Chris,
African or European?
Cheers,
Bob Haskell
AHCA 3000 MkI registrar
http://www.ciahc.org/registry_3000mk1.php
On 08/01/2018 08:37 AM, Chris Dimmock wrote:
> Hi Perry,
> I suspect the mistake you have made is that the calculations indicate
> the vehicle is stationary.
> If I use your same calculations for the same column of air (so let's
> assume the radiator opening is the same size as the fan diameter), but
> the car is travelling at 60mph:
> 60 mph = 5,280 ft per min, which is also the airspeed velocity of an
> unladen swallow.
> So then 3.1416 x .390525 x 5,280
> = 6,478 CFM
>
> Which is why you never have an overheating issue at 60 mph (unless
> you've put an electric fan in front of your radiator, as happened to
> several competitors at Bathurst)
>
> I'm not sure what this all proves, but it shows that you only generate
> 20% as much airflow stationary in your driveway at 3,000 rpm as you do
> driving at 60 mph.
>
> Best
> Chris
>
> On 1 Aug 2018, at 1:58 am, Perry <healeyguy at aol.com
>
>> OK this has been bothering me off and on for years and I never did
>> anything to try to calculate the Cubic Feet per Minute (CFM) of a
>> stock AH100 radiator fan. In light of the recent discussion of air
>> flow in the engine compartment
>>
>> (Some text borrowed from the net)
>>
>> According to the general fan law for axial fans, you will need to know
>> the fan blade diameter and pitch (blade tilt or attack angle) along
>> with the rpm to calculate CFM. An Austin Healey 100 radiator fan has a
>> four blades with a 15 inch (1.25 foot) diameter and an 4-inch
>> effective pitch (one inch pitch times four blades). This means that
>> each revolution of the running fan blows the 1.25 foot diameter column
>> of air coming passed the fan 4 inches.? There are efficiency losses
>> but we are talking generalities here . For the two ends of the
>> spectrum the engine is idling at 700 rpm or powering along the road at
>> 3000 rpm. The pulley setup on the AH100 is nearly the same diameter on
>> the water pump and crank, so the fan rpm is approximately the same as
>> engine rpm.
>>
>> Calculate the linear velocity of the air through the running fan. If
>> each revolution moves the air 4 inches, then 700-revolutions per
>> minute multiplied by 4 means the air is being moved at 2800 inches per
>> minute, or 233 feet in one minute. ?For 3000 rpm engine speed, the
>> answer is 12000 inches or 1000 feet.
>>
>> Calculate the CFM (volumetric flow of air) at 700 rpm. The volume of
>> the column of air described above is pi (3.1416) x fan radius squared
>> (7.5 inches or 0.625 feet squared) times the column length in feet.
>> This would be 3.1416 x 0.390525 square feet x 233 feet = 285 cubic
>> feet per minute at 700 rpm. At 3000 rpm the CFM is 1226.
>>
>> I knew that the stock fans do not move much air but my calculation
>> says it really is horrendous. Suspect my math is off somewhere. Math
>> teachers and mechanical engineers and anybody else for that matter,
>> Comments please.
>>
>> Perry
>>
>> To:
>>
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