Dual Spiral Hoop paipo fin concept...

General Discussion
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……If they are made very long (low aspect ratio ) the angle of attack which they can handle gets lower, an aspect ratio of 1:1 to 3:1 is a good place to start ……

Low aspect ratio foils are more tolerant of angle-of-attack (i.e. they stall at a higher angle-of-attack than a high aspect ratio foil). However, the maximum lift coefficient generated by a high aspect ratio foil (all other factors being equal) just prior to stall will be a little greater than the maximum lift coefficient just prior to stall of the low aspect foil. In short, the lower aspect ratio foil will have a wider range of angles-of-attack between a zero lift condition and maximum lift than will the high aspect ratio foil (but the latter will have the greater maximum lift).

 Hi MTB   That's incorrect   A long low aspect ratio tunnel is LESS tolerant of angles of attack                  <div class="bb-quote">Quote:<blockquote class="bb-quote-body">
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……Regarding " An internal laminar effect" there is very little of it, that’s the whole point of enclosing the water flow. . . it evens out the pressure in the tunnel, thus almost eliminating pressure layers and the friction between pressure layers which results from them moving at different speeds . . . . the aim is to avoid differences in pressure. ……

I assume that by “pressure layers” you mean three-dimensional “layers” of water of essentially constant pressure. If that is the case, then it is erroneous (or at a minimum, highly misleading) to talk about “friction between pressure layers” since isopleths of pressure are not congruent with the streamlines of the flow. [i] 

  [/i] <span style="font-style:italic"></span>  Firstly MTB  I said that the tunnel 'almost' eliminates pressure differences . . . of course there must still be some pressure differences and the isopleths of pressure will of course be congruent to the streamlines of flow   Secondly, you say that  if there is no pressure inside the tunnel then there can be no friction between pressure layers, and that thus it is misleading  to talk about friction between pressure layers. . . . .  but this misses the point, the point is that reducing pressure differences reduces friction between layers.      <span style="font-style:italic"></span>  <span style="font-style:italic">

Note also that pressure differences acting on the foil are what generate “lift” (i.e. the component of the force on the foil that is normal to the free stream velocity) – hence avoiding pressure differences, means avoiding the generation of lift.

Not exactly. . . . pressure differences are a necessary consequence of lift generation, it is the redirection of flow which creates lift, and if the flow can be redirected with less pressure difference then we have a lower drag foil.

Tunnels and annular wings redirect a large quantity of water with less differences in pressure than with flat plane wings hence the better lift/drag ratio.

Bernoulli’s principle isn’t ideal for visualising enclosed tunnel fin behaviour.

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Go with whatever works for you, Roy. I’ll stick with what I read in aeronautical engineering books and papers based on empirical observations (e.g. from wind and water tunnels and real world experience), analytical derivations/calculations, the results of numerical simulations (computational aerodynamics and hydrodynamics), and simple physical considerations and constraints (such as the continuity requirement).

As one simple example, calculate and plot out the spatial pressure isopleths and then compare with the streamlines of flow and you will find that the two are far from congruent–the most obvious difference being the principal major axis of the pressure isopleths is in the spanwise direction while the principal alignment of the streamlines is in the chordwise direction.

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Low aspect ratio foils are more tolerant of angle-of-attack (i.e. they stall at a higher angle-of-attack than a high aspect ratio foil). However, the maximum lift coefficient generated by a high aspect ratio foil (all other factors being equal) just prior to stall will be a little greater than the maximum lift coefficient just prior to stall of the low aspect foil. In short, the lower aspect ratio foil will have a wider range of angles-of-attack between a zero lift condition and maximum lift than will the high aspect ratio foil (but the latter will have the greater maximum lift).

I know this is digressing a little from paipo hoop fins…

But I’ve been wondering for some time about fins and the relative impact of changing chord lengths closer to the base and closer to the tip. Surfboard fins are decidedly non-elliptical in load distribution, with the brunt of the lift generated near the base (as in, if you change the base chord a little it has a much larger impact than small changes elsewhere, on a rail fin).

