An unorthodox seaplane hull, modell tests

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Himat

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I do draw, design, build and fly model aeroplanes when time permit. Here is a few pictures of a winter project before taxi tests.
View attachment 23500View attachment 23501View attachment 23502

After posting some pictures in the “How many of you guys fly RC Planes too?“ thread, I do chose to answer the question in a separate thread.

What does it look like after taxi test?
The taxi test did go well, in the sense that I learned something of it!

Due to faulty weight estimate the plane did sit lower in the water than designed. At low to medium speed water displaced by the main hull hit the aft outrigger mounting tube. This threw spray into the prop. Above a certain speed this was no problem as the forward main hull then out of the water.

The airplane accelerated tracking straight and with no large change in trim angle with speed as designed. Visually, there is no indication of a pronounced “hump” speed. As the plane accelerate it lift gradually out of the water.

One trait emerged, when slowing down the forward part of the outriggers on two occasions did dig in and the plane “ground looped” or “broached” depending on if you use aeronautical or nautical terms. I suspect the rigging angle and lengthwise placement of the outriggers caused this.

At the time the model is on the workshop table and some modifications carried out. If there is interest and no objection, as this is a model airplane, I will try to update with more information as the testing do progress
 

Jay Kempf

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After posting some pictures in the “How many of you guys fly RC Planes too?“ thread, I do chose to answer the question in a separate thread.



The taxi test did go well, in the sense that I learned something of it!

Due to faulty weight estimate the plane did sit lower in the water than designed. At low to medium speed water displaced by the main hull hit the aft outrigger mounting tube. This threw spray into the prop. Above a certain speed this was no problem as the forward main hull then out of the water.

The airplane accelerated tracking straight and with no large change in trim angle with speed as designed. Visually, there is no indication of a pronounced “hump” speed. As the plane accelerate it lift gradually out of the water.



One trait emerged, when slowing down the forward part of the outriggers on two occasions did dig in and the plane “ground looped” or “broached” depending on if you use aeronautical or nautical terms. I suspect the rigging angle and lengthwise placement of the outriggers caused this.

At the time the model is on the workshop table and some modifications carried out. If there is interest and no objection, as this is a model airplane, I will try to update with more information as the testing do progress
What is the rational against the normal tip float? Is there a benefit you are going for of some sort. I like the way the whole thing looks...
 

autoreply

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Are the outriggers meant as wave-piercers?

Are you familiar with the (Dutch obviously ;-) ) axe-bow? I've always fancied the idea of axe-bowed outriggers and a flying semi-submerged-wing that starts to plane with sufficient airspeed (removing the drag of conventional steps)
 

Himat

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What is the rational against the normal tip float? Is there a benefit you are going for of some sort. I like the way the whole thing looks...
The rationale against tip floats, none particular but I did not think they would perform well on this design. One design goal has been to make a hull that is as stable as possible while afloat. For a model size airplane or light plane I consider that tip floats is not an optimal solution. The other goal is to design a seaplane hull without a traditional step.
 

Himat

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Are the outriggers meant as wave-piercers?

Are you familiar with the (Dutch obviously ;-) ) axe-bow? I've always fancied the idea of axe-bowed outriggers and a flying semi-submerged-wing that starts to plane with sufficient airspeed (removing the drag of conventional steps)
No the outriggers are not “wave piercer”, but rather long and narrow planning hulls. I do not know about the “axe bow”, or at least have not heard about that name before. One of the design goals is to do without the traditional seaplane hull step(s), another is to arrange the hull keel line/ trim angle in a most favourable way. Give me a few days and I will try to get time to write a design description.
 

Himat

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A little background:
I have since I was a kid been reading about, drawing, designing, building and flying model aeroplanes. After playing with model aeroplanes for at least thirty years I might be called an aero-modeller. Growing up on the shore of a lake, floatplanes that now and then visited strengthened the interest of that part of aviation. And the lake was a convenient place to take off and land my models, hence a special interest in seaplanes. This led to reading more about airplane floats and hull design.

