midddd Flashcards
what are the Six-Component Balance
LIFT
DRAG
Side force
Rolling Moment
Yawing Moment
Pitching Moment
formula for lift in a plane wire balance (L)
C+D+E
Drag formula (D)
A+B
Side Force (Y)
F
Rolling Moment (l)
(c-D) x b/2
Yawing Moment (n)
(A-B) x b/2
Pitching Moment (m)
E x c
The two reference frames used
frames of the body axis
frames of the wind axis.
A thii
frame of reference is called
stability axes.
In wind axes, Note
that drag is in ______, direction
negative x
in wind axes lift is in______,
negative z
direction is in ________direction
is the side power.
positive y
_____have x, pointing in the sky, z, pointing down, and y pointing in the sky
towards the right. If the test segment is not horizontal then a suitable local convention needs
to be adopted. We view vectors which indicate components of forces used in_____
wind axes
______ are bound to and travel together with the model.
Axes of the Body
VERTICAL FORCES in skew
Z + A sin(epsilon) - C Cos(delta) - E = 0
HORIZONTAL FORCES in Skew
X+ A cos(epsilon) - C sin(delta) = 0
Z axis skew
m + Ec = 0
3 component balances:
LIFT
DRAG
PITCH
2 component balance
LIFT
DRAG
A wind tunnel balance is an incredibly complicated device and its design and
construction are much better left to_______
engineers of balancing than to engineers
of tunnels.
They are placed outside
the model, inside or
outside the wind tunnel
chamber test section, but
they always introduce some
interference in the wind
flow.
External balance
They are placed inside the
model; thus, no interferences
are introduced in the wind
flow by the balance
components, but a mechanical
support for the model is
always needed to maintain it
in the test chamber and change
the model orientation if
desired.
internal balance
Enumerate Model Mounts
Two Strut
Three strut
Single strut
sting
Any wind tunnel balance is of necessity a
compromise between the_________ and _______
required maximum
load capability of all components and the
accuracy required for minimum loads.
For an internal balance, the solution to this
problem is much easier, as many balances can be
planned and installed for various load ranges, and
one chosen for the desired loads. A danger with an
inner balance is that the loads of the model can
unintentionally exceed the ability of the balance.
For external balance
For an external balance it is important to use a
thorough analysis to try to account for and form of
model and test for which the balance will be used
over a number of years.
The estimated loads a balance has to bear are
primarily a_______
feature of the size and speed of the
tunnel test section.
If a tunnel size is doubled, the size of the
platform and the loads are increased ______ for a
set vehicle-to-tunnel-size ratio, platform attitude
and tunnel speed.
fourfold
In general usage four types of external
balances were used. These balances are______
main load-carrying
members-wire, frame, yokes and pyramid.
The model was usually mounted inverted so that the
aerodynamic lift applied to the weight to prevent
the wires from being removed, as the resulting
stress can never be reduced to zero.
Wire Balances
There’s a strong tare drag on the wires that was
hard to accurately determine. The wires appeared to
split, which would result in the model losing.
Wire Balances
turned out to be much less stable and
flexible than the alternatives and since the very
early days of aeronautics have not been used
extensively.
● The space occupied is very large.
Wire Balances
These balances provide
mechanisms for changing the
angle of attack and yaw and
transmit the model loads down
into a system of linkages
that separate them into force
and moment components.
Platform
yoke
pyramid Balances
One of the most troublesome problems of wind tunnel
balances is the
lack of rigidity
______in the balance may move the model from
the resolving center and invalidate the moment data
or nullify the balance alignment so that part of the
lift appears as drag or side force
Deflections
_______must be kept down to where they are
negligible or they must be evaluated and accounted
for in the data reduction process.
deflection
is the greatest source of
deflection.
mounting Mechanism
The only way that the wind tunnel engineer can solve mounting mechanism is
to use high-modulus elasticity
materials for the strut.
Early pivots were _______in wind tunnel
balances.
Knife edges
Because the edges
of the knife can be damaged
by shock loads and can bear
loads only in a compressive
direction, they have been
replaced by____
Flexure pivots
benefits of flexures
The benefits of flexures include:
1. They may be designed to withstand loads between
the coupled members in either direction with no
loss of motion.
2. They are practically frictionless and thus reduce
the symptoms of hysteresis.
