Spacewalker half scale 168" electric

[Ted Oram]

Hi Andy......yes I know what you mean by the ÿoungsters"flying all over us......the 1/4 scale SW weighs 6.8 kgs so it does not float in...best landings are to keep a bit of power on till the last couple of feet altitude.
I have some more photos of the inside ...I will dig them out and post them for you tonight (here time).
The big advantage of the 2 seater SW is that there are far more built this way and more colour schemes to chose from......I was lucky enough to be able to contact the owner in the US for my 1/4 scale and he was most helpful with photos etc of the full size.

Cheers
Ted

[Andy Boylett]

Hi Ted,
Interesting that your 1/4 SW weighs 6.8kg. Mine weighs 6.9kg would you beleive.....but it is electric motor and batteries!!!


As it is Bradford display tomorrow I have had to interrupt my usual routine of building a house in my back garden - well supervising some young lads I have got building it for me. They have got 1500 tes of clay to dig out, then a whole house to build.

Anyway, I spent a little time today fitting the control surfaces so that at least they are hanging by their hinges for the display. They were a little fiddly but the end result is really excellent, with a lovely smooth, strong and very positive hinge. These are individually made hinges, each with a ball raced pivot. These are the original fittings that I made/used. The left hand ally piece and the steel tube on the right were epoxied in place. [The picture shows brass tube, but steel was used in the end] The ally 'clevis' is a tight fit in the steel tube...........

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After all the discusison we had re the nasty little clip that retains the hinge pin in place, I had intended to use a pucker circlip on them. However, the groove on the end of the pin was just too big for the circlip and was obviously made for the 'nasty little clip'. Now when assembling them I found that a nice little ss cap-head screw with a nyloc nut fitted nicely.....

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[Alan Cantwell 1131]

That hinge is not running on screw thread is it?

[Andy Boylett]

That hinge is not running on screw thread is it?

Hi Alan, no it is running on a small roller bearing. Each hinge is a tiny manufactured unit with a 3x6x2.5 ball race in the middle.........

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[Andy Boylett]

It was together for the first time since covering today. So much so that I had to borrow a knife to remove small areas of covering that were blocking pieces that slot together!

Also, Mr Eccles produced his sets of scales and we had a weigh in. This came in at 35.5kg. It still has a cowl, some servos and some batteries to fit so likely to hit its target of 38kg.

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[Andy Boylett]

I have now started to fit the servos and make up the linkages. The rudder is driven by 2 servos housed in a Hobbyking aluminium mount. I am using the HobbyKing Turnigy high votlage, 33kg.cm digital servos. If one of these servos was to fail live so that it was essentially stuck but still under power then the other servo would try its best to fight it. The result would be 2 servos drawing maximum current and a big standoff with no movement. I have therefore used the design I saw my inspector Ian Turney-White using in one of his planes to allow each servo to do its own thing. The difference here is that I have had to re-design it so that the servos can be inline rather than side by side. here it is:

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You will notice that the link that drives the control surface is not attahced at the centre of the floating drive bar. The off-set to one side is what allows it to provide movement by either servo when one fails.
Because of the situation it has to use an angled design that does not give perfect symetry (as the other side by side servo design does). I have set it up in Autocad and it gives he following performance:
With both servos operating at 30 degrees either way, it gives rudder movement of 28 deg left, 32 deg right.
With rear servo dead, 18 deg left, 23 deg right.
With forward servo dead, 10 deg left, 22 deg right.

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And this is the underside

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I will post actual movements on the next post.

[Andy Boylett]

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Normal movement with both servos operating......

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Movement with left servo stuck at central position.....

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[Andy Boylett]

Movement with right servo stuck at central position.....

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[DAVE JOHNSON]

Andy--thats really ingenius,havnt seen it done that way before but this could be used in other instances --DAVE

[ian redshaw]

Have you tested the set up when either servo fails 'floppy'? Thats one of the disadvantages of the conventional cross bar set up, if a servo fails and has no mechanical resistance then the good servo tends to simply move the floppy servo and not the control surface as intended, as the easiest to move part of the linkage is the floppy servo.
Also, this type of linkage can lead to a fair chunk of slop as the various components can 'roll' about the bolts. I find that doubling up one or more of the linkages massively reduces this. On your set up, if you packed up the rear servo by the thickness of the main crossbar, you could double up the linkage between it and the crossbar, effectively sandwiching the rear servo arm and the crossbar between two links, getting rid of the jointed 'dogleg' linkage and eliminating any potential roll between at least two of the main components. I'm no having a go, I'm all for linkages that means servos aren't hanging out in the breeze!!
Also number 2, please tell me you making a cowl and not leaving it off for advertising purposes!!?? The Spacewalker is classic homebuilt shape that is spoilt by the lack of cowling.

Ian.

