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Where to shim?

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Horst Sergio
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Re: Where to shim?

Postby Horst Sergio » Wed Nov 06, 2019 8:57 am

corrected my recommendation, thanks to flying grandpas next post
matanshapira wrote:
Wed Nov 06, 2019 6:26 am
You mean you put a shim under the front screw of your front wing? This makes the lift go a bit towards the back, right? Wouldn't it increase the drag?
As your wing is on the downside of fuselage you would have to add the shim behind the backscrew, but then how you said both right. And the extra drag will compensate a bit the other effect of moving back the pressure point, so I just would recommend this if you love drag and want to go slower. :wink: And I think the foot placement will not change so much as maybe needed.

And to add this:
I would drill a new hole also through the LF carbon fuselage. As both holes should have around 6 cm distance I think it would be enough to add just another hole in the same distance towards frontwing and then go with the strut one step forward as you already and originaly can do with the stabilizer. I don't think that even the LF carbon fuselage is such a piece of construction art, that the new hole will affect the stability more than the existing holes already do. Just be sure to do it well with proper tool.

If you additionally lower just slightly the stabis negative pitch (shim about 0,3 mm behind back screw, here you may use a plastic piece), you will end up in a very easy way with about 8 cm foot stance towards back. If you then are able to go a bit faster you can add another 2-4 cm :D

@Kamikuza: Reading the questions and the other answers before answering yourself could increases the precision of your own answers :wink:
Last edited by Horst Sergio on Wed Nov 06, 2019 9:14 am, edited 1 time in total.

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Re: Where to shim?

Postby flying grandpa » Wed Nov 06, 2019 9:02 am

matanshapira wrote:
Wed Nov 06, 2019 6:26 am
You mean you put a shim under the front screw of your front wing? This makes the lift go a bit towards the back, right? Wouldn't it increase the drag?
Yes, if you are mounting your front wing above fuse.
Use hard foam 3 mm thick, sanded on one end more to taper it.
At high speed drag is higher, so max speed is lower (I prefer lower max speed when wave riding)
But low end may be even better due to lower fuse drag (it works at lower AoA then).

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Re: Where to shim?

Postby Peter_Frank » Wed Nov 06, 2019 10:14 am

matanshapira wrote:
Wed Nov 06, 2019 6:26 am
flying grandpa wrote:
Wed Nov 06, 2019 12:15 am
I simply shim in:
4. Front wing - fuselage connection.
This reduces front pressure, improves pitch agility a bit, reduces max speed a bit. Usually there is no need for other adjustments (1; 2 or 3)

You mean you put a shim under the front screw of your front wing? This makes the lift go a bit towards the back, right? Wouldn't it increase the drag?

flying grandpa is correct, this is an alternative way, to obtain the same as 1+2.
So choose what is the easiest for you, it will end up the very same.

BUT, you need to shim the front wing, AND the rear wing, to have unchanged stability, and to move the trim a significant amount.

If you shim the front wing up, and the stabilizer up the same amount, you get the same effect as 1+2, meaning less front foot pressure, and unchanged stability :thumb:

I dont recommend only shimming the front wing, as most foils today are trimmed in the sweet spot I believe, and having a stabilizer that pulls too much down (which in old terms should be more stable) is a PITA :o and IMO not more stable, but tend to give kangaroo rides and problems when changing speed.

Also, if only shimming the front wing, you wont move the "trim balance" much, like you would if you shim both, because the stabilizer will increase the front foot pressure, if only the front wing is shimmed.
You are also right matanshapira, it will increase drag a bit, and you will lose lift and start a tad later when the stabilizer pulls more down, if you only shim the front wing. But you can not feel this small change IMO.

But shim the front wing and the stabilizer the same amount is my advice, if easier than shimming fuselage-mast-board :D

8) Peter

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Re: Where to shim?

Postby revhed » Wed Nov 06, 2019 8:53 pm

matanshapira wrote:
Wed Nov 06, 2019 6:33 am
Those carbon fuselages from LF are not accurate at all and I already noticed that there's a twist when I bought it but they said it's within the acceptable range.
So I wonder what Liquid Force considers acceptable fuse twist tolerance? :roll:
At least for me and most I know, if by sighting the F wing and stab from level, board inverted on ground, either from front or behind, if you can see not in plane and perp to the STRUT, then not ok!
R H

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Re: Where to shim?

Postby matanshapira » Wed Nov 06, 2019 10:09 pm

revhed wrote:
Wed Nov 06, 2019 8:53 pm
matanshapira wrote:
Wed Nov 06, 2019 6:33 am
Those carbon fuselages from LF are not accurate at all and I already noticed that there's a twist when I bought it but they said it's within the acceptable range.
So I wonder what Liquid Force considers acceptable fuse twist tolerance? :roll:
At least for me and most I know, if by sighting the F wing and stab from level, board inverted on ground, either from front or behind, if you can see not in plane and perp to the STRUT, then not ok!
R H
When I put it on the ground, 1 of the 4 wing tips is not touching the ground, LF said: "The heads are glued into the fuselage by hand, so there will always be a slight twist.". When I put the fuse itself on a straight surface it appears aligned. I'm really not sure how to verify that the fuse is not introducing any change of angles with home tools.

