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Nov 11, 2019, 04:17 PM
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Build Log

BIMBO Twin Otter 480 Build


INTRODUCTION
When I (re) started my RC flying hobby back in the spring, I was surprised just how far everything has moved on since my attempted RC glider flying some 40 years ago on 27Mhz. It took me a while to choose a trainer glider and then I chose the Night Radian from a couple of options recommended by my local RC shop. It flies very well and I was impressed with the Spektrum SAFE gyro - like having my own buddy box on board. A couple of club members later recommended that I progress to a powered aircraft with ailerons. I chose the RIOT and installed a SAFE receiver and programmed it to suit the RIOT and my changing level of competence. It's great because it can be switched off in flight for me to increase my skills. Although nicknamed a "chicken stick" by more experienced club flyers, this has helped me gain experience without too many dives into the gorse. I am now considering using a SAFE receiver for the maiden flight of my Twin Otter 480 when it is completed. It might save a few tears.
Deciding on a twin engined aircraft for my first scratch build might seem to be jumping into the deep end of things but I liked the look of the Otter and it used to be part of our local Exeter FlyBe fleet. Reports on flying the Otter were encouraging and its design like the flat bottomed wing does not look over complex.
Having printed off and marked up I.P's build notes and read through several build threads, I feel that I am getting a better understanding of how the Otter build will progress. One of the threads also helpfully provided a CAD file for cutting. I initially investigated having these parts laser cut but then decided on investing the money on a scroll saw as the parts do not seem too complex and the wing ribs should hopefully be able to be cut in bulk having a constant chord. I have now printed these onto card to be used as my initial template.

PREPARATION
Having decided the Otter 480 would be a more practical build and transport size over the 600, I decided that the work bench space in my garage needed re-configuring for model building. Although I have several strip lights, the lighting could be better so I have installed a 6k Lumin twin LED strip above the bench - much better. I have also recycled some 18mm ply and wardrobe batons to make a flat building board and jig based on a Model Flying design. I have forgotten how cold my garage can get in the winter so I have switched on a portable dehumidifier and that has taken the chill off as well as providing a source of washing water.

MAKING A START
I am following the sequence set out by I.P and will be incorporating a few minor mods that I saw on various threads such as part panelling the sides to enable window cut outs. I am thinking this will also help handling at the field. I am not convinced the printed plans are 100% on scale as my sticks appear slightly thicker. This may be due in part to imperial to metric equivalents for Balsa. But as long as I keep to my conversion chart that 3/16 = 5mm, my build should be consistent. The next stage will be to complete the left and right panelling and gussets prior to preping the jig for the initial boxed assembly.

FUSELAGE ASSEMBLY UNDERWAY
My DIY building board /Jig has been a real / essential aid to aligning the fuselage. The box section is quite flimsy so I think without the jig a banana would result. Even so, I decided to replace cross piece No.14 as the tail was going slightly out of line at that point. I have followed advice from our club member, Joe who builds a lot, by dabbing CA if necessary to get a grip then wicking in Aliphatic. This seems to have worked well. I glued the main body and tail area first and put an initial tension on the front - nose part to get the longerons used to a quite sharp bend. Not sure if the plan intended cross pieces of sheeting to seal off the nose and tail but I decided to use 3/16" (ish) sheet. The next stage is to cut and fit the top formers. I have printed the CAD/PDF formers file that another RCGroup member prepared onto card and am now comparing these and adjusting to the plan as the printed plan is very slightly to a smaller scale. e.g. 3/16 stingers are a tad smaller on the plan. This coupled with modern metric equivalent sizes of balsa is keeping my vernier gauge busy.
Last edited by RobertDaw; Nov 21, 2019 at 06:00 AM. Reason: Further progress to report
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Nov 11, 2019, 05:04 PM
Complete the Illusion!
35Mhz's Avatar
Robert, You will find that the plan is not 100% to scale in all respects, it a case of 'making' parts that fit, lots of measuring and adjustment, but the end result is well worth it, best to cut parts as you go, rather than to the plan.

Tim
Nov 12, 2019, 03:20 AM
Slip the surly bonds...
Sopwith Mike's Avatar
Great start Robert, looking forward to the build.

Mike
Nov 20, 2019, 05:22 PM
build like there is no 2moz
wallis_100's Avatar
Great start so far :-)
Nov 21, 2019, 04:53 AM
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Thanks all for your positive and encouraging feed back on my build. And for the the tips which go a long way in smoothing out my learning curve.

