of proposed aircraft and engine components.
During a test, the model is placed in the
of the tunnel and air is made to flow past the model.
of tests can be run in a wind tunnel.
Some tests are performed to directly measure the aerodynamic
on the model.
The most basic type of instrument used in this type of testing is the
We must measure six components, three forces (lift, drag, and side) and three moments
(pitch, roll, and yaw), to completely describe the conditions on the model.
For some tests, only
three components (lift, drag, and pitch)
On this page, we will discuss an idealized measurement
of the three forces and three moments of a six-component balance.
As shown in the figure, an idealized fighter plane model is suspended by wires
from the ceiling of the wind tunnel.
In reality, there are several different ways to
a model in the test section, and the measuring devices may be located
the model, or on a measuring
outside the tunnel. The choice of location for the measuring devices and the
type of mount affects the
for the model.
In this idealization, there are six gages
labeled A through F, that keep the model correctly oriented inside the tunnel.
The gages could be electronic
that measures a force by the stretching of an electrical element in the gage
and an application of
to the change in electrical resistance of the element.
Or the gages could be simple, mechanical
that measure the force by the stretching of a spring.
A test is conducted in the following manner.
With the tunnel turned off and no air passing
through the test section, the
of the model is determined as the sum of the forces from
gages A, B, and C.
The tunnel is then turned on and air flows over the model. The model generates aerodynamic forces
and moments that changes the readings on the strain gages.
is given by:
L = A + B + C - W
drag (Dr) is given by:
Dr = E + D
The side force Y is:
Y = F
If there is no
rolling moment, the values of A and B are equal. If there is a rolling moment
(RM), the value is equal to:
RM = (A - B) * a / 2
yawing moment, (YM) is equal to
YM = (D - E) * c / 2
pitching moment, (PM) is equal to
PM = C * b
This mounting system is an idealization and the components must be kept perpendicular to
one another to give the proper measurements. For instance, if the model moves downstream
when the wind is applied, then the A, B, and C gages will be measuring some portion of the
drag force in addition to the lift force. This complication explains why most wind tunnel
mounting systems involve rigid tubes and not suspension wires.
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