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Thread: Hovercraft Design

  1. #9
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    Default Re: Hovercraft Design

    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 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
    Last edited by Keith Oakley; 19-Apr-10 at 10:37 AM.

  2. #10
    John2
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    Default Re: Hovercraft Design

    Quote Originally Posted by atters View Post
    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

  3. #11
    John2
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    Default Re: Hovercraft Design

    Quote Originally Posted by Keith Oakley View Post
    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 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

  4. #12
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    Default Re: Hovercraft Design

    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
    Ian Brooks
    Gloucester, UK
    http://www.hoverclub.org.uk

  5. #13
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    Default Re: Hovercraft Design

    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.
    In the beginning there was the Cube; I know where is came from and it has the power to create Hover life. But with great power comes great Fun.

  6. #14
    John2
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    Default Re: Hovercraft Design

    Hello Ian,

    Quote Originally Posted by Ian Brooks View Post
    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.


    Quote Originally Posted by Ian Brooks View Post
    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.


    Quote Originally Posted by Ian Brooks View Post
    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.


    Quote Originally Posted by Ian Brooks View Post
    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


    Quote Originally Posted by Ian Brooks View Post
    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
    Attached Images Attached Images

  7. #15
    John2
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    Default Re: Hovercraft Design

    Quote Originally Posted by atters View Post
    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.
    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
    Attached Images Attached Images

  8. #16
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    Default Re: Hovercraft Design

    Quote Originally Posted by John2 View Post

    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
    Ian Brooks
    Gloucester, UK
    http://www.hoverclub.org.uk

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