And it is clear that the fin/foil has an enormous interaction with the bottom of the board, the plane to which it is attached, which may explain the disproportionate impact of the base chord.

Things are rarely as simple as they seem, low aspect foil function is poorly explored in three dimensional theory or empirical studies, and low aspect foils that have the trailing edge 1.125 inches from the plane’s discontinuity even less so - except specifically on surfboards. And the theory and empirical findings that people attempt to apply generally are not straightforwardly applicable to low aspect foils nor foils attached to planar surfaces with an inch to the end of the plane.

And then you have the added complication that the planar surface is, well, a planing surface, whereas the fin functional is hydrodynamical.

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……And then you have the added complication that the planar surface is, well, a planing surface, whereas the fin functional is hydrodynamical.

Comment:

Could you elaborate on what you mean by this statement? Absent the presence of a stalled condition, or ventilation, or cavitation, the physics and general principles of the two would seem to be very similar.

[Supporting details for my comment:

A planing hull or a fin aligned at an angle with respect to the flow of water toward it creates a pressure force (lift) on the planing hull, or the fin, as a consequence of imparting a component of momentum transverse to the direction of the oncoming flow (i.e. the free stream velocity).

The (approximate) equation for the lift created by a foil (that is not in a state of stall, ventilation, or cavitation) is:

FLf = (1/2) x (rho) x (Af) x (Velocity)^2 x (CLsF) x (AOA) x Rf

where:

FLf = lift force(foil)

rho = density of the fluid

Af = planform area of the foil

velocity = speed of the flow past the foil

AOA = angle of attack

CLsF = lift coefficient slope-foil (= lift coefficient per unit change in the AOA)

Rf = aspect ratio factor(foil)

The (approximate) equation for the lift created by a planing hull is:

FLp = (1/2) x (rho) x (Ap) x (Velocity)^2 x (CLsP) x (AOA) x Rp

where:

FLp = lift force(planing hull)

Ap = wetted planform area of the planing hull

CLsP = lift coefficient slope-planing hull

Rp = aspect ratio factor (planing hull)

Thus a foil and a planing hull operating at an angle of attack relative to the zero lift angle both generate lift and share the same dependencies that affect the magnitude of the lift that is created.

Now, if we assume that the foil planform area is the same as the wetted area of the planing hull, and that both are operating in the same fluid, have same angle of attack, and the same speed of flow past them, we can compare the relative lift generated by each of them for the same angle-of-attack:

FLf / FLp = (CLsf / CLsp) x (Rf / Rp)

Experimental studies show that the aspect ratio coefficient for a planing hull, Rp, is approximately:

Rp = Rp*

where Rp* is the aspect ratio of the planing hull.

Experimental studies also show that the aspect ratio coefficient for a foil with an aspect ratio, Rf*, is approximately:

Rf = (Rf*) / (2 + square-root(4 + Rf* x Rf*))

Planing hulls typically have small aspect ratios (e.g. << 3). So do typical surfboard fins (especially for multi-fin boards). In the case of fins, it is easy to show (from the equation above) that as the aspect ratio of the fin becomes small, the equation for Rf simplifies to:

Rf = Rf*/ 4

… i.e. an identical functional relationship to that for a planing hull. Hence a planing hull and fin of small aspect ratio differ in the force that they can produce by approximately the ratio:

(FLf) / (FLp) = (CLsF) / (CLsP x 4)

Experimental measurements indicate that the ratio CLsF/CLsP is approximately 8.4 (for a flat bottom hull). Thus for a foil with a planform area equal to the wetted planform area of a planing hull, the foil will generate approximately twice (2.1 x) the lift per unit angle-of-attack.

That shouldn’t be totally surprising since only the bottom of the planing hull generates lift (imparts downward momentum to the water passing under it), while both sides of the foil generate lift (again, by the downward deflection of water). Thus to a first approximation, a planing hull and a foil (not stalled, ventilated, or with cavitation) share the same physics and processes (and differ primarily only in the number of wetted surfaces deflecting water).

