Nigel R Posted January 23 Share Posted January 23 Orthus Mythical two headed dog. The brother of Cerberus; the guard dog of the Cyclops. More soon... Quote Link to comment Share on other sites More sharing options...
Nigel R Posted January 23 Author Share Posted January 23 The mythical Orthus had several distinguishing features; for starters he clearly had two heads. But he was also depicted with a tail that had a sting like a scorpion or even a snakes head on the end of the tail. A bit of a beast. Orthus is a project I've had on my wishlist for a while. What will my Orthus be? Orthus will be a twin engined airframe - like his mythical counterpart with two heads. I'm currently undecided between using a pair of Irvine 39, OS 46 SF, or OS 56alpha. But what to do about his other feature, the tail that ends with a sting? Something different is called for. Here I decided a little while back that I would have a go at making this airframe with a 'tail end feature' - twin fins. Why twin fins? Simple - I have had good experience on another project with twin fins providing an airbrake option, by pulling both rudders inboard by 45 deg, allowing for a steeper descent into shorter strips. It's like having a parachute hung out the back. Most airframes would use flaps for a similar function; twin rudders provide an alternative option. 2 Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted January 23 Share Posted January 23 Interesting Nigel, when It's built don't fly if there's an Hercules present. 😉 1 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted January 26 Author Share Posted January 26 Some more background. I've built several twins. The first was a conventional layout (build log - Double Trouble) and the second was a double fuselage layout (build log - Doppleganger). The twin fuselage is the one with the rudder airbrakes. Aesthetically, I was quite pleased with the fuselage shape on Double Trouble... ...so I'm going to base Orthus on Double Trouble. Not a rehash mind, more an evolution. Double Trouble had a constant chord wing and was very mild mannered. This time around, being happier with twins in general, I'm going to use a slightly spicier tapered wing for better aerobatics. I found the overall size of the airframe made transport easy enough (62" span) with a single piece wing. Obviously it will look a bit different with the twin fins. As an aside, I was very tempted to do a kit bash from the Galaxy Warlock... ...but, it's a big bigger than I wanted, it is 74" span. 3 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted January 29 Author Share Posted January 29 I've now taken the plunge, doubled my mortgage, and ordered some balsa from SLEC, as I'm reasonably certain of most of my construction details - given that I've already made a very similarly sized twin with the same fuselage shape / nacelles. However, I have already started some work on the model using some wood I had "spare". I figured I would try and work the part of the model that I was least sure about - which at this point is the twin tails. Wishlist: * Two independent rudders on two servos * Able to move a full 45 degrees for useful airbrake function * No weak control links * No messy external hardware I figured I had a few options for controlling the rudders: * Something with bellcranks and pushrods * Snakes of some sort * Closed loop being routed, somehow, from fuselage through the tailplane itself then on to the rudder * a torque rod, going from inside the fuselage, to right out near the rudder, with a short link rod to the rudder itself. * burying some servos in a tailplane or fin I ruled out bellcranks quite quickly, although they'd probably be functionally ok, they were quite large and I'd not be able to hide them inside the structure. The structure would need to be quite deep - much deeper than I'd hoped. I made a test piece with closed loop, but I wasn't very happy with the very tight curves I found were needed in the guide tubes. I very nearly used torque rods - I got as far as making a pair of double ended rods up from 3/32. This was promising, but, after making them I tested how rigid the rod was, and with the (quite long) lengths involved, I found they were too flexible for my liking. I didn't want any issues with flutter in flight, or for the torque rods to be too weak to sustain a knife edge type situation. I ruled out using individual servos - the really flat servos are too pricey for me. This pretty much only left snakes. To check viability of these, I worked out what the largest, shallowest curve I could use whilst still containing one snake in each half of the tailplane. Short version, this appeared to be viable. And I also figured, if I was a touch crafty and with some careful building, I could also hide the rudder horns safely inside the tailplane structure. I'll put up some pictures soon - they'll explain more than my words have! 1 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted January 29 Author Share Posted January 29 (edited) This is the tailplane internal structure. It will be sheeted with 1/16 later on. The quarter circle I carved out to accept the snake is (I hope) obvious 🙂 At the tip of the tailplane, toward the rear of the structure, is the pocket I am building to hide the rudder horn. More on that soon. Edited January 29 by Nigel R Quote Link to comment Share on other sites More sharing options...
