Zirolli B25 Mitchell build

[David Jones]

For the past couple of years a small group of us have been talking about building a large model as a joint project. It was discussed numerous times and then shelved, that is until Alan Greaves took the bull by the horns and decided he was going to invest in a B25 Mitchell. I would build the airframe, he would glass cloth and paint it, and Mike Whitehead would sort out the radio and electronics installations. Alan Cantwell has volunteered his engineering skills as and when they become necessary.

We looked into the possibilities of it last year, taking advice here and there and finally decided on the 118” Zirolli plan, with a laser cut kit, probably by Belair. The plan was ordered first and was given a group mulling over. It was decided at the outset that, as this would be our first really large model, and a twin to boot, that we would keep it simple, erring on the side of practicality and reliability, rather than concentrating too much on scale detail. As long as it looks right in the air, we won’t be too bothered about exact scale detailing. It won’t be entered into any static competitions; we just want a good flier.

I decided to start with the tail feathers and here is where we made the first alterations. We were surprised to see the rudders were operated by one servo via bell cranks, these would be discarded and each rudder and elevator would have its own servo. The wooden hinges and horns would be discarded and replaced by Robart fittings. (More about that later, there has been some change of heart).

The kit was ordered from Belair, followed by a wood pack from SLEC. The first thing about this particular plan is that it is blown up from the smaller version. Belair have cut the kit as per the plan, resulting in non standard size cut outs and notches which, as the build progresses, becomes a bit of a pain to say the least.

So, enough of the drawn out intro’ lets get to the workshop. The fins are a straight forward construction consisting of a frame work of 1/4” thick stern posts with 1/8” ribs and 1/4” square leading edges. Blocks were inserted at the hinge points and where the trailing edge of the tail plane inserts. This is then sheeted with 1/16” balsa, with carved balsa block for the top and bottom tips.

[David Jones]

The rudders are built up with a 5/8” leading edge and 1/8” ribs, tapering to 3/16” x 1/2” at the trailing edge, with pre cut profiles for the top and bottom. The plan recommends blocking the trailing edge up on the board, lay sheet balsa in for the ribs and then sand to final shape. I didn’t like this method so I decided to build the outline of the rudders, cut the ribs to shape and then glue them in place. The rudders, like the elevators, will eventually be covered with Solartex.

[David Jones]

With the fins and rudders completed, my next task was the tail plane and elevators. The tail plane is of basic construction, but the 1/2” thick tip ribs are short of the end of the trailing edge spar, and are shaped to fit the curvature of the fins. The remaining length of the spar inserts into the fins to give a more solid fixing. The plans show only two hinge points for each elevator, which may be joined with a wire joiner. It was decided that each elevator would have four hinges, and would be controlled independently from each other.

The elevators are built up in the same manner as the rudders. As yet, they are not finished to final shape. (Having since seen the photos of Colin Hammonds’ Mitchell, the rudder and elevator hinging may be re visited).

[David Jones]

It was now time to turn my attention to the wing. Zirolli supplies several A4 pages of building notes with the plans, each step having its own tick box to make life easier. Although not as thorough as a booklet that comes with more popular kits, they are very useful and save a lot of head scratching.

The 10 foot long wing can be made up into several configurations and attachment to the fuselage. We have decided that it will be made into a three piece wing, two outer panels just over 36” span each, with a centre section just less than 48” span. The centre section will incorporate the engine nacelles / retractable undercarriage, and will be permanently attached to the fuselage.

Construction of the centre section commences by joining the two main spar halves together, and then slotting in ribs 5 and 7, which form the nacelle sides, and rib 6 which extends past the trailing edge, forming the “spine” of the nacelle fairing. 1/2” x 1/4” hardwood spars are then glued to the top and bottom of the main spar. The nacelle bottoms are then added along with a sub former and then the remaining ribs are attached.

All these parts are 1/8” lite ply and the interlocking slots are pre cut. They should go together with ease but, because the slots are cut oversize, the result is a very “floppy” arrangement, necessitating numerous pieces of packing to make the joints tight and more rigid. (Though not shown on the plan, I am adding 1/2” triangle fillets to all the major load bearing joints around the nacelles and end ribs).

