ANNUAL GENERAL MEETING. - vic.qhover.com · ANNUAL GENERAL MEETING. Yes it's that time of the year again. Come to this month's meeting and vote in your new committee. Have your say

March 2007

.ANNUAL GENERAL MEETING ' . ' Yes it s that time of the year again Come to this month s meeting and vote in your new

. . , ? committee Have your say on what is to done this year All that and more When

Wednesday the 1 4 th of March at 8 o'clock p.m.Box Hil l Co mmun ity Arts Centre

47 0 St ation S tre et Box Hi ll.Be ther e!

Feb's fly in was a fairly low key affair. Many of the usual suspects arrived for a fine day flying. The East end of the lake was very busy and many of the boats and PWCs even ventured into the shal-low area in front of Paradise Point.

Tony M's. much vaunted new hovercraft's buoyancy test and our first chance to get a gander at the SEV was a no show. The test having been done previously. We did get to see some pics. though. This is going to be quite something to behold when it is complete.

Peter S. also held back on debuting his updated craft but did come to the lake with Paul M. just to show that there was no truth in last months report.

Tony's new hover???? No it's the Club's Discoverhovercraft

Potted History of Hovercraft continued

SKIRTS:

The original skirt designs were many and varied: examples ranged from brushes to folding semi-flexible material - even plywood was used; but the first skirt that we would recognize, was just flexible material attached to the outer edge of a craft, the lower edge of the material stopped from blowing away by being attached to the hull with ropes or chains. The air trying to escape from under the hull blew up the skirt into a "C" shape.

Naturally a complete bag with a top and bottom attached to the hull quickly evolved, but the method of inflation was done in two different ways; in one, tip air from the lift fan was fed directly into the bag with the majority of the air from the fan going directly into the plenum [the space beneath the craft]; this gave a very stiff bag but with problems if the bag became damaged, very similar to an air-lubricated inflatable dinghy. This system is still very popular in North America where it is perfectly adequate when used on long expanses of smooth water. The alternative is a bag into which all the air from the lift fan is fed, and the lubrication air is let into the plenum through holes cut in the skirt or through a gap in the hull. These two types are known as "no-flow" and "full-flow" bag respectively.

The bag skirt design was used initially on both the larger commercial craft and on some smaller designs, but particularly on the larger craft it was too bouncy and the drag on anything other than a smooth surface was unacceptable. The next evolution on the larger craft was and is the H.D.L. "loop and finger" or "bag and finger" type skirt - a series of convoluted pieces of open-ended material "fingers" attached to the bottom of the "C" or bag type skirt which became the standard design for the larger rough water use amphibious Hovercraft.

In contrast to the American craft the majority of early British racing craft used the full flow bag skirt, the stiffness of which could be adjusted to the driver's requirements by virtue of the outlet hole size. Experimentation with loop and segment designs was tried but lacked the necessary stability for high-speed use; experiments were also tried with straight segments and although having less drag than the bag skirt, stability was a problem. In racing terms this meant that the bag skirt was slower in a straight line but more stable in the corners.

The skirt that evolved was in effect, a segment cut through both the loop and finger Profile skirt to include one complete segment and the same width part of the loop: this is the basis of the segmented or finger skirt which, with various modifications, is the most commonly used skirt on all European craft up to maybe 20 feet in length. Before leaving skirt design I should mention the design of Monsieur Bertone of France, whose conical "jupe" (skirt) was used on the ill-fated Sedam cross channel craft. To use this design [for which the British Government had no patent] a series of large individual material cones are arranged across the bottom of the craft and individually fed with air. A peripheral spray skirt is used obscuring the jupes from the casual observer. I can think of one craft where this spray curtain looks similar to segments. In many respects the previous comments with regard to skirt design are the visual changes, and as with a lot of things hovercraft, what you see is only part of the story.

CommitteePresident–    Tony Westwood   

0418 147 233 [email protected]

Vice President­    Col Dainty   0427 343 262 [email protected]

Secretary­    Stuart Wale   (03)5940 2080 [email protected]

Treasurer­    Reece Isaac   0414 568 180 [email protected]

Ordinary Committee MembersKen Osmond 9723 2030 

Peter Streader 9764 3448

Paul Dempsey 9580 2832

Newsletter Editor­    Reece Isaac   

submitions for publishing to [email protected]

Webpage­ www.ahfvic.tk 

WebGroup­ groups.msn.com/vichover

Monthly Meeting: The second Wednesday of every month at 8P.M. at the 

Community Arts Centre 

470 Station St. BOX HILL

Registration: Registration Numbers with the “HV” prefix are allocated by the 

club.  Apply to the President for application forms

HULLS:

Particularly on light hovercraft the hull designs have gone through many changes, the use of a bag skirt meant there was plenty of room for a boat-like hull with inspectable buoyancy, chambers often filled with ping-pong balls or plastic bottles. The hull, usually made of plywood [cheap but not good in the long term], could be made to float well and was easy to construct, and by this time planing surfaces which give a self-righting effect in the event of a plough-in had become a safety requirement of the HCGB.