Nigel R Posted January 29 Author Share Posted January 29 (edited) Here I've sheeted the tailplane. This is a closeup of the trailing edge of the tip of the tailplane, along the hinge line with the elevator. You can see the snake now embedded in the structure. The snake emerges into this rectangular pocket (which I haven't sheeted, for reasons). The clevis will live inside the pocket, as will the rudder horn. Hopefully this will be better illustrated a bit later on. The sheeting over the pocket is done using a 1/32 ply insert. I figured that would give me just a touch more space inside the pocket. This shot is from the sheeted side of the above structure, with the ply insert on the bench. More soon. Edited January 29 by Nigel R 2 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted January 30 Author Share Posted January 30 (edited) A few more shots of where I am up to with the business end of the inner tailplane workings. Snake entry into pocket... ...and the other side of the tailplane... ...and a different angle, now with sheeting on both sides (well, except for the ply insert) ... ...with that (nearly) done, time to put the tailplane itself down and make some fins and rudders. The above process actually took quite a long time - the few working prototypes I made didn't come for free (sadly). I also spent a bit of time playing with the dimensions of the tailplane to find out what the widest curve I could get for the snakes was. This was a case of balancing off the span and chord of the tailplane, and a bit of suck-it-and-see to find out what curve would fit. In the end, I think I wound up with about a 6" radius in the snake, and the tailplane itself is about 20" in span. As I'm shooting for ~150 sq in on the total tailplane/elevator area, that gives 7-1/2" average chord, of which around 1-1/2" will be elevator, leaving 6" chord on the tailplane... Hence the "about 6" radius" on the snake itself... Edited January 30 by Nigel R Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 2 Author Share Posted February 2 (edited) Time to make some fins and rudders, or vertical stabs as they say over the pond. Internal structure from 1/8"; I'm adding a hard point of ply to receive the M3 threaded rod: Starting to get some 1/16" sheet on the rudders and taper the 'ribs': The rudder's LE has the ply scarfed to some 1/4" balsa: Rudders now sheeted: More soon... Edited February 2 by Nigel R Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 2 Author Share Posted February 2 (edited) ...and some sheet on the fins... I then put some 1/4 x 1/2 around the outside. Later on, this will be sanded to a nice rounded outline: Time for a mock up! Fins pegged to the tailplane with cocktail sticks: ...looks good so far, I think. You can see the ply insert on the rudder is lining up nicely with the tailplane 👍 And a view from the top. At this point I have now hinged the fins/rudders, dry fitted the M3 rod, cut the nylon horn down a bit, and opened up the TE of the tailplane in order to allow free movement. Phew! Sadly I forgot to take a picture with the snake in place, but, it's all nicely enclosed in that pocket. I'm rather pleased with my efforts so far. So much so I promptly stopped work on the thing for about three weeks! This pause in building activity was the point I spent the kids inheritence on a small stack of balsa and started drawing out the wing in CAD. Edited February 2 by Nigel R 2 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 2 Author Share Posted February 2 And to catch up with progress to date, the wing in devWing: and the internals: The taper is fairly mild - the tip is around 2/3 of the root. I'm using strip ailerons, of 1-1/2" width. There's a secondary spar there to help carry the load around the cutouts for retracts - if I've done my sums right there is space to get a 3" main wheel tucked away. 1 Quote Link to comment Share on other sites More sharing options...
