What do cooling fans do?

Discussion in 'Engines' started by cherry blossom, Jun 2, 2009.

  1. cherry blossom

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    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?
     
  2. A4K

    A4K Well-Known Member

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    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...
     
  3. Matt308

    Matt308 Glock Perfection
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    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.
     
  4. Airframes

    Airframes Benevolens Magister

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    Good, concise explanation Matt.
     
  5. michaelmaltby

    michaelmaltby Well-Known Member

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    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
     
  6. Colin1

    Colin1 Active Member

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    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.
     
  7. michaelmaltby

    michaelmaltby Well-Known Member

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    Thanks Colin.

    Michael
     
  8. Stitch

    Stitch Banned

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    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.

    [​IMG]
     
  9. michaelmaltby

    michaelmaltby Well-Known Member

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    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
     
  10. seesul

    seesul Active Member

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    #10 seesul, Jun 3, 2009
    Last edited: Jun 3, 2009
    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?
     
  11. VALENGO

    VALENGO Member

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    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.
     
  12. cherry blossom

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    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?
     
  13. Colin1

    Colin1 Active Member

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    #13 Colin1, Jun 4, 2009
    Last edited: Jun 5, 2009
    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.
     
  14. Colin1

    Colin1 Active Member

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    #14 Colin1, Jun 4, 2009
    Last edited: Jun 4, 2009
    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.
     
  15. VALENGO

    VALENGO Member

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    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.
     
  16. cherry blossom

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    While looking at the very nice image posted by Stitch, I speculated that the performance would suffer as air was taken in after it had been used for cooling. It seems that Kurt Tank may have had the same idea as I found that the U7 variant of either the Fw 190A3 or A4 (internet sources differ) had external intakes (see pictures at The Focke Wulf FW-190, his aces,variants and victories. - Page 3 - WW2 in Color History Forum). Did the change work and if so why was it not used generally? If it did not work, why not?
     
  17. Matt308

    Matt308 Glock Perfection
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    Great post stitch. I too didn't realize that the oil radiator had airflow created by the venturi effect. This would explain how the fan was likely most efficient at idle/taxi operations to keep the cylinder heads cool. And this is where the radial engine was most susceptible to heat build up if not appropriately managed.
     
  18. vanir

    vanir Banned

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    Ram effect is mostly a device for raising engine critical altitude, especially useful in single speed superchargers with a relatively low throttle height for the WEP rating. Although it seems to raise output above the WEP throttle height all it's really doing of course is raising the WEP throttle height.
    For multispeed/multistage supercharged engines ram effect is more about high altitude performance, though it can be used to offset performance drop off in-between supercharger gears.

    I believe the external ram intakes actually reduced maximum speed performance on the Fw190A due to drag increase for a relatively insignificant gain in maximum altitude performance.
     
  19. Gibbage

    Gibbage New Member

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    Also the ram air intakes would be taking in cold air. The air intakes for the supercharger on the BMW are behind the engine, so it is constantly ingesting hot air from the cylinder heads. Using cold air is a LOT more officiant in terms of engine performance, but as stated, but big intakes will negate that effect. Most Allied radial aircraft took in air away from the engine. The P-47 and F6F took in air with a big scoop on its chin, and the F4U and Sea Fury on the leading edge of the wings.
     
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