View Full Version : tip and splitter clearance

erbus - n/a
19-Aug-06, 04:44 PM
... finally have engine mounted and blade length set as best as possible. However, tip clearance is 6-7mm around one side of the duct and splitter clearance is 14-15mm on one side (whole engine is slightly twisted on craft - but difficult to correct).

Are these clearances too close for comfort ... engine, gearbox and fan are mounted on very stiff mounts.

Any advice most welcome



srn4 - n/a
19-Aug-06, 11:12 PM
6-7mm sounds ok. Some have slightly less, but thats fine...

Are you saying that the distance between the actual splitter plate and rotating blades is 14-15mm? If you are then this is not safe.....

The blades flap a lot under load and you should have atleast 50mm between the rotating fan disk and the splitter....IMHO...

Derek Sweetman - n/a
20-Aug-06, 08:35 AM
I agree 6-7mm tip clearance should not cause a problem.

But the blades when under load pull forward slightly so 12-20mm clearance from the splitter will be ok.

Too big a gap between blade and splitter plate can cause the lift air to leak back which can cause problems - depending on how sparingly your splitter plate is set!

Hope that helps


erbus - n/a
20-Aug-06, 09:56 AM
Thanks for those, quite different on splitter gap. i only ask as i can close it all up today or strip it all down and try and get a few more mm. from what you say is it my understanding that the blades, under load, can move forward? if so, when off load do they then 'over' spring back which is what my clearance is to accommadate?

i will have a look this morning and see if i can easily get a little more clearance.

thanks for the definative clearance on the tips though.

much appreciated


srn4 - n/a
20-Aug-06, 12:29 PM
Perhaps my 50mm was a little bit over the top.....Just flex the blades a bit and see where they are going to move to when they bend and then use your own judgement.

Don83000 - n/a
20-Aug-06, 02:22 PM
I have always run a splitter clearance of 18mm at the blade tips with no problems on a 900mm duct.

erbus - n/a
20-Aug-06, 06:38 PM
many thanks for all the help....

hopefully will get it back in the sea next weekend with improved lift now everything is aligned regarding duct and splitter...



kach22i - n/a
21-Aug-06, 02:47 PM
I'm putting some vinyl cove base strip on my modified splitter (1989 Scat II), and letting the glue set, will be taking the duct tape off soon. I will then tape the edges to check for wear and to clean up the airstream.

My understanding is you can not get too close. That under speed the blades are drawn foreward. If you plow in or stop quickly the blades are said not to whiplash backwards - I really hope this is correct.

Sean Atterbury - n/a
23-Aug-06, 08:29 AM
What is that blue stuff?

I am looking for something like that to chuck between my fan and duct as the clearance is no as I would like it.

Can you fibreglass over it?

kach22i - n/a
23-Aug-06, 12:19 PM
The blue stuff is a compressible foam wich is almost impossible to glue to. It's called "Sill Sealer" or sill plate sealer.

This is an experiment which I will probably have to remove with mineral spirits.

So far I have experienced sucking forces which result in the material being lifted up and cut by the fan.

Also in areas outside the fan any debris entering the duct or exiting the duct takes out small chucks of foam with it as it has no impact or tearing resistance.

I don't know if it's my imagination, but it seems a lot more quite at idle, no different at full rpm's unfortunately.

Don't try this at home folks.

It's just a learning curve us slow folks are on. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

EDIT: This topic (clearence) came up before, see link below

http://hovercraft.org.uk/FUDforum2/index.php?t=msg&th=53 (http://hovercraft.org.uk/FUDforum2/index.php?t=msg&th=534&start=0&S=0f084326bf15cf5c9a8d58059fe6fa52) 4&start=0&S=0f084326bf15cf5c9a8d58059fe6fa52

<table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote:</td></tr><tr><td class="quote">
i reckon 10-13mm is a sensible clearance if you're not concerned about peformance, any more and you are throwing air away needlessley

any system that deflects more than that needs to be more rigid

splitter gap will benefit from being close, blades will deflect forwards under load and don't spring back noticably during deceleration - a 50mm gap is safe enough again without compromising things dramatically

this is just my opinion based on my experience others will differ - take what everyone says and then make your own mind up

good luck


erbus - n/a
23-Aug-06, 04:12 PM

i originally did ask the question and raise that topic but ...

after making a new engine mount and reshaping the duct to make it 'more round' i found my clearances a bit more stingy than i had hoped for. increasing the clearances means a fair more amount of work so was just asking if what i ended up with would be ok (to get more clearance means stripping everything out and unfortunately the grill forms part of structure shaping the duct and everytime i remove it things change slightly hence hoping to stick with what i've got)