Spacewalker half scale 168" electric

[Andy Boylett]

Wing joining.

Just for completeness on this thread, here are the joins in the wing being completed as per advice from John Rickett when I asked how to do it under technical threads...

Here was the problem, wing with a large 26" chord joins up to its middle section but has a 2.5mm gap.

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So, shiny packaging tape applied to one half and a very light smear of vasaline added for extra measure. Then I very carefully put a little fillet of car body filler (P38) around the edge od the other half. I found a very small fillet works best because the filler is quite stiff and does not compress that easilly when the wings are brought together. When I assembled the wings I found a few swift taps with a large hammer in an appropriate place (there was a nice solid place on the centre section) bedded the filler in nicely ...

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And here is the finished join, absolutely perfect, no gap at all anywhere

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[Andy Boylett]

The Forsythe saga (sorry, I meant clevis)

Here is a picture from an engineering catalogue showing the standard endings for 3mm to 20mm rod ends for pnuematics fittings (high impact velocity and vibration). I think this shows quite nicely the options available. A bolt would not normally be used (unless you can get straight shank) because the thread quickly flattens and then becomes loose.

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The circlips I ordered to replace the spring clips on my Protech Clevises arrived and myself and Phil tried them on the clevis pins..

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The photos show the spring slips supplied with the fittings alongside a circlip I ordered.

They are not bad, but they are also not briliant as the circlip is .35mm thick and the spring clip is 0.58mm thick. Since the groove in the clevis matches the spring clip it leaves the circlip a little loose from side to side. Also, I could only fit the circlip with the use of a magnifying glass as it was too small for me to handle....when fitting an external circlip it must only be opened up just enough to clear the shaft it is going on to - If it is opened more it will be permanently deformed adn then be loose. This could not be done well out on an airfield. Conclusion, use the circlips on the elevator and rudder hinges (that will likely never be removed) but not on the aileron control link.

So, I started searching for some of the retention clip type pins (top left in the top photo) and while doing this I came across the specification for the 3mm fittings. Is anyone aware that the load rating for many of the 3mm clevises and threaded rod we use is 15kg?.....yes, just 15kg.

Also while seaching I found that the big rc suppliers (extreme size planes etc) all supply their fitting in m3 or m4 depening on size of plane. Now this is quite interesting because it ties in with me finding the spec for 3mm fittings is a little borderline and i should be using m4.

Now the next finding. If I go to m4 fittings we are now in the size range of all the standard engineering component manufacturers who usually supply from m4 upwards (m3 is obviously mostly just made specially for RC). Check these out for example, just £1.43 for an m4 fitting...
http://www.ondrives.com/bearings-clevis-components-m4-m16.html
Nice fittings.

Also, m4 ss threaded bar is stiff enough to use (for short push rods) without having to encase if carbon tube.

[Andy Boylett]

Our local club "bring a model night" is in 2 days time so I am starting to complete the assembly of the plane ready to display it.

Now we have decided how to retain clevis pins I have assembled the rudder and fin. Right back on page one of this thread I showed pictures of how the scale hinges for the rudder and elevator would be made. The only change to this was at Phil's suggestion to not use brass tube, but to use a harder material. The raised the problem that brass tube was available with a 5mm bore to suit the hinges but nothing else I cna find has a 5mm bore. Hence, I have just truned up on my lathe some of the smallest items I have ever made. These are stainless steel tubes, made from ss 10mm bar. I drilled the 5mm hole first and then turned it down to 6.5mm OD - very carefully . I had to do it in legnths of 40mm at a time, with a rolling centre in the end to hold it in place. Here are the 9 tubes ready for fitting...

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I have now epoxied all the rudder pieces in place..

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The reason the fin is sat in the living room is because I find that after using epoxy on hinges it helps to sit flexing the surface back and forth until it is all set, thus avoiding the hinges sticking if any epoxy did get in the wrong place.

I think the finished hinges really do look just like their full size versions..

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[Phil Clark]

A dab of oil on the hinge pivots priot to bonding prevents anything sticking Andy...............

[Andy Boylett]

A dab of oil on the hinge pivots priot to bonding prevents anything sticking Andy...............

Hi Phil,
Thank you. I have also tried vaseline which is supposed to stay where you put it. The reason I don't like using either of them is I have had a problem with oil (and vaseline) getting where I didn't want it and then stopping the sticky stuff sticking. I have also found that if even a tiny spec of oil gets on the woodwork then the covering will not stick either (when using iron on film). Hence I now opt for keep them clean, but moving, which works fantastically.
Cheers
Andy

[Phil Clark]

I always cover the surfaces 1st, seal with dope, then bond hinges.......the hinge holes can simply be covered over, then open up and tidied and sealed before the hinges are glued (far less faff than tryng to cover around pre installed hinges). I apply oil with a tiny paint bush, so never apply very much....never been an issue because the timber is covered & sealed so the oil cannot soak in.

[Andy Boylett]

Thanks Phil
Yes, I can see doing that way has its advantages. I might have to break the habit of a life time and have a go the other way round ......I have always completed the hinges before covering and used a hinge type that can be dismantled for covering. I would say I have 95% of the time used iron on covering (and sit in the living room doing it!) and it is much easier with th esurface off. I particularly like the kavan giant pin hinges because the pin is relatively easy to remove and put back.
Cheers, Andy

[Andy Boylett]

I have been experimenting with 2 different designs for the tail wheel support. The full size plane has a large curved spring with a fitting to carry the trailing rear wheel. The centre of this wheel is 300mm behind and 224mm below the rear edge of the fus. So, I need to build some sort of 'srping' that will carry the rear wheel 150mm back and 112mm down.

