Fiesler
Storch on Finals
Cockpit
detail
Fiesler
FI 156 Storch
Building the Fiesler Fi 156 Storch
The
North West of England has a very active Warbirds Group, a loose collection of
modellers interested in building and flying Scale Military Aircraft of any type
or age. If it flew in military colours you are in. With advancing years flying
fast jets or fighters is getting a bit of a challenge, so something more sedate
and visible (large) was called for. I am currently building a Westland Lysander
and whilst on a visit to the Shuttleworth Collection at Old Warden (housing the
only flying Lysander in the
Since this model was started Black Horse Models (Ripmax) have introduced a 112" (2850mm) ARTF model. It looks superb and so it should be at £500. Then you have to add a 28-33cc petrol motor, radio etc. or you can build your own.
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Fiesler
Storch on Finals | Cockpit
detail | Fiesler
FI 156 Storch |
This is not a beginner’s model, either in the building or the flying; do not proceed unless you are a competent builder and flyer. The build is not difficult; there is just a lot of it with many pitfalls and tricky bits. A high degree of accuracy is required. Using conventional hinges on the wings would make it a lot easier. The undercarriage also needs skill in wire bending, soldering and brazing. As the build proceeds photo's will be added to augment the notes. Good Luck.
Due to its odd design it is suggested that the Wing Centre Section is completed FIRST followed by the Outer Wing Panels, Tailplane and Fin. The fuselage can be built last.
Wing Center Section
It is essential you build this first, taking great care to ensure the whole assembly is perfectly accurate, lined up and true, you are almost working to thousandths of an inch, as any errors are magnified in the 85" span and its alignment with the fuselage. The rest of the model seems to revolve around this one component. Laser cut parts would be a great help, any offers. Make sure your chosen 6.4mm (1/4") spars fit snugly into the 8 slots in the dihedral braces. The completed wing panels must slip in easily without any stress or distortion before final alignment and glueing. The completed wing centre section is used to locate the two 6.4mm (1/4") dowel holes (see Note 3) and 6mm wing bolt in the fuselage formers F6 and F7 (Ripmax part No. F-MG122). Make sure you get the wing incidence correct and the wing square to the fuselage. When adding the completed wing panels to the center section make 100% sure your building board is flat with no twist etc. The wings must be flat and of equal incidence. This is VITAL as once the wings are joined up to the center section and fuselage formers F6, F7 are in place any errors are very difficult, if not impossible, to correct.
Wing Panels
The Fi 156 wings were designed to fold back for storage but I have only seen one photograph with the wings folded.
These are of conventional construction and have no dihedral. They do however have unique problems of their own. The first is the center section described above which is BELOW the lower surface of the flat one piece main wing, a single 85" span. Then we have external hinges and leading edge slots. It is doubtful if these slots will actually work as per the prototype, so they are best regarded as cosmetic. (see Note 1) The ailerons are of the “Frise” type, that is the lower edge of the “up” going aileron projects downwards into the airflow causing drag and hence yaw, this counteracts the so called "adverse yaw" produced by the “down” going aileron, (where lift equals drag) these are not differential ailerons, (see Note 4) but have a similar result keeping the aircraft's heading true during a turn or when correcting a roll.
To ensure a perfect fit build each half panel onto the completed centre section. Do not glue at this stage, placing the panels to one side.
To ensure perfect fit and alignment the flaps and ailerons are best built integrally with the wing and cut out later. Do not forget to add the boxes and slots for the hinges, horns and the leading edge slots, also make sure you know where they are located. You cannot start poking around inside the wing after it is completed. (been there, done that) The method used is to build all these in, slit off the control surfaces, then glue and slide in the completed hinges. I used single sided copper clad PCB laminate (the high quality glass fibre material to Spec FR4 (Maplin Part No.WF40) not the cheap phenolic resin FR2) with 1/16” brass rivets soldered to the copper laminate. Make sure the copper clad surfaces are on an outside faces, then you can solder the rivet shanks to it. Both ailerons are fitted with dummy "mass balance weights" to stop flutter. Make sure these do not foul the wing underside on the down going ailerons. These are shown in scale locations. The ribs R2 (12 off) are built up from 3 layers, 0.8mm,1.6mm and 0.8mm with F1, A1 and the slot tabs contained within the 1.6mm inner core. Make the middle layer of the ribs so that F1 and A1 can be slid in and out, only fixing permanently once the wing is off the building board, line up F1 and A1 during gluing to ensure perfect alignment. (See Note 2) I used a laser and a piece of this thread pulled tight.
