Author Topic: T Foils  (Read 55840 times)

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Offline Will_Lee

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Re: T Foils
« Reply #30 on: February 18, 2008, 05:42:58 PM »
Stand by for some ASCII Art!

If you sail with the rudder loaded (ie a bit of weather helm), then there is high pressure on the leeward side of the board and low pressure on the windward side:

(looking from the bow, with the boat on Stb tack)


     Low   Rudder    High
____________________________
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
                  V


Imagine a T foil on it's own in the water (not attached to anything): When it is loaded, there is low pressure above and high pressure below:

Low Pressure     -  -  -  -  -  -  -  -  -  -  -  -

               T foil    =================
 
High Pressure     +  +  +  +  +  + +  +  +


Now put the two together and you get:

____________________________
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      --         | |          0
--  --  --  -- | | 0  0  0 0  0  0  0
                 | |         
 =================
 
  +  +  +  +  +  + +  +  +

The minuses combine to become double-minuses on the windward side, but on the leeward side they minuses and the pluses cancel out meaning less lift from the foil (and less lift from the rudder too I think).

The major problem of this is the large twisting load on the junction of the T foil and the rudder.

This problem can be eliminated by moving the T foil up the rudder a bit. (About 300mm? I don't know, sorry). This leaves this situation:

      -          | |          +
      -          | |          +
      -          | |          +
      -          | |          +
      --         | |         0
 --  --  --  --| |  0  0  0  0  0
================
 0 0 0 0 0  | | ++ ++ ++ ++
      0          | |          ++
      -          | |          +
      -          | |          +
      -          | |          +

Here you can see that the extra low pressure on one side is balanced by the extra high pressure on the other: No twist!

Other solutions to the twisty problem are:

1) Build the T foil really reallly well.
2) Don't load the rudder too much.

Hope that lot helps,

Will



Offline Phil Alderson

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Re: T Foils
« Reply #31 on: February 20, 2008, 07:55:03 PM »
I have been trying to figure out how far along the T-Foil the influence of the rudder goes,

I have not found anything useful yet other than Biplane wings should be at least one chord width apart.

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Offline Will_Lee

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Re: T Foils
« Reply #32 on: February 20, 2008, 11:08:32 PM »
Kevin is your man for questions of that kind.....

Offline Ghislain Devouthon

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Re: T Foils
« Reply #33 on: February 21, 2008, 11:19:41 PM »
Old .... and new rudder.

Now I just need to decide where to put the T foil ?
I don't want to put it at the tip for obvious structural reasons.

Should the T foil be at the front on the leading edge or at the back on the trailing edge ?

The rudder is about 16 cm wide while the foil is 12.
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Offline Will_Lee

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Re: T Foils
« Reply #34 on: February 22, 2008, 08:09:56 AM »
Nice looking new rudder!

I think that from a hydrodynamic point of view it doesn't make too much difference, however if the T foil is further forward then you can load it up more. Also it is probably a good idea to have the thickest part of the rudder and the thickest part of the T foil intact, so a short slot on the back of the T foil and a short slot on the front of the rudder would allow you to fit the two together neatly, while preserving the thickest (strongest and stiffest) part of both.



   

 
« Last Edit: February 26, 2008, 08:13:27 PM by Will_Lee »

Offline kevin_ellway

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Re: T Foils
« Reply #35 on: February 26, 2008, 07:34:32 PM »
I've been doing a lot of CFD analysis of T foils, centreboard and rudder interactions and on planing hull trim. So here is a quick summary of findings

1) T foil sections
=============

Best to use asymmetric section with about 10% t/c. Speer H105 is a good section because it gives low drag a zero lift and has an extended drag bucket at higher lifts.

2) T foil planforms
===============
Assuming you don't want a span >900mm, use an approx. ellipse. You can get approx elliptical loading by having a trapezoid with a taper ratio (tip/root) of around 0.5. Make tips rounded.