I started building a 'leaf' spring with laminations of a saw I cut up. This has actually turned out really well but it is quite heavy so I decided to have a go at making a composite one.I drrew out the shape of the curve I wanted to make and then cut out a 'mould' in a piece of wood. The mould is lined with plastic and has a stainless steel tube built in - this will be the pivot and support for the tailwheel later. I did a bit of searching on the internet and worked out that I would need 26 layers of glass strand and 3 layers of carbon to get the stregnth I need. There are also 20 smaller pieces of glass added in 2 places for additional strength - this is around the ss tube and at the start of the curve where the highest stress level is.

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Here it is all laid up and then clamped. I used to work in advanced composites development making high glass content moulded grp components so I know a little about clamping force. If you go too high the resin is squeezed out and the glass content will increase (as a percentage). Unclamped the glass content is around 20 to 30%. Fully clamped the glass content can go up to about 84%, but at this level the part is weak and will delaminate easily. The maximum stregnth is at around 70% glass. The best way of achieving this is to make a number of parts and use different clamp pressures. However, there is an approximate guide to getting it if like me you only want to make one part.....
- Use of lots of clamps, spaced a little apart.
- increrase pressure on each one slowly and gently...keep a count of how many turns you do on each clamp, starting from when it was just touching.
- watch the resin being squeezed out from the sides...at first it will be only resin, then as pressure increases a little glass will start to come out....you really have to use a small piece of wood to investigate around the edge of the mould so you can see when the glass starts to come out.
- After you see glass starting to come out, keep incrreasing pressure about 15% more (if you have done 10 turns on each clamp then go another 1.5 turns).

Here is the mould as I opened it...

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You can see the built in ss tube there.

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[Phil Clark]

Polyester or epoxy resin Andy?

[Andy Boylett]

Polyester or epoxy resin Andy?

Polyester.

[Phil Clark]

Put me right, but I've always undertood polyester to be a lot more brittle than epoxy, so epoxy would be better suited to this application would it not?

[Andy Boylett]

Hi Phil,
The main reason I chose polyester is that I did the layup in 3 goes until I got the overall stiffness right and polyester lets you do this whereas epoxy less so. Also, as we discussed n the phone, the epoxy resin I have (the sp106) does not like to wet out carbon fibre.

In terms of properties. Polyester is brittle in solid form, but not as brittle as the glass mixed with it in the composite! Once in a composite it is really quite tough and resistant to splitting. Some years back a lady drove into the side of my kit car (GRP body/chasis) and it totalled the front of her car...bumper, both wings, lights. She hit the side of the bonet area on my car, which was all grp. I got out expecting to find disaster, only to find no damage at all...not even a little split .

[Andy Boylett]

Tailwheel assembly is now finished and appear to be about right. As Phil said to me, we will have to wait and see what happens when it 'bumps' the floor

Here is the complete unit..

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The wheel is the right size but temporary until I either find a suitable one or make one.

The steering unit is home made and the aluminium wheel holder is attached to the steering bolt by a silver soldered plate hidden inside..

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You can see the layers of glass and carbon quite nicely in the photo above. From the top it goes carbon, 16 layers glass, carbon, 10 layers glass, then carbon again.

I might have to do something similar at the top end if the steering arm spins - it is only held in place by 2 nuts..

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[Phil Clark]

With the model on it's wheels Andy, there is going to be A LOT of load on the steering servo as the tailwheel yoke is mounted at 90 degrees to the composite spring, so with the model in a nose up attitude on the ground, it has a fair degree of forward rake........ideally, you need a rearward rake (caster) so that as soon as the model moves forwards, the wheel will in effect self centre straight........the way you have it, it'll want to try and twist through 90 degrees all the time and the servo will be constantly loaded trying to prevent it.

Phil

[Andy Boylett]

Thanks Phil, I'll make the yoke so it angles back 20 or 30 degrees, a'la a caster. Cheers Andy

How's this?

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I have moved the steering as well so it is attached directly to the wheel assembly..

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[Andy Boylett]

All hinges now finished, with the elevators completed...

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There was quite a lot of efort involved in making these hinges but they are really positive. There is full ballrace in each hinge providing alovely smooth operation, even when I heave as hard as I can on the edge of it while moving it. Also, because the hinge uses a machined clevis fitted over a 3mm aluminium plate, there is no lateral miovement at all, no wiggle, no wobble.
Here is the original photo of the hinge parts. The brass tube shown is now made in ss and the spring clip has been replaced by a circlip...

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[Mike Booth]

I had no idea that there was so much science in circlips. Im a splitpin man myself, no chance of pinging off.

[Andy Boylett]

I had no idea that there was so much science in circlips. Im a splitpin man myself, no chance of pinging off.

Hi Mike,
It was just the engineers playing ping-pong with each other - we like doing that I agree with you that splitpins are good

I like the splitpins, but they are also difficult at this size as they are fiddly - also as phil says, they would need replacing if reused becuase they break very easily.

For the control clevises I am now using 4mm ones which are more robust and have the 'retention clip' as per the photo at the top.

For the hinges, these are 3mm and I have got the circlip on at the moment, but I might have a go at drilling a 1mm hole in the end of one of the pins.

[Andy Boylett]

Well I managed to get it assembled for my local clubs 'Bring a Model Night' yesterday..

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You can see the tail wheel assembly quite well above. This has already proved to be no good. There are 12kg of batteries, all of which go right at the front, in front of the main wheels. Without these batteries in place the weight at the back is rather large as there is no counter-balance....and the tail wheel gets squashed.

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My son Sam always liked to play 'Tetris' where you fit shapes into a space. Here is my car after we managed to get the whole half scale plane in...

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No room for Sam though ..

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[Nick Reeves 3055]

wings on the roof???