What do cooling fans do?

[cherry blossom]

Some WW2 radial engines, such as the BMW 801, had a fan to "assist" in cooling. Do these actually suck in extra air in flight or do they simply speed up the flow of the same quantity of air through the engine? If the quantity of air is the same, how does the fan assist the cooling?

[A4K]

I'm not an expert, but I'd say they suck in extra air to an extent, like a jet engine, but more probably the function is to 'chop up' the air a bit (also like the multi rows of turbine blades in a jet engine) to provide a smoother airflow to the engine, making for a more efficient cooling process.
The guys who didn't have defence cuts put an end to their Air Force training (*ahem*...) will be able to tell you for sure...

[Matt308]

When they originally tried to stuff the radial engine into the narrow profile cowl, they encountered overheating problems. So short of expanding the cowl diameter (and thus introducing more drag) it was decided that a fan would increase airflow. This is especially important at idle/taxi speeds, as radial engines are notorious for overheating if not well monitored. This same technique was copied by the Russians and Japanese on some of their fighters.

[Airframes]

Good, concise explanation Matt.

[michaelmaltby]

Correct me if I'm wrong, but in the case of the BMW engined FW-190 was the cooling fan not associated with a nose cowling oil cooler that consisted of lots of tubing. In such an implementation surely the fan's role is to direct a CONSTANT/CONSISTENT flow of cool air over the oil radiator regardless of aircraft's speed or attitude.

MM

[Colin1]

Originally Posted by michaelmaltby

Correct me if I'm wrong, but in the case of the BMW-engined Fw190 was the cooling fan not associated with a nose cowling oil cooler that consisted of lots of tubing. In such an implementation surely the fan's role is to direct a CONSTANT/CONSISTENT flow of cool air over the oil radiator regardless of aircraft's speed or attitude

Michael
I think the annular oil-cooling arrangement of the 801-engined Focke-wulf was only effective when the aircraft was in the air and brought into effect by the forward motion of the aircraft; the cooling fan on the other hand was only effective when the aircraft was taxying or ground-running.

Cooling 55 litres of oil would be a big ask on the cooling fan.

[michaelmaltby]

Thanks Colin.

Michael

[Stitch]

Originally Posted by michaelmaltby

Correct me if I'm wrong, but in the case of the BMW engined FW-190 was the cooling fan not associated with a nose cowling oil cooler that consisted of lots of tubing. In such an implementation surely the fan's role is to direct a CONSTANT/CONSISTENT flow of cool air over the oil radiator regardless of aircraft's speed or attitude.

MM

Actually, the fan was INSIDE then annular oil radiator, so it was only directing air over the cylinders themselves, not the radiator; it worked better when the airplane was moving, and the air was getting "sucked" through the sliding nose.

[michaelmaltby]

As a follow-up - how did these radial fan + oil cooler engines hold up in battle damage compared with in-line glycol cooled engines like Merlins and Allisons? Surely one vulnerability was traded for another.

MM

[seesul]

What was the oil operational/minimum/maximum tempreature?
Did the oil cooler have an termostatic valve for the temp regulation?
What kind of oil (specification) did they use back then?

[VALENGO]

Originally Posted by cherry blossom

Some WW2 radial engines, such as the BMW 801, had a fan to "assist" in cooling. Do these actually suck in extra air in flight or do they simply speed up the flow of the same quantity of air through the engine? If the quantity of air is the same, how does the fan assist the cooling?

As the cross section is not increased, the fan suck extra air and speed it up thru the cowling. The higher airspeed = the more turbulent flow = the more efficient heat transfer.

[cherry blossom]

Looking at the very nice image posted by Stitch, I suspect that one of the major roles of the fan is to direct the air outwards. The arrows show the air flowing in reverse through the annular oil radiator after passing through the fan. Thus the BMW 801 may have most of its cooling air directed towards the head and valves, which need most cooling, while the centre of the engine may be partially cooled via the oil cooler. However, I am not sure how the intercylinder baffle ring interacts with the fan to give good cooling of the rear cylinders. I remember reading that there was a 1941 modification to the exhaust of the Fw 190 that significantly improved cooling. Does anyone know how?

I am not sure if the fan's effect of making the airflow turbulent is important. Surely air that has passed a propeller is already turbulent? I am also not sure if the fan causes more air to flow between cooling fins. The problem is that higher speed means lower pressure. Of course, what we really need to know is the pressure difference, which will depend on the speed at the front and the back of the cylinders.

Finally, the BMW 801 always had relatively poor supercharging. Was the basic reason that the supercharger air had already been heated and any ram effect removed by being used for cooling?

[Colin1]

The Fw190's operational debut was marred by cooling problems, the principal of these being the second row of the 801's cylinders which were beset by run-away head temperatures.
The cooling fan was introduced to channel air to the rear of the 801's nacelle and vent it at the rear, thus providing the second row cylinders a heat transfer medium. Its specifications were

32" (81.3cms) diameter
12 blades (each blade of aerofoil section)
magnesium construction

running speed
1.72 x crankshaft speed
3.18 x propeller speed

Air from the interior of the nacelle exited the cowling via two circumferential slots controlled by sliding gill rings, the rear ring controlled airflow past the second row of cylinders, the forward ring controlled the reverse flow of air from the interior through the oil coolers.

As Bristol discovered with its Hercules engine, the cooling fan was most effective during ground-running, at take-off and at low speeds, becoming less effective as airspeed increased with cooling responsibilities being more readily satisfied by the forward motion of the aircraft.

[Colin1]

Originally Posted by seesul

What was the oil operational/minimum/maximum tempreature?
Did the oil cooler have an thermostatic valve for the temp regulation?
What kind of oil (specification) did they use back then?

The answer to the second question is yes but really, any internal combustion engine will have some thermostatic means of controlling oil temperature; without it, it would be difficult to get the oil to it's optimal running temp and in the Soviet Union, practically impossible without baffling the front of the nacelle in the manner of the Polikarpov I-16.

[VALENGO]

Originally Posted by Stitch

Actually, the fan was INSIDE then annular oil radiator, so it was only directing air over the cylinders themselves, not the radiator; it worked better when the airplane was moving, and the air was getting "sucked" through the sliding nose.

I always believed the same about this, but looking closer at the picture discovered something: the air flows thru the radiator but in forward direction, you can see little arrows showing the tortuous direction. Everyday brings something to learn.