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Sep 29, 2017, 05:52 AM
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Build Log

Foka 4 scale 1:3,75 from Fliegerland


In 1960 the Polish factory SZD made another throw at high performance gliding in an attempt to break the German hegemony in this domain. Although still of wooden monocoque construction, their radical Foka (seal) design was extremely pure and streamlined (laminar flow over wings and fuselage), still looking futuristic more than 50 years later. It not only looked good, but after a couple of months gained 3rd place in the 1960 standard class world championship. It later astonished the glider community when the 24 year old Jan Wroblewski entered such 15m Foka4 in the open class and became world champion during the 1965 UK held championship, the one and only time a standard class glider won the open class title, and this in an aerobatic capable (+6 -3G) production glider. Production of the SZD 24 Foka4 ended in 1968 after 204 gliders, of which only 137 found an export customer. It acquired 7 world records with a 1:34 glide ratio, a maximum takeoff weight of 322kg and an initial speed range between 62 and 250km/h. Some are still flying nowadays but are limited in speed to 160km/h and Gís after some disastrous in-flight structural wing breakups.

My Foka shortly after getting airborne on its 3rd flight

Acquisition day 1

At the end of the 2017 flying season I stumbled upon following advertisement on the popular German RC website.
Since the early sixties I had fallen in love with models of the very elegant Foka4 glider in the catalogs, but never possessed one. With my recent involvement in the towed scale-glider community I sometimes saw such models in either 3 or 5meter span, but the 4m models were either unavailable due to age or not for sale. With the ease of transportation in the garage compartment of my camper and increasing difficulty of manhandling heavy models, the 4meter span version was on my dream list and that advertisement made me aware that Fliegerland still produced such model and had all kinds of spares for it in case of mishaps.

With a CFK fuselage and rudder, plus styro-Abachi with CFK D-box reinforced wings with mechanical spoilers, the quality kit sold for a hefty 850 euro. The one for sale seemed immaculately finished including a cockpit interior and pilot, was equipped with quality servoís and reported to have made only 4 flights. After contacting the seller it became clear he flew at the German model site at Wey, where I had attended large model-glider gatherings in the past. A few days later the president of the BiGGS association took me to the sellerís home in his large van, and we soon discovered that Mr Manfred Klinger had been the K in the well known HKM glider model brand. Although not involved anymore, his hobby room was still full of parts from his former business and that immediately assured us about the quality of assembly of the Foka gleaming as new on the table. I even couldnít find traces of the model ever having flown but he ascertained us it flew well and with the iron wing joiner was capable of nice aerobatics. Besides feeling rather heavy I could find no flaws nor play in the controls. It was obvious the model was worth the asking price of a new unassembled kit without electronics and after exchanging the information about the throws and powerbox, he removed his batteries and receiver, and we blocked the model in the van for the trip back home from Germany to Belgium.

Figures day 2

The port wing weighed 1502gr centered at 71,5cm out, the starboard wing weighed 1460gr centered at 70cm out. After the large registration marks under the port wing were removed this came back to 1446gr ! Both wings were 194,5cm wide, 30,5cm at the root and 10,5cm at the tip, calculated at 79,95dm≤ wing area and with a center section 12,5cm wide gave a total wingspan of 401,5cm. The wings were joined by a 16mm diameter plain steel 35cm long key weighing 562gr. The horizontal tail weighed 212gr.

The fuselage with just rudder and servoís weighed 3146gr. Add to that the canopy with instruments of 155gr, the pilot of 176gr, pilot seat shell of 64ge, the batteries of 300gr and a receiver plus vario of about 80gr and the complete fuselage ends up at 4133gr. Grand total of the ready to fly model becomes a hefty 7657gr, causing the wing load to reach about 95gr/dm≤.

Inventory day 2

Next morning I started making a detailed inventory of all the model parts and available information. The builder used 25mm up and down throw on the elevator, 20mm up and 10mm down on the ailerons, 50mm left and right on the rudder, and flew with a CG 85mm from the leading edge.

I noted following servoís: Elevator D-power DS-570BBMG (a 58gr digital relatively cheap servo with 7kgcm torque. All other servoís are reported to be analog to reduce power consumption. Rudder has Graupner C577 (42gr, 3,6 kgcm at 4,8v), towhook release by Graupner C4621 (very expensive 52gr 8kgm at 4,8v). Ailerons are Multiplex Micro mcV2 (expensive 28gr 6V capable 3,8kgcm). Spoilers are cheap Hobby King HK933MG (12gr 1,8kgcm 6V capable digital servos).

Inside the cockpit a Graupner PRX-5A linear powerbox is installed, allowing double power inputs of 2S Lipo/Life/Lion, 5 or 6 cell NiMh/NiCd, with selectable 5,1/5,5/5,9V outputs rated at max continuous 5Amp. The seller provided me with the original box containing various adapters and the booklet. The front of the fuselage contains two pockets for 5cell NiMh batteries as used by the previous owner. The 7,5cm deep 6cmx1,5cm pockets are separated by the pin operated 12mm diameter tow release mechanism, and both battery packs secured in place by a single Velcro strap. Lead blocked by foam can be seen flanking the pockets.

