Seaplane retracting gear.

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Himat

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As for cavitation some of Naca papers I have reviewed put the cavitation speed at 45 knots which is more than our light planes will see in the water...

For higher speeds a supercavitating foil is required...with a sharp LE somewhat like a supersonic foil...but with an overall wedge profile with a bluff trailing edge...this puts the cavitation bubble a couple of chords length back of the foil...
A side comment on cavitation seen from the acoustical engineer point of view. Cavitation is the water boiling due to local low pressure. The pressure limit before onset of cavitation depend on at least absolute pressure, rate of pressure change and water quality. Particles suspended in the water lower the cavitation threashold. (A powerful echo sounder might cavitate in port, but is ok out at sea.)
 

Dan Thomas

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Just to add that Stinton has a comparison chart of a Partenavia light twin in both landplane and float configuration...

The floatplane sufffers a 15 percent reduction in maximum speed...

Also a 30 percent decrease in climb...

Overall drag increase is estimated at 31 percent...

And all of this is for a sub-200 mph airplane...these numbers would steadily get worse as get into faster aircraft speeds...

So there is a high price to pay for water capability...the successful adaptation of hydrofoil technology is really quite a breakthrough...my hat is off to the Akoya designer(s)...
That loss in performance is not due to the presence of a step. It's due to the addition of massive floats to a landplane. This isn't a sensible comparison at all. The Cessna 180 I flew with floats lost some performance, too, but not as bad as the Partenavia. Any aircraft designed to operate off the water will have its aerodynamic drawbacks, even that fancy new one that has your interest. I'd like to see it proven first.

Dan
 

Himat

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Yes it is useful to keep in mind the definition of cavitation which is vaporization of water due to pressure dropping below vapor pressure...

On the upper surface of a cambered hydrofoil the pressure drop (due to velocity increase as per Bernoulli) may go below the vapor pressure of the ambient water...

The result is the formation of vapor bubbles...this in itself sounds harmless but it is important to understand that these bubbles quickly form and then implode causing shock waves which can damage the structure in question...be it a pump or valve or hydrofoil...

All of those factors mentioned by Himat relating to the onset of cavitation are true of course...but the main thing is the drop in pressure, which itself is a function of the velocity over the foil...

How much velocity are we talking about...?...well...because of the very high density of water...nearly 1,000 times as dense as air...we do not need a whole lot of velocity increase to see huge drops in pressure...

Remember Bernoulli's equation for pressure is P1 + 1/2 density * V^2 = P2 + 1/2 density * V^2...

So if we have water flowing through a small restrictor valve for example it would not take much speed increase to drop the pressure by a whole lot...

If the flow through that valve (or over the foil) speeds up the water by let's say 20 meters per second...it means a pressure drop of 1/2 * water density of 1000 kg/m^3 * 20^2 m/s = 500 * 400 = 200,000 pascals or 200 kPa...

Since the boiling pressure of water at 15 C is 1.7 kPa and atm pressure is 101 kPa it means dropping the pressure by 200 kPa would put us well into cavitation...(we would need a pressure drop of 101.3 - 1.7 = 99.6 kPa which equates to a speed of about 15 m/s)...

20 m/s is only 45 mph...

Of course that speed increase over the top of the foil would have to be relative to the speed at which the foil is traveling through the water so overall it seems the Naca figure of 45 knots before cavitation seems to make sense...
I have not checked your numbers, but I there is another mechanism at play too.
If the foil vibrate, it do work like an acoustic transducer. The velocity and amplitude of the vibrations will then set the cavitation treashold.
 
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Jay Kempf

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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
Dan, we are talking skin pressure from V/Vo correct? What powerful vacuum? A mechanism is a point load so it has to be light weight and the door attached to it has to be very stiff to control the distributed load. Not true with vacuum or pressure in a bag distributed over a large surface not amplified by a mechanical advantage. And you could have a latch mechanism although I deemed that unnecessary. We are talking about a small hinged mechanism like an unpressurized cargo door behind the point in the hull that would take the hit from a tree that would be horizontal behind a hard bulkhead.

