Denman Island, British Columbia
Airspeed Indicator |
This is an airspeed indicator for my car. It provides an indication of the speed of the slipstream relative to the car. In calm winds, at standard atmospheric conditions, the airspeed will be the same as the car's groundspeed, indicated by the speedometer. Discrepancies between the two are caused by a wind, either naturally-occurring or as a result of the slipstream of another vehicle, or by non-ICAO-standard conditions, such as extreme cold.
The airspeed indicator uses a small fan with an integral tachometer output, originally intended for cooling computer CPU chips. At a sufficiently high windspeed, the fan self-generates enough voltage to produce a signal at the tachometer output. A wind speed of about 25 km/h is required to overcome the "cogging" effect of the brushless motor and to generate enough voltage to operate the tach sensor. This is adequate fo use in a car.
The output signal of the tach consists of alternating high-impedance and ground states at a frequency of two cycles per revolution of the fan. With the addition of a pull-up resistor (see schematic below), the signal becomes a train of high and low voltages suitable for input to the tachometer.
The fan for the Airspeed Indicator is mounted behind the grill of the car, ahead of the radiator. Since the mounting, the grill, and the surrounding hardware all affect the airflow through the fan (referred to in aeronautics as "position error"), the mounting must be completed before calibrating the instrument. Any changes in mounting will require recalibration.
Since an automotive tachometer expects to see two pulses per revolution on 4-cylinder mode, it is capable of reading fan RPM directly, up to 8,000 RMP. Experimentation revealed that an airspeed of 160 km/h (easily achieved with 100 km/h groundspeed plus a 60 km/h headwind) produced a fan speed of about 12,000 RPM. Fortunately, the tachometer could be switched to 6-cylinder mode, extending its range to 12,000 RPM.
Calibration was accomplished by driving at a constant speed in both upwind and downwind directions, and averaging the readings obtained. Provided that the runs in both directions were performed within a few minutes of each other, and that the wind was not too gusty, the average reading could be associated with the car's groundspeed. Similar calibration runs were made over a broad range of groundspeeds. This provided a table of RPM v. windspeed data pairs, which were then used to calibrate the tachometer dial.
The fan's design speed is around 1,000 RPM. I have operated the airspeed indicator for several months, with typical highway speeds producing fan speeds of 8,000 RPM for extended periods of time. This includes numerous hours at or near 12,000 RPM in headwinds. So far, I have observed no harmful effects from this extended high-speed use.
Home
Copyright © 2005 Keith Walker
Last modified: 3-Jan-2005