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What is tailwalking and what is wheelies?
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dsgrntlxmply



Joined: 16 Jun 2010
Posts: 255

PostPosted: Mon Aug 01, 2011 10:03 pm    Post subject: Re: What is tailwalking and what is wheelies? Reply with quote

noshuzbluz wrote:

This is a brief moment of too much lift. Wheelie/Tail Walking. Which ever you prefer to call it.


Why is the wheelie due to anything that the fin is doing, rather than to forces generated by air against the rather large surface area that the board is increasingly presenting once the nose begins to lift?

A board can do the same thing bone dry on the beach if I am carrying it and get it in a poorly chosen orientation relative to the wind.
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DanWeiss



Joined: 24 Jun 2008
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Location: Connecticut, USA

PostPosted: Mon Aug 01, 2011 10:27 pm    Post subject: Reply with quote

Just to clear up a few misapplied terms present in this thread.

First, cavitation rarely -if ever- happens on a windsurfing fin. Cavitation is where a sudden drop in water pressure causes air bubbles to form. The water is, for a moment, literally boiling against the surface of whatever caused the air to precipitate out of the water. That something is usually a rapidly spinning propeller or an abrupt bump along a very high speed powerboat's hull. Simply, a windsurfer does not generate sufficient pressure differential to cause cavitation. What we experience is either stall, where the fin can no longer resist the pressure causing its angle of incidence and it slides sideways, or, more commonly, a windsurfing fin can loose most of its lift component through ventilation. Ventilation is when the air above the water is drawn down the fin for any number of reasons and we suddenly spin out.

Ventilation is somewhat close to cavitation insofar as each involve air on the fin surface but are truly different in cause.

Second, tail walking is a term of art that describes a situation where the board's nose rises wildly due to a lack of mast base pressure and excessive fin power together insufficient to keep the board's nose down The entire board is ripped out of the water and appears to stand nearly on its tail. Another necessary component is a very high flow around the fin, enough to cause the fin to roll upward when the nose rises. You sometimes will see tail walking after bearing off to pass underneath a competitor in a slalom race when sailing overpowered. In most cases, tail walking can be overcome only be very strong and talented sailors who constantly sail on the edge of destruction. Tail walking is a hugely dramatic situation.

Getting the nose to lift, even a decent amount, when overpowered is not nearly as catastrophic. To me, it comes down to degree, tailwalking being of the higher order.

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feuser



Joined: 29 Oct 2002
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PostPosted: Tue Aug 02, 2011 9:31 am    Post subject: Reply with quote

rigitrite wrote:
What you're describing is deflection, not lift. It may seem like lift, but it's not. You wouldn't say that a canoe paddle or a boat rudder creates lift when you use them to turn the vessel. The effect you're describing is real, but it's the actual molecules of water hitting the fin and imparting a force to cause motion in another direction. Same thing happens with the sail: actual molecules of air hitting the material and imparting force to the board, but that's only like 20% of the power in a sail on a beam reach. LIFT is what powers the sail, and were your sail symetric, like a fin, then you'd barely be able to move.

The question you really wanna ask is: If the fin creates no lift (it doesn't) then how come a huge fin can make you tail walk even when you can barely plane? (we all know this is true.) The answer is counter-intuitive, but makes sense.
...


Just to clarify:

The main force acting upon a fin is actually lift, not deflection. As it has been stated, the force of the sail causes an angle in the water flow around the fin, which by itself creates an asymmetry with a resulting pressure differential and a perpendicular force called lift.

Deflection is the force of dynamic displacement measured by the mass of the displaced medium times the vector (length and direction). Since a fin does not create a hole in the water behind it, we know that the 'deflected' water is reunited with the laminar flow of the opposite side of the foil right at the trailing edge of the fin. That means that the sum of deflection vectors is near zero.

Incidentally, when we spin out, the fin stalls out (usually too high of an angle of attack); the fin then actually does create that hole in the water (filled with turbulence & bubbles). Lift is momentarily replaced with deflection alone (the side facing the oncoming water molecules still acts as a deflector).

If you have ever spun out you can attest to the difference in kind and effectiveness between the two phenomena.

Just thought I'd throw my 2 geek cents in there...

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bred2shred



Joined: 02 May 2000
Posts: 981
Location: Jersey Shore

PostPosted: Tue Aug 02, 2011 11:08 am    Post subject: Reply with quote

DanWeiss wrote:
Simply, a windsurfer does not generate sufficient pressure differential to cause cavitation. What we experience is either stall, where the fin can no longer resist the pressure causing its angle of incidence and it slides sideways, or, more commonly, a windsurfing fin can loose most of its lift component through ventilation.


This is exactly right- spinout is caused by ventillation or stalling, not cavitation. Windsurfers are not capable of developing enough pressure differential to induce cavitation.

Ventillation occurs when the fin generates enough pressure differential to literally suck air down its low pressure side. This is typically the result of operator error. You hit a bump, the board lifts off the water briefly, the endplate effect of the board is lost, and air is sucked down causing you spin out. Sailing dynamicaly by absorbing the chop with your knees rather than sailing statically will help prevent ventillation.

Stall is typically the result of using either too small a fin or a beat up fin. If you use too small a fin, the incident angle gets too high to maintain laminar flow on the low pressure side of the fin. The flow separates and you spin out. Or, if the leading edge of your fin is damaged, this can induce flow separation (similar to ice on an airplane wing). Choosing the right fin and maintaining your fins helps prevent stall. Weeds or junk on your fin will also cause stall.

