Design & Build Sport Twin

[Nigel R]

Right, now that's dry, time to cut the wing saddle out. Template is stuck down in the appropriate place (LE above TE by 1/16" to give a half degree of positive incidence):

The shape is marked with a pin and a lot of holes.

Cutout is roughed out with a dremel sanding drum, then a curved sanding block.

After I did one side, I re-used the pegs (previously used to keep doublers and sides aligned) and transferred the wing section to the second side, separated them, rough cut the second side, put them back together and finished off the sanding.

The saddle is now a good copy of the wing seciton. I cut it oversize by 3/32" or thereabouts, to allow for the wing joining bandage. Any gaps will end up filled by silicone sealant, after covering.

I have a bit more prep left to go with the sides - longerons, doublers around the tailplane saddle, a few uprights.

[Nigel R]

The last week has seen a bit more progress.

The most nerve wracking operation of the whole build:

[Nigel R]

Hacking a hole in something that just took 35 hours to put together was enough danger for one week.

So I cut some parts for the nacelles. Sides first, just like the main fuselage these are 1/8 with ply doublers

[Nigel R]

And some formers. These have gone through a test fit to get the size just right:

[Nigel R]

And I made the main formers for the fuselage and did a test fit in the jig:

 

at this point I've also added the longerons and some uprights, and some 3/8 square to reinforce the tailplane saddle. 

Edited By Nigel R on 01/06/2019 12:10:54

[Nigel R]

I now need to get the nacelle firewalls set up for the engine mounts and plumbing - but as I've run out of m4 hardware that will have to wait for the postman to bring another packet of tee nuts and machine screws.

[Nigel R]

Well instead of twiddling my thumbs waiting on the postman I got on with glueing up the fuselage:

 

A jig is one of the best workshop tools you can buy or make, I think.

Yes, the longerons are meant to be stickup up past the top of the sides. They do that in order to give something for the rear decking sheet to land on.

Annoyingly, the 1/8 ply former at the front of the wing has warped. The top half has gone wonky. Never mind. Hopefully no one will notice! The bottom 1" of that former has been reinforced with a strip of 1/16 ply to beef up the dowel hole, and that bit at least is nice and square.

The rear of the fuselage is like most of my builds, which means it gets a few crossmembers, cut and fit from 3/16 or 1/4 scrap, while it is sat in the jig. I don't know if they really add much strength by the time it is all finished with decking and sheeting and all that, but they certainly help hold things in place during the build process (i.e. keeping the fuselage sides upright and parallel and so forth).

While I was in and around the tail end, I put in the tailpost block, a lump of 3/8 soft sheet sanded down to size and stuck with aliphatic (like the main formers).

All good so far (apart from that warped former). Lots to do yet but happy with the last weeks progress through the job list.

Edited By Nigel R on 03/06/2019 09:44:25

[Nigel R]

Got the decking on. Took a while to get the shape matched up to the canopy I am gong to use but otherwise went well. I soaked the sheet and it seemed happy enough to go around the curves.

[Nigel R]

I am using three pieces of 3/8 to make the front deck. I got the first two into place:

[Nigel R]

Next step is to flatten the top of the two pieces I just fitted, then add the top piece (more 3/8). Then a bit of carving.

[Nigel R]

I added the top slab to the front decking.

[Nigel R]

I also added a slab underneath the nose area:

[Nigel R]

A fair amount if this timber will be sanded away. Right now the nose is looking quite chunky!

[Nigel R]

Also had a bit of a think about the tank install in the nacelles:

[Nigel R]

I think I am going to end up with the neck of the tank in line with, and the fuel lines coming through, the centre of the mount. Nothing particularly unusual. With the engine upright it is a bit lower than optimal however there isn't much I can do about that. The nacelles are not very tall!

[Nigel R]

I've been doing some engine install jobs this week. First, getting the mounts on the firewall:

The small ply discs are only there to make the firewall thicker so the tee nuts do not poke through to the other side. I turned then up on a drill.

Yes, the holes are meant to be in different places on the two firewalls... The starboard engine has right thrust.

I also drilled the fuel line holes. I'm intending on using standard tanks with a two line setup. Some other jobs were done too, drilling the engine mount and soldering a threaded extension on to a couple of lengths of wire cable ready for setting up the throttle run.

I also got the nacelles together:

It is going to be quite cramped in there. If I'm honest I wish I'd made then a bit longer. If it comes to it the First dodge will be to get a pair of slec square tasks or sullivan slant tasks. If I still can't fit everything in satisfactorily I can always make more, I guess.

