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Jun 15, 2016, 05:46 PM
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Build Log

Restoring a 1980 Grob 103 Twin Astir II

1) the search for a Twin II and a first analysis.

After having neglected the Belgian Air Cadets for decades, a friend of mine suggested I came along with my models for a courtesy visit. Instructors and cadets were very interested and they offered me some time and space to perform demonstrations on the vast Nato reserve airfield of Weelde. An even better deal was that they offered me a couple of winch launches in a Twin Astir, their standard trainer. I try to renew that visit every summer and got the envy to make a model of a Twin I flew in. Because theirs were purchased secondhand on the German market, not 2 of them are painted the same. For ease of seeing it in the air, I opted for the PL91 which in 2013 still sported all-orange flight controls and wingtips.

All BAC Grob 103's are of the Twin AstirII type. A search for a glider model in the 4meter class only came up with Twin Astir III types which have a much different wing shape. Early 2014 I saw an old second hand Twin II for sale by a Dutch modeler and purchased it. Luckily I did because I have encountered none since. That was the good news, the bad news was that the old yellowish model sported rough repairs on the tail and a canopy that held together with Scotch tape over numerous long cracks. The wings were in a nice custom linen bag that also contained wingtip extensions. The 350 euro I had to pay for it was steep considering the overall condition of the 3m55 span model and the very old servo's that were in it, but again, I haven't seen another Twin II advertised since.

The model was kept in stock for two years before I realized that having a spare glider for the 2016 season could come in handy if either the Ka8 or Blanik got serious mishaps. In the meantime my search for a canopy got me a slightly larger full assembly from Germany for 40 euro. I think my model might have been a German WIK kit from the eighties but that is still a wild guess inspired by the WK letters on the starboard vertical tail. The clear canopies are glued over a fully empty blue tub that covers all the goodies in the fuselage front. Following picture shows the badly damaged old canopy and the larger new one that will have to be trimmed to fit over the small tub.

Making up the inventory revealed a serious list of shortcomings. The towhook consisted of a side slit affair that rapidly damages the nylon tow-loops. The release servo was a Robbe digital S181 from which I could not find data regarding strength. The casing around the front wheel had partly separated from the floor, indicating a serious crash on the nose had occurred. Elevator and rudder servos are huge and of Digicont brand, but neither moved when hooked to the tester. The separate aileron servos in the fuselage are Robbe S171 but have different arm (lengths). They work smoothly, but the single big red …... spoiler servo is erratic in operation. The wings looked good but there was no system to hold the thermal tip extensions or short aerobatic tips in place. The extension connection wires bend too much for my comfort but with everything glued solid in the wings I cannot yet see how I'll correct that. The leading edge of the port wing is cranked slightly back where the aileron starts, the starboard wing leading edge is completely straight. The fuselage structurally looks remarkably good but the side hinged plastic rudder has been compressed to the point of permanent deformation with bent and even broken plastic side surface. Worst of all was the tailplane on which a crude attempt to repair it left gross marks without a clear indication of what exactly had been broken. This is a picture of the underside as it was delivered to me.

The Oracover was in a sorry condition, half loose and where the tailplane sits on top of the vertical tail, a kind of very hard automotive type polyester compound had been plastered very roughly over what? A single screw was supposed to keep everything in place and felt very solid. The top of the stab looked a bit better but here again the whole balsa surface felt flimsy, without ribs and not very airfoil shaped anymore. At least the exposed wood showed that it had not been broken, but the condemned useless screwhole extremities puzzle me. The aft one is almost on top of the rudder and could impossibly have engaged in the vertical stab, the front one just takes on a pin to keep it centered.

2) Restoring the subassembly's

As the wings looked relatively good I decided to at least for the first trim flight to leave them as as they were and first completely overhaul all the rest. The stripping of the original German markings went well with the use of a sharp knife, tweezers and liberal amounts of cleaning benzene. The typical Twin Astir red and blue patterned panels on the nose were a different story. Those had been factory painted and the only way to remove them was to sand them away. This had to be done because the orange that is used for the BAC gliders and tugs doesn't cover too well and the paint lines would have continued to show through even after using a primer. The more fundamental could then be taken care of.

