Seaplane retracting gear.

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autoreply

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Well talk about poor form...

Why do you engage in a discussion and then run away in a huff when you have no technical leg to stand on...?
Because I have no interest in a single-sided "discussion". You didn't responds to most of the things I said and asked and the rest were ridiculed. You're welcome to do so, but that's the end of my participation in this "discussion".



And now back on topic, meaning amphibians.
 

Himat

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And now back on topic, meaning amphibians.
Well, then I'll cut, glue and sand some foam on my model. Not full size but I do hope progress will be at a higher pace than most self design homebuilts. I'm not convinced it's "the solution" to seaplane design, rather a test to demonstrate that a seaplane don't have to have a steped hull. I'll post it in an RC forum with a link here when I have tested it.
 

Topaz

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Saying anhedral is destabilizing is like saying wing sweep is destabilizing...it is a nonsensical statement...
Anhedral most certainly lessens roll stability, the same way dihedral enhances it. Forward wing sweep lessens roll stability, while the much more common rear-ward sweep enhances it. Vertical placement of the wing also affects roll stability. Low wing lessens it, high wing enhances.

We use dihedral, anhedral, sweep forward or sweep back wherever it is necessary to achieve some objective...
Exactly, which is the point AR was making, by my read. Rearward-swept-wing, high-wing transports would actually have an excess of roll stability because of those two features, so anhedral is incorporated by the designers. The goal is to reduce that roll stability and avoid Dutch Roll, amongst other things.

As for the hydrofoils in question, I have no comment. While the anhedral of the main 'foil would certainly lessen roll stability while carrying the aircraft's weight, there appear to be some kind of small 'foils mounted at the tips, as well. Those may be set up to oppose the effect, or even overwhelm it. Maybe they're some kind of little tip ski. I don't know enough about the design to even guess.
 
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Topaz

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...that Piaggio Schneider cup racer from the 1930s was also proven to work...using same hydrofoil concept as Akoya...but they never ironed out some of the other technical issues apparently...
To the best of my knowledge, that aircraft never actually flew. The Wiki entry on the P.7, at least, quotes a source saying that it did not. And its hydrofoil arrangement appears to be substantially different, with larger, more conventional hydrofoils (with significant dihedral) at the bottom of the two wing-like "legs".

Again, not commenting upon the Akoya, but the Piaggio vehicle (while facinating) may not be the best means of verifying or refuting the configuration.
 

Dan Thomas

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I would like to know how many of the posters in this thread have actually flown anything off the water. And how many have ever flown anything at all.

Too often I see some bizarre (and some very old) ideas pushed as being far better than what we have. I know that what we have seems archaic and clunky, but as far as I know neither the air nor the water has changed since the Wright brothers and still behave exactly as they did in 1903, and behaved that way when many, many, many ideas were tried.

For one, you need to fly a floatplane or amphib and see what even a three-inch chop does to an airplane. In the Cessna 180, the wingtips flex up and down two inches or more while on the step in a three or four-inch chop. You NEVER see such flex in landplane ops, even on a rough runway. Water is really tough on stuff, and a six-inch chop would tend to break much structure. Even a day spent water-skiing on a rough lake is informative; take a tumble at 35 mph and see just how hard that surface is and how bruised up you can get. It hurts, man.

Then try to land that amphibious aircraft structure on hard-packed, drifted snow, or rough ice, and see just how long it would last. Skiplanes use the usual landing gear legs with their springs or oleos or flex of some sort and still get thumped around without the tires to absorb the rapid rattles.

Some flying lessons are in order for some HBA folks, I think. Try to be open-minded; the most difficult students are the ones who already have their minds made up. They are unteachable. When I was an instructor we used to have to let them go; they were a danger to themselves and the airplane. It's amazing how much perceptions change once a student encounters the air (and water) in a very real sense.

Dan
 

orion

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OK, I guess I have to jump in here for a bit - regarding the anhedral hydrofoil wing geometry on the Lisa amphib, this is the one special case where the configuration is actually beneficial and believe it or not, is actually quite stable. We have to remember here that the hydrofoil wing is not submerged for most of its operation and therein lies the trick. If an anhedraled wing was fully submerged then yes, it would be unstable, as would most variations of the theme.

But in this one particular case we have an exception. First, it is important to note that the wings are surface piercing - they are fully submerged only for a very short period as the craft accelerates. Also notice their size - they're quite a bit larger than one might suppose you'd need for a light craft as this. This is actually a good illustration of what I mentioned earlier: A fully submerged foil is quite powerful and efficient as long as it maintains some minimum depth. As it nears the surface that lift diminishes rapidly, especially if an end plate (the body) leaves the water at the same time. If this unique condition isn't properly designed for a very damaging proposing motion can onset, especially in a craft that might be a bit underpowered to start with. Usually this condition has to be corrected with either sufficient power or a larger wing. But that too has penalties so by introducing an angled orientation, you can tailor the hydrodynamic forces in such a way that he transition to pre-flight mode is relatively smooth with only minor penalties.

