This page contains references to online plans to build a radio controlled 1/10 scale model hovercraft, designed and made available by Mark Porter at www.model-hovercraft.com.
This model is a good project because the plans are simple, the design works and it is adaptable to suit individual needs and resources (see the page of links on Mark's Model Griffon web page to other sites showing variations on the basic design). I decided to build this model because there is a lack of commercial models available, and I also enjoy self build projects. I hope this page will supplement and therefore enhance the original plans by giving a few hints and ideas. I've split my constructional hints into a few main sections to keep things simple. Apologies to people viewing this from outside the UK for the references made to UK only suppliers. Here is a photo of my craft finished and in action.
One word of warning before you embark on the project to build a hovercraft - do some research into suitable locations to run the finished model (especially for water - over land is less of a problem, but you'll need somewhere flat such as tarmac that is free from obstructions/ vehicles/ people etc). This may seem obvious, but you may be surprised at how many restrictions there are on where you can operate radio controlled boats, particularly if you don't live near the coast. Most sites of open water that are not rivers are either designated nature reserves, or are privately owned, or if they are publicly owned for recreational use, there still may be requirements for licensing to use craft on them. I'd recommend investigating local r/c boat clubs too - some will only allow sail, or at the least may frown upon things that are a bit out of the ordinary like hovercraft, but occasionally you'll find an open minded bunch of people. If you are lucky, the local authority may have a play pool or boating lake specifically set aside in a local park, but these are rare. I've had real problems finding suitable venues even though where I live is surrounded by gravel pits and small lakes, so I write from experience.
One of the difficult things in making the hull is keeping all the measurements accurate so that everything matches up correctly. I highly recommend using a set-square, protractor, metal ruler, sharp pencil, sharp knife, and good light to make all the measurements and markings. Also, for the duct holes, a circle cutter is essential to create a perfectly circular hole and to minimise finishing effort. A manual circular hole cutter which will go up to 210mm diameter is available (in the UK) from Screwfix direct (~£10 - code 10003 at www.screwfix.com), but this can only really be used on plywood up to ~4mm thick, which is OK for this project as it happens. If you choose not to use a circle cutter, then make sure you cut the hole with your saw slightly too small and then sand it out to the right size. A circular belt sander attachment for an electric drill is good for this, as you end up with a near circular hole and no odd lumps in the edge. For cutting balsa and thin plywood, the sharper the knife the better, as this creates clean cuts. For thicker ply, you can use a junior hacksaw or a coping saw, but it takes a lot of patience to get a straight line with these. I wouldn't recommend an electric handheld jigsaw, as these run a bit too fast to maintain a good cut. A bench mounted jigsaw may be useful, but I did not have access to one of these so cannot really comment.
For materials, some of the ones specified were unavailable. I know Mark intended the design to use very thin ply in order to keep the weight down, but I got away with slightly thicker ply for the main hull section, and also used 10x10mm balsa (again due to availability) without any great problems with weight, so don't panic if you can't find the exact ones specified. The main weight issue will come from batteries, motors and radio gear.
The majority of the hull is easy to construct and the plans are sufficiently detailed to knock one out quite quickly. Do take time to get the glue joints right though as even slight warps and misalignments will multiply up when you try to pull it all together. Don't forget to varnish and/or paint as you go along, as this will help seal the wood and protect it from warping if the atmospheric moisture content changes (not a problem if you are constructing inside a house, but for us shed/ garage dwellers, more of an issue).
Obviously, you'll need to keep the deck of the middle section open or hingeable in order to access the radio gear and batteries etc., but it might also be worth considering making the rear deck removable (albeit via more secure fixings) so that repairs can be effected and access be made available to the duct motor mount (if you've made this separate - see below) and rudder control mechanism. I recommend screwing the rear duct in place rather than gluing, or alternatively (but less secure) using brass flat hooks.
This is the most complicated part of the whole thing, so it's worth taking your time over. First cut your ring out - here, the circular hole cutter really comes into it's own, though with thicker ply, it's harder work. Don't forget to cut the outer edge first, then the inner part.
I came to realise that the inner wall of the duct is a cylinder, but the outer wall is a cone (well, duh!). If you are making a cone (see section diagram at http://www.btinternet.com/~mark.e.porter/2tdx_duct4.html), you cannot use a rectangular sheet of ply - it just won't roll right (try it with a sheet of card if you don't believe me). What you need is a slightly curved, or bent, rectangle. The way to work out the pattern for the conical part of the duct is based on simple geometry. You will find this easier to do if you have a CAD package on your computer (e.g. TurboCAD LE which is free from www.imsisoft.com). If you're not using CAD, you'll need a very large piece of paper - for cost effectiveness, I recommend plain lining wallpaper which is about £4 a roll which gives you loads to play with so it doesn't matter if you mess up a few times. What you need to do is first accurately draw a 1:1 scale side section view of the outer wall cone - for which you'll need the start and end diameters and the width of the duct. At this point, it is worth saying that you should get hold of the props you are going to use first, because you might not be able to get the ones specified by Mark, and therefore you may have to adjust the dimensions to suit whatever prop you can get (see next section). So, you'll start with a diagram of a trapezoid like that below.
