Zirolli B25 Mitchell build

[David Jones]

The ply hinge plates and balsa blocks were made up and the brass tube bushes were glued in. These were then dry fitted in situ and the piano wire hinges were inserted to check for alignment. Once satisfied that all was correct, the hinge blocks were glued in and the 1/16” balsa top sheeting was added and weighted down while the glue dried.

[David Jones]

Each rudder and elevator will have its own servo. It was decided that the elevator servos would be mounted central, rather than at the inboard ends. Alan wanted then mounting upright, so the top of the servo would protrude out of the tail plane similar to the aileron servos. I made up some false ribs between SR4 and SR5, reinforced them with balsa triangle and added hardwood mounting points. Due to the lack of depth in the tail plane, this method of fitting the servos meant that they protruded from the bottom surface by almost 3/4 of an inch and just didn’t look right.

[David Jones]

This idea was scrapped and the servos were mounted on their sides on a 1/8” ply plate. The false ribs were removed and hardwood bearers were epoxyed to ribs SR4 and SR5, the ply plate acting as both servo mount and hatch cover. The servo wires were then added and the bottom surface of the tail plane was sheeted.

[David Jones]

The elevators were then made up from 1/8” balsa ribs and false leading edge, 1/2” x 1/4” trailing edge and 5/16” ends. The main leading edge is 5/8” thick, with a groove cut down the middle to accept the piano wire hinge. Again this groove was cut oversize to accept the brash tube bushes which run the full length of the elevators. Slots were then cut to match up with the hinge blocks. Once the leading edge was rounded off the hinge slots were lined with 1/64” ply. In due course the rudders and elevators will be covered with Solartex.

That’s as far as I’ve got with the build to date. I’ll be away from the workshop for the next week but then I should be back in to finish the tail plane and mount the fins.

[David Jones]

I left off where I had completed the elevators. All that was required now before the fins could be fitted to the tail plane was to accommodate the rudder servos. Hitec metal geared mini servos are to be used and they will sit upright in the fins, with the servo head sticking out of the inboard side under the tail plane.

[David Jones]

It was now time to attach the fins to the tail plane. In order to ensure they were square with the tail plane, both back to front and top to bottom, and that they would be parallel with one another, I drew reference lines at the end of my building board to act as datum points.

I weighted the tail plane down on the board over the lines and ensured the centre line of the tapered trailing edge was level by packing both ends up. A fin was then dry fitted and I ensured that the centre of the leading and trailing edges were the same distance from my datum line. I then checked the fin for vertical accuracy by using an incidence meter.

The trailing edge of the fin is not vertical to the trailing edge of the stabiliser, but leans back at the top by 3/16” between the top and bottom hinge points, this relates to 1 degree on the incidence meter. After numerous checks and measurements and being satisfied that all was correct, I glued the first fin to the tail plane with 30 minute epoxy. Because of the slower curing time I was able to recheck everything and pin/tape the fin securely until the glue went off.

[David Jones]

The attachment of the second fin was accomplished in the same manner, but I had to use a short piece of board clamped to the bench to cater for the overhang of the first fin. Square datum lines were drawn on the piece of board as before and the second fin was attached. The rearward slant was measured both by incidence meter and visual alignment.

[David Jones]

One job I forgot to do was the air scoops on the outer wing panels. For some reason I thought they were on the centre section, so a quick phone call to Alan and I got the panels back before he started to glass cloth them. I made up the front face from 1/8” lite ply, cut some aluminium gauze to shape, and then mounted them on a piece of 1/8” balsa that I’d painted mat black.

A slot was then cut in the leading edge of the wing panel and the front of the scoop was glued in position. Pieces of scrap balsa were used to flare the scoop into the underside of the wing. A bit of filler was then added and rubbed down. It’s not much, but a bit of scale detail does add to the character of the model.

