Twin Comanche

[Wolfie]

Hello

 

First thread here, so hopefully this is the right place.

This is going to be an 'old school' scratchbuild of a PA-39 for electric power.

Old school as in lots of pencil and graphic construction rather than CAD.

 

Comments, critics and suggestions welcome.

Plans were drawn a couple of years ago as a 10x scale up from some NASA 3 views giving a 55 inch span. I finally got around to plotting the fuse sections in card.

Most of my builds start this way. All sections are plotted and cut as halves as it garantees some symetry.  I recut these in 3mm balsa and laid up the first fuse half. The wing saddle is in 2mm sheet and the primary stringers are 4mm square.

 

 

 

[Wolfie]

Having made the first side, I use a simple wood jig to mount it upsidedown and then build the second side. There are short pieces of 3mm sq that act as alignment between each half former.

 Second half formers are glued in and 4mm main stringers added.

It was then removed from the jig and the intermediate stringers in 3mm sq were added. The curves were just a bit too tight for the 4mm stuff.

There are other photos in the album which give other views as well. I have just posted the main ones here.

 

[Wolfie]

The 3 views were not so helpful for the engine nacelles but armed with a selection of photos I think I have a rough idea of how they should look. Half of the wing is drawn up.

As it's always useful to have a reference point for getting things square, the tail was made up from 10x4 and 4mm sq.

As far as the wing design goes, I have opted for a semi-symetrical airfoil at the root and a more or less flat bottomed section at the tip. This results in a reasonable quantity of washout. The wing shape with the section between the fuse and nacelle meant that the rib sandwich is only the ribs from nacelle to wing tip. The root and intermediate rib are contructed sections with a little adjust to fit.

 

[Tim Hooper]

Wolfie,

 

Welcome aboard!

 

You're obviously a skilled and experienced modeller in that you've made that fuselage look very simple in its construction.  Will you be planking or skinning it later on?

 

tim

[Wolfie]

The wing structure is 4mm sq stringers with shear webbing in 1,5mm. TE is 1,5mm sheet with a single 4mm sq stringer. Intermediate LE is 4mm sheet that will later have another 10mm piece on top.

The LE is also skinned in 1,5mm sheet. Outboard of the nacelle its straight but inboard the surface is a complex curve but the 1,5mm sheet seems flexible enough to cope if you go slowly.

 

[Wolfie]

The nacelles are just 3mm sheet formers in each plane that will be planked. Firewall is hard 3mm sheet doubled with 1,5 ply.

The observant will notice that the wiring hole is not in the right place!

The top TE skins have been added as well as the first half around the nacelle.

[Wolfie]

Having completed the first stage of the starboard wing (and therefore having learnt how it should have been done...) The 2nd wing goes quite a bit faster.

 One area for improvement was to make 2 indentical ribs at the nacelle position and glue the primary nacelle former between them. I now have a surface to skin to on both sides.

For joining the 2 wing halves it seemed easiest to me to glue the root rib of the second half to the completed 1st half and then build up to it.

It just remains to add TE and LE skins to the port side.

Then the firewall is jigged up 0°/0° and glued.

 

[Wolfie]

Hi Tim,

 

'They' warned me that you would try and convert me to electric... I told them it was too late

Yes the fuse will be planked in 2mm sheet. I was just about to start that last night but various other chores came up.

[Wolfie]

Swiftly followed by the put it all together and see if it's square check.

 

 

[Wolfie]

The wiring is run through the LE skinned section. Each of these ribs had 12mm holes cut for the power runs. I use a short piece of 12mm brass tube that has been filed down sharp at one end to cut the holes.

The motors are a pair of 2217-7 outrunners.

A pair of nacelle sub-formers have been added at the spar. The nacelles are skinned from the firewall back to these and then will be solid balsa from there back.

I wont bore you with pics of all the stages of planking (its like watching paint dry).

The tail parts of the nacelles are just soft 6mm sheet cut to outline and then sanded to fit.

I will complete the nacelle planking once all the electrics have been checked out.

[Danny Fenton]

Super stuff, really nice to see some more scratch building. It definitely looks like a Commanche

 

You have a single nacelle load bearer, does the leading edge take some of the sideways loads? And also how are you going to tackle the retracts surely they will pass right through that nacelle profile section?

 

Cheers

Danny

[Wolfie]

Thanks Danny

 

I don't really consider the centre nacelle former as the primary load bearer. The planking creates a very strong monocoque and the forces are distributed over a significant area of the wing.

I'm not considering undercarriage on this one as I intend to use it in the back garden. She can just belly flop on the grass.

 

[Wolfie]

Finally got started on planking the fuselage at the weekend.

 

Starting at the window edge of the roof and working inwards. Final piece was shaped rather than having lots of joins at the centre.

