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Author Topic: Rudder control  (Read 2492 times)

tassie48

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Rudder control
« on: April 23, 2018, 12:22:58 am »

I just read while doing research on my WW2 1/24th scale PA-2 German Flower class Corvette that the rudder had a rating of 321 in Tons Going Ahead and 349 in Tons Going( Taken from Astern from the Anatomy of the Flower Class Corvette Agassiz book) which is a hell of a lot of pressure and got me thinking I always use twin arms on my rudder to servo linkages one to push and the other to pull then looking at other model I see a lot just using a single arm to the servo my early model a twin arm way back in the 1960s with old Macgregor rc gear I asked the guy I purchased the model from and he said that it makes sense to keep even tension on the servo as a free horn on the servo is wasted ,and having both arms working is a no brainer to use .
He called it value for money engineering so I have for the past 49 years used this system and will always do so I understand on some models space is the issue and a single arm is need just would like to no if other modelers use the two arm system like me from a seed planted when I was a kid to today still using the two arm servo system tassie48     
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derekwarner

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Re: Rudder control
« Reply #1 on: April 23, 2018, 01:43:14 am »

tassie....this is a very complex issue, however simplified.......

'the mechanics of the rudder system is such that it could withstand an opposing force of approximately 1/3 of the vessels [weight] displacement....and within these limits would act as a rudder'

This being [in round figures] a Corvette of ~~1000 tons, and the rudder structure resistance ~~300 tons   

So if the Corvette were helmed at speed greater than the limit, the vessel hull would commence to be pushed sideways without steerage  :((

Fully agree, for greater mechanical reliability, twin rods between the servo and the rudder........during a turn, one rod is in compression & the other in tension & visa versa in the opposite direction........balance of forces  :-))

Derek
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Derek Warner

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Martin (Admin)

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Re: Rudder control
« Reply #2 on: April 23, 2018, 04:12:44 am »


 ..... never ceases to amaze me what you can learn here on Mayhem!

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Peter Fitness

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Re: Rudder control
« Reply #3 on: April 23, 2018, 06:42:55 am »

Some of my early model boats have one arm to the rudder, the later ones have two, not because of any engineering knowledge, I just thought it would be better to have two.


Martin, I've learnt heaps of stuff on Mayhem, and elsewhere, my biggest problem is remembering it :embarrassed:


Peter.
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Bob K

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Re: Rudder control
« Reply #4 on: April 23, 2018, 07:50:13 am »

We do indeed learn much on Mayhem.  I had never previously considered how much force the rudder is subjected to, and my previous smaller boats have only had a single arm / rod connected to the server. Never seen a problem, but that maybe shows how good the servo / linkage system works under load.

For my current 29kg displacement ship I was advised to use two arms /rods, which I did.
This also goes to show the importance of using an HD servo with metal gears on larger boats.
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roycv

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Re: Rudder control
« Reply #5 on: April 23, 2018, 08:18:31 am »

Hi, very interesting, never thought of the displacement of the boat as associated with the forces acting on the rudder.  I just thought of the speed of the water from the propeller and the faster this was the smaller the rudder blade area.  I have experimented with a handheld motor in a bowl of water and used a knife blade in the propellor wash to simulate a rudder, I was surprised at the forces involved.

I usually make my own model boat rudders and normally go for 1/3rd. of the rudder area ahead of the rudder stock, does this small balancing effect come into play?

What about the conditions for going sideways in a model.  Does this happen?


regards Roy
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derekwarner

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Re: Rudder control
« Reply #6 on: April 23, 2018, 10:14:40 am »

Here is an example of a vessel with the helm to port.......but skating toward the stdb water

Clearly this US Carrier has not lost guidance......but just on a little larger plotted turn to port than the computer controlled  :-X

So if you exceed the design limits , the rudders will not fall off.......but just not act beyond their limits

Derek

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roycv

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Re: Rudder control
« Reply #7 on: April 23, 2018, 11:03:53 am »

