Seaplane retracting gear.

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orion

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How about retractable hydrofoils...?

For the weight of an airplane they could be truly tiny to make enough lift to get the airplane out of the water...

If you look at the size of the huge Boeing 929 hydrofoil which carries up to 400 pax and look at those little wings that provide the lift to get that 200 ton boat out of the water it is truly amazing...

Even human powered hydrofoils are twice as fast as any conventional rowing craft...I think the flyak (hydrofoil kayak) goes something like 25 mph which is really moving on water...and on muscle power no less...
As long as they're submerged, hydrofoils are quite effective and efficient. However, as you near the surface you run into a series of significant penalties that make the transition to free flight very difficult and problematic. As the submerged foils nears the surface the flow over the upper skin can easily cavitate, creating a very damaging environment that over time can pit and erode even the best stainless steel.

Furthermore, the drag rise of the water/air interface is a segment of the operating envelope that has to be designed very carefully, especially as said foil starts to breach and initiates conversion from submerged lifting surface to a surface planing ski. The complications of this realm are further compounded since as the foil nears the surface it starts to lose lift - in the transition from submerged foil to planing skin it can lose over 50% of the lifting capability.

These are the key performance based reasons why we don't see more hydrofoils in this type of operations. Then, to make things worse, you also have the issue of accidental contact with the water. If in improper attitude, this contact can be worse that landing a float plane with its gear down. Lots of expensive noises.
 

Himat

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I think the general consensus is that the step is in fact a major source of drag on an otherwise clean shape...

As for a retracting step...some research has apparently been done years ago...here is a Naca report...

http://www.google.com/url?sa=t&rct=j&q=aerodynamic drag seaplane step&source=web&cd=8&ved=0CE0QFjAH&url=http%3A%2F%2Fwww.dtic.mil%2Fdtic%2Ftr%2Ffulltext%2Fu2%2Fa801229.pdf&ei=paWLT6KpB9DusgbGxJXOCw&usg=AFQjCNE1BQCGDQ8FkbBFY7qtWbG52rVFtw&cad=rja

In the introduction the report states...

"While the seaplane — particularly a long—range seaplane — is in flight, the step may account for an important fraction
of the parasite drag."

And here is a pretty neat idea for a low-drag seaplane...have the main wing double as the hydrodynamic surface...

Seastryder – A new personal amphibious craft » Blog Archive » SeaStryder: Ground Effect, Flight & Landing
And the NACA report goes on to state that the movable flap reduses aerodynamic drag by "2 to 3%". That for å flying boat at the time, flying maybe 200knots. Now, in a light homebuilt aircraft with a mission profile with rather short flights, is it of any benefit? Maybe, maybe not. I would rather do like it's done to the Seastryder and Centaurusplane, get rid of the step.
 

Himat

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I agree the transition from submerged to surface planing could be difficult...but I don't think cavitation at these slow speeds (relative to a propeller speed) would do much if anything...

The main thing that impresses me about hydrofoils is the huge decrease in drag compared to a conventional hull...

But there is also the transition from the floating hull...to the point where the foil lift is carrying the ship...

I don't think you could just plop a cigar-tube fuselage in the water and expect to get up on the hydrofoils by applying power...

So you would still need a hull with marine features like chines...a v-shape bow...etc...

Once you are on the water wings then it would be a much shorter takeoff distance to rotation speed ...due to the low drag of the water wings...

I don't think the transition from water wings to air wing would be very troublesome...at a certain speed the air wing will make more than enough lift to pull the airplane and the water wings right out of the water...

Interesting to see that picture of the Lake with the water foils...there has to be some more info and probably a few more experiments in this area...
There is one more thing against hydrofoils. Hydrofoils are not that practical. There used to be hydrofoil fast ferries where I live, now they are all catamarans.

The issue?
Floating debris. With a hull or ski you bump it and hope it slides underneath. (Without punching a hole.) With hydrofoils you probably hook it. If unlucky the hydrofoil is thorn out, that might leave a bigger hole than just hiting a floating object.

Next, beaching. Again, something prodtruding from the hull is not a that god idea.
 
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craig saxon

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I found a good report on that Lake and some other hydrofoil seaplanes...written by the designer of that hydrofoil Lake...