Next, continue the sloping lines of the trapezoid upwards making sure the angle is accurate (if you're not sure, use a protractor to help if you're doing this manually). Eventually, the lines will meet and you'll end up with a very tall triangle. The side of the triangle will be the radius of a very large circle. The next stage is to draw an arc from the bottom corner of the trapezoid with the centre at the apex of the triangle. This arc needs to be as long as (Pi times the larger duct diameter) + 15mm. Then, draw another arc from the top corner of the trapezoid, again with the centre at the apex of the triangle and the radius equal to the length from the apex to the top of the trapezoid. This length will also need to be equal to (Pi x smaller duct diameter) + 15mm. You should be able to draw a line through the two end points of the arcs back to the apex of the triangle. The resultant shape is the pattern for the cone. (click the image below to enlarge it - it's very large, so beware, but it's not to scale)
Just cut it out of the paper, or if using CAD, print it out at 1:1 scale (you'll probably need to do it on several sheets and join them up again after printing, unless you happen to own an A0 plotter!) and then stick this over the sheet of ply and mark & cut around it. Now when you roll the resulting cut ply, it will roll into a cone that will fit the duct perfectly (if you've done your measurements properly).
When you've glued the duct walls to the ring, you'll be ready to move onto the leading edge strips. Here, I found it useful to calculate the angle of the cut (7.5°) and the length of each of the 24 pieces, which will vary according to the diameter of the duct ring. When you've cut and glued the strips in place, you might find it looks a bit messy - or not, if you're very careful - but what I did was to first sand down the corners, and then fill in any gaps with flexible wood filler, and then sand again. This way, you end up with a nice smooth leading edge ring.
The rudders I found to be a bit of a pain. I used small plastic hinges which I fixed into the horizontal bars, since this gave me convenient fixing points, but you may find a better way (I saw one version which had vertical bars slotted into the rear of the duct as well as the horizontal ones). The main difficulty with the rudders is getting them to move in a parallel fashion (this is not covered in the original plans). In the end, I found the method that worked for me was to use a thin strip of aluminium, drilled with three 2 mm holes which are spaced identically to the centres of the rudders. Then, I cut three thin rectangular holes (just wide enough to pass the aluminium strip through) near the tops of the rudders, but in line with each other when viewed from the side (due to the middle rudder being taller, the hole had to be lower down on this one than on the outer rudders). The metal strip was then slotted through the three holes, and 1 mm gauge steel wire was then pushed through the balsa of the rudders from the top, down through the slot and passing through the drilled holes in the metal strip, and then down further into the rudder balsa again. This was the only was I could see of getting proper parallel motion from the rudders. You could of course fit the strip at the bottom of the rudders, but I didn't want it to interfere with the control mechanism. If you have a better suggestion, please let me know and I'll update this section.
Now, once you have parallel rudder motion, you only need to control one rudder to control them all - I suggest using the middle one since it comes down to near the base of the duct, so is less visually obtrusive. The control rod mechanism for the rudders is a bit a a fudge job as well. I used another piece of steel rod bent in such a way as to embed in the balsa, go through a hole in the deck floor and then be hinge joined to the push rods. See the following diagram for details. Once I'd got the rod connected to the rudders, I used a ball joint and push rods connected through a bell crank in order to get the motion right for the servo mounted in the centre of the craft. Again, here you may have a more creative solution (I know some people use cable controls for instance), and there are no right and wrong approaches.
Mark suggests mounting the motor into the duct permanently using a horizontal balsa bar arrangement. I would recommend against this for two main reasons. One, if you ever want to change the motor for a different size or type, you'll have to rip the mountings out and make new ones, and second, if you need to do any maintenance or repairs to either the motor or the duct, it is easier if they are separate. If you look at a real Griffon, the motor is fixed on a tubular mount, so I feel it is better to re-create this for the sake of both practicality and authenticity. It's easy to do as well. All you need to do is go down to the plumbing section of your local DIY store and pick up a short section of 21mm overflow pipe, and a 34mm right-angle bend coupling. Also, pick up some PVC/plastic glue while you're there. Look at the following image to see what I mean.
