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John2
17-Apr-10, 06:14 PM
Hello all,

I'm new to the forum. My name is John.
My wife and I have been looking high and low for a hovercraft design to build, but they all look very "hovercrafty" in our opinion (no harm or offense intended to any one design) and our aesthetic eye wants something more streamlined and visually impacting, like a fighter plane or rocket car, so she asked me to design something that is more along the lines of a performace cruser lightship.

I was wondering if anyone would be willing to collaborate on assisting me with my design for a hovercraft which I hope will transition to ground effect. I have been working on conceptualization for about 8 months now and wanted to get expert (hands on) opinions for the three iterations so far before moving on to the next level.

The design is still open for changes if required but understand that the design isn't just a quick paper napkin that lacks all the engineering principles and required mathematics to make the design feasible, so please point out changes believed to be required with signifcant reason for life or safety and not just because it "looks" wrong, unless it is wrong.

There is still a lot left to do such as analyzing the aerodynamic, volume coefficients, and hydrodynamics stability and scantlings etc, but a lot is already done. Questions will trigger those results in the interest of reducing bandwidth use, so please ask.

I'm attaching quick pictures for discussion. Any opinions and advise would be greatly appreciated. Here is the first of three designs in the running.

Thanks
J

John2
17-Apr-10, 06:17 PM
Here is second of third design in the running for critical discussion.

J

John2
17-Apr-10, 06:19 PM
Finally, the third of three iterations.
Questions? Comments?

J

curtis5420
18-Apr-10, 10:33 AM
looks like you want to spend lots of money, on something that looks good, without much thought about how it works, or why.

what are you going to be building your rocket ship out of? what are your design critiria? i.e payload, operating conditions?

so how many people? what sort of size do you want? engines, what have you in mind? materials? what facilities, do you have?

none of your 'designs' look much more than un-engineered concept images.

so lets have some details, of what you want to achieve.

Ian Brooks
18-Apr-10, 01:18 PM
Hello

The design of a hovercraft is really quite difficult... Successful hovercraft designs tend to look workman-like as a result of the underlying Engineering, and I would suggest that you do a little background reading. Try these:

1 - Hovercraft Design & Construction, Elsley & Devereux, 1968, old but still relevant introduction. Copies come up on ebay from time to time.

2 - Theory and Design of Air cushion Craft, Yun & Bliault, ISBN 0 340 67650 7. 2000, the most up to date text there is.

What is your professional background? I suspect that a successful aesthetically pleasing design would be the result of a collaboration between an Industrial Designer and an Engineer.

Ian

John2
18-Apr-10, 01:52 PM
Hello Curtis,


looks like you want to spend lots of money, on something that looks good, without much thought about how it works, or why.

No one wants to spend a lot of money, well, not anyone that I know anyway. But, "a lot" of money in any instance is relative to the beholder because if you look around, you will find that even small hovercrafts are deca thousands in cost and they are bought and sold all the time. I have done a preliminary cost and work study on the design and have a general idea of what it will cost and how much time it will take to build it.

Thank you for your kind words...they do look good don’t they? I prefer the low profile twin thrust design myself because the windage was reduced dramatically and the frontal area was cut by 35%, reducing the thrust engine requirement by 20hp, which led to weight reduction, which had to be reiterated again, which led to a reduction of lift engine reduction. Drag is a real drag! But, the reduced profile with the single larger fan would be much simpler and efficient since belts or 90 degree gears wouldn't be required.

I believe I have dedicated a lot of thinking into the designs and I won't share all of it in the interest of conserving bandwidth, but I feel confident saying that I have a really good idea of how a hovercraft works and why.


what are you going to be building your rocket ship out of? what are your design critiria? i.e payload, operating conditions?

It is my intent to build out of balsa/fiberglass above the waterline and foam/fiberglass below. I have found that this combination is the best for strength to weight ratios. I had to make some design assumptions in order to generate a weight study so I used a tolerance of 1/460 for spans, 101#ft^2 (force 5 hurricane) for the scantlings above the LWL and 360#ft^2 for the scantlings below the LWL and the tub, just in case she lands hard or floatsam floats out of the trough. Safety factors are included obviously.

We're not young anymore and we tend to seek the "comforts" such as heat and keeping our heads out of the rain and the bugs out of our mouths, sooo, she is to be designed as a "comfortable" performance cruiser based on criteria set by a certificating agency such as Lloyds, ISO CE etc. I want to design her to "inshore" criteria just in case we want to go out to do some deep sea fishing. If you like, I can attach the guidelines for perusal, which are substantial. Her displacement is approximately 2090# which includes about 180# for fuel (24 Gal), and 800# payload leaving an empty weight of 1110#.


so how many people? what sort of size do you want? engines, what have you in mind? materials? what facilities, do you have?

Payload is about 800#. Human weight is not static, so for sheltered waters or land locked use, I would say 3 passengers and a pilot for low performance fun. Reduce by 1 passenger and I think stability would allow use up to the 3 mile lines (inshore) for mid performance fun. Take it to 1 pilot and 1 passenger, and she would be in her element to offshore conditions on a good day obviously. If I can get her to become airborne (ground effect), it will mean a maximum of 1 pilot and 1 passenger unless the power to weight ratio is increased significantly. My definition of "offshore" is that distance which takes less than an hour to travel to be in safe waters.

