Jim.Thompson's blog View Details
Posted by Jim.Thompson | Apr 05, 2020 @ 06:17 AM | 5,503 Views
A buddy and mine picked this up at the local scrap metal heap at the tip.
After dismantling and inspection, I decided it was in surprisingly good order inside, despite the rough exterior condition.
I have cut it down in the usual way, increased the compression ratio and adjust the port timing to suit.
I made a spark plug adapter to fit a CM-6 plug, as the 14 mm plug that it came with stuck out to far near the propeller when the lead was fitted.
Running on AvGas and Stihl Ultra oil @ 30:1.

First run today, after some starting problems. Some tuning and fiddling required, but it goes ok.
17 x 6 propeller.

Short video here:

21cckioritz (0 min 20 sec)

Posted by Jim.Thompson | Feb 11, 2020 @ 03:49 AM | 4,638 Views
I was given a fuselage mould by a flying buddy. It was a thin mould made for joining in the old "cold join" , rolled tape method.
I have been modifying it to make it suitable for putting it in a vac bag and venting the bladder to atmosphere.
It is nearly done; pictures of that later.
Meanwhile, I have been working on framing up the four wing planels, using the so called "laser method".
Some pictures attached.

Top Cut of one outer wing panel:

Top hot wire cut (1 min 1 sec)


I now need dryer weather to do the composite work. We are experiencing torrential, flooding rain here.

Jim.
Posted by Jim.Thompson | May 04, 2019 @ 07:02 PM | 4,384 Views
I have adopted the routine of modifying the templates for hot wire cutting from that produced by (in my situation) Profili2.
Profili2 template print outs show a sharp change of gradient from the airfoil to the chord line at both the leading edge and the trailing edge.
I shape the LE with a curved, smooth entry to facilitate easy starting of the wire transit. It is inclined to get stuck with a sharp change of template shape here (or anywhere for that matter).
The very small extra foam can be quickly sanded off after the cut is done. If care is taken with the shaping of this curved starting section, little or no extra foam is left uncut.

At the TE, I continue the airfoil contour gradient beyond the TE, as distinct from sharply changing the contour to the chord line. This minimises or avoids any "scalloping" (loss of foam mid panel) of the TE and provides a faithful result.

One other practice that I am at variance with Phil Barnes vac bag instructions is in the type of top template used. If I recall correctly, he advised the use of a bottom template with a full airfoil template sitting on top of that to do the top cut.
Not only does this obviate the above advantages, but also can result in the wire slipping between the two template parts and completely ruining the foam blank.
This at least, was my experience, and I now only use a separate top cut template.
A typical one can be clearly seen in the foam rib hot wire cutting video posted in this blog below.

Jim.
Posted by Jim.Thompson | Dec 25, 2018 @ 07:38 PM | 8,290 Views
This is my very first Arduino project.
Thanks to Hans Meijdam on this site for the design, guidance and assistance: http://www.modelbouwforum.nl/threads...-vario.199904/

The only problem I encountered along the way was Linux Mint permission problems. Once I got around them, all went according to the instructions.
I have encountered "Permission Denied" problems before in my otherwise excellent Linux Mint 17.3 operating system. Don't ask!.............

It is amazingly sensitive and sounds like a full size sailplane vario.
Posted by Jim.Thompson | Dec 10, 2018 @ 04:19 PM | 7,247 Views
The advantages of this method are:
1. Light and strong.
2. Ease of construction.
3. Quick and cheap. Foam for the ribs can often be found from recyclers.
4. Accuracy. All ribs cut together in the one set up. Cannot get them better aligned than that.

For my next constant chord wing using this method, I plan to construct the spar first. It will go something like this:

1. A foam core tapered in thickness will be hot wire cut. Dimensions will be the airfoil thickness minus the carbon caps, minus 1 mm for sanding/fairing balsa. Lets say, nominally 35 mm high and 8mm tapering to 4mm thick, (for example sake only).
2. This core will be faced in 1 mm Pauwlonia ( but could be balsa or basswood et. al.) with vertical grain orientation.
3. End grain of the timber is then sanded down to the foam.
4. Pre-made carbon spar caps are then added to the top and bottom of the assembly. These will be layed up using common uni-carbon/laminating epoxy which is cheap.
The thickness of these caps are tapered out to the tips.
5. The caps are glued on and the assembly wrapped with Dyneema, Kevlar, or other similar high strength braided line. Then all given a light application of thinned epoxy resin.
6. The "laser method" hot wire cut ribs are cut with a slot to take the spar. The building board might be in two parts, to enable separation of the two halves to allow the spar to be lowered into place, glue applied, then the two halves brought together.
7. The joiner boxes will be glued into the main spar and the assembly wrapped again.

I fully expect this assembly to be super light and super strong.