 

The second starting point was at the back of the wing root where there is a transition from nearly square to rounded.

 

Then working from top and bottom alternately until there is just a small triangular fillet required.

 

 

 

[Wolfie]

Take a break from planking for a while and sort out some other details.

 

 The fin is a framework of 4mm sheet, 10x4 and 4 sq. This has been skinned in 1,5mm sheet:

 

There was a lot of head scratching about how to fix the wing. In the end I went with something I last used 35 years ago on Beagle Airedale model.

The spruce tongue on the front of the wing slots into a U shape locating box in the fuse. The locating box is made up from 8mm sq on a load spreading plate of 2mm ply. Align the wing in situ and then the box can be tack glued in place. Disassemble and coat in CA.

 

The LE has been reinforced with 4mm filets around where the spruce tongue is located. The rear fixing will be a pair of 4mm nylon bolts.

 

And this is where we are as of this morning:

 

 

 

 

[Bob Cotsford]

lovely bit of planking.  I like a planked structure for the strength it gives and the curves you can create, but doesn't it seem like hard work at the time! 

I like the idea of the wing-tongue box that can be adjusted to fit on first assembly rather than the more common slot in a ply former, a neat idea I'll have to remember.

[Wolfie]

Thanks Bob,

 

I only really 'discovered' planking about 6 years ago when making a Lockheed Starliner model and I've been hooked ever since. It really is amazing just how strong a monocoque you can create this way.

Agreed that it can be terminally boring at the time, thats why I usually try and leave lots of little job just to break the monotony  

[Wolfie]

Completed the underside of the nose last night and made a start on the underside of the rear cabin. Added the servo bearers for rudder and elevator.

 

 Put everything on the scales including 3S 3000 LiPo, servos, Rx, ESCs, motors, props, spinners etc.

1080gm. 

At a rough guess I think this will end up around 1200gm (42oz) finished. I haven't worked out the wing area yet but the loading should be rather less than I had planned.

[David Ashby - Moderator]

Watching with interest Wolfie, really like the model, nice work sir!

[Wolfie]

Thanks for the kind words David.

When I sold up all my RC equipment and models in the mid 70s I still had a shortlist of models that I wanted to have a go at. The Twin Comanche is that last one on that shortlist.

Absolutely no idea of what I'll do after this.....

[David Ashby - Moderator]

I do like civilian types - Cessna's etc. they seem quite undermodelled. 

[Wolfie]

Final one for the week.

The planking for the underside of the cabin is now complete and the wing saddle is doubled with 3mm sheet.

A transverse plate is glued in across the main 4mm sq stringers just below the cabin windows. With this in place the bottom of the fuse former in the centre is cut away along with the longitudinal bracing. These parts where only there for support during the initial build. This now leaves a large open area above the wing.

The plan is that the LiPo pack will be fitted in a box above the wing centre assuming I've got the balance checks right so far....

 

Next update in early January.

 

Merry Christmas to all.

[Wolfie]

In between all the eating, drinking, and demolishing and rebuilding a fireplace there has been a little more progress. It's so much easier working with balsa than 20cm square oak beams

 

Start off by fitting the rudder and elevator servos so that the control routings can be established.

 

The rear wing fixings are 4mm nylon bolts and captive nuts in fuse. 

 

Then back to planking again. Gets rather tricky around these sorts of curves and it does help to pre-form the planks by running your thumbnail down the back of them while twisting to the shape needed.

 

Again the final triangular piece can be cut to shape  and glued in. The the approximate shape is traced onto the surface and the excess is removed. Will only get the final shape when the windscreen is wrapped around.

 

 

[Wolfie]

The tail has been skinned with 1,5mm sheet same as the fin.

 

Then the rear fuse/fin fillet is made up from some 8mm sq and scraps of sheet.

 

The tail sheeting is slotted to give the alignment for the fin and the centre 8mm sq pieces do the same. Then its sanded to shape. I have some pieces of brass sheet that can be placed on the tail for protection whilst sanding to shape.

 

Finally the rudder is hinged and the fin strake made from a laminate of 4 and 3mm sheet.

 

It's starting to look like and aeroplane.

 

Sanity check shows that CG at this point in the build is around 35/40% MAC. Putting the 3S LiPo pack above the wing centre near the fuse bulkhead brings this forward to less than 20% MAC. Hopefully this means that I should be able to make a wing centre battery box and get the balance without adding ballast.

 

[Ton van Munsteren]

Wolfie ,

 

That realy is a beauty and a very clean build keep up the good work.

 

Ton

[Wolfie]

Thanks Ton,

 

I just wish I had more free time to be able to build. Only seem to find a few minutes here and there between working on the house and looking after our animals.

 

If you could see the state of my workshop this morning, you would not describe my building as clean