Author Topic: Shimming 400 tail pics  (Read 1191 times)

headmount

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Shimming 400 tail pics
« on: October 24, 2021, 07:07:17 PM »
I don't know about you but I get confused when people use terms like positive and negative angles.  I relate more to; lift the leading edge or lift the trailing edge. 

Here's how the 400 rear wing seats into the black fuse with no shim.  As you can see, even with the screws tightened down there a gap toward the rear, which seems to whistle, which bugs me as I go where I don't want to be drawing attention.

Soo... to lift the leading or trailing edge , that is the question... with a 999 and short fuse. 

Second shot is with a single layer of plastic I harvested off a big empty green container for kitty litter, this eliminated the gap toward the rear back half of the fuse/rear wing connection and eliminated the noise.  Hard to tell if this lifted the leading edge or not.  It appeared like it just created a better seat and with the shim being equal throughout the length of the attachment , would the leading edge be the same as no shim?

Third shot is with a double layer toward the front half (still single in the rear) which I think is lifting the leading edge of the stabilizer up a tad.

I didn't do a rear shim yet.  But it's pretty easy once you have a generous out of the shim.  Any excees you can trim by drawing a razor blade along the side with the top and bottom being a guide. 

headmount

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Re: Shimming 400 tail pics
« Reply #1 on: October 24, 2021, 07:15:03 PM »
Here's my 400 with the turned up tips (not the HA400).  It seats perfectly like the Incan stones at Machu Picchu.

headmount

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Re: Shimming 400 tail pics
« Reply #2 on: October 24, 2021, 11:04:54 PM »
A clearer shot of the single shim throughout.  After I thought about it I think it does lift the leading edge more than the no shim pic. 
« Last Edit: October 24, 2021, 11:17:49 PM by headmount »

Hdip

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Re: Shimming 400 tail pics
« Reply #3 on: October 25, 2021, 08:20:49 AM »
You'll know when you ride it. If you lift the rear edge of the stabilizer you get more lift. So in the scenario you just laid out, you are lowering the rear edge of the stabilizer which will mean less lift. I find those kind of things are easier to go by feel as my eye isn't accurate enough to judge the angles.

headmount

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Re: Shimming 400 tail pics
« Reply #4 on: October 25, 2021, 09:23:00 AM »
If you lift the rear edge of the stabilizer you get more lift.
You get more lift in the front foil, not the stabilizer.  The rear stabilizers are set to provide some downward force on the rear end of the fuselage which causes the front wing to gain more upwards angle of attack and lift.   The downward stabilzer angle comes at the expense of increased drag. The Axis minus 1 shim reduces the downward angle of the rear stabilize, reducing drag.  It's a matter of degrees and between minus 1 to -.5 is probably optimum.  My challenge is to find out how much I've changed the angle (up in this case).
« Last Edit: October 25, 2021, 09:27:31 AM by headmount »

Califoilia

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Re: Shimming 400 tail pics
« Reply #5 on: October 25, 2021, 10:16:42 AM »
If you lift the rear edge of the stabilizer you get more lift.
You get more lift in the front foil, not the stabilizer.  The rear stabilizers are set to provide some downward force on the rear end of the fuselage which causes the front wing to gain more upwards angle of attack and lift.   The downward stabilzer angle comes at the expense of increased drag. The Axis minus 1 shim reduces the downward angle of the rear stabilize, reducing drag.  It's a matter of degrees and between minus 1 to -.5 is probably optimum.  My challenge is to find out how much I've changed the angle (up in this case).
I agree that the pluses and minuses labeling of the various shims can be confusing between foil companies, mainly because some companies' tails mount on the top of the fuse, and some mount them on the bottom.

Whether it's actually correct or not, I simply look at them as either adding or subtracting 1o of lift to the entire system's flight att.

So if/when I want less in the system I use a -1, and when I need more lift I throw on a +1 shim. Since I believe you have a 3D printer, the links are for the downloadable files so you can make some shims, and know exact degrees you're putting in or taking out with those shims.