Initially segments were fitted to the same cross-sectional bag skirt design of hull but usually with a more rounded bow shape. The lift air was pumped directly into the plenum through a duct, usually positioned on the front of the craft and in front of the driver: but unlike bag-skirted craft the hull needed to be correctly balanced in order to lift up squarely, otherwise the craft would have a tendency to sit tail down with all the lift air escaping from under the front fingers. This situation was alleviated by the fitting of some internal ducting to the rear fingers, or a small bag that jacked up the rear of the hovercraft allowing the plenum air to flow backwards. There were still drawbacks in the system such as the lift fan being vulnerable in the event of the craft ploughing-in at speed and water hydraulicking up the lift duct; also from stones flicked up and into the fan by the front fingers.

The next step in hull evolution, which had many spin-off ramifications, was the full-flow hull system in which all of the lift air was fed into a chamber running around the entire periphery with a dedicated hole located just under the top of the top skirt attachment of each segment. The advantage in lift air efficiency is best quantified on two similar sized craft, one of which will adequately use 20 horsepower for lift when fitted with a segmented skirt, whereas a similar craft with a loop segment skirt can use considerably more horsepower. The reason takes us right back to Sir Christopher's original peripheral air jet system. As the outer face of the skirt is angled inwards at approximately 45 degrees the air is jetted down each segment from the feed hole in the hull, creating an air curtain extending beyond the edge of the skirt, thereby reducing the skirt friction, but probably more importantly, as the air is being jetted inwards under the craft it significantly reduces the air loss in comparison with the other system. To the casual observer the main difference is the reduction in spray from craft fitted with this type of skirt.

This type of hull which also acts as a duct, or air box, means that the location of the lift in the system is less critical and therefore it lends itself to a single engine / single fan [integrated] concept which enables small fun craft to be made more cheaply; in fact so successful was this concept that for many years this design was the basis of the two lower

LIFT & THRUST

I have indirectly mentioned lift systems; there are a few basic guidelines to consider. The maximum height at which the craft hovers from the surface is approximately one eighth of its width (any more than this will affect the stability of the craft) and the larger the footprint of the craft relative to its weight the better, a design weight of between 10 and 15 lb per square foot will enable the use of an axial fan.

The axial fan is very good at moving a quantity of air but not so good at generating pressure, therefore the airflow efficiency will drop away very quickly with only a small increase in back pressure. If a craft is undersized or overloaded and the skirt pressure is too high the skirt stiffness will cause excessive drag and wear as well as making it difficult getting the craft over hump [up on plane]. A bigger fan or more a powerful engine will not overcome an inherently bad design.

It should be mentioned that as craft get bigger e.g. 25 feet or more in length, a higher skirt pressure in combination with a skirt geometry giving a more flexible performance e.g. a loop and segment type, will usually use the centrifugal fan, because:

A. The overall craft footprint is bigger and the depression in the water when the craft is at sub hump speeds is proportionately shallower than that of smaller craft.

B. The increase in craft size brings with it its own stability.

C. The increase in skirt pressure will of course be at the cost of increased friction, spray, noise, and fuel consumption, but have the advantage of a greater payload for a given size of craft.

The choice of a ducted axial fan will depend on a combination of factors; the h.p., available flow required etc., but as a basic guideline for any axial fan the smaller it is and the more blades it has the more pressure it can produce - and the more noise! The reverse is true of both lift and thrust fans, the bigger diameter they are with the least number of blades, the more efficient is the air flow and again for a given h.p., that reduces noise.

Of course this is an over simplification, but it helps to explain why as integrated thrust / lift fans get bigger the lift efficiency will drop away, requiring either very fast rotating fans or a lot more blades: both inefficient and noisy solutions.

As with all things, simplicity of design cost etc., will play a part in the designer's options but I would suggest that any designer doing their job properly should start with what they perceive as the most efficient package.

The readily available range of powerful two-stroke engines have meant that even the crudest designs will work, but an efficient design will enable less suitable [from the power to weight of respective] engines to be considered e.g. automotive four-stroke petrol or diesel. The compromise has to suit the use to which a craft is being used; for example operators using two-stroke engines on commercial operations do so because for a given sized

hovercraft the payload is increased in comparison with a similar craft using heavier engines. On the other hand for people building craft of their own design or from a kit, the use of a suitable second-hand automotive engine can considerably reduce the building cost.

