The Ben Buckle Junior 60

This Ben Buckle Junior 60 story began in 2009. Its purchase was inspired by the achievements of my friend Phil Bevan. A like-minded model aero enthusiast, and an avid builder, it was he and Les Boone (of HP V.T.49 fame) who had helped me to build the kit, in spite of which, owing to slovenliness on my part, wasn’t covered for at least three years.

Figure 1.1 Close-up of the business end of the J60

The Junior 60 was designed in 1946 as a free flight model by Albet Hatful. The setup of the business end is a reflection of that era. An OS 30 (cubic capacity of 5cc) 4-stroke single cylinder reared up in front of the windscreen. And a 6oz fuel tank sat next to the pilot. Throttle lever and fuel lines ran through the front bulkhead (see Fig 1.1). This was practical, though inelegant, and I was itching to improve the whole set-up, given that we were more used to building fine flying scale models.

There ensued about four fun-filled years of flying in Northwick Park, (see Fig. 1.2) and some careless crashing, but nothing severe enough to break it completely, mainly due to the flexibility of the rubber-band secured wing and tail section. One of its famous crash landings, witnessed by many, was on a berry bush in the adjacent Golf Course, much to the surprise of a young French woman who was about to tee off. This must have put her off her stroke because she erupted like Vesuvius and we were convinced that we’d seen the last of our aeroplane. With hearts racing, we galloped over, and only after profuse apologies, and the timely intervention of the Groundsman, and her boyfriend, did we finally tease it from her clutches in a relatively un-damaged state.

We generally flew on a Saturday morning. The wind speed threshold, above which, I was no longer comfortable to fly, was about 10mph. Then in October 2015, the inevitable happened. A build-up of

castor slobber in the carburettor (which we found retrospectively, owing to the sharp observations of our cousin Michael) had caused a weak fuel draw into the engine, which in turn led to an unpredictable performance of the OS 30. A dead-stick event soon after take-off had meant yet another, hurried landing, and this time the impact was severe enough to break the tail-skid and wooden propeller, and buckle the RHS piano wire undercarriage leg. This was disappointing, and although no further damage was evident, the years of wear and tear, and fuel contamination of the wooden structure, convinced us that a thorough re-vamp was necessary.

Figure 1.2 – J60 flying over Northwick Park

I relished the thought of crafting in wood and aluminium, all the ideas I had accumulated over the past five years, and I started with the rear fuselage and tailplane, which was unsatisfactory for several reasons. It had to be lighter (to obviate giving lead-weight a free ride, as this J60 required 100g of lead-weight, to improve its ‘fore and aft’ balance, and flyability), required larger control surfaces, for more effective low speed control, and a steerable tail-wheel.

I decided to enlarge the elevator by about 20%, by encroaching into the tail-plane area. The rudder was enlarged by splitting into two panels, above and below the tailplane, utilising effectively, the rear part of the tail-skid. This gave a total enlargement of about 35%. The lower panel was connected with the upper, using a 22swg piano wire joiner. Most of the spars, apart from the tail-plane trailing edge, from which the hinges hung, were built-up hollow sections.

The hollow section is designed like a box girder. The top and bottom spars, (with grain running along their length), are cut from sanded 3mm balsa sheet. The perpendicular webs are cut from sanded 1.5mm balsa sheet, (with grain running vertically), and glued on the outside. I apply glue to both surfaces and clamp the joint with light pressure, where possible, until dry. Naturally the loads are

borne by the upper and lower leaves, however, the web provides stiffness against bending or twisting forces, and further reinforcement is possible by adding more leaves to the inner surfaces of the spars, (whichever is in tension) where needed.

Figure 1.3 – New tailplane with steerable tailwheel, and two-panel rudder

Figure 1.4 – New tailplane joined with new fuselage section

If it’s a particularly long beam, a short leaf may be added to the bottom and/or top spar in the vicinity of its maximum bending moment. This would be in the middle, in a simply supported beam, and in the root, if it’s a cantilever.

