four element prop

ron45
ron45 Registered Users Posts: 8
I saw a vid on utube on two double blades made from a two by fours on a table saw. There were many things wrong with this presentation to my way of thinking. Fairly sloppy workmanship and using what is probably the lowest grade wood in the world. But the basic idea intrigued me because it made for a simple joint to join the two double blades. This technique, on a table saw resulted in two blades with a 15 degree pitch from a single two by four. But the two sides of the blade were parallel instead of being an air foil. How much of a reduction in efficiency would result from flat blades? Would it effect start up speed in a negative way? I have heard it stated that most variations in blade design count for very little difference in output. How true is that statement? I include the link to the series of six vids.

http://www.youtube.com/watch?v=uT2y5JJ24f4

Comments

  • GreenPowerManiac
    GreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: four element prop

    The question is: How many attempts did he make to achieve one good prop ?

    I have 5 hand made props. None are wood because they are HARD TO BALANCE and are unpredictable in weather even if sealed properly. Wood warps, bends, expands, contracts to the elements 10 times more than steel or plastics. If you wish it to last a week then don't balance it.

    My methods are as follows:

    Use 6" PVC and cut the shape you wish. Always cut the pitch so it rotates in a clockwise direction (large tapered end curves up towards a clockwise rotation), so when you fixture the prop with a lock washer and nut it doesn't come off. Each blade MUST be perfectly balanced at the same weight. All components that fixture to the blade must also be the same (balanced weight). Any deviation from center line will cause vibration when spinning. Once balanced, I'll spin test them at low speed. The blades must not stop at the same point every time, this indicates imbalance. Spin several times and allow to stop. I'll test a dozen times or more until all 12 tests are different stopping locations.

    The more blades, the easier it is to balance. Two blade prop must be perfect. Three, a bit more discrepancy. Four, may be the easiest to balance because of the 90 degree angles. Five, six, or eight, you must be nuts and have a lot of time on your hands.

    This guy looks good on video, but is no artist in reality.
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  • ron45
    ron45 Registered Users Posts: 8
    Re: four element prop
    The question is: How many attempts did he make to achieve one good prop ?

    I have 5 hand made props. None are wood because they are HARD TO BALANCE and are unpredictable in weather even if sealed properly. Wood warps, bends, expands, contracts to the elements 10 times more than steel or plastics. If you wish it to last a week then don't balance it.
    ******************
    Thanks for the input. I only consider wood because I hadn't thought of pvc and I hate that stuff. Long story not relevant here.

    I'm new enough to all this to be confused by your description of how to do the pvc blades. Got any pics posted anywhere of close ups of your props? That would make your advise more clear to me. I have ordered the Otherpower Homebrew turbine book. I am a nubie to most everything. Lots of nomenclature and basic science to learn about. I expect your answer will be come more useful when I've done some more reading. Thank you for taking the time to help. I do like the idea of alternative prop material because of all the reasons you state.

    "This guy looks good on video, but is no artist in reality."
    You got that right too. I think maybe he was trying to keep it short.

    I'd still like to know [ from anyone ] more about the effect the shape of the blade has on power out, rpms, startup speed, stability etc,

    Ron
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: four element prop
    hand made props. None are wood because they are HARD TO BALANCE and are unpredictable in weather even if sealed properly. Wood warps, bends, expands, contracts to the elements 10 times more than steel or plastics. If you wish it to last a week then don't balance it.

    The airplane industry hung wood props on aeroplanes, and they lasted more than a week. There must be a lesson or 3 there.
    I can imagine an unbalanced prop at 3,000 rpm would have some effect on the plane.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: four element prop

    My old plane had several wooden props. Worked find (plane was next to the California Coast and used to be tied up outside for years). Vibrated enough that * probably could not feel the difference if it was out of balance.

    I also ran an aluminum prop. for a few years. Was for a larger engine and my 65 HP could not get up to full RPM on ground run-up... However, it was a whole bunch more efficient. During climb, it performed about the same as the wood props, but way out performed the wood during cruise. I really liked it.

    Propeller design (to do it right) is a pretty sophisticated field in its own right. Designing one that operates efficiently over a wide range of wind speeds is impossible. You either have to pick a compromise or use variable pitch designs (like the more expensive aircraft do).

    Engineers and Scientists are still working on perfecting the "ideal" wind turbine blades even after these many years.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: four element prop

    Maybe that's the core of a good turbine, get a blown airplane engine, save the crankcase, and make a new drive shaft to fit in the bearings. Stick a biplane prop on it, and switch on the appliances.

    So, what made airplane engines not grenade with a prop a little bit out of balance? Why does it matter so much with small wind power ?

    Heck, WW1 & II, planes made it home with chunks shot off the prop, sure was not balanced anymore.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: four element prop

    There are a lot of people on the Otherpower forums that make their own wood props. A few used chainsaws to carve the blades! :cool: Don't know if anyone there made four-blade props like you describe, though. That site is primarily concerned with DIY, particularly for wind turbines although there are other topics as well.

    http://www.otherpower.com/ - Has lots of info on it, they even have a few books you can buy.

    http://www.fieldlines.com/ - Discussion forums. The site can be a bit slow to respond at times, they recently did an upgrade and are still ironing out some bugs...
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: four element prop

    Drive by any small airport where small planes are tied down. (Pretty rare these days!) In the old days, you would find that most of the planes had the props "parked" horizontal. The reason that I was taught was that the prop would absorb moisture evenly on each blade if they were horizontal, tending to keep them in balance. If the blades were left vertical, the bottom blade would gain more moisture than the top. Seems to make sense to me.

    As for planes coming home with holes shot out of the blades, I bet that was pretty rare, especially if the damage was very significant. I have a 3 bladed ceiling fan for example, and each blade weighs exactly the same according to Susan's kitchen scale, but I cannot run the fan on high speed because it looks as though it is going to shake itself off the ceiling. The is a fan of ~36" in diameter, turning maybe 200 rpm. Think about a wind turbine with a 60" rotor spinning 1000 rpm, one blade a couple of grams out of balance, and bingo, all kinds of problems get amplified.

    Tony
  • GreenPowerManiac
    GreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: four element prop

    Ron,

    What's the point of giving a close-up picture of the blades when you don't like PVC ?

    I'll post a few anyhow.

    They are very messy to cut, sand and balance despite having to weigh each blade on a digital postal scale.

    These have been very durable and have survived a couple bad incidents. Notice the blade rotation is counter-clockwise. One design flaw overlooked, now corrected.

    Once, the entire prop spun off and was laying in the weeds, undamaged. Another time, same thing except it hit my pole barn roof before landing on the ground, still untouched and put a good dent in my barn trim.
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  • vcallaway
    vcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Re: four element prop
    you would find that most of the planes had the props "parked" horizontal

    Probably because I was in a different region. The private airstrip I worked at as a kid had a different idea. A vertical prop meant we were to fuel the plane. We rotated them horizontal after fueling. I was never told why they were kept horizontal. I was just a dumb kid then anyways. :D