SLOPING OFF...
...the Newsletter of Christchurch and District Model Flying Club for...March 2025
FOKKER DVIII, by JIM RUFFELL
I do enjoy reading others build projects, so please accept the following as belated payback on my behalf.
The Fokker has been on my to-do list for a while, a simple high wing and boxy layout, with minimal scale detailing, and varied colour schemes. My aim was to try, new to me, inexpensive, non traditional constructional methods and materials, namely scratch building with foam sheets in various thicknesses:
With that in mind, a search of the internet uncovered a project by Joshua Orchard based upon the very same aircraft:
(https://www.youtube.com/watch?v=tqLcqSwETPc )
Joshua’s build fitted the bill entirely and gave confidence to proceed along a similar path.
I followed Joshua’s lead and looked for an existing plan to be resized, the main criteria being a reasonable scale outline. I settled upon an old free flight plan designed and drawn by Cal Smith and available as a .pdf download from Aerofred:
(https://aerofred.com/details.php?image_id=81322&name=#plan )
The plan fairly easily adapted to foam construction with key parts well drawn.
The next step was to decide upon an overall size for the model, ¼ scale was perhaps too big for my storage and the plan size of 56” span a tad small for pleasant flying performance. Another problem was how to determine the likely weight of the completed model. I decided to approach this in a roundabout way and produced a spreadsheet, which broke down the model into major components and weights. A sometimes tedious process, but I learned a great deal – you might want to skip the next paragraph!
First step was make up several 6” 3mm foam squares, (4 squares = 1 square foot) and coat each with various finishes, none, glass & epoxy, glass & water based varnish, glass and Decopatch, selected panels were also given a colour coat in automotive spray paint or household emulsion. I then weighed each square using a cheap jewellery gram scale and converted to ozs/square foot. (I find visualising wing loadings in lbs/square foot easier than metric equivalents). Assuming you’re still with me, the next step is to calculate area (square footage) of fuselage sides, wing skins etc. (the entire model would be covered in 3mm sheet foam) then do similarly for bulkheads, tail (6mm sheet) and wing ribs (9mm sheet). Other parts were calculated similarly, such as the wire struts, tail plane, cowling etc. Motor, battery and rc-gear weights were taken from published figures.
Eventually, I finished populating and adjusting the spreadsheet, I’d calculated weights for 1/5 and 1:5.5 scale models. 1:5.5 scale fitted well with my chosen motor and battery combination, and a wing loading of 18 – 19 ozs/square foot. The resulting model having a wingspan of around 60” required a small enlargement of the plan (108%) when printing. The motor & battery combination was based around a heavy lift drone motor of 400KV, 500W continuous rating, turning a 15x10 wooden prop (near to scale diameter). The motor, a 22 pole outrunner type, being short with a large diameter, was easily hidden within the vast radial cowling, its weight well forward and battery directly behind, another consideration for such a short nosed aircraft.
For prototyping electric power configurations, I generally use Propcalc, a tip here if you don’t care to subscribe, use the demo version, don’t worry about the motor type or size, just select a motor with similar KV. You’ll need to make allowances for the motor being either overloaded or under-loaded, but at least the flying performance may be assessed for model overall weight, and prop sizes accordingly. See example below, not the correct battery or motor, but it’s in the ballpark.
Construction began with plywood doublers, firewall and 2 intermediate bulkheads in an open top/bottom structural box, combining all the major stress points, e.g. motor mount, undercarriage mount, wing struts & battery box. The motor sat on the face of a smaller box projecting from the firewall, sized to contain a 4 cell 3000mah pack vertically, with access from below via hatch above the wheels. Wing and wheel front struts could then be attached to the firewall using small cable ties, the second intermediate plywood bulkhead mounted the remaining struts in same way. This structural box then effectively skinned with 3mm foam fuselage sides and 6mm foam bulkheads from the cockpit to the tail (no longerons) to form the fuselage. Tail plane & rudder were cut directly from 6mm foam with 6mm balsa strips to elevator & rudder hinge breaks (to accommodate hinges) and 1/16” balsa capping of leading/trailing edges for ding proofing. The fuselage was impressively stiff and light, once turtle decks and fuselage underside completed.
The wing generally followed the plan, but with more ribs and cut from 9mm foam, a new deeper section (120% Clark Y), carbon fibre spars, fully webbed with 9mm foam and ply reinforced bolt on wing fixings. Individual aileron servos mounted on screwed in trays with the wing tips fashioned from Cellotex/Rohacell insulation foam blocks, a very strange material that weighs little, cuts easily and may be sanded with a finger, while still remaining quite rigid! If you refer to Joshua’s wing construction, he used a similar material for wing ribs, spar webs & tips. I deviated slightly from Joshua’s overall wing build process. Wing skins were made slightly oversize (measured the length of the rib external faces for largest and smallest to obtain root an tip skin chord), fixed the ribs to bottom skin first and held everything in place with masking tape and weights on a flat build board while the glue dried. Trimmed lower skin to size, fitted webs, and internal details like aileron facings, spars etc. before fitting top skin, overlapping the lower skin, tape and weigh down again while glue sets. Cut the ailerons from the completed wing, the LE & TE were formed where the two skins overlapped; TE requires a little sanding or angling off to avoid the thick foamie TE look. 3 wing panels in all, L&R outer, plus centre section. Well worth reviewing Joshua’s wing build video (Part 2). Once again, all impressively light and stiff.
Regarding adhesives, for foam to foam, I used Gorilla glue, very light, foams into gaps, quite strong (stronger than the foam) if a little brittle. As Gorilla glue foams while curing, difficult to cut or sand when set, so trim excess before cured. For foam to wood, PVA in all but high stress areas, where either Gorilla glue or epoxy (used sparingly) is advised. For final finishing and gap filling, Wickes light weight filler or similar, cheap, easy to sand and weighs very little.
