Thinking that a pilot can steer a WIG without extra training
WIG behave different than conventional airplanes. There are many examples in the past where a pilot, who flew normal airplans, did the opposite reaction as needed in a WIG. That was for instance the reason of the distruction of a large Russian Ekranoplan. The pilot did react to a situation by adding more power. The Ekranoplan rose about ground effect. Became unstable. Hit the water pretty hard. If he had reduced the power, he would have been able to tell the story.
On the Homebuiltairplanes-forum they talked about what happens when a WIG leaves ground effect. ORION says:"When a WIG moves away from the surface beneath it several things happen at once - here the trick is that all these things must happen in a coordinated orchestra or the craft can became unstable ending the maneuver in an extremely catastrophic manner. As the wing moves higher there is a slight loss of lift due to a decrease in lift coefficient (dependent on geometry and aoa); there is also an increase in drag. This is good since the combination would initiate a motion back down to previous height. But this assumes that the craft was designed correctly and that the variables that are a function of height change have the proper characteristics so that this can happen in a predictable manner.
Then you have a very rapid Cp shift, the resultant of which is at about 50% MAC in ground effect and closer to 25% MAC out of it. The Cp shift behavior is height as a function of chord based and is one of the more sensitive aspects of the transitioning phenomenon. Can play real havoc with trim requirements in a typical WIG operational envelope.
Then you have to play with the variables of trim and how the wing's behavior affects the tail. For instance, as the craft rises, imagine the combination of the Cp resultant moving rapidly forward and the horizontal tail suddenly being subject to even a small amount of downwash.
So yes, ideally the WIG would settle back down to the surface but that is much simpler to say than do.
Detego's video is a good basic primer on some of the WIG history (specifically Dr. Fischer's work) but is about a decade or more out of date. The craft that became Flightship never apparently fully completed the IMO sea trials however was most recently purchased by the Singapore government (who self-certified it) in order for it to be developed as a commercial venture. I have a current customer who spent about two years personally visiting and flying the current world-wide "fleet", such as it is. His evaluation of the Flightship was that it was unstable and untrimmable except in the most ideal circumstances.
The large Hoverwing is currently being developed by the Koreans but at this point in time there is not much really known about the program except that is being built more like an airplane or Russian Ekranoplan rather than a marine vessel - the whole thing seems to be formed of hundreds of panels, all riveted together. Given the pounding these things can take, does not seem like a smart approach."
Then you have a very rapid Cp shift, the resultant of which is at about 50% MAC in ground effect and closer to 25% MAC out of it. The Cp shift behavior is height as a function of chord based and is one of the more sensitive aspects of the transitioning phenomenon. Can play real havoc with trim requirements in a typical WIG operational envelope.
Then you have to play with the variables of trim and how the wing's behavior affects the tail. For instance, as the craft rises, imagine the combination of the Cp resultant moving rapidly forward and the horizontal tail suddenly being subject to even a small amount of downwash.
So yes, ideally the WIG would settle back down to the surface but that is much simpler to say than do.
Detego's video is a good basic primer on some of the WIG history (specifically Dr. Fischer's work) but is about a decade or more out of date. The craft that became Flightship never apparently fully completed the IMO sea trials however was most recently purchased by the Singapore government (who self-certified it) in order for it to be developed as a commercial venture. I have a current customer who spent about two years personally visiting and flying the current world-wide "fleet", such as it is. His evaluation of the Flightship was that it was unstable and untrimmable except in the most ideal circumstances.
The large Hoverwing is currently being developed by the Koreans but at this point in time there is not much really known about the program except that is being built more like an airplane or Russian Ekranoplan rather than a marine vessel - the whole thing seems to be formed of hundreds of panels, all riveted together. Given the pounding these things can take, does not seem like a smart approach."
WIGs are not airplanes
As i mentioned in the leading page, if a WIG can leave the ground effect and stay above it, it is no longer registred as a boat, but as a airplane. I found a article on the Homebuiltairplanes.com-forum about this.
