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Pumping a foil - short versus long and stab size

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Pumping a foil - short versus long and stab size

Postby Peter_Frank » Mon Apr 12, 2021 1:59 pm

I saw this video:



And noticed he found the longer fuselage and bigger stab could pump better, as he got more glide between pumps, because of more lift (he said).

It does not sound quite right.

My take on this is, the longer fuselage and a bigger stab, will give you better pumping leverage, so you increase the speed much more efficient when pumping - thus you got better glide between pumps as a result from this.

As a bigger stab does usually not lift more, in stable glide - on the contrary, it maybe lifts down, reducing overall lift a tad.

The downside of longer fuselages and/or bigger stabs, is the complete foil will turn slower and feel a lot less lively, not good...
Except for beginners maybe.

What do you guys/girls say?

8) Peter

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Re: Pumping a foil - short versus long and stab size

Postby Dontsink » Mon Apr 12, 2021 2:07 pm

I think what we feel is happenning and what is actually happenning are not always the same.

My theory (based on nuthin' really) is that longer fuses/bigger stabs will make the foil "self steer" through the pitch changes as load factor is increased/decreased by the pump action.

So the rider does not have to control the pitch angle so much and can just pedal away,with the stab keeping the front wing at the optimal angles.

Because as you said,a bigger stab cannot be more glidey,it's more downwards lift and more drag.

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Re: Pumping a foil - short versus long and stab size

Postby jumptheshark » Mon Apr 12, 2021 2:54 pm

Trade offs are hard to pin down.

The idea of elevating the main wing and gliding it's descent is the main driver in pumping, so aspect ratio and foil thickness of the main wing is likely a major factor to pumping efficiency. Analogy being bird flapping its wings. The tail wing is what it is and has an impact no doubt. If it is big enough, it likely can produce propulsion as a factor of it resisting pitch input and translating that rider effort into some propulsion. Dolphins pump the tail wing for propulsion and not their pec fins. For pumping a hydrofoil, there is interplay between the two with trade offs for different forms of foiling.

In stable flight, the rear wings profile pulls down, but once the foil is turned hard, the tail wing exits the streamline of the main wing and its area resists pitch change. That can be felt in hard turns as rear foot feedback. I like that sensation of drive in hard carves that I am imagining comes from the stab.

I think most thrust and glide come from the main wing, but a not insignificant amount of thrust can be generated with the stab while pumping.

Short fuse would require higher frequency pumping action compared to longer. Smaller stab would also favor a higher frequency. There would be a point of diminishing returns where too big or too long are counterproductive and are simply too slow to pump or simply add drag.

Definitely think the interplay between main, stab and fuse length are all in the alchemy to how the foil "feels" in: stable flight, carving and while pumping or gliding.

I would think in the end we will see specific differentiations that best suit particular types of foiling.

Wingers and surfers will trend to higher aspect main wings and smaller stabs on short fuses (for advanced riders) to max glide and improve the maneuverability when riding 50+ L boards.

Windsurf foiling will want higher aspect main wings with longer fuse and smaller stabs. They don't really pump at all, but want efficiency to make up for it.

Kite foillers will be looking for different trade offs depending on style. Mod aspect main wings and stabs are acceptable for free riding because we have more power on tap and a kite that takes a portion of rider weight. Short Fuses for maneuverability but less overall need for efficiency as pumping is more a choice than necessity. Longer fuses for learning. Smaller stabs for speed, yet also for maneuverability, but with a trade off in how the foil carves. I can't help feel the stab plays a pretty big role in the character of how a foil carves.

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Re: Pumping a foil - short versus long and stab size

Postby Matteo V » Mon Apr 12, 2021 4:45 pm

Dontsink wrote:
Mon Apr 12, 2021 2:07 pm
Because as you said,a bigger stab cannot be more glidey,it's more downwards lift and more drag.
"Downwards lift" on a stab occurs when the foil is traveling in a straight line. But that same foil section can produce upwards lift at a different AOA.

Think of a jet fighter that rolls inverted, and trims its AOA so that its upside down wings produce lift in the opposite direction compared to when they are operating in the normal orientation. This demonstrates that both upwards and downwards force can be produced by almost every foil section if AOA is adjusted accordingly. It is useful to note that while symmetrical foil sections (50/50) are equally efficient in generating upwards and downwards force, asymetrical foil sections are not.


