para kay col8 Flashcards
balances should be ___ & ____
accurate and reliable
things to consider in 3d balance calibration
body axes, wind axes, wind axis components, velocity
types of body axes
lateral, longitudinal, vertical
3 types of forces in body axes
drag, lift, side force
6 COMPONENT BALANCE
Lift (L), Drag (D), Side Force (Y), Rolling Moment (l), Yawing Moment (n) , Pitching Moment (m)
LIFT FORMULA
C+D+E
DRAG FORMULA
A+B
SIDE FORCE FORMULA
F
ROLLING MOMENT FORMULA
(C-D) x b/2
YAWING MOMENT FORMULA
(A-B) x b/2
PITCHING MOMENT FORMULA
E x c
OTHER TYPES OF COMPONENT BALANCE
three component balance, two component balance
an incredibly complicated device, much better left to engineers of balancing than to engineers of tunnels
wind tunnel balance
placed outside the model, inside or outside the wind tunnel test section, always introduce some interference in the wind flow
external force balance
placed inside the model, no interferences, mechanical support is always needed
internal force balance
model mount of external balance
two strut, three strut
model mount for internal or external balance
single strut, sting
compromise between the required maximum load capability of all components and the accuracy required for minimum loads
wind tunnel balance
danger in inner balance
model can unintentionally exceed the ability of the balance
important to use a thorough analysis
external balance
has to bear are primarily a feature of the size and speed of the tunnel test section
estimated loads
if a tunnel size is doubled
platform and loads are increased fourfold
four external balances
wire, frame, yokes and pyramid
usually mounted inverted
wire balances
why are wire balances mounted inverted
so the aerodynamic lift applied to the weight prevents the wires from being removed
much less stable and flexible, used since in the early days of aeronautics, consumes a large area
wire balances
these balances provide mechanisms for changing the AOA and yaw and transmit the model loads down into a system linkages
platform, yoke, and pyramid balances
minimizes deflection in linkage type
rigid strut
interaction loads can be reduce to a small value
choosing proper linkages
can be encountered in high speed flows
deflections
___ is more when three point strut support is exhibited
interference drag
one of the most troublesome problems of wind tunnel balances
lack of rigidity
____ may move the model from the resolving center and invalidate the moment data or nullify the balance alignment
deflections in the balance
must be kept down to where they are negligible or they must be evaluated and accounted for in the data reduction process
deflection
greatest source of deflection
mounting mechanism
solution to mounting mechanism
use high-modulus elasticity materials for the strut, have a deep and stable structure
large number of ____ are the main components of an external balance and are built to have minimal deflection under high load.
levers
knife edges
early pivots in wind tunnel balances
benefits of flexures
may be designed to withstand loads between the couples members in either direction with no loss of motion, practically frictionless and thus reduce symptoms of hysteresis, can survive treatment which is relatively harsh, practically wearproof; remain constant for an infinite period of time
wind tunnel measurements (pressure)
manometer, barometer, pressure transduces
balanced by the weight of the liquid column
manometer
used to measure atmospheric pressure
barometer
converts pressure to an analog electrical signal
pressure transduces
why do we calibrate
ensure accuracy, control error, help assure precise measurement
properties measured during calibration
air speed, floor angularity, turbulence measurement
determines airspeed inside test section
pitot tube
done when there is no model inside test section
setting the speed
usually determined by measuring either static or total pressure
tunnel speed
subscript for bellmouth or settling chamber
L
subscript for the region before testing section
S
usually measured throughout selected regions of the test section with the use of pitot-static tube
dynamic pressure
calculated from a barometric pressure measurement, and a test-section temperature determination along w/ the equation of state
density
variation of q in the working range of the jet
less than 0.5% from the mean, corresponds to a 0.25% variation in velocity
has to be uniform across the cross section
flow
can be calculated using sphere type and claw type yaw meter
flow direction
desirable variation
less than +-0.10 degrees
maximum variation
+-0.20 degrees
measurement of irregularity in the flow
turbulence
used to identify whether the flow is laminar or turbulent
reynolds number
previously used as the primary way to measure relative turbulence of a wind tunnel
turbulence sphere
common way to measure relative turbulence of a wind tunnel
hot-wire anemometry
can be measured using rakes of pitot tube
tunnel boundary layers
