[Nick Flint Posts: 5070 Status: Offline]

Otter Thrust is Multy wing 7W blades.
6 Blade hub harbouring 4 blades of 750mms length, ie diameter of 1500mms, ie two empty spaces
Absorbs around 30 BHP with a pleasant blowy noise  .
The  blade/blade/space, blade/blade/space set up assures the noise is broken up into multiple whooshes.
Tis most agreeable to be near. 

[Eric Yeoman Posts: 779 Status: Offline]

"My Otter blades at root are 200mms! tapering to 110mms."
 
Hi, I find this most interesting as I'm starting a new project as soon as I get my shed organised, major sort out. Are these 5Z blades? I'm assuming 54". I'll be using a 24hp thrust engine & would like to know the pulley sizes please. thanks. Another thing the 4z blades , ( I think that's correct ), have disappeared from the Multi-Wing catalogue, down here. Will 5Z blades work as lift units? thanks again.  PS. this will be  a 16' Surveyor twin engine machine.

[Al Posts: 1008 Status: Offline WWW]

Reason I asked about thrust fan is I may build another hover, but at the moment it is a "May"  and not a definite.  For ease of acquiring and significant cost difference, I was thinking use a fan rather than a prop for thrust.  £ to $ plus import duties etc is going to make buying an American prop rather expensive in comparison to a multiwing fan.

[Nick Flint Posts: 5070 Status: Offline]

I guess in OUR cruising ducts the internal shape is best described as the upper surface of an aerofoil section, with the outer duct surface generaly parallel to direction of travel. Thats how I carved it!
This results in good smooth curve into the bell mouth ensuring less turbulent air ingestion in front of the fan/prop.
Yes John, MY KingFisher propellor was noisier than my Otter, rather than quieter as they promised! The main advantage (use) of the duct is to fasten the mesh to so as to protect against prop ingestion. A large tip clearance also results in less likely blade strike which so OFTEN results in racing hovercraft duct explosions - often as a result of striking the ground or another craft with force.

The "Aspect ratio" of a prop/fanblade (or wing) is important. ie the ratio of wing/blade Chord to Length of wing blade.
The longer  and thinner a blade or wing is- the MORE efficient, but the trade off is always against a consequent loss of lift/thrust- (per unit length)!
OWL wings (Very Quiet) have a superb  chord to length ratio (ie stubby) with feathered trailing edge to reduce noise (pardon the pun)
My Otter blades at root are 200mms! tapering to 110mms. (very LOW aspect Ratio - AKA "OWLISH")   
Ive just looked it up to remind me!


My Koolprop KingFisher 1  Prop blades were nearer 100mms down to 75mms, HOGH aspect ratio and as such, as John suggests the higher RPM needed for  required BHP absorption and thrust resulted in a high RPM when the WACKY WACKY noises began to intrude. 
These blades resemble the wings of a soaring edge record winning sailplane. Long and thin!

On KingFisher 1 there was NO thrust duct as such - just an open frame to allow alleged increase in thrust afforded from sideways ingestion which allegedly occurs. Side ingestion was tested by me with trepidation and a piece of loose wool! Yep- it was true, and I have some pants to prove it. 

As cruisers - we "cruise along" - hence the name    with soporific smiles in the sure and certain expectation that our blades are unable to indulge in duct strike.
Ive seen racing blades cut with 5 mms clearance in the presumed expectation of more thrust efficiency.
Running the fans so FAST at racing meetings was then a recipe for fan sale profits. Ive also seen the queue outside the fan blade supply trailer at race meetings.!!!
I always raced with approx 15 mms and NEVER had a blow out.
40 mms is bigger and safer!
Ive raced.
Ive cruised.
Cruising is better, safer, quieter, cheaper, greener, and more socially enjoyable over two weeks. You cant fish whilst racing. 

[Al Posts: 1008 Status: Offline WWW]

Yep - its' not a conventional "duct" (same as most hovercraft!).   It's designed to offer aesthetics/protection AND, importantly, to help reduce thrust loss caused by the excessive twist on the fan blades.  Unlike a lower-twist propeller fans throw a huge amount of air sideways (in an air duct they are designed for, it doesn't matter as it's all inside the duct anyway!).
You would get more thrust using a larger open prop BUT craft size limitations make that impossible.  Commercially available smaller props generally have a small chord (width) so need to be spun much faster to develop thrust (i.e they are noisy!). Just like everything else it's a compromise based on what is readily available commercially!

So does it matter much on what shape the chord of the duct is?  (e.g. flat bottom or equal chord etc).  I'm imagining an aircraft wing that has been formed into a circle, some wing patterns give more lift when aircraft is correct way up (Clarke Y section) good lift at slow speed, where as others give good lift whether the aircraft is the correct way up or flying upside down.
 

[John Robertson Posts: 5275 Status: Offline WWW]

Yep - its' not a conventional "duct" (same as most hovercraft!).   It's designed to offer aesthetics/protection AND, importantly, to help reduce thrust loss caused by the excessive twist on the fan blades.  Unlike a lower-twist propeller fans throw a huge amount of air sideways (in an air duct they are designed for, it doesn't matter as it's all inside the duct anyway!).
You would get more thrust using a larger open prop BUT craft size limitations make that impossible.  Commercially available smaller props generally have a small chord (width) so need to be spun much faster to develop thrust (i.e they are noisy!). Just like everything else it's a compromise based on what is readily available commercially!

