The Dave Platt Models Jungmeister is a quarter-scale biplane of traditional fabric-covered wooden structure.
The wingspan is 66 inches and it requires a 15cc 2-stroke or 20cc 4-stroke engine and four-function radio.
The weight should be between 10 and 14lb. I am told that the design was originally published in the late 1970s and that
the model is intended to be sport-scale rather than an accurate replica.
The plans as supplied are comprehensively detailed, on one large sheet, with more than adequate instructions. Alternative lines are drawn to allow the building of the low-backed prototype or the standard production shape with a higher, more rounded rear fuselage.
The quality of the materials supplied is excellent and the standard of cutting of the parts is good. The glassfibre cowling is good quality.
The fuselage is based on sheet balsa sides, held together by pre-cut plywood formers at the front and balsa further back. Balsa sub-formers fill out the shape with sheet and block balsa providing the outer skin at the nose and fabric-covered balsa stringers everywhere else. The engine is mounted upright on hardwood bearers incorporated into the nose structure. A glassfibre engine cowling completes the front end which will comfortably take a .90 2-stroke. When I tried to fit an O.S. fs.120 4-stroke (Mk.1) I found that the rocker covers fouled the top of the cowling and would have required an unsightly hole in a highly-visible area. The fuel tank fits into a purpose-made tunnel through the firewall immediately behind the engine, where it is readily accessible for maintenance. The centre-section struts are made from piano-wire and are bolted by saddle clamps on to plywood hard points within the fuselage, becoming a permanent fixture. The struts are faired with balsa. A fixed tailwheel leg bent from piano wire is bound and glued to a ply plate fitted in the rear fuselage. There is a huge open cockpit which gives plenty of scope for adding detail.
The wings are straightforward to build. Each wing is built in three panels which are then permanently joined.
The upper centre section has mountings for the struts, consisting of hardwood blocks which are drilled and fitted with blind nuts on the inside. The centre section struts are terminated by electrical eye connectors soldered to the struts, through which the mounting bolts pass. This method of attaching the top wing is effective and secure, but fiddly to set up as you need to jig the top wing in place (the jig is shown on the plan) and solder the end fittings with the model assembled. Using a tiny butane torch to get sufficient heat I managed to singe the spars and set fire to the leading edge of the top wing!
The lower centre section has wooden dowels and nylon wing bolts for attachment and incorporates the rear undercarriage leg mounting plate. The lower centre section is balsa skinned.
The interplane struts are formed from aerofoil-section aluminium tube. Bicycle spoke ends are epoxied into the tube, which clip into nylon fittings screwed to spruce blocks built into the outer wing panels. No provision is made for the attachment of scale rigging wires.
The tailplane, elevators, fin and rudder are each built as half ribs glued to a 1/8 inch sheet balsa core. These are then shaped to section. A large (and heavy) block of balsa fills in between the top of the tailplane and the fin. The whole tail assembly is permanently glued to the fuselage after covering. Again there is no provision for the attachment of scale rigging wires.
The undercarriage is soldered up from pre-formed piano-wire legs, the area between the front and rear struts then being filled in with balsa sheet. The undercarriage is attached by saddle clamps to the fuselage underside and lower wing centre section, and needs to be detached from the wing when the model is dismantled.
Doped nylon covering over the open structure and glass cloth and epoxy resin on the sheeted areas completes the model. More modern heat-shrink covering materials may be used instead.
There is plenty of space for the radio installation. The rudder is driven by closed-loop cables, the rudder horn being linked directly to the servo. The elevators are driven by a hard balsa pushrod or plastic snake rods. The aileron servo fits in the lower centre section, driving the bottom ailerons via pushrods and bellcranks, or curved plastic snake rods. The upper ailerons are connected to the lower ones by rigid wire rods.
Flying the model
This is a fast little biplane. Take-off is brisk, needing very little rudder correction at all. The climb is steep, with plenty of power in reserve on a .90 two-stroke. In flight the model is stable and responds excellently to the controls. The four ailerons provide a high rate of roll and all the appropriate manoeuvres may be performed satisfactorily. With full rigging wires, the landing approach is steep and the speed must be kept up to ensure full control all the way down. There is little tendency to nose over on take-off or landing and the model displays no inclination to ground-loop.
To me this model appears to be substantially over-engineered and a lot heavier than it needs to be.
There is no need to use thick balsa sheet fuselage sides where a built-up frame would be stronger and lighter. It is done just to unnecessarily simplify a part of the basic structure which wouldn't be difficult for a skilled modelmaker anyway, and this is no kit for an inexperienced builder. The hardwood block used to mount the front undercarriage legs and hold the wing leading-edge dowels is far too big and heavy for the job. The balsa stringers used on the rear fuselage look thick and clumsy and will be prone to damage while the model is being transported. This problem may be resolved by reducing the depth of each stringer by 1/8 inch and capping it with a piece of 1/8 x 1/16 inch hardwood, fitted edge-on.
