|
Application of the fan laws for a change in RPM is straight forward. From an existing data set of performance points, simply calculate the ratio of the target speed to the speed at which the initial data was determined. For each data point, multiply the CFM by this ratio. Multiply the PRESSURE by the ratio squared and multiply the POWER by the ratio cubed. In order to determine the performance at a given point, simply use straight line interpolation. Review the first row of the figure below for an example of the calculation for a change in RPM from 1750 to the lower speed of 1700. Notice the calculation is carried out for every performance point. |
|
|
Small changes to the diameter of a given prop result in predictable variations in the performance of the fan blade. These changes can be calculated by determining the ratio of the target diameter to the initial diameter. As in the case of variations of the speed, each performance point must be calculated. For FLOW, use the diameter ratio cubed. For PRESSURE, use the square of the diameter ratio and for the change in POWER, use the diameter ratio raised to the fifth power. Examine the first column of the figure for an example: change from a 30 inch diameter prop to a 29 inch diameter prop.
If multiple performance criteria are changed, simply apply the corrections sequentially to each point as in the lower right corner of the figure above. |