<< > > bcgardner wrote:
 > > >
 > > > Paul,
 > > >
 > > > You wrote: "You will have to stop the fan blades from spinning when the
 > > > power is off to the fan motor."
 > > >
 > > > While I can see that this would be a good idea in order to reduce wear
 > on
 > > > the bearings, are there any other reasons for doing so?
 > > >
 > >
 > > No but it will really zing at 200 MPH due to dynamic pressure
 > > being 104 pounds per square foot as opposed to only 26 pounds
 > > per square foot at 100 MPH.
 > >
 > Paul, while still intending to take your advice to figure out a way to stop
 > those blades from spinning, doesn't the difference in dynamic pressure
imply
 > that there is a "right" side to put a fan on a radiator?
 >
 > If one put the fan on the side away from the natural airflow (a puller fan
 > rather than a pusher), wouldn't the fins and core of the radiator serve to
 > diffuse and slow the airspeed to more manageable or less harmful levels?
 > Couldn't that be a general conclusion about placement of fans to assist
 > cooling?
 >
 > Barry Gardner
 > Wheaton, IL

 Yes I would put it on the back side but Jim Rahm put them on the front side
as
 he had more room there probably.

 BTW the dynamic pressure drop won't be a whole lot through the rad. Right at
 the moment I don't have those numbers.
  >>

If the duct cross section is the same before and after then the dynamic
pressure is the same by definition of continuity.  The reason to put a fan in
front of rather than behind is for higher output and longer life of the
*cooler * electric motor.  The reason to put one after is to use a smaller
cross section fan without having to make room for a long diffuser from an
upstream fan.  The downstream fan can have a rapid transition duct.  FWIW I
think cooling a helicopter with an electric fan is a mistake.  I would rig a
direct drive fan.  The weight and power usage of a suitable electric setup
would be much more (considering the alternator, etc)

Jeff Spitzer

 
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