[Andy Boylett]

Hi Ian, good points, thank you.
If the other servo fails floppy then yes, the other one pushes it. I am hoping that that is smoewhat down the list of failure scenarios. We are using 2 seperate receivers and each controls one half of the servos. The primary function of this link is so that if one servo loses its signal then it goes to fail-safe (centre positon) and is still under power. The other servo then continues to fly it. It may be possible to add some sort of spring system to limit the 'floppy effect' if one servo is slack. I'll have a think about it.

As far as slop, there isn't any at all. We measured it last night because that was our main concern. We had the tiniest, nearly undetectable amount of movement with the servos live and us trying to move the rudder bar. This is because I have used high quality car rose joints (4mm) and 4mm ss threaded bar on the links. The other joints are closely machined holes with smooth shank bolts in them. There is no 'roll' as you describe between the various parts as I have used 14mm wide, 5mm thick ally strip. It is a very close sliding fit to the servo arms, which are themselves machined ally.

Cheers, Andy

[Andy Boylett]

Andy--thats really ingenius,havnt seen it done that way before but this could be used in other instances --DAVE

Thanks Dave. I only made it because I was talking with Nick Henderson who was asking me about the servo connections I am using as he is using the same rudder servo tray from HobbyKing. We were trying to work out how to do this for the in-line servos.

[Andy Boylett]

Following on from Ian's point about sloppy servos earlier, I have had a think about how to control the 'floppy' servo. It is really quite straight forward. By playing around with the 2 serovs and watching what the floating ally cross bar does I have noticed that if one servo is held in centre position then there is a maximum and minimum angle that the crossbar makes with the link-bar to the left servo. In other words, If I made the crossbar so that it has built in end stops for the angle between it and the link-bar then even if one servo goes floppy it does not matter as the end stop keeps it from going the wrong way.

[Andy Boylett]

Thank you for your support guys.

I have been back to the drawing board (literally!) to see if I can refine the linkage design for 2 servos so that a "floppy" servo can be prevented from stopping movement of control surface by the other.

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The top left drawing shows a 2 servo in-line set-up with 25mm arms. Servo spacing is 75mm and distance to the driven arm is 75mm. The long drive link is slightly over 150mm because of the angle, due to the link being attached at a point 20% off-centre ie 20 mm from the end of servo 2's arm.

Top right shows normal movement of both servos through an angle of 30 deg.

Bottom left shows movement with servo 1 fixed at centre and bottom right with servo 2 fixed at centre. The actual angle of the driven arm is shown at the top of each picture. The drawings prove what I suspected from moving the one I have already made....that there is a minimum and a maxium angle that is needed between the two link arms. You can see that the angles are 74 deg and 103 deg in both cases of servo failure.

What this means is that if I design the join between these two links so that these angles cannot be exceeded, then if the dead servo is "floppy" it does not matter as the linkage stops it from turning when it should not.

I am now manufacturing 2 sets of linkages because both of my elevators are also driven by 2 servos each. I am going to use this in-line design because it avoids the need for slotting that is required in the side-by-side version and the rudder one I have built has no slop in it at all. I am going to use the same linkage on both. This should now give me a way of dealing with: a receiver failure; a power on servo failure; and a power off (floppy) servo failure. It won't deal with a rouge servo that drives to one end of travel - but that does seem to be rare.

[Robin Woodhead]

Andy,

Try this for a simple and cheap double servo set-up that does not jam even if one servo stops, you still get 50% of moverment; picture show the set-up in my Caribou fuselage for the elevator control rod.

[Andy Boylett]

Nice setup Robin. That looks like it is relatively easy to make and set up. I assume there is a slot in that triangle where it is sandwiched.

Do you get the problem that Ian raised about a 'floppy' servo. If one servo failed and went floppy so it could flop easilly wherever it was pushed, would your other servo then just push it round?

Cheers, Andy

[chris_bell]

With Servo's burried like this in the Fus.

Is there any way of alerting you to the failure of one servo - Or is it that you have to be paying close attention when your checking your movements on each surface each time to spot a failure ?

(stunningly nice job BTW and a nice looking model - although i might be biased as I have the 1/3 fly baby)

Chris

[Robin Woodhead]

Andy,

So far never had a Hitec servo fail (touch wood), yes there is a slot in the triangle and it has simple bush to prevent jamming on the bolt threads as it slides. Never checked if the servo goes total failure, may try to check this weekend and let you know.

Robin

[Andy Boylett]

Hi all. I still can't do any building, or flying. Remember that finger I broke at East Kirkby? Well after a month it wasn't healing so I now have steel pins in it. Another 3 weeks like that, then they take the pins out. Then I have to not use it for another 4 to 6 weeks .....................and look what happens to your bandages when you try and do stuff....................

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[Mike Booth]

Andy, is this really in good taste and viable forum content?