I google searched the board "3 blades" those and more. allan down underA life lived in fear is a life half lived.
The big snag with 2 blades is the vibration during yaw. Any scheme with a tail suffers from this. With pitch control and servo yaw the problems of 2 blades are much less and for small machines it could be acceptable. It should be easier to hold 2 blades in step rather than 3 for a pitch control mechanism.
If you need to go to a smaller diameter for 3 blades then you need to do the same for 2 blades.
What matters in the end is tip speed ratio. If you were desperate for speed you could push 2 blades to slightly higher tsr, but I would not try for tsr higher than 7 for home made blades and you can manage that with 3 blades.
I am not really sure what performance you can get from planks, the ceiling fans seem pretty useless and no commercial machine has ever adopted such things. I would imagine that if you use a slower and more costly alternator and worked at tsr4 or below you might get some results, but even then they are less strong for a given weight than sensible blades. I don't think they will be comparable to pretty poor constant pitch constant width blades made from pipe, the curved surface of these does seem to work better in lower winds.
Flux[ Parent ]
Rob[ Parent ]
Adding gearing and increasing torque by widening blades would let you run with very low speed indeed, but the extra mass of the blades will most likely give your pitch mechanism and blade roots far more stress than if you went for a reasonable speed prop with direct drive.
Pitch control mechanisms are excellent if well designed, but in the hands of the inexperienced I think there is much more chance of failure than with simple carved wooden props. Most of these can survive very high speeds and if you keep the load on them they rarely fail. I haven't seen drawings of your pitch mechanism but some of the ideas I see far from inspire me and a windmill is the best fatigue testing thing available.
Theres some pics of the pitch control there...
Guess ill try making the planks into airfoils (clark Y)
What I meant by yaw torque, is that when its going fast and the tail moves it really seems to slam the tower kinda like a rattle because the blades are moving faster and they don't seem to like the sudden change in direction,...maybe i should video it next time and put it on you tube maybe i can get some better input...
Thx
I can certainly see why you are afraid to let it reach any speed.
The shudder during yaw is inherent with 2 blades. If the site is clean and it yaws slowly it may be reasonable but if there is any turbulence it will shake like crazy.
You have a lot of mass and inertia in a heavy prop of that size. Ideally if the pitch mechanism was perfect you could damp the tail movement to slow the yaw and reduce the judder. I have not used 2 blade props above 8ft and with reasonably light wooden blades the problem is acceptable but I wouldn't want to go much bigger.
Pitch control is nice but it is really a refinement. It should be based on 3 blades if at all possible.
I am sure making your blades something close to Clark Y will improve the speed and output immensely and if you can reduce the mass, especially at the outer radius I don't think the yaw judder will be worse than with the heavy planks at lower speed. Whether you can live with a 2 blade machine at that diameter depends on your nerve and the local wind conditions.
The dynamics of rotating 2 blades Vs 3 Blades has been known since before WWII. It can be explained. I doubt it can be explained to those ignorant of apparent wind, angle of attack, stall, lift, negative lift, centripetal force and furling in less than a small book. Some reading this, perhaps most, will be ignorant in one or more of those fields. But, by WWII, manufacturers of airplanes figured out 2 blade propped large planes would tear up stuff and placing a 3 blade prop on would cure some of those problems.
There have been many attempts to build Variable Pitch wind turbines. Some time back "Dinges' and I put together all of the VP patents we could find in one repository:
http://www.anotherpower.com/gallery/Variable-Pitch
Jacobs, the accepted master of the art, admitted having problems with larger sizes in his later patents. If one were attempting to build a new VP system, they would be well served to study what is in the patents.
Ron Adventure is just bad planning." -- Roald Amundsen [ Parent ]
You can pull all the power pullable out of the wind with ANY number of blades - even one (with a counterweight for balance, like a pinwheel made with a maple leaf with a pin through its center of rotation on a stick). This is because blades affect wind both upstream and downstream, for a considerable distance. If the next blade-pass occurs about the time the slug of slowed air has blown downstream by its own effective thickness, you get it all.
Now this means that, for a given TSR, the fewer the blades the wider they must be to affect a deep enough slug of air. But at a TSR of 6 or so even a two- or three-blade mill has very narrow blades when it's sized to get all that's gettable out of the wind.
One-blade has lots of problems. (A big one is that it's hare to get the center of drag to match the center of rotation. So it really shakes the mast. Another is the yaw vibration - see two-blade.)
Two-blade is pretty good except for the yaw vibration problem: A mill is a gyroscope. Turn it one way and it tries to tip back, the other way and it tries to tip forward. With a two-blade the angular inertia varies sinusoidially with blade angle. Yawing it while the blades are horizontal makes it try to tilt HARD, while they're vertical it doesn't make them try to tilt at all. A yaw takes many rotations of the blade to occur, so it vibrates hard while yawing - and windmills spend most of their time yawing to one degree or another as the wind constantly shifts. This effect goes away when you have three or more blades.
So three or more blades are pretty much fine in all respects. More than three blades means more blades to construct, shape, mount, paint, balance, etc.
The REAL efficiency for a fuel-is-free machine like a windmill is power-to-investment ratio. So we usually make them with three blades to maximize that measure of efficiency.[ Parent ]
