March 2004

Grab handles

The HCGB regulations for racing hovercraft state that the craft must have four handles fitted, two each side, front and back. In the Constructor’s Guide it gives options for the handles that need to be fitted around the edge of the hovercraft, and says to warn helpers not to lift the craft with them. Why not ? Surely you can make the hull strong enough, with strong enough handles and fixing points so that the handles can be used for lifting???

I could have bent up some aluminium tubing into handle shapes, but precision tight bending is difficult, especially to make 4 identical handles. So I decided to make the handles from spare lengths of webbing. I drilled through the outer frame near the rear corners and tapped the holes to take M8 button headed Allan screws. I then cut some short lengths from 2" wide webbing loops that were rated to 1.5 tons (thanks Meirion), and to prevent the webbing from fraying gave the ends a quick blast with a hot-air heat gun.

To cut the 8mm holes I first of all tried drilling them, but this didn't work as the fabric "fluffed up". Next I tried piercing them using a soldering iron, which worked really well. The bit of the iron melted a hole through the webbing, and simultaneously sealed the frayed end. I could then fit the straps to the frame using wide washers and Allen screws. As the fixing hole needs to be some way in from the end of the webbing I wrapped each end around the side of the frame, so clamping them top and bottom.
 

Rudders

I decided to build a twin balanced-rudder system, rather than use a single rudder. Although you can buy premade rudders they aren't cheap and I wanted to build my own. The idea behind the balanced rudder is that it pivots around a point about a third of the way in from the leading edge. As it turns, the airflow provides a little assistance, so that the steering effort from the driver, (and the stress in the control cables) is reduced.
 
 

I thought the easiest method of construction would be to bend a single sheet of aluminium into a rudder shaped sandwich using my small sheet metal bender (£40 from Machine Mart), and then fill the inside with foam to make it more rigid. The rudder isn't a structural component, but it does need to be strong enough to withstand the airflow. I thought 1mm thick aluminium would be OK as it was going to act largely as a skin.The plan was to fold the sheet to give a 10mm radius - a nice rounded leading edge, and then join the trailing ends together. I could then  insert a short length of aluminium U-channel into the top and bottom open ends to act as the outer of the pivot.

I drew 5 parallel lines, 7mm apart, to mark the fold points, and then put a 45 degree bend on each line using the sheet metal bender. This gave a good approximation to a rounded edge, and this was finished off by beating the fold over a length of 19mm tubing.
 
 
The rudders pivot on lengths of 12mm studding, running from the top of the duct, through the inside of the  rudders, and down to the splitter box. I made some brackets from a strip of steel, bent to match the curvature of the duct at the point where it was to be fitted.

I plan to replace the long M10 bolts with stainless button headed allen bolts, and trim the studding back to its minimum length.

 


There are a variety of ways of controlling the rudders e.g. a solid steering rod, or flexible bowden cable. The single cable method uses a stiff centre which can both pull and push, but these cables tend to be heavier, and they are supplied made up to certain lengths and can't be easily cut down to size. I went for a dual cable system, using lightweight bike-brake cable, where the cables can only pull, but they work in opposite directions. One cable pulls the rudders to the left and the other pulls the rudders to the right. A mechanical link between the two rudders means that they always operate together, at the same angle.
 

rudder bottom mounting

The bottom of the rudder pivots bolt onto the top of the splitter box, via two reinforcing discs.



Another engine frame

The first engine frame had a few problems - the steel tubing warped, the rad frame was too close to the lower pulley, and only one side of the engine could be adjusted up and down. I tried "de-warping" the steel tubing, by hitting it with the lump hammer. This only succeeded in making one hell of a racket and denting the surface of the tube. I didn't bend it though. There was only one way of sorting it out, I needed to make a new engine frame.
 
 
The second frame was of very similar design but corrected the problems of the first. In particular it had adjustable mounts on each side.The water pump would be attached to the rad bracket upright on the exhaust side.

The tall sloping uprights attach to the left side of the engine and the "bridge" supports the bottom of the radiator.


Trailer

Of course to get the hovercraft to a race I was going to need a trailer. As the hovercraft is 10 feet long and 6 feet wide, I was after a fairly large trailer, and the trouble is that large box trailers are usually built to carry a heavy load - and therefore cost a packet - but I only wanted to carry around 150kg. The obvious answer is some sort of boat trailer. Now boat trailers aren't the world's most complicated bits of machinery, but they do tend to cost a lot of money for what is really just a few lengths of steel tubing, two wheels, and a load of U-bolts.

A look around the second hand market showed that there were indeed trailers to be had, but needless to say they were all the wrong size.

What's this I'm reading on this honest to goodness real-engineering website? He's gone to the trouble of building most of a hovercraft and now he's thinking of buying a trailer rather than building one ????!!!!

Yes it's true. With the start of the racing season just 4 weeks away I decided that I didn't have time to build my own, so I headed off to the local trailer depot, and came home with a "Hallmark Merit 1". It's a nice trailer alright, but it was the best part of £500, and it didn't even come with a jockey wheel or light board!

Well serves him right.

Indeed, but it did need to be modified.
 

rollers on trailer

Once home, I unbolted the rubber keel rollers and the "bunks" (which are meant to support the side of the boat's hull) and their uprights, and layed the bunks on the cross members (to give something for the floor of the hovercrft to lie on) then lowered the hovercraft hull down onto the trailer. It almost fitted, but the load platform was just a bit low, and the side planing panels were clearly resting on the trailer's wheel arches. No problem. I removed the trailer's cross-members from under the main side rails, and re-attached them onto the top of the side rails, thereby raising the load platform by around 4 inches - perfect. I then drilled the cross-members and bolted the bunks into place more permanently.


The hovercraft will normally be hauled onto the trailer from the rear so I refitted the rubber keel rollers to the rear-most cross member. The spacing of the rollers was such that the floor of the hull would then be supported across its width when loading and unloading. An extra bunk was needed to support the front of the hovercraft floor. Closer inspection of the bunks showed that they were simply 4" x 2" lengths of timber, clad in a kind of tough carpet material. As I had a length of that timber handy I cut it down to size and covered it with some spare ribbed-rubber matting, and bolted this to the trailer.


So, due to the bolt-together construction of the original trailer it was a simple matter to turn it from a boat trailer into a quite different hovercraft trailer, without the need for many other parts. Of course all this work on the cross-members meant some severe drilling of 10mm holes was required, so any warranty on the trailer went straight out of the window.
 
 

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