An Affordable Avro - Part 1

Dave Batchelor describes his modifications whilst building a 98" Vulcan V-bomber to the Tony Nijhuis plan.

Following the demise of my MB339 back in 2016, I was left with (thankfully) a spare Jetcat P100 turbine looking for a home. I could not afford an ARTF model to put it in at the time so the idea of building something came to mind.

I wanted something scale and with a light wing loading as tearing around at 200mph is not really my thing. I wanted something that would look nice, fly in a scale manner and be affordable - not much to ask! Looking in the magazines I came across Tony Nihjuis’ advert for a 98” Vulcan, which would suit my turbine and flying style fine. I had not built anything from wood for some years and, funnily enough, the last plane I did build was from a free plan in RCM&E for a small pusher, also from the pen of Tony Nihjuis, called a Can-Do. Before this it was rubber powered models from Keil Kraft years ago!

Looking at the size of the finished model, which is about 2.5m long, I had to work out where was I going to build it and how was I going to transport it? Armed with a tape measure I went into my workshop and found no practical surface to build it on and, out in the van, I found that I could not get it in the back of there either - not a good start. I then found a thread on RC Universe, started a few years back by a guy in Norway on building the TN Vulcan. This gave me a lot of inspiration, as he had built his with a removable nose, so if I could do the same it would at least mean I could transport it in the van. Other people were following his lead and starting writing about their own Vulcans.

Back in the workshop I looked at the current layout and began to formulate a plan whereby I could move the milling machine and produce a large enough space to build the model. I built an L-shaped flat bench top onto which I could sit a large solid fire door, on which I could build the plane. As the door was not fixed down, I could (just about) slide and rotate it so that I could work on either side. Happy with the knowledge that it was all feasible and only a lack of building experience could hold me back, I placed the order for the plans, wood pack and vac-moulding set.

The full-size Vulcan has two engines on either side of the fuselage. The model uses a single turbine, centrally mounted, with the outlet in the rear fuselage extension that on the full size is the ECM (Electronic Countermeasure Module). A bit of a cheat, but this is not a super scale version and whilst four engine Vulcan’s have been modelled, they are much bigger and cost a fortune. In the sky, it takes a keen eye to spot the difference.

The plans arrived first and the full-size drawings were on two 3m rolled up sheets. When I first saw them unrolled on the living room floor, I wondered if I had bitten off a bit more than I could chew! But as the fuselage is built up from lots of formers and some interlocking pieces it quickly becomes a solid shape built on the bench, with only frequent reference to the plans. On Tony's website is a sequence of build photos, which provided me with an invaluable insight as to what lay ahead.

Making A Start
I cut out and cleaned up all the laser cut formers for the fuselage and began gluing. Cyano was used extensively in the build photos but I used Super Aliphatic and epoxy on the central and stressed areas, especially around the turbine and undercarriage mounts.

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Separating the parts from the extensive wood pack.

The fuselage is built initially over the plan and very quickly builds into a skeletal Vulcan shape. The nose and tail section parts fit together nicely. When I put it all together for the first photo, I remember commenting that this should be a quick build... Wrong!

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A skeletal frame is quickly assembled.

Before long, the build photos have you covering the top and bottom with sheet balsa, including the area of the two fuel tanks. The two tanks are shown on the drawing but no details of how they are mounted are given. It seems that the tanks on the prototype are built into the fuselage and are not removable. But I do not like anything that is not accessible, so I made the two tank installations removable before covering them with hatches.

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Making an additional F5, with neat keyholes courtesy of the rotary table on Dave's milling machine.

Looking at Tony's prototype photos, I was wondering just where and how everything inside gets fitted and how it is serviced. Having already planned a removable nose, I decided to make an inner frame cantilevered off of the front of the strong nose wheel area. This would be covered by the nose section and only be as long as needed to keep the overall length short enough for the van. I emailed Tony about my plans and he said that the front is not structural, so no problems.

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Focus on the removeable nose.

I made an additional former by copying the one I planned to have as the final forward fuselage former. Attaching it had me scratching my head, but I settled on four cap screws in the fuselage front, with corresponding curved slots in the rear former of the nose, with the nose cone fitting in a bayonet fashion. This was an easy job, thanks to having a rotary table for my milling machine, which enabled the four curved slots to be accurately cut so that the screw heads engage through the holes, then the entire nose rotates and locks into place with a sprung hatch release at the bottom. Although a rotary table is nice to have and made the job easier, careful measuring and drilling and filing will do the same job.

With this part done, I continued with sheeting the fuselage and nose area. The nose and tail sections are covered using dozens of individually cut tapered pieces of sheet, which took ages. They are then sanded to leave a round smooth shape.