Size right now is 16 feet by 8 feet for all iterations, with wings out to a span of 20 feet (wings were not shown because I have not decided on which foil to use or calculated the required area) I haven't decided on the engines yet but I need 25-30hp for lift with 1 inch hover-gap and 90-120hp for thrust, so I have allowed 140# and 300# installed respectively. Facilities are like any other, marinas, public ramps for trailers etc.


none of your 'designs' look much more than un-engineered concept images. so lets have some details, of what you want to achieve.

Well, that may be because you don't have all the information and looking at only 9 photos can be a bit deceiving sometimes, but don't haste to conclusions until al the evidence is in.

I would like to achieve an aesthetically pleasing, streamlined safe, comfortable and efficient design that we can enjoy and cruise with, some flexibility to go beyond the established performance limits on occasion.

So…which iteration would you consider knowing what you know so far and why? See any changes that may make construction simpler or increase comfort? No, I do not want to just design a platform and attach a fiberglass Lambo Diablo body kit. I though about it and we rejected it, for now anyway. :) We will use the gull wing doors however.

Thanks and enjoy your weekend.
J

John2
18-Apr-10, 03:39 PM
Hello

The design of a hovercraft is really quite difficult... Successful hovercraft designs tend to look workman-like as a result of the underlying Engineering, and I would suggest that you do a little background reading. Try these:
1 - Hovercraft Design & Construction, Elsley & Devereux, 1968, old but still relevant introduction. Copies come up on ebay from time to time.
2 - Theory and Design of Air cushion Craft, Yun & Bliault, ISBN 0 340 67650 7. 2000, the most up to date text there is.
What is your professional background? I suspect that a successful aesthetically pleasing design would be the result of a collaboration between an Industrial Designer and an Engineer.
Ian

Hello Ian,

You are of course correct on all counts. The reason why hovercrafts are difficult to design is that unlike an aircraft which operates in air or a vessel which operates in water or the vehicle that operates on land, the hovercraft does all three. The density difference between air and water is 745 times, so that gives you an idea of the different mediums which must be designed around. Dirt, is 2.5 times that of water…almost a liquid.

Hovercraft designs tend to appear "workmanlike" as you point out for other reasons as well and not just the underlying engineering. Sometimes, they need to be workmanlike, but not always. Other times, the engineering is so thorough that it is taken to a level that if a thought could build the machine, it would take no more than a whisper to make it happen. MOST of the time, it is because it is easier and cheaper to stay with the proven and go with the imperially accepted, than to take everything back to first engineering principles. It has been my experience that the economic factor drives the design and that 99% of all manufacturing companies that require engineering services are run by the almighty accountant. In this instance, I don't have to worry about the accountant squeezing every penny out of the job to design or build it and can therefore do what I want.

I have read quite a few authoritative works in aeronautical and nautical fields in my lifetime and have read an even greater number of everything else between. I found the Elsley somewhat outdated, but to be honest, it is a work of pioneers. The other by Yun I have in my library.

Your suspicions that a successful, aesthetically pleasing design would be the result of collaboration between different professions is accurate, but don't exclude the guy in the trenches. I have found that an untrained eye is able to identify the obviously wrong while many swear up and down that the numbers and calculations add up.

Sooo…what do you think about the iterations? 1,2 or 3? Why? Any changes? As a cruiser design, would you like to see a sink with water or possibly an ice box? What about a porta-potty? I know we of the male gender can just contribute to ocean height over the side, but the squatters have a problem with that idea.;)

Thanks
J

atters
19-Apr-10, 06:41 AM
I do not think any of the designs will be what you are after, take for eg. the deep see fishing. Where are you going to put your Rod ? as well as it looks like there is very little room to move about. As a cruiser to scamper up and down the river, GREAT but for anything other than that, working with the little info you have given, I am not so sure.

Do not get me wrong, the design looks fantastic, but not all that practical. Do you perhaps have one that has a view of the roof/roods open, that will be the real test, plus on those hotter days, one might one to fly with the top off, how would that work?

2c

Keith Oakley
19-Apr-10, 10:29 AM
I'm all in favour of radical new designs and the great thing about a hovercraft is that almost anything can be made to actually hover as long as it has essientially a flat bottom, is light enough, has enough lift air and pressure, and an adequately sealed skirt. Even horribly boxy caravans and oil storage tanks have been made to hover. But there is a difference between just hovering and working well. Absolute performance is only tested in racing however. Much cruising performance depends on where and when you use it and is 'in the eye of the beholder'.

The existing designs have evolved for a reason though. The open cockpit designs have evolved from the UK & European inland racing scene where the ability to turn fast, trim the craft etc by moving the pilots weight around is key. The crosswind effects and increased weight of high sides and enclosed cockpits are unwanted. Hidden under the skirt are sloping planing surfaces to enable the hull to hit the water or ground forwards sideways or backwards without hopefully rolling the craft over. From the low height of your skirt I'd guess you haven't incorporated these.

But many American craft have evolved differently. With little racing and the wide open expanses of the Missisippi to explore their cruising craft are bigger, have more enclosed cockpits, big props etc. Your designs are very similar to the popular UH series of craft - www.hovercraft.com (http://www.hovercraft.com) They also have WIG (Wing in Ground Effect) versions of many of their craft. I'd suggest suitable reading would be one of their plans even if you build something very different.

