View Full Version : Fan Blade Problem

Sean Atterbury - n/a
26-Jan-06, 04:40 AM
http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif Hi all.

I cant see what's happening,,,

Last night, set the blades to 45 and checked everything, clearance wize by hand first, it all misses by at least 10mm, then started her up and when ideling at 1800 revs there is a noise like the blades are hitting something, but at 2000 revs the noise goes away.

I checked and grinded more away and the result is the same, I cant seem to find where it is catching, I cave checked everything. Or should I just let it idle at 2000 ?

What would you do?

Is there a way to check it while running, like puting a piece of tape over the edges of the blades.?

lazza - n/a
26-Jan-06, 06:02 AM
Is there any rub marks on the duct where it shows signs of the blades hitting ? I would of thought personally that at 10mm tip clearance the chances of them rubbing would be slim unless there is excess movement in the fan frame. I have roughly 5 mm clearance on mine and now that i centered my fan properly they dont rub even at 6000rpm. Just wondering if something as simple as rubbing a bit of chalk on the tips would be enough to show if they are rubbing ? But if its anything like mine you would notice rub marks in the duct naturally anyway.

It's not something silly like the fan mesh rattleing is it ?

I'm not the expert here but thats my 2cents worth , and it cost ya nothing http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

Sean Atterbury - n/a
26-Jan-06, 06:25 AM
NO Not there, the whole fan around against the duct misses 5 mil but the fan is hitting something like the splitter or the fan frame, but there is ample clearance all over, it would make the most sence that it is hitting the splitter somewhere, the fan and splitter are now 15mm apart and the fan fram 30mm.

I see that there is some scratch marks on the verry tip of the blade, a bit mor investigating is in order.

I braught the craft to work so any questions can be answered ASAP.

Thanks. http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif

team black - n/a
26-Jan-06, 07:44 AM
The blades could be moving forward (ie lifting) until centrifugal force pulls them straight again, in other words, they probably are hitting the splitter

andycollins - n/a
26-Jan-06, 08:03 AM
I wonder if a car timing strobe light would help show where the trouble is?

Might be worth a try.

Otherwise look for

Places that can be reached by a blade pulling forward or untwisting under load.
Places that can be reached by a blade when the fan frame is pulled under load.
What happens to the splitter plate when it gets blown about. There will be a lot of buffetting and also the pressure may bend it.

Hope this helps.



Sean Atterbury - n/a
26-Jan-06, 08:24 AM
I Have been fulling and pushing on the blades when checking then, am a bit scared to pull to hard though, will quickly try the frame pull,......

OK.. fram is solid as a rock..

The splitter is tlayers if steel sheet rivited together, it can not flex any more and it is wedged in the mount like this [ so it does'nt flatter around much at all.

The fan mesh story could be a likely suspect as it is big and round as well as being home built. (Quite proude of it too)

Remember I have put some of that expanding foam around the inside because my clearance was not right, so now that will just rub away, the noise is more a nocking sound, like the blade is hitting.

Its a lot better now that I have taken off some of the splitter but still its a worry.

Took the craft to the exhaust people just now to get the exhaust bracket made, now it looks verry nice.

OK back again, I think that there where the duct and the rest of the craft is joined, there is the problem.. The chaffing is right at the tip of the trailing edge of the blade, thus somewhere there will be the problem.

jon_curtis - n/a
26-Jan-06, 08:31 AM
fan blades can move forward and backwards, to hit the frame and splitter.

the blades will move forward under load (high revs) and can wip back under rapid decelerration and wack the splitter!

your 503 should be ticking over at 2000 to 2200 and top out at about 6000 to 6200 static. (if it was in a microlight)6500 going along.

if it is ticking over rough the blades are probably wipping about a bit as the engine runs rough at low revs. get it to tick over at around 2000 to 2200 and see what its like, the tips of the blades should be 50mm ish from the splitter, with the route of the blades 10mm!

if the tips are too close its them that are hittin!

the fan frame should be a couple of inchs forward of the fan at least, the blades can move forward by that amount.

is the fan balanced? how did you balance it? if not balanced that will not help http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

Sean Atterbury - n/a
26-Jan-06, 08:40 AM
OK, My 503 came from a microlight and idles at 1800, however if I let her idle at 2000 then the noise goes away, I think your theory is on the money.

