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Rutan Ski-gull

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cowlove

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I assume out on or near wingtips as you surmised, and I assume large units such as these : Turnigy RotoMax 150cc Size Brushless Outrunner Motor

These should be equivalent to 12-13hp, but they are about 5.5 lbs each.
Intrigued by the scant info on the Skigull "sustainer/docking" engines, I just purchased the ~10HP little brother of this motor, and a 27" prop. Not sure what exactly I'm going to do with it, other than hopefully not hurt myself.

jeaRn2l.jpg

I wonder if the Skigull with her docking engines require a multi-engine rating?

Jim
 

narfi

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What are you guys thoughts on the skis, the idea fascinates me but I am sure I do not know all the advantages and disadvantages that they hold. (except that one of them broke in testing)

The idea itself seems simple enough though I am sure there are lots of specifics involved in their implementation. Without any real info yet on the skigull (that I have found) it seems the fuselage is much more aerodynamic than a typical flying boat.

What do you guys think about the topic?
 

Topaz

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I'm not qualified really (seaplanes aren't my thing, and I've not studied them much), but I think this may be one of Rutan's ideas that doesn't really work out. I mean, the notion is great - land on anything - but everyone else that tried them has had trouble with hydro-skis, and the ones on the Ski-Gull look awfully fragile in what photos I've seen. I suspect breaking them is going to be a common-enough occurrence that they'll at least need to be redesigned. Not really fond of the small wheels. For an airplane designed to "land anywhere", it seems, again, to be something prone to rock damage on an unpaved strip. Or is he only going to land on pavement, water, or snow?

Maybe I'm completely wrong, but that's my "thinks" on the subject.
 

narfi

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I was thinking more for just water applications as a means of keeping the fuselage 'cleaner' as an alternative to the stepped hull.

I am not a fan of the tiny wheels either.... but for pure water it seems interesting, and for out of water I would want something capable of gravel.
 

Topaz

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I was thinking more for just water applications as a means of keeping the fuselage 'cleaner' as an alternative to the stepped hull.
All the aero primes who worked in seaplanes in the 1950's - especially Convair - seemed to have had a passionate love-affair with hydro-skis during that period. While there was some successful testing, none of the designs ever made it to production. Seems like there was always some problem or other that kept it from being a practical solution.

If Rutan can pull it off, great. He'll be the first.
 

bmcj

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Convair - seemed to have had a passionate love-affair with hydro-skis during that period. While there was some successful testing, none of the designs ever made it to production. Seems like there was always some problem or other that kept it from being a practical solution.

If Rutan can pull it off, great. He'll be the first.
I tend to agree with you, but if anyone has a history of pulling off unlikely firsts, it would be Burt Rutan.
 

Jay Kempf

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All the aero primes who worked in seaplanes in the 1950's - especially Convair - seemed to have had a passionate love-affair with hydro-skis during that period. While there was some successful testing, none of the designs ever made it to production. Seems like there was always some problem or other that kept it from being a practical solution.

If Rutan can pull it off, great. He'll be the first.
Rutan will make it work. He already stated that the skis were not made the way they will be done in the final version and that they need to be toughened. The wheels I think were not meant to be for day to day ops. This is a seaplane. If it needs to be landed in the grass next to the runway the idea is that you can get around at an airport if you need to and the wheels are cheap and easy to replace and you keep a box of them onboard. It was always a compromise. At least that is from my monitoring of what's been available. Skis provide great damping in rough water. Not sure if I remember anyone doing a taildragger with front skis before. They will be robust enough for water. Land we shall wait and see. I suppose you could put some bigger wheels and tires on the back of those skis as well later if that proves necessary. Overall the thing seems to be doing quite well as a prototype. Not many mods being talked about yet. But it doesn't have much time on it yet either.
 

Aesquire

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The Sea Dart was sorta a taildragger with skis. Had serious vibration issues but modern smart magno fluid shocks would help.

Riveted metal skis on a heavily built jet fighter ( very dense packed mass ) vs. Composite skis on a light plane.

Definitely a work in progress. Best of luck.
 