For me I'd be concerned that the planing surface design of your craft is right, that the overall static fore and aft trim is right, particularly with different payloads, and that you can dynamically trim the craft, particularly against crosswinds; because the tight confines of the cockpit won't allow pilot movement. I guess you won't have the skirt shift systems used on larger craft to perform that role and your elevator will only be effective at speed particularly if its high and out of the prop airflow.

I too operated a 16 foot 4 seater homebrew design for many years and although it had higher cockpit sides, much like a powerboat, I found I looked over the windscreen most of the time because it was plastered in sand/mud and the open 8*4 foot passenger space with movable seats was essiential to cope with varying payloads and dynamic loads.

Good luck
Keith Oakley

John2
19-Apr-10, 03:56 PM
I do not think any of the designs will be what you are after, take for eg. the deep see fishing. Where are you going to put your Rod ? as well as it looks like there is very little room to move about. As a cruiser to scamper up and down the river, GREAT but for anything other than that, working with the little info you have given, I am not so sure.

Do not get me wrong, the design looks fantastic, but not all that practical. Do you perhaps have one that has a view of the roof/roods open, that will be the real test, plus on those hotter days, one might one to fly with the top off, how would that work?

2c

Hello Atters,

Thanks for your observations. I haven't started to think about the rod-holders and other quality of life items yet, but I guess, I can place those just about anywhere. If you remember correctly from my prior post, I intend to have her operate in ground effect, ergo the tight confines.

Why tight? Well, when a craft "plows in" or lists on to her ear, or worse, pitch-poles, the energy is substantial but it is borne into the structure, and hopefully dissipated by crumple zones. While in ground effect, the same is true, except that velocity in ground effect usually exceed 50 mph. When speeds exceed 50 mph, the energy is not just substantial, it is significantly substantial. Enough that it will kill you cold dead if the craft were to fall from effect and nose dive into a concrete wall of incompressible water.

Yes. There are those flying around in ground effect as if everything is a okay, but know that they are making a big mistake and take their life and the life of others in their hands when they put on those little antics. Our bodies are just not made to decelerate that quickly and it is better for the energy to flow through our bodies than to have our bodies flung into something harder than us. I don't even agree with hover crafting at any speed without belts to keep you from becoming airborne. Remember...the fall never kills you...it is the sudden stop.

Another reason is because the accommodations are not for live-aboard, so any excess volume is just dead space and weight. The length is the minimum I can design or feel comfortable for offshore work, well, to my own abilities. You are however correct that I could make it a bit wider and taller by about a half-inch. Costs will increase by the cube and windage and frontal area by the square, so, this change is not just linearly detrimental, but, I ahem, agree that maybe I may have been a bit paranoid regarding stability and safety.

Finally, I assumed that the craft would capsize and go turtle since she is designed to be unsinkable. That platform is almost impossible to right again, so I have designed it so that one of the wings can be control flooded until she sits on her ear with the cabin floating her high, then just sit on the other wing (creating a lever) while the flooded wing gets pumped out and the craft is righted back on to her waterline. With the right install on the motor (fuel injection, electronic ignition etc), the motor should survive a dunking long enough to start up when she is righted. If not, then at least a small kicker outboard can be installed as get home power or a dry cabin away from the elements can be had until rescue arrives.

I thought I posted plan views with the canopies excluded. Check the (b) pictures.The doors are set-up as split gull wings, but I can change those if need be. I will provide some more pictures as you have requested as soon as I can generate them. I know I haven't provided a whole lot of info yet, and my apologies are extended to all, but it is on its way as it is requested. Anything else you would like for me to post for your perusal?

Thanks again
J

John2
19-Apr-10, 09:55 PM
I'm all in favour of radical new designs and the great thing about a hovercraft is that almost anything can be made to actually hover as long as it has essientially a flat bottom, is light enough, has enough lift air and pressure, and an adequately sealed skirt. Even horribly boxy caravans and oil storage tanks have been made to hover. But there is a difference between just hovering and working well. Absolute performance is only tested in racing however. Much cruising performance depends on where and when you use it and is 'in the eye of the beholder'.

The existing designs have evolved for a reason though. The open cockpit designs have evolved from the UK & European inland racing scene where the ability to turn fast, trim the craft etc by moving the pilots weight around is key. The crosswind effects and increased weight of high sides and enclosed cockpits are unwanted. Hidden under the skirt are sloping planing surfaces to enable the hull to hit the water or ground forwards sideways or backwards without hopefully rolling the craft over. From the low height of your skirt I'd guess you haven't incorporated these.

But many American craft have evolved differently. With little racing and the wide open expanses of the Missisippi to explore their cruising craft are bigger, have more enclosed cockpits, big props etc. Your designs are very similar to the popular UH series of craft - www.hovercraft.com (http://www.hovercraft.com) They also have WIG (Wing in Ground Effect) versions of many of their craft. I'd suggest suitable reading would be one of their plans even if you build something very different.

For me I'd be concerned that the planing surface design of your craft is right, that the overall static fore and aft trim is right, particularly with different payloads, and that you can dynamically trim the craft, particularly against crosswinds; because the tight confines of the cockpit won't allow pilot movement. I guess you won't have the skirt shift systems used on larger craft to perform that role and your elevator will only be effective at speed particularly if its high and out of the prop airflow.