She revs to 6500 no problem and stays around that mark, ok she used to rev there, I have changed blade andgles and still do not know what we will get now, however with the blades at 40 and at 35 it oly reved to 6500 anyway, thus I feel a realy good motor as over reving will not occur.

My blades are at the moment 10mm at the centre from the splitter and 20mm at the tips.

Should I make them more at the tips?, seems kinda much te me.


So.. How do I set the carbs so that idle is now at 2000?

jon_curtis - n/a
26-Jan-06, 09:08 AM
is your splitter a curved plate? or flat in relation to the back of the fan? got a pic? 1 inch at the tips could be where is wacking.

whats your pulley ratio? how many blades do you have in there,

the engine over reving to past 6500 is not really to do with the engine, wether its good or not, its to do with the load on the engine from the fan. also the tick over speed will have more to do with the fan pitch and less with the carb setup! the carb or carbs? (which is it) is very important, but the engine must have the correct load, for it to tick over and top out properly.

is it twin carb?

whats the pulley ratio and how many blades? what dia. then can give you an estimate as to the correct fan pitch, then it takes time tweeking the pitch, running the engine, playing with the carbs untill the correct tick over and top end is found.

Sean Atterbury - n/a
26-Jan-06, 10:34 AM
Wow So many questions.

OK Multi-wing 9 blade hub with 6 blades & 850mm diam.

Rotax 503 twin carb dual ignition.

Ration 1 - 2.25 via HTD belt

Splitter streight with curved sides to stop escaping air when on full open. Hinged at back with door hinges.

hope this helps,

hope the pic gets there too.

jon_curtis - n/a
26-Jan-06, 02:07 PM
ok, do you know how to set the carbs up so they are balanced?

first if all if you have the pitch pins in, you dont need them, with the hubs all bolted up you wont need them 50 hp will not turn the blades in the hubs,

when it comes down to it only a mm or so on each blade will mean the difference between a couple of 100 rmp on tick over and top end.

i would take a little off the outer edge of the splitter if i was you. my splitter is flat, and it dont make much difference to air leaking past when it is raised. the gap at the tips is a good 30mm i think.

set the fan blades approx 40 degrees, and run the engine, get it up to temp and see what top end it will pull, like i said before you want to set it so it pulls about 6000 to 6200 when static as when your moving it will be higher due to the forward movement. if it goes past 6500 then pitch each blade up a little couple of mm, and try again.

this can take ages setting the fan, then running the engine then setting the fan, etc, untill it pulls 6200 approx static.

you can pitch the blades past 45 degrees if need be, but dont go past 50 degrees.

once you have it set at top revs, then you play with the idle screws on the carbs to get the tickover right, but you will find it will tick over around 2000 any how.

Sean Atterbury - n/a
27-Jan-06, 04:31 AM
OK, I know only a little about the carbs and they are as they came from the guy in Cape Town, so balanced, jes, I think they will be. The motor came off a working microlight.

Where are the timing screws?

I have already run the craft with the blades set at 40 but it reved to 6500 very easy, does'nt seem to matter what I make the blades,she reves to 6500, will see on Sunday what happens at 45, I also like having the pins in, I feel better with then in, I do not trust my eye and so the pins will stay leaving me with a 45 angle max, there is'nt a cut out for 50.

thanks http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

jon_curtis - n/a
27-Jan-06, 07:57 AM
ok dont listen to me! you dont need the pins, and i doubt very much your carbs are balanced!

regards the pitch, as i said mm will effect the fan matching, and the fan pitch has everything to do with the max revs the engine can pull. this you will learn when you look at the rev counter when your flying downwind the engine is doing 7500 revs and you have melted a piston.

regards your eye and getting the correct pitch, go to the multi wing website and there is a table showning the measured distance from edge of the blade to the centre of the hub, so you can measure exactly what angle each blade is at.

Sean Atterbury - n/a
30-Jan-06, 04:45 AM
http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif She gets 6 200, yay Perfect. Have posted a different problem now.