ATTN FLUX - (RE: Blades, TSR, Length)

[valterra]

Flux -

In an earlier post regarding translating blade calculator numbers into real life, you stated the using the numbers given at .7 radius and then a smooth taper work well.  The original post is at:

http://www.fieldlines.com/story/2008/6/18/31816/4723

You also said (and I totally missed it):

"Making the blades with no taper or twist based on the calculator figures at .7 radius works just about as well."

Am I reading that right - that if I simply use the .7 radius numbers at FIXED pitch AND width, then those will work just about as well?

For a 4 degree angle of attack (7 foot, TSR 6), according to Alton's calc, that is:

 5.1 degrees, 5" chord, .67 thickness.  

According to Ed's Calc, it is:

 5 degrees, 3.79" chord, .57 thickness.

I didn't include the "drop" since that is determined solely by the angle and therefore isn't relevant.  

There is obviously some variation there between calcs, but if I just forewent all the trouble of trying to explain to my woodcutting guy all the angles and twists, then one of these number sets would suffice?  

I'm a little partial to Alton's only because I could use most of the width of a 2x6.

You'd mentioned when helping me with my motor conversion that using 7 foot, TSR 6 blades meant I wouldn't have to be so critical about the root anyway.  So I am wondering if it is worth all the fuss.

THANKS AGAIN.

[Flux]

I have never made blades with no taper but others do including Bergey.

I tend to use a similar approach to DanB at Otherpower and taper linearly from tip to root and increase twist linearly from about 3 deg at the tip to what the board dimensions dictate at the root using the first available reasonable board size.

I would be quite happy to stay at constant pitch of 5 deg but I would be inclined to have some taper to reduce the width at the tip and I would be reluctant to keep constant thickness. It needs to be thin at the tip but you need strength at the root.

If you want to keep things simple you could take your 2 x 6 and make the face towards the wind a constant angle of 5 deg. Calculate the drop at the trailing edge or use a protractor. Taper it to about 3 1/2" at the tip ( I think I would do it symmetrically rather than just cut the trailing edge). Reduce the thickness to about 3/8" at the tip and increase thickness to about 1" at about 1ft radius linearly. Then blend the 1" into the full thickness to form the root which will be nice and strong at 2" thick.

Mark a point 1/3 the way back and keep this your maximum thickness and form a curve on the back that goes back to a sharp trailing edge and with a more rounded leading edge.

This drawing should show a Lucas freelite prop built in a similar way but with the back surface being a circular curve ( no doubt cut on a spindle moulder). Even that works ok but I think a bit of an approach to an aerofoil rather than a circular section would be better for the back. This is a 2 blade cut from one plank, you only need one half of it.

Hope you can follow what I mean, look at the Otherpower props to see if you can follow Dan's drawings and descriptions and combine them with mine. It should work ok.

Flux

[Simen]

I think the design here was easy to follow and scale, to my needs:

http://otherpower.com/blades.html

Simen

[valterra]

Thanks, Flux.  I was seeking clarification of what you'd said on the other post, about how blades with no taper work almost as well.  (See original post).

I will go with your idea before about setting chord, etc., that you mentioned in that post also, similar to what The Dans do.

Feel free to add any further clarification.

THANKS Again.

[Flux]

In terms of performance I doubt that the taper makes much difference. For  a blade made as an extrusion or a pultrusion as Bergey use then this would be an obvious choice. For wood I like to keep the mass down at the tips so it seems natural to reduce width as well as thickness. Also you can have close to the ideal chord at tip and .7R. If you go parallel then I think I would take the .7R chord width and have it too wide at the tip.

Having too much chord width seems to do little harm but having pitch angles too coarse really holds the thing back.

Flux

[valterra]

Sorry, Flux.  What did you mean by going parallel?

[Flux]

No taper, keeping the same width all the way along the blade.

Flux

[valterra]

Thank You.  If one builds blades using no taper, then go with the .7 number even though it would result in what is traditionally  considered overly-wide tips.

Thanks.