STOL Amphibious Aircraft Using Ground Effect

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sanman

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We've all heard of aircraft exploiting wing-in-ground effect. Here's a college prof using some wooden models to demonstrate:


Here's a real life aircraft called the Airfish:


So this design looks interesting, and I was wondering if it could be the basis for a small ultralight amphibious STOL aircraft that would be able to land either on water or even on unprepared ground like a bush plane.

I was thinking that the key to this would be variable wing geometry. The wings would have to be able to shift from their downward-canted position to a level horizontal position.

So that downward-canted position that we see is able to trap the air underneath to improve the ground effect, which is great for STOL. But once the aircraft is airborn, it would be better for the wings to tilt up to a horizontal position, which would then look like a reverse-delta-wing. This would reduce the drag and enable faster, more efficient flight.

To tilt the wings, I guess you'd need some kind of piston-like actuator-strut. Maybe the bottom of the wingtips could have fat tundra tires attached.

Can anybody think of anything else? Comments?
 

bmcj

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If you are shooting for STOL takeoff, wrap a hovercraft skirt around it and power lift off the water before starting your takeoff run. If you want a STOL landing, I’m sure that landing in the water provides excellent “braking”.
 

bmcj

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Or you could just buy an Avid Flyer on floats. Keep in mind that this is an early model Avid with the small Rotax engine.

 

malte

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I was thinking that the key to this would be variable wing geometry. The wings would have to be able to shift from their downward-canted position to a level horizontal position.
That is technologically most interesting and surely an exciting feature to design and build, but it will inevitably add a lot of mass to the aircraft. And mass is the main enemy of STOL capability.

[...] But once the aircraft is airborn, it would be better for the wings to tilt up to a horizontal position, which would then look like a reverse-delta-wing. This would reduce the drag and enable faster, more efficient flight.
[...] Maybe the bottom of the wingtips could have fat tundra tires attached.
Letting fat tundra tires hang in the free stream is counteracting the flight efficiency you are looking into.


Here's the deal:
During the takeoff run, you have three parts of lift. If you are slow, you are in displacement and relying on buoyancy (hydro-static lift). When you pick up speed you mainly exchange buoyancy for hydrodynamic lift, which is then exchanged into aerodynamic lift

I made this graph for my masters thesis on calculation and optimisation of hydrodynamic characteristics and loads of seaplane fuselages. It shows the relative parts of these three lift sources for the seaplane during takeoff in principle (There is more to it and more lift-shifts during a real takeoff, but it shows the basic idea).
1614162102110.png
Taking a look at drag is a bit more complicated. This graph is from the 1938 Sottorf-paper "The design of floats". He got it right back then!
1614163935560.png

So if we're using the ground effect, we are lowering the stall speed of the aircraft within the ground effect altitude limitations. The Dornier Stummelflügel (Stubs) are supposed to have some effect along this way, and also the Lake with its huge flaps so close to the water:

The Pöschel Equator took this to an extreme, as does the predecessor from Equator aircraft or the Berejev 103:

1614164239680.png
Pöschel Equator


1614164371427.png
Equator XCursion

1614164493305.png
Бериев Бе-103

All of these concepts try to escape hydrodynamic drag by lowering stall speed and getting away from the water quickly. To a certain degree they succed.

Even the mentioned hovercraft bottom was tested on a Lake:
1614164636618.png
(However, it proved to be very heavy and bulky and not at all aerodynamic).

To assist in a STOL hydrodynamic takeoff, you want to get rid off as much hydrodynamic drag as possible. While it has been a huge material and hydrodynamical problem for a long time, the utilisation of hydrofoils can assist in achieving this goal. Aircraft like the Lisa Akoya solely rely on hydrofoiling to get airborne and if you look at the sailing folks, we have learned a lot about those items in the past.

But to assist takeoff in confined spaces, STOL is not the only option. Good manoeuvrability can play an important role, too:
 

Riggerrob

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The original poster asked about ekranoplans. These were developed in the Soviet Union by an Italian-born engineer named Bartini. During the Cold War, NATO intelligence officers spread rumors about the "Caspian Sea Monster" , which was series of progressively larger Ekranoplans.
Ekranoplans primarily fly in ground effect, making the far more fuel efficient than airplanes and much faster than boats ... even planning boats. They can even "fly" over very smooth land.