Admittedly there are differences in the details of the flow patterns, but their effects on gross properties such as lift are generally small (e.g. the factor of 2.1 above, instead of 2.0). ]

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Go with whatever works for you, Roy. I’ll stick with what I read in aeronautical engineering books and papers based on empirical observations (e.g. from wind and water tunnels and real world experience), analytical derivations/calculations, the results of numerical simulations (computational aerodynamics and hydrodynamics), and simple physical considerations and constraints (such as the continuity requirement).

Hi MTB, You are still trying to tell me that a low aspect ratio tunnel handles higher angles of attack than a high aspect ratio tunnel, or are you ignoring that part of the conversation because you were in error ?

It is definitely the case that a low aspect ratio tunnel is less tolerant of angles of attack than a high aspect ratio tunnel.

Regarding real world experience of tunnel finned boards, you imply that you have some. . . . is that true ?

If we could clear up the first basic and very important point then perhaps we could move on to the pressure discussion. . . . . it’s all very well blustering about unspecified academic treatises etc, but unless you can exhibit a basic understanding oof how tunnel fins work then you have no foundation, and there are several very basic points which need to be understood, and which you have quite plainly misunderstood ( on this thread and the previous discussion ) . . . these need to be dealt with first in my opinion.

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Low aspect ratio foils are more tolerant of angle-of-attack (i.e. they stall at a higher angle-of-attack than a high aspect ratio foil). However, the maximum lift coefficient generated by a high aspect ratio foil (all other factors being equal) just prior to stall will be a little greater than the maximum lift coefficient just prior to stall of the low aspect foil. In short, the lower aspect ratio foil will have a wider range of angles-of-attack between a zero lift condition and maximum lift than will the high aspect ratio foil (but the latter will have the greater maximum lift).

I know this is digressing a little from paipo hoop fins…

Digressing a little ?

It’s a thread hijack !

On the subject of tunnel and hoop fins, a low aspect ratio foil is LESS TOLERANT of high angles of attack than a high aspect ratio foil .

Here’s a fun site for those interested in tunnels, hoops, and other alternatives to the ‘Flat earth society’ flat plane wing dogma: http://skyaak.blogspot.com/2006_05_01_archive.html

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I just stumbled on this thread and have to comment on the originality of thought in that initial design ( the paper mock-up at the beginning). It takes ideas to spawn advancement. That may or may not be THE idea, but it’s an example of the kind of thinking that will get us there. Kudos!! Very eye-pleasing organic curves there too.

I also like it because it’s even farther out there than the fins I ride on my personal fish. Maybe I can consider myself “normal” now…but I doubt it.

Mike

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With all due respect to John, the dual tunnel mock up is a step backwards. . . . it’s kind of like ‘improving’ the wheel by making it a more complex shape. . . not smart, the simple centrally mounted tunnel fin is already perfect !

.

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Hi MTB, You are still trying to tell me that a low aspect ratio tunnel handles higher angles of attack than a high aspect ratio tunnel, or are you ignoring that part of the conversation because you were in error ?

It is definitely the case that a low aspect ratio tunnel is less tolerant of angles of attack than a high aspect ratio tunnel.

Regarding real world experience of tunnel finned boards, you imply that you have some. . . . is that true ?

If we could clear up the first basic and very important point then perhaps we could move on to the pressure discussion. . . . . it’s all very well blustering about unspecified academic treatises etc, but unless you can exhibit a basic understanding oof how tunnel fins work then you have no foundation, and there are several very basic points which need to be understood, and which you have quite plainly misunderstood ( on this thread and the previous discussion ) . . . these need to be dealt with first in my opinion.

I’m claiming that a low aspect ratio tunnel handles higher angles of attack that an high aspect ratio tunnel (in the sense–as I said in my earlier post–that the onset of a stall occurs at a lower angle of attack for a high aspect ratio foil than for a low aspect ratio foil).

