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Canadair F4 Sabre XB812

But which squadron?

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Chris Barlow22/12/2019 23:29:42
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Further progress on the wings and the tank release mechanism.

I have gone a different way with this, opting for a direct servo release which is possible to implement after a wing has been built. Useful if you want to add hardpoints to an existing model too.

I started with a "normal" servo hatch/mount as used for the flaps and ailerons. To this I have added a pylon with a section removed and the sides reinforced with thin ply.

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The servo horn projects into the slot and will have a hook cut into it to pick up an M2 metal clevis which will be mounted inside the drop tank.

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The servo mount is then screwed into ply bearers mounted in the wing, glued to the underside of the skin but also to the spar and false leading edge for additional support.

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On most early Sabres the tanks were dropped with the pylon as a unit, later Sabres and the F4's used by the RAF used a fixed pylon and dropped the tank only. Considering most modern fighters also have fixed pylons this could be used on a variety of aircraft.

I have also installed all the required servo leads, just need to fit plugs to all these now! surpriselaugh

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McG 696923/12/2019 08:05:11
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2895 forum posts
1097 photos

Great progress indeed, Chris.

Very ingenious, your release system with the Quicklinks. yes

As Phil C wrote, you soon will be counting rivets... wink

Cheers

Chris

Chris Barlow30/12/2019 13:11:41
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1897 forum posts
1303 photos

I noticed it's been quiet on the build blogs over the holidays. Soon be the rush to catch up with only just over 5 months to go, or is that just me? Paint and detailing always seems to take me way longer than the actual build!

I do have a little more progress to report on my wings though. Front ply joiner has been glued on and wing dowels epoxied in. Ply plate to the rear of the wing, wing seat bolt plate blocked out with 1/2" balsa and bolts fitted. Time for a little bench fly! yeslaugh

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I also added a balsa fillet to the underside of the wing to blend it into the fuselage.

The plan calls for blocking out the wing to sit inside the fuselage but I have blocked out the fuselage wing bolt plate instead as this will allow the wing to slide out of place and snap the nylon bolts when it catches a tuft on landing, which it will do eventually! I have also trimmed the planked sides back 1/8" above the wing to allow the fitting of a balsa fillet joining the planked sides to the ply wing seat frame inside the fuselage. This also forms the foundation for the very slight wing to fuselage fairing at the rear of the wing root which I'll probably finish with filler.

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Soon be time for fine sanding, glassing and cockpit detailing! laugh

Martin Gay30/12/2019 15:37:59
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319 forum posts
244 photos

Looking good Chris. I assume your trimming of the fuselage sides will have kept the wing at the same incidence angle as shown on the plan.

Martin.

McG 696930/12/2019 17:19:18
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2895 forum posts
1097 photos

Your "little more progress" seems like an abyss of progress to me, Chris. surprise

But you're right, the blogs have been very quiet lately. Is it always like that during the Holidays?

I've been working hard lately but nothing compared to your results, I'm afraid.

I really like the way you joined the planking to the ply wing plate, but I'm afraid I'm not totally understanding your procedure... blush

Great work anyhow, young man.

Cheers

Chris

Chris Barlow31/12/2019 07:13:46
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1897 forum posts
1303 photos
Posted by Martin Gay on 30/12/2019 15:37:59:

Looking good Chris. I assume your trimming of the fuselage sides will have kept the wing at the same incidence angle as shown on the plan.

Martin.

I hope so Martin? surprise

The fuselage planking was trimmed back to allow the wing to seat onto the ply formers with a slight gap to the planking, checked by sliding a 6" steel rule along the wing to hit the ply inside the fuselage. The planking was then scribed to the top surface of the wing to form a 1/8" gap. 1/8" balsa strips were then butted up to the ply inside the fuselage and tack glued with cyano, the wing refitted and the balsa strips glued to the planking, finally the wing bolts were fully tightened to pull the wing back up to the ply formers.

After the glue was dry the wing was removed and the 1/8" strips were sanded back flush to the fuselage planking but left slightly proud at the rear to allow for the tiny wing fillet to the fuselage.

Same photo again to go with the explanation above. Hope this helps Chris?

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Martin, so if the wing is seated onto the ply formers I presume it is still at the correct incidence/position?

Martin Gay31/12/2019 09:28:59
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319 forum posts
244 photos

That is correct Chris. The inner keel pieces set the incidence angle for you!

Martin.

