Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: Martin (Admin) on September 01, 2008, 09:02:41 pm
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Why are model tiller shaped this way?
(http://www.modelboatmayhem.co.uk/forum/index.php?action=dlattach;topic=9014.0;attach=54103;image)
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So that if it goes too far the servo can still push it back around.
Glad to be of assistance O0
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Why aren't they just straight?
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duno maybe its to give more leverage ,if they were straight then you would have a max of 90 deg travel max .
daz
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I tend to make mine out of 13A plug legs. They're just straight and work. In normal use, the servo only has 45deg each side anyway.
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As I said, they are shaped to give the servo authority at greater legnths of travel.
If you at a 45 degree (pushrods) angle into a straight tiller, the servo has no authority, so needs more power to push it, however in that style, it can move it with ease.
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Aint that what i said ,i think err O0 .
daz
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The design is from ACKERMAN GEOMETRY principles
This is not that important unless you are into differing rudder angles (multi rudder configs) if you are into race car etc then yes it comes into play to get the inner and outer wheel turn angles correct.
as for rudders it is whether your arm is "over center" that matters so that an equal arc both sides of the center can be obtained by your servo movement.
servo arms rudder arms are available in straight as well as angular
IMO
Oh I do use the Ackerman system when I build twin rudders it gives a very good turning circle.
R
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Cos the back of boats are pointy?
I have never seen one fitted this way round before - always with the arms facing forwards. Ihad assumed that it was to give more rotation before the backward going arm thumped the transom
I havn't used them myself until tonight! I have just made the rudder and gear for my Higgins PT boat, and used one of these - it will go on with the arms facing forward.
andrew
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The arms in my Higgins are facing forwards too, just noticed that O0
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Cos the back of boats are pointy?
Clever! O0
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Yur, Martin, I'm a deep thinkuh
and i are a ingeeneer,two
andrew
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Dunno why they are shaped that way but I find that the lack of a centre that fits a servo a real pain in the tripes. All sorts of weird and wonderful servo attachments come in little bags, but not a decent size "tiller" arm.
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Hello,
I'm after a tiller arm like the one pictured, cane someone let me know a source of them?
Thanks
Mark
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Try here:
http://www.prestwich.ndirect.co.uk/hdwrrubbersetc.htm
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Thanks for the fast reply. :-))
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Dunno why they are shaped that way but I find that the lack of a centre that fits a servo a real pain in the tripes. All sorts of weird and wonderful servo attachments come in little bags, but not a decent size "tiller" arm.
good point Bryan
All the servo horns are too small I think they should come with some larger horns >>:-(
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After a little thought I now think the arms are angled to somewhat negate the effect of a rod traversing an arc so as to give the most advantageous result.
That sentence sounds a bit "odd" but it is the only way I could put it! Sorry.BY.
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Oh I do use the Ackerman system when I build twin rudders it gives a very good turning circle.
Pardon my ignorance, :embarrassed: but what is the Ackerman system? :o
Peter.
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Peter....courtesy of GOOGLE..........
Ackermann system
mechanics
Main
Aspects of this topic are discussed in the following places at Britannica.
Assorted References
use in trucks ( in truck: Steering )
Steering of trucks, with their relatively heavy loads, was a problem until power steering came into use in the early 1950s. Steering is always by the Ackermann system, which provides a kingpin for each front wheel. Maximum cramp angle of the front wheels is about 35 degrees. The minimum turning radius is dependent on the wheelbase. A few vehicles have been built with two steering axles in the...
However I suspect R/C servos as we know them had their origins for military applications...[seems strange but I would never take the time to count] ........ :-)) the number of splines on the servo output shaft has a one off location to the true cross axis of the pairs of arms or pre drilled holes on the servo arms or disc
But again just simple mechanics........if we use larger centers on the servo horns, we increase our rotational motion for any given requirement but also reduce the torque load on the servo...same applies in reverse for smaller centers on the servo horns.....Derek
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"Why aren't they just straight?"
Neat question. But judging from all the given answers it seems that nobody is really none too sure -- me included.
A lot of the fly boys are using the similar pull-pull system on their rudders, but mainly by emplolying cables, as opposed to using piano wire.
