Rumpler C.IV (oz2958)

About this Plan

Rumpler CIV. Free flight scale model of the WWI German two-seat reconnaissance biplane. Scale is 1/25.

Quote: "Jim Latham 's 1/25 scale WWI bipe for CO2 is easily adaptable to rubber power. Rumpler CIV.

Before we build our WWI two-seater, let's discuss the various types of foam available, for this material forms the basis of construction.

Foam is a most useful - and free - material which deserves much more widespread use than it enjoys at the moment. So far I have found three grades worth investigation; hard foam, as used for supermarket meat containers (effectively a free material), green or blue foam from refrigeration engineers (you have to pay for this) and the soft, almost pulpy material which is used for packing highly desirable toys like stereos, TV sets, and so on. This can be a most expensive material if you by stereo just for the packing.

Compound interest: Fuselages with compound curves are easy to reproduce with foam. Cut formers to the finished shape and fill-in between with strips cut from a food container. PVA is the best adhesive. When thoroughly dry, carve and and the fuselage to shape, the cover in non-porous tissue. Pre-shrink the tissue, or the foam may bow in under the tension. Local stress points may be reinforced with extra tissue patches inside the fuselage. The hard, shiny surface commonly found on this type of foam should be sanded away to ensure a satisfactory glue bond. A spreading of epoxy around fixtures such as undercarriage legs and cabane struts will add further strength. Flat-plate motor mounts need to be keyed in place with short lengths of balsa or they may break away in a heavy 'arrival' I have experienced this.

Blue (or green, sometimes) refrigeration foam can often be scrounged form an engineer - if you can find one replacing parts! He will be only too happy to get rid of it. If it is your lucky day you'll get a free motor and fan too. This relatively dense foam is a handy balsa substitute for carving pilots, cowlings, canopies and so on. Its lack of 'grain' makes it a very forgiving material. Extra surface strength results from covering in small patches of tissue, fixed with diluted PVA, paper-mache style.

White 'packing' foam is the lightest of all, but its cellular structure will show through subsequent covering. It is also weak when flexed, but its greatest strength or virtue is as a large jigging block or former to keep assembly square during fuselage construction. It can easily be poked out afterwards. If you are economising on purchase of stereo equipment try you local 'chippy'. Fish comes packed in this type of foam, which is simply thrown away afterwards.

I have yet to try wall-lining foam - the type that comes by the roll - but despite some initial misgivings (it is again weak) I accept that it too has substantial uses for aeromodelling, as Steven Midson has shown in his 'Foam Fun' series in this magazine.

Any snags? Only few. Don't let dope, cyano, balsa cement or, worst of all, polystyrene cement come into direct contact with foam. If the model gets wet, beware! The strength factor goes way down. Don't use an excess of PVA or the weight will rise unnecessarily. Lastly, don't stand the stuff without wearing a face mask. Now let's build the Rumpler!

The full-size Rumpler CIV was highly praised as a performer at altitude. It looked a good bet for CO2 or rubber power, so with no further encouragement I scaled it up to 1:25 scale, which gave a wingspan of 19-1/2in as you see here.

Construction: The first step is to build all the laminated bits, leaving them to dry as long as possible. It any are out of true, replace them! The fuselage is a conventional box structure. Nose area and rear decking is planked with foam cut from food containers, attached with PVA, then sanded to shape when dry. Cover the foam areas with pre-shrunk tissue (this merely means treating the required amount of tissue to a light spray with water). Open areas of structure are covered with tissue as normal.

Undercarriage and cabane struts fit into aluminium tubes epoxied to the fuselage structure as shown. Balsa fairings are slotted to accept the wire, shaped, filled if necessary, and covered with tissue.

Wings: These are built using shaped upper and lower ribs, spars laying centrally within. Although this looks fiddly, it is actually just as easy to construct as a conventional wing of this size, and certainly cheaper. Build a 'male' mould carving from hardwood. Cover with clingfilm and build a watertight box around it. To create the 'female' part pour fibreglass resin, well laced with inert filler powder, into the space. When set, remove and clean up. Cut sheets of 1/32in balsa over-length, laminate, glue and sandwich between the mould halves. Apply pressure with rubber bands. Sand the leading edges (LE) flush before removing from the mould; then slice to required thickness

Next pin down and glue LE, TE and wing tips. Pack up to the LE to suit the upward curve to lower rib. Cut and fit lower ribs. Glue spars in place. If you're going for ailerons, fix two strips 1/32in either side of the hinge line. Glue in upper ribs - and there you have a wing. Cut ailerons free, capping the hinge line with 1/32 sheet. Sand everything flush. Cover with tissue; warp in 3/16 washout at the aileron tips.

Tail feathers: No problems here, but note the inverted tailplane camber.

Assembly and rigging: Offer up cabane struts to fuselage and solder at top. Remove and fit balsa fairings. Glue in place on fuselage and fit top wing. Use the jig to fit lower wings. Add interplane struts, leaving the monofilament fishing line rigging until after test flights. This is because it may be used to tweak out any warps, or increase dihedral if really necessary. A useful tip is to gently pull the fishing line through glasspaper to remove its shine; glue is then accepted much more easily.

Engine cylinders are rolled paper tubes. The exhaust pipes are built up by tissue-wrapping a wire rod, bent to shape. Sand when dry. The main exhaust is soft balsa. Bent card is used for the radiator with card of balsa louvres added cater.

Finishing: I chose to cover my Rumpler in Jap tissue, lightly doped. The early, mauve/olive green camouflage with light blue undersurfaces is much easier than trying to paint lozenges! The German insignia is easily created from black and white tissue; glue into place only when satisfied! As a reference source, the February 1975 issue of Scale Models has some very useful photographs of this aircraft.

Flight trials: At first, flying the CIV was a nightmare - climb was invariably followed by a curving dive. If high enough at first, the model would curve down beyond the vertical. Once it achieved inverted flight, actually attempting to perform a bunt. What to do? Angles of this and that were altered, weight transferred here and there; all to no avail. The model finally gave up the ghost.

But surely, it had to fly. Sweepback, wing washout long tail moment arm, adequate dihedral. Everything seemed in its favour.

Another model was built and exactly the same flight pattern resulted. The only avenue left unexplored was that the engine clutter up front might be causing eddies or blanketing the apparently adequate tail surfaces. A temporary increase in fin area and - bingo! And that is why there is a seemingly unnecessary underfin. Now the model flies as if on rails in flat, right-hand circles. So will yours! "

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