8.2 Aerodynamic Balances
187
and the so called gauge factor is the ratio:
k =
ΔR/R
ΔL/L
=
ΔR/R
ε
The variation of resistance, due to the deflection is measured by inserting the gauge
as one of the resistance branches in a Wheatstone bridge, but direct measurement
is rarely done. In most applications, four gauges fixed to the deforming element of
the balance constitute the Wheatstone bridge which operates under the forces or the
moments exerted on the beam by the aerodynamic loads from the strut or model
supports. The components of each force are then deduced from relations obtained by
applying the Wheatstone bridge equation, which will not be given here. Thus using
the arrangements shown in Fig. 8.4, it is possible to measure a force applied to an
unknown point from the bending moment in a beam and therefore determine the
aerodynamic forces and moments, including the rolling moment, using two bridges.
For the direct drag measurement, the arrangement in Fig. 8.5a can be used. This
rather poorly sensitive method is not suitable for measurement of low drag, where the
method based on a deforming parallelogram is preferred. This arrangement further
amplifies the forces to be measured and hence the accuracy of the measurement is
increased (see Fig. 8.5b).
(a) Measurement of a force whose
application point is unknown
(b) Measurement of the bending
moment in a section
(c) Measurement of forces and moment
(d) Measurement of the roll moment
Fig. 8.4 Arrangements of strain gauges mounted in a Wheatstone bridge
187
and the so called gauge factor is the ratio:
k =
ΔR/R
ΔL/L
=
ΔR/R
ε
The variation of resistance, due to the deflection is measured by inserting the gauge
as one of the resistance branches in a Wheatstone bridge, but direct measurement
is rarely done. In most applications, four gauges fixed to the deforming element of
the balance constitute the Wheatstone bridge which operates under the forces or the
moments exerted on the beam by the aerodynamic loads from the strut or model
supports. The components of each force are then deduced from relations obtained by
applying the Wheatstone bridge equation, which will not be given here. Thus using
the arrangements shown in Fig. 8.4, it is possible to measure a force applied to an
unknown point from the bending moment in a beam and therefore determine the
aerodynamic forces and moments, including the rolling moment, using two bridges.
For the direct drag measurement, the arrangement in Fig. 8.5a can be used. This
rather poorly sensitive method is not suitable for measurement of low drag, where the
method based on a deforming parallelogram is preferred. This arrangement further
amplifies the forces to be measured and hence the accuracy of the measurement is
increased (see Fig. 8.5b).
(a) Measurement of a force whose
application point is unknown
(b) Measurement of the bending
moment in a section
(c) Measurement of forces and moment
(d) Measurement of the roll moment
Fig. 8.4 Arrangements of strain gauges mounted in a Wheatstone bridge