Pops

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My neighbor worked last winter at Lakeland, FL in restoring the Sea Dart for display at Sun-in -Fun. It was in very bad shape.

Dan
 

narfi

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ski1.jpg

With a larger wheel like this(not suggesting nose gear necessarily... tailwheel/skid maybe) but forward hinged into the sponsors.... one switch to land regardless of sea or land or snow sounds like a safe idea..... no flipping on water landing because you screwed up and put the wrong gear down. Heck even if you forget to put it down at all it should be 'safeish' on land or sea. (At least able to walk away from it)
 

Topaz

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View attachment 48852

With a larger wheel like this(not suggesting nose gear necessarily... tailwheel/skid maybe) but forward hinged into the sponsors.... one switch to land regardless of sea or land or snow sounds like a safe idea..... no flipping on water landing because you screwed up and put the wrong gear down. Heck even if you forget to put it down at all it should be 'safeish' on land or sea. (At least able to walk away from it)
Just don't slow down on water. ;)
 

autoreply

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What are you guys thoughts on the skis, the idea fascinates me but I am sure I do not know all the advantages and disadvantages that they hold. (except that one of them broke in testing)

The idea itself seems simple enough though I am sure there are lots of specifics involved in their implementation. Without any real info yet on the skigull (that I have found) it seems the fuselage is much more aerodynamic than a typical flying boat.

What do you guys think about the topic?
L/D (of the hull) is the metric. Orion was rather taken by them and discussed them at length on HBA. Turns out L/D of a ski is in the same neighbourhood as L/D of a planing hull.

Displacement hulls can be made to work for an amphibian, but only if it's pretty slow and really long. Outside that realm, you're into extremely narrow hulls (say 30 ft long, 5" wide), planing or skis.

By then skis make a lot of sense. A huge fraction of the weight of boats/amphibians is in the skin construction that has to be sturdy. Ridiculous if you have simply raise it above the water so it only has to survive big impacts at low speeds (say 10 kts), instead of going mighty fast (say 60 kts).


'
I also thought that statement to be a bit questionable since the prop still needs to clear the boom. But in reality, compared to something like a SeaWind or Lake Amphibian, the thrust line is actually lower since the prop does not have to clear a full sized fuselage section.

Regarding the performance of the ski, the story is not that straight forward. True, if you look at a ski as a low aspect ratio flat plate, the data suggests that the l/d might be lower than a more conventional hull however this design is not that simple. Richard Raoke has been working with this idea for quite a few years and from the glimpse at his results I got some years ago, the performance is actually measurably better. One of the areas where it presents an improvement over a conventional planing configuration is that when properly configured, the hydroski nearly eliminates the bow wave drag a more conventional planing hull has to deal with in the initial acceleration. Then, as the ski transitions to operating on the surface, it acs more like a wave piercing hull configuration than that of the more conventional planing one. As such, it essentially slices through waves, providing a much smoother ride.

Regarding hydrofoils, while it is true that the l/d can be substantially better than planing shapes and even the hydroski, that is true only for a narrow range of speeds and loading. As the foil accelerates the drag rise is quite rapid, which essentially requires the hydrofoil to transition from a submerged surface to a surface planing one, and therein lies the problem. As the "wing" nears the surface the thinner depth and higher speed leads to ventilation of the foil, which decreases the lift by more than 50%. This then requires a lot of power to essentially punch through that part of the run so that the surface can transition to a surface planing regime very quickly. Failing to do so, as soon as the lift decreases the foil plunges back under the water, increasing the lift again, going back to the surface, and on and on. The resulting porpoising motion could be quite destructive.

There have been several attempts to use hydrofoils on aircraft and on WIGs but all have confirmed that it really doesn't work. The worst case actually destroyed the aircraft and killed the crew.
 