3) Position on rudder
================
The T foil lift is of no benefit below about 6kts and so should be a zero lift below 6kts boat speed - see Y&Y article for reason.
If your boat is in bow down trim at say 7kts, best place is as close to the water surface as possible to take advantage of the upward flow off the bottom of the hull. Most cherubs are too fat in the bows to do this and start to adopt a bow up trim. One this happens, the near surface advantage is lost.

My thoughts are that it is more practical and easier to fix T if it is at bottom - makes launch and recover easier. See also 4)

4) Aspect ratio of rudder blade
========================
Given the normal ratio of rudder to CB sizes, the foils will give best lift to drag when the rudder is unloaded (i.e. practically neutral helm when boat is upright).

The trend on I14s for long skinny rudders makes no sense if the rudder is only lightly loaded. This is because only the induced drag (the drag caused directly by producing lift) is reduced by aspect ratio. If the rudder is lightly loaded, then the induced drag is very small.

For control at low speeds, pre start moves, and general control, you can do for a wider chord (25 cm or so) and a short blade. If you stick the T on this, it will be pretty near the surface anyway.

5) Using a T foil
=============
For any given speed, a hull will have an optimum fore and aft trim angle that will give least drag. It is important that this is maintained when using the T foil rudder. When at planing speeds, too much trim will be less detrimental to speed than too little, so don't pull the T on so that the bow is in the water. You must be able to place your weight sufficiently far aft to maintain the same optimum trim as you would without the T.

6) Notes on optimum hull trim
========================
When a boat is planing, the optimum trim angle depends upon the lift coeff of the hull's bottom. If your boat has a fair bit of chine rise in the back, trim will be around 7-8 degrees . For a real flatty like the Dog, it will be nearer 5 degrees.

Zero lift on a symmetric foil is at zero to the water flow. So on the Dog this would be at about -5 degrees to the boat. Zero lift for an asym foil is about -3 degrees, so this would be a -8 degrees on the Dog.

In practice, you will never use a T foil a negative lift.


7) Notes on nosediving
==================
If your boat is nosediving or sailing too bow down with the T foil fitted compared to when it is not fitted, then the foil is producing lift.

The balance of a Cherub offwind is a precarious one because of the ratio of rig height to boat length. The C of E of the rig with the kite is about 45% of the way up the mast. This produces a moment trying to pitch the bow down. You try to counter this by moving aft.

If the drive from the rig (with kite) was horizontal, practically any Cherub would become unsailable as the wind increased. Fortunately, the drive is not horizontal. If you have a long pole and a highly raked mast like on Atum the drive is upwards at about 34 degrees to the horizontal. This reduces the pitch down moment and lightens the boat, as does the T foil. (See WB sails website article on Lifting the bows of a Cherub)

If your boat is really divey, then you need to increase the rake, increase the pole length and lengthen the wheelie bars.  If you imagine the line of the drive vector from the rig and extend this line backwards, it will eventually cross the line of the boat at sheer level. If, where it crosses, is in front of where you can stand (lots of rake and long pole), then the boat will never dive. If it's well behind you (short pole, upright mast) you'll be sailing a sub.

I hope these comments help.

Offline Phil Alderson

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Re: T Foils
« Reply #36 on: February 26, 2008, 09:12:22 PM »
Excellent comments Kevin Thanks for those.

Regarding the fore and aft position I have heard that you should avoid having the high pressure zones of the two foils in the same place, so you should either put the T back behind the thickest section of the main foil, or sticking out the front so the main foil is aft of the thickest point on the T

Does this make any sense or is it just a myth?
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Offline phil_kirk

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Re: T Foils
« Reply #37 on: February 27, 2008, 12:44:58 PM »
Thank you Kevin,

I believe that when you talk about trim angles  you are assuming that everything is  angled relative to the water surface.  In practice as the boat trims up on to the plane downwind and accelerates the helm (grinnning from ear to ear) is not thinking about re trimming the t foil so that it is at zero degrees and may sail with the foil at the 5-7 degrees trim up angle of the boat. So in practice the helm has to re-trim the foil at a negative angle to the boat to avoid going swimming.

