I spoke today with a fellow who is both car and aerodynamic-savvy. He
explained that a crankshaft driven fan suffers from inefficiencies which
become significantly worse than you might expect when operated at high
speeds ( ie full throttle.) At lower speeds, the air moves over the fan
blades in a fairly orderly and predictable manner. But as the RPMs increase
the air is increasingly unable to follow the fan's contours and begins to
show disorganized turbulence, which results in a great deal of energy being
expended to overcome the resulting drag. The fan doesn't push more air, it
merely stirs it around like crazy. Engineers have overcome this, to a
degree by driving the fan via a belt at a speed significantly less than
crankshaft RPM. They can increase the fan blade pitch for satisfactory low
speed air moving performance, and make the blades somewhat flexible so the
pitch 'flattens out' at higher speeds. I was told that 10-15 horsepower is
a very realistic figure for the power to drive a crankshaft mounted fan at
very high speeds.
Having learned that, I apologize to anyone who may have been offended by
my skepticism, shown below....
Bob W.
-----Original Message-----
From: R. John Lye [mailto:rjl6n@server1.mail.virginia.edu]
Sent: Wednesday, November 03, 1999 6:46 AM
To: bwesterdale@edax.com; triumphs@autox.team.net
Subject: RE: horsepower
Bob W. asks:
>Not to be too much of a skeptic, but 18% seems rather high-- if you had a
>100bhp motor to start with, would it really take around 18 HP to run that
>tiny little fan?
<snip>
>Your thoughts?
18% may be a touch high - however, I can relate J.K. Jackson's story and
numbers here. He had a TR-6 on a chassis dyno and ran it with the fan,
then he removed the fan and remeasured. Simply removing the fan gained
him 13% more power on a TR-6 engine. The TR-6 fan is plastic rather
than metal like the TR-3/4 fan, but it has more blades so those probably
balance out a bit. Therefore, I'd guess that a TR-3/4 should see a
similar performance gain as the TR-6 did.
I hope that helps,
John Lye
'59 TR-3A, '62 TR-4, '70 GT-6+
email: rjl6n@virginia.edu
homepage: http://avery.med.virginia.edu/~rjl6n/homepage.htm
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