Orion, the Privateer is an interesting case because instead of a single faired hull you have basically floats. The overall configuration has much interference drag over a say a single fuselage conventional aircraft. The fineness ratio of the floats and their frontal area is small which is good and means that the step is relatively small compared to say the fuselage frontal area and even though you are going after commuter airliner speeds the steps are as you estimated not worth eliminating.

For a flying boat like design or say a turbojet commuter that if it could be designed for similar commuter jet speed like say an old Citation 500 a step could be a significant portion of the fuselage frontal are and a significant portion of drag at cruise.

Context is everything.
 

orion

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We've gotten a ways off the subject at hand but I'll input one more post, this time with a numerical example that is based on actual historical data and our Privateer program. First off, comparing the separation off a cylinder to a float step may have some levels of illustrative similarity but is in reality incongruous simply due to the differences in proportion and effect.

There have been numerous papers and design references published on the performance attributes of stepped hulls or floats, many dating as far back as the days of the Schneider Trophy races (which were all float planes), held from about the mid teens through the early 1930's. More detailed and exhaustive work was done for or on behalf of the US Navy all the way through the sixties. Hulls, hull geometries and variations have been studied in various forms, culminating in a sizable historical database that is now for the most part available to anyone looking to design in this realm. Most of the work can be found in a variety of archives; some is even summarized and referenced in some of the most popular texts in the industry, including the books published by Sighard Hoerner, amongst others. The example I'll use is from the latter, the numerical values being derived from the aforementioned Naval programs.

In examining the effect of a step type discontinuity, the referenced programs examined the drag characteristics of a typical high finesse ratio hull (as would be used on a float) by starting with a clean body of revolution and then measuring the drag rise as various hull features were added to the basic shape. The first modification of the clean body was the addition of the step. The example shows a drag rise of nearly 50% - CD0 went from .056 for the clean body to .075 (reference area was normalized at the projected frontal area of the original body of revolution). As "gordonaut" implies, this is a dramatic penalty from the smooth shape.

Interestingly enough though, it is not as great a penalty as is incurred when incorporating the "V" hull and the fore-body chine - starting with the stepped fair shape, that drag rise is just short of 200% of the stepped starting point. The example clearly shows that the step is a lesser penalty when compared to costs the other hull features create. But back to the example.

Looking at the Privateer, we'll take the reference areas of the sponsons (12.72 square feet - both sponsons) and normalize the aforementioned drag penalty of the step to the reference wing area (283 sq. ft.). The CD0 of the basic airframe is approximately .025 so when we back out the normalized coefficient for the step, we see that the drag rise due to the discontinuity in the hulls, for the whole airplane, is calculated to be only about 3.4% of the total drag.

This however assumes that one uses the same step size as was used in the original tests, which we did not. The historical data shows that there are optimal step size ranges for a given hull geometry and operational parameters - our design uses a geometry that's on the lower end of the scale, which explains why our analysis reveals a smaller penalty incurred due to the presence of the step.
 

autoreply

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I think the Akoya makes all of this discussion moot since they have made the hydrofoil work very nicely...
Hold on, you are assuming far too much. The Akoya is a research project primarily and an exercise in design secondly, all subsidized by government funds. I have seen at least 3 flights from the water documented on video. Nice, flat, unobstructed, windless water. That's like saying you have built a good aircraft if the wings don't fall off when you first start the engine.

Think of roll-yaw coupling when "on the fin" for example. What happens if you have a little bit of roll? The "fins" axis cross below the C of G. Is it stable in roll at all? I'd love to see it work, but it's still a totally unproven concept. Unfortunately, their holidays coincide with mine, so visiting hasn't been possible yet...


Interesting comments, "orion..."

I'm not familiar with the seaplane mentioned which is apparently under construction...but the specs say a cruise speed of 215 knots at FL150...

So this is not the high-speed airplane that we are talking about...if you were to test these floats at 300 and 400 knots I think you would find very different results...
Why? IAS and Reynolds is exactly the same at 300+ knots and those typical cruise altitudes...
 

Himat

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We've thought about it. So what would it take to retract skis, floats, or vary the hull so that the gear doesn't create "a lot" of cruise drag. Weight would likely be more, given the engineering challenge. Still I'm curios what the more innovative and experienced designers know/think about it.

