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MG MGA - Fan Blade Pitch Angle
|I must stop having these 'out of the box' thoughts in my idle moments.|
I have always been a strong advocate of the original spec radiator as a cure for many of the heating woes owners suffer. Last night I got thinking about whether there may also be other contributory factors that we have never discussed.
I got thinking about that item that drags down engine power - the fan. Back in the 50s all the companies were looking at sport car acceleration times to 60mph and topping that 100mph barrier. One way to give a marginal engine improvement may have been to reduce the fan pitch angle. That might be okay for UK cars in our temperate climate, but what about exports to the USA Australia, South Africa etc. In those cases a more coarse pitch angle (at the expense of outright performance) may have been considered an overriding necessity. And so to my question; is there any evidence to suggest that our fans were produced in 2 pitch angle options, fine pitch for the cool climates and coarse pitch for the hot climes? Also, was the fan pulley always the same diameter - larger pulley for the home market (less revs) and smaller pulley for export (more revs)?
My car is an export version with a 37 degree pitch at the blade tips. Anyone care to measure theirs and knock my ponderings for six (English cricket term, equivalent to a home run)?
|Interesting point Steve. I read not so long ago that the optimum fan blade pitch for cooling was around 28 degrees and went out to check mine and found it 40 degrees. The article author tried different pitches and various distances of fan from radiator.|
I think my car might also be classed as an export even though it has always been right hand drive because some months ago I got a Heritage Certificate and under Specification it said 'RHD export' and under Destination it said 'Personal export delivery'. ..................Mike
Having looked through the MGA Service Parts List I note that only one number exists for the MGA which probably puts paid to 50% of my thoughts. However, 6-bladed fans were used on other B-engine cars and it would be interesting to see if they are the same as ours. For example, on the NTG site I noted that their 6-bladed fans are listed for a host of cars (including the Magnette) that does not include the MGA. Makes me wonder that if there was a difference some got taken from the wrong bin during assembly at Abingdon.
I will keep delving. Trying to find a Magnette Service Parts List PDF. No luck yet.
|When reading this I thought of a non standard fan I purchased in the 70s and still have hanging on the wall of my garage. It is variable pitch fan for MGAs. I bought it new in the early 70s from some accessory shop. The idea with this was to reduce drag and improve mph and or increase power. It has 4 rather than 6 blades that are about the standard angle when still but when revs increase the spring loaded blades can flatten by about 50%. I did have it on the car for many miles and it certainly didn't cause any problems but there again I don't know how effective it was or how many degrees the blades actually flattened by. Perhaps I will give it another try.|
|Mike and Paul|
I did wonder about the 37 degrees being rather coarse. From my flying days we used to look at 19 to 21 degrees angle of attack for optimum lift. That's what we used to aim for when pulling hard in combat and coming into land. Above that angle the air starts breaking off the aerofoil section, wing lift efficiency starts dropping away noticeably, leading eventually to pre stall shaking and buffeting. My basic logic suggests that the fan blade follows the same aerodynamic rules. With that amount of continual buffeting at the 37 degrees it will lead to eventual blade fracture. Any aerodynamicists out there to put me right?
|I had a thought and I was right it is in the 1973 Derrington's catalogue. I will leave you to read the spec and claims.|
For ones who are not familiar this company was very well regarded and were the people who sold the original alloy crossflow head for MGAs.
|These guys produced the original (I believe) for A series minis,etc. Looks remarkably similar to your attachment Paul.|
I went out into the garage intending to measure the blade angle of my original standard metal fan ut unfortunately, I couldnt remember where I stored it.
I have though taken some pictures of the NTG plastic fan that I fitted to replace the metal one so you can see just how curved the fan blades are.
The NTG fan is 14 inches in diameter, the blades are 2 1/4" wide and when fitted it dropped the running temp by 10 degrees F at the expense of being much noisier in action.
I will try to upload some pictures so you can see the blade profile.
I will also keep looking for the metal fan.
PS If I may ask, how are Mrs Gyles hips coming along
|This is the plan view of the NTG fan
She saw the specialist today. Down to one crutch for one week then off solo. Cleared to drive, play golf and tennis. Amazing all this modern medical technology. Up to now she has never played golf and tennis in her life! More importantly she has also been cleared for vacuuming, cooking, ironing and shopping.
(hope she doesn't read this!)
|The modern version of your Aerofan is a Flexfan. It uses flexible metal rather than springs. Much lighter.|
|I tried really hard to buy a "Flexolite" metal fan from the USA couple of years ago but couldnt get any information to find out if they made one to fit the B-Series engine.|
I liked the idea of a metal fan with blades that flexed and "feathered" (the blade angle flattens out) at higher revs to save wasting engine power.
