[joncurtis Posts: 2068 Status: Offline]

I should be able to help with a cfd analysis.

[Ian Brooks Posts: 3551 Status: Offline]

Unless you were presented with real data then I would treat that with some scepticism. The papers I have read on the subject are all clear that length is important. The exit flow must have had time to achieve the maximum velocity before it leaves the duct otherwise it behaves like an open prop, I'd for the static pressure to have reduced to atmospheric. However, there are several things going on in duct design that can confuse the issue, such as the effect of suction on the inlet side. Duct design is complex and it really needs computational fluid dynamic analysis before it can be understood and optimised. I wish I had the resources to get on with it.

[Nick Flint Posts: 4866 Status: Offline]

Ref ducts---I was "told" recently this length may not be as impotent as believed earlier. hmmm - discuss---

[joncurtis Posts: 2068 Status: Offline]

With regards duct length, what's the graph look like, half the length of the diameter half the percentage increase? Or not that simple?

[Ian Brooks Posts: 3551 Status: Offline]

The graph



Comes from 'Air Cushion Craft' by Yun and Bliualt. I just added a few of our own data points out of curiosity.

What the graphs tells us is that thrust per hp is proportional to the cube root of the disc loading, where the disc loading is the diameter squared over power. Or put more simply, bigger is better!

Incidentally, this tells us that a ducted fan may be expected to make 13% more thrust than a similar sized open prop. However, a ducted fan should be as long as its diameter to achieve this gain.

Ian

[Eric Yeoman Posts: 724 Status: Offline]

[quote .   Also the Otter hovercraft seems to be the best overall design of craft around.  Is it possible to purchase one of these in any form.    Thanks Andrew


Not yet,but, when they are....the line starts behind me... 

[Ian Brooks Posts: 3551 Status: Offline]

Ref the Otter, unfortunately we don't have any plans to offer it commercially at this time. Having said that, we are continuing to refine the craft and the associated  technologies, so there may be future developments. If you are interested then let us know.

[ahaddow Posts: 14 Status: Offline]

thanks for that Ian.  Dont worry about the units I will sort this out for myself.   

[Ian Brooks Posts: 3551 Status: Offline]

The X axis is 'design parameter' which is basically diameter / sqrt (1000/power). I would need to to go check the units. Basically it allows us to compare various props and fans on the same graph. What you can see is that our installations are generally pretty poor! But we knew that, mainly it's due to compromises  to keep noise down

[ahaddow Posts: 14 Status: Offline]

Thanks for the reply John.   The graph is on page 2 of the general hovercraft pics.   The interesting thing about this graph is that it contains experimental as a well as theoretical data.  I have  used the other programs you mentioned and found them very interesting but how well do they predict actual craft.  Any thoughts on the Otter avialability?   

[John Robertson Posts: 5136 Status: Offline WWW]

Hi,

Not sure what graph you mean (do you have a link)?  You can model the performance of various craft (including static AND dynamic thrust) using our performance calculator or the pro performance calculator ( X axis is speed from both) - there's also a propeller thrust tool.

[ahaddow Posts: 14 Status: Offline]

Hello everyone,   this is the first time I have posted  on the forum.   There is a graph in the gallery of specific thrust which I think Mr Brooks posted.  Could someone tell me what is represented on the x axis.   Also the Otter hovercraft seems to be the best overall design of craft around.  Is it possible to purchase one of these in any form.    Thanks Andrew