Tailwalking is caused by using too big a sail, too big a fin, and/or too big a board. Being overpowered on a large sail causes you to reduce mast foot pressure allowing the nose to rise up. Too big (deep) a fin causes the board to torque, or "rail up." And using too big a board allows wind to get under the nose and generate enough lift to fly the board upward.

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techno900



Joined: 28 Mar 2001
Posts: 3900

PostPosted: Tue Aug 02, 2011 12:57 pm    Post subject: Reply with quote

feuser,

Quote:
The main force acting upon a fin is actually lift, not deflection. As it has been stated, the force of the sail causes an angle in the water flow around the fin, which by itself creates an asymmetry with a resulting pressure differential and a perpendicular force called lift.

Great description, and one that is easy to understand.

Then to carry the logic a bit further, then if a board is railed while the fin has low pressure on the windward side, then the fin will want to continue it's "flight" to the surface, possibly rolling the board over unless the sailor exerts pressure/weight on the windward rail to keep it down.

Anyone having raced or sailed a longboard aggressively will know what it's like if overpowered with the daggerboard down = rollover. It's the same with any board with a fin and or daggerboard, with the determining factors being board width, fin length and board speed, assuming the angle of water flow around the fin is creating lift.
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raffar



Joined: 23 Dec 2007
Posts: 47

PostPosted: Tue Dec 09, 2014 10:30 am    Post subject: Reply with quote

I liked bred2shred explanation, particularly the last paragraph in his post.

Different angles to the discussion:
One:
We have been taught when "Pumping" a large-finned formula board in marginal winds to include a rotational or skid motion. Among other factors this exacerbates the bend on the fin with every thrust causing the fin's windward side to bend toward the surface. The resulting the wing lift vector force has an upward element and the board lifts in jumps with every thrust minimizing the board's "wet area" and its resistance. This has an overall positive effect that helps getting on the plane ever earlier, which is obviously desirable as opposed to the tail walks mentioned earlier in this thread .
Two:
Counter point - Why fins aren't angled forward? I always thought the trailing edge of the fin angle is the one that generates the lift.
Three:
Hydrofoil fins.
Four: Do weed fins generate more/less lift?
$0.02
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bred2shred



Joined: 02 May 2000
Posts: 981
Location: Jersey Shore

PostPosted: Mon Dec 15, 2014 12:33 pm    Post subject: Reply with quote

raffar wrote:
Four: Do weed fins generate more/less lift?
$0.02


A weed fin can generate an equal amount of lift as a traditional or upright fin. But the question should be "does a weed fin have the same lift to drag ratio as an upright fin?" And the answer is - no.

For a given amount of lift, the weed fin will generate more drag than an upright fin. This is because its heavily swept shape is not as efficient as that of an upright fin. However, this is only true in clear/non-weedy conditions. A weed fin will have a much BETTER lift to drag ratio than an upright fin when used in weedy conditions (i.e., when the upright fin is covered in weeds).

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DanWeiss



Joined: 24 Jun 2008
Posts: 2288
Location: Connecticut, USA

PostPosted: Mon Dec 15, 2014 2:10 pm    Post subject: Reply with quote

IF anyone cares, the only reason I hedged on Bernoulli as taught in high school is the explanations assume without any basis one thing: That lift is caused by a foil splitting a fluid will cause molecules to flow faster on one side than the other so they meet each other at the trailing edge to create a pressure differential. Bernoulli isn't wrong, but explanations of what he wrote are dumbed down for mass consumption.

Let's assume that a fin's leading edge meets two water molecules at the same time. One molecule ("A") diverts to the left, the other ("B") to the right. Each travel on opposite sides of the fin. Those teaching Bernoulli claim but rarely explain but did not prove that molecules A and B will reach the trailing edge of the fin more or less at the same time. This flawed assumption undermines Bernoulli's conclusions because the teachers essentially ignore the existence of mass and inertia of the fluid.

Newton's explains differently. Foils for water are designed knowing that water's viscosity never changes. In other words, water's viscosity is the same, more or less, whether in fresh or sea water. Unlike air, water does not compress. And air's ability to compress results in greater mass and inertia of the fluid as a foil passes through it. That's why the trailing edge of most airplane wings -especially flying slower speeds- tip downward. The air displaced is pushed down and continues down, thus allowing the wing to function. The air must be deflected downward in order for a plane to fly. That's confirmed by the law of Conservation of Energy. Changing viscosity explains why airplane wings are designed to change shape depending mostly on speed and sometimes upon pressure changes, each of which changes the viscosity of the air. Newton's ideas on lift can be reduced to two words: air deflection. More mathematically, Force = accelerated mass or F=mA. Bernoulli doesn't contradict Newton but describes it using air velocity.

Much more complexity faces us when trying to describe why fins and sails, for that matter, create lift. Consider the Coanda effect states that moving fluid must be attracted to a nearby surface. The water from a faucet bending to flow around the convex surface of a nearby spoon is a demonstration of the Coanda Effect. It also means that the fluid will flow around the side of the foil with significantly less flow. Rotational flow and vortices seem to be what's actually making wings work but present far too much complexity of thought and computation to make for a 10th grade physics class.

For those interested in reading a more accurate explanation, check out http://amasci.com/wing/airfoil.html#expl

Tons of links on the website. This page gets down to the nuts and bolts, and much of my writing takes source from that direct link.

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