I think the main reason for the V-shaped Ackerman is to counter any glitches or snags as the control surface is moved through its arc. You get a nice, silky-smooth control of the surface if the 'V' end is facing AWAY from the servo. Having it the other way around means you would probably run into all sorts of problems. Those same problems can occur if the Ackerman is straight.
My take is that it's a lot to do with geometry. For instance, if it were straight, you would likely notice one side of the cable would stay nice and taught throughtout its traverse, while the opposite side becomes sloppy (and vice-versa).
An engineer I ain't. All I know is that Mr. Ackerman's invention works.
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Will someone please tell me what "Ackerman Steering" has to do with rudder tiller arms in model boats?
I can understand its use in road vehicles where the wheels have to follow different radius circles when turning if you want to avoid any element of sideways motion between the tyre and road surface, but in a boat?
The reason some control horns are angled forwards on model aircraft is to get the linkage in line with the hinge line and thus produce even movement. True, in some cases, like ailerons which can benefit from more "up" than "down", this is not wanted.
If this shape of tiller arm was any advantage than all model manufacturers would use it, which they clearly do not.
GlynnG
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I suspect that the Ackerman thing comes into play where there is more than one rudder, same as the difference between steering a Reliant Robin and a Morris Minor. It does make some sort of sense when you consider that the inboard rudder should get a greater deflection than the outboard one to describe the tighter radius in any turn.
I haven't got the drawing stuff out to check the geometry, but it might also be that there is a different response between left and right which MIGHT be significant in a single prop setup, or not, as the case may be.
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Peter....courtesy of GOOGLE..........
Thanks, Derek.............I think {:-{
Peter
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I did a google for ackerman and found this link
http://autorepair.about.com/library/a/1g/bl489g.htm
which explains the important bits that brittannica leaves out, and itself has a link to an RC car forum where it explained fully. The differential toe-out on lock would appear to apply equally well with rudders in fluid as with front wheels on land.
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Why are model tiller shaped this way?
(http://www.modelboatmayhem.co.uk/forum/index.php?action=dlattach;topic=9014.0;attach=54103;image)
Isn't it so that the arms don't go past 'the point of no return'?
Or that point where instead of the servo pulling the arms the right way, it instead pulls it the other way. [it's the same principle as the timing of a spark plug on a piston/crank... it always fires just after top dead centre, never bang on dead centre as the combustion may send the piston down and force the crabkshaft the wrong way.]
No?
[LIONs rudders are a doosy... they are right up in a tight 'cruiser' stern and they incline in towards each other so that my tiller arms, less than an inch long, have had to have their collars filed down or they'd rub against each other. It's actually 17mm from the outer tip of the arm to the centre of the rudder post and I had to make 'em myself. mecano collars with a bit to ali/tin soldered on. They sound great but if you've seen my soldering, you'll know it's 'mass' that holds the tiller firm, not the sturdyness of the joint.
The story is actually far worse than that but that's all I'm prepared to share publicly. ;-) I'll give you a clue... I only had what looks like a 1/8th drill when I made them.... and the linkage is 1/16th? rod!!! LOL When I used to go test sail it, it would sail like this: --> ~~~~
90° turns ended up being 110°!!!
My disgrace is not in doing the best I could do with what I had at the time, the disgrace is how I've fixed these oversize holes. LOL
Now you know why there's a big deck hatch above the rudders. :-)
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Actually you are incorrect there, ignition timing is always set a few degrees BEFORE top dead centre to allow the fuel/air mixture to fully burn just after the piston goes over top dead centre thus providing maximum force as the piston starts to decend , the piston is forced over TDC by its own momentum and momentum from the crank and fly wheel..
Mark.
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Actually you are incorrect there, ignition timing is always set a few degrees BEFORE top dead centre to allow the fuel/air mixture to fully burn just after the piston goes over top dead centre thus providing maximum force as the piston starts to decend , the piston is forced over TDC by its own momentum and momentum from the crank and fly wheel..
Mark.
That must be why I could never adjust the points on that yamaha 100 I used to have. Does it make a difference if the engines 2 stroke or 4?
TCC... whose knowledge of mechanics matches his soldering abilities. ok2
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By fireing before TDC you're compressing the bang so you get more power, same with a steam engine. Fire it too early and the bang is the conrod. BTW, I think in Martin's picture the arm is the wrong way round, the lead should be the same way round as the A/C piccy.
Regards Ian