Norman Masters says: "There are two stages to ground effect: span dominated ground effect which is a reduction in induced drag and chord dominated ground effect which gives an increase in lift. The long chord is to maximize the height of the lift boost of chord dominated ground effect. If your WIG is capable of sustained flight out of ground effect it must be registered as an aircraft and the pilot needs a pilot's license but if it is not capable of flight out of ground effect it is legally a boat. By designing the WIG boat to fly in chord dominated ground effect the height above the surface is automatically limited to 1 to 1.3 times MAC so there's no doubt about its legal status."
Using hydrofoils on WIG
Don't trust what i say, you simply need to see this video to understand why it is a bad idea.
In the past they did full scale test with hydrofoils on seaplanes.
If you listen to this video, you hear a guy talking who was involved into these test on the Goose with hydrofoils. But he also mentions that the Goose was tested with skies. And he tells that those skies were more promising than the hydrofoils. So ...maybe it would be good to consider those. Haven't found WIGs yet with skies.
But than this video below makes you believe it is possible to use hydrofoils. Hmmmm. I see that they studied TEN years the hydrofoils to get it right. So ... don't think you can do it overnight.
I found a lot of discussion about using hydrofoils on WIGs in my favorite forum, HomebuiltAirplanes.com . Member Orion seems to know WIGs from own experience.
Inverted Vantage :
"orion, apologies if you've heard this question before (I'm sure you have), and I'm not very educated on boat design...what research has there been on using hydrofoils as boosters to push the hull up out of the water, reducing drag and thus reducing the required thrust to pull the vehicle completely out of the water?"
Orion:
"It has been explored and is considered a promising (our concept uses the idea for instance) however it is not popular because a similar idea was incorporated into a test version of the X-114, which crashed because the pilot forgot to retract the foils and they dug in during a turn, destroying the craft.
The other problem is that in order for them to be fully functional they have to be submerged, even as the craft leaves the water. This however means that they're likely to be submerged or in water contact as the craft approaches the takeoff/cruise speed so the system does require some form of retraction so as to allow a low drag transition to flying speed. Our system is looking toward an automated retraction - without that the scenario of them inadvertently contacting the surface is very real."
Orion:
"There were only two cases of hydrofoils on WIGs that I've seen - one result was positive the other not so much. There was a Japanese POC scale vehicle that used small hydrofoils on the hull. These retracted at cruise. In this case the foils were successful in that they demonstrated a much quicker transition to flight.
In the case of the X-114 the hydrofoils did indeed get the hulls off the water much sooner but the take-off run itself ended up being substantially longer than on the conventional hulls. The accident killed the program so no analysis was conducted as to why but the guess was that the aft foil (located on the wing trailing edge at the craft's centerline) raised the vehicle too high thus reducing the angle of attack, which in turn resulted in the need for more speed before the craft actually flew.
Also, I was wrong about the foil actuation it turns out - the foils on the X-114 were fixed, not retractable. At low speed their impact with the surface was OK due to their incidence however, at cruise speed where the craft flew at a lower angle of attack, the incidence angle of the foils was actually slightly negative, which is why they caused the serious crash. As they contacted the surface they immediately sucked that side into the water, disintegrating the structure.
The idea of using a water drive for take-off assistance may be useful but will have to be done carefully since it needs to be in operation until well into flying speed. If it cuts out just as you leave the water and all you'll be left with is the small air engine, the drag of the higher aoa, plus any inadvertent contact with the water, may actually settle you back down into the water. At best this could be annoying - at worst it could set up a porpoising motion, which could be quite destructive."
"orion, apologies if you've heard this question before (I'm sure you have), and I'm not very educated on boat design...what research has there been on using hydrofoils as boosters to push the hull up out of the water, reducing drag and thus reducing the required thrust to pull the vehicle completely out of the water?"
Orion:
"It has been explored and is considered a promising (our concept uses the idea for instance) however it is not popular because a similar idea was incorporated into a test version of the X-114, which crashed because the pilot forgot to retract the foils and they dug in during a turn, destroying the craft.
The other problem is that in order for them to be fully functional they have to be submerged, even as the craft leaves the water. This however means that they're likely to be submerged or in water contact as the craft approaches the takeoff/cruise speed so the system does require some form of retraction so as to allow a low drag transition to flying speed. Our system is looking toward an automated retraction - without that the scenario of them inadvertently contacting the surface is very real."