Chris Barlow14/01/2020 00:54:25
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1897 forum posts
1303 photos

Thanks Martin, that's what I have! laugh

I have been playing with other models so not much progress on my Sabre, except for tonight when Andy Meade kindly emailed me his STL file for the drop tank.

Now I can understand if people work with CAD and 3D design every day may find the following a minor task but to me the technology is amazing.

I opened the STL file in Autodesks' Slicer app and sliced it 1 horizontal component and 7 vertical components with the interlocking model (it'll become clear in a moment) I then saved the plotted file as DXF and opened it in Fusion 360 to convert the DXF into a DXF file that could be read by Inkscape. (I don't have a clue why but Inkscape doesn't open some DXF files unless I do this step)

In InkScape I imported the DXF file and converted it to G-Code, and sent it to LaserGRBL software for cutting. Now it'll make sense!

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Once the eight parts had been cut they were slotted together..

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I know it may seem trivial but I'm so pleased with that and I did nothing. It was all done by the computer! laugh

Next I'll add a top and bottom stringer, M2 threaded rod and clevis and plank it with 3/32" balsa. Sand and glass and add tail fins!

Martin Gay14/01/2020 08:42:39
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319 forum posts
244 photos

Very nice Chris.

We were working towards drawing up a drop tank for the forum, but ours was a generic, circular shape.

Can you email the dxf files to me please, whilst I await a reply from Andy.

Martin.

Chris Barlow14/01/2020 16:27:52
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1897 forum posts
1303 photos

When I was checking the length and width of the tanks I didn't realise they had a flat top to them either! Good job we have some nerds looking out for the details we miss! winkyes

I will email you the DXF cutting files from the slicer app when I get out in the shed later but please remember these have been generated by the software for creating a laser cut, slotted together model.

The slicer software is a free download from Autodesk and will slice up 3D STL files however you want then, stacked slices, slotted etc. You can also arrange the slices whichever orientation and however many you want which could be quite useful for creating a balsa frame from a 3D model.

Martin Gay14/01/2020 18:40:01
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319 forum posts
244 photos

Understood Chris,

I am just waiting on Andy to send the stl file to me!

Martin.

Chris Barlow14/01/2020 23:39:53
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1897 forum posts
1303 photos

I thought I'd just add a bit more to the explanation of creating the file for laser cutting, mostly in picture form! yes

All links open in a new window for you. yes

This is the STL file created by Andy for 3D printing, loaded into Autodesk's "Slicer for Fusion 360" software (free for non commercial/hobby use)

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Using the settings in the left pane it was sliced 1 X 7 as an interlocking model...

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The plotted cutting files you can see on the right were saved as Autodesk DXF cad files and opened in "Fusion 360" (Free for non commercial/ hobby use)

Sheet 1

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Sheet 2

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After removing the border and text these were again saved as DXF files and opened in "Inkscape" (free software, again) and laid out to fit a 100mm wide balsa sheet and converted to Gcode (instructions) for the CNC laser.

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The Gcode file was then opened in "LaserGRBL" (free laser cutting software. Do you see a pattern here?.)

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Then all that's left to do is connect the laser, click the green play button and the laser cuts out the parts in about 2 minutes.

It really is that easy and as you may have noticed, free, except for the modified Eleks Maker CNC laser cutter!

McG 696915/01/2020 07:42:35
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2895 forum posts
1097 photos

Amazing, Chris. surprise

You really make it look easy.

But, I guess I'll stick to the Dog's balsa planking for a while... wink

Cheers

Chris

Chris Barlow19/01/2020 03:55:48
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1897 forum posts
1303 photos

Lots of learning on YouTube Chris. laugh

Having started messing with the drop tanks I got distracted and whilst they were on the bench I couldn't resist planking them to see how they look.

I installed the threaded rod into the tanks to allow the fitting of an M2 metal clevis for the drop mechanism. The clevis can be adjusted in length to allow a snug fit to the pylon.

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After planking the clevis will protrude to fit into the pylon.

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The tanks weigh just under 40g before finishing which is comparable to Andys 3D printed tanks at 36g before internal structures to allow "hanging"

Mr Meade also came up with an excellent idea of cutting tapered planks for the tanks using the laser cutter which I just had to try. 20 minutes drawing a plank calculated to match the radius of the tank and 6 minutes to cut 13 planks.

