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Control Surface Horn Positions - 8/11/2012 8:55 PM   
moggyman64


 

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From: spalding, UNITED KINGDOM
Status: offline 		Hi All,

I have been away from the hobby for quite a few years and now I am re-aquainting myself with building and model set up. I have been looking at photo's of various planes and also on looking at some new plans that I have aquired and I have noticed something that I hadn't noticed previously.

Dont know if this has been covered in a previous thread so my apologies if it has.

The mojority of the control surface servo horns that move up and down and side to side are fixed to or very near the leading edge of the control surface.,  I know many people will say that this is the thickest piece of wood or similar to get a good fixing for the horn. But in my opinion this is putting extra strain on the servo or making it work harder than necessary which in turn would drain more power from the battery(s) than necessary. It is my understanding that something with less resistance would move easier and freely. For example if you have a pull pull rudder set up and have the wires too tight then this will put strain on the servo plus battery. 

I have attached a brief sketch to illustrate what I mean. For example if you have a door which is ajar by 45 degs and try to push it in the directions at positions 1 and 2 then it is extremely harder to do (you need to use more than one finger) with a lot of effort than if you were to push the door using positions 3 and 4 ( you can do it very easily with one finger) with a lot less effort. Please try it yourself.

Therefore, if the control horns were moved in toward lets say a 1/3 or 1/2 of the dept of the surface instead of near the edge,  obviously with some additional strenght ( kevlar, wood etc ) then would this not benifit both the servo and the battery.  

I would appreciate any comments you may have.

Regards


Keith

PS. First time to upload a photo so hope it works


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RE: Control Surface Horn Positions - 8/11/2012 9:10 PM   
BarracudaHockey



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Status: offline 		For symetrical throw you want the control horn on actuation point on the hinge line.

As for strain on the servo, they make servos well into 400 oz of torque and extremely small, high capacity batteries so what you're worried about really isn't an issue.

For surfaces that only move one way, flaps for example, you can move the pivot point towards the trailing edge to get more throw.

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RE: Control Surface Horn Positions - 8/11/2012 9:25 PM   
KitBuilder



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Status: offline 		what your describing is a horn with a very long length to the hinge line. IE.. moving the horn halfway back but keeping the pushrod connection at the hinge line. This won't work as the rod will be pulling against the horn and if it does... the rod going into the fuselage would need lots of leeway.. it certainly would not work in the push direction as the surface would then be in the way. The connection point to the surface needs to be near the hinge line. The door example you describe orks because your only dealing with the push..directly "toward" the door. If you try the same test with the push or pull in a direction parallel to the door and not perpendicular to it with a connection "at" the hingeline.. it will be actually easier if the connection point is near the hinge line

standard 40 oz servos have no issues moving surfaces with no excessive drain.

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RE: Control Surface Horn Positions - 8/11/2012 10:03 PM   
Rodney



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Status: offline 		Your comment on the pull-pull is not correct either. On pull-pull you always want a bit of positive Ackerman if you can not guarantee a perfect zero Ackerman set up as then you never strain the servo by getting a tight line as you move off neutral. The positive will not hurt you and negative definitely will strain your servos.

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RE: Control Surface Horn Positions - 8/11/2012 10:16 PM   
moggyman64


 

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From: spalding, UNITED KINGDOM
Status: offline 		Thanks chaps for the replies,

I am not making statements but merely voiceing some observations.

regards

keith

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RE: Control Surface Horn Positions - 8/11/2012 11:15 PM   
da Rock



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Status: offline 		Rigging must be done in a certain way or the surface will deflect more one way than the other.

Pushrod
Horn's connecting hole
Hingeline

If you draw a line along the pushrod to the point where the connecting rod attached to the horn,
then continue to draw your line to the hingeline,
your line should have drawn a 90degree angle.

The rigging in both these setups will give equal deflections.

Move the horn back as you suggest in these two setups, and what happens to the line you'd draw? Both cases would result in the surface moving far less in one direction than the other.



