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Thread: It hovers

  1. #1
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    Default It hovers

    <font size="7">[/COLOR] it hovers



    after 2 years in the garage it finally picked up its skirt and flew...



    Found a couple of issues though, most can be sorted via adjustment or aditional bits (sigh)... but the engine set up isnt right i dont think...



    it ticks over slightly high 2500 as opposed to 2000

    It revs to 7000rpm peasy easy... but i,m only on about quarter throttle..



    at about 4000 it starts to inflate the bag, and gets bouyant at about 4500 and picks up (hovers) at about 6000 but that leaves me very little to play with... 7000 to get some movement(eek)



    i,m guessing that the blade pitch needs to be changed. trouble is i.m not to sure which way?..



    any ideas?





    Mart




  2. #2
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    Default Re: It hovers

    Hi Mart



    you need to increase the pitch, and quite a long way by the sounds of it.



    Just check forst however that you are getting full carb slide movement on the throttle lever, you need to ensure at zero throttle the slides are as low as possible, and at full throttle the slides completely clear the throat of the carbs.



    Going back to the blades, the easiest way to remember which way to adjust blades is to think of being of being in a small boat with your hand in the water, if you hold your hand flat facing the direction of travel (high pitch) you will slow the boat down, and conversely, with your hand sideways on to the direction of travel (low pitch) the boat will go faster.



    So to make the engine run slower, you need to twist the blades so that they 'bite' more air.



    A simple way to adjust blades is to clamp a hacksaw blade to the frame somewhere so that it just touches the leading edge of the blades at the root. Then you can setup a gap or overlap with the blades accordingly. Slacken the bolts and turn the blades so that they just miss the end of the blade and then clamp them up again. This means the blades will all be very close in pitch and you can have a reference for subsequent adjustments.



    I hope that makes sense?!



    For your Rotax, you are aiming for a static rpm of 6,300 to 6,800.

  3. #3
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    Default Re: It hovers

    Whats the maximum angle of attack on the blades?



    i had a quick looksee earlier, but concentrated on the carby bits...



    if i stand and look at the fan from the front of the craft, i've got a large surface area facing me (well it looks that way:)) if i turn the blade any more it may be at 90 degrees ..?



    if this is the case, is that ok or... what are the other options for slowing down the engine?





    Mart

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    Default Re: It hovers

    Hi



    Whats the type & number of blades, duct size, and the ratio (gearbox or pulley)?



    If you let me know I can do a few sums.



    Ian

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    Default Re: It hovers

    Hi Mart,



    the maximum angle is probably 50 degrees (I expect that will get a few comments!!)



    From the front of the fan, the more blade you can see, the faster the engine will run, remember the water analogy and the fact that the blades will be moving 'sideways' around the duct, not forwards.



    Let me know fan size and pulley/gearbox ratio and I'll check the likely angle you will need.


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    Default Re: It hovers

    Quote Originally Posted by Iceman
    Hi Mart,



    the maximum angle is probably 50 degrees (I expect that will get a few comments!!)



    From the front of the fan, the more blade you can see, the faster the engine will run, remember the water analogy and the fact that the blades will be moving 'sideways' around the duct, not forwards.



    Let me know fan size and pulley/gearbox ratio and I'll check the likely angle you will need.





    Aha My interpritation of your description was slightly out, in that case i need to decrease the visible portion, which as you said will increase the surface ares presented to the air ( i think)



    i will work out the fan & other bits tommorow,



    the bits i do know





    Multiwing 5Z



    12 blade hub, I think 6 used



    not sure of bottom pully size or belt at this stage



    Mart

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    Default Re: It hovers

    Well i started stripping down this evening...





    the throttle now runs smoothly, when its fully open, i cant see the slide at all, it sits about 5-10mm above the airfilter apature, when released, it sits fully on the jet seat.



    I,m in the process of turning the fan blades, the data from that side of things is..



    Multiwing 5z blades x6 (12 blade hub)



    HTD 1440 8M 50 3527DS belt



    22teeth lower pully 90teeth upper pully/hub



    Mart

  8. #8
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    Default Re: It hovers

    Hello again!



    Fan diameter is the missing dimension now!




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