The right-angle 34mm pipe is cut away, and the straight overflow pipe is glued in place into the hole. A small hole is drilled in the straight section to take the motor wires, and I wedged the motor in place because it is lightly smaller in diameter than the pipe (and this also allows for alignment of the fan in the duct). Next, you make the base - I used a combination of ply and balsa, the ply providing good support for the pipe and the balsa giving height. Again ,don't forget to drill a hole for the wires to exit through. The bottom of the overflow pipe is glued into the base piece using epoxy resin, and then the motor mount can be screwed into place from underneath in front of the duct once it's mounted on the rear deck. Incidentally, mounting the duct is not particularly easy, but I used two straight pieces of ply glued to the sides of the duct, and used a set-square to ensure the face of the duct was vertical, and then used a balsa pillar to prop up the back of the duct (see the diagram in the duct section).
I sought out my props based on availability and on what I had read about hovercraft. In full sized craft, the duct fans are usually multi-bladed, the theory being that more air is pushed through per revolution of the motor if the fan has more blades, although in order to do so, more power is needed from the motor. With this in mind, I decided to go for a 4 blade prop, rather than a standard two blade variety, as this seemed more likely to generate the sort of air flow needed, and meant I could get away with using a Speed 400 motor instead of the larger and heavier (and more power consuming) RS 540 and Speed 600 varieties. I picked Graupner 6" x 3" four blade pusher props for both the lift fan and the duct fan, which have a smaller diameter than the ones recommended by Mark, but make up for their size by having two extra blades. As a result, I made my duct and lift hole that much smaller. If you trawl the model shops, you can sometimes get specific duct fans, but these tend to be very small, and nowhere near enough potential air flow, and once you get to the scale we're working on, you only get two, three or four blade props. Some people have experimented with centrifugal fans taken from PC coolers, but I've yet to hear of successful results. Don't forget your prop adaptors - it can sometimes be difficult to get the right one for the job.
There are no explicit instructions for attaching the side decks to the main hull, but you'll find that there is a very narrow ledge upon which to fix them. In an ideal world, because of the lightweight nature of the decks, it should be sufficient to glue the decks on these ledges. However, in practical reality, it's likely that this won't be sufficient enough to provide a robust fixing. To improve matters, I added four balsa support struts between the outer edge of the side decks and the bottom edge of the hull. This provide added rigidity without adding much weight. Use epoxy resin to attach these as PVA will not be strong enough under tension should the side decks experience an upward pressure relative to the hull.
If you have made your skirt well (particularly if you've stitched it), it should be a tight fit on the hull. Attaching it to the thin edge of the side decks and front and rear ends of the main decks is relatively easy, but once you start to pull it onto the curved corners, the tightness of the skirt can start to tell. Because of this, you'll need a strong fixing to attach it. You could of course attach it to the underside of the decks and hull, but this leaves the 'rim' exposed and it become tricky to negotiate the side walls of the hull where they angle upwards towards the front and rear of the main deck. I used some very narrow carpet fixing tape (double sided) which I happened to have knocking around the place. This is devilish stuff, but pretty strong. Be careful to line up the skirt edges so that they are central to their attaching part - in other words, the long parts of the skirt need to be attached centrally to the long edges of the craft, and the short parts likewise to the ends. When you come to try and get the corners to attach, you'll probably find that there is too much tautness to be able to pull them up far, but this doesn't matter as the adjustment can be made where you attach the bottom edge of the skirt to the underside of the craft. You may find that contact adhesive is as (or more) effective, but whatever you use, it'll need to be strong, flexible and durable.
I made my cabin primarily out of balsa sheet and balsa square section strips. I'm not going to go into details, because as Mark points out, the design and measurements needed will vary in practice from machine to machine (plus, as can be seen in Marks Griffon Gallery, the attention to detail you want to give will make a big difference into construction technique). What I would draw your attention to, though, is the area around the lift fan. Obviously, for the fan to blow air down, it needs a good supply of air to blow. Initially, I tried what I had seen on other Griffon model craft which are attempts to mimic the vent ducts on a full sized machine, but these alone proved woefully inadequate and choked the fan. So, I opened up the back end of the cabin 'lid', again as I had seen in other models. This improved things, but there was still a noticeable difference between having the cabin on or off, and whilst the fan was going, as I fitted the cabin, there was a definite suction. I decided the only way to provide enough air to the fan was to open a hole in the roof of the cabin directly above the fan. This made all the difference. Instead of leaving the hole open, which is both unsightly and dangerous, I covered the hole with a fine aluminium mesh (the type used to bridge gaps in car body work prior to using a filler - available from Halfords or other car maintenance stores) which I held in place with balsa strips glued with epoxy. The mesh allows sufficient airflow to provide significant enough lift to cope with minor bumps and lumps (i.e. a lift on my craft of approximately 3cm).
Another point about the cabin is that it will add weight which can affect the balance of the craft (not really enough to affect lift), so it is worth leaving the positioning of batteries and radio gear until after the cabin has been fitted in place.
All drawings/ graphics and text on this page (c) Copyright Jason Collins-Webb, 2003