Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Starman, May 20, 2010.
What happens when a WIG craft leaves ground effect? Doesn't it just settle back into it?
One would hope.
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 Fischer's work) but is about a decade or more out of date.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.
Thanks for the explanations, Orion.
There are three classes of WIG craft, determined by a combination of wing span and power available. They are:
1. can not rise out of ground effect.
2. Can jump out of ground effect for short distances.
3. Can fly out of ground effect and stay out.
The first two are legally classified as boats and the third as an aircraft.
The million dollar problem to solve is the serious stability problem caused by the craft pitching nose up as it rises, which is the opposite of what you want it to do, and is why hydroplanes flip over so easily.
My solution to this is based on the way standard aircraft are stable (enough) while in ground effect, but it requires longer wings and span dominated rather than chord dominated ground effect.
Your summary is concise and right. But current marketing thought is that Class A WIGs will never be a suitable product basis due to limited maneuvering (most operate in a manner similar to a hovercraft so are considered unsuitable to more crowded waterways; also tend to be loud if of the PAR configuration - Amphistar for instance).
Class C WIGs are airplanes so no-one is likely to certify them as WIGs.
The reverse delta as originally developed by Dr. Lippish actually does work (when applied correctly) however most programs tend to modify the delta shape to something less than ideal and as such, even those programs continue to be plagued by the stability issues.
If the design a WIG Craft does not consider the "airplane" components, it will never fly because you will need to know some knowledge about aerodynamics. So, I think it is reasonable to design WIG Craft by using some elements from airplanes as well as from ships, which make WIG Craft an amphibious craft. Even if you are building a class A type. Unfortunately, it is not so easy to fuse this two together and make it works.
WSH500 and HW20 are using turbine engines. Is it a wrong engine choice? Even though turbine engine has good Power to Weight ratio, which I think it is why they use the turbine, it is not meant for the sea environment. I started to question on the TBO for turbine in these WIG Craft.
You don't get the point - of course the WIG is a flying craft so flight technology and science must be used to design said craft - that's obvious. But it cannot be configured in the same manner as you would approach a conventional aircraft; they're two different animals.
One thing that is noticeable on the Lippish WIGs is the very low aspect ratio. I wonder if it would still be as stable if the span was extended while keeping the same chord. I realize a longer span is less practical, but aerodynamically it should be the same, shouldn't it. Come to think of it, I wonder if there are any real advantages to a higher aspect ratio anyway.
In this realm the benefits of additional span are minimal but the penalties can be substantial (operationally). There is a pretty good overview of the Work by Lippish by an author named Borst. Not really detailed but does have some good historical background.
I'd go for the 3rd choice....and have a longer chord center piece in IG .
Then it's not a WIG and you'll need to build, document and certify the craft as any other commercial aircraft. In other words, there is no benefit to that approach.
It would seem to me that two wings, the front higher than the back, would solve the "nose up" pitch problem and carry more load while remaining narrow enough to manuever in tight places. Just a thought off the top of my head.
OK, so we've got the stability problem solved, by sticking with the tried and true Lippish reverse delta. So now the question is why won't the sorry suckers take off in a light chop?
It seems to me the problem must be one of lack of power so they need to add some brute force horsepower to these boats to make them more useful.
In order to increase the flying height, to deal with rougher water, the solution seems to me to be to make a larger wing and decrease the wing loading. A larger wing will create more water drag though so it will require evenmore power to take off. The problem with that is that a combination of less wing loading and more power might turn it into a type C WIG, which we do not want. It certainly is a problem, isn't it?
Yep, certainly is.
The reason the concepts didn't take off is really varied as a function of the configuration in question. The HW for instance was not able to achieve a stable enough cushion so that more of the craft cleared the chop and the water contact. It just wasn't able to reach flying speed.
The Jorg craft apparently had a similar problem (excess drag) so they weren't able to get onto plane to accelerate to full speed.
According to the one contact I had then, the Chinese craft were the worse of all - although they did look like they had sufficient power, for some reason they were able to only come up on plane but never transition to flight.
The design of these is very critical since the configurator must match and balance between cruise requirements and those of takeoff. This controls everything from foil choice to incidence to allowable CG to hull trim angles. Some of these may work in perfectly flat water but the variation introduced by the chop could be sufficient to knock it off that ideal.
And BTW, the wing is really not supposed to be in the water at take-off so the extra area should not cause an acceleration penalty. But of course this is not always possible - I noticed the Flightship trailing edge looks to be probably a foot under the water at the dock.
I always thought this type craft would have a niche up in the frozen waste lands, carrying cargo across the Frozen Lakes, etc.
Yes but only c-type would work...Kaspian Seamonster etc work only in large water areas.
That does sound like a good idea, for really big lakes, or for smooth snow covered terrain.
OK, then this calls for giving away the next part of my secret plan You know those powered canoes that are used in Asia (I can't recall which part) that have the engine mounted up in the stern with a long straight shaft, in line with the crankshaft, that goes back about ten feet or so the prop in the water. Those engines are on a pivot and used for steering. My idea is to use a similar setup for a take off engine, with a long shaft, and allow the engine to pivot up and down so that the prop will follow the water surface and stay in the water, thus giving constant thrust in spite of waviness of the water. Since the prop can lower as the craft rises it will allow the craft to clear the waves and get up to flying speed before the take off engine is shut down.
That's a good idea to keep the wing out of the water. I saw that the X-112 and X-114 had no fuselage and so the wing provides the flotation in back. I see that the Airfish 3, which appears to have been the most successful, does have a fuselage which keeps the wing out of the water
What do you think of placing a Autogyro blade on a WIG boat to create lift?
No one has tried this yet?
Once the blade is pre rotated by the motor, it continues to rotate as a function of the air speed rushing by it and creates lift with no power applied, sufficient to lift 1,000s of kilos.
It also creates no down draft.
Auto gyros fly at 180 kmh using a Rotax motor to drive the forward propulsion propeller. The vertical lift blade is pre rotated to achieve rotational speed, then the motor drive is uncoupled and applied to the propeller. The same well established system could be used in a WIG craft.
With a 1,000 kg free lift from the rotating blade even less power could be applied to drive the WIG craft forward.
What is it like in Southern Ecuador, by the way? Is it worth moving to? Is it really as inexpensive as people say? Is it secure?
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