Hints, Tips and Technical Things : A Need-2-Know of all things Hovercraft
Air and Pressure
Buoyancy Foam, Body Language, Beginners
Centre Of Gravity
Drag effects
Exhaust gas, Environment
Fans [lift]
Gas
Hulls, Hover Height
Lift Problems
Mountings, Motors
Noise
Pressure and Props , Plough-In, Planing Surface
Resistance
Skirts, Materials & Design, Stability Issues, Safety
Towing
Understanding
Wind
Xcitement
Y because we like it
Air :
Hovercraft need compressed air to support them, The pressure is very low, at about 2 inches of Water Gauge.
To get an idea how much pressure is available get about 12-15 ft of garden hose or clear plastic hose full of water , hold both ends together and level with each other, water to the top of each end.
With your mouth, blow air into one end ----- Gently -----, water comes out the other end. Do this until the water is about 1 inch below each end. Now puff into one end gently until the water just gets to the top of the other end,
Thats how much pressure a multiwing fan produces. Very good fans will move about 6 inches of water, you can use the same method to move 6 inches, it just takes more pressure
Remember when measuring the actual lift pressure that you must take into account the water distance down the hose and the distance up the hose and add them together, this is approximately the pressure in inches or milimeters of Water Gauge.
Buoyancy Foam :
This must be fire retardant Closed Cell Urethane foam. There must be sufficient buoyancy to support the weight of the engines and all other non floating devices in the craft, people float engines don't. When cut, the foam exhibits millions of tiny half bubbles on its surfaces. Water soaks into these half bubbles by capiliary action and the surface very quickly becomes waterlogged and very HEAVY.
If the foam is left in the open air the water eventually evaporates but it takes time. Pour-In foam sticks where it touches, including skin, clothes and tools --but-- with vibration over time the foam deteriorates at the surface it is adhered to and a gap is formed which attracts moisture/water which, as above, soaks the surface making the hull heavy.
If your hull is heavy, you have to remove the soaked foam, dry the cavity and replace the foam. To meet Survey requirements all buoyancy foam must be encased to prevent moisture build up. Keep in mind that buoyancy foam is not light and depending on how it is mixed can add many Kilos to your overall weight. It is acceptable to use the air trap method for buoyancy but this has to be well designed into compartments, I can send you a seperate page on this.
Centre of Gravity :
This is important both fore and aft, and sideways. Your hovercraft must be balanced corectly.
Drag :
Basically 3 kinds of drag to overvome.
Surface Drag over waves and ground, Form Drag, the effort to push your craft along. And the drag caused by the cushion between the ground and the hull.
Exhaust Gas :
Must not be released into the skirt system or cushion. and not through the thrust prop or duct.Must be where it will not affect you or your passengers.
Lift Fans :
The most common fans seem to be the Multiwing and Breezax brand of air conditioning fans. They are relatively inexpensive and they have been used in hovercraft for many years. The manufacturers will not recognise or approve their use in hovercraft so it is a matter of 'enter at your own risk'. But they work.
Keep a close eye on the fan blades and the hub sections. The blades rub on the duct and wear. Sand and water deteriorate the blades, Cold hardens the blades. Cracked or damaged blades must be replaced and the fan re-balanced. When I was into model aircraft, we used to boil the nylon prop blades for a few minutes in water to resoften them, this however makes them too soft for hovercraft use and is not recomended.
Remember that the fan is running at thousands of RPM so any out of balance will vibrate severely.
It takes a lot of wasted power to drive these fans because only the tips and about 4" in from the tips does any work, the remaining part of the blade is quite useless, causing the air to swirl around and between the blade roots, this absorbs POWER like you wouldn't believe and gives nothing in return.
The maximum pitch for these blades is around 20 - 25 degrees near the tip because the blade, toward the root pitch increases to about 40 - 50 degrees, quite useless for hovercraft purposes. I developed a lift fan which operates in the severe back pressure and bounce back air of hovercraft
The Hub is huge and the blades short, it gives around 3-4 times the air volume and pressure of the air conditioning fans, but the hub design is critical and the blade air foil section is critical also. So they are made for each particular hovercraft design, they are not a 'one stop fits all'.
When there is a larger market for these fans we will further refine the design for mass production.
The Hull :
The hull carries you on a cushion of air trapped under it by a flexible skirt system. You've read about this elsewhere so I won't go into it here.
