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Jim.Thompson's blog
Posted by Jim.Thompson | May 21, 2014 @ 04:48 PM | 1,954 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:
http://www.rcgroups.com/forums/showthread.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.
Posted by Jim.Thompson | May 02, 2014 @ 04:17 PM | 2,113 Views
I built this foam plank heavy with view to Dynamic Soaring it. I hope to travel up to one of the DS sites where some of my flying pals fly regularly. There are no DS sites to be found around where I live.
This plank has an AUW of 1850 grams, which makes it a bit of a heavyweight.
Span: 1500mm
Airfoil: PW51

The finish method went as follows:
1. After the fuse and wing were shaped and sanded, I applied spackle in the usual way. Sanded, then another coat etc. till smooth.
2. Then I screeded a coat of PU glue over the surface. This really made the spackle nice and hard. Sand, second coat. Sand.
3. Spray adhesive, then two layers of crossweave tape diagonally opposing.
4. Lay flat with covering iron. Sand lightly.
5. Spray adhesive, then covering film to decorate.
6. Finally, a layer of 30 micron laminating film.
7. The elevons were bagged glass over balsa, with some light uni-carbon strips top and bottom at the TE. This made for nice and stiff elevons.
8. The fin is vac Bagged foam core. Carbon/glass.

I hope to get out to Mt. Borah for a test fly tomorrow actually! Great forecast.
Our winter season of westerlies has begun.
Posted by Jim.Thompson | Apr 09, 2014 @ 12:30 AM | 2,365 Views
These pictures show how I build the box section fuselages - I hope!
The EPP beds that the foam cores come in is cut down to 10mm sheets using a hot wire and a couple of spacers on the bench.
After the sides and top are cut out, I line the inside with 80/20 glass and epoxy.
The wood that I'm using for the bulkheads is recycled Douglas Fir venetian blinds that I have cut into strips. These are glued together and first tacked in with CA.
The final assembly is done with PU glue.
Posted by Jim.Thompson | Apr 06, 2014 @ 12:56 PM | 2,215 Views
I flew my new little 3 servo plank out at our favourite beach headlands. It flew just fine without an problems. Smooth and agile. I did make a very slight CG adjustment, but ended up setting it back to where I had it from the bench set up. I look forward to flying it out at Mt. Borah in some good lift.
My flying buddy Andrew even took a short video of the launch here:

Jims Plank (0 min 11 sec)

Posted by Jim.Thompson | Mar 24, 2014 @ 07:26 PM | 2,310 Views
During the short breaks in the humid rainy weather of our wet season, I've designed and built a new 1200mm span 3 servo plank.
The wing core was cut from some low density scrap foam scrounged out of a skip at the dump to a PW106 airfoil. I chose the higher camber PW section as I want this one to be as light as I can achieve and be a bit of a floater. Also, I wanted something that will turn a bit quicker than my larger planes, fly in light conditions and be suitable for one of our local beach headlands.

Wing:
Root Chord: 220mm
Tip Chord: 160mm
LE sweep around 16 mm to keep the 22% chord line straight.
I first applied a 20mm strip of bias woven carbon (using a double carrier sheet), to the LE of the wing and allowed 3 - 4 hours for it to start to cure. Then proceeded with the layup below:
Layup is two layers of 76 gsm plain weave, one @ 0/90 and the other @ 45/45.
Approx 1/3 span overlap of each of the second layers to constitute 3 layers for this inner part of the wing.
Three servos to separate the ailerons a bit and provide better efficiency.

Fuselage:
This is made from scrap depron sandwiched between layer of glass/epoxy. I cut out the depron and lined it with 80/20 Fin:100 gsm glass. Then built it up into a box section structure using bulkheads where required. Then sanded to a rounded sort of shape and covered with two layers of 50 gsm glass plain weave fabric/epoxy.

Fin
This was hot wire cut to a NACA06 section from some scrap blue foam. Then bagged with two...Continue Reading
Posted by Jim.Thompson | Jan 23, 2014 @ 01:03 PM | 1,995 Views
I've been fitting outboard bearings to the servos in my flap installations in the last couple of builds. It makes for a very tight servo installation with residual backlash and slop down to the absolute minimum. In fact, it is only the slight bearing backlash remaining; with very good quality servos, such as MKS, this would be absent.
I'll submit a picture of one out of one of my Wompoo's. It's difficult to take pictures of them in a new operational wing, due to access difficulties etc.
This was a very cheap DIY way of doing it. I just made up a laminate sheet and cut out the bearing supports from that. The bearing is a press fit into the little laminate block and it is glued to top and bottom skin. I similarly link both skins to my servos to avoid flexing of the skin where the servo mounts.
I've tried several bearings from several sources. Very mixed quality and availability and cost. The ones I have now are cheap and available from Banggood. A$2.32 each delivered. These are 3x8 mm and will suit the bigger metal geared servos with a 3 mm horn retaining screw. All that is then needed is a longer screw and a suitable spacer between the bearing the horn. And of course, the support block as above.