Four servos are required per wing and the linkage is totally contained within the wing. Due to the unusual external hinges the aileron and flap horns cannot be fixed in the normal underwing location and are set on top of the moving surfaces. The 85" wings are deep enough to contain standard servo's and arms. This gives a very scale appearance but does need some building skill to get it all working smoothly. Due to lack of space, drawings of the servo mechanisms in the full down position are shown on the Formers Drawing Sheet No.3. Note that the hinge pivots for the flaps and ailerons differ. Jerky ailerons are not acceptable, usually caused by warping wings or hinges not dead in line. (See Note 5) To ensure perfect hinge alignment a number of assembly jigs were made and these are shown in the photo below. Do not forget to add the mounting points for the struts, which MUST be used on the wing and the tail as they are designed to carry flight loads. Due to its complexity much of the hidden detail has been left off the plan. It is a typical “D” box construction with a flat bottom "Clark Y" type airfoil, with cap strips on all the ribs and a sheeted inboard section. You could add sheet in-between the spars with the grain vertical. Hollow out the tips as shown to reduce weight, or use carved foam. The strut details are for guidance only. Establish the exact length of the struts before covering and fix the attachment points (4) to the main spars at the points shown. For scale accuracy the angle between the struts and the wing should be 25 degs. The upper ends of the vertical sub-struts just push into small brass tubes fitted to convenient ribs.
To ensure perfect hinge alignment, do not fit the TOP 1.6mm sheet to the flaps or ailerons, fit the hinges and pivots using the jig shown in Photo (12) When you are satisfied with the alignment epoxy in the hinges and fit the top sheets. Before fitting the top sheet you can also fit the dummy mass balance weights and servo horns. (See Note 6) Note that W1 (wing seating) must line up flat with W6, W7 and W8. Fit W6 and W8 then slot in W7.
The ailerons are rather odd (what is not on this model) having two widths. Ribs R1a and R2a are extended (on the ailerons) to accommodate this feature. Where the wide part meets the narrow, squeeze the top and bottom together, glue and sand to blend.
Slots (not Slats)
These are built up from 8mm balsa sheet and sanded to the section shown. Fit 5 1.6mm ply tabs and slot these into the gaps in the front of ribs F2 (the laminated hinge ribs) Fit the slots as a final operation after covering.
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Wing
Centre Section | Wing
Centre Section | Part
Completed Wing |
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Part
Completed Wing | Flap
and Slot Hinge Jig | Selection
of Hinge Jigs |
Tailplane and Fin
A little known feature of the Fi 156 was its adjustable incidence tailplane. This could be adjusted in flight and a slot can be seen in the fuselage at the leading edge. Not really practicable for a model
Before you start work on the fuselage the tailplane and fin should be completed. The tailplane and fin both have the same basic construction, a 1.6mm sheet core (pre-cut the large holes first) with ribs and spars glued to either side of the core. Sand to a symmetrical airfoil section. Fittings are required for the tailplane struts and the closed loop rudder horn and the elevator horn, a simple brazed assembly is the only solution here. See detail on plan. The snake (a carbon fibre snake is recommended with appropriate support down the inside of the fuselage) for the elevator is buried inside the fuselage and the snake and snap link will require fitting BEFORE the rear fuselage is closed up. Fit a rubber sleeve to the snap link to ensure it stays closed, it might even be a good idea to solder it closed, it should never need to come off again.
Assembly of the Tailplane and Fin to the Fuselage.
The assembly of the tailplane and fin presented a problem, a bit like the chicken and the egg, in the end I adopted the following method. If you look on the fuselage plan you will see a red line at the tail end. Complete the fuselage BELOW this red line including the complete tailwheel assembly Trial fit the elevator assembly ensuring it is parallel to the wing. Sand the lower 9mm tailplane seat to achieve this, once you are happy with the alignment fit the control snake, slide down the fuselage and glue the complete tailplane/elevator assembly in position. Now complete the build with the addition of the fin and rudder assembly ensuring it is truly vertical to the wing and tailplane. The fuselage can be covered in Solartex later. The photo's below show the completed assemblies. Click on these for a full page view.
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Fin
Assy ready for covering | Tailplane
ready for covering |
Fuselage
Start by making two sides from 1/8” sheet; note the scarf joint at the rear section. Cut out the 1/16” ply doublers and glue to the sides using contact adhesive. (contact adhesive is preferable as it does not warp the balsa) Make sure you do a LH and a RH side. Place the sides together and sand to an identical size. Fit the 3.4mm sq. longerons.