Wonderous Mountain
I'm not an experienced aircraft designer, but at least in my model aircraft I try to be innovative. Some experience I have gained too. From a healt and safety stand point, it's probably best that I test fly my flying innovations with radio control. (Now and then RC equals remote crash.)

Jay Kempf's retractable step sounds like a workable idea and the inflated bag might even provide some shock absorbtion on landing. It's a question about if improved performance versus weight and complexity. As for operating on snow or ice, both float's and seaplan hull's can probably be designed to work in these conditions too. The float's on Norwegian Heinkel 115's pre WW2 had ice runner's and reinforcments to adapt them for operation on ice.

A booklet, "Naval Fighters number twenty three: Convair XF2Y-1 and YF2Y-1 Sea Dart", describes the testing of the ski equiped fighter prototype. Ski('s) instead of a stepped hull have been done and probably can be done better today. Again, separet ski's for snow or ice is probably not needed if operation from snow and ice is acounted for in the design.

Hydrofoils, Orion have pointed out the general arguments against the use of hydrofoils. Autoreply pointed at an important issue with how the Lisa team have executed it, the hydrofoils have anhedral and that might give stability problems. What happen if you sideslip the Acoya on landing?
 

autoreply

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Say What...?

Dynamic pressure at 215 knots and FL150 = ~100 lb/ft^2...

Dynamic pressure at 400 knots and FL300 = ~200 lb/ft^2...
No, I did't say that. 215 kts @ FL150 = 170.5 KTS IAS, RE=1.58, 300 kts @ FL300 = 183.6 IAS, Re=1.45E6

Exactly the same regime in my book...
Do you know for sure the ins and outs of the Akoya test program...?...if you do then I would be interested in hearing some technical detail...

From what I could see they have a seaplane that is taking off nicely in what looks like average water conditions...
Tell us then, which video have you seen and where can we see it? The only ones I have seen incorporate perfect water, without the slightest chop. In fact, the videos where you can actually see it taking off weren't disclosed till fairly recently, which stimulates my "suspicious" mode even further.
I don't understand your point about roll yaw coupling...I assume you are talking about the hydrodynamic characteristics here while the craft is operating in water...

I do not see any reason why the anhedral of those water wings would be anything but stabilizing...
For exactly the same reason a taildragger or any wing with anhedral is destabilizing. In a medium that's 700 times denser than air, that might get interesting really quickly in less than perfect circumstances, like a bit of crosswind, a bit of roll angle on touchdown or some small waves..
 

Himat

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From what I could see they have a seaplane that is taking off nicely in what looks like average water conditions...

I don't understand your point about roll yaw coupling...I assume you are talking about the hydrodynamic characteristics here while the craft is operating in water...

I do not see any reason why the anhedral of those water wings would be anything but stabilizing...
Average water conditions depend a lot on your wherabouts. If you have seen the Centaurus model/Gull UAV rough water operation videos it's more in line with what water handling I would like to have.

About the stability and yaw coupling of the hydrofoils, I do see a cause of concern when it comes to water handling. What hapen if you land with a sideslip?
Same, what if you do a high speed turn while on the water?

A hydrofoil is a wing and for stability it should the have ordinary V-form. Now, on the Acoya there is a lot of sweep back on the hydrofoils, that act as on any wing as dihedral. Time will tell if they have got the balance right. And I'm eager to se some more close up videos as the plane accellerate from standstill to lift off. Anyway I do like the design and do hope the Lisa team have more luck with their Alcoa than Piaggo had with their P7 Schneider racer. (The hydrofoil undercarriage is not a new idea.)
 
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Jay Kempf

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Water is actually ~800 times denser than sea level air...but I guess 700 is close enough for this level of discussion...

There is no reason that anhedral is destabilizing...otherwise we would not have aircraft that make use of it...like the Antonov transports for example...

Those water wings are going to be quite stable in water...don't forget that the airplane will rise out of the water with speed as the lift from the hydrofoils increases...

As this happens the wing surface area decreases automatically as the airplane lifts out of the water...that is the ingenious effect of anhedral for these wings...it is quite beautiful design...

At some speed the airplane will actually pull those water wings right out of the water and will ski on those turned up "winglets..."

Beautiful...