I too operated a 16 foot 4 seater homebrew design for many years and although it had higher cockpit sides, much like a powerboat, I found I looked over the windscreen most of the time because it was plastered in sand/mud and the open 8*4 foot passenger space with movable seats was essiential to cope with varying payloads and dynamic loads.

Good luck
Keith Oakley

Hello Keith,

As a generality, I will agree. Do you really think the design is radical? I was thinking “different” because as you already said, anything can be made to work with a hovercraft, but I don’t believe it is so far outside the box that it is radical. The lines for the cabin
have been drawn in conical sections for aerodynamic cleanliness and the base is drafted for hydrodynamic efficiency for transition to ground effect, but those are common practice. What do you find radical? Is it good or bad to you? Why?

I could not have said it better myself. That is the reason that I have asked the community to provide their “own, hands on” experience and observations. Nothing beats that kind of input. As you have pointed out, and many times it is overlooked by many, there is a significant and quantifiable difference between all out racing performance, cruising and, performance cruising. For me, cruising must be as quiet as possible within reason and slow enough so that you can smell the roses and see the ducklings all in a row, but, you definitely want the performance in place for those times when you need to perform and get the hell out of some idiots way or get somewhere fast and furious!

My first post might not have been fair. I am not against “hovercrafty” hovercraft. It’s just not the design we were after and since not everyone likes to have the same car, then we decided that our hovercraft does not have to look like everyone else’s just because it’s
the trend. I like their design and the evolution current hovercraft have survived. In particular, I like designs from abroad better than I do from home base, (reason why I’m in this community). For example, I have given thought to “turning fast” by experimenting with a rudder that is actuated to drop or lift into the water in the required direction as the operator attempts a left or right turn effectively spinning the craft on a dime, but that adds weight and complexity which can be avoided by planning ahead and giving oneself
enough time and room.

The height of the skirt at this stage is 12 inches. Currently, I expect her to provide 12 inches of hard structure clearance with an additional 1 inch of gap height. The slanted bulkheads making up the hull are in place, it just can’t be seen because of the skirt, but that is not the only way to preserve her traverse or longitudinal stability. Volume above the
waterline helps to dampen submersion as well and proper placement of the center of gravity and center of flotation in relation to the center of buoyancy is extremely effective. Losing skirt lift outside of the bank will also assist with reducing lean, but I suspect
those systems are best used in larger displacement craft.

Missisippi? Google the Great Loop. That is just one estuary. Unlike my brethren, I have always been a minimalist and never believed that any person could occupy more than 343 cubic feet of space at any given time, so why live in 450,000 cubic feet of space? The enclosure and larger prop's however, are good things for many reasons, so those should be provided as often as possible, regardless. I have studied several established and reputable companies, including Universal, and I have also studied individual amateur designs. I don't think that my design can hold a match to Universal's product lines, but I thank you for the noble comparison. I have seen their WIG in action as well as the WIG by Weber. Both very nice and appealing designs. My favorite thus far researched? Hover Shuttle. J

Nope, no skirt shifting systems. KISS is the order of the day. Regarding the elevator, it is a bit more complex than can be possibly explained in short sentences but, the bottom line has to do with take off/landing speed, the moment arm and the volume coefficient. In other words, the rudders and elevator must be designed to perform with authority at those lower speeds opposed to cruise speeds. That's why they are usually huge. As far as putting them in the prop wash, well, the consequences for a rudder stall due to engine failure is a sloooow turn and poor authority but, for the same failure while airborne means a certain climb followed by a nose dive and imminent death, so keep the elevator in the free stream and design accordingly so that authority is evident and operation is smooth instead of influenced by the greater forces in the prop wash.. I may move the elevator still, and may even make it a canard, but that decision can be made on the final iteration.

So, knowing yet some more, which would you prefer to designate for future work and why? Any changes or suggestions?


Thanks
J

Ian Brooks
19-Apr-10, 11:08 PM
I've been following this thread with interest. I won't comment on ground effect flight - thats not in my experience.

The graphics show some interesting bodywork, but of more interest is whats underneath that. You mention that the overall weight would be in the region of 2000lb, with a length/width of 16ftx8ft. This will place the cushion pressure a little over 16lbf/sqft - which is on the upper side of the normal range. What do you expect the hump drag to be? You show two design options for the thrust fan/prop. Given the installed power I wonder if you have yet determined the thrust from each option? These two numbers will begin to determine whether the craft is likely to work as a hovercraft. If you give us the numbers, we can comment on the likely success of the design.

The skirt has the appearance of a bag skirt. Have you considered the details of this yet? What will the bag pressure be? It would be interesting to discuss how the design will control plough in (plow in your neck of the woods!)? Have you considered some type of responsive skirt, or will you use compartmentation (or maybe both)?

Cheers for now
Ian

atters
20-Apr-10, 08:48 AM
How would the lift system work ?

What motor do you intend using and where would it go ?

I like the idea of flooding the one side to right it again but then would that seal-able area double as buoyancy when floating, something like that cargo ship that sinks to load stuff.

I was at sea just a few weeks ago, Not So Sure I would want to try land a ground effect craft on those waves, but that's just me. :eek:

John2
20-Apr-10, 07:47 PM
Hello Ian,


I've been following this thread with interest. I won't comment on ground effect flight - thats not in my experience.