Robert
Last edited by RobertDaw; Nov 21, 2019 at 08:39 AM.
Nov 30, 2019, 11:39 AM
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Build update


First of all, a thank you for all the encouraging comments about the start of my build and also to the many build log photo's of the Twin Otter and various techniques used. These have all been extremely helpful as this is my first scratch build and decades since building a more simplistic aircraft kit. My local model shop build night have also been a excellent source of advice on tools, materials and techniques. My garage / workshop has quite an assembly of forum photo's of the fuselage hung up on the walls to help me confirm or otherwise my conceptualisation of the plan.

FORMERS & SHEETING
My DIY jig has been keeping the fuselage very rigid and hopefully straight as the build progresses. So far, it has not been a chore cutting out the cardboard former templates from the PDF - CAD file, adjusting sizes to my actual need then cutting and trimming the balsa formers. I deviated from the build instructions slightly by adding the short vertical sticks between the rear top stringers after fitting the rear top formers - which i made slightly deeper to fit inside the top stringers. I was not sure if the rear deck could be sheeted with on 5" plank so I bought some card and tried that first and great - no creasing. Last night I water sprayed the outside of the 5" wide plank then roughly rolled it over the top of the fuselage, pinned and clamped and left to dry overnight (a forum idea). This morning I trimmed to width using pins as a width guide then fixed using Locktight 60 seconds adhesive, followed by a lot of masking tape and a couple of clamps. With the wide choice of adhesives, I hope this was a good choice. I might run some Aliphatic along the inside when the construction comes off the jig. I am quite happy with my first sheeting attempt as there are only a couple of minor seams that will need a touch of lightweight filler. A little disappointed with the 5" sheet that is actually 2 sheets pre-glued but slightly distorted on the join.

HATCH
As suggested somewhere, I used cling film to avoid the adhesive joining the hatch frame to the stringers. I contemplated making the roof frame separate but decided that with a supporting block, it should be firm enough to sheet - remembering to chamfer the sheeting to give a smoother fit to the front edge. I think my hatch top formers are a little thicker than plan and I decided to deepen them to also fix to the cross members to add rigidity (another forum idea). Also from a forum, I found a dashboard photo so I have printed that to the correct scale to stick on later. Unlike the forum source, I wont be adding a back light for now. A scale appearance, a little scale detail and fly-ability are my priorities. However, I fancy fitting wing and landing lights as I think this will also help flying orientation on a dull day - if I can remember which side the red is.

I am now recruiting for two FlyBe test pilots but not wanting to spend the top model shop prices, this may take a little while.
Nov 30, 2019, 12:38 PM
Slip the surly bonds...
Sopwith Mike's Avatar
Great progress.
Nov 30, 2019, 12:46 PM
Complete the Illusion!
35Mhz's Avatar
Great progress Robert, great to see another one using yards of masking tape, an invaluable clamping means. A useful tip is to use one of those sellotape heavy weight dispensers, is so easy to rip off a length of 25mm tape with one hand , useful when your holding the piece to be glued with the other!

if you have a local branch of Wilko there only 2.75
Dec 01, 2019, 06:31 AM
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Thread OP
Thanks both.
Useful tip on the Wilko dispenser. I found my clamps and other stuff in our Newton Abbot Wilko.
Dec 03, 2019, 03:11 AM
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CaseySP's Avatar
Quote:
Originally Posted by RobertDaw
First of all, a thank you for all the encouraging comments about the start of my build and also to the many build log photo's of the Twin Otter and various techniques used. These have all been extremely helpful as this is my first scratch build and decades since building a more simplistic aircraft kit. My local model shop build night have also been a excellent source of advice on tools, materials and techniques. My garage / workshop has quite an assembly of forum photo's of the fuselage hung up on the walls to help me confirm or otherwise my conceptualisation of the plan.

FORMERS & SHEETING
Nice work - have been subscribed since you first started this thread.