Hello I look at your picture and I would think that this is a case of a large mass being turned, but the momentum of the ship is stiil going in an onward diresction. This would suggest the boat is going sideways but it is more a case that the rudder is never going to direct all that ship's momentum in another direction until the screws start to exert backward forces in the new direction.
I am happy to be corrected here.
It is an interesting area I had not considered before.
regards Roy
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John W E

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Re: Rudder control
« Reply #8 on: April 23, 2018, 11:22:49 am »

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Peter Fitness

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Re: Rudder control
« Reply #9 on: April 24, 2018, 06:36:31 am »


I usually make my own model boat rudders and normally go for 1/3rd. of the rudder area ahead of the rudder stock, does this small balancing effect come into play?

regards Roy


I believe it does Roy. I'm no engineer, but I think having such a rudder may reduce the load on the servo to a degree, and make the rudder more effective. I'm open to contradiction, however. I also make all my own rudders, as well as prop shaft assemblies.


Peter.
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dreadnought72

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Re: Rudder control
« Reply #10 on: April 24, 2018, 06:45:58 pm »

You're right, Peter. Hopefully there's a diagram attached to this.  :embarrassed:

Water's flowing vertically up the page in each section, the bow is pointing dowards.

In 1., we've got 30 degrees port rudder. As water is being forced to change direction by the rudder (to flow off the top-right of the diagram), it's been accelerated: mass x acceleration is a force, and in this case we call the force lift. Force is a vector, and it can be broken down into constituent vectors any way we like: I'm going for drag directly along the keel of the boat (a force slowing us down), and, at a right angle to this, sideforce (a force attempting to pull the rudder to the side). These three vectors make a right-angled triangle.

2. We have a balanced rudder, with some of the rudder in front of the rudder stock. The rudder generates lift, sideforce and drag from both sections of the rudder. It's not a very well balanced rudder, as the bit aft of the rudder stock does most of the work. These two sections are joined by a red line going through the rudder stock, and I've added a green line measuring the distance between the drag vectors, and a blue line for the distance between the sideforce vectors.

3. Looking at the drag, the total amount of drag remains the same (adding the two drag vectors nose-to-tail would equal the overall drag vector in diagram 1. But we've made a sort of see-saw (two moment-arms for the physicists) - while the overall effect of the drag is a torque trying to rotate the rudder anticlockwise, the torque from the forward section of the rudder counteracts this to some extent.

4. Sideforce: again the total amount is the same. This is, of course, the force shoving the back of the boat sideways. And, again, there's a tendency in this diagram for the rudder to rotate anticlockwise, countered somewhat by the sideforce created by the forward section of the rudder.

So ...

Balanced rudders reduce the torque on the rudder, and consequently reduce the force required to turn them in the first place = good engineering practice.

And if our rudder linkages snapped - there was suddenly nothing to counteract these two torques on the rudder - the balanced rudder would return to a centred position.

One of the risks with an over-balanced rudder (that's one where the forces on the front of the rudder are greater than the rear) is that it's inherently unstable. Whether steering port or starboard, the torque is continually trying to drive the rudder away from its normal, centered position. More work for a servo, more risk of an incident.

Andy




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roycv

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Re: Rudder control
« Reply #11 on: April 24, 2018, 07:10:03 pm »

Hi thanks for taking your time on this.  Just a small point but a rudder area just aft the rudder stock only operates in half of the prop wash where as a balanced rudder goes into the other part of the prop wash.  Will there not be more forces acting on the rudder?
I originally constructed semi balanced rudders as back then, on dry batteries, the prop wash was much smaller and I wanted to have greater control of the model boat.
regards Roy
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dreadnought72

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Re: Rudder control
« Reply #12 on: April 24, 2018, 08:24:54 pm »

...a rudder area just aft the rudder stock only operates in half of the prop wash where as a balanced rudder goes into the other part of the prop wash.  Will there not be more forces acting on the rudder?


Yes, Roy - and the bit nearest the prop will be proportionally more affected by the propwash: so maybe the one-third rule is a good rule of thumb.


Andy
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