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Mar 06, 2010, 09:58 AM
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Trevorh's Avatar
Build Log

Cliff Charlesworth T61 Falke Motor Glider


Although I have been writing up my model builds for some time on my website, this is my first attempt at a build thread. I will try to keep it as real time as possible - if only in the hope of getting a few hints and tips along the way - preferably before I have gone wrong!

(Update, April 2010: The T61 has now been added to my website here)

I have built a number of electric sport and scale models from plans, but this will be my first motor glider. I haven't finalised the choice of prototype but rather fancy the red and white colour scheme of G-BUIH.

Cliff's T61 plan is to 1/4 scale, giving a span of 150in and a wing area of about 1600sq ins. No AUW is given but a target wing loading of around 1lb/sq ft implies that 10-11lb would be a good outcome. I was therefore rather dismayed when the bundle of wood I ordered weighed about 21lb! Much of this is spruce and ply and, as one whose formative building years date back to the days of 1200mah NiCds and ferrite can motors, I am used to avoiding such heavy materials wherever possible.

However, with such a high aspect ratio wing, one area where I felt I should not compromise was the main spar so this is where I started.
Last edited by Trevorh; Nov 20, 2010 at 12:55 PM. Reason: Updated web links
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Mar 06, 2010, 10:03 AM
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Trevorh's Avatar

Spar Building


The top and bottom spars are of 1/8in spruce, 1/2in wide for the inboard section then tapering to 1/4in at the tip. With a sharp knife, I just about managed to cut the tapering pieces from a sheet of 3mm spruce, then spliced them onto the straight inboard sections.

Sorry, I couldn't quite get the camera far enough away to get the full length of the spars in but it is already clear that my building board could do with being a couple of feet longer!
Mar 06, 2010, 10:30 AM
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vonJaerschky's Avatar
Great subject Trevor! At that size it should be a real soaring machine. I suppose she won't be coming to Chilliwack with you. I'm watching with interest.
Mar 07, 2010, 10:13 AM
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portablevcb's Avatar
Also interested. Almost got those plans too. 1/8 scale would be 75", just about right
Mar 08, 2010, 02:52 AM
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Trevor,

I had one of those sets of plans but couldn't go forward with the build for other reasons. In correspondence with Cliff memory says that he was suggesting a built model weighing around 12 -15 pounds fitted with IC. Again from memory he said that people had wound up with some aircraft in the 20 pound region but no reason for that to be the case.

I can imagine your bundle of wood weighing what it might but you'll cut a lot of material away. Much like kits that weighed a heap until all the spare was discarded.

Good luck as it is a lovely aircraft. To check on "images" within your search engine as there are a mass of beautiful schemes.
Mar 08, 2010, 05:31 AM
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Trevorh's Avatar
Thanks guys for your encouragement. Perhaps I'll revise my weight target to 15lb. I'm not expecting contest-winning soaring performance so as long as the wing loading doesn't get too high it should be okay.

Spar building continues with the fitting of the 1/32 ply outer sections of the front and rear spar facings. The balsa spacers are my addition to help keep the spars aligned during the process - with 1/64 ply wing skinning, there is no scope for smoothing out any waviness in the spars!
Mar 08, 2010, 06:09 AM
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paulbw's Avatar
Hi Trevor,
I'll follow your build too - this will certainly have presence as it floats slowly past! I freely admit to resorting to a dust mask and sander the last time I had to come up with tapered spars!
Paul W.
Mar 12, 2010, 07:20 AM
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Trevorh's Avatar
The plan shows the brass joiner boxes supported in two ply formers fitted inside the spar. I didn't fancy this so instead used a system of balsa wedges as shown in this photo sequence. Because of the large glueing area, I tried not to use too much epoxy so stuck with Aliphatic resin and 'cyano tacking' for most of it. There is still the option of glass/epoxy wrapping the spar if I get cold feet but it does all look pretty solid.

The other mod I made was to leave a gap between the brass tube and the piece of spruce blanking off the end. My thinking here was that any debris that may find its way into the tube has at least got somewhere to go.

Next step is to cut some ribs, so I may be some time!

