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Message
Humm,
Rusty, I think a fan might be useful for ground
operations, but once airborne, my calculations indicate it would likely
hinder airflow more than help it.
Here's the rationale.
Normally, duct design calls for slowing duct air
velocity down to 0.1 cruise speed or 0.3 Climb speed (Horner). So if
your cruise climb is 120 MPH IAS then the air flow in the duct should (ideally)
be around 0.3 * 120 = 36 MPH. That is 36 MPH or 52.8 feet/sec.
With a core with a face area of say 200 sq inch.
That would give 52.8 * 200/144 = 73 cubic feet/sec or 73*60 =
4400 CFM of air flow through the core. I personally doubt a fan would do
anything more than hinder that much air flow. If you are not getting
that much air flow, then I believe attention to the ducting would be more
productive than adding a fan. But, that is just my opinion
based on back of envelope calculations.
Again, for ground and taxi operations, I agree, a Fan
would undoubtedly help, but I don't believe it would help once
airborne.
FWIW
Ed
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----- Original Message -----
Sent: Wednesday, August 08, 2007 9:16
AM
Subject: [FlyRotary] electric fans
Greetings,
I've only been skimming
through the cooling posts, but I haven't seen much talk of electric
fans. I was thinking about this during my drive time yesterday, and it
seems to me that we have exactly the same need for an electric fan as a car
does.
- Packaging limits size
and location of radiators.
- Ducting optimized
for normal cruise speed, fan compensates at low
speeds.
It would seem to me
that the best situation would be to optimized your inlet and
ducting for normal cruise flight, then use a fan during
climb. The
drawbacks would be weight, drag, and perhaps bearing life on the fan, but the
positives could make up for these I
think.
From what I've read, it's
common to use a switch to tie the motor leads together, which keeps the
motor from completely freewheeling when unpowered
in cruise. It would seem that you can reduce the drag and
bearing problems that way. I would also think a clever guy could provide
a switch mode that allows you to generate alternate electricity from the
freewheeling fan.
Just something old
to think about again.
Rusty
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