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RE: Control Surface Horn Positions - 8/11/2012 11:22 PM   
da Rock



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Status: offline 		Notice that the horn on the bottom wing in the picture is already moved away from the hingeline? It was done so that the surface would deflect equally. That is a standard configuration found on T-tail models.

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RE: Control Surface Horn Positions - 8/11/2012 11:37 PM   
frets24


 

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Status: offline 		Moggyman,

Expanding on Rock's excellent tutorial, it is worth mentioning the distance the between hole used in the horn and the hinge. Since the horn is at 90* from the surface you are effectively transfering the force at a distance from the point of rotation. Using your original post premise, that would translate as using the horn positionhole closest to the hinge would be like pushing the door from position 1 or 2 and a hole further out on the horn would be as in position 3 or 4 on the door. Your original idea is right...pushing the door/surface from closer to the hinge is much harder. You just need to take the 90* arm into consideration. Just imagine the same distance from the horn's position hole to the hinge as the distance from the hinge to the point on the surface.

see modified da Rock pic below

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< Message edited by frets24 -- 8/12/2012 5:18 PM >

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RE: Control Surface Horn Positions - 8/12/2012 11:59 AM   
moggyman64


 

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From: spalding, UNITED KINGDOM
Status: offline 		Thanks da Rock for taking time to do the sketch and a super explaination and also thanks frets24 for agreeing that there was at least some logic behind my observation. In my mind I was making sense but I forgot about the 90* angle of the control arm.

However I am still not convinced that having the pull pull rudder control wires overly tight as said by rodney would not be a strain on the servo, I am of the impression that with this type of set up you should have the wires with a slight tension and with no slack, otherwise it would put unecessary strain on a servo which over a period of time would be detremental to the servo and possibly the control horn mountings themselves.

From day one when I was learning how to setup models I was always told by people with experience that you should never apply unecessary strain to a servo by way of overly tight or possibly any fowling down through the fuse or firewall. Maybe I have just been away from the hobby too long and things have changed.

Regards

Keith

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RE: Control Surface Horn Positions - 8/12/2012 12:28 PM   
BarracudaHockey



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Status: offline 		Pull pull cables should have tension but not be taught.

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RE: Control Surface Horn Positions - 8/12/2012 12:54 PM   
moggyman64


 

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Status: offline 		Thats my understanding of it barracudahocky,

Keith

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RE: Control Surface Horn Positions - 8/12/2012 1:54 PM   
jester_s1


 

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Status: offline 		Pull pulls should have enough tension than any wiggle you do at the surface goes directly to the servo instead of being taken up by the lines when at neutral. That means they are tight enough to be straight and no more. Going tighter than that doesn't do anything for you and will help the geartrain wear out faster.

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RE: Control Surface Horn Positions - 8/12/2012 3:07 PM   
Rodney



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Status: offline 		I've been using pull-pull on all my large models (both elevator and rudder) for 20 years. I have found that you want the lines just taunt enough to have no slack (but very little tension) when at neutral. I always make sure I have a bit of positive Ackerman in the setup (where the line not being pulled as you move off neutral goes slightly slack). This NEVER causes a problem and saves wear and tear on the servo as being much easier on the battery drain. The slack line when off neutral will not cause flutter or any other problems as the air load prevents this.

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RE: Control Surface Horn Positions - 8/14/2012 11:20 PM   
KitBuilder



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Status: offline 		I use a second double arm to handle the pull pull. The servo just moves the arm... it knows noting of whats down stream.

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RE: Control Surface Horn Positions - 8/15/2012 12:26 AM   
moggyman64


 

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Status: offline 		Hi kit builder, any diagrams

Keith

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RE: Control Surface Horn Positions - 8/15/2012 6:01 AM   
countilaw



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Status: offline 		The double arm  setup takes all the stress off   of the servo  including the servo mounts.     

Frank




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RE: Control Surface Horn Positions - 8/15/2012 1:59 PM   
Rodney



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Status: offline 		I also use an arm (in this case a pulley) to isolate the servo arm from the pull-pull lines. However this still does not negate the need for some positive Ackerman (or at least neutral) to prevent excess strain on the servos as the control surface is moved off neutral.

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