1961 West Side Drive, Rochester, New York 14624.
Retired Project Engineer in charge of air flow and pressure standards for the Taylor Instrument Cos. and author of 'The Engine Wasn't There', 'Of Engines, Governors and Nomenclature', and other articles.
THE BAKER FAN
The Baker Fan was developed by the Baker Co. as a load for running-in new engines and not as there seems to be a tendency to expound at the Reunions, as a device for the measurement of horsepower. There are two reasons why these fans are inadequate for such use. First, their performance is air density dependent, and second, the speed is relatively insensitive to the power input.
At higher elevations, the air is rarefied (less dense) and the fan spins more easily. Therefore, the power input to the fan would be less for the same rate of turning.
The normal air pressures at different altitudes are well known. From those values, I have calculated the change in air density with altitude and, in turn, the change in the amount of power required to drive the same fan at the same speed at three specific locations. If 50 horsepower were required to drive a fan at a particular speed at Rochester, N. Y. altitude (510 ft.), then only 48 horsepower would be needed drive the same fan at the same speed at Olean, N. Y. altitude (1438 ft.) and 40 horsepower would suffice at Denver, Col. altitude (5219 ft.).
Minor biasing of the performance of such fans is also caused by day-to-day fluctuations in barometric. These changes would generally be of the order of less than 3 percent either plus or minus from the norm.
That the fan is subject to the biasing effect of air density changes is only one of the shortcomings as an indicator of power input. The power consumption of such a fan varies (at least as a first approximation) as the cube of the rotational velocity. Expressed in non-mathematical terms, as a more powerful engine tends to turn the fan faster, the resistance of the fan to being turned increases sharply and the more powerful engine fails to turn it very much faster than a less powerful one.