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|On the water at Eaton Park, Norwich.||At rest.|
In 1984 I decided to stop using IC engines altogether. I redesigned my development model and built this model from balsa, and used RS540 motors for lift and thrust.
The building of this model stems from an interest in hovercraft which dates back to the mid-sixties when I first saw one leave the sea and crawl across the road. I have since built several glow engine powered models but these are not suitable to operate in a park in the middle of town, mainly because of the noise they generate. After moving to an area with a good boating lake I decided to make something a bit more socially acceptable but still unconventional in the shape of a RIKO hydrofoil. Unfortunately this underwent conversion to a submarine after coming to the crunch at the side of the local boating lake.
My next project was to build an electric hovercraft and depart totally from convention. The subject is the British Hovercraft Corporation SR.N5 which, with its bigger sister the SR.N6, has become arguably the most successful hovercraft built. The SR.N6 is basically a stretched version of the SR.N5 and has an extra four ribs on each side deck. This plan can be used to construct either craft, and in this case the SR.N6 would be seven and a quarter inches longer than the model shown here.
I decided to construct an electric model and set a running time of fifteen minutes as a target. This not only meant that the model would have to be light but the skirt had to be efficient. This resulted in a structure predominantly made of balsa sheet with plywood formers to attach the standard RS540 motors.
The first problem was to find a suitable motor and fan combination. I tried to use an RS540 driving a small centrifugal fan from a hair-dryer but this just resulted in the cells getting very hot and the model refusing to lift off the ground. No chance of getting a long duration here! Next I tried a selection of propellers and some cooling fans which I bought from various radio shops. I set up the kitchen scales with a square foot of ply as a baffle board to find out which propeller or fan would produce the most thrust. A three blade 6x4 propeller produced a force of eleven ounces against the board but was not going to fit in the neck of a scale duct (the real one is waisted). After a bit of head scratching I moved the propeller to the mouth of the duct. At first sight this was not a very elegant solution, but it works well. The air is then fed through the skirt into the area below the Hovercraft. The amount of airflow thus produced is ample and compensates for the inefficiencies which are inevitable in such a small machine as this.
The hull is a box structure with two side beams running the length of the craft. The forward and rear cabin bulkheads are fitted between the beams, as is the rear wall of the buoyancy box. The underside of the hull is skinned with balsa sheet which is also extended sideways to provide the inner skirt attachment. Behind the cabin wall is a buoyancy box which, with the cabin, supports the craft if the lift motor should fail over water. The mounting frame which supports the lift and propulsion motors is attached to the top of the buoyancy box and the back of the rear cabin wall. Along the top of each side beam is a length of one quarter inch square balsa to carry the ribs for the sidebodies. At the front of the model the floor slopes up to meet the hinged bow ramp. On either side of the ramp are the inner walls. These provide extra strength in the case of head-on smashes. The back end also has to be rigid as it has to support the tail unit. This is one part of the craft which is particularly prone to accidental damage. The tail unit itself consists of two vertical surfaces with hinged rudders and two horizontal tailplanes. The rudders act together and provide the prime method of controlling the machine. The tailplanes are used to provide pitch trim control, and are set to the best position by experiment. I made the hinge pins for these from cocktail sticks.
The sidebodies are made from one sixteenth inch balsa ribs and are skinned underneath with one thirty-second inch sheet. The reason for this is that the air cushion supports this area and also it helps give a scale appearance. The ribs are set one eighth of an inch into the rails along the side of the hull and by the same amount into the outer skirt attachment beams. When adding the sidebodies it is best to jig the outer edges on 3 3/4 inch blocks, particularly when making the bow portion. Details of the bow construction can be seen in the photographs - a picture paints a thousand words! The ribs for the bow section are cut from the remaining ribs. Panniers are provided at the rear of each sidebody, the starboard one containing the rudder servo. Originally I fitted the radio receiver and batteries in the port pannier but this isnít a good idea if you want to keep the equipment in good condition - everything gets wet. The radio is now in the cabin with a Mardave Mk II speed controller, which is used solely for forward and reverse control of the thrust motor. The cabin roof is made from balsa sheet on formers apart from the front which is carved from balsa block. Some experimentation was also required to find the right size for the dorsal fin as without it the model tended to weathercock into wind.
The obvious departure from conventional boat or aircraft modelling techniques is the flexible skirt. The type used here has been used on several of my glow powered Hovercraft and is reasonably simple both to make and to alter, if you do not quite get it right first time. Also most modellers have recourse to an expert with a sewing machine who will be only to pleased to be involved in the construction of the latest project! The material used is polyester lining which can be bought from most fabric shops and is marked out as follows. First the material should be pinned down evenly and divided into eight and a half-inch wide strips. Then one of the end templates is drawn around. Measure the A and B lengths along the edges of the strips. These will be the outer and inner edges respectively. Draw around the other end template to complete the section. This is sewn as shown in the skirt drawing, remembering to leave an overlap of about one quarter of an inch all round. It is wise to mark out one left-handed and one right-handed version of each section. This will make it easier to assemble the skirt.
There are no holes in either the base of the craft or the inner wall of the skirt to provide airflow to the cushion, as the material is leaky enough. I do not recommend making any holes as water will quickly be scooped up through them. The model works well with the skirt like this, but to prevent air being wasted through the outer skirt wall mine has a rubberised coating. This is easier to do than it first appears. The coating is applied by brushing or spraying the outer wall of the skirt with Copydex thinned with water.
After the first trials with an RS380 providing the thrust it was apparent that a larger motor was required. The RS540 was fitted and drives the two 6x4 props through a universal coupling.
Having completed the model my impatience got the better of me so an early attempt was made to cross the local pond. Going with the wind was easy, apart from the tendency to weathercock. It was much harder going the other way through the four-inch waves, whipped up by the worst weather we have had this year. As mentioned before, the main method of control is by using the two rudders. It is alarming to find that the immediate effect on the craft is only to change the direction in which it is pointing, particularly on land! It blithely carries on towards the nearest wall/yacht/duck. The trick is to yaw past the angle through which you want to turn, wait until the model starts to actually go in the right direction, and then straighten it up with opposite rudder. Performance is much better on land than on water, providing the skirt is dry. This sometimes makes transition from water to land difficult, and effective control is only regained when the airflow has blown the excess water away.
In making this model I have had to obtain a lot of information from various sources. As the SR.N5 was first built in 1964 most of this is historic, and I would particularly like to thank Hover Publications, the British Hovercraft Corporation and Rolls Royce for their help.
This model appeared, with a free pull-out plan, in the May/June 1986 edition of " Radio Control Boat Modeller " .