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Author Topic: Working Trim Tabs  (Read 2921 times)

6705russell

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Working Trim Tabs
« on: September 01, 2008, 09:07:23 AM »

Hi all,

I am looking to start on installing some kind of pneumatic set-up to operate the trim tabs on my 1/12 lifeboat, i saw a brief outline of this in one of the magazines using the compressed air chamber from a Technic Lego kit, as anyone got any ideas on this?

Many thanks

Russ
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RipSlider

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Re: Working Trim Tabs
« Reply #1 on: September 01, 2008, 10:17:54 AM »

I've also been playing with this idea becuase active trim tabs are just about *the* most efficient way to add speed to a Deep V hull if you want a fast boat. The Steven's Institute reckons on a 35% speed increase.

While I haven't got around to sorting out kit, there are a few things to think about>

1) Power: The trim tab will need to be pretty strong, and the system that supports them - and the section of the hull that they are attached to, will also need a good deal of strength. The tabs need to push the bouyant bow of the boat down, and this will need a good deal of force.

One mechanism I was thinging about was to to drive each tab from a 10kg servo. 2 would giv som factor of 20kg's of force, which might be enough.

2) speed: There would seem to be significant benefts in mixing the response rate of the trim tabs to the speed of the boat. forward speed will be proportianal to the number of times per minute that the trim tabs need to support the weight of the boat. The trim tabs can be viewed as dampers to an ossiclating motion. By keeping the damping at roughly the same rate as the motion-to-be-damped, there is a significan increase in stability to be gained

3) "Knowing whats happening". In order for the system to be "active", there needs to be a sensing mechanism to know when the boat is pitching, so as to activite the dampers. This system also needs to be proportional - you don't want the dampers at full arc when the boat has only tilted slightly. This - I think - can be done pretty simply with a Heading Lock Gyro - from the model Helicoper world - trned on it's side - it ill also give propertional response, and is cheap enough and very light.

4) Option for trnasverse damping. Active damping gives the biggest benefits - in terms of speed, stability and fuel saving, for straight ahead motion. However, there is also significant benefits in transverse damping. I.e if the boat tilts to port , the damping on that side kicks in to attempt to level it. Ths will give a minor sped increase, but a bigger stability increase, although this increase will be directly propertional to speed - the faser the boat is going, the better this mechanism will work.

Essentially it's just another heading lock gyro, pointing on a transverse axis, and in theory is easy. The issue is that there will rarely be 100% transverse tilts without some form of forwards tilt as well. At this point, you need a micro controller which has code to decide which "tilt" to give priority to controlling. Or built two sets of dampers.


Anywho, dampers are something I definately want to play with. But I've not yet worked out a "cunning plan" to make it work well.

If you do come up with a plan though, the rewards are pretty big though.


Steve
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RipSlider

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Re: Working Trim Tabs
« Reply #2 on: September 01, 2008, 02:34:55 PM »

Good lord.

That spelling is utterly awful.

My sincere and unreserved apologies to all who waded through the above.

I was going to post something else, but my ear's are red with shame now, and I've forgotten what it was.

Steve

<<edit - I remember now:

Don't be put off by the above. A couple of servo's, a mixer and a gyro will see you have a fully functional system. And the benefits are huge. When you get it tuned right, the boat will go an awful lot faster, but at the same time be an awful lot more stable. And the batteries will last longer as well. You should definately go for this! >>
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6705russell

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Re: Working Trim Tabs
« Reply #3 on: September 01, 2008, 03:30:41 PM »

Know kneed to apolojize   :D

I think the servo route is easier because with the pneumatic set-up the air tank is going need to be continually charged up with an air pump.

Russ


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johno 52-11

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Re: Working Trim Tabs
« Reply #4 on: September 01, 2008, 04:26:36 PM »

Hi Russ

What class of lifeboat are you building? My mate Robert has working trim tabs on his Severn Class Lifeboat that are controlled by a standard servo you can see some pic's of the difference it makes here [url][http://www.lifeboatmodels.co.uk/severnatspeed.htm/url].

he has all the rams connected together and a linkage to a servo.
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BJ

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Re: Working Trim Tabs
« Reply #5 on: September 01, 2008, 05:06:48 PM »

I can offer you TWO Bhuler 12v 80mA motors which rotate at 60 to 80 RPM and are geared at about 113:1. Put an eccentric on the shaft and that will be POSITIVE control for a trim tab. They have a small gear "locitit-ed" on but a little heat will solve that!
You cannot rotate these motors against that gear ratio.

If you want and are willing to cover the postage them send me a PM but do note that I have my bucket, spade and tickets in my hand so there will be a delay in getting them.
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grasshopper

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Re: Working Trim Tabs
« Reply #6 on: September 01, 2008, 05:15:39 PM »

I don't think that fore and aft pitch needs to be damped by  a gyro controlled system - speed (throttle position) related might be sufficient.
As for the need for a 10kg servo, the amount of angular deflection is quite small, therefore full travel on a standard servo and a suitable levering system would give the servo sufficient mechanical advantage.

Trim tabs are available from Graupner https://shop.graupner.de/webuerp/servlet/AI?ARTN=2342 and could probably be modified to be radio adjusted

An alternative trim tab that's being used for aero / hydrodynamic trimming on full size boats and aircraft is the Gurney flap - and that requires less mechanical effort to deploy.
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6705russell

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Re: Working Trim Tabs
« Reply #7 on: September 01, 2008, 07:47:46 PM »

Hi Russ

What class of lifeboat are you building? My mate Robert has working trim tabs on his Severn Class Lifeboat that are controlled by a standard servo you can see some pic's of the difference it makes here [url][http://www.lifeboatmodels.co.uk/severnatspeed.htm/url].

he has all the rams connected together and a linkage to a servo.

Hi there Johno, i am building the Arun, i have a seperate thread on the build, going slow at the minute with other builds on the go but progressing slowly, is there achance you may be ableget some pictures of the set-up?

Cheers

Russ
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DickyD

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Re: Working Trim Tabs
« Reply #8 on: September 01, 2008, 07:57:48 PM »

Hi Russ

What class of lifeboat are you building? My mate Robert has working trim tabs on his Severn Class Lifeboat that are controlled by a standard servo you can see some pic's of the difference it makes here [url][http://www.lifeboatmodels.co.uk/severnatspeed.htm/url].

he has all the rams connected together and a linkage to a servo.
Your link doesn't appear to work so how about this one  http://www.lifeboatmodels.co.uk/severnatspeed.htm
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johno 52-11

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Re: Working Trim Tabs
« Reply #9 on: September 02, 2008, 03:39:55 PM »

Hi Russ

What class of lifeboat are you building? My mate Robert has working trim tabs on his Severn Class Lifeboat that are controlled by a standard servo you can see some pic's of the difference it makes here [url][http://www.lifeboatmodels.co.uk/severnatspeed.htm/url].

he has all the rams connected together and a linkage to a servo.
Your link doesn't appear to work so how about this one  http://www.lifeboatmodels.co.uk/severnatspeed.htm

Thanks DickyD

Looks like i got the cursor in the wrong place when I pasted the address in  :embarrassed: :embarrassed: :embarrassed: :embarrassed:
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BJ

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Re: Working Trim Tabs
« Reply #10 on: September 02, 2008, 04:29:53 PM »

If anyone wants to read about trim tabs then go to http://www.lectrotab.com/     and their link is                                 www.lectrotab.com/Tab%20Force%20Formula/TabForceFormulaRev081606.pdf - for LectroTabs TRIM TAB DEPLOYMENT FORCE CALCULATIONS.
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