Radio Controlled Hovercraft SRN1

So this was one of my Lock down builds, and in anticipation of building this and other models before the lock-down started I purchased loads of 3mm (1/8”) plywood and 1/8” Balsa wood.

Basically, I love hovercraft's! I think they are an awesome vehicle; they can go between water and land with ease. This particular hovercraft is inspired by the first practical hovercraft the SRN1. Have a look at the Wiki page for more information on the SRN1, but in its simplistic form it has a propeller mounted horizontally in a duct which blows air down into a plenum and then causes lift. Some of the air is allowed to escape through ports which allow jets of air to push the craft forward or backwards. As a prove of concept the SRN1 worked very well but required a lot of power to get it to hover and that’s where the skirt comes in! Adding a skirt allowed the SRN1 to hover much higher and improved its moveability.

My model is very similar. However right from the start I had decided to add two motors and propellers into the duct which counter rotated and hence would balance out any yaw effect from the use of only one motor/propeller. Like the SRN1 air is allowed to escape into ducts and directed forward and backward by servos.

1/8" or 3mm plywood about 8-10 square foot.

1/8" or 3mm balsa wood 4 planks 6" by 3 foot long.

Foam block for the duct. Off cut from a 4" think sheet used in building insulation.

Two brushless motors. Turnigy. D2822/14 1450Kv

Two 30 Amp ESC's (generic) hobbypower 30A loaded with SimonK firmware.

Two servos. Corona CS238MG Metal Gear Servo (4.6kg / 0.14sec / 22g)

Two propellers 6050 Bullnose CW and CCW.

Skirt material PU coated Nylon 4oz

Lipo 11.1V (3 cell) 1800mAh * 2

So Foam is a very enjoyable material to work with its quick and easy to shape on a lathe. However it is very messy and the dust gets everywhere. So make sure you use a dusk mask and have some kind of ventilation running. I happened to have purchased a couple of kitchen extractor fans which were to allow me to fibre glass in the garage and stop the fumes going up into the kitchen! So I used one of my fans to get rid of the dust from the foam.

To turn foam on a lathe i made up a 3/8" plywood plate which was screwed to the backing plate of the lathe. then I screwed 1" screws into the foam to hold it onto the plywood. However I also supported the front face using the tail stock pushing against a piece of plywood onto the foam. The best tool I found for cutting into the foam was a small sanding drum mounted in a handheld rotary tool.

You can see in the pictures I have made the duct in two pieces the main duct was first cut to the correct Outer diameter and then re-enforced with one layer of fiberglass cloth. Once this had set overnight the inner diameter could be turned out (just slightly larger than the 6" propeller). The bell-mouth was firstly shaped to the outside pattern and roughly shaped on the inside and not fully knocked through, once the bell was glued to the main duct the middle was knocked through and the inside finished with sand paper. Then the bell was re-enforced with fiberglass tissue.

Next I mounted the twin motors, take a look at the pictures as it shows how I have achieved this. I used two sections of aluminium angle and mounted the motors on either side using the motor mounts that came with the motors. You may notice that in the picture of the motors in the duct that both motor cables go the same way, I decided this wasn't going to be easy to do so made each motor cables go a different way as shown in the other motor pictures.

Then I glued two pieces of wood to the outside of the duct and carefully cut into the foam to slide the aluminium through the duct. I mounted the two 6" propellers to make sure everything was lined up.

This model was really made up as I went along. But the basic dimensions were set at the start. The plywood I buy comes in foot widths so I decided the model would be 2 foot in diameter and have 24 sides. To start with I printed out a few pages of the deck plan and then stuck them together to make up the full size, my printer only prints in A4 so it needed 3 pages to be stuck together to get the required pattern. Once I had the full-size pattern this was stuck to the top piece of ply with a glue stick. And to make things easy I decided to cut the 4 quarters out at the same time.

Once cut out the lightening sections were filled with balsa wood, this is simple to do but just takes a bit of time. Wiping the glue residue of with a damp cloth helps to keep everything neat. Then the 4 sections were glued together. As you can see in my pictures I have used "suitable" weights to keep all the bits together.

The bottom deck was cut out in 6 sections as you can see in the pictures. I had an idea to firstly try using the model without a skirt which is why 3 of the sections have holes in them, however this didn't work so the holes were filled in.

The balsa inserts are an attempt to keep the weight down and basically reduce the weight of the plywood by half.

Next, we move onto the bulkheads which due to the fact the hovercraft has 24 sides I needed 24 bulkheads. As with the top deck I cut out 4 at a time but needed to repeat that 6 times! And let’s not forget that each bulkhead has a little support!

Lathe time! I decided that the air entering the duct needed to flow onto the bottom and not just hit the flat surface of the hull. I have had experiences of air being so badly affected by hitting the floor that it lost most of its Pressure/flow so this step seemed to be worth doing. So, I turned up a nice shape for the air to flow around into the hull. But then there is another problem? The air is twisting/spinning around so I decided I needed some vanes to straighten the air. This bit took a bit of designing but in the end, I decided to make the vanes out of fiberglass by laying up a couple of layers of CSM (Chop Strand Matt) onto a section of drain pipe (68mm diameter shed pipe) which I had cleaned and waxed. Once the fiberglass had set I removed it from the drain pipe and cut it into sections. Then I made up a top and bottom retaining platform. I honestly don’t know if this was necessary or if it makes a difference but I was happy with it (and that’s all the matters)

To make things worse I had decided that I wanted to keep the top deck removeable. so all the bulkheads were only glued to the bottom plywood. And the turned wood section also needed to be removable to allow for the propeller to be changed if needed.

