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> wrote:
>
> 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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