Multi-Blade props

[PeterDove]

Hi All,

Can someone tell me what difference having more than two blades makes for your propeller setup? Does it mean it turns easier?

Peter

[richhagen]

When the blade is static, the amount of starting torque is a function of the integral of the surface area and the angle of that surface area to the incident wind.  

Whether that surface are is spread across one or a hundred blades, it does not make a huge difference.  

A two bladed prop is hard to balance, as the blades tend to teeter a bit when they pass in front of the tower at the bottom of their path, and this creates a relative imbalance on forces on the blade and a resultant vibration.  This is less noticible with more blades, although generally an odd number of blades is recommended.  

The more blades that you have in an ideal turbine, the thinner the blades will be in the direction of the wind.  The thinner something is the harder it is to make it as structurally sturdy in that axis.  

Based on the above, and the fact that I cannot effectively balance a one blade prop, and even if I do, I have a large inertial weight hanging opposite to it that is not doing anything productive, that leaves me with a three bladed prop as the optimum.  

This represents my current thinking on the subject, others may differ.  Rich Hagen

[willib]

having three prop  blades gives you more torque than two prop blades

[bj]

    Richhagen, currently have six small (5 foot diameter) blades.  Never

thought of going to seven.  Problem was with harmonics.  Could issue

a Babshie whail at some rpms.  Don't want to annoy one close neighbor,

    But as I was advised by someone on this board,  avoid even

number of blades, to avoid harmonics, I Immediatly thought of one

less.  Maybe one more is a better choice.  Either way, the choice is

easy to develope, and will post.  

   In the meantime, thanks for the thought.

    bj

[richhagen]

Willib, I believe that would depend upon the width and angle of the blades.  If the blades were all the same on the two and the three bladed prop, then yes, the starting torque would be greater. Rich

[scottsAI]

Hello PeterDove,

Please read this:

http://en.wikipedia.org/wiki/Wind_turbine

In summery, one blade to two 6% more power.

Two to 3 another 3%, 3 and up few small %.

The blades are optimal smooth and designed to get the above.

Most practical applications 3 is the best compromise.

As the blade count goes up the rotational speed goes down, torque goes up.

Farmer wind mill has lots of blades for the low starting winds with high torque and low rotational speeds needed by the pump.

Have fun,

Scott.

[JSNH]

I remember reading an interview with the person who stared Jacobs wind charger back in the 30's. The interview was in the late 70's or early 80's. He said he could not understand why the department of energy was testing 2 bladed wind mills when he proved back in the 30 that they just vibrate the machines apart. He used 3 blades and they had less of a vibration problem.

[finnsawyer]

It seems to me you are assuming that the same blades are used in each case.  A proper regard for the physics would indicate that as the number of blades goes up, the width and thickness of the blades decreases, as each blade needs to capture less of the power in the wind.  Consequently, it should be a draw, except, perhaps, for effects like tip vortices.  But even these kind of effects should be reduced per blade as the blades get narrower and thinner.  In any case the efficiencies you give are so small as to be of no concern to the home builder.  

[scottsAI]

Yes, multi blade props are not much improvement to the home builder.

The article uses the optimal blade shape to achieve the improvements.

A big disadvantage of more blade is thinner blades = weaker blades.

Blade strength is a big issue.

3 blades seems to be the magic number!

Scott.

[Dennisd]

My reading (Also from the inventor of the Jacobs) on this is that a two bladed prop, during rapid yawing, has vibration problems for gyroscopic reasons. when the prop is vertical, it rotates easily around the tower. When the prop is horizontal, it is harder to yaw. this means that when the turbine yaws, a vibration at twice the props rotational frequency is caused. 3 bladed props always have some horizontal and some vertical component, so this effect doesn't happen.