Spacewalker half scale 168" electric

[Seamus O'Donnell]

Looks good are you following the full size construction

[Andy Boylett]

Thanks for the info Dave.

Seamus,
The full size construction is a welded tubular steel frame and wood added on in places. The wing is wood built around a square box spar. I discussed the build with the USA supplier of the kit for the full size and he sent me the drawings for it. Using these I loaded them into AutoCad and then produced my own design to give a perfect scale outline exterior. All areas, including cockpit and undercarriage are a faithful replica of the full size.......I am even manufacturing the undercarriage from SS tubing that I have sourced at exactly half diameter of the full size and the wing construction also follows closely the box spar design of its big brother.

The only thing I have not sorted out yet are the flight controls, the dash and a pilot.

Regards
Andy

[Andy Boylett]

Well, I had my first formal large model inspection by Phil on Wednesday evening and everything is OK to proceed. So, on with the rest of the fus.
Here is a side being made up from cyparis...

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Having not built something of this size before I am very keen to ensure it is built exactly straight. I have attached the rear sides to fus front with plastic sheet in the joint so that nothing will stick when I add glue. I have been jigging everything up by first setting up the fus front so that it is stting exactly horizontal side-to side and with the front firewall exactly vertical all with respect to the flat work surface. This way I have been able to use the work surface with a large set square to jig up the tail group before completing the building of the fus rear. This photo shows an 8 foot aluminium straight edge set along the centre line of the fus...

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And this pic shows all the use of the set square to jig up the tail group.....

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[Andy Boylett]

I am making the tail-group removal for easy transport so the next few photos show how I am attaching everything. This plane has strut wires between the stab, fin and fus so I don't need to worry about anything 'waggling' around.
I have made the stab so it sits on the fus and is located by 2 dowels. You can also see a hole in the stab where the leading edge of the fin slots through. You can also see a slot at the rear of the fus where the trailing edge of the fin will slot in.....

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To hold the fin in place (which in turn holds the stab in place) I have fitted a captive nut in the leading edge woodwork......

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This nut is then used to pull the fin down onto a fixed cross-member, from under neath the finished fus. This photo shows the cross-member made up ready for fitting to the fus....

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[Andy Boylett]

Today I have finished fitting the fin and then competed the upper part of the fus rear.
This photo shows the fin being slotted into place......

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And this one shows it fully in place.....

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The leading edge of the fin is slid in through a close fitting guide fixed in the fus to give it a solid vertical location. This photo shows this guide and at the bottom you can also see the fixed cross member through which the bolt is fitted to hold the fin in place.....

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And here is the fus as it now stands.....

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All 3.1m of it........

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[Andy Boylett]

Well here is the latest view of the model

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The fus rear is now finished and fitted to the front half wiht all fairings and trim added.

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The rudder is operated by a proprietry HobbyKing mounting, fitted such that the arms will operate the rudder from outside the fus...

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And underneth the fus there is an access door to get at the servos...

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[Andy Boylett]

The elevators are controlled by 2 servos each side with one servo controlled form each of the 2 receivers. Each pair of servos is connected by a linkage that allows both to operate giving full surface travel, or just one to operate giving half surface travel. Thus there are 4 servos, mounted side-by-side on 2 rails in the fus.....

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And then there are 2 removable hatches under the fus to be able to access the servos....

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You can also see on the above pic that I have inlaid a small hardwood strip on the lowest, most prominant part of the fus, just to help with hangar rash protection.

[Andy Boylett]

Now to the wing. I have had quite some debate as to whether to make the wing 2 piece or 3 piece. A two piece wing would give 2 panels of 7 foot by 26" wide, and eliminate the need for the wing joining tubes. I think a 2 piece wing could be nearly 2 kg's lighter. However, fitting the wing would become rather arduous as the plane would likely have to be held upside down. So, 3 piece it is.

Todays task, to make the wing centre section joiner, that sets the dihedral angle. This joiner also take a huge loading, with a 5g loop putting some 150kg load through this! It is made of a box section with cyparis spars top and bottom and 2 sheets of ply either side. Here are the 4 sheets cut ready for assembly...

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The pairs of ply are sheets are assembled thus....

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i weighed the parts before and after cutting the holes and the result was nearly 30% off.

[Andy Boylett]

OK, now I have built the centre wing joiner...

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And now I have started to build the wing centre section. This is about a metre wide and has wing tubes for the outer 2 panels to attach. Because I have used 2 wing tubes either side it is necessary to build the wing panels with the phenolic tube as complete lengths and then cut them later - if they do not end up completely paralell then they will not work!....

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And at the front are the attachments that latch over a cross-spar built into the fus (and there will be wing bolts at the rear)...

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[Andy Boylett]

Here is a closer view of the wing tube attachment, showing the reinforcing pieces next to the ribs....

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Because of the 2 wing tubes I have now started to build the first part of the right-hand outer wing panel onto the protruding tubes from the centre section. I have put spacers in so I can cut the phenolic tubes later....