With regard to thrust systems I have mentioned ducted fans: why ducted? Primarily for safety reasons when fitted with a 50 mm square metal mesh guarding the front of the unit and rudders, elevators and possibly vectors at the rear, the thrust system should be capable of containing any broken blades, belts etc., as well as preventing any human ingress. It is true to say that for a given diameter the ducted propulsion system is more efficient in thrust terms than an open-bladed unit, but it is noisier than, for example, a broad-bladed, slow running paddle propeller. Noise is energy, therefore the more power the unit has to absorb for a given diameter, the more noise it will generate and the amount of thrust per horse power will reduce. It might be necessary to use more blades to absorb extra power, as with the lift system the use of more blades will increase pressure and reduce thrust. But within certain limits some pressure is not a bad thing as it makes control surfaces [rudders etc] more effective. In conclusion, the compromise if available, is to fit the minimum number of efficient profile blades traveling at an adequate speed to absorb the power available.

After forty plus years of almost continuous development the ability to build from scratch a successful hovercraft still relies on common sense, to the extent that a successful hovercraft is the sum of its parts. A good hull, skirt or power unit will only be effective if they are connected to other components that can make use of each other's good qualities. Air density, wave characteristics etc., are constant, as are the basic physics with which we are trying to work: over the years we have bent the rules often by the use of complex skirt designs or by the use of lightweight engines etc., but there is always a trade-off - a price to pay, which in other operating conditions might make the craft unacceptable. So as my final observation, I would say, make a list of priorities for your hovercraft - define its operating conditions, payload, performance, convenience and cost.

Hopefully my comments will enable you, the reader, to make a balanced decision with regard to the compromises necessary to own the ideal hovercraft to satisfy your particular operating criteria.

Happy, Safe, Successful, Hovering.

Regards

Bill

On the Water will be aired on http://www.onthewater.com.auThursdays in Victoria at 7.30pm starting on March the 15th Hovercraft should be aired on 5th of April Community Channel 31http://www.channel31.org.au/ Other states may need to check station guides to see when this program will Air... Adelaide http://www.c31.com.au/ Sydney the show will be on at 2pm Tuesdays and 7.pm Saturdays and 4.30pm Sunday http://www.tvs.org.au/programming/index.html Perth http://www.accesstvwa.com/

What is a Hovercraft ?A Hovercraft (also known as an air cushion vehicle) is a machine that rides on a cushion of air.  A fan 

provides lift air whilst propulsion is via a fan or propeller.  They are fully amphibious and range in size from the 300 tonne giants that cross the English Channel to small single seat recreational craft.

In the light Hovercraft category there is a variety of designs from family cruisers to high powered racing craft. Craft can be purchased ready to fly, constructed from kits or completely home built from plans.

The Hovercraft's unique ability to skim over all types of surface provides the ingredients for a most exhilarating experience.

The Hovercraft ClubThe Club consists of a group of hovering enthusiasts and represents a wide range of Hovercraft designs.

The aim of the Club is to develop and promote the safe and friendly use of light Hovercraft for recreation and sport.  Through its members, the Club has many years experience in the design and construction of small Hovercraft and so can provide advice on what to do (and what not to do!).

The Club holds meetings on the second Wednesday of each month at 8 pm, at the Community Arts Centre, 470 Station St, Box Hill.  Meetings consist of; business, reports, organization of Hoverin rallies, and  regularly include videos and guest speakers.  Contact Tony Westwood President on 0418 147 233 for more information.

..........................................................................................................................................................................

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Next Fly day is at

Lang La ng by Reece Isaac

Sunday the 18th. is the day 10a.m. is the time. Lang Lang is the location. A pleasant drive down the South Gippsland Hwy. A kilometre or so past the Korrumburra turn off. Turn right for Lang Lang pier. Lang Lang at low tide is mud

flat. A boat ramp is available for use and the beach is quite accessible also. High tide on the day will be after 1145am. The plan is to fly around French Island anti- clockwise, if conditions are suitable on the day. Otherwise, just exploring the northern and eastern shores and the various rivers, creeks and drains.

The trip around French Island will use 2 tanks of fuel depending on how your craft burns fuel. My 264 on the 2 occasions did this with 3 up and 2 up. Carrying 20l in jerry cans and a refuel stop about half way saw me around with a little to spare upon return (how little I can't recall) Fuel cap 25l +20 in cans. Journey time 4hrs+.

The GPS trace of one of my trips around French Island is shown on the right. Wind generated heavy seas were encountered on the southern shore but the rest of the journey was quite good.

One proposal is to fly over to the north shore of Philip Island to Rhyll for a bite and a refuel.

There are usually many other boats out in the deeper water so it may be necessary to do some “giving a wide berth”.

So here's to good weather and calm waters. Hope to to see you there.

Lang Lang lower left