I am really impressed with design and there is no point in a lot of the pooh poohing I am hearing here...

This is a breakthrough for seaplanes and there is no reason that any issues that might arise should not be worked out...

It is a very elegant idea that is long overdue and has been nicely implemented here...there is no reason why this could not be made to work on a transport size airplane and then we would really have something groundbreaking for air travel...
I saw the reduction of wetted area of the hydrofoils on rising as a very clever thing too. Also the relatively vertical configuration with just a small traditional hydrofoil on the end is clever as this is at very high angle of attack while submerged before the thing starts to rotate to more planing mode. Once it rotates it is a true hydroplane with flight controls active to keep from spinning horizontally. They figured out a way to get a tail dragger type handling out of it. As soon as the thing starts moving the front end starts climbing and the rear stays planted until the flight controls can take over.
 

autoreply

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Water is actually ~800 times denser than sea level air...but I guess 700 is close enough for this level of discussion...

There is no reason that anhedral is destabilizing...otherwise we would not have aircraft that make use of it...like the Antonov transports for example...
Every textbook or the simplest math will show you that anhedral is destabilizing. That's exactly why the Antonov, many other high-wing aircraft and most fighters have it... otherwise they would be too stable. Not to mention that all hydrofoils in existence have horizontal or V-shaped foils (dihedral), not inverted V-shaped foils (anhedral). They would be highly unstable, even if surface-piercing.
Those water wings are going to be quite stable in water...don't forget that the airplane will rise out of the water with speed as the lift from the hydrofoils increases...

As this happens the wing surface area decreases automatically as the airplane lifts out of the water...that is the ingenious effect of anhedral for these wings...it is quite beautiful design...

At some speed the airplane will actually pull those water wings right out of the water and will ski on those turned up "winglets..."
In a perfect world they would; theoretically. In a real world, a small distortion, like a bit of roll angle or a wave will likely be highly destabilizing. Let's imagine the right fin touching the water. This will cause a lot of hydrodynamic drag and lift. Since the C of G is behind and above the fins, it will pitch down, yaw to the right and roll to the left. Especially with the engine in the back (large moment of inertia) that will might make for an interesting event.
I am really impressed with design and there is no point in a lot of the pooh poohing I am hearing here...

This is a breakthrough for seaplanes and there is no reason that any issues that might arise should not be worked out...

It is a very elegant idea that is long overdue and has been nicely implemented here...there is no reason why this could not be made to work on a transport size airplane and then we would really have something groundbreaking for air travel...
Unlikely you, I'm not critiqueless if something works once or twice let alone if it's only (publicly) been demonstrated in perfect conditions. That's not pooh poohing, that's a healthy dose of skepticism, based on solid arguments and some knowledge of the very subject...


Ow and can you share a link to the video with those "average water conditions"?
 

Himat

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I saw the reduction of wetted area of the hydrofoils on rising as a very clever thing too. Also the relatively vertical configuration with just a small traditional hydrofoil on the end is clever as this is at very high angle of attack while submerged before the thing starts to rotate to more planing mode. Once it rotates it is a true hydroplane with flight controls active to keep from spinning horizontally. They figured out a way to get a tail dragger type handling out of it. As soon as the thing starts moving the front end starts climbing and the rear stays planted until the flight controls can take over.
The wetted area reduction as speed rise is one of two possible ways to make a working hydrofoil vehicle. Either the hydrofoils are designed to rise out of the water and that way adjust lift to weight with varying speed or the hydrofoils are allways fully submerged and an automatic control system keep the ride hight. That as far as I have read and remember hydrofoil theory. To put small "skiis" at the end is a clever afterthought. (Notice, the "skiis" are not there in the first pictures and videos.)

Re watching the videos and stop it a few times it look like the "tip" of the hydrofoils are behind the main wheels. The dynamics of the main gear could then be different at planing speeds compared to wheeled operation. It's difficult to tell if the hydrofoil "tips" are far enough aft to make them work like the main wheels on a nose gear set up. Obvious the centre of hydrodynamic lift must be further forward at lower speeds to lift the nose out of the water first. At second thought, there is probably a lot about what makes this design work that is hidden in the details to the casual observer. Just look at those small fins at the bottom of the tail.
 
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