This nose job prevents the canopy opening to be made conform to the sliding method of a real Foka. The clear canopy is mounted on a plastic shell that is reinforced with wood and also contains a very nice instrument panel with realistic gauges and handles. Unlike the real glider, this pilotís legs are only visible aft of the instrument pod, the front of it is dark plastic covering the mounting plate for the servoís and powerbox mounted in the front of the fuselage. The canopy is held in place by a wooden dowel at the front under the nose panel, a wooden dowel at the back for lateral stability, and two strong magnets along the aft sides. No provisions were made to facilitate the grab of this solidly held piece for removal.

Under the canopy a loose plastic seat assembly with stick and realistic spoiler and trim lever can be inserted in the fuselage. It then covers a rubber mounted wooden oblique plate with Velcro to mount the receiver on, and also has guides for the antennas at 90į. The quarter scale graupner pilot is too small and without parachute almost disappears in his cockpit, definitely detracting from the scale proportions and general image of this nice glider.

Guides left and right inside the fuselage were glued at regular intervals all the way to the back to guide the pull-pull rudder cables and metal elevator pushrod to the tail. The horizontal tail is kept in place by a single M6 nylon bolt and wooden alignment dowel, but is not easy to screw in place because of the overhanging rake of the rudder assembly.

The wings are kept in place along the fuselage by MPX Unilock wing joiners and electrically connected via 2 loose MPX connectors. A 3mm steel rod at the back of the wing roots keeps the wing incidence angles fixed. There is only a molded tailskid and noseskid with a fixed tight fitting 80mm wheel to land on. This has a white plastic side-valve? that hampers free rotation and will probably be omitted. The wheel on the model has purposefully been re-positioned more forward of the CG (versus real Fokaís) to alleviate the front skid during takeoff and landing. This prevents ground damage if landing on hard surfaces, but spoils the overall elegance of the fuselage.

First impression day 3

Even sitting on the ground, Foka gliders give the impression of high speed flying, and to me are the most elegant gliders ever produced. I have the same feeling towards the SA210 Caravelle for airliners and Hawker Hunters for jet fighters. The large German registration marks detract the otherwise pure lines of this model. Luckily after some searching I found a UK registered identical colored Foka 4 and if removal of the German letters goes smoothly I probably will change the appearance to:

This real 1963 Foka 4 has been totally restored and flies with the UK Glider Heritage center since 2016

This second hand model was worth itís price (all parts and 2K paint plus Orastick amounted over 1400 euro) but I was disappointed about some servoís. All the rest is high quality work and the model looks stunning but features a straight instead of slightly concave wing intrados on a profile that is about half as thick as the real one, I wonder how well it will glide at that weight.

Just look how thick the wing of a real Foka is compared to the wing profile of my model

This is not a lightweight thermal model but a very solid aerobatic capable one. In order to eventually lighten it up a bit for pure thermaling or slope flying, I saw that Fliegerland can deliver a 50cm CFK wing rod and will probably order one together with a spare canopy. Iíll probably also replace the cheap wheel by a more solid Fema example and mount a fake small antenna at the back of the canopy to facilitate the opening of it. At least the pilotís head will also be replaced by a larger one to better portray the true proportions of this small narrow glider. There is sufficient space in the nose to accommodate a variety of batteries and changes in lead weights. Iím looking forward seeing this beauty fly and am sure not many other such Fokaís in that high visibility true color scheme will show up at the various events that I attend throughout the year.

Preparing for flight day 4

Searching in the scrap box I found the head of the Ka2 puppet (that I replaced with a 3D scan of my own head) and soon had it stretched over the head of the Graupner pilot. The proportions looked better but her hair was so high and the molded headset so thick that it didnít look right. Because it was a kind of rubber I took out the stuffing, scalped the top of head including the headset, tightened both top sides together with staples and used a typical glider hat to cover everything up. Without the stuffing her cheeks looked a bit narrow so I used the holes of the cutoff ears to insert stuffing that I let overflow to form white hair where the headset had been. The more rounded face looked perfect for the proportions and the hat also got stapled on the head.

Pilot with head transplant and hat, but still unattractive skin color, at least it fills the canopy better.

I then took the port wing and with the use of my hair dryer the huge German registration marks were easily removed. After wiping off with benzene no glue traces were left and no discoloration visible. Drilling a hole in the aft top of the canopy frame I was able to insert a length of black tiedown plastic ribbon that now looks like an antenna but in fact serves to much more easily unlatch the cockpit from its strong magnets at the base.