But this is not necessarily the main idea. Forward propulsion is the main goal of pumping. Thus reversing the direction of force on the stab is not done to lift upwards, but rather to provide the horizontal component of a mostly upwards force to forward motion.

If you want to simplify the situation......
Do longer or shorter dive fins produce more efficient thrust? Definitely an over simplification that may not necessarily scale infinitely, but thats much easier to conceptualize.

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Re: Pumping a foil - short versus long and stab size

Postby BWD » Mon Apr 12, 2021 5:16 pm

Long ago i read an explanation of sailing physics that likened the boat hull reacting to forces from one side made my the keel, and from the other side made by the sail, to an orange seed squeezed between the fingers - squeeze and it slips forward.
If spork were around he could tell us how this concept might apply to a pumping foil reacting to stab and wing forces - or not!

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Re: Pumping a foil - short versus long and stab size

Postby Dontsink » Mon Apr 12, 2021 5:40 pm

Matteo V wrote:
Mon Apr 12, 2021 4:45 pm
Dontsink wrote:
Mon Apr 12, 2021 2:07 pm
Because as you said,a bigger stab cannot be more glidey,it's more downwards lift and more drag.
"Downwards lift" on a stab occurs when the foil is traveling in a straight line. But that same foil section can produce upwards lift at a different AOA.

Think of a jet fighter that rolls inverted, and trims its AOA so that its upside down wings produce lift in the opposite direction compared to when they are operating in the normal orientation. This demonstrates that both upwards and downwards force can be produced by almost every foil section if AOA is adjusted accordingly. It is useful to note that while symmetrical foil sections (50/50) are equally efficient in generating upwards and downwards force, asymetrical foil sections are not.


But this is not necessarily the main idea. Forward propulsion is the main goal of pumping. Thus reversing the direction of force on the stab is not done to lift upwards, but rather to provide the horizontal component of a mostly upwards force to forward motion.

If you want to simplify the situation......
Do longer or shorter dive fins produce more efficient thrust? Definitely an over simplification that may not necessarily scale infinitely, but thats much easier to conceptualize.
Yes,but asymetrical foil sections are very ineficient creating lift on their "wrong" side.Lots of drag ,stall at lowish AOA etc..

I find it hard to believe that the stab adds any significant propulsive force when working at negative AOA's... but pumping is so on the edge energy wise that maybe a little bit makes a difference that can be felt by the rider.

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Re: Pumping a foil - short versus long and stab size

Postby Dontsink » Mon Apr 12, 2021 6:43 pm

As for the fin analogy, i think this same guy tried a stab mounted on a flexible extension...cannot remember exactly.
The results weren't spectacular I seem to recall but with so many variables involved it might be an avenue for future development.

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Re: Pumping a foil - short versus long and stab size

Postby Dwight » Mon Apr 12, 2021 9:36 pm

My feeling and guess..

As we move toward thinner and faster foils, WE become the foils worst enemy, by grossly over pumping, and disturbing the delicate flow over these thin, fast, foils.

So much so, the best in the world (Kane DeWilde and others like him) no longer rock the board, but just bounce evenly on it, to pump along endlessly, with what appears to be little effort.

So a longer fuselage, helps keep us from over pumping it. Making it seem to work better.
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Re: Pumping a foil - short versus long and stab size

Postby AndersP » Mon Apr 12, 2021 10:06 pm

Maybe a symetric stab profile would be good for pumping.
Good forward drive and low resistance both and up and down.

Still believe in lateral flex in the tail of the fuselage.
Once you find the right stiffness/softness combined with the right stab it should work for pumping.

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Re: Pumping a foil - short versus long and stab size

Postby bigtone667 » Tue Apr 13, 2021 12:51 am

I enjoy the simple fact that he actually does an empirical set of tests in benign conditions.

I have the same 1300/1150 HA wings and the 460HA/370/340 STABs and I prefer the 1300/340 and crazy ultra short for pumping in my little lake ripples. I think your local conditions can contribute to preferring one setup over another.
But I do like the standard fuselage if I want to go fast or learn new tricks.


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