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=AD0873851

Here's a picture of that Lake and another design the JR5G from Edo...
The retractable ski and David Thurstons hydrofoil was fairly extensively covered in this thread.
http://www.homebuiltairplanes.com/forums/general-experimental-aviation-questions/9843-amphibious-ops-retractable-skis.html
 

Himat

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Hey that Lisa is very nice...

Talk about clean...just those two little water wings in the front and tiny little ones out back...no v-hull, no chines, strakes nothing...and it seems to work just fine...
I am slightly sceptical about the Lisa hull performance. I do then think about seaworthines and floating debris. How do i work in waves?

Anyway, pretty it is, and it do demonstrate that it is possible to have a seaplane without a steped hull.
 

Jay Kempf

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Regarding why the step is not retracted, it is mainly because the step does not cause all that much drag and the retraction system, with all the associated systems and seals, would probably penalize the craft more than the drag reduction would benefit. While the hull's wetted area and shape certainly creates an addition drag count and penalizes the plane's cruise, in this case it really does not matter since long distance cruising is not what this airplane was designed for. But the penalizing drag on this airplane comes from a number of sources, of which the hull is only one.
The retractable step I designed was just a hinged flap with a bag in a shallow recess between it and the hull. Inflate the bag for the step. Deflate it to retract. No mechanism to seal. One hose goes up through a channel above the waterline so it can't even cause a leak if the bag fails. Is it increased weight, yup, but not much.
 

orion

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My answer was a bit off of the subject of light planes - I was paraphrasing one of the Navy's Technical Notes that summarized the work that investigated various attempts at neutralizing the step drag of the typical 1930's through the 1960's generation of amphibious aircraft and sponson designs. Since most were rather sizable, any additional systems had a measurable impact on the aircraft's empty weight.

One of the issues discussed was the inadvertent landing with the fairing in place (or step retracted) - although the landing itself might be OK, the loads generated by the water onto any hull structure is quite impressive. Whether extended or retracted (fairing or step itself), the structure must be able to deal with the load magnitude that is developed as a function of high speed water impact, not to mention the rigid locking mechanisms needed to keep everything in place. Just this alone can introduce sizable weight penalties.

Virtually every report concluded that although some improvements in the hull's step induced air drag can be made, the magnitude of said drag when considering the geometry of the typical hull equipped airplane is relatively minimal and as such, was not considered critical to the performance or function of said airplane.

We were able to verify much of this in the design of our Privateer program (www.privateerindustries.com). Our initial analysis determined that the step will introduce a performance penalty however even at 200 mph, said penalty was anticipated to be rather small. The manual calculations were followed by a CFD analysis, both for air and hydrodynamic performance. The air work pretty much confirmed our first cut evaluation and although some improvement could be seen if the step was somehow eliminated, the drag improvement was on the order of 2% or less (depending on design and baseline assumptions). In other words, not worth the effort or added complexity for a proof of concept prototype.
 

Dan Thomas

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The retractable step I designed was just a hinged flap with a bag in a shallow recess between it and the hull. Inflate the bag for the step. Deflate it to retract. No mechanism to seal. One hose goes up through a channel above the waterline so it can't even cause a leak if the bag fails. Is it increased weight, yup, but not much.
You would need vacuum, or powerful springs, to hold the flap up in flight. The air moving past it will suck it down. Landing gear doors suffer the same thing if they don't have strong mechanisms to pull them shut.

As Orion has pointed out, the savings are small. Anytime you add a system to retract a step or fill the void behind it, you add weight. Weight increases induced drag. And what then have we gained?

Aircraft designers, believe it or not, do think about such ideas and sometimes even try them, but in many cases they just sit down and do the math and find that it would be a waste of time. For example, retractable landing gear is pretty much useless on aircraft that don't cruise over 120 MPH or thereabouts. The reduction in drag is too small to justify the added weight and cost. Cessna built the 172RG that didn't gain much performance and some who fly it say it lost climb capability due to the extra weight, and that's even with a bigger engine to haul the stuff around. The fixed-gear 172 was a better deal for the money.

