[Healeys] Interesting (?) Photos

Perry healeyguy at aol.com
Tue Jul 31 09:58:01 MDT 2018

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.


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