Zirolli B25 Mitchell build

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David Jones
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Re: Zirolli B25 Mitchell build

Postby David Jones » March 14th, 2010, 11:25 am

The circular firewalls for the engines are bolted on to four rounded formers glued on the outside of the nacelle box. These were glued in place and, in order to ensure they would be in line to provide a flat fit with the firewall, two balsa sticks were temporarily glued across the face. 1/2” balsa triangle was then added to the rear of the formers for extra support. The insides of the nacelles have also been beefed up with balsa triangle to make the whole nacelle assembly more rigid and stronger. The firewalls were then clamped in position and the holes drilled for the 6 x 4mm bolts and captive nuts.

This is as far as I’ve got to date. Hopefully I will be able to get back on track with the build as family matters settle down. My aim next is to finish the outer wing panels, flaps and ailerons so they can be handed over to Alan for glass clothing.
Attachments
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B25 021 (Medium) (WinCE).jpg
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David Jones
Posts: 413
Joined: January 24th, 2010, 8:21 pm
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Re: Zirolli B25 Mitchell build

Postby David Jones » April 15th, 2010, 1:45 pm

The ailerons and flaps were completed and hinged. The outboard flap is hinged to the outer panel, but a further length of approximately 3 inches extends inboard to the nacelle, and is cranked at the panel join due to the gull wing configuration.
Attachments
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David Jones
Posts: 413
Joined: January 24th, 2010, 8:21 pm
Location: Rossendale Lancs
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Re: Zirolli B25 Mitchell build

Postby David Jones » April 15th, 2010, 1:50 pm

Before the top sheeting can be fitted, provision for the servo leads and landing lights has to be catered for. Paper tubes were made up and fixed in place in the pre cut holes in the ribs. For now, string has been used so that I can pull the servo wires through as and when the servos are fitted.

We are using 6volt halogen bulbs for the landing lights. Trials in bright sunlight show them to stand out more than enough. A ply plate was glued in situ between ribs 11 and 12 to act as a dummy leading edge and bulb mount. A ply sub rib was made up and glued in situ to form the side of the light bay. These were then beefed up with 1/4” balsa triangle.


The top of the wing panel was sheeted with 3/32” balsa. In order to ensure the washout was built in, a length of 1/4” square wood was taped to the bottom of the panel as per the plan instructions, and the panel weighted down while the glue set overnight.
Attachments
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B25 027 (Medium) (WinCE).jpg
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David Jones
Posts: 413
Joined: January 24th, 2010, 8:21 pm
Location: Rossendale Lancs
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Re: Zirolli B25 Mitchell build

Postby David Jones » April 15th, 2010, 1:51 pm

The 5/16” leading edge was then added and the panel sanded to shape. The leading edge was cut away for the landing light bay.
Attachments
B25 028 (Medium) (WinCE).jpg
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David Jones
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Joined: January 24th, 2010, 8:21 pm
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Re: Zirolli B25 Mitchell build

Postby David Jones » April 15th, 2010, 1:53 pm

There is no mention whatsoever on the plans for the landing lights, so I’ve made a design up myself. Sod’s law dictates that if you permanently fit something electrical or mechanical with no future access, it will break. With this in mind I decided to fabricate a removable module to give access to the bulb, should it fail.

The bulb will be mounted in the dummy leading edge and secured with silicone sealant, which will also act as a cushion against any vibration. A plug in module was made up out of 1/8” lite ply, with 4 x 1/8” dowel pins. The idea is that the module will be retained in place by virtue of a tight fit. The module was sheeted with 1/32” ply top and bottom, this was then shaped to an elliptical outline. The centre was sheeted with a curved “reflector” made up from 1/64” ply. A hole was cut to expose the bulb and the “reflector” was then covered with sticky back aluminium tape. All that remains now is for a piece of acetate sheet to be formed to make the lens cover. This will be done at a later stage.
Attachments
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B25 037 (Medium) (WinCE).jpg
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David Jones
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Joined: January 24th, 2010, 8:21 pm
Location: Rossendale Lancs
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Re: Zirolli B25 Mitchell build

Postby David Jones » April 15th, 2010, 1:56 pm

It was my intention to complete the ailerons and flaps next, but I realised I had to see how the flaps would fit against the nacelles so, back to the centre section. The 1/8” balsa formers were added to the box sections and sheeting the nacelles began. I started by adding 1” wide balsa strips to either side of the nacelle along the centre line on either side, the rest of the planking was then in 1/2” wide strips. Because of the compound curves involved, a lot of trimming and shaving would be involved, I preferred to soak the balsa in water with a drop of “Star Drops” in it, this, I have been told, breaks down the fibres in the wood to allow for some serious shaping and, when the wood dries, it retains that shape without any stress forces.
Attachments
B25 005 (Medium) (WinCE).jpg
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B25 011 (Medium) (WinCE).jpg
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David Jones
Posts: 413
Joined: January 24th, 2010, 8:21 pm
Location: Rossendale Lancs
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Re: Zirolli B25 Mitchell build

