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Mart366 - n/a
29-Apr-07, 08:44 PM
<font size="7">[/COLOR]http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif it hovers http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif



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

Iceman - n/a
29-Apr-07, 09:11 PM
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.

Mart366 - n/a
30-Apr-07, 09:22 PM
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

Ian Brooks - n/a
30-Apr-07, 09:30 PM
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

Iceman - n/a
30-Apr-07, 09:34 PM
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.

Mart366 - n/a
30-Apr-07, 10:34 PM
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 http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif 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

Mart366 - n/a
1-May-07, 08:33 PM
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

Iceman - n/a
1-May-07, 08:50 PM
Hello again!



Fan diameter is the missing dimension now!

Mart366 - n/a
1-May-07, 09:34 PM
Hello again!



Fan diameter is the missing dimension now!






will have to get that tommorow... but if its any help its a vortex storm with the big duct on the back http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif





Mart

Howard Ash - n/a
2-May-07, 12:47 PM
The duct on a Vortex Storm is 0.95m diameter.



We run a Storm with an inverted 503 with integral gearbox with a reduction of 2.58:1. We've found that 6 blades in a 12 hub set to ~45 degress will quite happily absorb the engines power without over revving.



A reduction of 90:22 sounds like too much for a 503...



Howard

Mart366 - n/a
2-May-07, 04:13 PM
As Howy says...





duct size 0.95M



its a 447 not a 503 though...







does this mean more money being spent? http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif





Mart

Paul Fitz - n/a
2-May-07, 06:34 PM
I am afraid so martin. Even with 12 blades in the hub you would not be able to absorb the nom. 40 Hp you have available. as the fan will only spin at c. 1590 rpm with the engine at 6500 rpm.



You will need to change the bottom pulley. Contact me by PM with your current email address, The one I have bounced.



Paul

Iceman - n/a
3-May-07, 05:52 AM
Just check your lower pulley again, 22 HTD is VERY small, I've seen a 24 and that was on the minimal size!



If you do need to change the pulley, you will need to raise the engine (relative to the fan centre) so while you are waiting for your pulley you can ponder on how to do this.



For every tooth increase in pulley size, you need to lift the engine approx 4mm, so you will have quite a big difference to make up. You may find that the next size belt will be closer to your existing setup and need less modification.



Nothing is ever easy!

Jamie Lewendon - n/a
3-May-07, 11:48 AM
Mart,



Sorry to remind you mart, but we told you when we sold it to you, that you would have to change the ratio to run a 447. I also offered to work out the ratio when you had decided on an engine.

For reference, your duct is 930mm dia



That craft with that frame was for a honda RS spitting out 100hp at 13k with the blades at 45 deg.



If you are lucky, you won't have to alter the frame, just get a different size belt.



Jamie

Mart366 - n/a
3-May-07, 03:59 PM
Hi Jamie,



i was 2 years ago http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif my memory isnt that superb....



quick question though... if the honda was putting out 100&#64;13k,



surely then 40&#64;6800 should be a doddle for the set up? or am i missing something?



can i take you up on your offer then of working out what i would need





regs



Mart

Jamie Lewendon - n/a
3-May-07, 06:15 PM
Mart,



see pm



Jamie