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Aerobatic Slope Soarer

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Martyn K17/04/2015 11:42:39
4541 forum posts
3169 photos

I have been toying with this idea for a while (at least a couple of years).

Basically, I want to design and build an aerobatic slope soarer which has the appearance of a modern moulded plastic missile but is actually constructed using traditional balsa/spruce and perhaps some glass cloth. My mind has been refocussed as there has been quite a bit of interest generated in Slope Soaring at our local club (South Cheshire RCS) - mainly thanks to Ian Jones - and we are nicely located between the Peak District and the mountains of Snowdonia and of course the Great Orme.

I can't afford a moulded machine (nor do I really want one anyway), I don't like foamies and kit options are limited and I actually enjoy building and flying my own creations. I do however like the appearance of some of the lovely 3m machines around at the moment.

The 3 view looks like this at the moment.


The wing planform is a double ellipse with a fully symmetrical wing section.

The section is a NACA 63010-mod with peak camber at about 35%. The incidence of the wing is +1.5, tail 0, I think that should be OK but I'll do some sums to get the CG correct and may revisit that.

root-wing section.jpg

It's 10% thick - about the thinnest I can manage using traditional construction methods, its not intended nor will be stressed for DS - just simple aerobatics - which is where I started when I first got into R/C flying back in the very early 1980's. Full circle perhaps.

I have had to tweak the planform slightly to get at least one straight line in the wing. The TE spar is straight and will be used as the datum for the wing construction. The mainspar will be a spruce/balsa laminate composite.

The LE will also be laminated. The wing will be covered top and bottom in1/16" sheet balsa.

The wing is 2 piece to simplify transport - thinking holidays here - it is also economical - designed to be built using 36" lengths of wood. The overall span - including those sheet tips is just a fraction over 2m.

The fuselage is even more challenging.

It is based around a 3mm spruce strip crutch and then planked with 3mm balsa. It will be built on a 4mm ply jig to hold it all in place and keep it straight while the upper planks are fitted.

A couple of snapshots from DevFus




The tail surfaces will be very traditional - cut from 6mm balsa sheet..

The wing will be a 4 servo wing, the inner 2 (or 3) bays will be flapped and then switched in as integrated full length ailerons when flaps not required.

I am planning to make DXF files available for all parts if anyone wants to copy it in the future.

Any interest in this?

Best wishes


Peter Miller17/04/2015 12:00:52
9007 forum posts
1040 photos
10 articles

Not a slope soarer myself (suffolk doesn't have much in the way of slopes!) but that is a lovely looking, sleek design

Martyn K17/04/2015 18:37:47
4541 forum posts
3169 photos

Thanks Peter

hopefully, I'll be able to maintain that shape when it gets converted into reality.

Martyn K21/05/2015 09:36:40
4541 forum posts
3169 photos

I have made quite a lot of design progress over the past few weeks. A set of parts has been produced on CAD and sent Dylan at Lasercraft to make a mini kit for me. This comprises of balsa wing ribs, ply ribs and liteply formers.


So while Dylan does his bit, I have started worked on the fuselage jig. I am using DevFus for the fuselage design, the fuselage will be built around a laminated horizontal crutch with the formers slotted into place. The program will produce a file for this and this is what it looks like.


The fuselage length is 1200 mm long (plus noseblock and rudder) and the spacing between them is 133mm.

I have chosen to cut 1,2,4,6,8,10 for my jig. If I need to add the intermediate ones later I can do.

The support frame is10.5mm square pine from B&Q and the jig verticals cut from 4mm builders ply

You don't need the T shape, nor the little notches (which are a count of the jig number). The side slots have been widened rather than cutting 4mm slots in side and base so it simply slots together.


These are the verticals


The board is an old shelf which was about 50mm too short so the back just overhangs. The lower 'longeron' is nailed to the base (against a straight edge and the verticals glued into place.




Apologies - out of focus. The side longerons added and glued into place checking all parts are laterally and longitudinally vertical/horizontal


A few gussets and copious quantities of glue, this ply does not absorb glue very well

More to come




Edited By Martyn K on 21/05/2015 09:38:59

Martyn K15/06/2015 13:41:12
4541 forum posts
3169 photos

Not a lot of feedback so far...

Still - onwards and upward Gungadin.

While waiting for the Kwik Fli IV epoxy to set, I thought that I would make a start on this, this lunchtime.