Dan
 

orion

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I think the general consensus is that the step is in fact a major source of drag on an otherwise clean shape...
Well, yes and no. This issue must really be discussed in context to any particular design or program - if anything, the conclusions are not all that clear cut. For instance, if you start out with a clean shape like a nice body of revolution, and then introduce a step or sharp edged discontinuity, then yes, the feature can significantly alter the drag characteristics of said body.

But if you're designing a conventionally hulled aircraft, the effects are not all that clear, especially when combined with possible complexities and unknowns of trying to configure something else for the particular mission. Let's face it, the hull shaped fuselage or sponson is not all that clean in the first place. As such, the consideration of the step can be made only in that context, in which case the addition of a properly sized step is not going to be a detrimental, or in most cases, a significant issue.

Sea planes are generally not designed for speed or airliner type performance - that's generally not the mission (at leas not since the Clippers and such). So if one chooses to go with a conventional hull, then we can discuss only those things that are connected with that package of choices and the repercussions they have on the airplane at hand.

Can we do better? Probably. The Lisa and the Shearwater are certainly departures from the conventional and show some potential benefits of thinking out of the proverbial (and at times, archaic) box. But both may also be accompanied by problematic operational issues that may not be clear at first glance. As such, any program that wishes to operate in this environment must make the appropriate choices based on the analysis of the situation, the historical database (which is pretty vast), and a risk assessment if going away from the conventional.

As far as the aforementioned step is concerned, given a hulled configuration, most of my data shows a step induced penalty usually well under 5%. It may be a little bit higher on conventionally hulled fuselages but is significantly smaller on properly designed floats.
 

orion

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I agree the transition from submerged to surface planing could be difficult...but I don't think cavitation at these slow speeds (relative to a propeller speed) would do much if anything...
Cavitation could be encountered at almost any speed and given the take-off requirements of the typical higher speed aircraft, it could be a factor. Some of the early design work done at Boeing saw damaging foil cavitation at as little as 30 mph. This turned downright destructive at seventy. (My ex-partner did much of the hydrodynamic work on the Tucumcari and the follow-on PHM programs, as well as on the Jetfoil). The higher speed cavitation pitted much of the aft section of the stainless steel foils, including breaking off sizable sections of the control surfaces.

Cavitation can also establish severe vibration events that can translate into the mounting and surrounding fuselage structures.

But there is also the transition from the floating hull...to the point where the foil lift is carrying the ship... I don't think you could just plop a cigar-tube fuselage in the water and expect to get up on the hydrofoils by applying power... So you would still need a hull with marine features like chines...a v-shape bow...etc...
Correct, although it is quite impressive how even small surfaces can lift at relatively slow speeds. The picture at the top right of our marine page (Marine) is the Westfoil hydrofoil. This was my ex-partner's last major project - he did the hydrodynamic work; I did the structural and the drive system (water and air). The picture shows the craft during it's first slow speed trial - the speed was kept below 25 mph to keep it hull-borne however even at that low speed we managed to clear the hull off the water (unplanned - surprised everyone).

Once you are on the water wings then it would be a much shorter takeoff distance to rotation speed ...due to the low drag of the water wings...
This is a very complex design exercise since the hydrofoil and the wings need to be properly matched to achieve that optimum set of characteristics to make it all work right. One of the biggest dangers during this phase is the high drag rise of the foil nearing the surface, accompanied by the sudden drop of lift. If this isn't handled properly the craft can quickly settle onto the water. But since it has speed, once the foil is fully submerged again it starts to rise, only to repeat the motion. This is referred to as porpoising and can introduce extremely damaging loads to a light structure as seen on aircraft. Even hydrofoils have seen catastrophic damage due to this phenomenon.

It is these less obvious features that have kept the hydrofoil applications away from much of the amphibious aircraft out there. It has been tried many times, each time resulting in performance that was not as good as expected, or resulting in some other repercussions that doomed the program.

I don't think the transition from water wings to air wing would be very troublesome...at a certain speed the air wing will make more than enough lift to pull the airplane and the water wings right out of the water...
Not as easy as you think. The issues of performance and trim alone have often proved detrimental. And as they say, if it were that easy, all these historical efforts would have yielded something other than the typical Edo float.
 
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