No, I do not have practical experience with tunnel fin boards; yes, I do have experience with annular wings. I hope that your basic understanding of annular wings that you will presumably now present will not be of the types suggested for how such wings might work that are presented in the web page you referenced:

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Here’s a fun site for those interested in tunnels, hoops, and other alternatives to the ‘Flat earth society’ flat plane wing dogma: http://skyaak.blogspot.com/2006_05_01_archive.html

An example of the type of “explaination” presented there that I’m referring to is:

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Is the Ring-Wing an anti-gravity machine?

As previously stated, my long-term goal with the Skyaak project is to demonstrate that the ring-wing design has little understood aerodynamic properties that may have certain advantages over the conventional airfoil. My friend Tony Sorenson, who is a self-professed weird-science nut, says that he thinks the Skyaak wing creates its own vortex as it flies, when spinning on its axis. He goes on to say that this vortex creates a low-pressure area inside the conical wings that provide a lift that is quite independent of the airfoil itself. Tony alluded to the mysterious ‘shape power’ of the ring-wing that creates an internal vortex, and goes on to say that the rapidly-spinning vortex inside a twister ( as in a tornado) has been demonstrated to affect the gravitational field. Apparently, the gravity inside the space where there is a rapidly spinning vortex has been shown to be reduced. I imagine that this may be the most notable property of the ring-wing design, if it is ever proven to be so. Much food for thought.

mtb

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……Firstly MTB I said that the tunnel ‘almost’ eliminates pressure differences . . . of course there must still be some pressure differences and the isopleths of pressure will of course be congruent to the streamlines of flow……

Go to:

http://www.esotec.co.nz/hb/HTML/CFD_F.html

Then check out the two figures:

  1. “Pressure Contours on Duct Surface” (pressure isopleths)

  2. “Streamlines around Duct Surface”

Now explain to me:

  1. How one can say that the “…isopleths of pressure will of course be congruent to the streamlines of flow” ?

  2. Pressure differences are “almost” eliminated ?

PS. Also check out the figure “Duct Stall Pattern” (as an example of low aspect ratio duct tolerance).

mtb

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I’m claiming that a low aspect ratio tunnel handles higher angles of attack that an high aspect ratio tunnel (in the sense–as I said in my earlier post–that the onset of a stall occurs at a lower angle of attack for a high aspect ratio foil than for a low aspect ratio foil).

No it doesn’t, a low aspect ratio ratio tunnel stalls at a lower angle of attack than a high aspect ratio tunnel

Perhaps you are thinking of flat plane wings ?

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No, I do not have practical experience with tunnel fin boards; yes, I do have experience with annular wings. I hope that your basic understanding of annular wings that you will presumably now present will not be of the types suggested for how such wings might work that are presented in the web page you referenced:

http://skyaak.blogspot.com/2006_05_01_archive.html

LOL no that reference was just included for fun, a bunch of guys making annular wing frisbees and throwing ideas around. .

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Hi Roy -

To be honest, it was a video of a flying hoop throw toy that was one of things that got me interested in the design. The skyaak looks pretty cool. Not sure about the inventor’s mental status though. After watching (and listening to) a couple of his video clips, it sounds like maybe he needs to get a life! If it was a surfboard he was grunting, groaning and panting about, I’d tell him to get over it and just go surfing.

That link does show a number of designs that IMO validate the hoop concept. Fortunately none of the designs pictured have to deal with kelp as an obstacle.

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I’m claiming that a low aspect ratio tunnel handles higher angles of attack that an high aspect ratio tunnel (in the sense–as I said in my earlier post–that the onset of a stall occurs at a lower angle of attack for a high aspect ratio foil than for a low aspect ratio foil).

No it doesn’t, a low aspect ratio ratio tunnel stalls at a lower angle of attack than a high aspect ratio tunnel

Perhaps you are thinking of flat plane wings ?

Yes. That’s true for “flat” (aka “planar”) foils as well as for annular (aka “ring” or “tunnel”) wings.