After what I saw on the wing I thought the removal of the registration on the sides of the fuselage would be a piece of cake but it proved different. The sticker removed easily from the orange painted fuselage but stubbornly stuck in the white stripe. When I finally got it loose I was astonished to see there was no white paint underneath but just the dull whitish fuselage. For whatever reason the builder had masked off the fuselage at the white stripe, then had it quality painted in 2K orange, took away the masking applied the markings at that time. Probably realizing the rather dull whitish fuselage looked poor compared to the gloss orange, he masked the orange plus the markings that were on the bare fuselage and then applied a thick coat of white to fill everything up. Needless to say, although it looked good when purchased, after removal of the markings it was just a mess with serious height differences along the paint remains around the removed markings within the white stripe.

Clearly visible horrible depth difference where the registration letters were removed

The white paint was so thick at most places that I was able to slide a sharp cutting knife underneath it to loosen bits and pieces from the fuselage. I thus decided to remove all the white paint from the fuselage portion along the total length of the markings and soften the angle both white extremities. This was a painstaking work that took hours and at places damaged the orange straight edge and caused some knife marks on the areas where the paint was reluctant to separate because it was thinner.

After removal of the paint by knife serious damage was visible in the plastic and orange line

I then put the fuselage on its side, masked the orange off and applied a thick coat of white Revell plastic paint by brush to fill in the depression between the orange paint walls and white edges. After a two hour drying period I removed the masking tape so it wouldnít stick on the sides while drying overnight.

That evening I also removed the wheel to measure it and its axle. Although it appears to be very strong, the tolerances in the fuselage cutoff are so tight that I decided to order a 78mm diameter narrower wheel with 5mm axis, but less that 30mm wide tire. After weighing much pros and cons regarding battery types and dimensions, I ordered two LiFe 2S2100 of unknown brand but except for their length, fit in the existing receptacles but are 1/3rd lighter than the Enelope 5 cell packs to which the pockets had been tailored.

Preparing for flight day 5

The morning after I used a very thin brush to correct the ungainly orange dent marks and must say that I was pleased with the result. I then put the fuselage on its other side to tackle the other half but here the cut marks in the plastic were a bit deeper and I had to apply a second coat of white before these were completely covered flat.

In between that I applied the small CBP markings on both sides of the tail and used vinyl paper and self sticking strips of chrome to imitate a front wooden skid with black leather compression skirts towards the fuselage. This is not only very scale like, but together with the removal of the large German registration on the fuselage, tremendously tapers it visually, accentuating the slender shape of the fuselage as a whole. Later on the skid will be painted because Iím afraid uneven terrain could rip the vinyl off rather easily.

The pilot was extracted and face plus hands were given more realistic skin colors, the glider hat became light blue and the glasses got a golden frame. I then removed the pilotís seat and Velcroíd a parachute to the back. The power box was taken out and green Multiplex plugs soldered on the battery ends in lieu of the JR servo connectors that had been used by the previous owner. The box was adjusted and tested and proved functioning. The previous owner had taken out his receiver but had not marked any of the wires, and because ailerons and spoilers ran through Multiplex sealed off wing connectors of unknown pin use, I used my 5v servo tester to identify and label every servo plug. There are seven in total plus two powerbox wires, one feeding directly into the hook release plug, a smart move regarding the 1,5A power consumption of this strong servo.

I then took the X8R receiver and high precision vario sensor of my Ka2 (that is out for months for repairs after a cartwheel landing in strong crosswind) and mounted those together with the antennas in the belly of the Foka. I made a copy of the Ka2 settings and relabeled it Foka 4 in my transmitter because both gliders need about the same control layout and audio features. In the meantime I got confirmation from Engel Modelbau that my batteries, wheels and 50mm marking letters were shipped so test flying it the following weekend will be possible.

Preparing for flight day 6

It took me the whole morning to adjust the neutral positions and throws of the flight controls and towhook. The port spoiler had internal mechanical adjustments and had to operate in unison and retract freely even with air pressure applied to it. For all that, I had the Foka completely assembled for the first time, but later had serious trouble separating the wings from the fuselage. My order from Germany arrived at noon a record 36 hours after I ordered it at the unchristian hour of 2am on a Sunday night. Whilst I loaded the LiFeís I balanced the wings and came out at 1477gr at 72,3mm for the port and 1474ge at 72mm out for the starboard wing. That got achieved by adding 23gr and 13gr of lead in the respectively inner and outer servo recesses of the starboard wing. I then changed the input setting of the powebox from NiMh to LiFe and tested the system again before attaching the unit to the fuselage. With the much longer batteries I had to drill out the wood that the builder placed at the back of the battery receptacles to prevent his Enelope batteries prom sliding too deep in the nose. This was anything but easy because these were only 15mm wide and the wood started 8cm deep.