Trevor

P.S. Any tips on how best to cut the steel joiner blade?
Mar 23, 2010, 10:20 AM
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Trevorh's Avatar

Rib Cutting


After trying to cut the first batch of ribs using the sandwich method, I eventually gave up and traced each profile in turn. It took me a week to cut out a set of blanks.

Some ribs - the three nearest the root and those supporting the outrigger wheels - are shown on the plan as 1/8in ply. However, to keep things simple at this stage, I have cut them all from 3/32 balsa and will apply 1/32 or 1/16 ply doublers where I think they might be needed.

So this is what six square feet of 3/32 balsa produces.
Mar 24, 2010, 09:57 AM
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Wing Building


I picked the brains of fellow modellers about how best to set about building this wing. Basically, with full depth spars, I was worried about loosing the alignment of the front and rear portions of the wing.

In the end, I placed each rib vertically over the plan and used a set square to mark off the front and back of the spar and the rear face of the false leading edge, trying to remember about the washout and the strange 'crank' towards the root of the trailing edge.

It came as something of a relief when at last I made a start glueing a few ribs in, using a length of aluminium angle as a dummy false leading edge. The batteries are just to make sure that the spar stays flat on the board and doesn't rock backwards if the fit of any of the ribs is a bit on the snug side.
Mar 24, 2010, 05:30 PM
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paulbw's Avatar
Hi Trevor,
I too find aluminium angle to be a very useful tool for keeping wing components in the right places. With a pre-built spar I suppose there is no option to use shear-webs as spacers for rib location?
Paul W.
Mar 26, 2010, 06:12 AM
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Trevorh's Avatar
I agree Paul. Although the box spar is undoubtedly very strong, I must admit I don't really like this method of building. With top and bottom spars notched into the rib, the rib looks after the integrity of the airfoil and the ribs also act as a jig when fitting the top spar. As you say, the shear web pieces can then be used to help to align and space the ribs. With the box spar method, the spar has to be built very accurately otherwise there is a risk that when you cut all those wing ribs in half, they don't match the height of the spar. So far though, it is working out reasonably okay.

Having got the false l.e. on, I think I will now bring the other wing up to the same stage before taking this one any further.

Trevor
Mar 27, 2010, 05:52 AM
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pentaxman's Avatar
I am going to watch this one with a great deal of interest.

I can't currently lay my hands on my 3822, but I think I have around a couple of hours flight time in a couple of these machines flown at RAF Swanton Morley in Norfolk.

I recall one having an electric start which made life easier than the other one with a hand crank inside the cockpit, mounted just under the instruments.
The flight pattern required the glider to come in under power so if the engine was shut off in flight (quite normal) then it had to be restarted - not so easy with a hand crank.

After watching many flights, it was easy to guess the hand crancked machines. They were the ones wobbling into the circuit!

Great start there Trevor, my only concern would be the apparent rather short length of the wing joiners.

Just clarify please, you have a oblong shaped brass tube fitted inside a balsa block which is the middle piece in the spar pictures?

How deep into the spar does this actually travel?

My only concern would be the gradual compression of the balsa above and below the spar box over time. Mind you this was not a particularly aerobatic machine so it should be OK.
Mar 27, 2010, 09:11 AM
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Trevorh's Avatar
The brass box is about six inches long. I know what you mean about the possibility of compressing the balsa. However the balsa wedges fore and aft of the brass box are glued to the ply spar facings so they are in shear and hopefully will reduce the compression load on the wedges above and below the box.

We shall see - eventually!

Have you anything on the board at the moment?

Trevor
Apr 08, 2010, 04:00 AM
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Wing Sheeting


Since the box spar is so torsionally rigid, I have decided to fit the 1/64 leading edge sheeting before building the trailing edge section of the wing. The skins were cut out, sanded rough on the inside and smooth on the outside, then joined. What really took the time though was sanding the ribs and false leading edge to get everything lined up. I always thought that 1/16 balsa sheeting didn't allow much scope for sanding out any ripples but 1/64 ply doesn't allow anything at all!

Finally though, I plucked up the courage to start fitting a skin.


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