The arrival of a somewhat different model on the patch recently has caused some interest, not least because of its unusual flying characteristics.   The model in question is a 2 metre span Fiesler Storch, very like a Piper Cub but with more ‘dangly bits’.

The full size version, officially known as the Fiesler Fi 156 Storch, is a two seat 1930s and WW2 observation / liaison aircraft with notable short take-off and landing capabilities.  Flying speed is from 30 mph to 125 mph.  Power is supplied by a single inverted air cooled V8 Argus engine delivering 240 HP.  Some versions are now fitted with radial engines. 

So why build a Storch?  It is makes a change from the ARTF and specialist aerobatic models regularly seen, it provided an engineering challenge so would not be boring to build, and it should be slow flying and stable to suit my experience level.  As the build followed that of ‘Skippin’ my first electric trainer, it was not needed in a hurry as I had to learn to fly first!  I did consider a Lysander, but that seemed just a little too complicated for only my second off-plan build in 30 years.  The total job to date (currently flight testing before completing the paint job and adding final details) has taken some 18 months, but with several other models being given priority in the build schedule.

The model originated as Traplet plan MW2733, supported by an article in Electric Flight International April 99.  Span is 79 inches and weight came out at approximately 6 lb including batteries.  The model was originally designed for a geared 600 can motor and 8-10 NiMh cells.  This was replaced with brushless motor and 3S Li-pos.  I was advised on motor type, speed controller, batteries, prop etc by Johm Emms, Puffin models.  Motor is a Mega 22/30/4 drawing 40 A at takeoff, speed controller is a Jeti Advance 40 Opto, battery a 3700 mAh 3S Li-Po, receiver is a Multiplex dual conversion with IPD (intelligent pulse decoding).

The model is of conventional built-up structure with an open frame fuselage using spruce longerons.  Covering is Solartex, which proved easy to use and looks the part for a fabric covered original.  Detail parts were transferred from the plan to balsa/ply sheets by photocopying the plan, then cutting and pasting the images using Spraymount spray glue.  A word of caution here - not all photocopies  copy at exactly 100% of original size.  Wing ribs were made as a block using ply templates.

 One type of construction used, that I hadn’t met before on models (common in full size), was the use of lamination for curved surfaces such as fin, tailplane and elevator outlines.  On this model these comprised four laminations of alternate 0.4 mm (1/64”) ply and 1/16” balsa.  These were easily made by cutting the strips of ply  and balsa, glueing a copy of the plan to a flat wooden shelf, then drilling holes and sticking in cocktail sticks around the inside of outline shape.  The laminations were then lightly sanded, coated with PVA glue and wrapped around the curve described by the cocktail sticks.  When dry the lamination was removed from the jig and sanded to profile.  The result is both light and strong and retains its shape well.  For tight curves, steam the wood first (I used a modified kitchen steamer).  Leading edge slats were laminated along 6 inch drain pipe to get the correct curvature.

Another innovation (for me) was the use of 0.4 mm (1/64”) ply as reinforcement at stress points and in laminations - it is easily cut with kitchen scissors, very strong and light - super stuff.

 Flap and aileron hinges were made up as per plan from 1.5 mm ply with brass bushes.  Carbon rods were used to stiffen u/c tubes and to mount the removable tailplane.

 Modifications from the plan include stronger (laminated) slats, servos fully embedded into the wings, a new flap drive method, a more realistic sprung undercarriage and a spring biased castoring tail wheel.  The open frame ailerons and flaps warped when they were covered, so these had to be fully skinned . 

Challenges?  Not surprisingly, sorting out the flap linkages and servos to give symmetrical operation at all settings (pretty vital) and the undercarriage design and build.

 What would I do differently? - Not use elastic bands and hooks to hold the wings against the fuselage.  Shear pins would make rigging much easier.  If I had known how slowly the model would fly, I would have made the u/c stronger (heavier).

 Results – On the first flight, with flaps retracted, the model tracked perfectly straight across the ground and was airborne very quickly.  No trimming was needed to get it to fly straight – very satisfying.  Height is maintained with about half power, it flies slowly, is stable but also is very manoeuvrable.  The glide is surprisingly flat, given the draggy nature of the design.  Using flaps requires some down elevator to maintain a comfortable flying speed, currently mixed in at 20%.  The castoring tail wheel allows scale type taxiing with the ability to turn on the spot by putting on full rudder and blipping the throttle – just as full size. 

What next?  - Learn how to use an airbrush and complete the detailing, now I know that it flies!

Next project – DH Dragon Rapide, Traplet plan again, based on original at the museum by Hurn airport (well worth a visit).

Advice for someone doing a build from a plan - Plans have often been reviewed in one of the hobby magazines about their time of issue.  These reviews usually contain extra building information, best sequences, explanations of complicated bits, pictures taken during the original build.  Very useful in deciphering plans which may contain contradictory information or be difficult to understand. 

Some plans can even have detail shapes for complete formers missing. 

Don’t expect plans to be more than semi-scale, unless claimed otherwise by the author.  Photos of a full size example often show up quite large differences between the shape on the plan and the shape of the original.  Be wary of altering wing or tailplane shapes, but fuselage cross sections, nacelle shapes etc can often be improved. 

 Useful sites -  www.leespring.com contains tutorial on springs and how to specify them, they also supply springs of all types.  Good for that sprung undercarriage.  Sources of bits – Cubitts Models in Trowbridge.

Martin & Sue H ,  Nov 2007