A long 10mm drill and extended knife did the trick but created a lot of wood mes sin the extreme nose. The batteries could now slide in 2cm deeper but still protruded a bit into the cockpit, necessitating Dremel work to very locally remove material from the thick plywood that formed the front plate for the clear canopy. When I got almost that far, one extra millimeter had to be scraped from the aft of the clear canopy and its wooden backplate in order to allow the canopy to fully close against the magnets. To ascertain the magnets had maximum strength I scraped off the thick paint coat that ran over it. All this allowed me to now have 4200mah battery capacity (for a complete weekend?) with the batteries totally hidden and locked tightly in the nose. I then fastened the MPX fuselage plugs to the baseplate to keep the wiring orderly during flight.

Top of new batteries stick out a few mm that had to be cleared from the front canopy plate.

During the evening I laid out a wing on the kitchen floor, parallel to the pavement alignment, then measured the imaginary wing prolongations at both ends and joined these with a long (yellow) ruler. A white rope was tightened along the midwing axis and the intersection point of both long lines gave me the MAC or Mean Aerodynamic Chord. Because the wing leading edges are perfectly aligned and are square to the fuselage center, a 90į angle to it allowed me to measure the chord (22,5cm and calculate the forward CG of 25%MAC at 56mm from leading edge, aft CG at 33%MAC being 75mm from the leading edge. I applied marks on the wing root to indicate that CG range. The most aft calculated CG is still 10mm more forward than what the previous owner used for flight, which will make for a very interesting first flight for me. Before going to bed I made the last fuselage orange and pilot brush touch ups.

Scientific measuring by kitchen materials to obtain the MAC line (green plastic triangle square)

Preparing for flight day 7

On the last preparation day I didnít apply the 50mm registration marks to the fuselage because the letter-type I had was too square and incorrect, it would have spoiled the otherwise gracious flowing lines of the Foka. A correct set of lettering might be ordered later. The lightweight wheels I had ordered came with a warning for heavier models to glue the rims with CA to the tires , that was the warning bell not to use them at least until my landings were consistent. I thus used the Dremel to flatten the white insert of the original tire so it now fits well and turns freely. Individual identification plaques (mandatory for models in Belgium) were then glued in the fuselage and below the removable canopy. I also put my cell phone number on those and a note to disconnect the batteries. If found after a mishap that could save a lot of work. I then powered everything up and set the failsafe positions in the receiver with neutral controls except some left rudder, hook open and spoilers open. I then made a backup copy of my models for my Open TX Companion and then installed the seat bucket to cover all the electronic. The pilot fit in nicely and looks a lot better than what was in it when I bought the Foka. The pilot body still doesnít look scale because in a real Foka one completely lays flat and fills the entire front with the tiptoes in the extreme nose and the head against the end op the canopy. It is obvious from the picture below that this figure is much too short but little can be done because of the positioning of the servoís and the ensuing instrument panel having been placed too much back, resulting in the stick being too far back whereby the pilot body cannot be sufficiently long and even with amputated legs is held in position against the instrument panel. All this unfortunately produces a false impression about the relative size of this very short, narrow and low glider.

The round hole used to stuff the pilotís trousers in, all this is covered by the long instrument panel

The last major job was the dreaded weight and balance. In fact the term should be reversed because we first need to balance it before we weigh. The first equilibrium showed a figure of 85mm back, way aft of the neutral stability. My idea was to maiden at 65mm or about 29% MAC, that is as far back as I feel comfortable with. Even this required a hefty 340gr of extra lead in the nose. Quite a bit of available space was already taken up by 3 loose lead pieces with a total weight of 550gr. The extra 340gr consists of various pieces of lead that were shaped and hammered so they could fit the space left between the batteries and fuselage sides. With this balance achieved the ready to fly weight of the Foka 4 amounts to 7700gr. An additional 57ge of lead was shaped to fit in the nose above the batteries and that can rapidly be inserted on the field to bring the CG forward by 4mm to about 27% MAC if necessary or for testing. Last important check was the hook release which functioned even with a pulling force of 10kg.

The rest of the afternoon was spent making a custom cradle out of 4cm thick isolation board. The width of it had to be exacvtly 20cm wide so it would fit in sequence between the Twin Astir, Ka8b and the Blanik, effectively using up the total width of the garage and thus blocking everything tight during transport. Unfortunately that meant that the elevator could not fit aside the fuselage. I first measured the camperís garage floor depth to see where the wooden movement stopping strip had to come in relation to the fuselage length. This then dictated the exact shape of the cradle bed and care was taken to allow sufficient length to allow the heavy wing key to be blocked in-between under the fuselage. First I experimentally found out the exact shape by using cardboard and scissors. Once I got that, I copied it on the isolation and used the figure drill to cut into shape. That is always a big mess because of the tiny particles that are statically charged and stick to everything. I got everything cut just in time before the sun settled down. Using emery paper I got everything smooth and using toothpicks and PU woodglue I spent close to midnight till the complete cradle was assembled and left to dry overnight. On the port side I made indentations so the horizontal tailplane is fixed with two rubbers alongside the vertical poles for stowage and transport.

