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Old Dec 03, 2012, 02:52 AM
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Pushrod Steering setup

If you have ever used Pull-Pull wire steering on a model, you will have encountered its major flaw. SLOP. No matter how well you set that up initially, the wires will not remain taught for too long! Any slight landing gear leg bend (or retract pin) means there will then be slop.
A second issue is that there is nothing to keep the leg in the straight position once the leg begins to retract and the wires go slack. So the wheel can turn off to a side and then bind/hit the wheel bay side etc.. Some people have added 'spring' ideas to assist this issue, but that has small issues too, plus is quite hard to set up well.

Meanwhile Pushrod steering gives a 100% solid, permanent, solution. So there is no contest...... "100% reliable", versus "Messy and fraught with issues". Pushrod versus Pull-Pull.

One main problem I see around is that very few people seem to be able to work out WHAT is required in a Pushrod system. and how to implement and set it up correctly, so as to arrive at that 100% working system!
So this Blog entry is to outline what I do for mine.....

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Old Dec 03, 2012, 03:04 AM
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The GEOMETRY of all the parts of your setup are what is important! Not hard to do, just important to set them all up correctly.
There are pics and videos lower down that show the various views of parts, and operation, required. So scroll down and check them out anytime you don't understand what just text is explaining.

Let's see a basic list:

1) First you need a control arm, as per all steering systems - but it only needs to be out to one side of the gear leg, seeing you only need one pushrod going to that.

2) The control arm needs what I will call a "Slide Rod". This arm has a control 'horn' on it that can slide along the rod. This is required for a few reasons that I will cover later.

3) Mount the retract using the retract mounting plate that goes to its "Top". The "Top" meaning the top when the plane is the right way up... the opposite end to the way the leg justs out. This plate is a solid sqaure plate, as opposed to the other side's plate that is more a "U" shape. Mounting the retract this way means you get nice clear open area down the side of the retract for the "Slide rod" and pushrod to operate in - and it also only nees a flat plate/surface top mount the retract which is far easier to mount a retract than the other methods "U" shaped mounting requirement.

4) A nice rigid pushrod assembly - I will outline some great bits you can get and use to make this well. Also useful if it is easily adjustable (which mine is).

So, on to the assembly and geometry.....
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Old Dec 03, 2012, 03:22 AM
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The first thing I will cover is the retract, its PIN, the STEERING ARM, the SLIDE-ROD, and the CONTROL HORN. (See the Pics as they come, below).

This retract shown is the 54g "Large Size" retract, from HobbyKing, and it comes with the steering assembly installed already! You can also get the "Medium Size" retracts like this. But not the "Small Size"... or not that I have seen.

So first take note of THIS retract set up, and later I will cover how to make all your own parts for one. Basically this is the form of end result you want to end up with, but you can do it in a few variations of ways.

THE PIN, or LEG WIRE installation - so it is captive and can rotate:

All of these retract types (and all retracts anyway?) have an Xmm hole in the Trunion and take a pin that goes into that, and leave X amount sticking out so you can bolt an oleo leg onto it.
But you might want to use a one piece wire leg.....

This pin has a circlip slot, and a circlip, at the top of the Trunion. This prevents the pin coming out, whilst allowing it to rotate for steering. This is a very nice way to keep the pin in! But it has a problem.... you have to DIS-ASSEMBLE the retract and get the Trunion out, to remove the circlip and pin!

Another way is to use the two grub screws that can come in from the pivot bush sides of the retract, into the trunion. To do this, you make a "U" slot around the top of your leg wire (or a pin) so that the trunion grub screws come "into" that "U" a bit, thus the leg cannot move up or down anymore and come out, but it can still rotate. You don't do the grub screws up tight, just enough to go into the "U" slot. Use BLUE loctite on the grub scews and then they will stay where you do them up to.

To make a "U" slot in the top end of your wire leg, or pin..... put it into an electric drill so you can have it spinning - put the drill into a vice if you need to and get it going. Then you can use a Dremel Cutting Wheel (like 1mm or 2mm wide wide) to grind/cut against the spinning wire leg (or pin) and it will make you a very nice, near perfect, groove of equal depth right around the leg. Don't go too deep!! You don't need a lot anyway.

So now you have your leg, or pin, installed and able to rotate.....