Dylan at Lasercraft Services has cut me a parts kit.. Thanks


Quality is excellent as usual, nice quality wood of the right density. Basically, formers are lite ply, root ribs are 1/16 ply and ribs are 2mm balsa plus a couple of extras

To (hopefully) prevent me ending up with a jig shaped fuselage, I have rubbed G Wax into the jig formers wood around where the longerons and fuselage formers will sit.

Have a dry fit of the formers in the jig and check that the slots are just the right size to take 3 of 1/8" spruce strip. Make any adjustments now - its easier before everything gets covered in glue from (speaking from very recent (in the past 45 minutes) experience blush)


Clamp the formers into place - making sure they are the right way up..

.. then start laminating longerons. The outer lamination sits outside the formers. This will be the sanding reference and also provide a little extra anti dinging (hopefully)

The joints are simply butted together, but all the overlaps are in different places. On mine, I have placed the outer laminate in the center and extended fore and aft..


The outer longeron is tapered towards the fin..


F6 is doubled up.. The central slot will be removed - this is where the wing joining tube will be located


The front 2 formers are lower half only - this is where the canopy will be..


Lots of peg clamps to hold it together until the glue sets..

More to come - but as you can imagine, it will involve lots of planks... The intermediate formers will be added next - probably tonight


Martyn K15/06/2015 22:36:11
4541 forum posts
3169 photos

Not sure why that last piccie appeared in the wrong order, it should have been this one:


This show the slot for the wing joining tube that will be opened out when most of the sheeting has been completed.

So, tonight's activities.

I slotted in the intermediate formers


They all fitted well, just a couple of clamps to hold them while the glue set.


An error in my CAD file meant that F3 was in 2 pieces so had to be joined. Not a total crisis, I'll need to correct that though

and then finally, I laminated the spine from 3 pieces of 1/8 spruce


The very front former (top of F2 in the photo) is false and will be removed to make way for a canopy. That one was intended to be split, Its purpose is to simply set the curve for the spine and will be removed when the planking is finished

I appear to have lost the top of F1 as well..

Still, you live and learn. At the moment its all jigged up while the glue dries. Hopefully start putting planks on tomorrow. Some of the curves have quite a tight radius so it is going to be quite a challenge to get a decent fit

More to come


Lindsay Todd15/06/2015 23:58:37
1657 forum posts
1627 photos

Very nice looking design Martin, sleek and stylish, Like the name too, another prod in the ribs to give slope soaring a a go. Linds

Martyn K16/06/2015 09:22:40
4541 forum posts
3169 photos

Thanks Linds - appreciated.

This mornings activities..

I started planking..

However, first though , I needed the former that delimits the rear end of the canopy. This looks like another CAD error on my behalf - or rather, I sent the wrong former to Dylan (from an earlier version) for cutting. The F11 was too short so needed to be extended slightly - which is what I did.


A simple balsa base with a spruce back. It will have a 1/32 ply face when completed so I am not too worried about it - just irritated


Then onto planking..

The curve around the upper deck is quite sharp so narrow planks in this area are required. Stripped some medium soft 1/8 balsa and sand a bevel on one edge. Glue into place and repeat for the strip on the same place - other side.


Lots of peg clamps to hold them together and also to maintain the curve of the backbone. I use PVA for the adhesive then leave it for about 30 minutes to allow it to dry.


Repeat for the next two strips - you can see this is going to be quite a long process and I promise you will not get an update after every plank is fitted.


Looking from the rear.

From F8 (shown) to the base of the fin will be soft block. The planks don't look brilliant, but the step is only about 1/32" and will be invisible when sanded. As the curve evens out, the steps will be far less noticeable.

As you can imagine, planking is quite a slow process - mainly waiting for glue to dry, but the next two planks are slightly wider - and the next two wider again. It will soon get done and its very rewarding to see it slowly get covered as the planking progresses


More to come


Edited By Martyn K on 16/06/2015 10:13:25

Martyn K18/06/2015 08:56:18
4541 forum posts
3169 photos

On with the planking. Didn't get an update yesterday but did reach a milestone last night

I completed planking the upper half of the shell and in the main I am very happy with it..