http://www.banggood.com/Wholesale-Su...1-p-58277.html

I've already posted a picture of one bearing installation on my Wompoo build thread, but here it is again:
Posted by Jim.Thompson | Sep 04, 2013 @ 06:35 PM | 2,867 Views
I'm planning on putting together a light 1500 mm span EPP plank at our coming Manilla Slope Festival. It will have a PW75 airfoil and I'll cover it with laminating film. This will be my first attempt at using Lam film.
I decided on the spur of the moment to make up trial fuselage using 12mm sheet offcuts from a former EPP wings sliced down using my hot wire.
I cut out the sides using a fibreboard template and laminated 200 gsm uni-carbon to one side of each (to become the inside of the assembly). In hindsight, I would use uni-glass instead due to any 2.4 ghz shielding concerns.
I then cut some more sheet to form the top and bottom of the fuselage and laminated this with 200 gsm uni-glass.
Some formers were made with some carbon strip which I had made some time previously to fit several stations.
The assembly was a bit of a fiddle, and some more thought is needed to get this sorted. But I used some hot glue for the first bottom/side join. Not a good idea, as this caused some sanding problems later on. However, it did provide a good start. The battery pack (4 AA cells) was hot glued into place and well supported as far forward as possible. The remaining side and bottom was glued up using PU glue and spiral wrapped with tape overnight.
I shaped and sanded the corners this morning and the result looks quite presentable! Nice and light and very stiff!
Posted by Jim.Thompson | Apr 03, 2013 @ 10:31 PM | 3,432 Views
I'm posting this here for my own reference and for anyone else who may be looking for it:

"K Factor" for tailplanes.

K= (Stab Area)x(distance between 1/4 chord points) divided by (Wing Area)x(Average wing chord)

Really nice flying planes have K~0.5
Trainers have K~0.6
Racers and high aspect ratio sailplanes K~0.4
K~0.3 is almost unflyable.
Posted by Jim.Thompson | Feb 14, 2013 @ 04:07 AM | 3,557 Views
I have the two wings ready for closing now.

edit: They are joined up and removed from the mould. It was a difficult release; I used wax only and no paint. Not a good idea. I'll at least prime the moulds next time when using wax only.
The weights are both around 390 grms. Not much lighter than the first prototype.
I was hoping for a lighter one, but don't mind much. The previous plane was a hoot to fly @ 2kg in big air! Given a guess, I would say this one might finish up at around 1.8 kg.
Posted by Jim.Thompson | Jan 05, 2013 @ 01:39 AM | 4,130 Views
I layed up one fuselage yesterday and there is another in the mould today.
I have 25 psi pressure in the bladder and the excess resin is audibly being pushed out.
I had success with the orange coloured Bunnings drop sheet plastic for the bladder. I found out my accident that it does not even require the wing fillet silicone plugs in place to suppor the bladder. It is sufficiently stretchy enough.
This plastic welded very easily using an 80 watt iron with the tip shaped into a sharp cone. I can move along the seam quite quickly and where required, I will go over a stretch if there is some doubt about the seam integrity. I know I have it right when it tears freely from the waste edges.

This fuse is layed up as follows:

1 layer of 100 gsm plain weave glass fabric @ 0/90 orientation.
1 layer of 100 gsm plaind weave glass fabric @ 45/45 orientation.
1 layer of 200 gsm uni-directional glass @ 0 orientation.
1 layer of 76 gsm kevlar back to about half the fuse length.
Extra pieces of glass to face the wing fillets etc.
Extra Uni-glass @ 90 in the fin for stiffness.
Carbon tow in the wing root arris's.

Finished weight for the first one: 200 grams.
I mixed approx 100 grams of resin.

This fuselage is nice and stiff and quite light for the service it will be put too. Given that planks land hard and fast, I need them strong!

I'm pleased. I'll try another layup after this second one is out of the mould. Maybe with a layer of 165gsm crowfoot glass, one layer of the 100 on the bias, and a part layer of the uni.

Jim.
Posted by Jim.Thompson | Dec 22, 2012 @ 07:19 PM | 4,310 Views
I did my very first vac resin infusion tests yesterday and the results were quite good. I had leaks on the first tests, but it did not seem to spoil the composite plate that I was making. Probably because the leak seem to bypass the layup.