Hull Attributes :
Basically a boat like platform with a cavity to carry you, but outside things are different.
The hull sides are angled to form 'Planing Surfaces', You can see these on the plans from any reputable designer, these perform several important functions and the angle is determined by the skirt design.
The angle of the surface for finger System is around 30 degrees and for a Bag system it is around 19 degrees, so why. (See Skirt Design)
Planing angles basically reduce the effect of Plough-in allowing the hull to skid across the ground or water without digging in and throwing you out.But thats not all.
There is a secondary planing angle between the main angle and the flat bottom of the hull, this is most effective at the front of the hull to further reduce the effect of plough-in by eliminating the 60 degree corner formed between the floor and planing angle of the hull. (see sketch)
Plough-In :
When a hovercraft travelling forward at speed starts to drop the bow, the lower edge of the planing surface and front floor get closer to the ground/water, this reduced gap starts a turbulence at that point which further reduces the hover height. Suddenly the front drops and contacts the ground/water and the craft comes to a sudden halt. You rev the engine, skirt pressure builds, the front lifts and off you go again.
Engine Mounts :
Check the engine mounts: Rubber mounts can work loose leaving the fan to hit the duct, wearing it away.
Skirts :
The better your skirt is sealed to the hull and the ground, the more air will be trapped, leaks caused by worn fingers or badly fitting skirts savagely reduce your lift pressure.
Skirt fingers wear and fray on the tips. When worn, the hovercraft loses hover height and you start hitting into things that you once could fly over easily, this damages the hull.
Skirt Materials :
At a pinch, you can use the light plastic tarp material for skirt fingers, it lasts a short while and is very inexpensive to buy. You can experiment with Urethane coated Nylon, but this frays if it is more than 180 denier.
The best material I have seen so far is the skirt material that Bill Baker Hovercraft (UK) uses.
You can also use the reinforced Vinyl used on Taughtliner trucks, you know, the flexible sides on interstate trucks and couriers.
Obtain the lightest material that has Ultra Violet protection coating.
Skirt Designs :
There are many designs but the most used are the bag, full finger, Loop Segment and Bag Segment skirts, you can find info on these in the books but with the bag and loop segment types, if you don't get the calculations right
before you cut material you will get a poorly performing skirt.
Skirt Geometry is a "Black Art" Get it right and it's good, get it wrong and you start again. It's all to do with Pressure Differential, the pressure difference between the atmosphere, the plenum and the cushion.
If you choose a loop segment design, in the belief that you will not need a plenum, Please Reconsider. A hovercraft seems to work better, more stable, if you have 2 pressures. A skirt/hover pressure that lifts the hull and load, and a slightly higher pressure in the bag or plenum.
I have a Loop Segment skirt on Hovergo (You've seen the pictures) and I have some difficulty in getting air flow to the back of the craft, with a front mounted lift engine. A Bag/Finger may have been better as the bag woould act as a plenum.
The Bag skirt has 2 different curves, the outer will form a half balloon (semi circle) shape because the pressure differential is high. The inner section curve is quite different, and forms a much larger radius (about 3-5 times larger) because the pressures are very similar under the craft.
The larger radius attaches much further under the hull so the angle is much less, otherwise the hovercraft would sit on the inner attachments and damage them.
The design for the full finger skirt is well known, I can send a copy if you need it, and designed for a planing angle of 30 deg.
The design of the loop segment skirt is quite another matter, the geometry is quite critical and the books are wrong in the way they design the system.
Design requires some mathematical skills with sin, cos and tan functions. I did all my design in TurboCad v5 in 3D, which took a little learning, but the accuracy was excellent, (The janome sewing machine was not).
Think Safety at all times
By-Standers, walls, rocks and trees don't bounce.
REMEMBER... if You-Knok-em --- U-Pay.
Always be aware that spectators have little or no idea about hovercraft and the need for plenty of space, particularly when arriving and landing.
Many other users, like Boaties, canoeists and board riders etc don't know or care that you will go sideways for a great distance while turning.
Always plan ahead.
Thrust Props and Fans :
Many hoverers are going over to 2 or 4 blade wooden thrust fans and 3 blade aircraft props like WarpDrive or similar.
These are much quieter when running but need to be larger in diameter, making the centre of thrust higher. They do not need a duct but you must provide efficient guarding to stop the bits if the prop fails and shatters and to stop people from putting their hands into the spinning blades.