Reading about the traditional or “orthodox” style of stepped flying boat or float hull my understanding is that this design ends with quite a few compromises:
  • The placement of the CG relative the step is if the forward hull is considered a planning boat hull to far aft, resulting in large trim changes with speed before aerodynamic forces overcome the trimming moment of the hull. These large trim changes also make the hull ride at less than optimum trim angle causing excessive drag.
  • The upward slope of the rear hull implemented to make rotation for takeoff or at least set the wing angle of attack to be able to take off again result in the planning surfaces riding at a less than optimum trim angle.
  • And then there is aerodynamic drag from the step.
This list is not comprehensive and open to discussion, it’s only my view.

Let’s continue to my seaplane hull design. Inspired by the NACA reports on “Planning tail hulls”, Andy Lennon’s “Sea Loon” design and the RC “flying hydro” craze this design is a mix of ideas. (Actually I have found a seaplane hull patent that I do think incorporate some of the same ideas.)
  • The central hull is long and narrow with a flat “skid” surface at the bottom.
  • The outriggers are ordinary planning hulls with the transom close to the aircraft CG. To throw spray away from the plane each outrigger is half a V-bottom with the deadrise outward.
  • The outriggers keel is set deeper than the central hull in such a way that with the hull planning on the aft of the outriggers and central hull with flaperons deployed the wing should be close to AOA of max CL.
  • The keel lines are set to give a trim angle close to four degrees while accelerating and planning. This is to give a best possible L/D on the planning surfaces.
 

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Holden

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Himat,

Looks like you have put a lot of thought into your concept. Maybe Rutan is copying you...see ski-gull thread. He is using a trimaran also, and so am I.

I like the flat skid idea. A lake is not always sandy and smooth. Perhaps a retractable ski would be good just like Rutan and I are doing. From what I understand, a ski should be about 20 times longer than wide for max lift over drag.

The problem with fixed narrow skis on the bottom as planing features is that water is not flat, but the airplane need to remain flat as it changes speed. When a wave swell hits, the hull skis should change angle and the cockpit remain at the same angle, just like a snow skier does naturally with his body. How could a snow skier ski well if he had to lean back and forward over each up and down section of terrain? Why would a seaplane be any different? Here is why most static hulls fail...same reason as a snow skier would fail if he were a rigid body...

By allowing the hulls (all three) to pivot and maintain a constant AOA to the water while the body remains constant to the air, you solve the problem of rough water...this is what I am doing...

Then there is the transition from static to dynamic where the outboard floats are no longer needed or wanted and contact is via the center hull only. This is determined by how well the wing control can keep things level and at what speed. Control is transferred from the wing to the outboard floats at some point before control is lost in the wing... make sense?

This is basically what I am going to do.

Holden
 

BBerson

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The ground looping ( water loop) problem mentioned in post 1 is likely caused by the outrigger half V shape, I think.
The half V outrigger with dead rise only on outside is prone to water loop.
It's like having massive toe in on wheels, which will ground loop nicely.

Reverse the dead rise to the inside of the outriggers, to make stable. ( yes this will make spray into prop, can't help that)
 

Aircar

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The centre of bouyancy looks to be too far forward at a glance and if the dynamic centre of effort of the hull is too far forward it will want to waterloop (and of course weathercock on water in a wind ) --digging in one float(sponson) can occur if the aircraft banks on the water unlike a monohull where the centre of bouyancy hardly changes . Molt Taylor's 'floatwing' idea is worth considering --lots of dihedral needed and the 'ground effect' tends to level the wings although high speed planing turns are comfortable . The ICON used the "Boeing clipper" style sponsons at the Cg (with all rotations taking place through them so that banking didn't change anything ) Some catamaran hulls are unstable and try to 'walk' with each hulling skipping at different times over waves --there is an excellent NACA video of a twin hull flying boat model 'cutting loose' on a towing tank test -I think I posted a link to it several years ago but a bit of googling should turn it up (and the classic Beaver on floats 'dance' that ends in a spectacular cartwheel and capsize was also posted on HBA way back (the film was set in Alaska ,something like "motherload" I seem to recall --the crash was unintentional but caught on film and too good to waste...
 

Himat

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Himat,

Looks like you have put a lot of thought into your concept. Maybe Rutan is copying you...see ski-gull thread. He is using a trimaran also, and so am I.
That’s true, I have been thinking about seaplane hulls for a long time and done a lot of sketches. To make a model and test have been and is great for learning too. When looking at the model and testing I have discovered a few things I didn’t think about.

