Jet Stream 500
14
Analyzing the test results:
The wind speed is displayed on the LCD panel as a measured wind speed. The static pitot
tube in the test area takes a sample of the active and passive air pressures and passes them
to a sensor that estimates the wind speed. This speed is used for two reasons. One is to
display the wind speed for your reference, and the second reason is for the controller to
control the speed of the fan. Without the wind speed control, the controller would not be
able to compensate for test variations and fluctuations in line voltage.
The Lift and Drag force calculations are measured with sensitive electronic sensors that
are capable of measuring up to 1.8 lbs. The measurements are averaged to remove
possible errors due to mechanical vibrations. As noted earlier, since the model exerts its
own weight on the sensors, this force must be subtracted from the measured force when
the wind speed is 0. The wind speed and drag force is also automatically zeroed at this
time to automatically calibrate the electronics before every test. This is to insure accurate
and consistent results on every test.
The L/D ratio is derived by dividing the lift force by the drag force. This measurement is
an important parameter in the study of airplane wing design. As you can see, if the lift
force is high and the drag force is low the model wing would be very good for gliders.
Inversely, if the lift is low and has a large drag, it would be very poor in a glider design.
The larger the L/D ratio the longer a glider can stay up. Varying the angle of attack or
placing the wing into a stall condition will vary the L/D for a wing. The L/D ratio can be
positive or negative depending on the direction of lift. If the lift force is in the positive
direction (upward) the L/D ratio is positive, but if the lift is negative (downward) the L/D
ratio will also be negative.
Adjusting the Angle of Attack:
The angle of attack can also be adjusted either before starting a test or while a test is
underway. To adjust the angle, wind speed should be zero before making the adjustment.
If adjustments are made with forces exerted on the model, the adjustment knob may not
operate properly. Be sure the wind speed is 0 before adjusting. The test arm may be
adjusted +/- 30
by turning the adjustment knob in front of the test bed. To make the
adjustment, gently push the knob in to align with the test bed and rotate the knob to the
desired angle. Lift and drag forces are directly affected by the angle of attack adjustment.
This is an excellent tool to demonstrate the effects of stall velocity and stall angle by
adjusting the angle until stall occurs.
Stopping a test:
Pressing the
key will return the wind speed to 0 MPH. Once the wind speed has
returned to zero, the test bed may be removed to substitute test models. If the tunnel is
not to be used for extended periods of time or the supervisor is leaving the room, be sure
to turn off the tunnel's main power to prevent unsupervised operation of the tunnel. As
the wind tunnel is capable of strong wind speeds, a supervisor must be present during
operation to prevent injury.