Large Model Association Forum

I guess most people know that I quite like the more unusual or lesser modelled aircraft, the new project is no different.

I guess first up is a very brief background on the aircraft....

The aircraft type first started off as the Douglas A-20 Havoc designed in 1937 by Donald Douglas, Jack Northrop and Ed Heinemann as an attack/light bomber and night fighter. It went through many differnet guises and was operated by not only the US but amoung others, France, Russia and the UK. By the end of its production in 1944 nearly 7500 had been built! (and only a handful remain today....none in flying condition I believe.)

The Boston IIIA was an adaptation from the French specified Boston DB-7A and was ordered by the RAF as a longer range light bomber/attack aircraft. It was given twice the fuel capacity, a pair of Wright R-2600-23 engines and gun blisters to the side of the nose. I understand that the IIIA had 7 guns...Four Browning .303's in the nose, two mounted at the rear gunners position and apparently a Vickers .303 in the ventral position, though I have not found any drawings or photos to support that. Beyond these the aircraft can be differentiated against other Havocs/Bostons by its individual exhaust stubs and an extended carburettor intake which came to the front of the engine cowling.

Operationally the Bostons took part in the attacks on the Scharnhorst, Gneisenau and Prinz Eugen (Operation Cerberus) and also the infamous raid on Dieppe (Operation Jubilee) in August 1942. A total of 570 aircraft saw service in the UK and it is described as a beautiful aircraft to fly, almost fighter-like in its handling. Seemingly the Boston became the favourite aircraft of those who flew it.

So the model....

Scale 1/5.5
Span 134"
Length 105"
Weight approx 70lbs (32kgs)

Power will be from two Zenoah 62's with Unitracts retracts and Futaba Radio gear (2 x 6014 rx's, battery backers) and a whole host of servos (about 23!) Construction will be very straight forward Balsa, cyparis and ply.

I am hoping that this will be very much an all-weather model, capable of flying in the usual strong blustery crosswinds and perhaps the occasional rough landing strip. My plan is to build a two piece wing (with a very short center section built into the Fus) with a belt and braces system of large diameter Ali tube to take the basic rigging and flight loads and steel straps across the spar joints to hold the wings in place and provide a bit more rigidity. Large 18mmx12mm cyparis main spars out to the engines will then be tapered down towards the tips. 2mm lite-ply (sanded down to about 1.8mm thickenss to save a bit of weight) wing sheeting and then a glasscloth finish.

The fuselage will be built as a simple cyparis box structure onto which the formers and then the outer sheeting will be attached. I like this method as it is quick to build and is very easy to keep things in alignment as the fuselage sits perfectly flat and true on the building until the very latter stages when the bottom gets sheeted/planked. Tailplane will be in two section to aid getting it in the van, ali tube again keeping things neat yet strong.

I designed the model using Adobe Illustrator. Not a great CAD program but it is the only program I (sort of) know how to use. I do the basic work such as tracing around 3-views, rib and former postions and locating engine, undercarriage positions. The final plans, though sufficient for me, are in no way good enough to sell. Most of the design is in my head and it would take up far too much time to get to get my thoughts organised and onto paper to a standard that others could understand.......!

A little bit of a start has been made today!

First job is to get myself a kit of parts cut out. Formers and ribs are all a combination of liteply and marine ply, 1/8" mainly. The plans are chopped up and glued onto the wood with Prit-stick.

Where I have double the parts to cut (such as ribs) I will tack glue two sheets of ply together and cut them out as one. That ensures all duplicated parts are identical.

Just the tailplane and fin ribs to be 'marked out' now and I will get cutting!

having followed your modelling trials and tribulations, i can only say WOW!!! and await further instalments

Thanks Alan,

A few hours later......



Believe it or not there are 20 fuselage formers there. Most have been cut into four sections that will clad the outside of the basic box structure...more on that a fair bit later!

The next job is to cut the cyparis spars to size.

They will start off as 19 x 12mm, but taper down to 10 x 6mm between the outer edge of the nacelles and the wing tips.


That is tomorrows job though...

Morning Steve--very interesting project---with the progress you are making,it should be ready for display at Gaydon !!!

Hi Dave....Ha! I am sure a part of it will be at Gaydon! I am on leave at the moment, so I am hoping for a bit of progress......

So.....tapered spars!

I am going from 19mm x 12mm to 10mm x 6mm in cyparis....

First...mark out your cuts



Rough cut them on the band saw...no need to be deadly accurate just cut to the outer edge of the line or beyond....make sure you don't cross that line though or you'll create a weak spot in the spar.



I clamped all four spars together to sand them...that way you are more likely to keep the sanding block square than just on one little spar. That is a 2ft long Permagrit snading block....excellent for sanding straight edges...and leading edges, sheeting etc...



I have a double taper....so the next cut was marked out (remembering that the spars are now 'handed' left and right)

Et voila! Tapered spars.

I dont know Steve could of washed your hands before the the last pic
Looking forward to seeing her fly, just remember, " think heavy "
Cheers

Andy

Hi Andy!

When you lift that stack of ribs..there is no doubting what I was thinking! ..Some lightnening holes will still be on their way though.