Video of a top cut:
Top cut left wing (1 min 35 sec)

Posted by Jim.Thompson | Dec 02, 2018 @ 03:37 PM | 8,122 Views
Link to the vimeo video here:
Silicone Hinge Demonstration (8 min 19 sec)

Posted by Jim.Thompson | Oct 25, 2016 @ 04:19 PM | 11,324 Views
This is the last method I have tried successfully. The advantage of it is that it can be moved on the mandrel before investing any composite materials in the job.

https://www.rcgroups.com/forums/show...3&postcount=23

I will take some pictures next time that I do one this way.
Posted by Jim.Thompson | Sep 02, 2016 @ 05:37 PM | 12,887 Views
I've been deliberating for months on how to build a dolly for launching gliders when aerotowing. I finally got around to making one, this is the result.
It needs a bit of adjustment to provide a slightly increased angle of attack to lift the glider off the dolly positively.
Some form of suspension would also be good; it would prevent the possibility of the glider being bounced off the dolly prematurely due to the bumpy ground.
Posted by Jim.Thompson | Mar 14, 2016 @ 03:52 PM | 13,304 Views
Here for reference and ease of finding. I get asked now and again for this and have to do a search.

https://www.rcgroups.com/forums/show...&postcount=311
Posted by Jim.Thompson | Mar 14, 2016 @ 03:51 PM | 13,222 Views
Here for reference and ease of finding. I get asked now and again for this and have to do a search.

https://www.rcgroups.com/forums/show...&postcount=311
Posted by Jim.Thompson | Jul 21, 2015 @ 08:01 PM | 13,992 Views
This is my latest project; a lightweight version of this model. I have the heavy one, as can be seen earlier on in my blog.
This one has foam core/vacuum bagged wings and tail surfaces. All hinges are kevlar, so called "live hinges".
The wing airfoil is the same as my Entropy airfoil, which is basically a TP 29 variant.
Four servos, servos in the wings as can be seen in the picture.
I like a rudder on these little models; it assists braking during final approach while landing using crossed controls, just like the full size often resort to. Besides, it is good for aerobatics including fully developed spins.
At this early stage without paint, it balances out at around 800 gram. I expect it to gain at least another 100 during painting. It will still be a good light to medium air weight.
Posted by Jim.Thompson | Jun 28, 2015 @ 03:56 PM | 14,645 Views
As most builders have found, or will find, supplied tube and rod of whatever material (carbon, aluminium or steel), come in a nominal size only. There is always, or mostly, a size discrepancy or variation from the specified size.
I now make my own receiver tubes to suit any supplied rods or joiner tubes, for this very reason.
However, there comes a time when a hole needs to be drilled in a bellcrank (for example), to fit a 3 or 4 mm factory made rod. Without a full set of numbered drills sitting in the shed on hand, it can be frustrating to drill/file/ream a hole out that will be a good fit with near zero slop.
One way around this, is to reduce the size of an off the shelf, hardware store twist drill. I use a disc sander as per the video. For final polishing, a grind stone works well too.

Reducing twist drill diameter (0 min 28 sec)

Posted by Jim.Thompson | Apr 07, 2015 @ 01:44 AM | 16,839 Views
I am building a replacement set of wings for my old Passer Thermo.
I used some yellow XPS foam from Bunnings here in NSW Australia. It cut extremely nicely using my gravity hot wire cutter.
I'll attach a .pdf document. Unfortunately, the dimensions did not come out in the file transfer from the .dxf file.
Each side is a two panel wing. The first panel is 600mm long. 200mm chord at the root and 160 mm at the break. The end panel is 300mm long with a tip chord of 120mm. The TE is straight like the original. Technically a swept back wing.

Video of the hot wire cutting of the wing cores here:

Cutting wing cores. (1 min 33 sec)

Posted by Jim.Thompson | Feb 15, 2015 @ 05:25 PM | 16,717 Views
Posted by Jim.Thompson | Dec 07, 2014 @ 02:50 AM | 16,297 Views
This is a link to a report and pictures of my gravity powered hot wire foam cutter:

https://www.rcgroups.com/forums/show...r#post16646458
Posted by Jim.Thompson | Oct 29, 2014 @ 02:24 PM | 16,185 Views
Description and pictures posted here:

https://www.rcgroups.com/forums/show...970768&page=21
Posted by Jim.Thompson | May 21, 2014 @ 04:48 PM | 17,718 Views
I have been busy learning to draw in 2D LibreCad, which is an open source CAD drawing system designed specifically for Linux. That has been a challenging but very rewarding adventure in itself.
Also, I've been working on the design of my next project; Entropy build thread here:
https://www.rcgroups.com/forums/show....php?t=2164114

Also I have been doing a bit in the shed building another light plank out of the remaining scrap foam that I salvaged from the tip. This one will be very similar to the 1200 I finished recently (flew again yesterday - very nicely too), but will be 1400 span. The max size panel that my gravity cutter can cut is 700mm, so that determined the size. I expect to be able to make this one to an even lighter wing loading that the smaller one. I've learned a few more tricks along the way, as happens.