Where it all started: Author with the 1:3.5 scale version of the Rhönadler 35.

Rhönadler 35

Part I: Design and Construction

For those who might be reading this article on a device with a smaller screen (like a phone), the images in this article are going to be really small. Never fear, though, you can tap on any for a full screen image at which point you can pinch-n-zoom to your heart’s content. — Ed.

The Rhönadler is one of the classic sailplanes from the era of the 1930’s, and by 1937 was the most widespread high performance machine of its time. No original examples have survived, although there is a recently constructed replica that now resides in the Wasserkuppe museum. Fred Slingsby based his Type 13 Petrel on this design, and you can see that the empennages of the two gliders are almost identical. Back in 2013, I built a 1:3.5-scale version, and it is from this that the new 1:3.9-scale version is derived.

Left: The fuselage is built by the half-shell method, the first half being built over the board. | Centre: Once all the cross bracing at the rear and the majority of planking at the front has been added, the half shell can be removed from the board. | Right: The wing joiner box mount is reinforced with a bracket bolted to a doubled-up former.

The fuselage is built by the half-shell method, whereby the first side is built directly over the plan, before being removed and the second side added directly to it. In order to keep the whole thing straight it’s necessary to 1) apply as much of the ply planking to the front as possible and 2) add enough diagonals and doublers to the fuselage rear to render it as rigid as it can be. This is a quick and reliable way to build monocoque type fuselages without all the fuss and extra work needed when using a jig — although sometimes, you don’t have a choice!

Left: Adding the top pylon facing. | Centre: Finishing off the ply planking at the front. | Right: Lead added as far forward as possible before adding the filler that will constitute the nose block.

The most important thing learned from the bigger version, when the wings parted company with the fuselage during a hastily contrived crosswind landing, was that the wing joiner box needed to be robustly attached to the fuselage. To this end, an aluminium bracket is bolted to a beefed-up former underneath to top surface of the wing mounting pylon, allowing the wing joiner box to be bolted to it. The ply planking at the front is smoothed over with body filler, and the temporary liteply formers inside are removed and the interior glassed with polyester resin and wing joining tape. The nose block is made up from three or four applications of filler.

Left: Planking and filling completed, with the temporary formers in the cockpit removed and the interior glassed. | Centre: The rudder is built ‘in the hand’. | Right: The complete tail unit prior to fitting.

The wings are a departure from my usual choice of airfoil, featuring a scale thickness at the root in order to capture the essence of the full size. This then transitions to my favourite HQ35–12 section at the tip, something I couldn’t have done without the services of CompuFoil, the wing plotting app (see Resources, below).

Left: Checking the alignment of the empennage relative to the top of the pylon. | Centre: Setting up the canopy framework. | Right: Inserting piano wire pins into the framework joins.

The spruce spars, 5x5mm in dimension seems too small for the task to the enquiring eye, but once the ply webbing plates have been added to the front of the spar, a secondary 5x5mm spar is dropped in front of the web plates to form a strong and very light I-beam.

Left: Early stage of wing construction: fitting the sub spars to form an I-beam. | Centre: Details of the wing joiner box. | Right: Making up the inter-wing fairing.

The fuselage is covered with Solartex (see Resources) the joins being hidden by means of brushed-on two-pack primer, sanded back. Once this has been done, two coats of the same primer are sprayed on and flatted before adding the final two-pack top coats.

Left: Smoothing the fit of the fairing with car body filler. | Centre: Wing nearing completion. | Right: The Rhönadler airframe, complete & ready for covering.

The flying surfaces are covered with HobbyKing (HK) film; matt clear for the open structures and white and red film for the trim colours over the sheeted parts.

Left: The Solartex finishes on the edge of the nose block. | Centre: The nose now with brushed-on 2-pack primer to hide the ‘Tex joins. | Right: Smoothing off the nose dry with a sanding block: 80 grit initially, then 120, down to 320.

With a four-and-a-half metre span, this is not exactly a small model, but at around 5.5kg, and even with that enormous wing area, it’s not difficult to rig or to launch.

Left: All-moving tailplane blades covered with HK film. | Centre: HK matt clear film on the wing. | Right: Using a card template to cut out identical trim colour for each wing.

Close-ups of the finished model

So how does she fly? Tune in for the upcoming Part II and the gripping conclusion!

©2013, 2022 Chris Williams

Resources

• CompuFoil — From the website: “the most recognized name in airfoil template software. Easily produce foam template, built up wings, leading edge shaping guides, and planforms…”

All images by the author. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.