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Foil Question for Mechanical Engineer

 
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eric

Since 13 Jan 2006
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PostSun Jul 15, 18 6:52 pm    Foil Question for Mechanical Engineer Reply with quote

So, some of us are using shims in our foils to play with the location of the center of lift with respect to our feet on the board. If an ME out there could watch this 6 minute video:https://www.youtube.com/watch?v=mVdSx5eNdUA on an easy way to do this I would be obliged if you could comment on what this does to the top, low end speed, other, etc. In other words, "No such thing as a free lunch." What am I compromising with this shim method?

Thanks,
Eric

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Nak

Since 19 May 2005
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PostSun Jul 15, 18 9:35 pm     Reply with quote

Some of the guy's reasoning is a bit off. rather than go into it, let's just fix his method so that you don't have negative consequences. You're changing two things when you shim in the manner the video shows. You're changing where center of lift ends up on the board, what you're trying to do, and you're changing the relative angle between the board and the fuselage--what you may not be trying to do. Think of it this way: what if you really shimmed the front of the fuselage up. You'd reach a point where you'd have to pearl the nose of the board to prevent the wing from porpoising up. Reverse it, and you'd really have to jam the tail of the board down to get it to lift off. Ideally, this relationship is good from the factory. If you're a better foiler than me you could play with this angle to make it perfect for you. However, you don't want to change two things at once--ideally. So let's say you just want to play with center of lift. You could shim the mast/fuselage as the video shows, and then you could shim the mast/board in the opposite--but equal angle--direction. This would move where center of lift was on the board while leaving the relative angle between the board and the fuselage untouched. You are in effect doing the same thing as sliding the mast in the board tracks.

I'd have to measure it out to see how much you're really changing the center of lift. My guess--and it's only a guess--is that changing the angle of the board makes you naturally lean more on one foot or the other. If you shim in the manner I've described and it doesn't change foot pressure as much as the video method, you have then proven that the shimming is just making it more natural to lean a bit more on a particular foot. If foot pressure between the two methods is the same, then you've shown that the change in center of lift is the causal factor.

BTW, all of this assumes we're talking about front foot pressure while up on the foil and NOT on the board or not while transitioning from board to foil. If we are talking about either of these two things--and NOT foot pressure while up on the wing--then what you in fact want to change is the angular relationship between the board and the wing. If it's taking too much front foot pressure to keep the board on the water, then you move the mast backwards, or if you can't do that then shim either the mast/board OR the Mast/fuselage. (Not both.) Shimming the stabilizer would have the same effect, but it would also change the speed the foil is trimmed for.

Also BTW, this is NOT based on my expertise in foiling--or lack thereof LOL. It's based on some engineering knowledge and 36 years professional experience managing aerodynamic trim forces.

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eric

Since 13 Jan 2006
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PostMon Jul 16, 18 7:38 am     Reply with quote

Thanks Nak.

I did not do a good job of explaining my question. I'm curious to know how the wings' initial "effort" to regain equilibrium, which the video says is "flat" (parallel to the surface) impacts other aspects of the foil's performance. My layman's perspective is that the system in my case is rotated forward with a shim under rear of mast at fuselage. The fuse will then rock back to regain its balanced position. In so doing, the wings are "helping my front foot" by taking on some of the downward push. In so doing, am I losing energy that would have been applied to the system elsewhere? Forward thrust, upward thrust, top end speed, low end lift...?

This method was presented as a way to achieve the same results as moving the mast forward or back, without messing with the relative angles of the front and rear wings with respect to each other (shimming wings), which seems like something you don't want to change if possible.

In my case, I am running the Slingshot 633 wing on the 36 Dwarfcraft. It's mindblowing!! So fun. The small DC has tracks mounted near the tail of the board, so
ironically this small board works very well with large wings. Almost perfect.

I wanted my front foot to come back just a bit more. I am tall, and like some nose rocker in front of me. I put one CC shim in, and bingo! Front foot sweet spot moved back about an inch. Feels PERFECT. Foil retains its sweet ride. Board feels perfectly parallel to water.

I am simply curious. In terms of performance characteristics, what, if anything did I sacrifice? That force that rotates the fuse/ wings back to level--where is that energy being taken from?

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grigorib

Since 19 Sep 2017
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PostMon Jul 16, 18 8:08 am     Reply with quote

If I’m not mistaken he’s using the shim to adjust triangle created between feet position (projected feet position) and center of pressure of the wing.
In aerodynamics a wing should have positive angle of attack (2 degrees in gliders I recall), for higher water density it might be different but if wing was not mounted properly by design (too low or too high AoA) this shimming would be essential.
Now if you do this shimming on a foil with a proper AoA you’d end up riding the board tilted nose up or down and that looks crappy. Instead I suggest to rather move mast/feet and use above shimming as a last resort in such case

Last edited by grigorib on Mon Jul 16, 18 8:17 am; edited 1 time in total

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grigorib

Since 19 Sep 2017
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PostMon Jul 16, 18 8:10 am     Reply with quote

But if your stabilizer AoA is not set properly and steering the whole setup too much up/down, which should be felt especially at speed, I’d suggest to shim the stabilizer

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eric

Since 13 Jan 2006
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PostMon Jul 16, 18 8:19 am     Reply with quote

Everything "feels" perfect and the board "feels" parallel to the water. Mostly a "Conservation of Energy" question from the perspective of the system counteracting the angle on the fuse/wings the shim creates.
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Nak

Since 19 May 2005
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PostMon Jul 16, 18 8:59 am     Reply with quote

eric wrote:
Everything "feels" perfect and the board "feels" parallel to the water. Mostly a "Conservation of Energy" question from the perspective of the system counteracting the angle on the fuse/wings the shim creates.


You're not messing with the trim between the stabilizer and the wing, so you're not changing the way the foil flies at all. If it feels right to you during takeoff, flight and landing then you're not losing anything. There's no change in the work being done by the foil, just a change in how it balances. At worst you've increased the interference drag at the fuselage/mast joint, but I doubt the difference would even be measurable.

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eric

Since 13 Jan 2006
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PostMon Jul 16, 18 9:13 am     Reply with quote

Got it!
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