I covered the entire airframe with a light glass cloth (18g), held in place with Decopatch glossy glue brushed on. This is a water based (PVA) adhesive made for decoupage, but works well over foam with glass cloth, lighter and easier application than epoxy or water based varnish. Decopatch is compatible with automotive spray paints or household emulsion. Insignia, cowling and tail artwork were made with spray paint and masking before lozenge camouflage. Applied lozenge style pattern on the fuselage (found a 1/32nd scale pattern used by plastic modellers and scaled it) and solid colours on the remaining surfaces. A4 sized stencils were created by hand for the 3 of 4 lozenge colours, using Cricut stencil material, a thin transparent plastic sheet with low tack adhesive, these worked OK but soon became ragged and leaked, necessitating hand retouching, for a larger area more stencils might be required. Painted the entire lozenge pattern area with a base colour of the lightest shade and then worked through the 3 remaining darker colours using the stencils, in order lightest to darkest. Magically, the base colour appeared as lozenges, after the 3rd stencil colour. Emulsion paint was applied with Dulux tester pots and integral roller, washed out and re-used as necessary with either B&Q or Dulux tester paint.
For those of you into 3D printed parts, I printed half a le Rhone rotary engine & Spandau’s from Thingiverse, rescaled and/or cropped in the slicer (Cura). The short outrunner required a spacer for the prop driver (see below), which after some hours I created a printable STL in FreeCAD, same for the wheel hub, as those I’d bought were a bit crude and heavy, although used the original tyres. Yet to print TPU tyres, have to draw one first!
The completed model came in at 2kg (4lb 4ozs), under budget (target to achieve wing loading - 4lb 6ozs), the planning spreadsheet predicted 4lb based upon calculated estimates, I’m just amazed how close all those numbers were. Based upon similarly sized models in conventional wooden structure and tex type covering, I reckon its at least 20% lighter.
During the build a number of key decisions were made regarding aerofoil selection, incidence, C of G, thrust line, washout & dihedral, and having now flown the model several times may be reviewed:
- Dihedral, plan showed wing flat across top surface, scale drawings show wing is flat through thickness. As the plan was for a free flight model, the slight dihedral added may be beneficial and wing panels could be finally assembled upside down on a flat surface. For RC, little benefit, so I went for scale. Subsequent flight tests indicated that the high wing configuration possessed a reasonable level of roll stability, more than I’d usually expect from a zero dihedral wing.
- Washout, plan gave approx 2-3 degrees of washout in outer panels. Personally, washout (twisted wing) is to be avoided at all costs (Eh? - Ed). Assuming a cambered section, an alternative is to transition to symmetrical section at the tips, but really this is only the same as twisting the wing. My preferred option for the wing section was to have a sharp leading edge at the centre, blunt at the tips. Looking at the Fokker, the wing planform was hardly likely to result in a tip stall anyway. Subsequent flight tests exhibited no tendency to tip stall, even under extreme provocation!
- Thrust line, plan indicated 2 degrees of down thrust, and about the same right thrust. I checked around with other plans and models of the Fokker, all suggested the same. Again, I hate messing around with thrust lines, so it was zero zero (Tsk tsk - Ed). Flight tests indicated I’d got this wrong, and thinking through the reasons, it was fairly obvious, short nose with motor well below wing. Down thrust was mitigated by mixing the throttle into the elevator, and as I write this, I’m wondering if the same would work using the rudder to reduce the need for side thrust. I’ve subsequently introduced approx 1.5 degrees of down and side thrust which has improved the model greatly, although still a work in progress.
- C of G, again amazingly, the C of G came out spot on to the plan, 30%, without nose weight, although I’m somewhat sceptical of the plan figure. The bad news was that anything more than 60% throttle would cause the model to go practically vertical, this all the more interesting during the maiden flight take off, when trimming had yet to be done, on chopping the throttle went straight into a stall turn! Needed about 5 degrees of down elevator to hold level flight at cruising speed. The pitch stability seemed otherwise fine. I’ve subsequently added approx 2ozs to nose, moving C of G to approx 28%, combined with the thrust line changes and handling is much improved.
- Incidence, plan has the ‘default’ Clark Y flat bottom level with tail plane. As I’d deepened the section for both handling and structural reasons, thereby increasing the camber, it seemed prudent to effectively raise the TE and level the geometric centre of the aerofoil with the tail plane. Given the amount of down elevator required for level flight, it could probably do with a bit more negative incidence, there is provision adjust the wing rear struts with shims to accomplish this.
Currently, the model flies nicely; flight timer is set for 8 minutes with plenty in hand. There’s work to be done on right turns, which I believe are due to of motor torque combined with the big prop. Left turns are fine, right is fine with right rudder as well; otherwise the model looks a bit odd turning right, dragging its tail low. Its very light, a consequence of which is that I happily cruise around at low airspeed, but with vague aileron response. Pick-up airspeed and the ailerons become responsive. Landing and take off behaviour require pilot attention with the high wing and narrow track, ground looping a well known issue with this type. Little torque effect on take-off as gets airborne quickly, not stalled yet, other than a stall turn. Loops are unremarkable.
All in all, what can you expect from a WW1 aircraft, and my admiration goes to those who flew these machines by the seat of their pants!
Acknowledgements:
Joshua Orchard – Fokker DVIII A quarter scale foamie YouTube video series.
Mike Roach – numerous foamie designs and source of handy tips and inspiration!
Gordon Whitehead – his book ‘Scale Aircraft models for every day flying’, ISBN 0903676087.
A practical guide to scratch building and sizing-up your subject.