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[Andy Boylett]

Now I have completed the basic structure of the wing centre section I have load tested it. I had aimed to measure the deflection for the load applied and hence be able to calculate the effective modulus and then be able to calculate its breaking load. However, with the method used to load it up I cannot measure deflection acurately enough as it appears to pretty much zero.

Load applied is 150kg at the wing centre rib, with the underneath supported across ribs 4 at either side....
This is the wood suppport underneath..

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Here is the set-up on scales ready to load up..

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And here it is with the first 60kg of weigths on, which was then followed by my 92kg standing on it

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I chose 150kg as this 5 times the weight of the complete fuselage assembly in a 5g loop. Considering there is lot more structure around the centre as well (and the fact that we do not intend to pull 5g) then this looks fine.

[Andy Boylett]

Now I have strated the outer wing panels. First need to build the spars. I am doing these as a box section, with 25mm by 12.5mm at the centre, tapering down to 25mm by 6mm at the outer end. Here is the first spar tapered.....

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And then the 2 box spars completed...

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Now, before building into the plane I have performed a simple test that will enable future designs to be perfected. It may be useful if a liabary of wing spar designs and strengths was held somewhere so that builders could call upon it?
Since my box spar is tapered, the effective centre load point will be 1/4 of the way out from the thick end. So, I have lifted the thick end up 6mm and then loaded the marked point with weights. I was intending to load it until there was no gap under it (a deflection of 6mm), but after applying 50kg the spar had deflected 2.8mm. The 'batteries' either side of the spar are just for lateral stability to stop it trying to twist sideways...

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And here is a close-up showing the gap still under the spar (that 3mm piece of ply still passed under easilly)..

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Now, armed with actual data I can calculate the modulus of the spar as built and refine for future designs.

[Mark Partington 2989]

Now I have completed the basic structure of the wing centre section I have load tested it.

Andy, just one (to me) error is that you've tested the spar upside down, the load paths/stress points are very different when you reverse the geometry.

To get a pretty accurate assessment, follow full size practice and test it the right way up, by suspending the spar by the ends, then hang the weights from the centre of the spar. To test the finished wing structure, you'd need to suspend the centre section by the joining tubes, unless you're using steel plates to join the spars at the join in which case test as 'just the spar' above.


Mark

[Andy Boylett]

Mark,
Thanks for the thoughts. There are 2 tests I did...

Re the centre wing joiner test....
The stress paths in this structure are identical whichever way tested, just that they switch around from tension to compression. So, what happens when testing the other way up is that the parts that were in compression become the ones in tension and vice versa. I have tested as though doing an outside loop, simply because it was easy to load up that way. I agree with you that in other wing structures you would get different results depending on whch way up tested, but this is because of the none symetrical nature of the wing structure and the fact that some materials behave differently in compression and tension. My wing spar and joiner is totally symetrical so it does not matter which way up it is.

Re the spar test...
It is totally symetical and can go either way up so makes no difference at all.

Cheers, Andy

[Mark Partington 2989]

My wing spar and joiner is totally symetrical so it does not matter which way up it is

Andy, I think you're confusing terminology, as what you call the joiner is the main spar in the centre section. Anyway, the paths would only be symetrical if it was flat, (so the following doesn't apply to the outer wing spar which is symetrical).

The simplest way to explain the load paths and stress points in shapes that have a change in direction, (as explained many years ago during the aeronautical engineering phase of my training, it was also demonstrated), is to imagine a piece of string across the tops of the 'V' shape, when tested upside down the string is in tension - helping to resist the shape collapsing, now turn it the right way up and the string is in compression - doing nothing to help the 'V' to retain it's shape.

Try it on a test piece if you don't believe me and you'll see what I'm saying.

[Andy Boylett]

Hi Mark,
No I don't think I was confusing anything. The centre joining 'spar is made with solid wood that is not 'string'. Unlike string, this wood can take load in compression and hence it matters little which way up it is tested. The 'string' example often used in aeronatics is I beleive because the lower part of an aeroplane main spar is designed to take a high tension load (like your string), but because it is relativley thin it cannot take a similar compression load and will buckle.....it is used to try and get peopl to understand designs with tension only components. The deisgn of my spar is the same either way up and the only difference is that is ikely that the side in compression will fail first because the ply will delaminate....but up to the point of failure it will still behave with a normal compressive modulus.

If you are using the string example to say that the friction between the floor and the 2 load points of the wing is assisting the load then I think it is negligable. The wing angle is 4.5 degrees and the load points are the edge of ply the ply ribs. Testing this way would usually be on rollers but in this case the angle was very low. If friction was absoltely perfect then the maximum 'assistance' by the ground would be a factor of 0.078 (tan45 deg) or less than 2%.

Cheers, Andy

[Mark Partington 2989]

No I don't think I was confusing anything.

Andy, I'm talking about this, which is the main spar in the center section, irrespective of it looking like a wing joiner.

OK, now I have built the centre wing joiner...

