|Feb 22, 2010, 11:16 AM|
Joined Feb 2006
what is the difference between 4 blade and 5 blade
fans?? having an extra blade does that create more thrust? Or having one less fan blade create more speed but less thrust?
|Feb 22, 2010, 12:46 PM|
depends on the fan and how it loads up. there's no right or wrong on this question, really. it's all in the science and manufacture of each individual unit and the work it performs. the manufacturers use fluid dynamics programs and all kinds of technical stuff to design and build a fan and, they are designed to perform their best work in a specific range. the blades and blade number are designed around these specs. so, while one manufacturer uses 5 blades, another uses 3, another uses 7 and so on and so on.
rating one against another by blade count isn't really an accurate way to measure them up against each other though, since it doesn't mean much based on the above paragraph. lets take a hypothetical fan into consideration. it has 5 blades and spins at 30,000 RPM and produces its best thrust numbers in this range. the blades, pitch and count all produce its maximum thrust at that RPM. now, if one were to change the count to 7 blades, the performance would "fall off".............the RPM and thrust would drop and Amps would rise. not really a very efficient way to make power. lots of heat would be the byproduct of wasted energy. so you can see that it's all in how the fan is designed and the conditions it is designed to operate under that is more the answer and not specifically changing the blade count to get better numbers.
I know there are companies that sell rotors with less blades for their units and this apparantly increases RPM and therefore, thrust. less mass will spin faster and get you more output, but there are tradeoffs with everything. there's no free lunch.
|Feb 22, 2010, 01:34 PM|
In most situations, pitch makes much more difference than blade count. The link below is to my thread discussing 6 and 12 blade scratch built 90 mm fans with equal pitch.
If you look at the numbers, you'll see they perform almost identically.
|Feb 22, 2010, 03:51 PM|
I agree with Doug, if high efflux is needed. Control line speed freaks used custom single blade props, for yrs w/counterbalances. The theory being the blade does not run in as much prop wash. Or so I have been told.
You want a plane to go fast? Reduce the number of blades and increase pitch, RPM, smaller dia. But you cannot have your cake and eat it too. Then you lose thrust with the smaller dia, unless you can pour in more power. More power is more weight. Like a dogchasing it's tail.
So a compromise is required between thrust, efflux speed, power available, etc. Lot's of blades do sound nice. esp. when getting into the higher numbers.
I went to a drag boat race yrs ago. The really fast ones use single blade. But they took a running start through the timer. Compromise!
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