Orion:
"There were only two cases of hydrofoils on WIGs that I've seen - one result was positive the other not so much. There was a Japanese POC scale vehicle that used small hydrofoils on the hull. These retracted at cruise. In this case the foils were successful in that they demonstrated a much quicker transition to flight.
In the case of the X-114 the hydrofoils did indeed get the hulls off the water much sooner but the take-off run itself ended up being substantially longer than on the conventional hulls. The accident killed the program so no analysis was conducted as to why but the guess was that the aft foil (located on the wing trailing edge at the craft's centerline) raised the vehicle too high thus reducing the angle of attack, which in turn resulted in the need for more speed before the craft actually flew.
Also, I was wrong about the foil actuation it turns out - the foils on the X-114 were fixed, not retractable. At low speed their impact with the surface was OK due to their incidence however, at cruise speed where the craft flew at a lower angle of attack, the incidence angle of the foils was actually slightly negative, which is why they caused the serious crash. As they contacted the surface they immediately sucked that side into the water, disintegrating the structure.
The idea of using a water drive for take-off assistance may be useful but will have to be done carefully since it needs to be in operation until well into flying speed. If it cuts out just as you leave the water and all you'll be left with is the small air engine, the drag of the higher aoa, plus any inadvertent contact with the water, may actually settle you back down into the water. At best this could be annoying - at worst it could set up a porpoising motion, which could be quite destructive."
The placement of the elevator
The elevator is huge on WIGs. They need to keep the pitch moment under control while the pressure point shifts between pressure points of ground effect and real flight (from 50 to 25% of the mean chord).
Kavin of the youtube channel Think Flight tells in one of his WIGs videos that he had to place the rudder higher to get one of his large RC models more stable. The elevator was in the ground effect area and that made the rise of the WIG complex as the elevator went from ground effect to above ground effect. That changed the pressure point in the elevator too. And you don't want those changing pressure point locations in the devise that is needed to keep the pitch moment stable.
Only focussing on wing and forgetting the hull
As many thinkers will do, is put a lot of effort to get the wing design as good as possible. But ... don't forget ... the hull and side skirts or endplates are VERy important for the time the WIG is still in the water.
I found at homebuildairplanes.com a discussion about the hulls of WIGs. It might help you in your steps to get that hull right.
ORION:
The biggest problem WIG craft continue to have in their development is simply that the hydrodynamic forces at play will drive the power requirement, more so than anything flight related. If we look at an idealized planing flat plate at a positive angle of attack, the optimum L/D occurs at or near 5 deg in relation to the waterline. The max L/D value at this point is approximately 5.5:1. But this is idealized and assumes a perfect flat surface. Introduce typical environmental issues and realistic hull design values, including things like dead-rise, the max L/D value will most likely be around 4.0:1 or even less. So, for your 3,000 pound craft you will be generating about 750 pounds of drag. Below this trim speed you will be in displacement and/or transition mode and after this point it'll be assumed that your wing will be providing lift thus reducing the weight the hull has to lift.
Then your dynamic effects come into play - as a rule of thumb, it's a good idea to have a substantial thrust margin at this point so as to avoid porpoising. For the sake of discussion let's round off the thrust required to get off the water to be at about 1,000 pounds. Given your two 40 hp motors with moderately small props, it's doubtful that you'll generate more than about 250 pounds of static thrust, give or take a bit. Maybe with an optimized low pitch prop you can do a hair better, but you'll still be woefully short of the mark.
And regarding your 250% increase in fuel efficiency in ground effect, simply stated, in your dreams. While it is true that a wing in ground effect does generate less drag and to a limited extent a bit more lift (requires very small h/c values) than in free air, remember that you're dealing with induced drag only. Because of speed and performance issues though, you generally have a large wing so your cruise lift coefficient is relatively small, thus making the induced drag number relatively small. You still have the fact that it's a low aspect ratio design and you are still carrying the rest of the vehicle. Those numbers do not change.