I waxed up some thick glass plate after taping the edges to keep them clean so that the acrylic filler would adhere to retain the bag later. Then I cut out two cores, one out of 1.5mm LRC (low resin content) Soric.
The second one, I cut the same size core out of Coremat, which is a regular fibreglassing core filler which I just happened to have in stock. I place a layer of heavy glass cloth (295 gsm) on top and bottom of each of these and positioned them on the glass plate. Then added a layer of peel ply, with the ends wrapped around pieces of spiral wrap in the usual way. I finished off the stack with a layer of infusion mesh. Inlet and outlet hoses were inserted the spiral wrap, one to pull the vacuum and the other to supply the resin.
I used a spray paint pot as a resin trap, copying "Ant's" example described in an earlier infusion thread.
I used Araldite LC3600 (it goes by several numbers??), and the ambient temperature at the time was 35 deg. C, so I didn't bother warming the resin.

I had some difficulty chasing leaks, but after they were (mostly) tracked down, the infusion went smoothly and plenty quick enough.
The bag was opened and the samples stripped from the glass this morning and look good. What was...Continue Reading
Posted by Jim.Thompson | Dec 08, 2012 @ 02:08 AM | 4,262 Views
I made a quick joiner mould today using two pieces of 12x12 RHS aluminiun and one piece of 40x40 aluminium RHS.
I used the 40x40 to space the smaller sections accurately. I waxed this to ensure it would stick to the assembly. Then I cleaned a piece of 8mm plate glass with acetone and masked off the 40 mm wide strip that would form the mould interior. Then I carefully ground the glass slightly to provide a good key for the epoxy that I was to glue the 12 x 12mm strips to, with my angle grinder. The 12x12 strips were cleaned with acetone and sanded rough on one face to provide a key.
Then I removed the masking tape and glued and clamped the thinner strips to the glass using the large piece as an accurate spacer. I glued them with epoxy and West Systems 403 added.
When the glue had mostly set, I removed the 40 x40 and cleaned the surplus epoxy that had exuded slightly out the inside corned of the joins with a chisel.
Now I have my mould for a 12 x 40 joiner bar!
Any combination of scrap sized material could be juggled to suit the application. This saves much time and money.

Jim.

When fully cured, I waxed and PVA treated the mould. Then when the PVA was totally dry, I layed in the bottom of the mould 12 rows of 50k carbon tow, suitably wet out with laminating resin and rolled as flat as possible.
The next day, I glued in a foam core with included plywood shearweb that I have previously cut to 9x36mm using my hot wire gear. This was carefully spaced equidistant from...Continue Reading
Posted by Jim.Thompson | Dec 04, 2012 @ 02:29 AM | 3,963 Views
I layed up a couple of new nose cones for my 2 mtr. Endorphin over the last couple of days.
The first one came out very well. I used wax and PVA. The second one is in the mould with the bladder at around 15 -20 psi pressure. This one has wax only and is the first time I have used only wax. The mould is well seasoned now and has produced 3 or more parts.

I'll pull it tomorrow morning and check the results. This one is 3 layers of 185gsm glass cloth.

edit: The second one done without PVA (wax only) pulled nicely! One of the benefits of a seasoned mould!

Jim.

edit: In the third pic below, the bladder can be seen folded to avoid it interfering with the lap join on the bottom of the part. Much simpler than making an insertion tube and fitting the bladder after the mould is assembled I think. A few dabs of glue stick will keep the plastic bladder folder untill it is inflated. I have not had one become sandwiched in the lap join.
Posted by Jim.Thompson | Nov 16, 2012 @ 12:58 AM | 4,069 Views
I don't think I ever posted a picture of my finished Kingcobra (P63) here on my blog. It's been flown at the last Borah Festival since this picture.

Jim.
Posted by Jim.Thompson | Nov 12, 2012 @ 08:39 PM | 3,787 Views
I've have some more insights into why my servos stalled in the plank "Circa" and totalled it.
The reflexed wing section (airfoil) on a plank flies straight and level when trimmed correctly. The reflex trailing edge part of the section provided negative lift to counteract the normal forward pitching moment of the aifoil as it generates lift.
So it follows that the control surfaces (and the servos) will have a load on them in normal, level flight, to provide this self stabilising force.
Which means that as the speed increases, the demand on the servos to maintain the level flight, reflexed inclination, will increase. As in a dive!
Then, if additional demand load is required of the servo to pull the control surfaces up, it might be pushed beyond it's limit. This will happen earlier in the case of a reflexed plank airfoil, than in a tailed aircraft with a conventional airfoil. (all other factors taken into consideration.).