The wing spars are almost full-depth 1/4 inch thick hardwood - very heavy, while the 3/4 inch square balsa leading edge is far too large. These are made this way to allow the wings to be fully cantilever, again presumably to simplify the model. Much weight could be saved by reducing the sizes of these components or replacing the spars with a spruce and balsa sandwich. Of course the model would then require functional flying and landing wires, but would anyone trying to build a scale model of this type of aeroplane omit the rigging anyway? Regarding the interplane struts, it is my opinion that the little nylon brackets which the struts clip on to won't last very long. If one breaks in flight you might have a problem finding a lost strut somewhere in the grass. Digging the damaged brackets out and replacing them on a regular basis will play havoc with the wing covering and paintwork, as they are attached below the surface. It would be far better to arrange a bolt-on method of attaching the struts, which will also provide secure points to which working rigging wires may be attached.
The tailplane, fin, elevators and rudder are all overweight. The 1/8 inch balsa cores used are completely unnecessary and very heavy. Perfectly adequate open-structure parts may be made, which are actually quicker to build than the components as designed - I designed my own tail parts and they came out exactly half the weight of the kit components, before covering. The weight saving at the tail will bring extra benefits in reducing any ballast needed at the nose - mine has none at all, thanks to weight saving at the tail. The tailfin would benefit by having its trailing edge spar extending down the full depth of the rear fuselage, rather than being simply butt jointed on top of the tail fairing balsa block. The heavy lump of block balsa used to shape the area between fin and tailplane may then be safely replaced with 3/32 inch balsa sheet pieces, again saving a significant amount of weight in a critical area.
How scale is it?
As this kit is designed to be a sport-scale model, you might expect some liberties to have been taken with scale accuracy. Fortunately for builders wishing to make the best of this kit most of these may be easily corrected, while a few will require a substantial amount of work.
Comparing the model to scale documentation, it becomes obvious that the rather old Aeromodeller scale drawing was used for the basic shape. This would be fine, except for the fact that this drawing has too much leading edge sweep on the tailfin, seen in the side view. This flaw is replicated on the model, but once you are aware of this it is a simple matter to draw the correct outline over the plan using a good side-view photograph for reference and build to the new outline. Compared to photographs, I also have reservations about the shape of the fuselage top decking, forward of the cockpit. I believe that this should be a little higher at the cockpit and slope down more towards the nose - it just needs the former forward of the cockpit building up a bit before the sheeting is applied.
The second problem is with the control surface hinging. The rudder is fine as the full-size has a simple hinge arrangement, but the elevators on the model are hinged at their leading edge in standard modelmaking practice. If you look closely at a full-size example you will see that the hinge line is set back from the leading edge, to provide mass and aerodynamic balancing. Doubtless the model will fly well with the elevators as drawn on the plan, but scale accuracy has been compromised for simplicity.
Similarly with the ailerons. The full-size has Frise ailerons, where the visible chord on the undersurface is significantly greater than on the top surface. Again these have been sacrificed for simplicity, along with the proper closed-loop pair of cables linking the upper and lower ailerons.
The wing section used on the model is different from the full-size. The undersurface should be almost flat, with a higher camber on the upper surface. Correcting this would require modifying the lower wing cutout in the fuselage and adjustments to the length of the centre section and interplane struts as well as cutting out a fresh set of wing ribs.
Finally, for a sport-scale model, the undercarriage works well. Unfortunately it completely destroys the appearance of the model when sitting on the ground. The full-size has oleo-sprung legs whose length and angle when viewed from the front is completely different in flight from when it is resting on the ground. The legs are also shrouded in an aerofoil-section fabric-covered frame. The springy thin piano-wire legs and balsa sheet core supplied do not attempt to replicate any of this and make the model look terrible. The only solution is to scratch-build a fully working scale undercarriage, which is an interesting task. Likewise, the model should have a sprung, fully steerable tailwheel leg, not a piece of bent piano wire with a wheel on it, although this is not obvious when looking at the model.
Jungmeisters have had very long careers indeed. They have been used as advanced trainers by the Luftwaffe, the Swiss Air Force and others and as aerobatic mounts by private owners in several countries. There are many choices of colours and markings which are widely documented. Add to that the conversions which have taken place, fitting the type with more modern radial or horizontally-opposed engines in completely different cowlings and you can see there is plenty of scope for producing a distinctive model.
Overweight, not as accurate as you may expect, this kit may still be turned into a fine scale model if you are prepared to counter the shortcomings by adequate research and have the ability to safely redesign the parts which need it. If you can do all this, though, why not just design your own from scratch? If you simply want a Jungmeister lookalike to fly and aren't too bothered about accuracy and detail, then this may be the model for you.
Thanks go to Mike Delacole for filling in some of the history of the kit for me.
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