Install the arm you are going to use - it needs to have that Control Rod coming out 'upwards' of it (towards the retract mount), as per the silver rod seen in the pic below.
The steering arm is snug against the trunion, but note that in many cases (not this one) you can need to have some washers in there so that the oleo leg is a bit more out from the trunion. This is when an Oleo diameter is 'too large' and if it was against thr trunion then the oleo top cannot 'get around' the retract corner as it retracts! You will find that out if it matters in your case ever! LOL

The Slide-Rod is needed because when the retract is retracted, that arm rotates 90 deg to then be in-line with the servo. It needs to be LONG enough to 'absorb' any maximum amount of 'error' that can be in the system. This will allow "error" to not matter in the pushrod length, and/or servo trimming for aligning steering direction. More on that later....
But this part is one of the most important parts of pushrod steering! If you don't do it correctly, you will encounter nasty issues - which I bet is what all the "Pushrod steering haters" ended up encountering, but never worked out how to prevent it!

The Control-Horn is the little black thing. To make the pivoting joint from Pushrod to the Slide-Rod. This also has some critical factors! It must be a nice 'slide fit' on the Control Rod - whatever slop it has, will add into the total 'freeplay/slop' of the system. You CAN make a ZERO slop system, if you make the effort to get all these parts right!!

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Old Dec 03, 2012, 03:52 AM
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Here we have the pushrods end Clevis installed. These are awesome Clevis items, from HobbyKing. I use them in tons of places!! They are about 50c each but well worth it! They make for very solid, accurate, connections for your control systems.

http://www.hobbyking.com/hobbyking/s...ntrol_Rod.html

For this part you need a clevis that will be an accurate fit to the Control Horn. This clevis has a 2mm bolt for its pivot. The Control Horn actually has a 2.5mm hole - so having them perfect is best, but in use I found this joint has no freeplay. I guess the thread of the bolt makes it that bit larger in diameter. So this all ends up a perfect fit!
If you were using some other Slide-Rod and Control-Horm - possibly made by yourself - you still do the same thing.... make it a perfect fit!

Another important part is the "face to face" meeting of the Clevis and Control-Horn. It must be snig so that the Control-Horm will always remain perfectly in-line with the Clevis/pushrod. Because this alloy clevis can be clamped closed, according to how much you do up the pivot bolt, means it will end up that snug "face to face" fit.
If you use a Nylon Clevis, you can use a silicone or heatshrink sleeve over it so that it closes up snugly onto the Control-Horn.

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Old Dec 03, 2012, 04:02 AM
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This pic below shows the servo - which you can mount anywhere that it can be fit in the pl;ane (even way down the back if necessary!) - and the pushrod as required for this setup.
The pushrod 'steps down' in this case because the servo height is a bit much for the setup. The final pushrod portion, which attaches to the Clevis, is best to be levelling out at the 'height' that the Control-Horn pivot will be at when the retract is retracted. You will see another pic of this alignment result soon.

You could also use a long pushrod, that is even somewhat flexibe, from a servo well rearwards in the plane, that just ends up with its ends as per this setup. Such a pushrod would likely travel in a "sleeve" so would just mount/support that exit end at teh appropriate height to achieve this alignment. The Dynam Me-262 does its pushrod steering ithat manner - I will put up a pic of that installation at some stage.

The Pushrod used here is 1.5mm piano wire, so you need a "perfect fitting" 1.5mm hole in the Servo Control Horn.
HobbyKing (once again) has a great tool to make the perfect Z-Bend in any wire up to 1.5mm !!! For just $7 !!
http://www.hobbyking.com/hobbyking/s...p_to_1_16.html

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Old Dec 03, 2012, 04:38 AM
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In these next pics you will see how the Servo to Clevis operation unfolds.
You adjust the length of the Clevis on the pushrod - NOT the angle of the servo arm!! - so that the Slide-Rod Control-Horn is HALF way along the Slide-Rod.
(well Pic 1 of mine is sort of half way along!)

There are two reasons for this:
1) If you make any steering trim adjustment to the system, the Control-Arm will still be some amount away from an end of the Slide-Rod.
2) If you DON'T disable the steering servo, via a mix with the Gear switch, the servo can still steer/move even with the landing gear retracted, because the servo motions will just slide the Control-Arm along the Slide-Rod and not hit an end which would stress the servo and mechanics of the steering system.
There is a video to show this later....

These three pics show the servo in a Mid position, and then at each end of the Slide-Rod - possibly occuring if the steering nees some trim, or if the servo was moved after the gear was retracted.

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Old Dec 03, 2012, 04:49 AM
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The next pic shows the gear extended.
You will have adjusted the Steering Arm to be 90deg to the aircraft, as well as the Servo Arm being 90deg to the aircraft. They need to be equal angles for the steering to be linear to both directions of steering. Though this is not absolutely critical to be perfect, as you will hardly tell a bit of non-linearity when you use Steering.

Sometimes, due to aircraft space limitations etc, you might choose to have an offset Steering Arm, or Servo Arm, or both. The only important thing is that once you ahve set up those arm angles, that you adjust the Clevis/Pushrod to be sure the Control-Arm is half way along the Slide-Rod when the steering adjustment with gear extended was straight (steering straight).

