Topic: Douglas A-20 Boston

I have always wanted to build the Boston since I first saw a picture of a model about 15 years ago in a modelling mag. I can't remember who built it...but it was about 70" span and just looked mean! Big fuselage and massive nacelles wih quite thin and elegant little wings. From what research I have already done...the pilots all claimed the Boston was a fabulous airplane to fly...more like a fighter than a light bomber.

Phil approached me last year and asked if I would like to fly the P-38 in the 'Fighteraces Team'.......I didn't need asking twice!

So now the time has come to think about the next project.....I knew it had to be something that would fit in with the Teams 'theme'. At the time Phil was still planning on the B-26 being a part of the team...so that give me a rough idea of the size the model needed to be.

I began by researching the aircraft as best I could. 3-views are a little thin on the ground but I ended up getting in touch with an American modeller (Jim Suchy) who has built an A-20 Havoc. He kindly let me have his 3-views (well, more like 6 views!) which are excellent and are what I have mostly based my model on.

Once I had these drawings imported into my favourite progam....Adobe Illustrator....I could begin to figure out just what size I was going to make the model.

I had a few constraints....

Must fit in the van (VW Transporter)
Be able to find wheels for it
Fit either 45cc or 62cc engines in the cowls
Be able to find pilots for it
Be of a similar scale to the other team members aircraft.

After a lot of swapping and changing I finally settled on 132" span which makes the model about 1:5.5 scale. I happened upon some Elite Force Pilots on ebay which are 1/6th (ish)....Wheels are a bit of a pain...but they came out dead on 8" for the mains. I also think I can just about shoe horn in two Zenoah 62's so I can keep up with the hooligans!

Hopefully the weight will be around 60lbs though I intend building the model to be able to cope with more than that (they always end up weighing more than you first think!)

I am still working on the plans, though they should be ready to be sent to the printers in a few weeks. One of these plans will be designed to be cut up and stuck directly onto the wood for cutting the parts out, the rest will be just a basic outline with a few details here and there so I don't forget what I was thinking further down the build!

Retracts are going to be Unitracts. By far the cheapest and I have very good reliability from Tony's units. They won't win any scale comps but are 100% functional....I can add some detail later on.

Radio will probably be twin Futaba 6014's (2.4GHz) and one of the simpler Powerbox systems to be able to handle the current going to at least 21 servos!

Can't wait to watch another one of your fantastic builds, Steve.

You must have a very patient and understanding wife.

Oh yes...she is!

Have been doing a spot of work on the plan when I get the odd spare moment.

I have been scratching my head as to how to have the model 'break down'. There are numerous problems with all the standard methods.

I firstly would have liked the model to have a 3 piece wing...with the center section carrying engines, retracts etc and removeable from the Fus....but from what I know of the Libellula the center section ends up too big and awkward to move around the workshop, furthermore, the cutout in the Fuselage is exactly where the bomb bay is and I don't think I would have enough strength left in the fuselage at that point.

The next thought is to have a two piece wing...plug into either side of the fuselage. Makes the wing panels a bit lighter and the fuselage MUCH stronger...but now I have to spread the loads of the engines and wheels (not forgetting the flight loads) through a join.  Not ideal. I have a pretty heavy duty wing joining tube in the workshop which is about 50mm diameter and about 2mm wall thickness. I am thinking that this should do the job? I'll also put a smaller (25-30mm) tube at the rear spar location.

Other than that dilemma the rest of the model is quite straight forward. I will have detachable tailplane halves and leave the fin permanently on the Fuselage, with a removable nose cone (the glazed bit) to gain access to rx's, batts etc.

Using tube joiners will always make it heavy because tubes aren't actually very good at taking bending loads. A better bet might be to use strip joiners joining the spar caps via vertical pins (ie bolts). It just requires flat steel strips which are screwed into the spar caps of each side (upper and lower) with precision-reamed or bushed holes. A bolt then passes through the holes and is nipped up with a nut.

You'd probably have a metal bush as a distance-piece between the strips so that the bolts had something to bite onto - using countersunk holes would also make it more "positive". With a little cunning the bolt locations could be all inside the fuselage (inserted from the bomb bay) and/or the nacelles (inserted through the U/C wells). As well as being lighter and more flexible in application it is technically better engineering, because you're not transferring loads through the ribs. The only danger area is the screw pattern into the spar cap, but there's loads of info in PFA/EAA design codes on how to calculate these.

There are variations using horizontal pins, but they rely on good quality welding and machining to implement.

Just a thought...

PDR

Yeeeaaaaa....good man  

Guess I'll just have to let you bomb um with your glazed nose as the B-26's replacement will have up to 16 forward firing .50 cals   (& rockets!!)