A spinning prop is mesmerising and people will investigate.
A high Thrust line increases the tendency for the hovercraft to nose down under power, and the torque effect being higher above the Centre of Gravity can cause the craft to drop the back corner opposite to the direction of rotation of the thrust prop.
To counteract this effect just offset the rudders toward the dropping side so that the craft tends to turn away from the droped corner, it doesn't take much but it works and when you are moving - straighten the rudders again.
Body language :
The art of using your body position to control a hovercraft --- doesn't work so well with large heavy craft.
While going in a straight line, move your body to the right or left, without moving the rudders, the craft will go in that direction.
If you move to one side and turn the rudders opposite direction the craft should move sideways without turning.
You can stand to one front corner and turn the rudders that same way qyockly, the craft will dig down on that corner and turn in its own length, on the spot,
Practice other tricks, have fun, carefully.
With a large craft the reaction time is slower and it's better to sif still and use the rudders to control the attitude of the craft.
If you feel the craft dropping to one side or other in wind, etc., rather than using body language to correct, try kicking the rudders quickly, toward the dropping side, the craft should quickly right it's self.
Engines and Mounts
Lift Engines
Do not be tempted to use light material for the lift engine mounts.
After all you are only adding another 5-6 kilos or 12 pounds to use heavier steel. The heavier the mount, the more it absorbs the engine vibration and the less it transmits to the rubber hull mounts.
It takes more effort for the engine to start the heavier engine mount moving before the forces are transferred to the shock absorber rubbers.
It doesn't make sense to use light materials then add weight to the front to balance the hull.
Thrust Mountings
These must be strong to absorb vibration and transmit the thrust forces and the cyclic and twisting forces of the thrust prop / fan.
If the frame is weak, it will twist and the drive belts will run off centre during high thrust loads, the fan blades may hit against the duct. the twisting forces are greater when using larger diameter propellers.
Skirt Systems
Most hovercraft use some type of finger skirt which is a general purpose very flexible ideal for rough and undulating terrain, choppy water, ice, mud and sand. If you want to hover on grass, fingers are less efficient than a bag skirt, which will flatten the grass reducing loss of lift air.
Bag skirts as are used in USA have more drag over undulations and rough water but are easy to maintain, so you see there are decisions to be made here. Generally the finger skirt is the most widely used system and there is a standard design for the fingers. Keep in mind thought that individual finger systems use much more material than a bag skirt.
The Hull can be made of plywood, fibreglass or aluminium, each has advantages and disadvantages. Most smaller hovercraft, 3 to 4 metres long are made of fibreglass/foam sandwich material or ply. A couple of Victorian hoverers have made successful small aluminium hovercraft, you need good sheet metal skills to do this.
Stability Issues
Hovercraft stability is a science all by it's self so I can address only the basics for your reference. The lift air must be contained under the hull by the skirt curtain.
Bags offer by far the most stable skirt system because of the way they deform when the side of the hull tips down, Bags can give a very hard ride and if over pressured will bounce.
Full finger skirts give a reasonably firm ride and are quite stable. When seperate fingers are distorted the leakage can cause collapse, hoverers have been thrown out of hovercraft when the hull contacts the ground or water because the fingers collapsed.
Bag/Finger is the next most stable and is used extensively on larger hovercraft because less material is used in manufacture and the bag forms the plenum.
Loop Finger is slightly softer in ride (but not much) than the full finger system. Less material again in construction but more difficult to attach the fingers. More later.
Copyright and Disclaimer
Hovergo Web Site is copyright, not that anybody would have any reason to copy it, but thats, that.
The information contained herein is broad sweeping and general information, it belongs to everybody, which is the reason for the page.
For too long the basic stuff has been disregarded and beginners as I was couldnt get to the nuts and bolts of Hovercrafting.
This is unacceptable if we wish to have an active group of knowledgable hoverers who care for theactivity and or business.
We are uncovering info to make anyone who wishes to be involved with hovercraft, aware of the things one should be aware of.
Check the disclaimer on the front page for other disclaiming stuff.The web site will be enlarged to contain all kinds of info on general subjects.
One page of great interest is the INFORMATION ABOUT AUSTRAILA page, please check it out.
Thanks for your interest.
eMail Hovergo SG for more information
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