[David Jones]

The engine nacelles on the centre section have now had a layer of glass cloth on them, and the glass cloth panels to cover the retract cut outs have been made up and only require cutting to size. The underside of the nacelles were cut away to allow fitment of the retract units onto the bearers. Because we are not making doors I didn’t want to remove too much material, so It was a job of cutting away as you go by lowering the leg and wheel assembly back until the wheel was fully retracted. The slots were then cleaned up, I was a bit disappointed at first with the amount of material I had to cut away but, when the model is finished and painted, I’m quite confident that they will not be noticeable unless you get down on your hands and knees.

[David Jones]

The centre section is going to be permanently fitted to the fuse and, in order to install it, the upper half of formers 7 and 8 have to be removed, along with the stringers. Once this was done the centre section was offered up. There was a considerable gap between the bottom sheeting and the wing seat. The instructions accompanying the plans state the leading edge should be 5/8” higher than the trailing edge which would give 2 degrees positive incidence.

I sanded 1/8” from the rear of the wing seats which minimised the gap between them and the centre section bottom sheeting, giving a snug fit. I couldn’t get my head around this 5/8” difference between the leading and trailing edges as it didn’t make any sense no matter where I took measurements from, so I opted to go with the 2 degrees incidence.

My work bench is dead level in all aspects. With the fuse sat on the bench and the centre section clamped in place, the crutch was showing 0 degrees along its entire length. Turning to the plan, I drew a centre line through W8, from the centre of the ribs’ leading edge to where it tapers to a point at the trailing edge. This was then transferred to the end rib (W8) on the centre section, and a piece of wood was clamped along the line.

With the crutch still showing 0 degrees, an incidence meter was placed on the piece of wood at W8 and it was found to be exactly 2 degrees positive incidence. I then checked the tank floor inside the nacelle and found it to be
0 degrees, the tank floor is square to the firewall, therefore no up or down thrust will be built in.

[David Jones]

There is still some work to do on the centre section before it is fitted to the fuse. When that is done I’m not just going to rely on glue, I’m going to fit some blocks and plates and bolt it in position as well.

However, before I do commit myself to permanently fitting the centre section, I would welcome any comments regarding my method of achieving the correct incidence. What I have done seems logical to me, but have I got it right?

Am I correct in assuming the crutch is my datum line?
Am I correct in the way I have found the centre line for the rib?
If both these points are right, am I correct in assuming that I have achieved the 2 degrees of positive incidence?

I await any comments with an open mind, and a broad pair of shoulders.

[David Jones]

The decision regarding “do we have a fixed wing or a removable wing” has always been a topic of conversation and lengthy discussion re the pro’s and con’s of the build. As far as the build is concerned it doesn’t matter, it’s as easy to make the wing removable as it is to have it permanently fixed. It’s when it comes to storage and transportation that the subject takes on a new meaning.

Alan wanted a permanently fixed centre section because he felt that it would be truer and stronger and, having seen the model at the various shows, it wasn’t really that big, was it? Well, seeing the thing put together on the work bench, you do have to admit, size does matter!!

The plans show that the wing is either, fixed and glued to both the lower and upper wing seats, with access to the interior of the fuse through the bomb bay doors or, removable, with the wing bolted to the upper wing seats, the section of fuselage below the wing being independent of the rest of the fuse.

Sticking rigidly to the plans and construction notes, it looked as though we were committed to having a fixed wing but, why couldn’t we have the wing bolted to the lower wing seats and have the top of the fuse independent from the rest of the fuselage instead?

I could stiffen up the bottom of the fuse in the bomb bay area by adding doublers to the formers and extending the stringers through a bay either side and then completing the sheeting to make it a solid structure again.

I decided first to install the hardwood blocks in the centre section as per the plan. These were adapted to accept the wing bolts from the top instead of from the bottom. The plan shows the wing bolts are screwed into tapped hardwood blocks glued between the parallel lite ply wing seats. I decided that I would fit two 1/4” plywood plates across the full width of the fuse and fit 6mm captive nuts.

The gap between the wing seats was then in filled with 5/16” sheet balsa, giving a 1” wide seat along both sides. This made the fuselage beneath the wing very strong and rigid, and I was able to dispense with the crutch cross pieces, giving better access to the interior of the fuselage.