What about interlocking blade spars?..............this is how the 2 centre section panels on Deak's old Liberator were done, one on the front, and a 2nd on the rear, sliding in detween a set of formers each and bolted through, accessed through the top turret hole for the front spar and the rear fuz break for the rear spar.........

That has a pair of ZG45's hanging off the front in long nacelles compared to the A-20, on a VERY high aspect ration (& quite flexible wing) and that survived knocking 200 flights before John left for the US.

Phil

Fantastic  Can't wait

I have never seen one of these, nor heard of one, however my first impression when looking at  the photos was a very nice wing shape, seems slimmer and much more 'fighterish' than 'bomberish'

Early days I know but what scheme have you got in mind?

Rich

Cheers PDR....yes that thought had crossed my mind too. I guess I am nervous of structures I have no experience with.....silly I know and not practical!

 My concerns were always in getting those steel strips properly screwed and bonded to the spars and passing the loads through the Fus without needing huge dihedral braces/spars. Having a tube spar at that point takes most of the loads...I just needed to add a little support to help stop the tube bending.........

I also wonder how easy the model would be to rig single handed. I would like this model to be easy to operate with just one/two people.

The blade spar is an interesting idea, Phil. A bit like fullsize gliders? I know that they are very easy to rig single handed. I could fit a peg at either end of the blades that would hold everything true while the main bolts go into place.....hmmm. Need to give this some more thought!

Guess I'll just have to let you bomb um with your glazed nose as the B-26's replacement will have up to 16 forward firing .50 cals  Evil (& rockets!!)

No, it's ok......you can go infront! Wouldn't want to be in the way if you got a twich in your trigger finger!

Early days I know but what scheme have you got in mind?

Hi Rich....

Yes, hoping this is going to be very fighterish to fly. I haven't decided on an specific airframe yet, but it will most probably be in DK Earth, Dk Green Camo with RAF roundels. Can't beat British! (though it is very much an American design! lol)

I'm assuming then that it will be a true British DB-7B (Boston III) rather than an asthmatic press-ganged French DB-7A?

PDR

hi steve, superb looking twin,  report in flypast dec 09 says theres  A-20G havoc/boston fullsize airframe arriving from australia and is beening displayed at Hendon in 2010..

woody

Looking forward to this  , always love watching your builds progress.  Thanks for taking the time to post your progress...


Cheers, Alex

Steve,

The spars on my homebuilt are in 3 pieces and made from sitka spruce, joined by steel brackets. The main spars have brackets front and rear, top and bottom, with the pins going in horizontally. The pins are AN5 bolts (so 5/16" diameter). The top and bottom pairs of brackets sandwich the spruce spar caps and are bolted to the caps with 8 x AN3 bolts each (so 3/16" diameter).

The rear spars are likewise attached, but the brackets are bolted to the spars using 5 x AN3 bolts each.

This structure is stressed and proof loaded to +/- 9G at 800lbs gross. There are videos of some Italian ejits looping the aircraft (which was plushly finished with 2 adult men on board - so gross weight probably nearer 1200lbs) on Youtube

http://www.youtube.com/watch?v=EY_03hSxAn4

for further evidence of what this fixing method will tolerate.

On the basis of this, my first choice would be exactly as Pete described. Steel strips 5" long with half a dozen screws into spruce spars would be more than man enough for the job. It would also instantly solve the obvious problem of needing a location method at the leading and trailing edge of the wing to prevent the thrust loads trying to slide the rear tube apart under power and the drag loads trying to slide the front tube apart under high speed power-off loads.

Having a steel strip screwed to a "Spar cap" top and bottom, in turn glued to beefy formers would sort out the fuselage attachments 

The blade spar is an interesting idea, Phil. A bit like fullsize gliders? I know that they are very easy to rig single handed. I could fit a peg at either end of the blades that would hold everything true while the main bolts go into place.....hmmm. Need to give this some more thought!

Basically yes...........

From memory, John had laminated ply & glass board 'blades' filling the space inside the front & rear shear webs between upper and lower spars (not sure how far out into the panel they expended though). They were obviously 'offset' against each other so when slid into the fuz between 2 formers, one blade sat in front of the other and it was all nipped up when 4 bolts were passed through said formers & blades. The system was duplicated front & rear.

Rigidity will obviously come from having the front & rear joiners as far apart as possible....John managed this OK as the B-24 wings are not heavily tapered, so the front & rear spar can be well spaced at the root.....the problem with the A-20 is the heavy forward sweep on the TE meaning if you space the spars far enough apart to give enough rigidity, as the spars have to be parallel for the 2 sets of blades to slide together, I'm not sure how far out along the span the rear spar would get before it breaks into the false TE in the flap/aileron location.