Re the sheeting - a spray of window cleaner with ammonia is usually great for helping the balsa becomes extremely pliable. You could almost tie it into a knot. The ammonia helps to soften the fibres (or the bonding between wood fibres depending on who you believe) - when it dries out the wood hardens with no real ill effects. The wood may possibly be slightly weaker, but it is only planking and not usually a stressed skin.
Last edited by CaseySP; Dec 03, 2019 at 05:14 AM. Reason: Spelling
Dec 04, 2019, 05:01 AM
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Thread OP
Thanks Casey and I expect that you are finding it very warm in Aus now and I hope that your area is not affected by any bush fires. Send a bit of that warmth to the UK - I certainly need it.
A great tip on the window cleaner and I enjoy learning about the tips and techniques that other builders are using. Something that RC books often don't include. Keep them coming all (please).
My next steps are getting my garage / workshop better insulated. I draft sealed the up and over door yesterday and stuck some foil covered thermal blanket to the metal door. Apart from my comfort the fridge/freezer cuts out when the ambient temperature approaches zero - apparently a feature in many. 5c inside this morning and my roof was not as frosty as the adjoining garage, so some benefit i think. Thermal panels between the flat roof rafters next. Off to Wickes later.
Dec 09, 2019, 03:46 PM
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Thread OP
A short building break has been taken to insulate my garage / workshop. I forgot just how drafty and chilly it can get. I have draft proofed the door, thermal lined it and cut 50mm insulation boards to fit between the joists. Hopefully the heater will be more effective now and the silver lining has really boosted the lighting level.
Dec 10, 2019, 07:23 AM
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CaseySP's Avatar
Quote:
Originally Posted by RobertDaw
Thanks Casey and I expect that you are finding it very warm in Aus now and I hope that your area is not affected by any bush fires. Send a bit of that warmth to the UK - I certainly need it.
Thanks for the thoughts - we are so far clear of the fires but the winds are bringing choking smoke over us. Pollution levels 10 times worse than Beijing and our kids not allowed to play outside at school. With the heat (38 degrees today) not a lot of fun.

Funnily enough I insulated our garage door a couple of years ago (80mm polystyrene panels) - mainly because the morning sun hit it directly from 8 am to midday, and temps in there could become unbearable in summer (and not safe to store models, lipos etc). Made a world of difference. Helps a bit in winter too - keeps it above freezing.

WRT to the Twotter, always been on my "must do" list, but thinking a short-kit would help my motivation a lot (days of cutting out ribs and formers are behind me). Would be a nice project with my lads.
Dec 14, 2019, 10:46 AM
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Thread OP

Bottom Formers next.


I don't think that I will get much free time this side of the new year for any extensive building so I am taking the opportunity to rejig the fuselage ready for the the bottom formers, U/C and planking. Also to check the build so far for alignment as any errors now will not right themselves later.
The tail taper seems fine / close enough to have a clean line onto the rudder later. There seems to be some minor twists in the tail section so I have clamped this area square and hopefully the bottom formers will keep this in place. I noticed a slight concave around B6 position so I have packed and reset the cross pieces at B6 & B7. I see from the plan that formers B5 - B10 are identical. I am close but my B10 is just beginning of the smooth taper line so I will leave that.
The nose section still lacks lateral rigidity so I will keep an eye on that and may add additional bracing although I still have to add planking, steering re-enforcement etc. I also realised that I didn't attach the canopy windscreen rail to the same contour as the roof so I have trimmed that off and will pay more attention next time.
The cut part PDF file has been a real aid. I have printed those onto card and have been checking dimensions to my model that was built over the plan which is slightly under scaled. As someone said - lots of measuring with the vernier and then adjusting the cut pieces prior to cutting out and transferring to balsa. In doing so I noticed the cord heights from B1 - B10 are all 1" so my old draughting compass has a new life redrawing the 'B' arcs. I have also cut out some templates to help get a smooth nose cone profile that will be cut from blue foam given by club member Joe.
I have just bought the U/C wire, clamps, collets etc and plan to include a sprung nose wheel based on an idea I saw on 'raysmodels' page. Why some suppliers still use SWG when most now use metric is a minor irritation to me when in my local model shop. That's another conversion chart I have donated. Local modellers are recommending I have a separate steering servo and not to share with the rudder to aid trimming and resilience.
Dec 16, 2019, 05:10 PM
build like there is no 2moz
wallis_100's Avatar
I fed both the rudder and steerable nose wheel from 1 std size servo through 2 flexible snakes.. no issues at all. the thin wire nose leg Ivan suggests works fine- I even had it replacable with a crank arm for using the floats to drive the water rudders


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