Well, I'm glad to hear the thread has caught your interest. If you have any questions regarding ground effect or anything else for that matter, please feel welcome to ask anytime. If I can't provide an answer, I will definitely make an effort to find one.



The graphics show some interesting bodywork, but of more interest is what's underneath that.

Simple is relative. What you consider "interesting" is nothing more than a simple airfoil which I have modified for this design. I may change it as I go ahead, but for right now, I think it is an excellent foil for the Reynolds with excellent characteristics and a "soft" stall. It isn't very noticeable but the camber line is shaped much like an S, except for my modifications. The bottom must be flat. Convex means suction which pulls the craft down like a venturi and concave means poor longitudinal stability which could mean a nose dive, neither of which are good for your health, although suction is the lesser of the two evils in the case of a hover craft due to the air cushion. I have attached pictures.



You mention that the overall weight would be in the region of 2000lb, with a length/width of 16ftx8ft. This will place the cushion pressure a little over 16lbf/sqft - which is on the upper side of the normal range. What do you expect the hump drag to be?

Oh geez. I remember calculating all kinds of drag, skirt, wave, aerodynamic profile, pressure momentum etc. I just can't remember the figure. I think that the greatest drag was at a Froude that was with the craft crabbing in yaw and perpendicular to a vector. I want to say somewhere in the 260lbf range. In any event, she should climb out of the hump before 12 mph. This isn't particularly critical because the design doesn't have to be out of the trough fast enough to gain an extra half a second to win the race. As you have already identified, the cushion pressure is on the high side and although I won't make the change now, it may be that I may increase length overall which reduces drag dramatically, but I doubt it will be stretched by much. The normal 10-12lbf/ft^2, would require an increase to 21-25 feet and that would be ohhh so very nice, but ridiculous for what we want to do and the weight would kill the ground effect criteria. Besides, you want a little resistance to every other wave out of frequency that wants to play drums with the skirt.



You show two design options for the thrust fan/prop. Given the installed power I wonder if you have yet determined the thrust from each option? These two numbers will begin to determine whether the craft is likely to work as a hovercraft. If you give us the numbers, we can comment on the likely success of the design.

Actually, there are 3. Thrust is to be provided by a 3 bladed prop and if it wasn't because they must be isolated from human contact, they would be ductless or shroud-less. I chose 3 blades because it is quieter. Undecided as of yet regarding the power-plant (although it looks like Subaru for thrust and kohler or tecumsah for lift) I decided to work with three diameters and then pitch and tip speed would provide the obvious choice at an approximate rpm. I chose 60 inch (shown), 54 inch (not shown) and dual 36 inch. As you already know, "dual prop's" only means double the thrust, for the same velocity obviously and the primary reason for choosing those was to reduce height, windage, aerodynamic drag and frontal area at the expense of additional weight and complexity . There must also be additional reserve thrust for the hump “threshold”. If I get 1.4 times the thrust required to get over the hump in moderate conditions, then all should be well.

On a positive note, I began to make all calculations at 76 mph, but, I took it down to round out at 70mph. Also, the hover gap that I’m calculating is 1 inch so cutting power back to attain ½” will help tremendously for hump depression reduction, and subsequently, hump drag. Properly calculated exit holes will reduce the hump drag as well. Lots can be done so that reserve thrust can get pretty close to 1.8 times the required hump drag thrust without increasing HP. Therefore:

Lift: 24-29 HP, 34-36" prop,
Thrust: 80-95HP, 60", 54" prop,
Thrust 120-150HP, 36" prop



The skirt has the appearance of a bag skirt. Have you considered the details of this yet? What will the bag pressure be? It would be interesting to discuss how the design will control plough in (plow in your neck of the woods!)? Have you considered some type of responsive skirt, or will you use compartmentation (or maybe both)?

Potatoe, potato…if a VP of the U.S., Dan Quayle can misspell it, we can use either plough or plow! J Here, you have me at a loss sort of speak. I have not given any thought to the skirt whatsoever other than it may be a good idea to make the lower section in fingers for maintenance reasons. I have been wrangling with this a bit because I’m found needing good clearance for offshore work, but also need to deal with the added bulk and drag beneath the foil while in ground effect. I have not looked at responsive skirts or compartmentalization. Have any advice or suggestions?

There appears to be significant fuss about plowing. How often does this “really” happen on its own? Does good piloting and seamanship reduce or eliminate this phenomena or is it cause and effect related mostly to reckless handling or competitive performance that pushes craft outside of the stability and performance envelopes? I’m currently researching this extensively and will provide my findings. This just seems like something that can be eliminated through proper design and piloting and I suspect that there is a lot that can be done to eliminate it. Shoot…the stealth bomber (flying wing) had systems integrated that prohibited the pilot from physically and willingly stalling the damned thing.

Okay, keep it coming. Just be patient if my response isn't quick. Gotta work AND be a grandfather too! Just got news that I will now be a granddad for the fifth time!

