Airspeed measured by instruments is referred to as IAS, indicated airspeed, whether true airspeed is referred to as TAS.
Fig. 1: the anemometer in position in the test room.
The first airspeed measurement device is a small anemometer. It gives airspeed with a precision of 0.1 km/h.
Iíve made an attempt to calibrate it, but it was not very meaningful. There is a link where the experiment is described (in French):
Although I couldnít reach speeds above 30 km/h during the experiment, it looks like the anemometer needs a correction by some ratio, with true speed against indicated airspeed being a linear function:
TAS = 0.8832 * IAS + 0.2
This is quite surprising because I would have expected IAS to be inferior to TAS.
One more thing to say is that the anemometer displays an averaged value of airspeed; it takes around 20 to 30 seconds so that the displayed airspeed gets stabilized.
The anemometer is positioned near the bottom right corner of the test room entrance:
Fig. 2: the anemometer seen from the entrance cone.
In this position, the anemometer is not in the influence range of the tested propeller, when the tested propeller is spinning in static conditions, wind tunnelís door opened.
Also, at this position the TAS should be close to the TAS near the test roomís center line. This will be discussed later in this report.
The anemometer will give me a reference value for airspeed.
Fig. 3: the Pitot tube in the test room.
The Pitot tube is connected to a data logger, this compensate for the low precision (1 km/h).
For the purpose of this experiment, the Pitot tube is fixed on a very cheap device that allows it to be positioned almost anywhere in the test room.
First, Iíve set it near the anemometer and run two series of measurements at increasing speeds, in order to compare the both measurements at similar positions.
Wind tunnelís speed is increased in four steps then shut down. A picture of the anemometerís display is taken every 10 seconds; speed is corrected with the above formula and compared with the logged Pitot tubeís speed.
We can see that the anemometer, as expected, needs some time to reach a stabilized value while the Pitot tube records any change in wind tunnelís speed quite instantaneously:
Fig. 4: Pitot tube and anemometer IAS during 1st run.
Fig. 5: Pitot tube and anemometer IAS during 2nd run.
The maximum speed for the wind tunnel is around 52 km/h, according to the anemometer, used as a reference.
We can see that the Pitot tube seems to display an under estimated speed, according to the reference speed. But in fact, it is hard to conclude because the Pitot tube and the anemometer are not exactly at the same position, the Pitot tube being nearly 2 cm more ďinsideĒ the airflow. At this point, and as shown later, the TAS might indeed be a little less.
So finally, I canít really conclude about airspeed measurement accuracy. Both seem quite coherent. The Pitot tube has this advantage that it can be put in an aircraft for test flights.
Fig. 6: view of the test room.
The entrance of the test room is a square of 30 cm sides. The Pitot tube is positioned under the longitudinal center line, the height is adjustable. The Pitot tube is at the same distance of the honeycomb as the propeller should be for propeller tests.
Iíve made 10 runs, at full speed, starting at 16.4 cm under the longitudinal axis and going up to the center line.
For each run, I compared the Pitot tube IAS with the stabilized anemometer IAS.
The maximal speed of the wind tunnel depends upon the wind tunnelís motor, which is losing power under its increase in temperature. So itís never really twice the same. The following values of IAS are corrected for anemometer reference speed set to 52 km/h. Those are the speed variations at different distances from the center line:
Fig. 7: airspeed distribution in the test room (propeller position).
This result can be compared with the equivalent found in the NACA report No. 73. In this report, similar tests were made for a 1 feet diameter circular section wind tunnel. On the graph below, curve A is at the entrance of the entrance cone, B is midway in the entrance cone and C is 5 cm behind the exit side of the entrance cone, so in a similar position as for the homemade test runs.
Clearly, we can see that airspeed is disturbed and very fluctuant, while NACAís model is very steady. More runs are required, but this is probably due to the poor quality of the honeycomb, the very short entrance cone and, maybe, to the effect of vibrations of the Pitot tubeís silicon tubes.
The airspeed measured at 10.3 cm of the center line is somewhat unexpected and strangely low compared to airspeed measured at 13.4 cm and 5.9 cm. Looking at the data from the logger we can see that itís also the run with the lowest disturbance.
The blue line oscillates mostly around 48 to 49 km/h while the two others have 3 km/h width oscillations.
Fig. 8: loggerís data showing airflow disturbance.
No runs were made at longer distance downstream the honeycomb, except one, with the Pitot tube on the center line, 30 cm from the honeycomb. The speed shows a little increase at this point, compared to the initial position, with a bit more than 53 km/h. This is coherent with what we can see on the D and E curves on the NACAís graph (D is 20 cm downstream the honeycomb, E is 46 cm).
Fig. 9: Airspeed distribution in the 1 foot model (NACA report No. 73).
The results of this experiment show that both the anemometer and Pitot tube give coherent value of airspeed.
Steadiness of the airflow should be improved, or maybe airspeed measurement can be improved (lower down silicon tubes vibrations?).
This report gives good references for incoming propeller tests.