I like the flat skid idea. A lake is not always sandy and smooth. Perhaps a retractable ski would be good just like Rutan and I are doing. From what I understand, a ski should be about 20 times longer than wide for max lift over drag.
Sure a ski undercarriage is interesting. Do you have any reference to the aspect ratio for a ski? I have searched but never found much useful information about dimensioning of water skis.

The problem with fixed narrow skis on the bottom as planing features is that water is not flat, but the airplane need to remain flat as it changes speed. When a wave swell hits, the hull skis should change angle and the cockpit remain at the same angle, just like a snow skier does naturally with his body. How could a snow skier ski well if he had to lean back and forward over each up and down section of terrain? Why would a seaplane be any different? Here is why most static hulls fail...same reason as a snow skier would fail if he were a rigid body...

By allowing the hulls (all three) to pivot and maintain a constant AOA to the water while the body remains constant to the air, you solve the problem of rough water...this is what I am doing...
If studied, the weight shift wing with a “Zodiac” hull might be considered an example where the hull follow an optimal AOA relative the water and the wing relative the air. I do think that’s one reason these vehicles work quite well as seaplanes. Still, even with the hulls “decoupled” from the wing, some wave frequencies might prove problematic. And then it’s the frequency, not the height of the wave that is the problem.

Then there is the transition from static to dynamic where the outboard floats are no longer needed or wanted and contact is via the center hull only. This is determined by how well the wing control can keep things level and at what speed. Control is transferred from the wing to the outboard floats at some point before control is lost in the wing... make sense?

This is basically what I am going to do.


Holden
If I do understand you correct, yes.
 

Himat

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The ground looping ( water loop) problem mentioned in post 1 is likely caused by the outrigger half V shape, I think.
The half V outrigger with dead rise only on outside is prone to water loop.
It's like having massive toe in on wheels, which will ground loop nicely.

Reverse the dead rise to the inside of the outriggers, to make stable. ( yes this will make spray into prop, can't help that)
Well observed. I’ll investigate this further when testing proceed. At the moment I am rebuilding the way the outriggers are fixed to the hull.
 

Himat

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The centre of bouyancy looks to be too far forward at a glance and if the dynamic centre of effort of the hull is too far forward it will want to waterloop (and of course weathercock on water in a wind ) --digging in one float(sponson) can occur if the aircraft banks on the water unlike a monohull where the centre of bouyancy hardly changes . Molt Taylor's 'floatwing' idea is worth considering --lots of dihedral needed and the 'ground effect' tends to level the wings although high speed planing turns are comfortable . The ICON used the "Boeing clipper" style sponsons at the Cg (with all rotations taking place through them so that banking didn't change anything ) Some catamaran hulls are unstable and try to 'walk' with each hulling skipping at different times over waves --there is an excellent NACA video of a twin hull flying boat model 'cutting loose' on a towing tank test -I think I posted a link to it several years ago but a bit of googling should turn it up (and the classic Beaver on floats 'dance' that ends in a spectacular cartwheel and capsize was also posted on HBA way back (the film was set in Alaska ,something like "motherload" I seem to recall --the crash was unintentional but caught on film and too good to waste...
I am not sure that the centre of buoyancy is to far forward, static the plane sit “level”. On the other side, I think your consideration about digging in one float is right. I have read about Molt Taylor’s floatwing and considered it, but at least for a full size airplane I think it would be difficult to combine with flaps. Optimising the Dornier (Boing clipper) style spoonsons might be a better way forward. Maybe combined with a central ski.

When “Googling” I found this title “Tank Tests of Navy 24 Place Seaplane Glider XLRG-1” with this video: Tank Tests of Navy 24 Place Seaplane Glider XLRG-1 - YouTube Is the plane in this similar to the one you think about?

The “Mother Lode” Beaver landing was easy to find. To design floats that don’t have this behaviour I think would be very difficult. The shape of the keel line might alter the risk some at the expense of performance in other aspects. Land an airplane on the nose wheel with a similar sideslip and a similar stunt could be performed, if not quite that spectacular on a hard surface.
 