Postby David Jones » April 15th, 2010, 2:00 pm

The nacelle was then sanded to shape, balsa block being used to form the end cones. The whole procedure proved to be a time consuming affair and, due to the shape and certainly the size of the nacelles, it reminded me of when I used to build model boats. If this thing doesn’t fly, it will certainly float.

That’s it for now, I’m having to leave the build for a couple of weeks to earn some brownie points by doing some long overdue decorating. This will also give us an opportunity to do some much needed research.
Attachments
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B25 019 (Medium) (WinCE).jpg
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B25 020 (Medium) (WinCE).jpg
B25 020 (Medium) (WinCE).jpg (12.07 KiB) Viewed 15032 times

David Jones
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Joined: January 24th, 2010, 8:21 pm
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 4th, 2010, 10:02 am

For those of you who have been following this build thread, and may have been wondering where I’ve disappeared to, here’s a quick update. Due to other commitments I’ve not been able to resume working on the model until this week. The final shaping and bottom sheeting around the cut outs for the flaps has been completed. The wing has now been handed over to Alan for glass clothing of the nacelles, outer panels and flaps and ailerons.
Attachments
B25 122 (WinCE).jpg
B25 122 (WinCE).jpg (11.94 KiB) Viewed 14859 times
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B25 125 (WinCE).jpg
B25 125 (WinCE).jpg (14.83 KiB) Viewed 14859 times

David Jones
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Joined: January 24th, 2010, 8:21 pm
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 4th, 2010, 10:05 am

We were able to spend a day with Keith Mitchell at Longhorsley at the beginning of May. He showed us the ins and outs of his model and imparted to us some very valuable pieces of advice, for which we owe him many thanks. The final outcome of some further deliberations is that the wing centre section will be permanently fixed to the fuselage. The fuse will remain a one piece affair, but with a removable tail plane. The undercarriage doors will not be operational, instead, a cut out will be made in the bottom of the nacelles to allow the wheels to retract and a removable panel will be made up to allow for future access. The same will apply for the nose leg.

It was now time to start the construction of the fuselage. A basic crutch is made up from 1/4” x 1/2” balsa, incorporating the 1/8” plywood wing seats. The diagonal bracing makes this quite a rigid structure. The crutch is then supported on blocks and the formers added, along with the tail plane seats and the battery/radio box sides in front of the wing. A few stringers are then fitted to give further support for the formers and to keep them square.
Attachments
B25 126 (WinCE).jpg
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David Jones
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Joined: January 24th, 2010, 8:21 pm
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 4th, 2010, 10:08 am

The bearers for the retractable nose leg are then fixed in place. The cut outs in the formers are too far apart for the Robart mechanism, so I made up a U shaped plate from 1/ 2” ply to fit. The formers were beefed up around this area and 4mm captive nuts were added.
Attachments
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David Jones
Posts: 413
Joined: January 24th, 2010, 8:21 pm
Location: Rossendale Lancs
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 4th, 2010, 10:09 am

The next stage of the build is to add some 1/8” sheeting below the wing seats so the centre section can be fitted. I failed to notice that the stringers in this area are hardwood. Due to the non standard size of the notches in the stringers, an order has been sent off to Stuart at Solutions for some Cypirus. Also, I can’t fit the centre section until some further work has been done on it once the nacelles have been glass clothed. During this lull in proceedings I intend to turn my attention back to the tail feathers and re hinge them as per the plan.

That’s it for now, hopefully my next up date will be quicker than this one has been.

Allan Griffiths
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Re: Zirolli B25 Mitchell build

Postby Allan Griffiths » June 4th, 2010, 5:30 pm

looking great so far, thats how we made ours, fuse and centre section one piece removable tailplane. i am currently doing the cockpit detail, Taking ages but will be worth it. then fitting the engines AGAIN. was fitting zenoha 45`s , now fitting two mintor 37cc.

I have gone for working gear doors, absolute nightmare. still not checked with air in the system yet but sure will be ok.