Lots of pins


Now that the rear planks have been trimmed, it looks somewhat better than the photo from a couple of days ago. Sanding at the stage involved just removing the 'edges' - it will be sanded down more thoroughly before it gets covered


And the front former (behind the canopy) after it has been trimmed and rough sanded. A little ragged on the LHS (of the pic) but acceptable


Now I can get it off the jig I can examine the inner surface. I am pleased with that - generally quite tidy. I took the fus (on the jig) down to our club night on Tuesday night (at the pub!). I was asked how I do the edge joins. I have done quite a bit of planking in the past. Obviously, when working around a curve, you need to bevel the edges to the planks to fit against each other. I think that the mistake is try and bevel both edges. Use a stripper to cut parallel strips - on this the main planks are 1/4 x 1/8 - cut with a stripper. I then place the plank vertical and trim a bevel using a razor plane. All done by eye - after a while it gets easy to judge the correct angle to hold the plane. The other side is left square. If you try and bevel both edges, you will simply end up chasing yourself round in circles. I have also stopped trying to cut tapered planks for the same reason. Just taper the edges where you need to fit them. It all gets covered up in the end so no-one will notice

This morning - after another bad nights sleeps and hence early start in the workshop, I removed the fuselage from the jig. Now in hindsight, it would have been much more sensible to have done the bottom first - there is more of it, however, by removing the middle jig supports, I have got it in the jig, cut the lower slot for the lower backbone and fitted it - this one is laminated from 2 off 1/8 sq spruce

Now being left to dry..


More to come..


Paul Nazaroff18/06/2015 14:40:23
7 forum posts

Looks like an excellent project Martyn. I have been thinking of building something similar using DevFus and DevWing. Would the DevFus and DevWing files be available? I prefer an all moving tailplane so would like to redesign that area.


Martyn K18/06/2015 15:02:41
4541 forum posts
3169 photos

Hi Paul

I used Profili for the wing. You are welcome to the files, I'll package them up into a zip file and let you have a download link


Paul Nazaroff18/06/2015 15:06:50
7 forum posts

Thanks Martyn

Martyn K18/06/2015 15:10:24
4541 forum posts
3169 photos

Forgot to mention

I considered an all moving tail and if I develop this design further - I am thinking of a 100" and 132" variants at the moment - which will have a Thermal soarer option, then I'll do an all flying tail. However for the slope, ruggedness takes precedent. The tail is quite high mounted and I was concerned about its susceptibility to damage. It would be very easy to modify except that there is not much space in the fin for an all flying horn



Edited By Martyn K on 18/06/2015 15:10:48

Martyn K19/06/2015 22:37:19
4541 forum posts
3169 photos

A little bit more progress today

Planking has started on the lower fuselage, its quite tricky around the bend by the fin post but apart from that is mainly more of the same..


One thing I nearly forgot to do was fit the pushrod tubes for the rudder and elevator. It would have been very entertaining trying to fit them after the planking had finished


At the rear


and at the front..

Note both these photos are actually upside down 0 hope they don't upset your sensibilities and/or OCD

More to come


Alan Pennington20/06/2015 07:19:38
174 forum posts
46 photos

It is looking great, await further updates

John Privett20/06/2015 09:54:33
5672 forum posts
219 photos
Posted by Martyn K on 15/06/2015 13:41:12:

Not a lot of feedback so far...

Only just spotted this thread... Looks good, I shall continue watching with interest! thumbs up

TheFlyingCrust20/06/2015 10:42:15
611 forum posts
67 photos

Just spotted this. The fus shape seems to take more than a nod to your love of classic aerobatic planes.

I shall be watching.


Martyn K28/06/2015 23:09:00
4541 forum posts
3169 photos

Just a bit more progress. The Kwik Fli has been hogging the limelight, but I have progressing the planking slowly


Just working down one side, adding one plank at a time and letting it dry. The pins are pushed through from one plank to the next making sure the line of the fuselage is maintained


A couple of days later.. This is as far as I can go. I need to get the other side this far - the next big job will be to fit the wing joining tube which sits in this slot


which will be opened up at both sides when it is fully supported by the planks.

Also need to add the rudder push rod

One area that I am bothered about, I don't think the fin is large enough and may redraw with the fin about 50mm taller. Not a show stopper at the moment though

More to come


Steve Houghton29/06/2015 13:57:06
510 forum posts
342 photos


Nice to see a 'totally traditional' craftsman at work. Loving all the planking, and its a great looking design. Very reminiscent of some of Keith Thomas's planked designs from the past (I still have the plan for Angelina, which I'll build one day), with a modern update.

Following with interest.

Martyn K29/06/2015 15:09:23
4541 forum posts
3169 photos

Thanks Steve

The planking is getting to me now.. Majority done though and its getting back to the interesting bits again


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