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Aircar

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That is indeed the video Himat - good sleuthing. I forwarded that link to John Brown in regard to his claiming that his carPlane is also amphibious (see his patent claims - in fact if you could post a link to it here that would help a lot ) I built his display models (for cost) and tried to converse with him about hull loads and stability on the water to no avail . (he proposes three sections of inflated airbags on his 'hulls' as the external planing surfaces --even Moller claimed that his "Skycar" was amphibious --it would have sunk into the water flooding the ducts at the very least and been totally inadequate for a score of other reasons .

I note that John has been giving more lectures to German audiences (the last set down for May30 at Braunschweig --it is only a matter of time before the fan and the proverbial meet.... (there is a photo of a red painted "1 : 4 " scale model on the CarPlane GMBH website --look under recent shows or press I think -you can see the awful inflow to the prop and the blunt pod trailing edges plus the spindly tail support etc . It is just TOO BLOODY BIG apart from anything else - I suggested to john that he attached an external 'scabbed on' skin to his own car - 8 feet wide and 16 feet or more long with huge overhangs and get a 'feel' for justy how cumbersome such a thing really is ( I was working for a firm building m,otorhomes at the time and well familiarized with the problems of sheer size --they do not loiter in busy streets or do the shopping in them --being based on delivery vans they are just as restricted in use . the carplane is extremely tight in the cockpits and even needs elbow bulges just to work (the wings are carried between the bodies which sets everything .

Rutan's 'fetish' with twin bodies (since Voyager) and possibbly on his current seaplane (amphibian?) is hard to fathom when the lack of any need for internal cargo or passenger volume at all is considered -the White Knight and follow on gigantic launcher . There was a brief fashion for gigantic twin hulled transoceanic airships just prior to and extending after WW2 but the SARO princess and others ended that era.

To an extent the behaviour of that water tank test is due to being TOWED - it is the 'kite dancing' effect -- and not neccesarily representative of a free body but the lateral rock and 'dutch roll' interaction is typical of floatplanes as well -maybe you could post a link to the "motherload" upset as well?
 

Himat

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That is indeed the video Himat - good sleuthing. I forwarded that link to John Brown in regard to his claiming that his carPlane is also amphibious (see his patent claims - in fact if you could post a link to it here that would help a lot ) I built his display models (for cost) and tried to converse with him about hull loads and stability on the water to no avail . (he proposes three sections of inflated airbags on his 'hulls' as the external planing surfaces --even Moller claimed that his "Skycar" was amphibious --it would have sunk into the water flooding the ducts at the very least and been totally inadequate for a score of other reasons .
....

To an extent the behaviour of that water tank test is due to being TOWED - it is the 'kite dancing' effect -- and not neccesarily representative of a free body but the lateral rock and 'dutch roll' interaction is typical of floatplanes as well -maybe you could post a link to the "motherload" upset as well?

The link to the "Mother Lode" crash:Mother Lode Crash Sequence - YouTube
Link to another NACA test of the seaplane glider: Tow Test of a Twin Hull Amphibian Glider - YouTube

The "Mother Loade" is probably a good example of how to not land an airplane. I doubt the hull design could do much difference.
The seaplane glider "Kite dancing" in the tow tank, I do think the test is representative of a free body that would be directional unstable.
 

Kingfisher

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Hello Himat and all,

this is a very interesting thread, since I am working on a similar seaplane project, which I have attached my patent file of. You need to scroll down a few pages to see the pictures. I'd love to hear your feedback.

Before I talk about my own design, I'd like to state that I consider the French built "Akoya" the most revolutionary seaplane ever. It's the most aerodynamically refined of its kind, because it does away with the planing hull altogether by replacing the landing gear with a set of incredibly simple fins (hydrofoils). I wouldn't expect this to be stable on the water, but the videos prove otherwise. Also it should be pretty good with rougher water, since the glider-like nose should pierce the waves, until the whole plane lifts up altogether onto these wing stubs, out of harms way.

Anyway, I must be sharing the twin-boom fetish, as my own design contains features of the P-38, one of my all-time favorites.

Similar to your design are the following features:
-Twin hulls
-Half-vee shaped cross section
-No hull step (or let's say, different step, inspired by NACA planing hull test reports)

Different is that the fuselage in the centre sits above the water (hopefully, unless I build it too heavy).

