Swept Area/Root Area - design definition

[adaml]

Dear All,

Have made a few sets of small wood blades now using the Alton Calculator as my starting point then using my own by eye, bodge it and run approach - so far seems to have worked out OK!!!!  Please forgive the ignorance of the following lol!

Question is:  If I want to carve out a 24" blade but need to leave 3" as the root for hub attachment - should I actually be cutting my timber at 27" to accomodate the root and leave the resulting 24" as the effective swept area?

Realise this may sound an odd question, reason for asking is that in the past I have calculated my blade length i.e. 21", 24", 26" etc then included my root section in this length.  The question then dawned that if I am trying to carve a 24" swept area blade, by including 3" as the root section, I am not actually doing this but making a 21" blade as I am loosing 3" to my root section!!  

Or have I lost the plot?

Thanks in anticipation.

[Flux]

I prefer to work from diameter then this issue doesn't arise. If you want 24 blades then it will be 48" diameter. Whatever you do for attaching to the hub gets lost in the system. If you make 24" blades with a 3" root extension then it will be a 54" diameter prop.

Increasing the swept area with root extensions is perfectly ok but the calculator needs to see it as a 48" diameter for calculating the chord and pitch. If you want to include the 3" root pieces then you should use 54" as the diameter for your calculations..

Probably it won't make a lot of difference except possibly for reduced speed but it is cheating to use anything other than diameter for power calculations as the power goes up as diameter squared and an extra few inches on the diameter increases power a lot.

The silly bit at the middle contributes nothing in terms of swept area.

Flux

[adaml]

Thanks Flux,

You have confirmed my thoughts.  e.g. 48" dia to key into the calculator, then if I wanted I could add a 3", 4", 5" etc root purely for attachment to the hub but the calculated diameter of the acutal blade would be 48".  I would not want to included the root pieces in the diameter calculations.

Just something I had been pondering over the last few weeks and was beginning to give me a headache!!  Also I felt I had been leaving myself short of root section for secure attachement to the hub.......

Adam.

[Flux]

Not sure you have got it now.

Anything you add to the root will actually increase the diameter. The calculator needs the actual diameter.

If you have blades 1ft long with a root of 2ft the radius will be 3ft. You need 6ft diameter for the calculator.

Flux

[adaml]

Thanks Flux,

My misunderstanding, am with you now - think I had "over-thought" it all a bit too much!

[Ungrounded Lightning Rod]

In addition to the calculators not caring about the hub area and only being interested in the radii at the various stations...

If you're calculating the swept area it's:

 - (pi * ((outer radius)*2) - ((inner radius)*2)))

So moving the active area of the blades outward greatly increases the swept area, rather than just compensating for the tiny area of the hub/root.

[Ungrounded Lightning Rod]

The bbs software turned the double-asterisk "square it" symbol into a single asterisk and switching the state of bold.

It should be (pi times ((outer radius squared) - (inner radius squared)))