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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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    $\begingroup$ 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.) $\endgroup$ – Ville Niemi Jan 22 '16 at 6:53
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    $\begingroup$ @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. $\endgroup$ – egid Jan 22 '16 at 7:24
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    $\begingroup$ @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. $\endgroup$ – Ville Niemi Jan 22 '16 at 8:18
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    $\begingroup$ @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. $\endgroup$ – J... Jan 22 '16 at 12:00
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    $\begingroup$ @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. $\endgroup$ – egid Jan 23 '16 at 21:49
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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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    $\begingroup$ Found several interesting things googling around. The one that most intrigued me was this: youtube.com/watch?v=yolgS1bn7P8 $\endgroup$ – slebetman Jan 22 '16 at 12:35
  • $\begingroup$ 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. $\endgroup$ – Kevin Reid Jan 22 '16 at 16:20
  • $\begingroup$ Same is true of all the other examples. I don't think I've seen a floatplane with foils. $\endgroup$ – egid Jan 22 '16 at 16:47
  • $\begingroup$ More on the Pc-7 (soon funded by Fiat and renamed Piaggio), but in French, and the original article by Giovanni Pegna, in Italian. $\endgroup$ – mins Jan 22 '16 at 19:18
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    $\begingroup$ @mins I just look at the pictures anyway $\endgroup$ – TomMcW Jan 22 '16 at 23:16
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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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  • $\begingroup$ 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. $\endgroup$ – Peter Kämpf Nov 27 '16 at 23:09
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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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