As far as the anhedral is concerned, here we have to take into consideration the motion of the plane. This is not a simple problem but the overall behavior can be summed up into a couple of basic ideas:

1) Any roll instability for takeoff is limited to only minor departures from the horizontal. As such, major roll input that would substantially offset the craft from the level is unlikely and would be stopped by the wing tip geometry anyway.
2) The actual motion the craft sees is not a simple roll about it's horizontal reference axis - the roll axis is actually quite a bit lower. If the aircraft does get upset in this mode the reaction is not like an airplane's where the wing's lift vectors would continue the roll in an unstable manner; the motion is more like a teeter, resulting in dipping the wing-down foil deeper into the water. The other foil actually stays pretty much where it was, although in some conditions it can go a bit higher.

So here is what happens - remember that the foils are only partially submerged and are generating a sizable fraction of the lift up 'till actual take-off. As the aircraft rolls, it dips one of the foils deeper into the water, immediately creating a sizable increase in lift. If the other foil leaves the water, it of course loses lift. This is the mechanism that makes this whole thing stable but it is important to keep in mind that his is really the only special case where this is true. Pretty much all other forms of anhedral are destabilizing in roll.
 

Topaz

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...So here is what happens - remember that the foils are only partially submerged and are generating a sizable fraction of the lift up 'till actual take-off. As the aircraft rolls, it dips one of the foils deeper into the water, immediately creating a sizable increase in lift. If the other foil leaves the water, it of course loses lift. This is the mechanism that makes this whole thing stable but it is important to keep in mind that his is really the only special case where this is true. Pretty much all other forms of anhedral are destabilizing in roll.
Interesting, and thanks for the information. The tremendous density difference is hard to grasp sometimes.
 

autoreply

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1) Any roll instability for takeoff is limited to only minor departures from the horizontal. As such, major roll input that would substantially offset the craft from the level is unlikely and would be stopped by the wing tip geometry anyway.
2) The actual motion the craft sees is not a simple roll about it's horizontal reference axis - the roll axis is actually quite a bit lower. If the aircraft does get upset in this mode the reaction is not like an airplane's where the wing's lift vectors would continue the roll in an unstable manner; the motion is more like a teeter, resulting in dipping the wing-down foil deeper into the water. The other foil actually stays pretty much where it was, although in some conditions it can go a bit higher.
We have to take all movement into account, not just roll or pitch (stability)

Let's assume the following numbers:
*50 kts
*1 sqft of fin submerged instantly by waves
*Cdfin: 0.05 (Still seems optimistic for a planing fin)

That means we have an instant lift of 29 kN, which will roll the design back to the horizontal. So yes, perfectly stable in roll if we don't look any further. Maybe even too stable, that's a 6G roll input, but that's another discussion.

But we will also have "some" drag. Roughly 1.5 kN, or about half of this planes empty weight. HALF. Since the fin is a considerable distance outside of the C of G and considerably lower, it will cause rather massive nosedown and left-yaw moments. In fact roughly the same as landing a taildragger like a Piper cub violently and with one wheel completely blocked from rotation. We all know how the latter would end...

Yes, in theory it would work perfectly, if you just ignore the above and my numbers could be off considerably as well. In reality, 8" waves are to be expected in any seaplane operation from my point of view, in which it at least should be possible to land safely. But until somebody has shown me a video of the Akoya flying in considerably more than 0.5" waves, I remain very skeptical for the above reasons...
 

Jay Kempf

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No it won't...

Because the lift which is 20 times greater (your numbers) is acting through a moment arm equal to the length of the center of lift on those water wings...to the CG which is quite a ways aft...the combined moment of the lift of 29 kN times that moment arm will easily counter any drag x the moment arm of the distance of the center of lift below the CG in the vertical plane...

You have not thought this through or drawn a free body diagram like I suggested...

Another thing is that you are talking about 8 inch waves submerging the fin momentarily and causing these forces...

This is wrong...waves are a sinusoidal pattern which means that half of the time the fin will be outside the wave...when we average the wave frequency it means that there is a half the amplitude so the fin would be counted as submerged continuously at half the depth of the wave...

There is a lot more wrong with your analysis but these are just some of the major points...
When landing and taking off one tends to put the wing in it's highest CL mode. Meaning that the fins (the horizontal portion) if optimized for takeoff and breaking then surface would be at a very small positive trim so that they would at approaching takeoff speeds and AOA the drag would be mimimum. While slowing down for landing or speeding up for takeoff you would be at a high AOA which would mean the thing would tend to plane on the end of the fin and skip like a stone before finally losing enough lift to submerge. Very low drag while skipping (planing). The placement of the fin relative to the CG changes dramatically between high AOA and low AOA just like a tail dragger. I give the designer credit until I can prove otherwise that they thought through all those modes and transitions and found a balance that works. From what it looks like to me this is a clever and subtle solution to an old problem. BWB and delta designs have similar overlapping competing mode design challenges. This is similar but different if that makes sense. At least that is what it looks like to me. Wish the designer would jump in and let us know the assumptions that went into the operating modes.
 

autoreply

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I thought it might be a worthwhile to crunch a few numbers on the Privateer amphib that was mentioned earlier...
You're making one big (invalid) assumption that basically nullifies the rest of the numbers.

Most of the Walters can maintain sea-level power above sealevel, some even up to FL120. Without knowing the power output at altitude, what you write has little resemblance to the real numbers...
 
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