Foka in its home/transport cradle, note fake antenna to facilitate canopy removal

Maiden Flight

Saturday 23 September 2017 after the fog lifted, I arrived at TMV where Bert performed a buddy check using my checklist. He had reservations about my chosen CG point but at the end we agreed that in this unknown case too much forward was less a risk than too far back. He also advised me to eliminate the aileron to rudder mix for the maiden. Dirk prepared his Bidule 111 towship and we discussed the options with the weak wind, accepting a full crosswind on the longest runway or go for a headwind on the shorter runway with a concrete active road on the threshold, in the end we agreed on the former. A break chord was inserted at my towhook and soon were ready to go. My takeoff seemed long, the Foka had no urge to get airborne by itself so I helped it with some up elevator. Either the forward CG or the thin wing were responsible for that and during the climb I kept on applying more and more aft trim but the Foka never seemed to want to stabilize higher than the towship.

I released at 300m after a very stable tow. I fan the elevator trim all the way up but the Foka kept a relatively high speed. It glided well and even without a single cloud in the sky it picked up thermals. Pitch and roll response felt, turn reversals were fast with little adverse yaw (even with the aileron rudder mix dialed down to zero). I then stabilized and entered a dive for the CG check. It recovered almost instantaneously, proof that the CG was close to the forward limit. With that in mind I safely could perform a stall, but had to apply serious back pressure for the speed to reduce. When it stalled, it resulted in a vigorous wing and drop. Recovery was straightforward but I wouldnít like to encounter it at lower altitude because of the excessive altitude loss. On request of Bert I repeated both last patterns to ascertain ourselves consistency of the results. Next check was operating spoilers of the Foka during a straight glide. Spoilers out increased not only the dive angle but also the speed, with spoilers in the nose rapidly raised into a moderate climb, a situation I donít like and would have to be corrected by finding out an appropriate spoiler to elevator mix.

It then became time to think about the landing. With the known flight characteristics I wanted to keep the speed high in the pattern and kill it with the spoilers in final. This approach worked well but with full spoilers the modelís sink rate increased more than anticipated and not wanting to retract them by fear of ballooning, nor apply too much backstick due to fear of stall, I ended up touching down hard about two meters short of the grass, in a low cultivated plowed area. Field inspection showed a small compression crack behind the cockpit and a crossmember that retained the pilotís seat from moving backwards to be loose. Although sunny, it seemed just too cool for the epoxy glue to cure so I abandoned the repair attempt, dismantled the Foka and drove home early because of other planned activities that evening. Back home I removed 180gr of lead from the nose, reprogrammed 5% aileron rudder mix and charged the LiFeís which only took 92 mah to replenish after all that ground setup, testing and maiden. Those Life batteries sould be good for an entire weekend of flying.

Further Adjustment flights

Next day I drove out again and with only 5 pilots on the field, conditions were even better to conduct further evaluation flights. The crosswind was still present but necessitated us to first operate from the opposite long runway. During flight nr2 I was surprised that even with the lead removed, The Foka still didnít get airborne sooner nor stabilize higher above the towship. With a mainly blue sky I found it difficult to see the bank angle of my model behind the towship, certainly I hadnít expected that from the completely orange wing surfaces but I admit, the chord is not very deep.

After disconnecting at 300m, I started trimming the model out again and was surprised I still needed a bit of up elevator even with the lead removed. First thing I did was to perform a CG dive check from which it only recovered very slowly, proof that I was close to the aft CG limit. On suggestion of Bert I performed another one after a longer stabilized hands off glide but that resulted in the same. Bert thought the CG was too much aft but some turns and stall revealed no nasty characteristics so I decided to further explore the flying qualities at that new CG before changing anything. It seemed to thermal much better although still at a serious speed, and the only change was that the nose pitched down even more during spoiler application, but didnít raise as much during retraction. With that in mind I entered the pattern and made a good landing using half spoilers.

Flight nr3 was performed without altering anything and the purpose was to catch thermals, which it did very well because I stayed up for 12 minutes before landing oblique due to the crosswind starting to blow harder. Flight nr4 was used to verify general handling with steep turns and rapid turn reversals. Because everything felt OK I even performed a looping before entering downwind. Final looked OK but a bit fast so immediately after crossing field boundary at about half a meter height, I rapidly opened full spoilers. That was not the smartest move because the model immediately developed a sink rate and some wing waggle, resulting in a hard landing during which the canopy separated. Close inspection revealed no further damage except that a 7cm crack had developed 27cm out on the port aft wing intrados. Being so much aft, the guys deemed it safe to continue flying but I elected to drill a 3mm hole at the end of the crack so it wouldn't propagate. Otherís tried with a sharp knife to remove some wood so both sides would lay flat again but to no avail, whatever wood had been used, it was thick and refused to spring back into position. No more aerobatics that day.

