§ 18. To determine experimentally the Static Thrust of a Propeller.—Useful for models intended to raise themselves from the ground under their own power, and for helicopters.
The easiest way to do this is as follows: Mount the propeller on the shaft of an electric motor, of sufficient power to give the propeller 1000 to 1500 revolutions per minute; a suitable accumulator or other source of electric energy will be required, a speedometer or speed counter, also a voltmeter and ammeter.
Place the motor in a pair of scales or on a suitable spring balance (the former is preferable), the axis of the motor vertical, with the propeller attached. Rotate the propeller so that the air current is driven upwards. When the correct speed (as indicated by the speed counter) has been attained, notice the difference in the readings if a spring balance be used, or, if a pair of scales, place weights in the scale pan until the downward thrust of the propeller is exactly balanced. This gives you the thrust in ounces or pounds.
Note carefully the voltage and amperage, supposing it is 8 volts and 10 amperes = 80 watts.
Remove the propeller and note the volts and amperes consumed to run the motor alone, i.e. to excite itself, and overcome friction and air resistance; suppose this to be 8 volts and 2 amperes = 16; the increased load when the propeller is on is therefore
80 - 16 = 64 watts.
All this increased power is not, however, expended on the propeller.
The lost power in the motor increases as C2R.
R = resistance of armature and C = current. If we deduct 10 per cent. for this then the propeller is actually driven by 56 watts.
Now 746 watts = 1 h.p.