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.
Flux[ Parent ]
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 ]