A number of more conventional flying boats have tried to use ground effect starting with an experimental troop glider commissioned for the US Marine Corps during World War. This glider had a conventional hull, but no stabilizing floats under the wing tips. Instead, its' lateral stability depended upon "float wings" that had the wing roots at the waterline.
Post war, Molt Taylor applied the "float wing" principal to his successful "Coot" kitplane. Coot had greatly expanded wing roots to ensure lateral stability. Several others copied Taylor's "float wings": Equator and Beriev 103. Only the Beriev's wings are low enough to provide significant ground effect lift.
Ironically, some Coots and Glass Geese have been retorfitted with small sponsons (water wings) at the waterline and near the step. These short chord water sponsons improve hydrodynamic lift at low airspeeds (e.g. too slow to plane on top of the water).
Disadvantages of "float wings" include difficulties docking at marinas as well as the risk of floatsom damage or burying a wing in a large wave.

Neither hydrofoils, nor hover skirts have proven successful. The latest experiments include hydro-skis as seen on the Australian Shearwater and Rutan Ski-Gull flying boats.
 

malte

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Neither hydrofoils, nor hover skirts have proven successful.
(The hover skirts did work, but are bulky and heavy, so they have serious disadvantages). I beg to differ on the hydrofoils. The Tests with the Buccaneer on hydrofoils gave an instant doubling of bearable wave height during rough water procedures. It is true, that NACA did not get it right during initial tests though and the engineers working on hydrofoils expressed a lot of doubt on the topic. Today, however, we are far more knowledgeable about foils, supercavitation, hydroelasticity, foiling materials, etc.
 

Riggerrob

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I've been on an amphibious kick lately but have more than a few unanswered questions, like how to seal the wheel wells and whether or not I really need a 90"prop. View attachment 107913View attachment 107914View attachment 107915
It is extremely difficult to make outer doors water-tight. Ergo, most flying boats concentrate on making the interior walls of wheel wells water-tight.
They also add massive drain tubes to quickly dump any water shortly after takeoff. Go take a close look at a Grumman Mallard and you will be surprised at how wide those drain tubes are!

My latest amphibian sketches place wheel wells above the waterline to simplify the hull structure. Main wheels remain half-extended from sponsons so they can act as docking bumpers. Meanwhile, I intend to steal an entire nosewheel assemble off an Aerocet float.
 

9aplus

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The original poster asked about ekranoplans. These were developed in the Soviet Union by an Italian-born engineer named Bartini.
Just small correction.
Roberto Ludvigovich Oros di Bartini was born in Rijeka during KundK monarchy. Father Hungarian, unmarried mother Croatian. He spent less than 3 years in Italy, Milan on Politechnica di Milano, than sent over to Soviet Union by his Italian camarads to assist in aviation development.
Rijeka is famous also on invention of torpedo by Ivan Lupis with further development and production by Wihtehead.
 

malte

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It is extremely difficult to make outer doors water-tight. Ergo, most flying boats concentrate on making the interior walls of wheel wells water-tight.
They also add massive drain tubes to quickly dump any water shortly after takeoff. Go take a close look at a Grumman Mallard and you will be surprised at how wide those drain tubes are!
[...]
That is correct, many amphibs use open wells where possible (i.e. in the wings or if the main gear is not part of the floating structure). The Lake, also has a considerable gap at the nose wheel door. It is tightened towards the floating compartments to the left and right of the nose gear, aswell as to the cockpit / main floating compartment aft of the nose gear.

IMG_4302.jpg


For Deltas concept, this question is way too early. The fine tuning of such a hydrogear will need tedious and careful testing and design, stability investigations, tank tests with both an aerodynamic (Reynolds) and a hydrodynamic (Froude) model. It is super interesting and I would be thrilled to read the engineering reports out of this.
 

Sockmonkey

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Here's the deal:
During the takeoff run, you have three parts of lift. If you are slow, you are in displacement and relying on buoyancy (hydro-static lift). When you pick up speed you mainly exchange buoyancy for hydrodynamic lift, which is then exchanged into aerodynamic lift

I made this graph for my masters thesis on calculation and optimisation of hydrodynamic characteristics and loads of seaplane fuselages. It shows the relative parts of these three lift sources for the seaplane during takeoff in principle (There is more to it and more lift-shifts during a real takeoff, but it shows the basic idea).
What I found most interesting about drag analysis charts like that was how it shows that a displacement hull with foils can actually have lower drag at all speeds than a planing hull as long as the foil lift kicks in before the big drag hump. Planing hulls tend to have the worst drag/speed ratio at low speeds and don't improve much until they finally get on plane.