In the end I went the twin electric fan route.
I just measured that NTG fan pitch angle on your photo. It's 26 degrees. That would tie in nicely with Mike's comments about 28 degrees being the most efficient angle. From your comments about it giving better cooling it would confirm my thoughts that our standard 37 pitch blades are too coarse, with the boundary layer air breaking off the blades, making them prone to airflow vibration and subsequent metal fatigue. Whilst I do not have any cooling problems I am considering, for safety reasons (metal fatigue) and a tadge more performance, in fitting one. The noise won't bother me; I get enough with the sports screen etc. Just turn the radio volume up a notch.
May be worth a clarification about the angle of attack limits we used in our fighters, F4 and Tornado GR1 in my case - the Lightning and Hunter did not have the luxury of an AoA gauge; you just pulled until you felt the stick lightly tremble. There was always a good buffer built in to our published limits to allow for ham-fisted pilots. So the 21 degrees we flew to would actually allow for another 3 degrees or so before the onset of the light stick tremble, leading to wing stall, making the 26 to 28 degrees spot on. It was only after using an AoA gauge in the F4 that I realised how close to the aerodynamic limits we used to fly the Lightning - ignorance was bliss!
|I have just done a bit of follow-on measuring of my standard MGA fan. The fan runs very true fore and aft (towards the radiator or engine). i.e. no blades out of alignment. However, the individual blade pitch angles surprised/alarmed me:|
I can understand the odd blade being out of alignment fore and aft after 60 years and many of us have probably tweaked the blades back and forwards to get them in line, but altering the twist (pitch angle) is another matter. Did they come out of the factory like this? I hope not.
I wonder what others find with their fans?
|Steve, the pitch angle has to be considered along with the air speed. So a coarse pitch becomes a lot finer when the approaching air is moving towards the fan.|
I don't quite follow the logic. Not saying you are wrong, just that I can't picture the theory in my brain. To my mind the fan blades are at a constant pitch angle to the in-coming air at right angles through the radiator regardless of the blade rotation speed and the air speed (suction plus car forward motion).
|THe faster the air crosses perpendicular to the blade, the less effect the blade is going to have in imparting it's energy into the air. At a certain air flow the fan is going to do nothing at all and beyond that will actually obstruct the flow. This equates to a perceived reduction in pitch angle, as Art describes.|
|Thanks for the explanation Graeme. I have been scribbling away drawing out the vectors and think I now have a grasp of what you and Art are saying. With the car stationary the blades cut the air at the 37 degrees. When the the car has significant forward motion the ram air through the radiator is at right angles, producing a resultant vector on the aerofoil section that does indeed reduce the apparent pitch angle.|
I found an interesting laboratory analysis on the web of a car fan in a shroud. They found that the optimum blade pitch angle when stationary or in slow moving traffic was 27.3 degrees with the fan 1 inch behind the radiator and a 1 inch clearance with the shroud. In their test this moved about 885 cu ft of air a minute. At 35 degrees this reduced to 865 cu ft.
So it seems we do have a very inefficient fan up front. Time for the NTG plastic fan or Moss asymmetric version.
A very inefficient fan: Just 2.3% less efficient than the optimum in the test set up. Its not that inefficient really is it and probably within the margin of variability in manufacture as your blades suggest.
I doubt very much if it was an MGA style fan that they tested so the figures are are just an example, although the percentage difference can probably be transferred across. You can take 2.3% whichever way you like. In a marginally cooled car such as the MGA that 2.3% can be the difference between boiling over or coping reasonably well in heavy traffic. For example, that 2.3% could make the bilge blower some guys have fitted rendered unnecessary.
For me, the cooling aspect is not the issue as I run all day quite happily at 175. Of greater concern is the significant pitch angle difference - 15%. That fan is quite a solid bit of metal when it comes to trying to twist those vanes. I would be concerned if many other fans have the same variance and it then proves likely to be a manufacturing tolerance issue. In my opinion out of balance vanes like that must be considered as a prime source of fractured blades we hear about.
|Steve, its like a variable pitch prop. On take off at low airspeed, use low pitch. As airspeed increases, prop tends to retard the aircraft, then use high pitch.|
The air flow direction that the prop sees is a vector combination of the forward flow and the rotational speed.
|Thanks Art. That puts paid to an old RAF aircrew phrase when you need to make an immediate and fast exit: "xxxx off in fine pitch"!|
This thread was discussed between 21/07/2015 and 23/07/2015
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