All the more reason for the servos to stall!

Comments, corrections and additions invited, as usual.

Jim.
Posted by Jim.Thompson | Oct 05, 2012 @ 12:16 AM | 4,314 Views
Sadly, my very good performing Circa crashed on the last day of flying after the slope fest had finished. It was a late afternoon flight, and I think that was a contributing factor. The batteries were a bit run down, but not enough to trigger the low voltage alarm on the telemetry.
I've written up a full report here: http://www.rcgroups.com/forums/showt...36870&page=167

After some discussion, we have drawn the conclusion that the servos stalled. They were Corona939MG and were most likely not strong enough for the task. I'll use stronger ones next time, most likely MKS6100 or better.

Jim.
Posted by Jim.Thompson | Sep 03, 2012 @ 11:02 PM | 4,501 Views
I've decided on a good method ( I think) for fixing the batteries into the nose of a plank (or any other fuse for that matter), which the nose is long with no canopy or nose cone.
I first determined the position to obtain the required CG position by taping them externally. Then I cut a strip of coreflute to the length that would support them and extend aft far enough to fix that. I fixed the batteries to the coreflute strip with hot glue and cable ties. Then slid them in, checked the CG and when right, I hot glued the after end of the strip where it was accessible.
The only thing remaining was to push up a cube of foam rubber between the batteries and the top of the fuse to prevent them bouncing up and down.

They can be removed easily if and when required.

Jim.
Posted by Jim.Thompson | Jul 19, 2012 @ 02:39 AM | 4,854 Views
I started the hinges on the new foam plank "Harlequin", but ran out of tape. The hinges are tension hinges. I used the tape and method recommended by my brother John.
It is a HobbyKing product described as :
"Ultra thin prismatic sticker sheet" and can be found here:
http://www.hobbyking.com/hobbyking/s...dProduct=11338

It comes in a sheet 180mm wide. I sliced stips off this 22mm wide. (180 long). Then removed the backing sheet from each piece and replaced it with 6 - 8 mm exposed. Then I took 2 of these and flipped one of them over so that the exposed sticky parts stuck to each other. This was then cut into 12mm wide strips. The stips were slid between the control surface and the drag spar, one of the backing strips removed and the strip stuck to the top of the wing. I had placed a strip of masking tape along the wing and control surface and the elevon to create a line to position each of these hinge pieces in a straight line. The second hinge piece was then slid in and stuck to the elevon, ie: alternating. When I had a set of four stuck on top, I turned the plane over and tensioned up each one in turn and stuck to its corresponding wing/elevon, as the case may be. I did four at each end like this to hold the elevon in position before continuing on the complete the entire hinge length.
The reason that I ran out of material is that I stuck the backing paper back on some of the film the wrong way up. It has a shiny side! I've ordered more from HobbyKing so I expect it will arrive within a short time. I want this plane flying in time for the slope fest in September at Mt. Borah, Manilla NSW Australia.

Jim.
Posted by Jim.Thompson | Jun 19, 2012 @ 06:24 PM | 4,963 Views
I now have everything that I need to complete my new foam plank.
Next is to fit the control horns and the rx. into the wing. The batteries and the switch is in the fuselage (well forward) and the rx. in the wing.
Hope to get onto it in the next couple of days.

Jim.
Posted by Jim.Thompson | Jun 08, 2012 @ 11:04 PM | 4,835 Views
I did managed to get some of the colour on the Cobra during our very brief dry spell. I noticed a couple of delamination spots that I missed, so I fixed them by injecting under the skin with a syringe etc. Now I just have to touch up the paint again. I used very narrow plastic spray painters masking tape to do the curve around the spinner. It worked really well, but pulled a bit of paint off. So now I have to touch that up as well.
I have the Soviet star decals from HobbyKing to put on after that and a flying buddy is sending me some vinyl window stickers. I don't know about panel lines, maybe I will do them, maybe just clear coat without.

Meanwhile, I have been building a foam plank for flying at festivals with crowded air space (Mt. Borah for instance). I've been a bit slack with my blog entries, but here's a pic of it in the jig with the fin socket gluing up. I made a socket out of some glass/epoxy by vac bagging it onto the bottom of the fin after covering it with cling wrap for a release. After trimming, I glued it in place in my old jig as per the picture.

The plank is 1500mm span, PW51 airfoil and looks like it will be AUW of around 1200 grams. That will equate to around 40 g/sdm, which will be fine for the good lift we most often get out at Mt. Borah, NSW Australia.
This one will have 2 servos only.

Jim.