Edit: Also, from the pics of your tail with the green shim you made, it has lifted the leading edge of the tail, and you now have a gap there between the tail and the fuse, and I think you'll get even more whistle from that than you would from the tiny original trailing gap you showed. That's just a WAG though going off of what I've had with some test tails we were playing with that didn't mount flush at the leading edge of the tail to the fuse very similar to that.
« Last Edit: October 25, 2021, 10:23:08 AM by Califoilia »
Me: 6'1"/185...5'1" Kings Foil Board...5'7" Kings Foil Board...9'6" Bob Pearson "Laird Noserider"...14' Lahui Kai "Manta"...8'0" WaveStorm if/when the proning urges still hit.

jondrums

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Re: Shimming 400 tail pics
« Reply #6 on: October 25, 2021, 10:55:06 AM »
counter-intuitively - if you lift the rear edge of the stabilizer - you get LESS overall lift.  But, you do move the "lifting center" further forward, so this FEELS like more lift if you don't move your foil back in the box or your feet forward on the board.

The reason I say you get LESS lift is that you've just increased the DOWN-FORCE on the back of the wing, which is the opposite of more lift.  The net total lift of the system has gone down.  That's why you feel more drag when you do this.  But you also get more stability because the lifting center is further forward of the center of effort.  Sorry for the semantics, but to me it matters.

headmount

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Re: Shimming 400 tail pics
« Reply #7 on: October 25, 2021, 11:30:07 AM »

Since I believe you have a 3D printer, the links are for the downloadable files so you can make some shims, and know exact degrees you're putting in or taking out with those shims.

Edit: Also, from the pics of your tail with the green shim you made, it has lifted the leading edge of the tail, and you now have a gap there between the tail and the fuse, and I think you'll get even more whistle from that than you would from the tiny original trailing gap you showed.
You mean this part right up at the front fuse/leading edge connection?  (pic) I don't have a 3-D printer so it is guess work.  I can make a cleaner shim up at that top little gap at the tip of the leading edge and close that gap.  In iPhoto I took some grid pic to try to compare the angle changes. That pic is with a double shim in front half.

Dwight (DW)

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Re: Shimming 400 tail pics
« Reply #8 on: October 25, 2021, 12:15:32 PM »
Itís possible the noise is not from the fit with the saddle.

I use a bread bag clip as a shim. Talk about gaps and poor fit!  Yet my rear wing is still totally silent.

Was experimenting with a tail shim per Kane Dewildes advice on the podcast. Shim more for taller mast, less for shorter mast, to maintain same perfect balance for all.  Best 96cm session ever. Although 1 mph slower shimmed for the 96. Much prefer perfect trim over speed. 86cm works perfect no shim.






Dontsink

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Re: Shimming 400 tail pics
« Reply #9 on: October 25, 2021, 03:00:38 PM »
Itís possible the noise is not from the fit with the saddle.

I use a bread bag clip as a shim. Talk about gaps and poor fit!  Yet my rear wing is still totally silent.

Was experimenting with a tail shim per Kane Dewildes advice on the podcast. Shim more for taller mast, less for shorter mast, to maintain same perfect balance for all.  Best 96cm session ever. Although 1 mph slower shimmed for the 96. Much prefer perfect trim over speed. 86cm works perfect no shim.

That podcast is gold.
It makes sense, taller mast means a bigger "lever arm" from center of thrust (wing) and center of drag (foil).

headmount

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Re: Shimming 400 tail pics
« Reply #10 on: October 25, 2021, 11:23:14 PM »

The reason I say you get LESS lift is that you've just increased the DOWN-FORCE on the back of the wing, which is the opposite of more lift.  The net total lift of the system has gone down.  That's why you feel more drag when you do this. 

But you also get more stability because the lifting center is further forward of the center of effort.  Sorry for the semantics, but to me it matters.
Your first part confirmed what I thought when I read Hdip's post.  But this second part I'm not familiar with; lifting center and center of effort.  Please elaborate.  Thanks.

 


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