Cut and drill the engine mounting plate from 3.4mm alloy and cut the engine bearers to length, drill to suit engine mounting plate, epoxy one bearer to the RH side and allow to dry, bolt the plate to the bearers and epoxy to the LH side, make sure the extreme ends of the sides match perfectly, when dry fit F2 with epoxy. Note holes are required in F2 for fuel lines, throttle snake and a remote glow wire, if fitted, drill all holes before fitting. The rest of the fuselage formers can now be fitted. See wing notes for the alignment of the wing seat and wing mounting bolts on F6 and F7. Also alignment of W1 in relation to W6, W7 and W8, not forgetting U1. The rest of the fuselage can be completed including all stringers, lower fairing and the tailplane mountings. (see above) All the lower undercarriage torque rods etc. are mounted on U1, epoxy in place. It is now time to fit the servo tray and fuel tank. 3 servos are required in the fuselage, rudder, elevator and engine. A closed loop system is used for the rudder and a concealed snake for the elevators. Run any pipes and snakes, closed loops etc. before the fuselage is closed up. Due to restricted space a custom fuel tank may be required, fabricated from thin brass sheet. With care the throttle servo wire can pass through a tube in the tank to increase capacity if required. A suitable tank is shown on the plan. Once the main fuselage sub assembly is completed we can add the rear sides, tailplane platform and underside. The rear underside should be drilled and tapped for the tail wheel assembly. For scale accuracy the cockpit internals are fabricated from 1/8” dowel and the insides sprayed with Luftwaffe cockpit green before the perspex cockpit sides are fitted. Note some of the perspex panels are hinged if you want REAL scale. The elevators are operated by an internal snake, this is impossible to connect once the fuselage is "closed up" Make sure your build quality is spot on and fit the tail wheel mounting plate U2 c/w tailwheel assembly as suggested above.
Click on the image for a full page photograph, clicking on the full page photograph will reveal an even large image.
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Bearers
glued in Vice | Partially
Completed Side | Engine
Bearers | Finished
part Side |
Undercarriage
Description for one side only, both sides are identical. Sheet 3 is a Front Elevation showing the location of struts and a general layout etc. Sheet 4 gives Parts Drawings in full size and a General Layout. These parts are drawn from an actual model.
This is an unusual undercarriage and gives the Storch its unmistakable character and also it's name, as it is long and spindly and has TWO positions, hanging almost vertical in flight, but rising upwards and outwards as it lands and the undercarriage takes the full weight of the aircraft and landing loads. It requires a bit of skill and accuracy in its fabrication if it is to function correctly. Springing is via torsion radius arms (B-B) linked to former U1. These are hinged at H2 to the lower leg (A) This slides in tube (E) part of middle leg (F), hinged at H2 to the upper strut (D) with an internal spring. This is anchored to the fuselage former F6 at (G). Legs (C-C) are fixed solidly above hinge H1 and anchored back to U1. As the wheel hits the ground the lower leg (A) rises inside tube, the spring is for location purposes only, (E) and swings out on the radius arms (B) (B) about H1 and H2. (D) and (C-C) do not move at all. All landing loads are taken by the torsion bars acting through U1. It can be seen that no load is transferred into F6. However, if the lower leg (A) bottoms inside tube (E) then some of the load is passed up to F6, which is NOT designed to take this load. Make sure you LAND rather than ARRIVE (C-C) and (D) act as stays only. The lower hinge could be fabricated from sheet brass but the design shown is simple, robust and quite small. It is virtually impossible to give EXACT wire lengths, make up dummy sets with an old coat hanger or florists wire and cut and bend to your pattern, using the drawing as a good guide. The following points should be noted. The lengths of the torsion bars should be identical, (shown as = = on the plan) to give equal springing, and the axis of the torsion bars should be a right angles to the leg assembly. (Note the main legs are angled forward so the torsion bars are slightly inclined down to the rear) I have now completed a full AutoCAD (included as Sheet 4 below as a .pdf file) drawing of the leg assy. using a substantial spring as the main (knee) hinge, cover with this heat shrink tube if required. Procuring an exact spring for this application is more or less impossible (4mm i/d & 2mm wire dia) so I wound one by hand onto a 3.8mm mandrel. This is easier than it looks, wind the coils tight & it will spring open to a tad under 4mm giving an interference fit on the legs. Solder in place if required or secure with a spot of Loctite. (do not braze) The bottom hinge is spilt along its center line as it is impossible to bend two 4mm piano wire legs to 10mm long x 44 degs once fitted inside the brass hinge. Make up a simple bending die from two pieces of 12mm sq. steel bar, note the radius on the holes. See photo below. Click for a larger image.
