September 2003

As supplied, the K&M 900mm duct has a panel underneath the front of the bell-mouth which forms a bulkhead at the rear of the hull. This needed to be cut to size to suit the width of the cockpit, so I trimmed the panel and drilled it so it could be fixed to a couple of pieces of angle, which were themselves screwed to the T-section which braces the sides of the cockpit.

With the duct in place it started to look more like a real hovercraft, which made me think about the pulley arrangement. The pulley ratio is calculated from the engine speed and the fan speed. The hovercraft club puts a limit on the tip speed of the fan blades, to keep noise levels down. For 4Z blades the limit is 168m/s, which is half the speed of sound! This tip speed, equates to a rotational speed of 3565 rpm.

The TZR engine (which is race tuned) might reach around 11,000 rpm so the lowest ratio would be 10000/3565 = 3.086:1 I donít want to run the engine or the fan that fast, so I go for a 60 tooth top pulley and an 18 tooth bottom pulley giving me a ratio of 3.33:1.

Buoyancy

Thinking of buoyancy for the future I try out some aerosol-can polyfilla foam. It looks the biz, but word on the street (HCGB bulletin board) is that itís no good for hovercrafting. So I order some 2-part polyurethane foam from Glasplies, in Southport. This is supplied as two cans of chemicals (hence the 2-part). You measure equal amounts into separate containers, then when youíre ready you pour them both into another container and quickly mix them together. You then pour the mix into the space that needs filling, and the liquid foams and expands hugely. Oh but you must do this in reasonably warm weather, and not in a cold garage at the end of SeptemberÖ

Splitter plate.

Integrated hovercraft work by splitting 1/4 to 1/3 of the airflow from the single fan into the skirt. The problem with this is that at low engine revs, the air flow is low and so you get a low hover height. At high engine revs you get a high air flow and high hover height. Of course what you really want (for any given surface conditions) is a constant hover height, but thatís the drawback of an integrated craft.

I decided to build an adjustable splitter plate. One day this might be automatically controlled (using the rev counter signal and a motor driven height adjuster), but in the first instance manual adjustment will help get the split ratio right.

The splitter box sits under the duct and directs some of the airflow downwards and into the chambers inside the hull. The box was made from 1.5mm aluminium sheet, screwed to strip which is welded onto the frame tubes extending back from the cockpit sides. The rear strip was made from a length of 50mm aluminium with large holes cut out to lighten it. This time I cut the holes using a bi-metal hole saw on the drill press. Itís an easy job, but it generates a lot of swarf!

The splitter box side panels include a complicated profile to fit into the front of the duct. To get the splitter box to fit was just a case of trial and error, snipping away a bit at a time. The rear and top of the splitter box were made from a single piece of 1.5mm sheet, bent round to 90 degrees. This was joined to the side panels via a length of curved angle. Unfortunately, bending angle in this direction is very difficult, as youíre trying to thicken the metal in places, so instead I cut a series of notches in the side which will allowed it to take-up the relatively tight bend, then I planned to weld it all up again.

Well thatís the theory, but as the angle was relatively thin and narrow, and my welding is more suited to thick pieces, it took a few goes. The end result wasnít pretty, but itís functional, and as it will be hidden awayÖ
 
 

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