In what they call half-foil boats, the foils don't lift the hull completely out of the water. This is done so that the hulls provide pitch and roll stability instead of needing active systems or oddly shaped less efficient foils, so they get the majority of the benefits without the downsides. AFAIK a pretty good way to do that is to have the foils stretching between a set of catamaran hulls.

Displacement hulls are said to have all-around better seakeeping ability than planing hulls, so if if you had a half-foil cat, I'm thinking you could theoretically have a ship with the best of everything without being too complex.
 

Tiger Tim

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I was wondering if it could be the basis for a small ultralight amphibious STOL aircraft that would be able to land either on water or even on unprepared ground like a bush plane.

Can anybody think of anything else? Comments?
My gut reaction would be to size the reverse delta to get the plane to break free of the water quickly at MTOW and have a more conventional wing to do the bulk of the work in flight out of ground effect. The conventional wing could either take the form of long dihedralled tips at the ends of the reverse delta or as a sort of upper biplane wing if you’re constrained by span or don’t like having the wing roots so close to the water/ground.

I imagine a fat reverse delta like Lippisch used isn’t ideal in cruise so you’d need a bit of power over a conventional plane. The question then becomes whether that extra power would allow the plane to jump off the ground just as fast if you ditched the weight and drag of the ground effect delta.
 

Dan Thomas

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Ground effect doesn't involve "pressure trapped between the wing and the surface." The proximity of the surface does two things: It interferes with the size of wingtip vortices, and those vortices cause drag, so drag goes down in ground effect. The surface also reduces the angle of upflowing air at the leading edge, reducing the angle of attack and thereby lowering stall speed. Upflow is caused by the low pressure on the top of the wing, and when close to the surface that air doesn't want to come up because it would mean generating a low pressure between the leading edge and the surface.

1614526316229.png

See the smoke lines in the wind tunnel rising as the leading edge approaches? The effect can extend a long way forward of the leading edge. Wind tunnel testing has to take this into account, as the upper and lower walls of the tunnel will affect everything. The tunnel has to be pretty deep.
 

Sockmonkey

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Ground effect doesn't involve "pressure trapped between the wing and the surface." The proximity of the surface does two things: It interferes with the size of wingtip vortices, and those vortices cause drag, so drag goes down in ground effect. The surface also reduces the angle of upflowing air at the leading edge, reducing the angle of attack and thereby lowering stall speed. Upflow is caused by the low pressure on the top of the wing, and when close to the surface that air doesn't want to come up because it would mean generating a low pressure between the leading edge and the surface.

View attachment 108041

See the smoke lines in the wind tunnel rising as the leading edge approaches? The effect can extend a long way forward of the leading edge. Wind tunnel testing has to take this into account, as the upper and lower walls of the tunnel will affect everything. The tunnel has to be pretty deep.
There is a certain amount of compression lift depending on the aspect ratio of the wing.
There's span-dominated ground effect and chord-dominated ground effect.
 

Dan Thomas

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There is a certain amount of compression lift depending on the aspect ratio of the wing.
There's span-dominated ground effect and chord-dominated ground effect.
That trailing edge needs to be very close to the surface to get that. This research paper, https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=11477&context=rtd details wind tunnel tests that put the trailing edge down to very low altitudes, down to 0.1 and 0.0 chordlength. Some pressure rise was noted. In the real world that isn't practical. My Jodel's trailing edge was less than a foot off the ground, on a chord of over five feet, or 0.2. Only a KR-1 or KR-2 might beat that.

1614622687643.png

Most sources will point out the ground's interference with the tip vortices and the upwash AND downwash. When I taught Aircraft Systems we dealt with the pressure effects of diverging and converging ducts, which is completely counterintuitive and students have a hard time with it. Turbine engines use those principles all through the engine, so unless one understands them, one will never get understand the turbine. A wing at a high AoA , at some distance above the surface, will have some converging-duct effect that could lower the pressure under it, but I could find no research that measured it. The above-referenced essay did not deal with those distances, only the small gaps, and of course when the gap gets really tiny the flow stops and pressure will rise.

1614622364042.png

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One less-than-super-scientific article that deals with upwash and downwash as well as tip vortices: What Causes Ground Effect?
 
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