I like the logic on length of spinny poles and rig rake to surviving downwind.

Slippery (a dog) with her new longer pole feels more controlable down wind than before.  I haven't used the new wheelie bars yet.

Offline kevin_ellway

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Re: T Foils
« Reply #38 on: February 27, 2008, 05:31:34 PM »
Thank you Kevin,

 :)No problem

I believe that when you talk about trim angles  you are assuming that everything is  angled relative to the water surface.  In practice as the boat trims up on to the plane downwind and accelerates the helm (grinnning from ear to ear) is not thinking about re trimming the t foil so that it is at zero degrees and may sail with the foil at the 5-7 degrees trim up angle of the boat. So in practice the helm has to re-trim the foil at a negative angle to the boat to avoid going swimming.

 :)Sort of. If you consider the axis of the boat to be the line between the base of the transom to the base of the stem, this will trim up by about 6 -8 degrees when you plane. So if you have a symmetric foil it will look to be a say -7 degrees relative to this line to get zero lift. Note also that an asymmetric foil produces zero lift when it is about -3 degrees to the water flow. So in above example, you'd need the foil at say -10 to boat to get zero lift.

You never want to have the foil pulling the back of the boat down. This just creates more drag, so the pitch down force of the rig increases and you go really slow!

I like the logic on length of spinny poles and rig rake to surviving downwind.

 :)It really works both mathematically and practically. If you have enough rake to the rig as a whole, the boat will go bows up as it goes up a wave increasing the rake and thus lift still further. This puts the line of force in front of the boat's cofg. The boat will then float (rather than crash) down as you go over the peak. That's what 18s and 12s do.

Slippery (a dog) with her new longer pole feels more controlable down wind than before.  I haven't used the new wheelie bars yet.

The solent is really choppy. D Beans was a real nose diver until I increased the pole length and in particular, added the wheelies. You could also try a shorter gantry to bring the lift from the T foil farther forward.

The big problem with pre 97 boats is not really the rocker, but the amount of planing surface - it's too big and the bows are too fat. So when the bows go in, the wetted surface shoots up, the resistance increases, the boat slows and the pitch down from the rig increases, pushing the bows down still further and so on, until the stern overtakes the bow. I actually have some software that simulates this.


Offline phil_kirk

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Re: T Foils
« Reply #39 on: February 28, 2008, 01:30:44 PM »
Thanks again for the really clear explanations. 

We sail slippery at quite a low combined crew weight 115-120 kg so the boat planes quite early.  We are still  getting used to sailing in breeze so probably haven't needed to get so far back yet.  I haven't fitted a T foil yet but your post above will help us choose our final soluiton.

We haven't really sailed the boat in much chop so I suspect the wheelie bars will get some use at Weston if there is some breeze.

Offline Ghislain Devouthon

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Re: T Foils
« Reply #40 on: March 04, 2008, 08:53:05 AM »
And now the finish product.  ;D

Please note : also Bilbo is on the background, I haven't painted him  ;).
« Last Edit: March 04, 2008, 08:54:59 AM by Ghislain Devouthon »
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Offline Will_Lee

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Re: T Foils
« Reply #41 on: March 04, 2008, 02:32:31 PM »
Shiney! I like the broadened roots of the T foil.

W

Offline Tim Noyce

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Re: T Foils
« Reply #42 on: March 04, 2008, 08:00:21 PM »
TRES BIEN!!  ;D

Offline Stuart Hopson

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Re: T Foils
« Reply #43 on: March 04, 2008, 10:59:08 PM »
Very nice! What do we think the optimum span of the T should be? as i'll be cutting down the prototype Atum Bom uber wide special soon.

Offline ross_burkin

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Re: T Foils
« Reply #44 on: March 04, 2008, 11:23:10 PM »
Surely that depends on the shape, thickness and depth?
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