Tail Talk
The prototype was built around a Wren 80 turbine, though the model is claimed to fly on 60 to 120 size turbines. I was going to use my P100, so I spoke to Dave Wilshire at Motors and Rotors about the thrust tube, which needed building in at this stage. The one Tony Nihjuis recommends for the Wren 80 was a lot smaller than Dave recommended for the P100, so I went with Dave's suggestion as a smaller one was likely to cause heat build-up and recirculation of exhaust gases due to pressure at the outlet (apparently!) A bespoke twin wall tube was ordered from Germany and when it arrived it seemed enormous. I had to remove a lot of material from the tail section bulkheads to accommodate it and I built back the strength with carbon tows glassed inside the tail cone.

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The JetCat P100 was fitted with a large thrust tube to help prevent heat build-up and recirculation of exhaust gases.

The tail fin is supposed to be built as one part with the fuselage and while transporting the model with the tail on is no problem, moving the fuselage around in the workshop was going to be tricky and hangar rash was inevitable. So, again, I took the lead of RCU contributors and decided on a plug-in tail.

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Dave's plug-in tail mod helps prevent hangar rash.

By this time, I had made contact with another RCU Vulcan builder, John, who lives in Harlington, just off the M1. He was about a year in front of me on his build and we met up for a very interesting talk. He was able to help me with a few areas, including the plug-in tail, and saved me valuable thinking time. I had incorporated some ideas that he liked but had already gone down a different route.

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Tail reinforcements include additional glass fibre and ply formers that extend down inside the main frames.

The tail fin as designed simply sits on top of the top sheeting, with part of the tail going through the skin and around the top of the spine. One of TN's build photos shows the tail ripped off in an incident, so I incorporated some additional glass fibre and ply formers that extend down through the turtle deck and inside the main frames to carry the stresses further down into the structure.

Bogie On Down
The recommended landing gear is electric retracts, with custom four-wheel bogies for the mains and twin wheels for the nose leg. We found later, when John was ready to do taxi trials, that the suspension in the main legs bottomed out completely with the take-off weight of his model (the springs were too weak) and he managed to crack one casting.

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Parts for one set of main undercarriage leg, oleo and bogie.

I also wanted brakes on my model. The TN gear did not have brakes and when flying from grass, as Tony does, I would agree. But when flying from a hard runway there is often the need to hold before entering the runway and also the idle thrust would carry it a long way before being slow enough to turn after landing.

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Main undercarriage leg fully assembled.

I know you can get very nice electric retracts with brakes from the likes of Electron, but I already had a couple of wheels with air brakes from the MB339 (sob), which were almost the right diameter and so I decided to use these as the nose wheel pair. The tyres were too thick, so these were turned down on the lathe and then had the tread re-cut. My thoughts were that with twin wheels on each leg of the main gear to brake (the trailing wheels are not really load bearing, so are more for show really) if these were individually braked, the force on one side would be halved if one of a pair broke contact with the ground over bumps etc, causing a slew to the other side. With twin nose wheel brakes, they are so close to the centre line that a brake imbalance would not be noticed. Dave Wilshire at Motors and Rotors rooted around in his box of legs and supplied me with a set of Behotec legs, which needed slight modifications, but proved perfect and I built two bogies to suit them using eight ‘jet’ wheels from HobbyKing. The bogies themselves are simple aluminium frames, which pivot around the front axle.

The nose leg brakes worked a treat, but I still had a problem to solve with the suspension on the main gear bogies. The bogies are pushed down when in the air on the full size, for the simple reason that when they retract forward, they end up horizontal within the thickness of the wing. Mine had to do the same and the original solution is to have sprung ‘shock absorber’ units mounted behind the main oleos that push the bogies down when off the ground. These are attached to the top of the oleo at one end and the bogie at the other. The travel on these has to be very long to fully extend down enough to allow the gear to retract properly, but also to absorb the travel of the oleo when it is compressed. John had struggled with the springs for his as any springs that were strong enough to hold the weight of the bogie when retracted were too strong for the travel involved.

I thought long and hard about this as I too was faced with the same issue with my home-made legs. What was needed was a suspension unit that was fixed to the moving lower part of the oleo, but this just couldn't be. Suddenly it came to me that a torsion spring, wound up on the front axle, could do all the holding of the bogie in the retracted position without being affected by the oleo travel. I made several springs from piano wire, wrapped around a mandrel in the lathe (rotated by hand) and soon had two springs per bogie, one left hand and one right hand, which worked perfectly. My suspension ‘shock absorber’ units on the bogies are simply a tube in a tube and are only there for decoration.

Nose Wheel Steering
The next issue was nose wheel steering. By design the servo is directly connected to the nose leg by wires (pull-pull) and sits just under the retracted nose oleo. Lack of space and a central frame former at the front dictated that this would need to be a micro servo but there were two things I did not like about this:

The wires were going to be very slack when the leg was retracted, and the confined space made dealing with this slack difficult.

When running fast on the ground it is very likely that one nose wheel or the other will lose contact and cause a side load on the nose oleo. The shock of this would be taken directly by the servo, and if it failed...

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Steerable nose-leg retracted...

