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  #16  
Old 10-05-2012, 01:37 AM
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Quote:
Originally Posted by Zulfiqar View Post
considering the above excellent explanation - why am I seeing the blades bend backwards when I rev the engine with clutch engaged. Is it due to the blades having a rounded profile on the tips of the blades?
It absolutely shouldn't appear that way unless there is some kind of an optical effect going on due to strobing or something else

Fans and props often show bad distortion when filmed





Actually, what is probably happening is the fan is quite flexible because it's plastic. The ends of the blades are moving faster than the inner portions and are twisted to have a higher angle of attack, thus under greater stress. The airflow over the blade is going to try to straighten out the rear portion more than the front portion (less angle of attack) because the pressure differential is greater toward the rear and generally the blade is thinner. That movement would appear to be toward the front.
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87 124 cooling fan upgrade SERIOUS issue-prop1.jpg   87 124 cooling fan upgrade SERIOUS issue-prop2.jpg  
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Last edited by mach4; 10-05-2012 at 01:39 AM. Reason: spelling
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  #17  
Old 10-05-2012, 01:55 AM
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I can do that with a pencil

In the ground picture, what does the shadow say about what's going on with the prop?

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  #18  
Old 10-05-2012, 07:18 AM
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I suspected there were other forces in play there.

Thanks for the lesson, very interesting what is going on.

Mach4 are you a pilot?
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  #19  
Old 10-05-2012, 09:50 AM
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Quote:
Originally Posted by jay_bob View Post
Mach4 are you a pilot?
Yes, but not current. Flew for the Navy and also hang gliders.
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  #20  
Old 10-05-2012, 10:37 AM
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Thanks for the nice lesson. Let me explain what Im seeing

lets take data as

fan clutch engaged
engine speed at 2000 rpm

Because the fan is spinning fast it resembles a disc - If I rev the engine and look at the fan from the side then it appears to flex (bend) towards the engine when looking at the edge. After reading this excellent piece Im going to paint a few marks on the fan blades then try to eyeball it. You learn something every day.

Can this phenomenon occur because the fan is not actually set into the shroud - In my car it kind of sits half inside the shroud with big clearance on the sides

I have another situation I could never understand - this is with old jap fwd cars, their radiator fans when brand new have straight blades and are pretty quiet too, when they age the blades warp into a potato chip profile and end up kind of flat at the edges - They sound pretty loud at that point and dont move much air despite being at the same rpm as new.
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  #21  
Old 10-05-2012, 11:53 AM
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Quote:
Originally Posted by vstech View Post
what you are seeing is more than likely an optical illusion from the spinning blade profile flickering in and out of your visual markers...
+1. The engine cooling fan is not a propeller on an aircraft. The blades move very little, if they move at all. If this were the case, you could make the blades hit the radiator simply by revving to ~3000rpm in park. Again, I'm almost positive you experienced what I've seen elsewhere, with the bent radiator support and tired motor mounts combining into unpleasant contact.

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  #22  
Old 10-05-2012, 03:39 PM
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I'm ditching the mechanical fan all together and going all electric. Cold Tstat + Efan with early trigger worked on the M103 for 30k miles, it'll work on the OM603.
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