What scheme is yuor going to be? mine is grumpy which was based at duxford. well was when i started this build. a long time ago!! TOO LONG. :)

Alan Cantwell 1131
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Re: Zirolli B25 Mitchell build

Postby Alan Cantwell 1131 » June 4th, 2010, 9:18 pm

if its up to me, three tone blue, aka pacific colours, same as the duxford one in the museum, but knowing this lot i fly with, it will probably be bright red :shock:

David Jones
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 5th, 2010, 11:05 am

Thanks for your comments Alan (G), the colour scheme is yet to be decided upon. The silver and yellow, as per Colin Hammond's is really nice, but there's something about invasion stripes that makes a warbird stand out. As far as working doors etc, we were thinking of including them but, after a day with Keith, we have decided to ditch them, too much pain for so little gain. We'll probably go for working bomb bay doors, though a removable hatch is still a possibility and again, an easier option. As I mentioned at the start of the thread, our intention is to produce a model that flies right and is as reliable as possible. This is our first venture into a twin engined model so it's quite a steep learning curve, and a big model to boot! We won't be going down the road of ultra scale detailing, it will be more like "stand off scale" so that it looks right in the air.

As far as your comments Alan (C)....do you mean Post Office red or fire engine red??? Or we could do it Ferarri scarlet with a silver prancing horse on the fins and a black and white bar code down the fuz sides...not exactly scale...but different!

Dave J :lol: nes

Allan Griffiths
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Re: Zirolli B25 Mitchell build

Postby Allan Griffiths » June 6th, 2010, 6:29 am

Hi Dave i will put some pics or my build up along with yours if you dont mind?
i am using a hatch instead of working bomb bay doore under it will be all UP air valve sequncer, air tanks, RX`s switches. batt will be up front for balast.

David Jones
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 6th, 2010, 10:16 am

No problem Alan, like they say, two heads are better than one.
Dave jones.

David Jones
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 27th, 2010, 11:12 am

As the centre section will be permanently fixed, access to the inside of the fuselage will be via the bomb bay. We have decided not to include working doors, but to have a removable hatch instead. Sub formers were added at the front and rear ends, and the hatch area was then sheeted while in situ, before being cut away and sanded to shape.
Attachments
B25 139 (WinCE).jpg
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B25 142 (WinCE).jpg (13.07 KiB) Viewed 14612 times

David Jones
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Re: Zirolli B25 Mitchell build

Postby David Jones » June 27th, 2010, 11:13 am

The rest of the fuselage was then sheeted below the level of the centre section. In order to fit the nose leg in position, and for the leg to retract, a large section of sheeting was cut away, this will be covered by a removable plate, either made up from thin aluminium or rolled ply. This will make the cut out less visible, but will still afford access for routine maintenance.

That’s it for now, I’m waiting for the centre section to be returned so further work can be done on it prior to its installation into the fuselage
Attachments
B25 143 (WinCE).jpg
B25 143 (WinCE).jpg (11.5 KiB) Viewed 14613 times
B25 144 (WinCE).jpg
B25 144 (WinCE).jpg (12.24 KiB) Viewed 14613 times

David Jones
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Re: Zirolli B25 Mitchell build

Postby David Jones » August 22nd, 2010, 9:33 am

Due to a bereavement in the family, I’ve not been in the workshop much of late, hence the delay in the build. Things are starting to get back on track and I’m managing to do a bit here and there.

This has given Alan a chance to be getting on with glassing the nacelles. They’ve had a layer of cloth on them and one coat of resin. He is now laminating the covers for the retracts on top of the glass finish, which is protected by cling film. When these are done we’ll cut away the underside of the nacelles to allow the retracts to be fitted, and cut the laminated parts to size to act as dummy doors.


While he has been doing that, I’ve returned to the tail feathers. They were hinged with large Robart hinges, but we’ve decided to hinge the control surfaces as per the plan, which incorporates the correct scale outline. I’ve managed to adapt the fins and rudders and they are now hinged as they should be, with piano wire, running through brass tube bushes, providing the hinge line.
Attachments
B25 145 (WinCE).jpg
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B25 147 (WinCE).jpg
B25 147 (WinCE).jpg (14.88 KiB) Viewed 14200 times

David Jones
Posts: 413
Joined: January 24th, 2010, 8:21 pm
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Re: Zirolli B25 Mitchell build

Postby David Jones » August 22nd, 2010, 9:36 am

I’d already built the basic skeleton for the tail plane, so I don’t have to worry about amending what I’ve already done. As it’s going to be a removable unit, I’ve added solid balsa infils in the centre. The idea is that the tail plane will be retained by dowels at the front and bolts into captive nuts at the rear. The trailing edge of the tail plane sits behind F13, which has a slot cut in it for the central rib, a corresponding slot has been left between the infil and trailing edge to allow the unit to slot onto F13.
Attachments
B25 148 (WinCE).jpg
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