Message text written by "Michael D. Porter"
>All the above is true, and there's a further consideration. Blade pitch
is designed into a fixed blade as a compromise for the widest range of
operation, but this does not mean that the blade operates at the highest
efficiency at all speeds of rotation. Ideally, the blade pitch should
change with rotational speed<
<
The Tradeoff is in that the fan capacity is needed only at idle. At any
engine speed above idle the fan is pulling more air than necessary and
represents a HP load on the engine. Putting on a higher capacity fan will
increase iar flow at idle but also increase the HP drag on the engine at
speed. My 79 Ford had a fixed fan with flexible blades that would flatten
out at speed and pull less air and hence consume less HP. It still made a
horrible racket.
Many American cars (and some late model Spitfires and TR7's) had a
thermo-clutch incorporated with the fan which would allow much slippage
when cooling was not needed.
And, indeed, the electric fan is a wonderful thing in that it can be
controlled independent of the engine speed and switched off all together if
there is enough ram air while at speed.
Dave Massey
57 TR3 (With engine driven fan)
71 TR6 (With both engine and electric fans)
80 TR8 (with dual fans and a high/low speed control system. Works great!)
|