Does anyone have any quantifiable experience of how much more efficient the 1952-54 6 bladed fan is over the original alloy1948-51. 5 bladed fan?
is there a significant gain in cooling?
Thanks,
George
I don’t have figures , but note that they seemed to had done away with 2 bladed propellors on Hurricanes and the like before WW2 [ or there-abouts]
The pulley for the 5 blade fan is I believe something like 5" diameter (I don’t have one to measure) where the pulley for the 6 blade fan is about 4" diameter (the best I can measure mine as it is installed), therefore the 6 blade fan rotates at something like about 25 percent faster than the 5 blade for any given engine rpm.
The 5 blade fan is about 13" diameter. The 6 blade fan is 13-1/8" diameter.
The amount of air moved by a blade at any given rpm is related to the blade area, blade shape, angle of attack, and radius of the center of area from the axis.
Those are difficult parameters to quantify, so if, for the purposes of this discussion, we pretend they are equal, the 6 blades will move 20 percent more air than the 5 blades for any given fan rpm.
The number of vanes in the water pump was also increased from 4 to 5 with the change in fan and pulley.
The original thermostat when open is supposed to block off the flow through the bypass hose, but it does not block it off completely, some can leak past.
In engineering terms, this is a heat transfer problem. These cars were ok when they were new, but now they are over 65 years old, and many were run for years with straight water without anti-corrosion additives, so be aware of crud buildup inside the head, intake manifold, block and radiator which reduces the flow and heat transfer.
The diameter of the fans [ by different alloy blades] varied a bit . Often nothing to do with science but being able to clear things like bottom rad outlet and even the bonnet.
The SS100 had its fan mounted lower [ via a different pump body casting]. Once production of the 100 stopped after the war , the saloons basically the same as pre war had a new pump that moved the fan upwards.
The alloy blades are often very tired so I drew up a CAD drawing to cut new ones. But divided it half way along so more can be added or removed to et the max amount of blade length depending on room .
I have made the assumption that the ends of the blades won’t reach supersonic speeds.
The spacing of the blades also varied . Some at 60 deg apart some at varying distances, I suspect this was to make replacing the fan belt easier. after someone had got sick of skinning knuckles…
What I found interesting in a trivial way was that th early SS steel spider at the fan centre had a smaller distance apar t of te 4 holes. But for some reason this was increased with later cars But they must have kept the same tooling as on the later ones you can see half of the holes where the bigger centre hole has been punched.
Absolutely useless info but I like to deconstruct the manufacturing methods.
Hi Rob:
Given your engineering acumen, what are your thoughts on the following: Terry Larson once mentioned to me that he put a spacer between the fan and pulley in order to bring the fan blades closer to the radiator core, arguing that this improved cooling on the XK120.
Chris.
A single pass xk120 radiator shows a big temp differential across the face of the radiator (cooler at the sides). If more cooling is desired, Any reason against trying a triple pass down-flow core? It utilizes the core more efficiently by keeping coolant in the radiator a little longer and providing a larger effective radiating area.
I have about 9/16" of space between my fins and blades. Having once had the fan work itself forward on the pump shaft and chew up the radiator, that always seemed close enough to me.
Closer would also mean louder, and there is a minimum, any closer and you get localized turbulence (eddies), lose some of your efficiency.
It would be better to come up with some sort of shroud, like most modern cars.
I have also seen 120s with foam along the top tank so all the air entering the grille goes through the fins, none over the top.
Some have raised up the rear of the bonnet to let air flow out there.
The uneven (though symmetrical) spacing of the 6 blades was to counter a phenomenon called beating, one of the reasons helicopters are so loud, but with the 8 blade fan of the XK140/MkVII they seem to have abandoned that consideration in favor of more air flow. The 140/VII fan is 16" dia but I suppose one could modify it to work in a 120.
Based on my previous life as a propeller designer, the most important parameters are : diameter, RPM, pitch and blade area ratio, BAR.
BAR is the fraction represented by the actual projected area of all of the lift producing surfaces divided by the area of the circumscribing circle.
In terms of performance, number of blades is of little significance, but it is a bid deal as soon as vibrations are considered.
Karl
On my 120 the 6-blade fan is already pretty close to the core. My engineer’s observation would be that there would be much more to gain in efficiency by adding a fan shroud. Other than a non-critical shop air mover, you would never see an industrial fan without an inlet shroud. The rotating fan blades move air by creating a low pressure zone on the back side of the blade. If the airflow at the end of the blade is not restricted you waste some of the fan’s potential in end spill. One of the improvements in cooling on 140’s and 150’s came from the fan shroud.