Flight nr5 was conducted from the shorter cross runway and I was apprehensive for the final over the concrete road (that has to be guarded for cars and even pedestrians). With the previous landing in mind, programming a spoiler elevator mix would have been a logical move, but that is something I rather do at home on the Taranis companion instead of in the field. I therefore made a mental note to operate the spoilers very slowly during the approach, increasing progressively as deemed necessary. This worked very well, also on the sixth flight. The gliders then were forced to stay on the ground because of a nosewheel/prop problem of the towship.

The verdict

I am very pleased about the acquisition of this model. It flies superbly and thermals much better than expected. Due to its thin airfoil it flies differently than my other gliders and needs speed during the tow and all other phases of flight. The orange color is authentic and very visible on the ground, but doesnít contrast well in clear skies plus the thin lines of the Foka donít produce much contrast area. Iím convinced some more fine tuning and flying experience with that magnificent glider will provide me the joys I like in this hobby. It is sufficiently light for me to carry one handed over any distance.

Back home I assembled the model and reverse engineered it on the balancer to find out that I had flown with a CG at 75mm or 33% MAC, further back than Iím used to but still a tad short of the 35% neutral stability point and feeling very much as the sweet spot for this glider. I marked that spot on the fuselage and then programmed a spoiler-elevator mix in the TX that I tested on the companion and on the model, initially using 5% mix but easily adaptable in the field during further refinement flights. The crossmember had come loose again and this time was afixed using my preferential PU woodglue. The wing also got repaired but the origin of the damage is still not clear to me. The wood had been pushed in only at the start of the crack. That could not have been caused by overstress during the looping (stretch), and neither during the hard landing (compression). It could have been caused during the hard landing on the low crops at the end of the first flight and not discovered until the inspection of the wheel after the second hard landing. A while later Bert suggested the cause could have been the impact of the canopy that had separated during the second hard landing and was found just aft of the port wing with a dent in the frame. If that was the case I shouldn't be afraid of the damage. Anyway, repair was straightforward and I am not afraid that it weakens the structural integrity of the model. The small superficial fuselage crack of less than 2cm was hollowed out with the Dremel till the solid underground and I was amazed how thick the fuselage was, no wonder this Foka is heavy. I then filled that 3mm cavity with UHU Endfest, a very strong slow drying 2 component glue. I then programmed my TX with a mix between spoilers and elevator plus a mix between the towhook closed and the elevator, the latter hopefully having the model fly a bit higher behind the towship without me contantly having to apply up elevator or retrim during the tow. During next flight the amount of both mixes will be adjusted till a handsfree pitch input is found during tow and spoiler operations.

The adhesive decorations that I had applied on the skid had not coped well with my first landing in the plowed field. Earth remains had penetrated between the self adhesive material and the skid. I decided to remove all the self adhesive and leave the skid as delivered. I have some wild plans about eventuallyredecorating the model to portray OO-ZEU, a Belgian Foka that participated in Sabena markings in various international competitions during the sixties. That is also the reason that I might not elaborate further on the UK registration marks of my Foka in its present colors. The skid will be painted at that time instead of using adhesive materials. In the meantime I might order a factory CFK wing key to check the difference (in weight) and a spare canopy. To finish the original canopy I delicately cut the rather flimsy plastic and installed a functional scale vent window on the side. As a last touch I added the scale red tuft on the canopy top.

To deep instrument panel is positioned too far back to cover the modelís electronics below it.
Last edited by BAF23; Jun 07, 2018 at 04:28 PM.
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Jun 07, 2018, 04:55 PM
The sky is the limit
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Later updates

Autumn 2017 update

A weekend later we had a kind of summer again and I took the Foka to the field. It flew so well that despite the blue sky the first flight lasted 24 minutes. I performed a total of 5 flights that day which I used mainly to refine the spoiler elevator mix and the towhook elevator mix. The latter positioned the model at the correct height above the towship during climb. The combination of Fliegerlandsí thin airfoil and very little incidence (compared to the full size glider) still wasnít enough to bring it sufficiently higher than the towship even at the high required towspeed so I dialed in some up elevator mix as long as the hook was in a closed position and that resolved that issue very well.

With 2 of the landings still too hard I noticed back home that the fuselage behind the canopy now had a small crack at the other side and that the wheel cover assembly again showed a slit at the front. I repaired both but was tired of the same happening again and decided to glue a piece of hardwood between the bottom of the wing key tube and the top of the wheelbox. The idea was that the force of a hard landing couldnít compress the fuselage at that point anymore and the wheel box would stay fixed to the fuselage. Furthermore I was convinced that a lot of the force came from the 562gr weight of the plain solid steel wing key. That could be necessary for the aerobatics but definitely was an overkill for thermal gliding. After some searches we found a Dutch retailer who delivered 2mm thick hollow carbon tubes in 16mm and 14mm. After sanding the contact area and gluing, both tubes fit nicely into each-other and became one very solid tube that for the same 36cm length weighs a mere 48gr, almost twelve times less than the steel rod and thus half a kilo lighter.

