TOPIC: HOVERCRAFT

 

So having decided to design and build the F3 craft, I needed to decide on 3 key areas;

 

What hull material (and size)?

How many ducts?

What engine?

 

HULL

The obvious material was timber, ply. But it is heavy, doesn't like moisture and can be damaged easily. The top machines use advanced fibres and epoxy resin but this requires a pattern, mould tool and vacuum bagging etc. etc. Expensive and messy. I had produced a monocoque GRP car in the seventies which, for the competition shell, had a GRP roll cage - FIA approved! So I knew my composites.

But the final choice was HDPE - high density polyethylene. Lighter than water, bullet proof - used by SWAT teams and to armour plates vehicles. Slippery surface and self coloured so no painting. To assemble the hull required hot air welding. Clean and simple. And a eureka moment occurred when I found HDPE round drainage pipe which made for a tubular chassis with built in buoyancy.

So 10' x 6', the general size for this class. I estimate that the hull will weigh, bare, less than 30kg (66 lbs).

 

DUCTS

Now this is an interesting subject, one, two or three? The basic integrated design has one main duct and bleed off for the lift. Cheap and cheerful. But technically unsound for top performance. So I would have a separate lift duct at the front. As I see it, there needs to be a large wodge of air at the front and the craft then flies over this as it moves forward. Then, for a long time I favoured twin thrust ducts. Better control as they are outboard, lower profile for cross winds and, according to my calculations, more thrust per HP than a single. Contra rotating fans would be brilliant. And I looked at rotating the ducts for steering. Hydraulic drive (too heavy). But eventually I opted for one duct. Simpler. lighter and cheaper.

The actual duct design took much research on the internet and I think I have a good starting design. Thank you, Marc!

 

ENGINE

The class allows for a 500cc 4 stroke or a 250cc 2 stroke engine. With two engines you can mix and match. Now having come from the motor industry, real engines are 4 strokes. 2 strokes are for motor mowers and strimmers. So I started with a Honda CBR 400 4 stroke which gave me 50cc for a 2 stroke lift engine. But the Honda lump gave you a hernia just looking at it! So I then went to a Kawasaki ZZR400. Just as bad. Then I had another eureka moment, I discovered the 2 stroke! And after much research I opted for the Honda CR250 single cylinder engine, regarded by Honda as the finest 250 single ever. I can go into the spec in detail, but it weighs just 34 lbs and has an estimated 60 BHP at 10,800 rpm. One point of interest for you gentlemen is that I am using a Lectron carb at great expense.

 

So those are my basic parameters that were developed over 4/5 years. I then studied basic craft design to research every detail down to a single washer and nut. I was seeing what was trying to be done, how they did it and, of course, could it be done better. I have what I call my 'one percenters' - small changes that can't do any harm but should do better. And I have few 'ten percenters' - major redesigns that should be a distinct improvement. So, any comments?

 

 

 

 

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Author / Member : paul.haussauer

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