Sport Twin #2 - Duo FS

[Nigel R]

Several years ago I built a twin. It was essentially a regular low wing sport aerobat with the engines mounted on the wing. I've rather enjoyed flying it, and I felt like making another one.

 

That said, I wanted to make it a bit different to the last one. I toyed with the idea of putting a couple of bigger engines on something large enough to carry retracts and flaps but as usual simplicity (and a desire to actually finish the project within the space of a year) won out, so instead of going more complex I went with a different configuration - and this time I sketched out a few assorted variations of a model with two fuselages. Model size was to be around a 'compact 60' sort of size and weight - I already had the powerplants for the thing, two OS 40 Surpass, not the world's most powerful engines but (a) nice runners and (b) I already have them. The total capacity of 13cc feels about right for around 6lb or thereabouts of aerobat. If it proves underpowered I can try and shoehorn in a pair of OS 48 or 52 - I think they mount in the same space.

 

The wings on this one came out at a nice round six foot, albeit with a fairly narrow chord of about 10" at the root, for a total of near 700 sq in area. The fuselages will be just about the right size to fit on a 36" sheet of wood - so that, plus a cowl, plus a rudder. Styling wise, I had two Pegasus Hornet canopies - that kind of determines the front end size and shape (fairly narrow) and the rear end (a nice decking behind the canopy). Beyond that the fuselage will be kept as simple as possible (after all, I have to make two) so no fancy shapes underneath or belly pans or any of that stuff. 

 

Two fuselages present a bit of a problem for gear. Either some central pod for a noseleg (too much build effort) or four legs (crikey, no), or the simple answer, taildragger. The other sticking problems are likely to involve where to put the radio clobber.

[Jon H]

A guy my dad used to fly with had a tail dragging twin boom twin with cockpit pod and i think it was called a double zero. Pair of OS25fp's if i recall and it flew well. 

 

[J D 8]

 A popular twin some twenty years ago the Slim Twin was a two fuzz good flyer.

[Nigel R]

Slim Twin was one of the inspirations.

 

The other was 'Duet' which can be found on Outerzone here. I considered building that, but it was just not close enough to what I wanted in lots of small ways; lots of mechanical linkages shown to keep the servo count down (e.g. ailerons using torque rods and bellcranks!), trike gear, a central pod.

 

 

[Nigel R]

A few photos of build so far...

 

 

 

 

 

 

 

 

 

 

 

I didn't take s lot of photos as the general constructing is the same as most of my sport models. 

Edited November 23, 2021 by Nigel R

[Nigel R]

 

And we're up to date. 

 

Fair bit to do yet...

[Nigel R]

[Nigel R]

A bit of progress - some shape added to the fuselages. Everything takes longer with two of them! Top of the sheet needs dressing and then a cap of 1/4" wood will be added.

 

I did try planking them but managed to make nothing but an unholy mess and ended up stripping off what I had done and starting over with my usual method of a single, wet, sheet of 3/32" to make the sides of the deck.

 

I made all the formers on the deck from foamboard, it's very easy and quick to cut and shape for this kind of thing.

 

I also made up the throttle cables (sorry, no pic, I'll grab one later if I remember). This took quite a bit of thinking about - the usual four stroke problem of throttle very close to firewall - but I eventually settled on using bowden cables, soldered into cut down M2 adaptors (so only 3/8" of the solder cup and about 3/8" of the thread) threading into cut down ball links (again, about 3/8" threaded portion).  I also sneaked the cable through the firewall at about 45 degrees, which, because of geometry, bought me a bit more clearance between throttle arm and the point the cable goes through the firewall. That arrangement allows the motor to be mounted normally, and full range of throttle from the linkage, without any large holes or bellcranks or anything. It's the best method I've played with so far for a four stroke. Unfortunately I can't claim any credit, I stole the idea from another forum...

Edited December 1, 2021 by Nigel R

[Nigel R]

Picture is better than all those words, for sure.

 

Original bits left, shortened bits right.

 

Edited December 1, 2021 by Nigel R

[Jonathan M]

Nice-Nice!

[Nigel R]

Everything is twice with this sort of thing ?

 

Servos (sort of) in place:

 

 

As ever I cut a bunch of 1/2" strip ply and cut off lengths to form the mounts, then stuff in some scrap wood (a use for the previously removed planking effort) to reinforce the joins. It all helps strengthen the wing saddle.

 

Two standard size servos for rudder and elevator on the left (which is the starboard fuse) and one standard for rudder on the right (actually port). Micro for throttle in both, for obvious reasons. It's a bit tight as I've not made the fuselages particularly wide or tall. However, that rear decking does give somewhere to tuck the bottom of the servo into, so the wing saddle is kept relatively clear.

 

I've also gone for the rearmost location for the servos, as the nose is not short, and I will - hopefully - not need the radio to be right up the firewall end to get the CG correct.

 

Plan is (as I usually do) closed loop for both rudder(s) and ye olde blue sullivan snake for elevator, probably the next jobs while the rear fuselage is still open an accessible.

 

The current quandary is how I am going to hold the two fuselages in place, when the wing is detached. I have to have something solid at the front otherwise the tailplane will be bust apart in very short order. Oh, and, what the heck I am going to do about joining all the avionics up, can I bind two receivers? It might be cheaper to use two AR400s instead of messing with fancy wiring harnesses.