Thanks
J

John2
20-Apr-10, 08:18 PM
How would the lift system work ? What motor do you intend using and where would it go ? I like the idea of flooding the one side to right it again but then would that seal-able area double as buoyancy when floating, something like that cargo ship that sinks to load stuff. I was at sea just a few weeks ago, Not So Sure I would want to try land a ground effect craft on those waves, but that's just me. :eek:

Hello there Atters,

He he...not sure that I would land on those waves either. I would either keep heading home and land wherever possible as soon as possible, or, I would have transitioned to hover at the first sign of trouble and headed in under reduced thrust. :)

The lift system is direct drive and independent. I'm not certain which motor will be chosen yet because I'm still trying to iterate different options, but it would go on the nose as denoted by the picture. The thrust system I still have to work out in my head, but it will be such that twin propellers turn in oppossite directions and powered by a single engine, with the possibility to "slip" the belt so that thrust for one propeller can be greater or lower than the other. Right angle gears have come to mind, but those are complex, heavy and quite expensive. Those however have not yet ben ruled out.

I just finished cutting in the lift duct into the pan so that I can develop the plates and analyze if it will be difficult to wrap. For those that dont know, "developing plates" is unwraping the object so that it is flat, much like the map of the world globe, and then the program generates offsets for cnc/lofting or full size patterns. I've attached a picture of the thrust duct and the lift duct developed into a flat plate for comparison. This can be done for any component.

Yes, the flood chamber's first and primary role is to be dedicated bouyancy. Hopefully and GOD willing, it will always do its job without having to see a single drop of water.

Thanks
J

Ian Brooks
20-Apr-10, 10:14 PM
Oh geez. I remember calculating all kinds of drag, skirt, wave, aerodynamic profile, pressure momentum etc. I just can't remember the figure. I think that the greatest drag was at a Froude that was with the craft crabbing in yaw and perpendicular to a vector. I want to say somewhere in the 260lbf range. In any event, she should climb out of the hump before 12 mph.

On a positive note, I began to make all calculations at 76 mph, but, I took it down to round out at 70mph. Also, the hover gap that I’m calculating is 1 inch so cutting power back to attain ½” will help tremendously for hump depression reduction, and subsequently, hump drag. Properly calculated exit holes will reduce the hump drag as well. Lots can be done so that reserve thrust can get pretty close to 1.8 times the required hump drag thrust without increasing HP. Therefore:

Potatoe, potato…if a VP of the U.S., Dan Quayle can misspell it, we can use either plough or plow! J Here, you have me at a loss sort of speak. I have not given any thought to the skirt whatsoever other than it may be a good idea to make the lower section in fingers for maintenance reasons. I have been wrangling with this a bit because I’m found needing good clearance for offshore work, but also need to deal with the added bulk and drag beneath the foil while in ground effect. I have not looked at responsive skirts or compartmentalization. Have any advice or suggestions?

There appears to be significant fuss about plowing. How often does this “really” happen on its own? Does good piloting and seamanship reduce or eliminate this phenomena or is it cause and effect related mostly to reckless handling or competitive performance that pushes craft outside of the stability and performance envelopes? I’m currently researching this extensively and will provide my findings. This just seems like something that can be eliminated through proper design and piloting and I suspect that there is a lot that can be done to eliminate it. Shoot…the stealth bomber (flying wing) had systems integrated that prohibited the pilot from physically and willingly stalling the damned thing.


J

Plough-in is perhaps the most serious vice of hovercraft in general - it resulted in the SRN6 overturning in 1972 with the loss of two lives. It persists in light hovercraft today, but is largely conquered in large commercial craft.

It occurs following a series of events which go a little like this:

1 - the craft is operating at high speed
2 - the bow pitches down just a little for whatever reason
3 - the bow skirt contacts the water, and then lays on the surface
4 - wetting drag on the skirt overcomes the skirt tension and the skirt begins to get pulled under the craft (called 'tuck-under')
5 - as the skirt tucks-under, the center of pressure moves backwards making the situation worse
6 - the process continues until the hull hard structure contacts the water
7 - at this point the situation is critical. The hull could plane, in which case there will be an uncomfortable period of heavy deceleration, or it could sub-marine in which is very serious and can lead to structural failure, capsize and/or ejection of pilot & passengers etc.

Up until step 6 things aren't too bad and can be recovered by a skilled pilot operating a well-designed craft. In a poorly designed craft, the fate of the occupants is sealed at step 3, they are swimming. In the best craft, this phenomenon is tamed and used as a sort of brake. My craft has vents, designed to deliberately promote a plough-in when I want to slow down.

Take a look at Yun & Bliualt, they have the best part of a chapter dedicated to this phenomenon and will help you design the craft. In a nutshell, the skirt system needs to begin responding at step 2 to prevent the plough-in. There are two main ways to do this - either the skirt is designed to prevent tuck-under and to move the center of pressure forward at 2) (SRN series bulbous bow skirts), or the skirt is compartmented such that the reduced hovergap leakage at step 2 causes the front compartment pressure to rise and oppose the pitch-down moment. This is how my craft does it - together with a hull designed to plane in the event of hull-water contact.

At 70 mph, your craft will be prone to plough-in and for safetys sake you'll need to be sure the effects are controlled. It is that certain - this will happen, and unless measures are taken at design stage, there will be nothing the pilot can do to prevent it.

It should be noted that side-ways plough-in when operating at high yaw angles is also possible, and for this reason the speed craft should be limited at high yaw angles, unless the pilot is able to make appropriate trim adjustments. In large craft this may be done by skirt-shift mechanisms, in small craft it is usually done by the pilot shifting his weight!