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The big advantage is that whilst I was fitting the first batch of planks the laser was cutting another batch. The disadvantage is the smoke and smell of burnt balsa, which followed me into the house to the disapproval of the wife & kids!

The tanks will be held to the pylons in a notch cut into the servo horn

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To test the release mechanism I had to wire up the servo leads exiting the wing. These were soldered to two Multiplex plugs each with 6 pins. I used a common positive and negative for the servos and used the remaining 4 pins on each plug to connect the PWM (signal) wire for each servo.

The soldered pins were then potted in hot melt glue using a brass sheet mold clamped with a zip tie. I coated the inside of the brass with silicone grease, clamped it around the plug and filled it with hot melt. The brass does a good job of dissipating the heat from the glue and is helped by dipping into some methelated spirit and allowed to evaporate.

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I know you can buy a mold to do the same job, but I'm cheap!

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One plug is coloured black with permanent marker to differentiate between left and right. The other end of the plug is soldered to Futaba servo plugs to connect to the RX

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It may look a bit clunky but those big blocks of hot melt glue are great for gripping when unplugging wings and protect the soldered joints perfectly.

So with the wing servos wired up and connected to a servo tester I could finally try the pylon release mechanism. The servo arms rotate forward to release so the air flow pushing back on the tanks should force them off the pylons. Just need to add some fins and anti rotation detents now!

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Edited By Chris Barlow on 19/01/2020 04:05:39

Phil Cooke19/01/2020 08:15:59
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2381 forum posts
1637 photos

Impressive stuff Chris, I cant believe how well the lasered planking 'sheet' idea worked thats fantastic - Andy should slap a patent on that one!!

I've just added a pin on my droppable pylons to stop them trying to 'rotate' in the airflow, how do you intend to anti-rotate your tanks with your set up here?? And did they drop ok in your bench trial??

McG 696919/01/2020 08:37:30
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2895 forum posts
1097 photos

... only one word, Chris > SUPERB...

Chris Barlow19/01/2020 14:48:22
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1897 forum posts
1303 photos

Thanks Phil/Chris

Yes the drop tests went fine. I also set the win at an upward angle of about 30 degrees to see if the clevis would jam if the tank came off at an angle but that was fine too.

I was thinking of a hemispherical dome on the tank fitting into a concave recess in the pylon for the ant rotation, or maybe just a very short 3mm pin? There is also the tank brace which I intend to fix to the tank and clip to the wing with a single magnet and steel washer. Dont want that too strong and prevent the tank from dropping.

Andy Meade20/01/2020 08:36:56
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2723 forum posts
698 photos

Looking great there Mr Barlow. You're welcome for the assistance - I guess the cheque is in the post? laugh

Chris Barlow20/01/2020 13:58:05
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1897 forum posts
1303 photos
Posted by Andy Meade on 20/01/2020 08:36:56:

Looking great there Mr Barlow. You're welcome for the assistance - I guess the cheque is in the post? laugh

I too tight for cheques!

Slopeside beer tokens? laughyes

Chris Barlow31/01/2020 22:42:52
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1897 forum posts
1303 photos

Well, what a disappointment!

I've been very busy at work this week and as it usually happens when you're busy time flies. In fact somehow I have lost a day thinking today was Thursday. I had planned to get in the shed by 6pm and get the top half of the Sabre wing glassed having spent most of the week glassing everything else, including a Jet Provost.

Unfortunately today isn't Thursday. It's Friday, and at 9pm I realised this. Friday night is indoor flying night from 8pm till 10pm, something I look forward to even more now my club field is muddy and inaccessable and the weather is pants! Flying with a different breed of loonies and comedians from the PSSA guys on Fridays is always a good laugh.

And now I've missed it!crying

Still, I have made a little more progress on the Sabre over the last week with parts being glass clothed. The last part of the airframe was the top of the wing and since it is quite a clear area I had decided to try using peel ply over the glass cloth for the first time. I usually blot and scrape all the resin off the cloth, let it cure then spread another resin coat over it with a card and finish with sanding and primer...and more sanding, and more primer...

I have wanted to try peel ply for a while as it is supposed to eliminate the second resin coat and the sanding required so with a refresher course of youtube videos when I got home I rushed out to the shed to give it a go, and missed the flying!

So I now have a stripey wing and will have to wait to see if I have messed it up or not!

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And a couple of fuselages basking in a warm shed curing!

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Yes, there is a "smudge" of filler on the JP's back where my 1/8" balsa went flat instead of curved! laughangel

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