Nothing more done yesterday as we went flying in the evening to finish off the test flights of the Libellula with the 2.4 Radio and Zenoahs installed....MUCH better!

Next job on the Boston is to cut the spar locations, servo cable holes and then some lightening holes in the ribs......

Finally got the ribs and braces finished off today. Surprising how much work there is in that little lot!



and a dry assembly run to prove that things fit where are supposed to.

Steve,

Great project - am watching it closely.

A-20's of thew 409th BG, 9th AF, USAAF flew from my local airfield Little Walden & one tragically crashed in the village where my son used to go to nursery and the mother of a new born baby out walking with her pram went to the aid of the trapped crew only for it to blow up killing her and all aboard. Fortunately the baby survived and still visits the tower regularly.

Good luck.

Jason
http://www.361fg.com

Hi Jason,

Yes, interesting story...would be interested in what colour schemes flew from Little Walden!

Managed to get a few hours on the Boston the last couple of days.



To keep the wing sheeting and spars in one piece across the entire span I have devised a new mthod of integrating the nacelle into the wing structure. These 1/4 ply brackets will slot into the ribs and over the top of the spars and allow me to bolt the nacelle on during construction. They won't be removable, I'll simply be using the 6 4mm steel bolts to help transfer the loads into the wing structure.



The center section was framed up last night using Hysol. You can clearly see my 'belt and braces' method for wing retention. I have had to 'break' this wing at just about the worst position so I need not only fly loads to be covered (the steel straps) but also shear forces from landings and the two engines...(the Ali tube). I am not sure that each system by itself would be sufficient, but together I am pretty sure those wing aint going anywhere!



The rest of the wing is being framed as I type, I positioned all the ribs equi distant so I only have to cut one width of shear webbing. I am using this shear web to set the spacing and also help keep the ribs vertical during the 'flimsy' stage of construction.

Today saw the framed up wing dry and ready for a light rub down.....



I had spent a long time working out servo positions, flap and aileron positions etc so all holes and neccessary hard points are installed during this intial 'framing up' stage....consequently the wing is now ready for the first sheeting to go on.

This is new to me...but I decided the sheet this in 2mm lite ply. This is slightly stronger than 1/8" balsa (and slightly heavier) but it doesn't need joining and has a much more even surface finish to it. I was thinking about sheeting each wing half in one go...but quickly realised that was a bad idea. I don't think you would get enough working time with the glue (Aliphatic used here) so I opted to sheet in two sections.....rear of the main spar and forward of the main spar.

I cut the sheet to exact size using my plan (carbon paper between the plan and the sheeting...and marked off the flap and aileron positions too.

All weighted down with pins, clamps and homemade mini sandbags.

The second wing panel has been framed up now.



1/16" ply webbing on both sides of the main spars out to the nacelles helps to keep things rigid and 3/32 balsa webbing out to the wing tips...

Aileron and flap servo boxes are installed and just need the hard points adding for retaining the servos:

The retractable landing lights are now provisionally installed....one on each wing, just outboard of the engine nacelles. I am using 12volt mini halogen bulbs (20watt) that are easily available from DIY stores.. I'll have a 12v NiMh (4000mAh) battery pack drive both of these with a brushed speed controller as the 'on-off' switch. I should get just over 1 hrs operation from one charge...which should be more than enough for a weekends flying.

To start with I made a little sketch to get the shape of the offset hinges just right before transfering it onto some 1.5mm paxolin sheet.



I tacked four laminations together and cut them all out on the scroll saw.



I made up a small set of parts for each landing light...which will use the brass tube as a torque rod. That way I can get the light servo under the same hatch as the outer flap servo.....saves time and strength!



A lite-ply base is glued between the ribs with a smaller square of lite-ply to locate the light unit. This smaller square will be the part of the final sheeting..I thought it was easier to get the sheeting to fit around the square than around the light....

The whole assembly has now been tacked into place with cyano....tonight I'll go around everything with a bit of hysol and fit some locking pins through the brass tube to stop things from ever coming loose.



The light in the retracted position....

And deployed....

A little more sheeting and wiring inplace.

Wiring is Heavy Duty Silicon twisted cable with Ashlock connectors. I like this system as all wiring is exactly the right length and the connectors cannot be assembled incorrectly at the field and have a locking system too.....about as failsafe as it gets.

Looking really noce steve!!

Allan

Progress has been a bit slow lately, but I have been working in short bursts!

I decided to make a start on the fuselage. I started by laying some painters 'dust covers' over the plan and then building a basic warren girder structure from 9mm cyparis and balsa. 1/16" ply reinforces the area around the wing cutout and the bomb bay.



Formers installed for nosewheel retract unit, and wing mounts. I added a couple of extra formers to help keep everything in shape around the cavernous bomb bay. When you look at the bottom of the model, the length of the nose gear doors and bomb bay takes up most of the bottom of the fuselage....not leaving much structure around to stop it all flopping about!



The fuselage was then assembled over a plan view, inverted, to ensure everything was 100% square and true. More 9mm balsa and cyparis cross members were added to keep the box shape stable.