Naturally, the dimensions of the box-girder spar, in-spite of their built-up section, is no bigger than what is on the plan. I’m sure carbon or glass fibre could help in this respect, and would be advisable for the larger models, however, these materials largely remain outside my experience so far.

A similar structure was used for the fin, and at its base, where it meets the tail-plane, where covering with solarfilm was problematic before, I used balsa shavings (from a David Plane), atop a light supporting frame, which I then stained and fuel proofed to achieve a taut surface. This had not been trialled before, but the experiment was successful in the covering of compound curves, achieving the desired visual effect, and gaining the required weight saving without the need for steam-softening, had thicker sections been used. The Balsa shavings can vary in thickness, being dependent on a number of factors, such as the pressure exerted by the David Plane and the grade of wood. If it’s too thin it may tear. If it’s too thick, (say above 0.1mm) and it becomes less easy to work.

The whole assembly, including the steerable tail-wheel and aly control horn, weighed about 60g, significantly less than the 100g of the original assembly, and I eagerly anticipated a significant overall weight saving. See figure 1.3 above.

For the fuselage, the plan asked for a basic ¼’’ sq. geodedic structure. Clearly there were opportunities for yet more weight saving, and the new longerons and supporting uprights were reduced in cross section in the rear half of the fuz. Hollow sections were used for the diagonals. See figure 1.4 above.

Figure 1.5 – the front end of the Junior 60 was recoverable and re-used.

Figure 1.6 – Shows the new engine set up so that the front upper cowl line is preserved.

Figure 1.7 – Developed for reliable running in an inverted arrangement, the setup is peculiar to the rotary head equipped HP V.T.21

The next improvement was to the front end, which, being sufficiently fuel-proofed, was recoverable, and was dismembered from the old fuz, from just behind the firewall. See figure 1.5 above. An HP V.T. 21 (single cylinder 4- stroke of 3.5cc) was mounted in an inverted arrangement, so that the front cowl-line was preserved. See figure 1.6 above. The HP V.T. series of engines do not have poppet valves like conventional engines. Instead a rotary head (with a port) is used to uncover the inlet tract, glowplug element and exhaust port in turn. The combustion noise is much attenuated, a distinct advantage at our flying site, where noise has been a source of friction for a very long time.

Bench tests with the HP V.T. 21 commenced with GN10, a fuel with 20% oil content and 10% nitromethane. The high oil content was recommended for effective operation of the rotary head. Soon after the running-in period, the setup was developed for reliable running in an airframe. A high-mounted tank, exhaust back pressure and a peri-pump were required before it would run at elevated inclinations – a vital requirement for take-off and climb at full power. An on-board AA glow battery was also installed to ensure a reliable tick-over and low-RPM performance – the weight penalty was minimal and worth the performance enhancement it gave. See figure 1.7 above.

Figure 1.8 – A new centre section was built to accommodate the fuel tank in a high-mounted position.

To duplicate this setup in the airframe, the fuel tank had to be mounted as high as possible. There was ample space in the wing centre section, and I was fortunate to find a compact, flat fuel tank (from an old kit) during a routine rummage in Mayoor’s model shop in Harrow. See figure 1.8 above.

A new centre section wing was built, and the original wing panels were re-assembled onto this. Ailerons were also fitted to give roll performance and make it a bit more interesting to fly. The dihedral was reduced considerably giving the model a more aesthetically pleasing look. A new colour scheme of orange, white and yellow also contributed to a more striking appearance. See below in figure 1.9.

Figure 1.9 – Ailerons and reduced dihedral gives a more pleasing look.

All that remains now is to apply some finishing touches and complete the cowling. However, in spite of this, there was nothing to prevent the maiden flight and this commenced on Saturday 17th October 2020. This flight was different for many reasons. The sight, sound and smell of a glow fuelled internal combustion engine in a model aeroplane is such a rarity these days, where electrical appliances increasingly rule the skies over Northwick Park, that it raised much interest and amusement with the rest of the modelling fraternity.

Test flights and subsequent trimming of the control surfaces will be discussed in part II,

Sean Mendis


28th Oct 2020

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