Topaz

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Bringing this back to the SkiGull (I still think it all fits, but we're starting to drift), does anyone have any illustration and design description of the skis used on that aircraft in particular? I'm sure it's probably in the long video of Rutan describing the design near the beginning of this thread, and in that case, does someone have the timestamp for the beginning of that discussion?
 

justifidejoe

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All the aero primes who worked in seaplanes in the 1950's - especially Convair - seemed to have had a passionate love-affair with hydro-skis during that period. While there was some successful testing, none of the designs ever made it to production. Seems like there was always some problem or other that kept it from being a practical solution.
I'm no expert, but I was under the impression that the Navy lost all interest in hydroski/hydrofoil research once aircraft carriers for jet-powered aircraft came into being? I think the Navy was the primary funding source for these projects.This video says the sponsors that came to observe flight trials barely paid attention to the flights.
The video also makes it seem like both hydroskis and hydrofoils improved takeoff performance of the Grumman Goose. Of course these tests were done with a flying boat that retained its original hull, so you can't say a smooth fuselage with a foil/ski would outperform the hull alone.
Lake also did a bit of testing, with an apparently favorable opinion of the hydrofoil.
 

Topaz

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I'm no expert, either, but from what I've read of the period, the Navy of the 1950's was interested in blue-water operations, and how do you get a reasonably-sized seaplane of any kind to operate in 5+ foot swells? A ski can help, but isn't going to enable those kind of operations either.

With regard to a hydrofoil, I can only say what Orion had to say about it, which was, basically, "fine, until you hit a log." A planing hull might get a dent, but 'foils will be ripped clean off. A planing ski would probably survive a light hit, but snagging something as the aircraft was coming up onto the ski might get ugly.

In short, these are nice ideas if you can operate in an ideal world with smooth water and no debris. Real-world tends to be a little more messy.
 

Riggerrob

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I think the folding wings were for docking purposes, not trailering. I could be wrong.
...................................................................

Agreed. Folding (high) wings are primarily for docking.
A friend used to use his Volmer Sportsman to pick up chicks, but grumbled that he could not dock at the marinas with the best restaurants.
So the challenge is to build a flying boat, with a large cabin, that is as easy to dock as a Cessna floatplane.
Skigull and the Dornier SRay are among the few small flying boats that meet both those criteria.
Skigull's folding wings allow it to dock at most docks and marina slips.
In the long run, I envision a marina with a row of Skigull's parked along the fence. A customer phones (e-mails?) ahead to reserve one. A fork lift launches it down the ramp. Dock hands refuel, pre-flight, warm-up the engine, etc. The wealthy playboy flies off for a romantic weekend.
 

narfi

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So the challenge is to build a flying boat, with a large cabin, that is as easy to dock as a Cessna floatplane.
Skigull and the Dornier SRay are among the few small flying boats that meet both those criteria.
largeCabin.JPG

Large cabin?......
I am excited to see more about the skigull and how it performs, but I think it is a pretty big stretch to call that a 'Large Cabin'.....
 

Doggzilla

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View attachment 48868

Large cabin?......
I am excited to see more about the skigull and how it performs, but I think it is a pretty big stretch to call that a 'Large Cabin'.....
Compared to what? Most aircraft are built for midgets. Do the controls dig into your knees like a Cessna? No? Then its large enough.

The quickie was actually known for being pretty roomy compared to the sub 6 foot competition.
 

autoreply

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I'm no expert, either, but from what I've read of the period, the Navy of the 1950's was interested in blue-water operations, and how do you get a reasonably-sized seaplane of any kind to operate in 5+ foot swells? A ski can help, but isn't going to enable those kind of operations either.
5+ ft waves are easy with either a planing hull or a ski. The US2 routinely operates in 10 ft significant and 15 ft peak...
With regard to a hydrofoil, I can only say what Orion had to say about it, which was, basically, "fine, until you hit a log." A planing hull might get a dent, but 'foils will be ripped clean off. A planing ski would probably survive a light hit, but snagging something as the aircraft was coming up onto the ski might get ugly.
Most planes get on the step (planing) at pretty low speeds, even with floats or hulls. With a ski, the planing area and thus the planing speed can be lowered quite a bit.
At a given speed; a ski will always fare much better than a planing hull; it has way more "give", since they're sprung and given the small area you can afford to make them extremely sturdy. Unlike hydrofoils which indeed have a catastrophic failure, a ski with a bit of give will simply bump over anything hard, even logs that are catastrophic to both hulls and hydrofoils.
 
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