But here's the basic problem, you need power to get off the water but you then don't really need it to cruise (BTW, I think a more realistic cruise speed goal for you will be around 80 to 100 mph). On a large scale craft like the Russian KM that means you carry ten large turbofans for take-off but then you idle eight of them for cruise.
On a smaller vehicle like the failed Flarecraft you will then have a tendency to overpower the craft, which can greatly destabilize the vehicle (long story here and not a very pretty one).
The difficult task in designing a WIG is simply that there is not enough reliable information out there to do a good job of it. If you're designing a plane there is tons of data, text books and reference papers that you can dig through to get just about anything you need. For WIGs there is virtually nothing. I've been doing WIG development consulting for over about fifteen years now so I've seen most of the programs in the Western hemisphere. Some have potential and some are downright scams but all have the same issues - not enough data and not enough money to do it right.
The bottom line of this is that the project you have in mind is ambitious and there is a lot more to it than a cursory glance might reveal. I certainly do not wish to dishearten your effort but just want to make sure you understand what you're getting into. People and organizations have been after a functional WIG craft for years, especially a Class B. So far though, no luck. FlightShip came closest but it too ran out of money, mainly due to creative bookkeeping by the developer. That program did go through sea trials and certification but once the money got pulled, it just disappeared. Last I heard it was sold to someone in Turkey but there it seems to have gone nowhere.
The hull L/D value is for an established planing configuration. To get there though you have to deal with your bow wave so getting up on plane requires a lot more thrust than the planing L/D value might suggest.
STARMAN:
Thank you for the planing boat L/D figures, that helps. One thing I wonder about is the ideal planing angle - are you pretty sure that the ideal angle is 5 degrees? Previously, and over a decade ago, I had read that the ideal angle is 10 degrees. Possible 5 to 10 is close and is the ideal range? 10 degrees though would mean a little bit less wetted area in a hypothetical ideal situation.
Orion:
The ideal planing angle usually resides between 4.5 and 5.5 degrees. Yes, there is some variance as a function of Froude Number and finesse ratio but for a conventional hull, that's about it. Beyond that the drag rise is fairly substantial. Also keep in mind that the hull imposed trim angle will initially be much higher as you approach the planing speed so the actual amount thrust you'll need is dramatically greater.
The biggest problem WIG craft continue to have in their development is simply that the hydrodynamic forces at play will drive the power requirement, more so than anything flight related. If we look at an idealized planing flat plate at a positive angle of attack, the optimum L/D occurs at or near 5 deg in relation to the waterline. The max L/D value at this point is approximately 5.5:1. But this is idealized and assumes a perfect flat surface. Introduce typical environmental issues and realistic hull design values, including things like dead-rise, the max L/D value will most likely be around 4.0:1 or even less. So, for your 3,000 pound craft you will be generating about 750 pounds of drag. Below this trim speed you will be in displacement and/or transition mode and after this point it'll be assumed that your wing will be providing lift thus reducing the weight the hull has to lift.
Then your dynamic effects come into play - as a rule of thumb, it's a good idea to have a substantial thrust margin at this point so as to avoid porpoising. For the sake of discussion let's round off the thrust required to get off the water to be at about 1,000 pounds. Given your two 40 hp motors with moderately small props, it's doubtful that you'll generate more than about 250 pounds of static thrust, give or take a bit. Maybe with an optimized low pitch prop you can do a hair better, but you'll still be woefully short of the mark.
And regarding your 250% increase in fuel efficiency in ground effect, simply stated, in your dreams. While it is true that a wing in ground effect does generate less drag and to a limited extent a bit more lift (requires very small h/c values) than in free air, remember that you're dealing with induced drag only. Because of speed and performance issues though, you generally have a large wing so your cruise lift coefficient is relatively small, thus making the induced drag number relatively small. You still have the fact that it's a low aspect ratio design and you are still carrying the rest of the vehicle. Those numbers do not change.
But here's the basic problem, you need power to get off the water but you then don't really need it to cruise (BTW, I think a more realistic cruise speed goal for you will be around 80 to 100 mph). On a large scale craft like the Russian KM that means you carry ten large turbofans for take-off but then you idle eight of them for cruise.