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Hi Mark,
The centre joining 'spar is made with solid wood that is not 'string'. Unlike string, this wood can take load in compression and hence it matters little which way up it is tested. The 'string' example often used in aeronatics is I beleive because the lower part of an aeroplane main spar is designed to take a high tension load (like your string), but because it is relativley thin it cannot take a similar compression load and will buckle.....it is used to try and get peopl to understand designs with tension only components. The deisgn of my spar is the same either way up and the only difference is that is ikely that the side in compression will fail first because the ply will delaminate....but up to the point of failure it will still behave with a normal compressive modulus.

The string is an analogy, used to demonstrate the difference between trying to narrow the angle between the vee and trying to increase the angle. The highlighted part isn't really accurate as the spar booms are nearly always the same cross section and material, (for various reasons that are basically irrelevant to this), as you say - the upper in compression and the lower in tension, which are joined by one or more webs acting in the shear plane forming either a box, 'C' shape or 'I' beam. The design is different, but the construction is the same either way up, and the ply will more likely fail at the point of the one of the 'diagonals' where you've cut the holes in it, due to not having a continuous grain along the shear path.

If you are using the string example to say that the friction between the floor and the 2 load points of the wing is assisting the load then I think it is negligable. The wing angle is 4.5 degrees and the load points are the edge of ply the ply ribs. Testing this way would usually be on rollers but in this case the angle was very low. If friction was absoltely perfect then the maximum 'assistance' by the ground would be a factor of 0.078 (tan45 deg) or less than 2%.

It's nothing to do with friction as testing would be carried out with the part suspended. In your case the centre would be 'fixed' and an 'upward' load applied to the ends trying to 'straighten out' the vee, in my case the load would be applied upwards trying to 'close the vee, and that's where the difference in the outcome arrives.

Edit: All this discussion is purely academic of course as this piece of the Forth Bridge you've built isn't going to break under normal, or even fairly extreme, flying. The real weak point is the two ali tube joiners, or rather the wing structure that supports them, (a pet dislike of mine as they put the stresses away from the spars, they do have their uses though, short locating tubes).

Ask Phil Clark to show you the S/Steel plate joiner system, much lighter and they put the loads and stresses back where they belong, i.e. on the spars.


Mark.

[Tony Collins 1073]

As I am just an ordinary mortal, with ordinary concepts, I am totally bewildered as to why someone would deliberately weaken what is possibly the most important structural part of an aircraft in order to save an absolutely ignsignificant
amount of weight which would not make a hap'orth of difference to the wing loading on an aircraft of this size, but a very significant difference to the strength of the structure. I am referring of course to the wing joiner/main spar, call it what you will, and also the wing ribs which look have had an awful amount of material removed. Please enlighten me someone.

[Tony Collins 1073]

Sorry. Double post

[Andy Boylett]

Hi Tony,
no problem, I will explain. The main joiner is made of cyparis spars top and bottom, joined by ply 'shear webs' either side to form a box. I originally designed the plane so that the centre joiner would be the thing that joined the 2 wings together, sliding into the 2 sides. I then decided that a 2 piece wing was too big (each 7 foot by 26") and opted for the 3 piece. Hence the 'joiner' became part of the centre wing structure - for which the ribs had already been cut.

At the centre the cyparis spars take no load. Away from the centre the cyparis starts to take the load because as the joiner tries to bend the top and bottom spars try to be different lengths to accomodate this, thus if the plane is doing an inside loop the lower spar is in tension and the upper in compression.

I called the ply a 'shear web' in quotes as it is not strictly a shear web. At the centre it is the structural component taking all the bending load of the 2 wings trying to fold up in an inside loop, or bend down in an outside loop. To do this requires the 4 layers I have used. The main area of stress for these webs is right at the centre. As you move away from the centre the load starts to transfer from the webs to the cyparis. This is why the cut outs near the centre are shaped as they are, following the stress 'lines' as the load transfers out to the 2 spars (the triangle shaped cut out). Once more than 150mm away from the centre I could have dispensed with the heavy ply and gone back to a much thinner ply shear web. Rather than try and modify all the ribs I opted for using the ply as cut and removing the low stress areas to make it light. The 4 ply pieces weighed over 2lbs (985grams) before cutting and I managed to remove 30%! I have left full thickness where there is a rib attachment or a wing attachment, then removed material in-between. However, it is still a very robust centre section!

As for the ribs, they are virtually as per the full size, which I have been trying to replicate. All the ribs are 3mm lite ply apart from the end ones where wing tubes are and these are 6mm. The load carrrying part of the ribs , like the spars, is the outer portion. The inner part of a rib does relatively little. The material left in th einner part of the ribs is to 'hold' the outer parts in place, attach them across to the opposite sides and provide stability. I did all the design work in electronically in AutoCad and SLEC were going to be supplying and cutting all the ribs. This meant that I had the option to design the ribs like the full size, with a complicated cutting pattern because I knew that SLEC would be cutting them directly from the electronic files. The weight reduction was more than 50%.

I now have the left hand wing panel together, but with no sheeting, no cap strips and no leading/trailing edges. As this size of wing is new to me I have been testing everything constantly as I go. I have tried twisting the wing and it feels pretty good already.

Happy to try and answer quetsions, its still a learning path for me as I haven't builtone this big before.
Cheers
Andy