[Nigel R]

Bit more on the nacelles - getting the servo mount sorted out:

Tight. One to remember for next time, more space for the servo.

And a quick fit check now the nacelles are glued:

Getting there!

[Denis Watkins]

Wow Nigel

You have been ahead of the game from the start of this project, and there is the result

Don't skimp on the horizontal stabiliser

There is no need for a weight saving surface on this one

The horizontal stabiliser is subjected to huge forces, so build it rough and tough

[Nigel R]

Thank you Denis

I believe the tailplane will be strong enough. It's a fully sheeted structure, with several 'spar' members; solid sheet elevator. You are right it is not the place for a flyweight structure, the model will not be short of power when done.

There is a long way to go yet! I reckon I am about half done. Although at this point the whole thing is really starting to take shape.

[Nigel R]

Some slow but useful progress this week.

I have mainly been working the throttle links. The bowden cables are cut to length and made up with a threaded adaptor soldered on, the outer sheath is in place, appropriately supported with a few balsa blocks near the servo and both installs have been tested for full and free movement. I've gone with ball joints at the engine end of things and those screw on type links at the servo end.

I decided at this point that I wasn't going to rebuild the nacelles, but didn't like how the bottle tanks were fitting, so I bought my way out of the problem with two 6oz SLEC maxi tanks, which result in enough clearance for the servos to operate freely, and. allow me to completely surround the tanks with a layer of foam - which is a much better deal than I would have got using the standard bottle tanks. Lesson learned, don't skimp on the space for the gear to be installed in (should have known better, if I'm honest ). Another couple of inches in length would have made for plenty of space.

The second whoops of the week was the discovery that the starboard nacelle, with offset for right thrust, would not have clearance between the silencer and the side of the nacelle. I would have to cut into the corner of the firewall to clear the silencer. I guess I could have gone the woodwork route, but this was another credit card solution - Just Engines came up with the goods here (and very quickly - always good service from them ) in the shape of a pair of 3/4" extensions, which gets me a reasonable clearance without hacking the woodwork about.

Not sure how I could have easily avoided this (I suppose the engines could have gone in sidewinder, but I wanted them upright for ease of starting - and anyway but the time I'd noticed I'd already done the firewalls).

After spending my out of those problems, the last bit of the week's workshop time was spent on more straightforward matters - getting the main part of the 3/8 sheet top on the nacelles and stuffing in a load of tri stock reinforcement. Just like the fuselage, these will soon be in for a dose of razor plane and sanding block...

The tail end of the nacelles will also get some 3/8 sheet. Inside, tri stock everywhere:

Next job - two cowlings to come up with. It'll be thick sheet and tri stock again - nothing unusual planned there.

I just hope I've bought enough thick sheet...

[Jon H]

Depending on the sizes you might be able to 'vac form' your own cowls. All you need to do is make one wooden cowl plug from as much solid material as you can muster. Then cut the bottom off a 2l drinks bottle, shove your plug inside and hit it with a heat gun. The plastic bottle will shrink nicely around your plug and once you cut it free you can use it again for cowl no2.

If you are looking for a 100% perfect finish then this probably wont do, but if you want quick cheap and functional then it might be worth a shot.

[Nigel R]

That's a thought. How would you finish a shrunk bottle? I'm not too familiar with the technique (read: never done it).

[Nigel R]

Made a started on some cowling. First job, cut some noserings (turned on a drill), and then sort out the cutouts in the plastic spinners. As there were four cutouts I made a sanding tool from some scrap balsa shaped to the cutout I needed and just started scraping away at the spinner until I had four identical cutouts.

When that was done I could get the engines bolted in, with the nosering stuck to the spinner. I borrowed an idea from one of Pete M's build blogs - use some scrap 1/8 between the spinner the nosering, the nosering is held in correct position.

I've only got as far as the side pieces here (3/8 sheet):

That looks decidedly odd with the perspective, but the motor on the left is dead ahead no rightthrust!

Anyway. Now the sides are in (and it took a while to fettle them in to position) the bottom can go on, at which point everything should be nice and solid, and I can remove the motor and put some triangular stock in the corners and get the top block on.

Then time for some more carve and sand

[Nigel R]

Excuse the umbrella growing out of the port wing!

Got a good chunk of the block carve and sand exercise done. It's surprisingly hard work especially in the heat today! I will probably put the nacelles to one side now and get the nose on the main fuselage up to a similar point.

[Nigel R]

Changed my mind a bit and did another hour or two of sanding in the nacelles. Pretty much done with them now bar cutting out the opening for the motors.