MURPHY: POWER OF A’ TWELVE FOOT ‘POWER’ WINDMILL. 193 
The telation between load and wind velocity in (3) is for the 
useful load only. The mill also carries a useless friction load, to 
be considered further on. Now, if this useless load could be made 
to decrease as x decreases and become zero when x is zero, that is, 
the mill start in the lightest breeze then the curve AK would pass 
through the origin where x is zero and y is zero and be of the form 
x?==b/y (7) 
/ Most of this curve would differ little from the actual curve AK. 
Equation (3) would then be of the form 
bl'x (8) 
and (6) of the form yee kv x Ory x kv (g) 
that is, for the curve AK of the ideal mill the velocity of the wind 
wheel and also the useful load on the mill increases uniformly as 
the wind velocity increases. This curve shows the maximum power 
of the mill for any wind velocity, and in order for the mill to furnish 
it, it is necessary to increase the load on the mill ata rate equiva- 
lent to one pound on an arm of 35} inches, or an amount such as 
to yield rrz feet pounds perrevolution. It will then be found that 
the velocity of any part of the wind wheel is increasing at a rate 
given by the equation Vay RX (10) 
} R being: the distance in feet from the part to center of wheel, and 
vy and x being in feet per second. 
THR LOAD “CURVES (CR) ES AND Hs 
These curves are seen to resemble parabolas whose axis is par 
such a curve is of the general form 
by* COD) 
] allel to axis x. The equation of 
a4 and b are constants: for either curve. 
For the curve CR we have y==.2 when x19 And Vee sz wien 
tants. Hence a=8 and y==52 approximately and: (rt) becomes 
8--52y" (2) 
x being in miles per hour and y in horse power. 
If this equation be plotted it will be seen that it coincides with 
CR very nearly to about x==<20, bevond this itis above CR, and the 
distance between them increases with «ihe reason why C.Roitalls 
below the parabola is due to the reduction of wind. area or govern- 
{ ine of the mill, | The wind ts constantly changing its direction and 
ee 
) the wind wheel does not respond quickly to those changes of direc 
q . . . . 
} tion. It is pulled into the wind by a spring fastened to the tail, 
i and the quickness of its response to changes of direction depends 
\ on the tension of this spring. The greater the wind velocity the 
j 
i 
\ 
f 
i ’ 
k 
ii), 
ee 
