Before designing my mk 1, I hadn't seen any model hovercraft, and I find it interesting how closely the result resembles the layout of Mark Porter's Griffon design.
The design criteria were:
My reasoning went like this:
Fairly cheap suggested building it on the basis of motors no more than 400 size, which seem to be the point above which motors become disproportionately expensive. It also meant 7.2V (6 cell) buggy packs for batteries, because I had some of them lying around from other things.
For a 400 motor, a 6" propeller seems a suitable sort of size for direct drive. Make the lift arrangement a 6" propeller on a 400 motor with a vertical shaft, running in a duct.
Stick that duct at one end of a punt-like (for for ease of construction) hull, making the width of the hull about the same as the diameter of duct, and the length sufficient to accommodate a 6-cell battery pack, receiver and some speed controllers without being too congested.
That's quite long-and-thin, and it looked like it should be wider, so just cantilever a deck from each side of the hull, and it still qualified as fairly small.
My background in modelling is in free-flight, mainly scale or semi-scale small rubber-power (peanut scale up to about 2' span), with occasional dabbles in CO2 power. All are conventional balsa-and -tissue type construction, though with some more modern covering material occasionally. Consequently, it didn't occur to me to build in anything other than balsa.
For the skirt, I found a discussion on the web (can't remember where) of various skirt types, and picked bag skirt as what looked simplest to get right. I used to make kites, so I had a load of ripstop nylon stashed away, and I'm happy sewing it (tack with copydex to stop pieces slipping over each other, sew on a machine).
Propulsion was by two 75mm diameter GWS ducted fans using 400 size motors, controlled by individual boat-type speed controllers driven by an on-board V-tail mixer. This was mainly because I thought it looked better than a (proportionately) large prop sticking up at the back, but it also simplified construction because there was no need for rudder hinges and linkages. The mounting was trivial - each fan had a couple of locating lugs glued to the cantilevering deck, and then a large cable tie went round the fan and through the deck to hold it in place.
Some aspects of detail were that I thought I'd need actual scoops in the wash from the lift prop to get enough air into the skirt. I also thought inclined side decks would look nicer than horizontal, though it does make calculating the shape of the skirt fabric panels more tricky.
A couple of photos of the hull before the cantilever decks are added, which
show the structure most clearly:
Prototype skirt made out of black plastic sack (it is hovering, but the
camera flash has frozen the propeller):
Finished Mk1 hovercraft:
...was not great.
Actually, that's not fair - it hovered ok and actually I was fairly pleased, since as a prototype it might not even have managed that. However, it didn't hover evenly. I think what was happening was that the lift effect on the deck was partly pressure underneath, but the direct prop-wash from the propeller in the lift duct was contributing a large part of the lift. That meant that the centre of lift was a long way forward of the centre of weight, and it was more-or-less impossible to balance with my configuration, because there was no way to get weight in front of the lift duct.
The controllers, propulsion fans and lift motor were transferred to Mk2A, leaving the Mk1 as a rather sad gutted hull:
back to the top hovercraft page
back to Ian's contents page
back to www.astounding.org.uk top contents page
To comment on anything (please do) email ian@astounding.org.uk