On a smaller vehicle like the failed Flarecraft you will then have a tendency to overpower the craft, which can greatly destabilize the vehicle (long story here and not a very pretty one).
The difficult task in designing a WIG is simply that there is not enough reliable information out there to do a good job of it. If you're designing a plane there is tons of data, text books and reference papers that you can dig through to get just about anything you need. For WIGs there is virtually nothing. I've been doing WIG development consulting for over about fifteen years now so I've seen most of the programs in the Western hemisphere. Some have potential and some are downright scams but all have the same issues - not enough data and not enough money to do it right.
The bottom line of this is that the project you have in mind is ambitious and there is a lot more to it than a cursory glance might reveal. I certainly do not wish to dishearten your effort but just want to make sure you understand what you're getting into. People and organizations have been after a functional WIG craft for years, especially a Class B. So far though, no luck. FlightShip came closest but it too ran out of money, mainly due to creative bookkeeping by the developer. That program did go through sea trials and certification but once the money got pulled, it just disappeared. Last I heard it was sold to someone in Turkey but there it seems to have gone nowhere.
The hull L/D value is for an established planing configuration. To get there though you have to deal with your bow wave so getting up on plane requires a lot more thrust than the planing L/D value might suggest.
STARMAN:
Thank you for the planing boat L/D figures, that helps. One thing I wonder about is the ideal planing angle - are you pretty sure that the ideal angle is 5 degrees? Previously, and over a decade ago, I had read that the ideal angle is 10 degrees. Possible 5 to 10 is close and is the ideal range? 10 degrees though would mean a little bit less wetted area in a hypothetical ideal situation.
Orion:
The ideal planing angle usually resides between 4.5 and 5.5 degrees. Yes, there is some variance as a function of Froude Number and finesse ratio but for a conventional hull, that's about it. Beyond that the drag rise is fairly substantial. Also keep in mind that the hull imposed trim angle will initially be much higher as you approach the planing speed so the actual amount thrust you'll need is dramatically greater.
Thinking "WIG IS THE SOLUTION"
If you read about WIG, it sounds like ideal. I have to admit ... i still think this as true ... if the water is calm. The next text from the Homebuiltairplanes.com-forum tell us about what happens if there are waves. Post dates from 2012.
ORION:
" The work done by Hanno Fischer was done through decades of the German government's funding of the technology. But about five years ago there was an invitational fly-off of the prototypes in development at the time. The craft included the Hoverwing, the vehicles by Jorg, one or two from China, and I forget the rest. In all there were nearly a dozen vehicles present. On the day of the test there was a slight wind and the lake developed a slight chop (a local WIG craft developer whom I've gotten to know relatively well was there) - not one craft was able to lift off the surface. That pretty much killed the government cash cow."
FLYOVER:
"Orion, it seems that everything has been tried to make a successful WIG Craft but with no success. I believe in that statement given the long history of WIG technology and so many people that had tried to build it but with no success so far.
I even started to think that WIG is no longer commercially attractive for over the sea transport."
I even started to think that WIG is no longer commercially attractive for over the sea transport."
ORION:
"Sadly true - I think there's a variety of reasons for the lack of technological prowess in these programs, most of it stemming from too many ingrained ideas. For instance, Virtually all Russian and Chinese programs are still based on the classic work done by Dr. Alexiev, utilizing rectangular wings. While this does have some mathematical advantages, from the stability and controllability standpoint it is problematic.
But I think the biggest problem is that too many developers try to approach this technology more as a flying amphibious airplane rather than a new water based technology transport. As long as one keeps thinking "airplane", the WIG concept is going to be mired down with in-optimal approaches that will fail. Oh well, all it takes is a few more million...."
But I think the biggest problem is that too many developers try to approach this technology more as a flying amphibious airplane rather than a new water based technology transport. As long as one keeps thinking "airplane", the WIG concept is going to be mired down with in-optimal approaches that will fail. Oh well, all it takes is a few more million...."
So WIG still has a lot of issues to be solved, but ... if the solutions could be found... it would be a super thing.