Edited December 3, 2021 by Nigel R

[RedBaron]

A possible solution is in the photo of 23 Nov showing the two sides with the tail and wing centre section in place.  If you had slide-on outer wing panels you have convenient chunks for transport/storage and easy assembly at the field? It also allows permanent wiring. If you don't want to alter what you have (it is only wood) then maybe a dummy centre section for the wings-off situation? But then that still leaves the wiring problem to solve. 

Sometimes one just has to choose the "lesser of the two weevils"

[Nigel R]

Might be a bit late for slide on panels although I think it may have been the smart choice when I started out.

 

Dummy section idea has much merit... Would need to support things carefully during assembly and disassembly. 

 

I'm kind of prepared for the wiring to be a bit of a task, i guess.

[RedBaron]

Nigel - Just one further philosophical thought.

It is a built up wing so cut covering appropriately with a thin saw, make suitable holes in ribs, insert tubes etc and replace covering

Often whatever seem the hardest when one faces a problem is actually the most satisfying solution in the end.

[Eric Robson]

Then there is the custom carrier cradle for transporting and supporting the two fuselages while the wing is off. But that is more work.

[Nigel R]

Almost there with the first throttle install. 

 

Hopefully shows what the shenanigans with cutting down the fittings have achieved by the firewall. First, full open:

 

 

And at throttle closed, still plenty of clearance:

 

 

Meanwhile, the servo end will be a bit like thus:

 

 

I'll probably need a longer servo arm to get the movement spot on, but otherwise that ensemble is about ready for soldering the m2 extension thingummy to the cable inner.

 

Might also remake the foamboard half-former that supports the end of the cable outer, the hole is not quite located correctly.

 

 

 

Edited December 7, 2021 by Nigel R

[Eric Robson]

I would imagine you are going to cover the fuselages separately Nigel, the Black Widow is very awkward now that it is built up Trying to get the inner parts covered. I would like to use 2 -4 strokes but I am going the safe way with electrics.

[Nigel R]

I hadn't thought that far ahead, but it's a good call Eric. There will be a lot of handling at that stage and it would be best to have the thing in smaller rather than larger pieces, as well as like you say, for easier access.

 

[Eric Robson]

Hi Nigel, I have seen a twin fuselage model that had the tailplane with a tongue and box at one end and a single screw to hold it together easily detached for transport.  

[Nigel R]

That's quite a leftfield solution, I like it. I guess I could revisit the tailplane and split it down the middle. Extra servo to make left/right elevator halves. Hmm.

[Jonathan M]

As long as the elevator halves join together again for flight then you'd still only need one servo.

[Nigel R]

Sounds potentially tricky to engineer with enough solidity - the low risk option would be two servos, which would definitely be ok.

[Nigel R]

A little progress.

 

 

 

 

Doesn't look a lot but it took a bit of thinking about.

 

Meanwhile, the throttles are fettled and soldered and done. 

 

 

Closed loop guides are next. Unless something else seems more important first. 

Edited December 13, 2021 by Nigel R

[Nigel R]

Got the closed loop guides in and started thinking about the battery. The nose is quite long, plus two four strokes... having the option to tuck it away behind the servos seemed a useful thing to arrange, before it was too late. The rear decking is large enough to fit a battery in so my thought was to make use of that space.

 

It can be neatly tucked it in some foam, thus:

 

 

and a ply plate keeps it all in place, keeping a clear space for the closed loop to work in, thus:

 

 

The plate is restrained by scrap balsa at the rear, and a pair of servo screws at the front (note cutout for servo lugs). Everything fits through the hole in the former at the back of the wing saddle. It's all a bit of a keyhole surgery job to get in and out through the former, but I'm not anticipating the need to remove it very often.

 

I need to arrange packing for the fuel tanks next, which will probably complete the fitting out, before closing the fuselages up with the underside sheeting.

Edited December 17, 2021 by Nigel R

[Nigel R]

Managed to get a bit of time in the workshop over the holiday period in between festivities. There were more small jobs left to do on the fuselages than I thought, before the underside sheet goes on. Often the way, you think you're done but actually there's a bunch of small stuff that takes a load of time to sort out.

 

Tidied up around the wing saddle area on fuselage, made the leading edge rebate on the centre section of the wing, drilled holes for the wing dowels:

 

 

 

Reasonably neat fit, quite please with that. Will not require very much silicon goop at all, after covering.

 

Put in some ply supports for the arms of the torque rod landing gear, reinforced the former that is drilled for the wing dowel:

 

 

Added a bunch of square strips to the corners around the front end, to reinforce all the joins:

 

 

Not really visible but I also pegged the firewall (well, kind of, I used my alternative method and cut a slot into the join with the dremel saw wheel, which was then filled with a scrap of 1/32 ply).

 

Arranged the packing for the fuel tanks, nice and snug, using some closed cell foam:

 

 

 

All of that lot can be inserted through the former at the front of the wing saddle, both foam and tank.

 

Added some blocks to be tapped for the wing bolts:

 

 

To be drilled and tapped (tap, soak in cyano, clean up, etc) later on during final assembly. These are made up from a sandwich of ply / 1/4 balsa / ply.

 

Fuselage job list is looking quite short now. Just about down to: underside sheet, cowl, fin fairing / mounting blocks. Then I need to work on the wing some more, to join the panels and sort the aileron servo details out.