[David Jones]

The top spars are then added along with the square leading edge spar and 1/8” false leading edge. I was disappointed to find that a lot of the pre cut notches were not aligned and needed to be re cut and packed. The ribs are cut as per the plan, so it is obviously the profiles that are not accurate. I found this to be quite a surprise, bearing in mind the reputation of the quality of Zirolli plans. The “before and after” photos don’t do the poor alignments justice, but some of the in fills can be seen quite clearly.

The assembly is then turned over and the bottom spars added. The next step is to sheet the bottom of the centre section with 3/32” balsa. I have decided to build the outer panels first, so that I can align everything up as a one piece unit and fit the joiner tubes while I have good access to the structure.

That’s it for now; I’ll do an update when the outer panels are completed.

[David Jones]

The outer wing panels are pretty much straight forward. The main spar is pinned to the board and the ribs glued onto it. A 1/4” square shim is then inserted under the rear of the ribs, about a third of the way in from the trailing edge, this causes approx two degrees of washout to be built into the wing. The top spars, leading edge and tip are then added, followed by the rest of the bottom spars. Although not shown on the plan, or mentioned in the instructions, I’ve added 3/32” balsa rib webbing along the main spar to give it more rigidity both in flight, and whilst still being worked on.

As with the centre section, the instructions recommend sheeting the bottom as the next stage but, as I’ve already mentioned, I want to fit the wing joiner tubes in first.

[David Jones]

I’ve put all three sections together as a trial fit and I am very happy with the results. The whole structure looks true and aligned, and both wing tips are exactly the same height high above the board when measured at the main spar.

So, that’s the basic skeleton of the wing completed, the next stage will, no doubt, be a lot lengthier as this will entail the sheeting and separating the ailerons and flaps. I’ll keep you posted.

[David Jones]

The wing panels are joined by an aluminium tube in a cardboard sleeve, and there are 1/4” dowels at the front and back to keep the structure aligned. The dowels were fitted first.

The plans show the wing joiner tube is 1” diameter by 12” long. As these plans are blown up, the tube needs to be at least 15” long to extend through three ribs both sides of the join. The tube supplied is 24” long, too long for one panel, too short for two, so another tube was ordered. Instead of having two scrap lengths of tubing, I have extended the joiner tube to four ribs each side of the join, which just happens to be 24” so, no waste.

The pre cut holes in the ribs are again oversized, but correct sized “doughnuts” are provided to achieve correct alignment. These are not shown on the plan, and only one per rib was supplied, so I cut another set myself and had a “doughnut” either side of each rib. The sleeves were inserted and the wing assembled together and clamped, leaving a slight gap at the joints so I could later cut through the sleeve.

The plan shows that if a straight edge is extended from the top of the outer centre section ribs (8), the tip should be 1/4” lower. I did this by clamping a length of aluminium square tube along the top of the wing to ensure correct incidence and alignment. Once satisfied that all was correct, the “doughnuts” were glued in position either side of each rib and the tube glued as well. Once satisfied that all was well, the sleeve was cut and the edges cleaned up. This process was then repeated for the other wing panel.

[David Jones]

I want to get the bottom sheeting on the wing panels as soon as possible to stiffen them up, but more importantly, to protect the rib trailing edges. Wielding something this size in a confined space does tend to lead to hangar rash. I could start sheeting now, but it would be easier to sort out the servo boxes and other “internals” first while I can access them from both sides.

First up is the wing mounting tab that sticks out from the root of the outer panel and locates onto a plate in the centre section. The outer panels are then retained by two M4 bolts into captive nuts each side. The placing of these also helps with the dihedral setting. The tab was glued to the outer panel then, again using the length of square tubing, I “jigged” the wing up to obtain correct alignment and dry fitted the receiver plate, both were then drilled in situ, ensuring that both plates would lie flat against one another when bolted together. The receiver plate was removed and the captive nuts fitted. The plate was then glued into the centre section.