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| Lower Hinge Detail | Torsion Bar Mounting (unfinished) | Bending Dies | Upper
Struts | Detail
of upper strut brazed joint | Completed
Undercarriage |
Tail Wheel Assy
This was built up from a short length of 6mm brass bar, threaded 6mm and fitted with a nut and washer, drilled 4mm & sleeved down to accept a piano wire axle. This can rotate thro' 180 degs. The photo's make the assy self evident. All that is required to complete is the leather gaiter. (old glove finger)
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Tail
Wheel Assy | Tail
Wheel Assy with Leather Gaiter |
Notes
Note 1:- Slots are fixed leading edge devices, Slats open in flight to create a Slot. See a full description of Slots at Wikipedia. See also my separate article on making Slots and Slats
Note 2:- I made a simple jig and built the pockets for the wing hinges and the slots into the ply ribs before assembly into the wing. The actual hinges must be fitted later to ensure proper alignment.(See photo and assembly jig) It is essential that dimension (a) is identical on the wing and the control surface, otherwise the undersides will not be coplanar. Note that these center's differ for the ailerons and flaps. Fitting the extended hinges to the wing proved to be a problem as they must line up perfectly for smooth operation. After a lot of trial and error I used the simple jig shown on Sheet 3 and Photo (11). You can use the jig to fit all 3 (per surface) hinges or fit the 2 outer ones and line up the middle hinge with a thin trace wire stretched between the outer hinges. (1.6mm piano wire is too flexible) Once the three hinges are fitted to the wing, the hinges on the moving surface can be fitted using slow setting epoxy and moved up and down to slip into a perfect alignment. Leave to set on the special grooved board. Without removing from the board, cover the tops of the moving surfaces with 1.6mm sheet. On the second panel I fitted the wing hinges into the ribs before fitting to the wing and this proved to be an acceptable method. The only down side is the hinges now project below the wing.
Note 3:- Removing a snapped wing bolt is virtually impossible without making a mess. Drill the center section former and fit thin walled 6.4mm bore brass tubes. The wing dowels can be made a tight fit into the tubes and secured with a spot of contact adhesive. If a dowel breaks it can be pushed out and a new one fitted. Do not be tempted to fit over large dowels. You want the dowels to snap, not the wing.
Note 4:- Differential Ailerons have different throws on the UP and DOWN movement. The DOWN throw being less than the UP throw.
Note 5:- It is vital that ribs R2 are perfectly square and vertical, any errors will cause serious mis-alignment with the hinges F1 and A1.
Note 6:- Control surface flutter, usually occurs at high air-speeds, and once an oscillation starts it can be self amplifying, destroying the control surface. To prevent this, the centre of gravity of the control surface is brought forward, by the use of "Mass Balance Weights" to coincide with the hinge axis. In this way any oscillations are self damping and the problem is solved. Why a slow flying aircraft such as the Storch needed these weights is another matter. It may have been due to the unusual hinge design.
When it came to fitting the hinges in the ailerons and flaps I discovered a problem. You cannot fit these in after you sheet the top surfaces and you cannot fit them in before, as they project 20mm below the underside and the pivot holes need to be perfectly aligned. The solution was to make a special jig from chipboard (see photo) with the hinges dropping into the lateral slots, and the 1.6mm alignment wire running in the longitudinal slot. Using this jig you can fit all the hinges and horns, confident they are lined up perfectly and then fully sheet the top surfaces. The "dummy" mass balances just slot in later as they are not critical. It's a lot of work but I could not come up with a better idea. Note that the center's of the pivots for the flaps and ailerons ARE NOT THE SAME.
Download .pdf file of Sheet 1 Fiesler Storch Fuselage
Download .pdf file of Sheet 2 Fiesler Storch Wings
Download .pdf file of Sheet 3 Fiesler Storch Formers
Download .pdf file of Sheet 4 Fiesler Storch Undercarriage
References
Fiesler Fi 156 Heinz J. Nowarra ISBN 0-7643-0299-x Schiffer Military History Atglen PA USA
RAF Museum Cosford
Luftwaffe Support Units 1933-1945 Barry Rosch Classic by Ian Allan Publishing ISBN 978-1-906537-04-3