model,
– the wind tunnel axis, linked to the wind tunnel in which the tests are carried out,
– the balance axis, linked to the system used to measure the aerodynamic forces and
moments.
Here the aerodynamic coordinate system, whose axes are thus defined, will be
considered:
– the longitudinal axis
− →
X a aligned with the velocity vector
− →
V ∞ , positive forward,
– the vertical axis
Z a normal to the horizontal plane
X a , positive upward,
– the transverse axis
− →
Y a normal to
− →
X a and
− →
Z a , positive towards the right hand
side of the plane (starboard).
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_8
183
Measurement of Aerodynamic Forces
and Moments
8.1 The Aerodynamic Forces and Moments
The presence of the flow around a vehicle exerts aerodynamic forces and moments,
which act at the centre of pressure. The problem posed to the aerodynamicist is to
determine these forces and their point of action relative to the centre of gravity, G, of
the aircraft. The equilibrium of the aerodynamic forces is thus reduced to a force and
a moment applied at G (see Fig. 8.1). These are the necessary information required
to calculate the aircraft’s trajectory and its motion around the centre of gravity G.
In the wind tunnel, the loads on a model can be measured by multi-components
balances, with respect to a reference position, the centre of reduction of the forces
whose position must be provided to transport the moment to any other point (frequently the mean aerodynamics centre which is at around the quarter chord position
in the case of standard aerofoil). This allows the calculation of the aerodynamic forces
and moment coefficients which can be therefore extrapolated to the real vehicle.
The aerodynamic coefficients can be decomposed into the Euclidean space, resulting into three forces and three moments as shown in see Fig. 8.2. The decomposition
being done according to various coordinate systems:
– the aerodynamic axis, linked to direction of the headwind,
– the aircraft axis, linked to the aircraft or the model,
– the wind tunnel axis, linked to the wind tunnel in which the tests are carried out,
– the balance axis, linked to the system used to measure the aerodynamic forces and
moments.
Here the aerodynamic coordinate system, whose axes are thus defined, will be
considered:
– the longitudinal axis
− →
X a aligned with the velocity vector
− →
V ∞ , positive forward,
– the vertical axis
Z a normal to the horizontal plane
X a , positive upward,
– the transverse axis
− →
Y a normal to
− →
X a and
− →
Z a , positive towards the right hand
side of the plane (starboard).
© Springer Nature Switzerland AG 2020
B. Chanetz et al., Experimental Aerodynamics,
Springer Tracts in Mechanical Engineering,
https://doi.org/10.1007/978-3-030-35562-3_8
183