Other considerations of skirt design would be water-skirt interactions. In theory, the wave-making drag is greatest at a Froude no. of about 0.56 when the bow sits on the peak and the stern sits in the trough of the wave, but in practise the worst drag often occurs at Fr=0.4, when the bow & stern sit on peaks, the mid-ship sits over the trough. Air escapes mid-ship and the bow and stern drop down into the water, causing massive drag from which the craft cannot escape. A solution is to have soft skirts that offer little resistance the the water, and to have sufficient thrust to accelerate smartly over hump before the wave system has time to properly develop. Should a craft get trapped sub-hump, a pilot must stop, allow the waves to settle and then try again. A well designed skirt with adequate thrust will never become trapped sub-hump.

Regards
Ian

John2
22-Apr-10, 04:24 PM
Hello to all,

I need to thank John for helping me get back on since there were some technical log-in difficulties. Thanks. Curtis 5420 did you get my pm? I'm not sure if it went through when I was logged off.



Plough-in is perhaps the most serious vice of hovercraft in general - it resulted in the SRN6 overturning in 1972 with the loss of two lives. It persists in light hovercraft today, but is largely conquered in large commercial craft.
March 1972, SRN6-012, 5 souls were lost. Combination of wind and sea capsized vessel due to pilot error. Investigation noted that the pilot chose to run on a beam reach near shore and the surf from beam on caused leeward skirt to tuck under in the trough causing craft to list beyond angle of vanishing stability. Very tragic and very sad. I do not have the full specifics, but that’s the way Yung and Bliault published it in their book and no further explanation is given for their direct cause of death. I’m willing to bet that those souls were probably not restrained and may have perished from severe trauma or drowned from being unconscious from severe trauma.



1 - the craft is operating at high speed
2 - the bow pitches down just a little for whatever reason
3 - the bow skirt contacts the water, and then lays on the surface
4 - wetting drag on the skirt overcomes the skirt tension and the skirt begins to get pulled under the craft (called 'tuck-under')
5 - as the skirt tucks-under, the center of pressure moves backwards making the situation worse
6 - the process continues until the hull hard structure contacts the water
7 - at this point the situation is critical. The hull could plane, in which case there will be an uncomfortable period of heavy deceleration, or it could sub-marine in which is very serious and can lead to structural failure, capsize and/or ejection of pilot & passengers etc.
Ian, you have an excellent memory sir. ;)
I know it’s like hijacking my own thread, and I’m sure that this issue has been belabored in this forum to death, but I think we should expand a little on this and I am hopeful that the moderator will not find that I violated any anti-hijacking rules.

Here is my understanding thus far. Lets first make some assumptions and these should be viewed as generalities. Let’s assume that all the required engineering is in place, longitudinal and traverse stability, skirt, etc etc. Further, let’s assume that all operational procedures are in place, emergency, pilot training etc etc. Finally, let’s assume that the worst conditions for plough in are in action, opposing wind and current directions, tuck under etc etc. We now only have to deal with action and reaction, which are always, equal and opposite for the engineering until design envelopes limits are exceeded, delayed and inconsistent for the operational and finally, unpredictable and non-responsive for detrimental conditions.

It is obvious, and as it has already been pointed out above as a series of events, that a progression takes place which increase in complexity leading to the “point of no return”. The specific and principle reasons for these events can be defined and measured. The bottom line is that any action that puts the craft beyond their angle of vanishing stability, whether a high thrust line or excessive wetted drag, is an action which will cause the crafts inertia to seek neutrality on the opposite side of the stability curve, which at worst is going turtle. This progression can be measured in time and upon closer observation, it will be shown that there are “signs” which if left unheeded, eventually lead to the next progressive step. The signs follow a pattern and recognizing these patterns is of utmost importance if plough in is to be eliminated, avoided, or if imminent, reduced in force for minimal effort for recovery. It would be a disservice to attempt to provide pure clarity on this issue in such a small piece of wavelength, so simple points will be made.

Test results by Yung and Bliault show that it takes up to 9 seconds to go from normal travel through a recovery of trim from a plough in and, the plough in occurs at 4.5 seconds. That is a lot of time and there are 5 stages that have “signs”. Finger makes contact with water, light, moderate, serious and unstable tuck under take place and then, plough in begins.

That is an awful lot for a trained professional pilot or prudent private skipper which are both charged with souls to just arbitrarily miss and not recognize. The signs for the progression needs to be identified and I suspect that the most important will manifest at the gap between skirt and surface because that is where it all begins when disturbed, in other words, when the finger touches the surface.



Up until step 6 things aren't too bad and can be recovered by a skilled pilot operating a well-designed craft. In a poorly designed craft, the fate of the occupants is sealed at step 3, they are swimming. In the best craft, this phenomenon is tamed and used as a sort of brake. My craft has vents, designed to deliberately promote a plough-in when I want to slow down.
Excellent observation and this is the reason that I always have one ear to the ground. Books are nice, but I’ll share something I believe to be true. “Not even a perfectly engineered bilge pump designed by the book can outdo the resolve demonstrated by a terrified skipper with a bailing bucket. This may not apply to full flotation structures, but you get the meaning. I will concede that I cannot prove otherwise, but I suspect that much can be done to prevent going for a swim before step 3 as you have indicated above if we look for the signs that establish patterns for progression and then counteracting them expeditiously.