When the gear retracts, there is nothing to make sure the leg stay straight ahead - it could rotate as it closes up.
To prevent this there are two ways that can come into play:
1) An oleo leg, could have a flat filed onto the surface that meets the retract motor side, and thus it folds to have that flat along the motor.
2) The Clevis/Pushrod height is controlled, by mounting/supporting it, to be at the correct height to make the steering arm end up level when the gear is retracted. The second Pic shows that situation - though in this case it is just sitting in mid air - not truly forced to be the correct height by any mounting or sleeve system. The bend(s) in my pushrod make this occur - but that is not a reliable way to use for it to stay correct ongoing!

Oftena small amount of steering 'rotation', as it folds into the wheel bay, does not matter. But if it does, then you have to get more serious with that Pushrod mount/support idea.

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Old Dec 03, 2012, 04:53 AM
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Video of this setup in operation. Showing the ability for the Servo "Error" to be absorbed in the Slide-Rod length.
You should be able to see how the Slide-Rod length must be enough to allow the full sttering motion Servo travel, when the gear is retracted. Otherwise the Control-Horn will hit an end of the Slide-Rod before it rusn out of Servo travel... an then things get 'over-driven'. Servo stalling... bent mechanics.... not good!
So make a longer Slide-Rod if necessary.

You can also see how the Steering Arm is "low" after gear retraction, due to gravity, and because I have no mount/support for the pushrod to make sure it 'holds' the Slide-Rod at the correct height. You can then see it has made the "Wheel", which is not actually there, tilt off tothe 'side' a bit (rotate). It is only a small amount, but it might matter in the aircraft it is installed in. If it does, then add the mount/support for the pushrod.

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Pushrod Steering Operation (0 min 42 sec)
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Old Feb 07, 2013, 11:48 PM
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A better video and setup by FG1972 !!
Using a linkage stopper to help assure the pushrod stays at the required height.
If the pushrod alter height during retract operation then the steering will turn the wheel some amount. I make the pushrod (1.6mm) the same as the servo hole I drill (1.6mm) so it creates a 95% rigid height control - bu the linkage stopper idea is even better!
You can also use a 'slotted piece of plywood' for the pushrod to travel through, which allows lateral motion still, but holds the height constant.

If the steering servo is on the 'leg retracting' end of the retract, the slider needs to be UNDER the slider rod.
But if the servo is at the other end of the retract leg, like in this video below, then the pushrod needs to be ABOVE the slider rod.

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Setting up nose wheel retracts (0 min 22 sec)
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Old Jun 12, 2014, 08:52 AM
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Alloy Slider

Quite a long time ago, when I didn't have a slider for the steering arm of one steering system, I made my own from a bit of scrap aluminium plate. That worked so great that I do that a lot, because almost none that come with steering systems actually work 'properly'!
To work properly the slider needs to have a suitable amount of 'tube' portion that slides on the steering arm rod. It also needs to be JUST big enough hole diameter to slide well BUT also not have slop. So, if you make your own you can MAKE SURE it is like that!

I start with a 3mm alloy plate and cut a strip about 6mm wide from it - so I can make a bunch in one go.
Cut off smaller pieces about 8mm or 9mm from that - so now you have a 3mm thick rectangle.

I drill down the 3mm 'edge' towards one corner, with a 1.5mm drill, to make the Slider Hole that will allow it to run up and down the Steering Arm Rod. I do it free-hand, and you need to be careful and patient, so you can drill straight and accurately - a little bit off error is OK, but you can't afford MUCH!
I then drill that out to 2mm, but this allows you to "force" a bit of re-aligning for any error your first 1.5mm hole had.

Then you need a 'Cross hole" that goes through the "Side" of the plate, at the other end to the Slider Hole. This hole is for the clevis from the pushrod to connect to. It is also a 2mm hole.

I have also made some Steering arms before, from 6mm aluminium plate - but lately I always use Lander Alloy Retracts, which have the Arm and Steering Rod. They also have a Slider that is pretty lousy really! Thus best to replace that with the home-made one.

Note in the Video of its operation - In this installation case I use a "Servo Offset" command (programmed in the TX) as soon as the Gear Switch is activated. This is to TURN the nose wheel a bit right away, because as it folds into the wheel bay the geometry makes it rotate a bit too and if you don;t straighten it up it can hit a wheel on the fuselage edge. The offset keeps it retracting straight all the way in.
In many cases the geometry rotation can't be prevented.
On Extension, the Servo Offset is DELAYED to happen after the leg is already extended by a fair amount.

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Nose pushrod steering setup - operation (0 min 12 sec)


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