This page describes design evolution from the Mark 1 design.
This time I looked at the web a bit. I found
Mark Porter's Griffon design,
which seems to be the most popular design of model hovercraft on the web,
perhaps because it's so well written up on that site. I nicked two points
from his design:
1: I built my hull from thin plywood (didn't occur to me before because I've
never modelled in plywood), and
2: I saw that you can inflate a skirt just by putting holes in the side of the
lift duct.
The second of these in particular I should have worked out for myself - the whole volume below the lift prop will be higher than ambient pressure air, and since you don't need much volume flowing (theoretically, don't have any flow) through a bag skirt, you just need that pressure. Holes parallel to the flow will give access to the pressure, and inflate the skirt quite happily. (Of course, if the flow out from under the hull is unrestricted, the pressure under will match that in the general atmosphere, and then (by Bernoulli) this arrangement actually sucks the air out of the skirt - but that doesn't happen when it's running properly.)
In addition to the two noted above, the changes to the design (and my reasoning) were as follows:
I was aware that the Mk1 hadn't really worked the propulsion out, so decided to make the propulsion system mount on a sub-frame that would lift out of the hull. The point of this was that if my propulsion arrangements didn't work, I could build an alternative sub-frame and slot it in. It also means that jet turbine propulsion system remains a possibility...(if you're not aware of them, wander over to Wren Turbines and have a marvel).
Since it was modular, I decided to build a very rough-and ready mount for the ducted fans from the Mk 1 (two 75mm diameter GWS fans), while I got lift sorted out. This is why it's the Mark 2A.
Construction was fairly routine, and there's a bit of a photo sequence:
Lift duct being assembled
Main hull parts cut
Lift duct installed onto hull bottom
Supporting framing around the lift duct
Side pieces of hull going on
Bow in place (I reinforced it - probably redundantly - but if the battery
broke free in a head-on crash this is what will stop it)
Decking going into place around lift duct
Cantilever deck supports installed. The side decks go on top of these, with
a perimeter piece of hard balsa to stiffen and toughen the edge
Skirt fabric marked out
Skirt being sewn
Complete skirt and hull waiting to be joined (by double-sided sticky
tape)
I made the design up as I want along, but most of the time I drew it before actually cutting wood. My work means I have access to CAD systems, and consequently, I have much of the hull drawn on CAD. Therefore, here are drawings of the basic hull and the skirt. The images as shown are not to a particular scale, but if you click on them you'll get a duplicate at the sale of 2 pixels = 1mm.
Hull
Skirt pieces. There's also a separate page describing
how I drew the skirt shapes.
An isometric of the hull
Lift was checked by loading the craft with cans from the kitchen the cupboard. Its capability surprised me - I ran out of secure locations for cans (and I didn't want one slipping into the propeller while it was running) before I actually ran out of lift.
With the propulsion unit fitted the hovercraft was much improved over the mark 1. In fact, it fully met my expectations, so (of course) I raised my expectations, and decided to improve it.
The initial propulsion system (plain GWS fans, 6 cell 'buggy-pack' battery) was quite good on dry land, but couldn't achieve hump speed on water (which means it was sluggish if it started from rest on water, but if it got up to speed then crossed onto water it stayed fast).
The GWS fans come with a 400 size motor, but it's nothing sparkling. In GWS favour, their quoted thrust figures are about right - running two fans on 6 cells I measured a total thrust of 225g, which is pretty much spot on what their thrust table claims. I guess that means the rest of the tables are realistic values too.
I adopted a two-pronged approach to more power. I took out the supplied motor, and put in some 'electramax 480BB' motors from Overlander. These are one of those 480 motors that if you take the flux ring off, they fortuitously turn out to be the same diameter, shaft diameter and mounting screws as a standard 400 motor, so go into the fans with no problems. Arguably, taking the flux ring off improves suitability for ducted fan anyway. I also bought a new battery, also from Overlander - a 7 cell 3000mAh pack from KAN cells.
With these changes, the fans become a lot more worthwhile - a touch over 200g thrust each on 7 cells (compared to GWS figures for the supplied motors on 7 cells of 139g). Note that I don't have intake rings on the ducts - which would probably give something like another 25% or 30% thrust. This mod gave the hovercraft just under twice the thrust (static test) and hump speed is now relatively easily achieved
I also fitted a nifty adjustable-pitch four-bladed propeller from Ramoser Technik + Design. This is great - it doesn't need a prop adaptor, it lets me fine-tune the pitch and comes in various blade profiles for different applications. Also, Christian Ramoser answered my email queries very quickly and when I ordered it the propeller arrived direct from Germany within a couple of days. The blades can be assembled into the hub as tractor or pusher, but if you assemble right hand tractor blades as a pusher, it will push with left hand rotation - fine for a 400 motor which runs as happily in reverse as forwards. My only niggle was it was slightly larger diameter than expected, so I had to trim about 2mm off the ends of the blades to fit in my duct.
Although only intended to be a proof-of-concept propulsion arrangement, the hovercraft was sufficient fun that I didn't get round to the 'proper' propulsion mounting for some time.
As well as running in local car-parks, and a flooded street near my home, I took the hovercraft on holiday. A breezy beach was its downfall. I'd had some warning when a fast into-the-wind run saw the whole thing take off and fly for a couple of metres at a rather higher altitude than it is supposed to before my reactions released the controls and it returned to earth. Shortly after, in a nice wide sweeping turn, while travelling sideways into the wind, it flipped over, and when I picked it up one of the 6-blade fans only had five blades. I suspect that when it flipped some lump of sand or something found its way into the duct and tore off the blade.
No photo of the incident, but here's me and my daughter (Rachel) on the beach where it happened:
On the bright size, that was the kick I needed to start on the proper back
end, and the Mark 2B.
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