Once i had got all the bulkheads into place I left the hull to dry overnight with a wheel on top!

Then i made sure everything was inline and started to glue on the balsa sides. Not a lot to say about this step! Just glue each one on and hold it in place using pins. You should be able to see in the pictures that there is a gap of about 4mm at the top. this is where the air will escape to fill the skirt. The 4mm represents an area just slightly bigger than 1/3 of the main duct area. The principle is that 1/3 of the air should be used to make the hovercraft hover and 2/3 should be used to move the craft forwards.

The final picture shows the completed hull with the deck removed.

I did fibreglass over the hull using glass tissue.

So this step is out of sequence. The truth is I didn't know if this hovercraft would work?

Firstly i tried the hovercraft without a skirt, and no way was it going to hover, so then I blocked of the deck holes so 100% of the air was going down. And still it wasn't going to hover. So then I taped over all the side holes so the air could only escape through the 3 in the hull. It became very light but I am not sure I would call it hovering, and the pressure in the hull was so high it popped the deck off!

So the actual next step was to make the skirt and confirm the hovercraft would hover with the two holes in the deck for the puff ports. Once I made and fitted the skirt I tried the hovercraft and it worked.

So now I could design and make the puffports. My contingency at this point would have been to block of the deck ports and run 100% air to make it hover then add extra motors on the deck to move forward. But that wasn't needed so the puffports were made!

I spent a while trying to work out how the ports were going to function and where the servo was going to be placed and how it would connect. Hopefully you can see in the pictures how I achieved this? The servos are mounted in the rudders and move with the rudder whilst the servo horn stays still. all the bits were made from 1/8" 3mm plywood apart from the rudders which were 1/16"

The servo wire comes out of the pivot point and so the whole puffport looks very tidy!

The skirt is one of the hardest bits to design and make, but this is my 4 model hovercraft so i have got it sorted!

The first pictures shows the intended shape is 3/4 of a full circle with a radius of 75mm. To make this happen the joints in the 24 sections of skirt have to have the correct shape to inflate as desired.

One i had cut out all the sections i sewed them together and punched the holes in the bottom bits. I decided to only punch every other section which works well. I believe that you should calculate the area of all the holes and make sure it is a lesser area than the area of the slots allowing the air into the skirt. That way there should always be a pressure in the skirt. If the air can escape too quickly then you might not get the hovercraft to lift.

The skirt material is PU coated Nylon 4oz.

In the past I have made up a frame to hold on the whole skirt, but this time I decided that wouldn't really work so I cut out sections of retainer which covered one full section of the skirt and half the section of the next door bits. I started with the top and one by one I added the skirt, firstly I stuck the skirt down using double sided sticky tape then clamped it down using the retainers. This bit took a couple of hours but it turned out good in the end!

Its a TANK? ok so maybe not but the general principle of controlling the model is the same as a tank.

• Both lift fans were connected together and wired to a spare channel on my transmitter, i then configured the fans to come on with a switch and adjusted the power by adjusting the End Point Adjustment on the transmitter.
• The right servo was connected to the Elevator.(UK transmitter so the elevator is on the right)
• The left servo was connected to the Throttle. (UK transmitter so the throttle is on the left)
• Drive it like a tank both stick forward to go forwards, both backwards to reverse, and opposite directions to turn or spin.

So this was a experimental hovercraft which was made to fill in some of the Coronavirus lockdown time. I knew it wasn't going to be very quick but it was a challenge!

And it worked, However as the video explains it only worked with the motors running at 100% and only then on very flat surfaces.

I had loads of fun making this machine and i am pleased it works. when we are allowed back into public places i will try it out in a sports hall where i think it will pick up speed nicely!

I hope you have enjoyed this build. In the next step you will find the plans and if you are mad enough to try and build this model then feel free to ask any questions.

So as explained this model was basically made up as I went along!. So the plans are very hard to follow. However I will get loads of requests for plans so please find the PDF's below. Follow the sequence with the required number and sizes as per the list that follows. The videos gives far more information than I could write. and you are more than welcome to have the design file, just ask and I will email it to you. (I can't add the file here)

1. Make up the top deck plan using the 3 plan sheets stuck together. cut out 4 sections from 3mm ply.
2. Cut out 3 bits of each (two different bits) of the bottom sections from 3mm ply
3. Cut out the bulkheads and little support frame again 3mm plywood you will need 24 bulkheads
4. Cut out the centre bits you will need 6 bits to make the hexagon to hold the cone.
5. Make up the vanes (12 required) using fibreglass on a drain pipe.
6. Make up the vane supports you will need 4 halves but the top has to suit the duct. and the bottom halve should suit the cone.
7. Stick all the above together.
8. Cut out the puffport bits, 4 sides required, 2 deck bottoms, and 4 sections which go at the sides of the puffport and 8 bits which go above and below the puffport outlets. All 3mm ply
9. Cut out the rudder, the 4 blades are made from 1/16 plywood, the front and back where the servo fits is 3mm.
10. cut out the skirt retainers you need 12 big and 12 small sections. 3mm ply
11. cut out the front and rear cabs. 3mm ply.