The wing key being installed slightly aft of the CG, this huge weight difference required a new weight and balance of the Foka. I readjusted the position of the lead in the nose to achieve a 70mm CG position to explore during the next flights. The elevator sensitiveness had been high and during serious steep turns I encountered high speed stalls with moderate aft stick deflections. All this caused me to be extremely cautious applying up elevator during the flare and I therefore decided to seriously reduce the elevator travel by attaching the pushrod at the servo in a hole closer to the pivot point. That of course influenced the spoiler to elevator and hook to elevator mix and I augmented those. Weighing the Foka I got 7170gr which calculates to a wingload of 89gr/dm≤. In the meantime I also took the time to order the UK markings from Calliegrafics and applied them. Next flights were test flights to validate the new configuration, CG, elevator throw and mixes.

Pic 4446

Spring 2018 updates

During Eastern the weather finally was suitable for gliding but after a couple of flights with the Foka I was pleased with the lighter carbon rod and it even withstood moderate aerobatics, but still wasnít happy with the overall performance and the streamline of the elevator in trimmed condition. Thinking it over resulted in yet another approach, adding 1mm of wood under the leading edge of the stab. The validation flights for that were made during a glider meet in the North of Holland and proved that I was on a good track. The Foka flew a lot slower and held up well in very weak thermals. The elevator was streamlined with the stab but the model hung much too low behind the towship and recovered too fast during the CG dive test.

Back home I removed weight from the nose and put the CG back to 75mm. Since long I had been eyeballing the possibility of adding the scale like end-plates to the wing and after a while I found sufficiently detailed pictures to (re)produce those using scrap 5mm plywood under which I glued 2mm steel rods. The purpose was dual, the end-plates only protruding from the underside of the wing formed an aerodynamic beneficial winglet (reducing vortices and thus keeping the over-pressure under the wing), plus creating a skid that prevents wingtip damage when landing on hard surfaces.

Pic 4656

Flying in that configuration gave an improvement in thermal capabilities and a lower landing speed. Mixing 6% of elevator on the closed towhook switch position (without time slow either way) brought the glider at ideal height above the towplane with no tendency to pitch at release. The model can now be flown in smooth air without touching the trims. It thermals very well even at slow speed and performs nice basic aerobatics (loops, rolls, cuban eight etc).

After flying the model for the last time with its UK markings during the 2018 BiGGS long-weekend meet in Tongeren, I removed all the stenciling and prepared the fuselage for the wider white side panels associated with the Belgian OO-ZEU in the colors it wore during the 1963 glider World championship in Argentina. The purchase of this modern standard class glider had been largely financed by Belgianís National airline Sabena and even if it wore standard orange/white factory colors, it sported a full array of period Sabena stickers on the fuselage and rudder.

pic real sab

Factory fuselage striping was either narrow or wide, and whilst the German and English versions I reproduced were narrow, this Belgian Foka had the high white variety. After taping off the not to be touched orange panels, sanded away the 2K orange paint as much as I could where the white had to come. I then applied two coats of primer and 3 coats of white to uniformly cover the previous background. Calliegraphics produced a nice set of stencils which were applied on the new fuselage colors and under a wing. Half June 2018 the Foka was ready to replace the crashed Twin Astir in my fleet. At a flight ready weight of 7,4kg the Foka now has a wing load of 92gr/dm≤


Sep 26, 2018, 03:03 AM
The sky is the limit
BAF23's Avatar
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summer 2018 update

A hard landing during spring caused the badly repaired wheel arch to half separate from the fuselage bottom so the previously applied glue was completely removed, a metal piece made to cap the thin ply wheel arch over a greater surface to spread the loads of the wing support pylon, and everything properly secured with polyester matting. After letting the model rest on its wheel during an open door weekend I noted it had deformed the tire so much that it even couldnít rotate anymore in the wheelbay, but after a few days it assumed its round shape again. During the meet in Pottes our respected English friend John told me how to inflate that tire after removing the white dot.

The extremely hot temperatures during the BiGGS meet in Pottes caused the Orastick on the wings and tail to wrinkle so much that I abandoned flying it. Luckily a few members of my club in Zwartberg assured me that it was possible to stretch Orastick just as Oracover, but using much higher temperature on the iron. This proved correct but it is always difficult not to distort the under laying surface especially when it is wood on a foam core. Only two not too prominent bulges marred an otherwise excellent result, much faster than removing the old Orastick and replacing it with something else or glassing the wing. Knowing the next BiGGS meet would take place at Battice where they have a tarmac 60x20m runway, I added an aluminum cover that can be screwed on the aft fuselage skid so as not to damage my model if I have to takeoff or land on a hard surface. That also allows my model to be used occasionally at my club in Zwartberg.