PS. I’m still waiting for you to explain to me how the pressure isopleths are congruent to the streamlines of flow (as you claimed earlier) based on the two figures available for the Hummingbird aircraft that I referenced in my earlier post.

…And as long as we’re talking about the annular wing/duct incorporated into the Hummingbird design, let’s examine some of its lift and drag characteristics (based on the specifications enumerated at that web site) in comparison with the planar wing also incorporated into the design.

Here’s the relevant specs from that site:

Wing area (planar wing only): 55 sq-ft

Effective duct lifting area: 31 sq-ft

Weight of craft: 1100 lbs (aerobatic condition)

Percent of weight supported by the planar wing in level upright flight: 65

Percent of weight supported by the annular duct in level upright flight: 35

Planar Wing loading = 0.65 x 1100 / 55 = 13.0 lb/sq-ft

Annular Duct loading = 0.35 x 1100 / 35 = 11.0 lb/sq-ft

Lift ratio = (Planar Wing loading) / (Annular wing loading) = (13.0 lb/sq-ft) /(11.0 lb/sq-ft) = 1.18

→ Planar wing supports ~18 percent more load per sq-ft of effective wing area than does the annular wing.

Skin friction drag is more important than induced drag when traveling at speed. Skin friction drag is proportional to surface area. So let’ see how the two wings (duct and planar) compare with regard to (relative) lift/drag:

Mean Duct Diameter: 91 inches = 7.58 feet

Duct Chord: 35.5 inches = 2.96 feet

Surface area of duct (both inner and outer skins) = 2 x 3.14 x 7.58 x 2.96 = 141 sq-ft

Surface area of wing (both sides) = 2 x 55 sq-ft = 110 sq-ft

Annular Duct L/D(rel) = (11 lb/sq-ft) / (141 sq-ft) = 0.078

Planar wing L/D(rel) = (13.0 lb/sq-ft) / (110 sq-ft) = 0.118

Relative L/D ratio (Planar wing / Annular Duct) = 0.118 / 0.078 = 1.52

→ Planar wing produces ~50 percent more lift per unit skin friction drag than does the annular wing (all other factors being equal).

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Hi MTB, Why is it that you earlier claimed (twice) that low aspect ratio wings handle greater angles of attack ?

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PS. I’m still waiting for you to explain to me how the pressure isopleths are congruent to the streamlines of flow (as you claimed earlier) based on the two figures available for the Hummingbird aircraft that I referenced in my earlier post.

The Hummingbird diagrams are of a double foiled, relatively thick high aspect ratio wing, as the aspect ratio becomes lower, the chord thickness of the wing is reduced, and the inside foil of the wing becomes flatter, the pressure isopleths inside the tunnel will tend to become more congruent to the streamlines of flow

Please notice that diagram shows that pressure is tending to equalise inside the tunnel ( as shown by the pressure isopleth running between the inside surfaces of the tunnel). This effect will be greater with a lower aspect ratio thinner walled tunnel

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…And as long as we’re talking about the annular wing/duct incorporated into the Hummingbird design, let’s examine some of its lift and drag characteristics (based on the specifications enumerated at that web site) in comparison with the planar wing also incorporated into the design.

→ Planar wing produces ~50 percent more lift per unit skin friction drag than does the annular wing (all other factors being equal).

OK then, let’s do that, keeping in mind that the tunnel fin is a half tunnel and because of this creates a beneficial vortex when turning which a complete tunnel does not.

I am glad that you have raised this point regarding the skin friction drag differences between planar wings and annular wings, as I was intending to answer it when you raised the subject in the previous thread

Although what you say is correct, in that the planar wing needs 33% less area to produce the same amount of lift as an annular wing, an important point is being missed: namely that unlike the planar wing, the annular wing produces lift in the horizontal plane as well, and this lift is very useful for surfboards.