Nick Cripps Posted February 2 Share Posted February 2 Looks good, Nigel, I'm interested to see how your twin fin & rudder arrangement works. I've been pondering a similar twin engine twin fin design for a while but always get hung up with how to actuate the rudders. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 5 Author Share Posted February 5 (edited) Thanks Nick. Sadly the proof of the pudding is likely to be a ways off, going by how long previous builds have taken me. Tonight's job was simply to stick the rib templates to some wood. Managed to squeeze them on to 3 sheets of Ecuador gold: it has to be said, I am not relishing the thought of the next bit, it is my last favourite part of an airframe build. Edited February 5 by Nigel R Quote Link to comment Share on other sites More sharing options...
Jonathan M Posted February 6 Share Posted February 6 Following with interest Nigel. How will you do the next, least favourite bit...? Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 6 Author Share Posted February 6 With a lot of cutting out by knife, and careful sanding... slow going! Nevertheless, that method is the quickest route I have to getting an accurate set of ribs when the wing is tapered (I don't like the sandwich method). (I haven't yet ventured into printing templates or getting ribs CNC'd or lasered) With a simpler rectangular planform, I'd (a) likely use foamboard these days, and (b) cut a lite ply template and then cut directly around that - which is dead quick and easy with foamboard ribs. 1 Quote Link to comment Share on other sites More sharing options...
Jonathan M Posted February 6 Share Posted February 6 Yes, that'll be a long session of 'mindfulness', but at least you've given yourself a flat-bottomed section! Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 6 Author Share Posted February 6 Indeed it will be 😄 The wiing section is fully symmetrical though. I've used a NACA four digit. I see that I forgot to mention, I am making the wing fairly thick near the root (17% IIRC) to help with strength and to hide the retracts. This tapers down to 12% at the tip. Another point I did not mention, I have placed several half ribs near the root and around the nacelle. With Double Trouble, I found I naturally wanted to hold the airframe at the root of the wing, and without some reinforcement the 1/16 wing skin is, erm, "delicate", hence adding half ribs for extra support in that exact spot. Quote Link to comment Share on other sites More sharing options...
Nick Cripps Posted February 6 Share Posted February 6 Assuming your leading edge sheeting is interrupted by the engine nacelles, can you not substitute 3/32 for the inner section? Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 6 Author Share Posted February 6 (edited) That could work (and yes, the nacelles will go back to the main spar). That said, I'll also be running some thin glass right out past the nacelles this time - I'm hoping that, coupled with the half ribs, will be enough to toughen up the LE root area Edited February 6 by Nigel R 1 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 7 Author Share Posted February 7 I got a few shots of the rudder assembly to better show the thing moving around 1 Quote Link to comment Share on other sites More sharing options...
Nick Cripps Posted February 7 Share Posted February 7 That looks neat. Fairing on the outside? Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 8 Author Share Posted February 8 Thanks Nick. It's a touch tight, I'm aiming to open up the slot a little, later on. As you can see I'm going to have to be really careful to get the hinges lined up when it comes time to glue them. The cocktail sticks are an aid to keeping things nicely aligned and might do double duty as slight reinforcement to the joint. External fairing - hadn't thought about it yet, but it's a good idea. Quote Link to comment Share on other sites More sharing options...
Nick Cripps Posted February 8 Share Posted February 8 You could put short tailplane extensions on the outside like those on the DH Vampire. PS - a few extra turns required on that studding in the end of the snake 😉 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 13 Author Share Posted February 13 Faced with the prospect of cutting all those ribs, I returned to the tailplane and instead cut and shaped and tapered the elevator. This was then hinged and bevelled. Whilst I still have some shaping of the fins and rudders to do I think I am almost out of excuses to avoid cutting the wing ribs. As usual for me, the elevator has harder wood at the hinge line and a hard point to take the screw on control horn. The rear of the elevator is simply soft sheet. Quote Link to comment Share on other sites More sharing options...
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