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One of the main problems with water landings is that you need a bigger engine to counter the various drags caused by the pontoons. The specific case of "run[ing] across our own wake to be able to get up on the step on glassy water" brings some interesting thoughts.

[B]asically a floatplane has to break the suction of the water - kind of like pulling your boot out of mud - and if the water isn't moving at all then it has to do that all by itself. If there are some waves, or a wake, or any kind of discontinuity in the water, then that gives the aircraft the momentary break it needs to overcome that initial suction. "The step" in this context refers to the step-up shape you see partway down the float (or hull); when you're "on the step" then you're using the part of the float forward of that step to hydroplane.

It is well known that water's significantly higher density makes high speeds difficult. A solution to that problem is the hydrofoil - a small underwater wing that raises some or all of a boat's hull out of water to reduce friction and increase speed.

So here is my dumb question: would placing a hydrofoil under a pontoon save fuel on takeoff? And how likely would the increased drag during flight cancel that advantage on a typical flight?

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hildred
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    A hydrofoil is a wing that would only really work while on water. All sea planes already have wings that work while either on water or flying. I find it very hard to imagine a situation where adding the lift to the wings you already need anyway wouldn't be more efficient than adding another set of wings that only work part of the time. Especially since sea planes spend much more time flying than they do taking off, which is the only part where the hydrofoils would help. (And only at a specific short part of the take off too.) – Ville Niemi Jan 22 '16 at 06:53
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    @VilleNiemi it's all about drag. There's a ton of power required to get up on the step and into planing mode, and anything that could be done to reduce that requirement would aid in overall efficiency. Engines are heavy; some of that could get turned back into payload. – egid Jan 22 '16 at 07:24
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    @egid actually it is not all about drag. You have to also consider mass, specifically the hydrofoil needs to be fairly robust and hence heavy or you risk fairly serious accidents when you run into waves. – Ville Niemi Jan 22 '16 at 08:18
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    @egid Yes, but there may only be a very brief window where it helps you break onto the step (ie: start hydroplaning). My instincts tell me that it would just become a massive sea anchor for speeds above that, and dragging a wing through the water all the way to takeoff speed seems like a hundred problems in one. Consider just the effective AoI and lift differential between the air wing and the water wing, especially at speed, and I'd be surprised if such a contraption didn't cause the plane to jerk and pitch uncontrollably everywhere in the envelope. Seems a highly unstable arrangement. – J... Jan 22 '16 at 12:00
  • It seems like a fundamental problem would be that the hydrofoil would create extra drag - a LOT of extra drag. So it would make it more difficult to get up to flight speed. Seems like it would do more harm than good. Seems like @Terry's answer indicated that the suction problem is really only a problem in a dead calm anyway – TomMcW Jan 22 '16 at 20:38
  • Why would the foil create more drag? These are devices that can lift many-ton ships out of the water. The size required for an airplane isn't going to be crazy. – egid Jan 23 '16 at 03:30
  • @egid, some drag during flight would be inevitable firstly as there would be lift induced drag which should not be much of a problem as lift is desired any way, then there would be multiwing turbulence and general turbulence, inertial drag as it has mass and so on. The hope is that since it is a small wing designed to create lift in the denser media of water that the total drag increase would result in less fuel usage than would be gained by reduced drag during takeoff would save fuel. – hildred Jan 23 '16 at 04:14
  • Drag and weight both count against fuel efficiency. I'd argue that foils probably add less drag than floats, and less weight than a more powerful engine. The net efficiency gain would be positive. The downside is harder control, significantly deeper displacement, and probably more maintenance. – egid Jan 23 '16 at 04:41
  • @egid I guess what I'm saying is you'd have more drag (pontoons plus hydrofoil) until you gained enough speed to lift the pontoons out of the water. So it would depend on how much speed would be necessary to do that. If it takes 60 kts to lift the plane out of the water and 80 kts to fly then it seems you'd expend more energy getting to 60 than you would save in the next 20. – TomMcW Jan 23 '16 at 21:27
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    @TomMcW I think y'all are grossly overestimating the amount of speed required for hydrofoils to work. Wikipedia suggests cavitation starts causing issues at 60kts; you can now buy a kayak called the Flyak that takes off at 10 km/h, and the US Navy tests someone linked below suggest that the research was productive, only being canceled due to a lack of interest in seaplanes in general. – egid Jan 23 '16 at 21:49
  • @egid Just watched the video slebetman linked. The narrator says that they did get improved efficiency with a hydrofoil and more with skis, so it would appear that I am wrong. – TomMcW Jan 23 '16 at 21:52
  • A hydrofoil could lift the aircraft body out of the water at a low speed (e.g. 20kn) and this would significantly reduce drag during the rest of the takeoff run. The hydrofoil could be surprisingly small and could retract like landing gear once airborne. Even sailboats can exploit hydrofoils. – Thomas Jun 27 '17 at 18:26
  • What I see as a problem is how to get the hydrofoil to pierce into water when the plane is lending and in a stable way. Too many things can go wrong during landing, e.g. strength required to withstand the impact, how fast the plane should "crash" into the water to prevent bouncing back from the water surface, and how to control the angle of attack (or decouple it from the AoA of the plane ) so that it doesn't shoot the plane back to sky. Without the hydrofoil controlled by a computer it's doesn't seem possible to achieve safe lending. – user3528438 Sep 13 '17 at 16:06

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Yes. I mean, sort of. Nobody's ever built a production hydrofoilplane.

