Product Tip of the Month June 1999

Product Tip of the Month

June 1999

Phototach

Fluctuating Readings at High RPMs

If you are balancing propellers and using the Phototach, you may experience erratic or fluctuating RPM readings at higher RPM settings. Th problem is most likely caused by the Phototach's inability to produce a trigger because the reflective tape is moving too fast. The Phototach must be able to "see" the tape for not less than 300 micro seconds to produce a pulse for timing. If you're using a standard 1-inch width of tape, you must take a couple of factors into consideration. As the distance from the center of the propeller shaft increases toward the blade tip, there is a linear speed increase at any point along that line during steady RPM run. In other words the tape may be moving too fast at 15 inches from the center of the prop shaft, but well within an acceptable speed when attached at just 7 inches out. If you are experiencing this problem, use the chart below to determine the width of tape required for your application.

First, measure the distance from the center of the propeller shaft to the location you intend to place the reflective tape.
In the chart below, select from the RPM column the first speed greater than the speed at which you intend to balance.
From this RPM number, proceed across the chart to the right until you come to the first number larger than the distance measured in Step 1. above.
From this point, follow the column up to the top to the minimum tape width required for your application.

As an example, use the following parameters: the distance from the propeller shaft to the intended tape location measures 25 inches and the balance speed is 2300 RPM.

Select 2400 from the RPM column since this is the first speed greater than your intended balance speed of 2300. From this number, follow the row across to 26.5, which is the first number higher than your intended tape location of 25 inches. From 26.5 follow the column straight up to the top--2 inches. This is the width of tape required for accurate readings at the intended distance and RPM level. (If your reflective tape is only 1-inch wide, place two 1-inch strips of tape side by side to create 2 inches.)

TAPE SIZE/LOCATION CORRELATION with RPM

RPM	1 INCHES	2 INCHES	3 INCHES	4 INCHES

1000	31.8	63.7	95.5	127.3
1200	26.5	53.1	79.6	106.1
1400	22.7	45.5	68.2	90.9
1600	19.9	39.8	59.7	79.6
1800	17.7	35.4	53.1	70.7
2000	15.9	31.8	47.7	63.7
2200	14.5	28.9	43.4	57.9
2400	13.3	26.5	39.8	53.1
2600	12.2	24.5	36.7	49
2800	11.4	22.7	34.1	42.4

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