[David Jones]

The same principle was used to mount the removable tail plane, though there would be just two 4mm wing bolts at the rear into a 1/4” ply plate fixed across the tail plane seat, and a 1/4” plywood tongue at the front, which would locate into a strengthened F12, similar to the main wing on a sport model.

[David Jones]

All that was required now was for a fine day at the weekend, setting the wing up for accurate fixing to the fuselage needed plenty of space, so indoors was out of the question. Alan came across and we set the model up in the back garden. I had already drilled pilot holes in the hard wood blocks, so all that was required was to sit the wing in the fuse and measure up to ensure it was central and square. Once satisfied all was in order, the wing bolt plates were drilled by using the pilot holes as a guide. The captive nuts were fitted and the wing was bolted in place. Measurements were taken again and we found the wing tips were dead on to within 1/16”, smiles all round.

With the main wing in place, we lined up the tail plane and drilled the two holes at the rear for the 4mm bolts. The captive nuts were fitted for that and the tail plane was bolted on. As this was the first time the model had been assembled, photos were a must and I apologise for the blurred images, I think the cameraman was a little over excited.

[David Jones]

All that remained now was to fit the ply tongue at the leading edge of the tail plane and to beef up the ribs in the centre section with some hardwood spars to spread the load of the wing mounting blocks. The model was then taken to be inspected by Dave Johnson who passed it with flying colours and I was now in a position to crack on with the build.

Before the centre section top sheeting can be added, provision had to be made for the servo leads and retract air lines. The retracts were temporarily fitted and the air lines made up. We were hoping to have two air tanks fitted in the centre section central bays, but after taking advice, it was decided that these would not give sufficient volume for the twin rams on the main gear, so it was decided that a large tank would be fitted on top of the wing.

I therefore made up a ply plate for the air control valve and servo, and this was fitted in one of the central bays. The wing servos will eventually be hard wired, and the lengths of heavy duty servo wire were threaded through the tubing and labels added.

All the wires and air lines will now exit the centre section through the bottom sheeting. The two central bays look a bit like spaghetti junction at the moment, but all will be tidied up in due course. All that remains now is to add the top sheeting and finish the nacelles, which I intend to do this week.

[David Jones]

The top sheeting has now been added and the nacelles finished off, the air intakes will be added after the wing has been glass clothed. The 5/16” balsa leading edge was then added and sanded to shape.

[David Jones]

The outer panels were then attached to line up the flap cut outs and the flaps finished off to fit. All the control surfaces were then dry fitted, the wing stood on its tip in all its completed glory and pictures taken, even the wife was impressed. While it was in this state, I couldn’t resist wedging the cowls on with a bit of foam, just to see what it will look like. I’m well pleased with the results and I know Alan will be.

I’ll get the wing over to Alan over the weekend so he can start to glass cloth it and I’ll get back onto the fuselage next week.

[David Jones]

Now that we have decided the centre section will be removable, the lower hatch area will have to be strengthened. The lower sheeting beneath the wing / hatch area was stripped back a full bay fore and aft, two new lower formers, F7 and F8 were made up and fitted. Stringers were added off set, so that they could be stitched into the original stringers and the entire section was then re sheeted with 1/8” balsa. The structure looks and feels as solid as a rock, and I’m quite convinced that it will be more than up to the job.

[David Jones]

The servo wires for the four tail end servos were put in place and labelled up. A ply plate for the nose leg (steering) servo was made up and fitted in situ before the top stringers were added. Formers F6 and F9, which will be the front and aft ends of the hatch cut out, were faced off to add some rigidity and to tidy them up. The top sheeting was then added. The top hatch area will be completed once I get the centre section back.

[David Jones]

It was time to turn my attention to the tail end and the fairing-in of the removable tail plane. The top section aft of the tail plane was made up from soft balsa block, carved and sanded to shape before being permanently fitted in place. Further sanding and shaping was then required to blend in with the elevators.