It's not a bad idea, but seaplanes are a relatively niche product these days and there doesn't seem to be a lot of innovation in the field. Back when there was, there was innovation everywhere so the idea of hydrofoils didn't stick.

The Convair F2Y Sea Dart, a 1950s prototype jet-powered fighter, used two 'hydro-skis' for takeoff and landing. It was more or less awful compared to the crop of land-based aircraft coming along and after a prototype disintegrated they (probably wisely) chose to shut the program down.

https://en.wikipedia.org/wiki/Convair_F2Y_Sea_Dart#/media/File:F2Y_Sea_Dart_2.jpg

There are some other prototype aircraft that used actual hydrofoils that I found after some quick googling. A company called Lisa Airplanes seems to have an LSA - the Akoya - that's under development using tech similar to the Sea Dart.

The 1929 Piaggio P.7 used hydrofoils, and was intended to compete for the Schneider Trophy, but never made it airborne. Interestingly enough it had a screw (marine propeller) at the tail and a proper aviation propeller up front, and the pilot would've had to do some juggling between the two.

enter image description here

It's likely there were others, but I'm running low on sleep.

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egid
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    Found several interesting things googling around. The one that most intrigued me was this: https://www.youtube.com/watch?v=yolgS1bn7P8 – slebetman Jan 22 '16 at 12:35
  • It looks like the Piaggio P.7 was designed for the main hull to actually touch the water, rather than using separate floats. This seems worth highlighting. – Kevin Reid Jan 22 '16 at 16:20
  • Same is true of all the other examples. I don't think I've seen a floatplane with foils. – egid Jan 22 '16 at 16:47
  • More on the Pc-7 (soon funded by Fiat and renamed Piaggio), but in French, and the original article by Giovanni Pegna, in Italian. – mins Jan 22 '16 at 19:18
  • @mins You could translate it for us. ;-) – TomMcW Jan 22 '16 at 20:32
  • @TomMcW: No problem for the French article using Google Translate. Unfortunately the Italian original article is too voluminous for the tool, but there are many graphics that are interesting. – mins Jan 22 '16 at 23:09
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    @mins I just look at the pictures anyway – TomMcW Jan 22 '16 at 23:16
  • That P7 looks lke it's have severe problems with the propellor. If the drawing is accurate it'd sit in the water, meaning the pilot can only spin it up once the screw has pushed the aircraft to enough speed to lift the fuselage onto the hydrofoils. And I don't even want to think about the corrosion problems that'd cause. – jwenting Nov 28 '16 at 07:15
  • Also, this thing is struggling toward production: https://en.wikipedia.org/wiki/LISA_Akoya – AEhere supports Monica Sep 14 '17 at 08:26
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It could make sense for a seaplane/amphibious to use a retract or semi-retractable hydrofoil (or 2 or 3) instead of the specialized fast waterborne planing hull. A simple barge-like hull (more aerodynamic, lighter/less robust since it's not hitting the water at 100+kts) would suffice up to maybe 15 kts. Much beyond that and the foil is starting to lift it. By 25, the hull is almost completely out of the water, clean out at 30+. Starting at about 80, it's an ekranoplane, up to take-off speed.

The inflatable hydrofoils also are promising.

See the tests of a Goose with skis and with foils

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https://en.wikipedia.org/wiki/Fairchild_C-123_Provider enter image description here

"Pantobase" ski allows rough ground, or snow/ice, or water operation, if the plane has appropriate controls and can float. As soon as it gets above ~30kts, it starts lifting, and the hull is effectively out of the water at anything above 50

Lockheed looked at C-130 seaplane adaptation, floats, boat hull, and skis were examined https://www.g2mil.com/c130seaplane.htm

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Lockheed (Secretprojects forum has an excellent thread) enter image description here

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John Frazer
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Aeronautical engineer David Thurston experimented with hydrofoils on at least one of his designs, I think it was the Colonial Skimmer. He stated in one of his books, I think it was Design for Flying that water is about 300 times the density of air, so a hydrofoil of very small size could lift an amphibious hull out of the water early in the takeoff run and significantly reduce overall drag. It also helped in handling higher waves, as the hydrofoil was on strut some distance under the hull, and the strut and foil could slice through waves with less impact than a hull.

I have a Phantom ultralight aircraft, I would like to experiment with inflatable floats with retractable hydrofoils. I believe the combination would be lower weight and drag than standard rigid floats. The step built in to the bottom of standard floats (required to break hull suction during takeoff) creates significant drag throughout flight. A retracting hydrofoil that is faired in flight should have less drag. And inflatable fabric floats are very light weight, and only have to handle water loads up to the point where the hydrofoil(s) lift them out of the water.

And if I build the entire assembly of poured concrete it will be very safe, as it will never get off the ground. Or water.

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    Actually, water is 800 times more dense than air. And hull suction needs to be broken when the hull starts planing, which is long before take-off. – Peter Kämpf Nov 27 '16 at 23:09