I finalized the angle of the tailplane but still found the elevator response too crisp so I increased the pitch expo to 25% and at the servo arm used the 2nd instead of 3rd hole out to reduce the throw.
A month later I flew it at the BiGGS meet on top of the old fort of Battice near Liege. The first day the wind was blowing hard and the Foka seemed the best choice of my models for that. I should have used the 1/2kg heavier metal wing rod but didnít dare to because at that time I did not know exactly how it would affect the CG (the rod is inserted 2cm behind the sweet spot 76mm CG position). Reverse measuring the CG with both rods after my return home showed it only affected the CG by 2mm so I now know that I can alternate between both carbon and iron rod without changing the ballast. Unfortunately the heavy rod causes the rubber tire to compress too much so this will have to be taken care of before attempting to fly it in that configuration.

A couple of hard landings resulted in no damage other that that of my ego, but I attributed it partly on the uneven ground surface, partly on my late reactions, and partly on the increased pitch expo that reduced the throw I used to apply previously during the flare. Back home I first replaced the trim deflection by a mechanical adjustment to the elevator rod length, then reduced the expo to 20% for the following flights. A slight incision in the 16mm carbon rod showed I further needed to round off the extremities of the copper tube in the fuselage and wings to avoid further damage. After that was done the model was ready again for further refining in flight later in the season.

Bat 18b

The last weekend of August I was invited with a few aircraft and gliders for the 20th anniversary of the RC club of Gemloux. The wind was blowing hard from over a hill and trees so flying was limited to solid machines. I opted for the Foka that performed well if kept at speed, but the turbulence during the first 50 meters of altitude gain during the tow was frightening. I should have used my steel rod instead of the carbon one but all went well and I performed some flowing high speed aerobatics for the crowd. Inspection after both flights did not reveal any obvious problems, the very slight indentation in the carbon rod had not become worse so my rounding-off of the copper sleeves had worked.

meeting 261

During the last annual BiGGs meet at SMAC St Truiden in September 2018 I opened the flying with my Foka. Even with mediocre meteo conditions (heavily covered sky and continuously shifting wind direction) I was getting 10 minutes flights and in between I further fine-tuned my Foka. I was able to let it fly much slower than before by playing with the stab angle and slowly reducing the lead in the nose. After every change I made a CG dive check but constantly got a slightly too quick recovery. I very nearly approached the perfect setup when disaster happened due to my own stupidity.

For the forth flight of the day I had again taken out a bit more lead from the nose and reset the elevator trim to neutral to start the tow (trim had been much up at the end of the previous flight). The tow went well but shortly after release (having to travel 50meters by foot to a safe pilot position), my earphone (around my right ear) dropped out before I had the chance to trim my glider for the new equilibrium. As I also wear expensive hearing aids and the earphone/sunglass combination tends to dislocate that as well when taken off, I stopped walking and instinctively took my right hand from the stick to check if my hearing aid was still in position behind my right ear. As this was the case I looked down to grab my earphone that was dangling on its cable and brought it back to my ear.

Looking up again to check my glider I was surprised to see it pointing down at an angle of about 60į with a lot of speed. I let go the earphone and grabbed my stick again, slowly applying a minimum of up elevator but already during the first second, before I saw any signs of recovery, the port wing snapped and the rest is history. The port wing was recovered intact and showed that the carbon rod had snapped at the place where it had showed that minimal damage after the Battice meet. The fuselage and starboard wing plunged straight down and crashed in a garden of a house where the impact pushed one of the garden-stones a few millimeters down but caused no further damage.
After we recovered each of all the bits and pieces spread all over the garden, we went back and took out whatever could be used again but that was not much.

Pic 2000

This was the end of that nice Foka. Morale of that story is that for any emergency the aviation law states that rule one is: MAINTAIN AIRCRAFT CONTROL, analyse the situation, take proper action. Why I inverted that basic rule remains a mystery, was it because my hearing aid cost more than the glider? Was it because I trusted my glider to glide without inputs like it always had done before (after being trimmed!), did I remove too much lead and the Foka became unstable and with the lower trim setting plunged into a dive and at the least up elevator over-corrected due to aft CG? Would an intact carbon rod have withstood the high-speed recovery attempt? (Irony is that the day before I got new rods to glue in each-other to produce a new wing-key).

Iím afraid all those questions will remain unanswered (or is it yes for all of the above?) and no matter what, my Fliegerland Foka is no more and on their website at that time they indicated a 6 to 9 month waiting time for a new production run. I therefore decided that the only quick solution was to order a 5 meter Foka4 version from a French amateur builder. Within days the deal was made for a kit with much prefabricated sub-assemblies which will permit me to have a Foka flying again early next season. A new thread will be posted on this blog when that larger Foka gets ready.

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