I've used the steel plate method for outer panels a few times now & it works V well, and one significant benefit is you can run both spars where they need to be for max strength, and simple bond/screw the steel plates to their root end without having to worry about having the 2 sets of plates parallel to each other.

They are however a bit of a faff to assemble, especially big panels single handed. Maybe you could incorporate a short 'tube' joiner as an alignment aid, before fitting the vertical load bearing bolts?

Phil

Am trying to visualise this but struggling.

Has anyone got any pics? I'd google it but don't really know what to google for.

Outer panel fixings on 180" Skyraider using 1/16" thick st/st plates (3/4" wide, 6" long) screwed/glued to the upper and lower spars with a vertical fixing passing though both plates secured with a nyloc on the underside. The same method can be used with plates on the front & rear of the upper and lower spars with horizontal fixings top and bottom.

Wow. that is so much easier than trying to fit a wing tube.

The general concept looks something like this:



Obviously it gets neater with countersinks in the main shear bolt. but you get the idea.

PDR

So it seems a fairly unanimous opinion!....Wing tube is out.....Steel joiners in.  Thanks all for your advice.

I am now drawing up som wing ribs (using Profilli as a guide) I have the imported into Illustrator and now tweeking them to have the spars exactly where I want and of varying sizes.

I'll run this by you all to see if it sounds sensible.

I have two main spars (top and bottom) of approx 20x12mm Cyparis (depends what the closest size Stuart at Solutions has), 1/16" ply webbing front and back. The main steel joiners will be glued and screwed to this with some local reinforcing (a la, the Skyraider photos) This giant spar can't go all the way to the tip (and doesn't need to) so I have feathered this spar to nothing just outboard of the nacelles and a new spar (12x6mm top and bottom) will be sandwiched to the first over the length of 25cm. This will then carry on to the tip...with 1/8" balsa shear webbing.

Rear spar to be a short length (3 rib bays) of 20 x 12mm and then fairing into 6x6mm cyparis to the tip.

The awkward bit is keeping the spars vertical to each other so that webbing attaches properly. The spars will have to sunk inot the wing ever so slightly and then built up to the surface with balsa.



Scale airfoils used...NACA 23018 at the root and 23010 at the tip....4 degrees washout as per fullsize!

I propose the main steel joiners to be 1/2" x 1/16" steel (5" long) and the rear joiners to be 3/8" x 1/16th.

Sound about right?

Phil, can you remember the sizes of Cyparis Stuart stocks?

He'll cut whatever you want him to cut mate........he did 25 x 20 for the main spar on the Raider.

Your timber sizes look more than up to the job......these are a fraction bigger than I used on the B-26, but that has 1/8" ply 'egg box' type shear spars back and front on the main spar, then conventional 1/16" ply webbing on the rear

Phil

Signing up, look forward to seeing this one come together Steve 

Steve

ooo yes, me too, some proper engineering too... 

Cheers guys....

Still working on the drawings....and I have one model to paint (a little Nijhuis Lanc for the evenings!) then I can get going on the Boston.

Think I have now sorted a supply of fat 8" main wheels (cheers Phil!) and  I hope to get my plan printed in the next few weeks. Beyond that I am slowly 'collecting' bits and bobs for the build as and when I see them on offer!

I shall proabably be using Futaba 9152's for the Ailerons and Elevators, Hitec 7985's on each flap section and a Hitec 7954 on the Rudder. The main reason for the Futaba's on the main controls are their centering ability (Hitecs are good servos, but not great at centering) and the fact that I have two already! I need one giant servo for that massive rudder, which might see a lot of deflection in an engine out situation and still quite large servos for the flaps though they will only ever get used at low speeds.

All other servos (doors, retract selectors etc) will probably be Futaba 3001's as they are cheap, reliable, and easy to repair if need be!

Steve! You're using proper petrol engines!!    You'll be converting the Comet to a Nimrod next!   

 

Now I told you not to go upsetting him Stephen 

.......petrol engines is one thing, but painting the Comet pink and making it look 'ugly'......I think that's maybe a step too far!! (not that it'll stop us trying though  )

Steve, I know this post is slightly off topic, but I saw this at the weekend and thought of you 

 

You're all heart!

 

Careful children....it'll all end in tears

You could roll off the top of those zoom climbs Steve....  

Slightly tweaked Comet

Hey...you're all ganging up on me now!

STuff the MR2 - it's been withdrawn from service. Modify it into an MRA4 or an R1...

PDR

Naahhhh, do the rare one

the AEW3

 

Naahhhh, do the rare one

the AEW3

The R1s ain't exactly common - there were only four built (and only three remain).

Of course if you really wanted to do an MR2 you could paint it as XV230, and add some pyrotechnics to the no7 dry tank bay...

PDR

XW666 could be taken to club splash-ins though