What craft do you have? Pictures of the vents or skirt? What must be done to plough in? Why would a need to plough in exist? Would it not be better to fishtail or do a 180 and make the velocity degradation tolerable?



There are two main ways to do this - either the skirt is designed to prevent tuck-under and to move the center of pressure forward at 2) (SRN series bulbous bow skirts), or the skirt is compartmented such that the reduced hovergap leakage at step 2 causes the front compartment pressure to rise and oppose the pitch-down moment. This is how my craft does it - together with a hull designed to plane in the event of hull-water contact.
This needs to be explored further. Have any pictures of the skirt? Are those compartments by design or did you incorporate the change yourself?


Other considerations of skirt design would be water-skirt interactions. In theory, the wave-making drag is greatest at a Froude no. of about 0.56 when the bow sits on the peak and the stern sits in the trough of the wave, but in practise the worst drag often occurs at Fr=0.4, when the bow & stern sit on peaks, the mid-ship sits over the trough.
Correct. That is exactly how I calculated for worst loads, simply supported, but, I have the craft upside down with full lift thrust pointing up. I wanted to make sure a capsize didn’t turn into one of those Star Trek moments where Scotty says “she’s breaking up Captain”.


So, any suggestions yet on iteration 1,2 or 3 for further development? Comments? Increase LOA?

Thanks
J

KipMac
22-Apr-10, 06:21 PM
Does any one know the address of the Plain English campaign HQ??
What is a velocity degradation for a start?

Kip

Philip
22-Apr-10, 07:26 PM
Does any one know the address of the Plain English campaign HQ??
What is a velocity degradation for a start?

Kip
Dont worry Kip. it was once explained to me as: get drunk, fall down, velocity degradation happens when you hit the ground!!

John2 and Ian keep it going although I think you both need to get out there and get some more practical experience:). EG some people on here have carried out capsize experiments in real life conditions.:)

BTW Did Scotty ever say "She's breaking up Captain" I think it may have been one of the later engineers.

Bryan
22-Apr-10, 08:42 PM
To plough in at 76mph would destroy the plaining surface and the people in the craft. To drive an open cockpit craft at speeds above 50mph is not for the faint of heart. The conditions have to be spot on. Flat water into a 6 mph wind perfect. We do not get blessed with that sort of weather often here. I would be thinking along the lines of shedding as much needless weight from the craft. Toss the gull wing doors and decrease that skirt pressure. A compartment skirt would be a must. Still it looks good. Just never seen that many good looking hovercraft that work very well. Interesting thread i wish you all the best.

kach22i
18-May-10, 09:46 PM
From an Industrial Design aspect I like the twin thruster option. However I have no idea where your lift fan is. I also think that getting in and out of the craft would be helped with some flat decks.

Here is a pretty hovercraft in my opinion, and it seems to be working for those Russian guys.

http://www.seatech.ru/eng/ships/hovercrafts/kater/kater.htm
http://www.seatech.ru/images_eng/ships/hovercrafts/kater/foto/4_big.jpg
The photo ID says: Kater


http://www.seatech.ru/eng/ships/hovercrafts/kater/kater.htm
http://www.seatech.ru/images_eng/ships/hovercrafts/kater/diz/1_midle.jpg
http://www.seatech.ru/images_eng/ships/hovercrafts/kater/plan/1_big.jpg
http://www.seatech.ru/images_eng/ships/hovercrafts/kater/foto/2_big.jpg

Philip
18-May-10, 10:28 PM
The engine position, fan arrangement and skirt look very similar to another Russian craft I've seen.45784579

kach22i
18-May-10, 10:32 PM
The engine poition, fan arrangement and skirt look very similar to another Russian craft I've seen.
Yes the Mars 600 looks to have been it's daddy.

AK-Invest I think was the Mars maker.

I have a long tread of nothing but photos and links in which it appears once in a while:
http://www.hoverclubofamerica.org/forum/index.php?showtopic=1801

Kawahover
20-May-10, 09:34 PM
John2

With all due respect I think you are making it WAY more complicated than it's really needs to be. The formulas needed to design a hovercraft have mostly already been figured out. There is much in free information and books to purchase with these details already established. It sounds a bit like you are trying to reinvent the wheel. A couple of questions for you:

Have you ever owned or operated a hovercraft?
Have you ever seen a small or recreational hovercraft in operation?

Someone had mentioned before the hull bottom design is critical. Flat is not an option. Again, no offense, it looks like you have some good ideas and I want to encourage that but hovercraft for the most part are amazingly simple machines with very similar operating characteristics despite their sizes, shapes and configurations. Design your lift system first and build the asthetics around that. I would recommend listening more to the people here who operate, build and work on hovers all the time. Their experience is invaluable and could save you lots of headaches..

Nate

Hovertrekker
21-May-10, 12:20 PM
Nate FYI - I think john2 is long gone from this forum

atters
21-May-10, 01:37 PM
Maybe he is like a Bill Gates, and take a bit of everyone else's idea and end up a Gazillionaire, all from what he has learnt from the forum...

Or because we hear nothing from him, lets hope he is getting busy in the workshop.;)