[David Jones]

Servo boxes were then made up for the aileron servos. The design is such that the whole of the top of the servo is exposed below the surface of the wing. The flaps are driven by either one servo (with bell cranks), or two servos in the centre section. An elaborate linkage with bent wire and tubes is then made up to drive the outboard flaps. We have decided that each flap will have its own servo; I’ll make the boxes up once the wing bottom has been sheeted. The model is fitted with Fowler flaps and the horns and linkages are all hidden inside the wing.

[David Jones]

It was now time to start sheeting the wing bottoms with 3/32” balsa. Starting with the outer panels, I firstly sheeted the leading edge from the main spar. I then made up skins by joining several sheets of 3” wide sheets and sanding them smooth before gluing them to the wing framework.

[David Jones]

The centre section was then sheeted; again I made up balsa skins and cut them to shape prior to gluing to the framework, starting with the outer most sections, outboard of the nacelles and then the leading edge from the main spar. Ribs 5 and 7, either side of the nacelles, had to be “cracked” and bent at the trailing edge to follow the contour of the nacelles. I glued a piece of stock balsa across the trailing edge ribs to maintain the correct angle while fitting the balsa skins. Sub ribs 5C, 7C and 2A, which are the flap ends, were then added.

[David Jones]

Before proceeding any further, I think it would be advisable to cut the flaps and ailerons out, starting with the inboard flaps in the centre section. Slots are cut in the sheeting between the bottom spars. The ribs are then cut and finally the spars and the flap section is separated from the main body of the wing.

[David Jones]

Hinge blocks are then added, the plan shows two hinge points, I’ve catered for three. All the edges of the flap section were then cleaned up and the hinge blocks rounded to profile. The trailing edge ribs of the wing were then sanded to profile and 3/32” sheet added to form the flap instep. Hinge block were then added to the inside of the wing.

[David Jones]

In order to ensure correct alignment, I “jigged” the flap section in position by gluing a piece of stock balsa along the top spar. Hinge points were then measured off and holes drilled for the large Robart hinges. In order to ensure the drilling angle was the same for all hinges, I made up a small gauge. I have fitted two hinges for the time being as I wanted to see the outcome before I go any further. I must say I am quite pleased with the results.

That’s as far as I’ve got to press. There are three more flap sections to cut out and then I’ll start on the ailerons. I’ll post my next update when they are done.

[David Jones]

I know it’s been a while since my last update, through a number of circumstances beyond my control, added to an illness in the family, I’ve not been in the workshop as much as I would have liked. Anyway, here’s what bit of progress I’ve managed to make.

I left off where I had cut out and fitted my first flap section. Having cut out the three remaining flap sections on the centre section, I fitted the pre cut ply horns to the two inboard flaps. I then sheeted the leading edges of the flaps with 3/32” balsa. As the radius of the leading edge was quite acute for wood of this thickness, I soaked it in warm water for a few minutes to soften it up. An angled slot was cut for the horn and the sheet was then glued in position. After being left overnight to thoroughly dry out, I sanded the edges flush.

[David Jones]

The next job was to sort out the servo bays / mounts. The horn, linkage and servo are all concealed inside the wing. As the servos were replacing a bell crank system I had to devise a way of mounting the servo. I decided to glue 1/2” x 1/4” hardwood rails, supported by 1/2” balsa triangle to ribs two and three. The servo would be mounted on a 1/8” ply plate which would be screwed to the hardwood rails.

[David Jones]

An access hatch was then made up on the underside of the wing. For any adjustment and routine maintenance, the plate can be unscrewed from the hardwood rails and the whole assembly removed from the wing.

[David Jones]

The flaps on the outer wing panels are operated by a wire linkage connected to the inboard flaps. We decided this was a non starter and that the outboard flaps would have their own servos. The outboard flaps were cut out and fitted in the same manner as the inboard flaps. As ribs 10 to the wing tip are 1/8” balsa, a lite ply doubler was made up and fitted to rib 10.

[David Jones]

I next turned my attention back to the centre section to fit the undercarriage and firewalls. The two main nacelle formers N3 and N4, which take the weight of the model on the ground, have pre cut holes for the u/c bearers. The centre holes were in the wrong position so I’ve beefed them up with 1/8” ply doublers, with the centre holes cut in the correct place to take the 1/2” x 3/4” hardwood bearers for the Robart retracts.