I have also found the axe-bow comment very interesting. My initial plan was to make displacement hulls very similar to a sailing catamaran (Hobie and the like), so they pierce the waves. However, the main issue with these is that they dig into the water, which flips the boat over, as I know from personal sailing experience. I therefore decided that the hulls should be planing, but be still very narrow (hight length to beam ratio), like Blohm and Voss BV238 flying boat.

A possible problem I might encounter, based on what I read here, is the ground looping issue due to the half-vee hull section. However, I do think that your outriggers being forward like a tail dragger land plane contribute to this. It's not necessarily bad, if it's easily controllable. Interestingly, the Akoya seaplane seems to have changed from a tricycle gear type fin placement to a tail dragger-like configuration in the later design, which is going in your direction.

Also I am worried about the tail fin area. I built a small scale balsa wood glider of my design, which was quite unstable around the yaw axis. I added two extra fins, so the tail is effectively a "W"-shape when viewed from behind. This cured the problem, but looks ugly.

Anyway, let me know what you think of my design, I would appreciate it.

Regards,

kingfisher
 

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BBerson

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Interesting forward swept vertical tail/engine mount. Good idea.

I don't however, see any need for sweeping the main wing. Might be tip stall prone.

Welcome to the forum, and thanks for the drawings!
 

Kingfisher

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Interesting forward swept vertical tail/engine mount. Good idea.

I don't however, see any need for sweeping the main wing. Might be tip stall prone.

Welcome to the forum, and thanks for the drawings!
Thanks for the feedback, BBerson. I share your concern regarding the tip stall. Here are my reasons why I designed my plane with swept back wings anyway:

-Centre of gravity: Plane would be harder to balance with an unswept wing. Nose would have to be fairly long.
-Looks. I reckon a plane looks ugly when the tail surface sweep does not match the wing, see the otherwise beautiful Comet airliner.
-Own experience with swept back flying wings and delta wing models. I found those are actually very good to fly at low speed with high angle of attack, although a swept wing with a tailplane may act differently.

Also, I built a 3 deg washout into the outer part of the wing.

My design is probably not going to excel from an efficiency / low drag point of view. However, I want this to be outweighed by versatility and good handling on the water as well as in the air, we'll see if it works. I'm also curious if the idea of using the wings as sails on the water will actually work.
 

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Himat

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Thanks for sharing an interesting design Kingfisher. There is one mayor drawback with aft mounted engines on small aircraft that probably will affect your design. To balance the aircraft the pilot and passenger need to sit forward of the airplane CG. In some cases rather far forward, this places the variable load at a considerable moment arm relative the CG. The weight of the pilot and passenger will then affect the CG a lot, something that can constrain the usefulness of the design.

Folding the wings vertical to be able to tie up sideways is a very good idea. Sailing the plane I’m not sure is that beneficial, but controlling the raised wings like sails might make manoeuvring afloat easier. Observe that the loads on the raised wings might come close to the flight loads.

The “ground” or rather “water” loop I think is very much a feature that all seaplanes will encounter to some degree. Actually, land a tri gear airplane on the nose wheel and it’s possible to ground loop a tri gear airplane. The detail design might make the aircraft more or less prone to “water loop”, but a really ham fisted pilot will be able to ground loop most anything.

Another patent that might be of interest to study is US Patent 5,277,383, Amphibian Aircraft by Tormakhov et al.

Comment on your last post will follow when I have studied it.
 

Himat

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Thanks for the feedback, BBerson. I share your concern regarding the tip stall. Here are my reasons why I designed my plane with swept back wings anyway:

-Centre of gravity: Plane would be harder to balance with an unswept wing. Nose would have to be fairly long.
-Looks. I reckon a plane looks ugly when the tail surface sweep does not match the wing, see the otherwise beautiful Comet airliner.
-Own experience with swept back flying wings and delta wing models. I found those are actually very good to fly at low speed with high angle of attack, although a swept wing with a tailplane may act differently.
Centre of gravity.
Visually the nose is shorter with swept wings, the moment arm of the pilot and passenger is more or less the same. With associated problems.

Looks, yes to look good there must not be too many confusing lines.
Seaplane design is in dire need of test data and experience with "unorthodox" designs.

Slow speed handling of deltas and tailed aircraft with swept wings are two different subjects. It is possible to make a swept wing handle well at slow speeds, but it will remain a compromize.

I do look forward to see the development and testing of your model!
 
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