[Nick Flint Posts: 5070 Status: Offline]

Im not sure "versions of the truth" applies here, as its not very important in the grand swath of things! ,,, but well spotted Al!
You are IMHO correct when you say a "thrust duct needs to be long, (er)" than the Otter.
I gather the Otter duct is mainly a guard against tree ingestion, and on this ONE account, I can report it was partially successful in that
instead of ingesting a tree, my craft just ingested a branch thus shredding a little mesh but my thrust fans remained operational to blow another day. It was all part of my sojourn up a lazy tributary flowing into the NE Loch Fyne (towards the Brewery) as I found i could get quite far up stream, but flowing with the water down stream I had reduced rudder authority. I just love exploring MY limits as well as geographically!
I gather the other aspect of blade tip clearance applies here, Which is about 40 mms. This is too big a clearance to prevent blade tip turbulence IMHO.
So I guess its best called " Guard against errant trees" and helps little in extra thrust generation, which we dont need anyway!
I call mine ---- "Gary the Guard" but "we dont talk about Gary".

[Al Posts: 1008 Status: Offline WWW]

I've heard two different versions of truth about the otter thrust fan     One is that the outer prop/fan guard is also a thrust duct, but I was always under the impression that a thrust duct needed to be quite long to be effective.


Other version of the truth I heard is that a thrust duct isn't needed and the outer ring around the otter prop/fan is just a guard


Can anyone shed some light on which is correct?


The use of large fan rather than a prop is a game changer, not only in ease of obtaining  the fan in the UK, but also financially on purchase price and lack of import duties etc.

[Nick Flint Posts: 5070 Status: Offline]

Yes the big black OTTER blades can deliver enough thrust at reasonable noise levels at low cost!
Life is spent learning, until you begin to know everything, just as you start to forget everything.


 

[Gavin Parson Posts: 170 Status: Offline]

Kingfisher 1 is near the top of my list of craft with quiet lift. I note what you say about the shortcomings of the thrust propellor and felt that the quietness of the lift was negated by the thrust prop. Indeed, fitting an Otter thrust setup would have made a very quite combo.
It's easy to carry on modding a new design of craft or a "different" setup when all you want to do is get out hovering. My Osprey 3 is like it. The experimental setup was very quiet but just a bit too complex and too heavy. It's being modded again but I want to have at least one craft I can use. Hence the purchase of the TS3.

[Nick Flint Posts: 5070 Status: Offline]

KingFisher 1 from olden times on Loch Fyne
Just checked- sold 4.5 years ago!
A super looking quiet craft, but oh so big to own and operate.
I could do much better with hindsight and experience, but I guess that ALWAYS happens.
1 I was considering a new BAG skirt as per SEV design as the fitted finger skirt material was already deteriorating,
and I could have attended to a number of "design alleyways" that were "less than ideal"
Part of the attraction to renewal was also to ditch my errors into the bin, and raise hover height, and decrease need for running repairs.
Bag skirt would have decreased low speed drag and help get over the hump.
Top speed would have improved as well as radically decrease plough in.
I got lots correct, and "good enough"


2 The thrust propellor was designed around rather inflated descriptions of the Canadian Koolprop.
In retrospect their inability to provide a thrust/ revs /power curve SHOULD have rung alarm bells.
"Too trusting"!?
I should have ensured low noise by relying on "big size = everything" rather than a manufacturers empty promises.
Plenty of hull width to accommodate a LARGE propellor.
I Didnt know and didnt think to ask, because I didnt know I didnt know!


3 Thrust engine buried inside hull for low noise, should have been higher (accessible and cool and shorter more easily managed drive train) rather than buried (inaccessible! and hot with FLOPPY belts), then noise reduced by shielding.


4 On the sea she was correct size and looked a treat.
5 On the road trailer , driving was testing especially for oncoming traffic   
6 Lift fan was astonishingly quiet, but in retro could have been mounted directly to engine, for simplicity and efficiency and lightness and cost 
7 Twin pot epoxy paint turned out superbly.


My excuses revolve around the fact I designed and started build before our club was formed, or joined!
Following advice and discussions - improvements were achieved sufficient to enjoy the craft operations.
I guess I sold it too early- ie before making the above improvements, but my Otter craft so much better fits my remit, that I am much happier with KingFisher 2


 

[Nick Flint Posts: 5070 Status: Offline]

Thanks for the updates.
Time flies when one is subject to pleasure

[Gaz Posts: 2387 Status: Offline Cruising around Cornwall WWW]

Gaz, do you still have this "somewhere"?
What state is it in?

It's a Sevtec Scout, ATM it's just stored outside Liskeard here in "Sunny" Cornwall. It's all but ready to use, needs a couple of hours of structural work & a checkup. We serviced it just before laying it up about 4 years ago so there shouldn't be a lot to do.
My local ATC Squadron has now  all but finished their new Surveyor and have handed the scout back to the club. As it is HoverClubs loan craft were currently looking for a new youth group to take it on, ANY TAKERS?
It will be heading for the Severn Hoverport later in the year once I have dragged Mr Brooks to Cornwall to help with skirt construction on my Squadrons new Surveyor, hopefully we'll have that ready to use in the spring.

[Nick Flint Posts: 5070 Status: Offline]

"Lara Craft" -designed / built by Gavin Parson - BLESSED by God - no longer owned by Kim and Val Taylor

[Nick Flint Posts: 5070 Status: Offline]

JANE FLINT on Porthmadog beach with her resident (obedient) chauffeur in KingFisher 1
Hes "one helluva guy" (I hear)
Other opinions are available.