I tried lots of different ideas of remote mounting a larger servo and eventually settled on what seems to be a complicated solution, but it deals with both problems at once. A standard servo is mounted perpendicular to the pull of the steering wires. The servo arms are connected to the nose leg via two bell-cranks to change the pull direction. The bell-cranks are mounted on a frame that pivots backwards as the nose leg retracts, keeping the wires tight. When the leg is extended the wires pull the frame forward onto a fixed stop. The final nose leg wires have expansion springs in them to absorb any shock loads. On the ground the servo moves but the nose wheels remain still due to the springs, but when moving the wheels steer fine.

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Steerable nose-leg extended...

To make all this, rather than struggle inside the fuselage through the nose wheel well, I made a mock-up of the internal structure and built it all outside of the fuselage before finally fitting it in the model once it worked.

Retract Power
Worried about the power required to retract the main legs with their heavy bogies forward into the airstream, I made another test bed to mount all three retract units and check the power available. This was rather disappointing as I found that with 100psi in the air tank the main legs would only just retract in still air, and after a drop of 15psi they would not retract at all. It was clear that a more powerful pair of main retract units would be required.

The next size up of Behotec units turned out to have the same size rams, so they would not help. Dave Wilshire had enough spare parts to put together one unit from Jet 1A and he lent it to me to try. The ram was far bigger, and the entire unit was more robust than the Behotec one. It was fitted to my test bed and proved to work really well, so an additional unit was ordered from Jet 1A. When this came the test rig was once again assembled, along with two air tanks to increase the volume available for the bigger rams. These worked very well and in still air I could get six cycles of the gear from an initial charge of 100psi.

Making The Delta
The wings were built on the plan. The plans say that the main and trailing edge spars must remain flat on the board until the ribs are all glued in. The ribs are supposed to have a flat underside section from the main spar backwards and sit flat on the board between the main and trailing edge spars. But it turned out that three of the ribs had a curved section top and bottom, so they could not sit flat on the board. The front and rear of the offending ribs all lined up on the upper surface, so they were glued in like this, leaving the bottom curved section of the three ribs clear of the base board. Subsequently the underside of these ribs had a piece of wood glued on and then sanded to make the entire bottom behind the main spar flat.

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Port wing being fitted.

The rest of the wing build went well but the elevator servos would not fit where they were shown on the plan as they had to lay down on their sides. The servos were mounted on ply hatches and these were mounted in a ply frame strung between two ribs. The two servos and the homemade retracting landing light were fitted to the wing before covering.

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Ready to skin.

The wings are built with part of the fuselage attached. There are two ribs with a 1mm gap where eventually the wing will be parted, leaving one part as an extension to the fuselage on each side. The wing is supported during construction by the wing tube outer and once this is started it will remain as one piece with the entire fuselage until fully covered and sawn off. There is no way I could do this in my workshop as the assembly would have to be moved about and also turned over to skin the other side. So, I decided to build it vertically, with the fuselage sitting on its side and the wing (with the fuselage extension) above it. Once one side was covered with 3mm balsa sheet it could (with some difficulty) be turned around to cover the other side. After this, a saw blade was run through the 1mm gap between the two ribs to separate the wing. They say that getting married and moving to a new house are the most stressful things you can do. But anyone who says this has never sawn the wing from his Vulcan! However, there was nothing to be concerned about and once apart the inner wing tubes were trial fitted, and the wing fitted back in place perfectly.

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Almost fully sheeted.

When it came to finishing the wings and fuselage it became clear that the end grain of the balsa sheet at the saw joint was going to be next to impossible to make a clean edge of. To overcome this, I bought some thin veneer to cover each side of the joint. Having cut holes to match the tube holes and other lightening holes, glue was applied, and two pieces of veneer were sandwiched between the wing and the fuselage. When dry, with the wing fitted and the waste sticking out on either side, the outer part was carefully sanded to the wing section and the joint was a perfect fit; any gaps between the end grain and the veneer were easily filled.

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Be brave! Using a pad saw to separate the outer wing.

By design the wing joiner tube is held in place by self-tapping screws fitted through the main gear wells. But I had already decided that when at the field I would not be turning the model over or laying under it to do anything, so all assembly had to be done from the top. The main wing tubes go into the fuselage from each side and cannot go right through because the turbine is in the way. I borrowed another idea from John to make the tubes screw into the main fuselage; a threaded bush is fitted into the end of the wing tube, which screws onto a bolt located inside the fuselage in the end of the outer tube.

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Wing parts after sawing in two.

This left only the outer wing panel to be secured to the tube. Here I borrowed an idea from my Xcalibur jet, where the wings have an internal ‘P’ clamp, which is tightened from above with a single screw once the wing is fitted. This all worked to plan, resulting in a very secure wing fitted entirely from above.

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Sandwiching the wing root veneers before gluing.

Next time, Dave finishes the build of his Vulcan and goes on the hunt for a suitable colour scheme.

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The final colour scheme will be revealed next time. No peeking at the first photo!

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