Let’s compare planar wings and half pipe annular wings which produce the same vertical and horizontal lift:

If we take a half pipe tunnel of area A, which produces vertical lift L, it can be seen that this fin will also produce horizontal lift 1/2L

Now to produce the same vertical lift with a horizontal planar wing we need a wing area of only 2/3A, but we still need to produce horizontal lift, so we must add a vertical planar wing of area 1/3A in order to do this. … . . . the result being that the total planar wing area needed to do the same lifting job is exactly the same as that used by the annular wing.

Given that this is the case, and that the annular wing has lower induced drag than planar wings, the annular wing will have a better lift drag ratio at all speeds, compared with a planar wing combination.

.

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Hi MTB, Why is it that you earlier claimed (twice) that low aspect ratio wings handle greater angles of attack ?

Barnes W. McCormick, 1979. Aerodynamics, Aeronautics, and Flight Mechanics. John Wiley & Sons. NY. p130-151.

To skip over the math contained in that discussion and summarize the results (excerpts from McCormick):

  1. “The effect of aspect ratio is to decrease the slope of the lift curve, CL, as the aspect ratio decreases.”

  2. “CL(max, finite aspect ratio) is only slightly less than CL(max, infinite aspect ratio).”

Hence a reduced slope to the CL curve (as a function of AOA) combined with nearly the same maximum CL value means that a greater range of AOA occurs between zero lift and stall.

There is also a discussion of delta wings (e.g. an approximation to the Bonzer side fins, or–more roughly–to fish keel style" fins) that you may find interesting. In particular, the figures on page 301, showing CL as a function of AOA for aspect ratios ranging between 0.5 and 2.0 show that CL is still increasing (in a linear fashion, i.e. no “round over” as typically occurs approaching the stall AOA) at the maximum AOA shown in the figures (25 degrees). By way of comparison, for a high aspect ratio wing (e.g. as found on a high performance glider), stall will typically occur at an AOA of 15-18 degrees (and “round over” typically begins at AOAs around 8 degrees).

PS. I will address the other elements of your last post in a separate reply.

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It is as I suspected, you are basing your statement on an analysis of planar wings.

With tunnel fins, a lower aspect ratio fin handles less angle of attack than a high aspect ratio fin

This example is important and also serves to illustrate that planar wing theory is not always directly applicable to annular wings.

.

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It is as I suspected, you are basing your statement on an analysis of planar wings.

With tunnel fins, a lower aspect ratio fin handles less angle of attack than a high aspect ratio fin

This example is important and also serves to illustrate that planar wing theory is not always directly applicable to annular wings.

.

And your reference for your statement is?

mtb

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MTB, I am the reference.

If you would like me to explain:

a) How I know that lower aspect ratio tunnels handle less angle of attack than higher aspect ratio tunnels, and

b) Why this is the case (In plain English), then feel free to ask.

Here’s a hint: It will help you to visualise extreme cases, visualise a very long skinny pipe, and compare this with a tall short tunnel, like a hoop. . . .the difference is immediately obvious, and has been confirmed many times in the water.

cheers

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MTB, I am the reference.

cheers

If you are the reference, I never have seen anything in your surfing videos that comes close to validating any of your conceded remarks. If I, or others watch any more video as your proof of how your boards work compared to others it becomes laughable. You seem well versed in intellectual mumbo jumbo, I’ll give you that, but please – the proof is in the pudding.

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Easternpacific I can assure you that a low aspect ratio tunnel handles less angle of attack than a high aspect ratio tunnel.

Your comments are entirely unhelpful to the discussion, I suggest that you try a couple of pieces of pipe of different length and diamter in the bathtub and find out for yourself how they behave. . . I won’t suggest that you spend 10 years testing them in the surf as I have done.

Calling me out on this one will only get you egg on the face, as I am correct . … . just as i am correct with the speed claims which you allude to.

I suggest trying a 3 inch diameter pipe a foot long, and a 6 inch diameter pipe 6 inches long. . . just whizz them in the bathtub. . . annular wing education for NEWBIES !!

Off to the bathtub with you Epac my boy

Lol !