Width of blade attachment to hub

[imsmooth]

I have a 5' blade being carved that is 10" wide at the root.  The wood is 2.5" thick.  How narrow can I make the part that attaches to the hub in order to reduce weight, without compromising strength? On the Otherpower.com site there is a 5' blade with an 8" root width having the end taper to 6".  This seems narrow to me for a 10" wide root.

[niall7]

hi iamsmooth

just my thoughts but i think if your using a good strong hub bearing i  wouldnt be too concerened about the weight issue ,when you get them balanced nicely they,l seem pretty weightless  ,light blades might spin up a little faster but heavier  ones would  store a little more momentium ,with the dimensions you mention and if your keeping the 2.5 thickness at the root it sounds like you have plenty of scope for  mounting them  , 8 x 2.5  still sounds pretty strong for a 5 foot blade

[Flux]

I agree, the weight is of no consequence, if your bearings and shaft can't support the weight it will not support the gyroscopic forces which are far higher.

For a 5ft blade you need something about 4" wide and over 1" thick to be strong enough if the tips are cut down to normal dimensions. You have enough strength to swing an elephant on the end so anything between your full plank and the dimension I gave will be perfectly ok.

Reducing the 10" may be desirable to get it on to a sensible hub, I see no real reason to reduce the thickness as you will probably cause stress raisers where you thin it and it will not be as strong as a thinner board in the first place.

Flux

[oztules]

Flux,

I think he means a 10' diameter machine made from 3 x 5' blades. not 3 x 2'6" blades for a 5"diameter machine.

This would make the 4"x1" at the root a little tight.... with some very nervous elephants.

.........oztules

[imsmooth]

Yes, Oztules, we are talking about a 10' rotor diameter.  I guess I will trim it to 8" for the hub attachment.  My hub is made from 2 3/4" discs of marine plywood, 14" diameter.

My bearings are strong, but they are standard sealed 25mm bearings.  Is there any significant benefit in getting sealed, high speed ceramic ABCE-7 bearings?  They cost a lot more.

[Flux]

With wide and heavily twisted blades the bending in the inner section will be minimal and as I suggested I wouldn't reduce the thickness as the stresses on the reduced section will be high. In this case keep it at 2.5".

For a blade with little twist it can easily be made from timber 1" thick and be strong enough for 10ft diameter. The bending stress will be distributed over the inner third of the blade. Nearly always failure comes from blade strike in the outer part of the circle. Root failure is rare and usually associated with stress concentrated in one point. I have never built wide and highly twisted blades but I suspect you have to be more careful with them at the root but as they waste a whole tree to get the chord and twist the root should be no problem, I still wouldn't worry about the weight and if I did try to reduce it I would reduce the chord rather than the thickness ( similar to what Dan does on Otherpower).

May be wise to keep the elephant off it though.

Flux

[Flux]

I believe this is an F & P or something based on it. I have no idea of the bearing assembly but my suspicions are that the bearings and shaft designed for a washing machine are very close to the limit or beyond it for a 10ft prop.

I doubt that the bearings are a weaker point than the shaft or housing and expensive bearings are probably pointless ( just avoid the Chinese junk bearings).

Flux

[ghurd]

I thought there was a 1% rule.

The shaft should have blades no larger than 100 times the shaft diameter.

A 1" shaft means 100" blades, max.

Anyone but me think that's right?

I have a 5/16" shafted ECM and this story is relative to what I do next.

G-

[Flux]

Not met it but that does sound perfectly reasonable. I wouldn't put anything bigger than 3ft on a 5/16 shaft and even then it wouldn't stand any overhang. That 1% rule does seem reasonable but I see people pushing their luck way beyond anything I would risk.

I wouldn't hang 10ft on a 1" shaft but if done carefully with little overhang and taper rollers and a stiff housing it may be more than adequate. With ball races in a flimsy hub assembly and lots of overhang I wouldn't want much over 7ft.

My guess is that the shaft is more at risk than any blade roots in a sensibly constructed prop.

Flux

[ghurd]

The 3' is probably over what it needs.  Even 30" may be too much.

May get something done today for a bit of output numbers.

Jerry Rigging it looks like quite a job.

G-

[hvirtane]

I thought there was a 1% rule.

Yes. That was the old rule of thumb by self builders in Finland and probably in Germany (, from where Finns learned it).

It means that for each meter of the blade diameter there should be one centimeter of the shaft diameter, if ordinary steel is used. Of course with modern car wheel shaft steels much thinner axle shafts can be used.

- Hannu

[scoraigwind]

The dimensions of a wooden blade at the root are important but the thickness is more important than the width.  Root failures are not so likely but extremely flexible blades are able to bend an amazing distance in search of the tower and to hit it in the event of extreme gyroscopic loads.

For me the gyroscopic loads are the ones to watch, and the shaft bending loads are much the same whether the blades are centred on the bearing or overhung, although it feels better to put the blades close to the middle of the bearings.

My minimum size piece of wood for a ten foot diameter (five foot long blades) is 5 3/4 by 1 3/4  timber/lumber section.  But the blade itself soon gets a lot thinner than that once you start on carving it.

On a nice site with steady winds the gyro forcs may not bite you but if you are exposed to gusts and lulls like most sites then there is the chance of the blades overspeeding and then yawing away and this results in very severe gyro forces.  It is for this reason that I have changed the direction of my yaw furling this year.  Maybe that means I can also use thinner blades...  But I don't think I will do the experiment.