3. They can survive treatment which is relatively
harsh.
4. They are practically wearproof; therefore they
remain constant for an infinite period of time.
Calibration takes at least ____ months for a
professional crew to calibrate the a new balance.
This time frame presupposes that sufficient shop
facilities are required for all kinds of
improvements, and acknowledges that several
modifications would almost certainly require the
first “calibration.”
3
involves loading the balance components
to see if they read what they should, testing the
system deflection, calculating the natural frequency
of the balance in order to prevent resonance, and
applying fluctuating load
Calibration
types of internal balance or type of transducers
strain gauges
piezoelectric devices
is compounded by
several items that allow the conversion of physical
forces into digital values that can be managed by a
computer. Several components comprise the DAQ andthe
selection of their properties will have an important
impact on the system behaviour.
Data Acquisition system
Main components of DAQ
● Mechanics
● Sensors or transducers
● Weight balanced sensors
● Pneumatic and hydraulic sensors
● Electromagnetic sensors
● Spring sensors
● Strain-gauge sensors
● Electronic amplifiers
● Wires
● Analog to digital converter (ADC)
● Software
try again
mechanics
Sensor or transducers
Weight and balance sensors
hydraulic and Pneumatic Sensors
adc (analog to digital converter)
software
wires
string sensors
strain gauges sensor
electronic amplifiers
electromagnetic sensors
why strain gauges are much used
all though piezos are rigid, steady loads are not well calculated
calibration purposes
loading the balance to show
deter.. coefficients of equilibrium
defining deflections as load features
to check repeatability in short prdt of time
test reliability in long prdt of time
defines sensitivity or mnm response loads
to detect hysteresis
are used to apply the loads. These
bars are indexed so that various values of moments
can be applied while holding the normal or side
force constant.
calibration bars
MODDDDD
4444
Pressure is balanced by the
weight of the liquid
column.
Manometer
Used to measure
atmospheric pressure.
Barometer
Converts pressure to an
analog electrical signal.
Pressure Transducers
Why do we calibrate
To ensure Accuracy
To control Errors
Help assure precise measurements
Properties are to be measured while calibration of test section
Airspeed
Flow Angularity
Turbulence Measurement
Setting the speed of the wind tunnel is done when?
no model inside the test section
can be put inside the test section to determine the airspeed
pitot tube
The tunnel speed is usually determined by
measuring either static or total pressure in
the settling chamber in front of the
contraction tube, as station L indicates,
and a static pressure in front of the test
P sub L + Q sub L = P sub S + Q sub S - Ksub1 x Q sub S
is usually measured
throughout selected regions of the test
section by means of a pitot-static tube.
Dynamic Pressure
is calculated from a
barometric pressure measurement, and a
test-section temperature determination
along with the equation of state.
Density
local velocities are then obtained
from:
V = sqrt of (2q/p)
may be added in the largest section
of the tunnel with spatial mesh densities
varied so that they are more dense in the
sections that correspond to high-velocity
regions in the jet.
Screens
The flow has to be uniform across the cross section with
________ when the test
section is empty.
all stream lines parallel to the tunnel axis
The flow direction can be calculated by using
Sphere type and claw type yaw meter
It would be desirable to have the variation less than
____. but it is often necessary to accept the best values
that can be achieved. The maximum variation should be
held to ____.
+-0.10°. an +-0.20°.
is a measurement of irregularity in the flow.
Turbulence
is used to identify whether the flow is
laminar or turbulent.
Reynolds Number
was the primary way to measure the
relative turbulence of a wind tunnel.
turbulence sphere
commonly now used to measure the relative turbulence of a wind tunnel
hot-wire anemometry
The critical Reynolds number
for the sphere can be measured
in two ways.
One method is to plot the
measured CD based on
cross-sectional area versus
Reynolds number,
is to
take the average of the
four pressures on the
aft surface of the
sphere and subtract this
value from the
stagnation value
The critical Reynolds number as defined by either force
or pressure measurements is then used to define a
_______ for the tunnel by comparing the
tunnel’s critical Reynolds number to the atmospheric
free-air Reynolds
turbulence factor
can be measured using
rakes of pitot tubes (left figure), which are specially
configured for the purpose or traversing devices (right
figure) fitted with pitot tubes, static tubes, or hot wire
tunnel boundary layers
