1926 RRG Prufling in 1:4 Scale

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This model was built a few years ago as my first for aerotowing. I had never aerotowed before so presumed a secondary trainer would be appropriate enough! RRG in Germany made plans available for the construction of these across Europe and in other parts of the world. The Prufling (examinee or student) was of simple wood and ply construction that any one or group could build with minimal tools and experience. They were mostly bungee launched on hillsides but one was even slung beneath a dirigible to be the first glider to be released at altitude in this manner. There is a nice model of one at the soaring museum in Elmira, NY. The full scale version had a 10.50 meter span so mine is ~2.63m or around 105". The 3-view from Martin Simons' book 'Sailplanes: 1920-1945' was used to draw the plans. The airfoil in the book was used. Martin even provided a contemporary cockpit controls diagram for reference to me via e-mail communication. That was very generous of him. Thanks Martin! The 'build' appears reversed but the original thread was lost thus the re-post with pictures of the completed model first. That way you can see what it looks like before wading through posts to see the end result. This poor thing ain't no sleek and sexy flying machine! It is different and I love the beauty of wood!





Drawing plans and starting construction with the rudder. It's a flat easy thing to make with sticks and 1/64 gussets.

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The fin will get built a little later after fuselage 'fleshed out'. A jig was made to match the side view curve of the lower nose bows. 1/16" balsa laminates 'keyed' into each other at their tips. When this assembly dried it was taken off the jig. The upper nose bow was built flat on the plan. These parts were used to make upper and lower fuselage crutches. The lower one was built over the upper one to maintain symmetry. These were then pulled off board and the cross members were added to build a basic box structure. A square was used to maintain alignment. Cabane supports were added. FG and epoxy added to the lower bow. The tow release will be in this area so might as well make it tough. It will need nose weight anyway!

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BTW... thanks to Andy Grose for the two photos shown above! Andy took many fine photos of my models flying! They are spectacular shots! Martin Simons provided the control system diagram so typical of these birds. This drawing was used as a starting point. The Prufling most likely(?) had a rudder bar in lieu of rudder 'paddles' as seen on the Primary types. I used artistic license to make everything functional. There was no way I could find pulleys small enough to loop the control cables as seen on the diagram. Compromises must be made. However, the control column and rudder bar will actually be connected via cables and pulleys elsewhere directly to the control surfaces. The servo will have links to drive the control column and rudder bar. In essence, if you can find a quarter scale pilot then the pilot would be able to fly this airplane if the servos were removed!

Rudder bar components shown. Pivot tube slotted and cable passes through. A small fork (not shown) got soldered to the cable. The fork will engage end of control stick for elevator control. Pivot tube bushings with brackets fabricated from brass. The stick pivots on a bracket soldered to pivot tube. Lateral motion of stick will operate ailerons. That action will be transmitter via micarta tiller at end of pivot tube. All brass was painted black. The control stick is wood, stained and with cord wrap for the handle. Clevis soldered on each end of cable for elevator connectivity.

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The curved forward keel was added to the fuselage frame. Supports were added to the keel. A skid hard point can be seen near the front. Others will get added, later. Servo rails and rudder bar mount added. A skid was made from music wire, painted then mounted. The control column mounted to keel. The ex-WWI pilot got in for a test fit. He was good with it! Elevator tiller parts. Control column connected to the aileron servo. The last picture show how the length of the cable and clevis connection was determined. A previous picture showed it fullu assembled in a finished state. These scratch builds are all about putting stuff on then taking it off for tweaking.

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Oooops! Left off the 'test fit' pic with the ex-WWI pilot. Wings started. They will have pull-pull cables from cockpit to the ailerons. A balance cable will connect each wing to make the 'loop'. Root ends have a single pulley. An inner rib will have two pulleys. One for the 'up' cable and one for the 'down' cable. The root ribs were faced and reinforced with 1/64" ply. Waxed paper keeps pulleys from getting glued to their mounts and makes for a nice 'spacer'. Just rip waxed paper out after epoxy cures. The last pic shows how the tapered tip ribs were determined. Just measure from the stringer at the proper rib location, transfer the measurements the cut the rib. A little sanding to fair everything and you're good to go!

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The Prufling is a collection of parts connected via brackets, bolts and whatnot. Here are some fittings. They were cut from sheet steel, hammered flat, filed smooth, cleaned and painted. The wing strut attach carry thru 'beams' are 1/8" music wire. The ends were heated cherry red then hammered flat, drilled, tapped to receive 4-40 bolts, cleaned and painted. The sanding bar was placed thus to help align the upper spruce spar while glue dried. Here is a close up of aileron pulley rib.

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More wing stuff. Here is a close up of one of two spar sockets each root end. A 'stub spar' with appropriate wing inter-connect fittings will get added later. Drag and anti-drag members added. They are 1/8" spruce. A balsa fairing strip added to the top of the LE spruce spar to be sanded fair. Support added to aileron pulley ribs. This is not shown on the three view but I didn't want that rib flexing under load. Overkill??? Microlux table saw used to profile balsa LE.

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More profiling of LE. After the parts were cut to fit they were glued to the wings. The basic structure of wings are done. Just need to add 1/64" cladding and stain.

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Can't have cables jumping off pulleys! Balsa and plastic tube segments added to help keep cables in place. The plastic tubes have their ends beveled in order to get a very close fit to the pulley cable grooves. The music wire was used to emulate the cable runs while glue dried. When the wings are disconnected and placed in storage the cables won't fall off the pulleys. Balsa infill added to fuselage edges then sanded fair. This is needed for 'purchase' when the 1/64" ply skinning added.

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Hard points for tail feathers made using dowel and threaded rod. Ooops! The stab mount is too high so it got cut free, trimmed then re-attached. Once stabs set in place they were braced with scrap while the strut ends' epoxy cured. CF tow used to reinforce nose at tow release area.

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Rear upper fuselage formers constructed. The rudder and elevator horn were made from music wire. They were bent to shape, soldered together and trimmed to length. Brass tubes were flattened one end, drilled for cable attachments, soldered to the music wire parts, cleaned and painted. These fuselage top stringer was taped to a brass tube then glued to the formers. This made sure it was dead straight. The front top hatch was constructed. The fin was constructed now that the fuselage plus rudder length known.

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Titebond Contact Cement (Weldwood Contact Cement is the same) used to attach 1/64" ply to balsa structure. Brush the cement onto each part and let dry. A sheet of paper is placed between the parts so that the proper alignment can be made. Once satisfied ease the paper out and the parts will start to adhere to each other. You get one chance to get it right! Don't even breathe while doing this! You could build a model with this glue but you'd be on pins and needles the whole time! On the wing LE a triangular block of mahogany was use to 'burnish' the ply onto the structure and around the LE. No heat or water was needed. The hatch got the same treatment but it was a whole lot easier to cover!

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This should be post #14 and the above one #15 but I got it out of order while posting. Nothing wrong with site except for person using it! Cable attachment shackles made from coat hangar wire. A whole slew of 1/64" gussets and cap strip stock made ready. The structures were stained. 'The Digger' is a great little tool to make hinge slots. I like it so much that when it got misplaced for a spell I made a replacement from brass.

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Ferrules cut from aluminum tubing. Their ends were flared so the cables don't chafe then they were painted black. The ferrules were let into the fuselage frame work and strengthened with CF and epoxy. A 1/64" ply cladding piece added. Card stock patterns made to size the 1/64" ply covering.

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More skinning. Balsa sheet glued inside fuselage. This will form lead shot receptacle after the ply skin put in place. Shot mixed with epoxy will be poured into the opening at the top inside 'lip' of fuselage.