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Re: Where to shim?

Postby matanshapira » Wed Nov 06, 2019 10:13 pm

Peter_Frank wrote:
Wed Nov 06, 2019 10:14 am
matanshapira wrote:
Wed Nov 06, 2019 6:26 am
flying grandpa wrote:
Wed Nov 06, 2019 12:15 am
I simply shim in:
4. Front wing - fuselage connection.
This reduces front pressure, improves pitch agility a bit, reduces max speed a bit. Usually there is no need for other adjustments (1; 2 or 3)

You mean you put a shim under the front screw of your front wing? This makes the lift go a bit towards the back, right? Wouldn't it increase the drag?

flying grandpa is correct, this is an alternative way, to obtain the same as 1+2.
So choose what is the easiest for you, it will end up the very same.

BUT, you need to shim the front wing, AND the rear wing, to have unchanged stability, and to move the trim a significant amount.

If you shim the front wing up, and the stabilizer up the same amount, you get the same effect as 1+2, meaning less front foot pressure, and unchanged stability :thumb:

I dont recommend only shimming the front wing, as most foils today are trimmed in the sweet spot I believe, and having a stabilizer that pulls too much down (which in old terms should be more stable) is a PITA :o and IMO not more stable, but tend to give kangaroo rides and problems when changing speed.

Also, if only shimming the front wing, you wont move the "trim balance" much, like you would if you shim both, because the stabilizer will increase the front foot pressure, if only the front wing is shimmed.
You are also right matanshapira, it will increase drag a bit, and you will lose lift and start a tad later when the stabilizer pulls more down, if you only shim the front wing. But you can not feel this small change IMO.

But shim the front wing and the stabilizer the same amount is my advice, if easier than shimming fuselage-mast-board :D

8) Peter
If I shim both front and rear wings, it sounds that it'll change the angle of the board, I will have the nose pointing down(undesirable). So I guess you meant that I can shim the front&back wings and also the mast-board connection to create a swept-back mast. Am I right?
Last edited by matanshapira on Wed Nov 06, 2019 10:26 pm, edited 1 time in total.

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Re: Where to shim?

Postby matanshapira » Wed Nov 06, 2019 10:22 pm

Horst Sergio wrote:
Wed Nov 06, 2019 8:57 am
corrected my recommendation, thanks to flying grandpas next post
matanshapira wrote:
Wed Nov 06, 2019 6:26 am
You mean you put a shim under the front screw of your front wing? This makes the lift go a bit towards the back, right? Wouldn't it increase the drag?
As your wing is on the downside of fuselage you would have to add the shim behind the backscrew, but then how you said both right. And the extra drag will compensate a bit the other effect of moving back the pressure point, so I just would recommend this if you love drag and want to go slower. :wink: And I think the foot placement will not change so much as maybe needed.

And to add this:
I would drill a new hole also through the LF carbon fuselage. As both holes should have around 6 cm distance I think it would be enough to add just another hole in the same distance towards frontwing and then go with the strut one step forward as you already and originaly can do with the stabilizer. I don't think that even the LF carbon fuselage is such a piece of construction art, that the new hole will affect the stability more than the existing holes already do. Just be sure to do it well with proper tool.

If you additionally lower just slightly the stabis negative pitch (shim about 0,3 mm behind back screw, here you may use a plastic piece), you will end up in a very easy way with about 8 cm foot stance towards back. If you then are able to go a bit faster you can add another 2-4 cm :D

@Kamikuza: Reading the questions and the other answers before answering yourself could increases the precision of your own answers :wink:
Wow, that's brilliant(adding another hole in the fuse)! I checked it and it'll fit well.
I'll try to shim the back wing on Sat session. Will start with 1mm(you wrote 0,3 mm, perhaps a mistake?)

All: this is a tremendous help and you created a great thread with insight and information that are very hard to find. It'll serve many foilers in the future.

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Re: Where to shim?

Postby Peter_Frank » Thu Nov 07, 2019 7:24 am

matanshapira wrote:
Wed Nov 06, 2019 10:13 pm
If I shim both front and rear wings, it sounds that it'll change the angle of the board, I will have the nose pointing down(undesirable). So I guess you meant that I can shim the front&back wings and also the mast-board connection to create a swept-back mast. Am I right?

Correct, shimming both wings, if supereasy, could be done to test that foot position trim is spot on.

And then you have to shim the board later, most likely.

Some flat boards have an odd "up" angle so you stand with your front foot bent - but if board too flat (no scoop) it might be necessary.

Can be solved by having the nose point a bit more down (meaning you might not need to shim, if you got one of these boards), but on cost of touchdown ease probably :wink:

Moving the mast position on the board is most likely the very best solution though, if possible :thumb:

8) Peter

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Re: Where to shim?

Postby Wazza Foil » Thu Nov 07, 2019 8:40 am

Shimming in a different way can simply be if you apply pressure with your toes, heels or flat feet or in straps or not.


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