stressed skin rotor

[ubud]

i saw some negitive responce to my stressed skin alum rotor blades, an had to do some emperical testing on my own, i clamped the rotor to the workbench, set tape on floor and at 1" mark on the tip. then started to load bricks out the blade, 20 of them (4.5#) total 90#, got about 7" of deflection,and some rise on the clamped center plate.

when i unloaded the blade it came back to my start point.

here are the pic's

















that last pic is the airfoil i've created

[paradigmdesign]

you may want to change the title to "stressed skin blades", usually when someone talks about a rotor in gennys, they are usually talking about where the magnets go.  

What length are the blades?  The reason I ask, is aluminum can be weakened due to vibrations, and a small rotor dia. causes higher frequency of vibrations (higher RPM).  What kind/thickness of alum are you using?

P.S. thoes spools of magnet wire in the background are making me jealous.

[Bruce S]

Ubud;

 Nice pics.

Cool way to tell if your planes are going to hold up on the hub.

I think what most are saying isn't the load handling ability put the constant "wiggle" factor.

Try removing the bricks and wiggle the blades the full bend distance and look to see if the Al starts to develop wrinkles or stress tares.

You may start to see the problem show up right around where the blade meets the conduit you have.

Cheers;

  Bruce S

[wdyasq]

The problem with aluminum blades comes from the continual flexing and fatigue properties of materials.  Not only does Aluminum have the problem, steel, fiberglass and carbon fiber also have similar problems to varing degrees.  

The windmill at Grampa's Knob Vermont and several DeHavilan 'Comet' aircraft failed due to aluminum fatigue.  I do not think any new alloys have completely cured the problems.

In the early 90's the Gougeon Borthers out of Bay CIty, MI did extensive tests on blade materials and fatigue .  The reports are published and available to those interested.

Ron

 

[ubud]

to answer some ?'s

blade length = 51.5 from center 44" of alum. .050" 9" chord width at root, 4" at tip

emt tube with 8" 5/8 grade 5 bolt jamed in, thru bolts 1/4x20 grade 8. root angle =19 deg, tipangle = 2 deg.

when trying to flex from the tip....all i do is move the c clamps trying to hold it to the workbench, see no flexing anywhere. stiffer than the preverable wedding (you know what).

wire, bottom right 10 # 14 ga.slated for 12pole dual plate, 14" dia.

top right 16 ga slated for 24 pole 18 coil vawt (gonna copy Mr Ed's)

left reel 44# 21 ga. (price was right, $2.09 a#) don't know where it will end up.

[sahlein]

As I stated before in another post, MATWEB.COM   lists over 400 aluminum alloys

in various tempers.

I believe that this gentleman's use of steel conduit as a "spar" is a novel idea.

Has anyone considered the use of aluminum SKINNED blades with a wood core that

has been impregnated with some kind of epoxy??

Perhaps a composite core would be easier to fabricate.

[windstuffnow]

  We used to load aircraft wings the same way to find the G load it would take before breaking. We used sand bags instead of bricks.  The point being, if you loaded it and unloaded it several hundred times, it would break from fatigue.  This loading and unloading, constant flexing from wind gusts and yawing will fatigue the metal and eventually break.  Not if it will break... when.  You can reduce the flex load on the sheet aluminum by filling the empty void inside with expanding foam.  This will help slow the process.   Also, if your using steel conduit and aluminum covering there should be a coating on both.   You will have problems with the rivets over time as well from electrolysis ( dissimilar metals).  It might be better to change the conduit to some heavy wall 6061 T6 and make sure you use aluminum aircraft type rivets.   Aluminum airplanes fly for a very long time if their built correctly, but they are inspected daily or at the very least before every flight.  Windmills usually don't get a "pre-flight" on a daily basis.

Just food for thought... watch them closely!

.

[Experimental]

   Hi Ubud,

   While all the things the folks have said , have truth in them -- I say, go ahead and run them !!

   Eventually those pop rivets, will probably loosen, and you may see reaction from the dissimilar metals -- but thats going to be a long time !!

Meanwhile, you can dream up ways of improving !!

    I might also mention, that I saw similar blades, on a commercially built mill from "way back" -- but don,t ask me what it was -- seems to me it was something from Australia !!

    I would worry more, about the conduit fatigueing, near the hub, and throwing a blade, than the rest of it -- but we have guys running blades off fans, and PVC and just about everything I can think of, including beer cans and plastic bottles !!

    You,ve built something, and that,s an accomplishment -- YOU DONE GOOD !!

    I enjoyed your pictures and will watch for more -- Have fun,  Bill H....

[niffa]

Hi Ubud,

Thanks for the post you have done a lot of good work that I may copy if you don't mind. I have a suggestion re you stressed skin on the blades. To transfer loads from the blade to the hub the alum' skin must extend all the way onto the hub. If as in your photo you connect the blade to the hub with only the spar the 1-inch section of spar will have to carry all the loads and WILL fail very quickly. If you extend the skin to the hub the skin can then carry most of the loads over a much larger area and so will be less likely to fail. I suggest you use epoxy glue and rivets at the root joints, as this will spread loads away from the rivets.

We can learn a lot from aircraft manufacture.

I agree with you, your picture is of a rotor.

Phil

[craig110]

Hey Ubud,

Sorry for sidetracking your diary, but I replied yesterday to your query about the solar cells (which had previously been sold to someone else), but the email got bounced back today due to the spamcop filtering.  Sorry about that, but I just wanted you to know that I didn't ignore your email.

Craig