First job was to completely strip the horizontal tail-plane and elevator. This was further complicated because I could not find a way to separate both from each other. Heavy sanding over the polyester clads finally got it into shape, span-wise depressions were filled with lightweight filler to resurrect the symmetrical airfoils. Pore-filler was then applied over the completed surfaces to increase their shape strength. After finishing with light sandpaper I got a much more resistant uniform surface that was ready to be primed in white before being covered with Oracover.

Getting the rudder into shape required the insertion of a triangular shaped custom balsa block through the a top corner large opening that shouldn't have been there. The lack of any inside structure allowed the balsa wedge block to be pushed into position with a long screwdriver. I made a small rectangular cut about in the middle where the block would come. First purpose was to fine position the block against the back of the rudder, thereby pushing the plastic sides back into their original shape. Second purpose was to allow me to apply PU glue on the block before the final slide into position. The operation went well, weighed peanuts and required minimal filler to obtain the original straight surfaces on both sides.

With the tail still open I saw a possibility to modify the rigid fuselage molded tail bumper to allow the use of a wheel as per real aeroplane. At my main airfield I operate from a tarmac surface so that tail bumper could have been gone after 25 takeoffs and landings. I found an appropriate 25mm plastic wheel in the scrapbox, Dremeled an adequate hole so it could be inserted through the inside of the fuselage, and drilled holes for a 2,5mm screw to pass through everything to complete the conversion from skid into tailwheel. The choice of a plastic wheel may appear odd but is fully motivated. Firstly plastic wheels are more narrow and it had to fit between the existing bumper walls. Secondly I recently encountered problems on my Blanik and large Ka8 because the rubber tailwheels had so much lateral ground friction that they sheared off the complete assembly during groundloops or whilst turning sharp corners during ground towing. The plastic wheel looks scale, allows free wheeling when necessary on any surface, but is also slick enough to slide laterally over any type of surface in case of groundloop. The system to actuate the elevator on top of the tail seemed solid enough and even more important, had no play in it, so it was kept the way it was.

Removing the side hook was no problem, but the nose cone had been weighted with small metal balls into a hot-glue type of substance that filled it all (for about 100gr weight). If I wanted a towhook to be installed at the front of the nose as per full size Twin Astir, all that gunk had to go. Prodding from the inside risked damaging the sides of the nose so I Dremeled off the tip of nose extremity from the outside first. That allowed my to drill an ever larger hole and thereby pushing the lead and glue out through the interior. When I had the 10mm channel all the way through, it was easier to remove the remainder of the side material with a screwdriver. A new pushrod was made for the length between the reversed nosehook assembly (because it hugged the inside of the nosetip so well that way and that had worked on the Blanik as well) and the original servo which seemed to have sufficient pulling force.

Liberal amounts of PU glue was then applied all around the inside of the nose, as well as on the conical part of the nosehook assembly. The latter was then pushed through as far forward as possible, exposing about 1cm of aluminum through the nose opening. The assembly was then allowed to dry overnight, the tail hung-up vertically with paper towels on the floor to collect excess glue that dripped off the nose. Excess metal was then Dremeled off and a file used to create soft corners around the nose opening. Because the 10mm tube is larger than the scale dimension and model gliders are towed higher versus the towship compared to their full size counterparts, I made the hook opening mainly on the underside portion of the nosecone so the hole isn't be that much visible and the aluminum tube ends up in the prolongation of the tow cable. Further anchoring of the hook and additional weights, plus solidification of the nose will be done only during the weight and balance stage at the very end of the restoration.

Investigating the dead servos revealed that at one point, extension wires had been soldered in reverse. The servos operated when the plug got turned around, yellow signal wire in the negative port and black wire in the signal port. After soldering everything the correct way (yellow into black and black into blue), I discovered that the servo tester caused the elevator servo to turn around forever in one direction. With electronically reduced throw the end courses were respected, but I didn't trust it anymore. Somebody probably discovered that before because a red sticker had been applied to the side of that servo. Luckily I found out that an old Simprop Contest servo in my spares box fitted perfectly on the particular Digicont servo chassis. The Simprop was then used for the elevator and the remaining Digicont moved to the less critical rudder position. Extensive run on the tester revealed no further shortcomings in their operation.

In the meantime I did cut the instrument consoles from the too large cockpit bathtub and used a combination of tape and filler to get them smooth (they were in fact two halves that didn't blend too well) and fit to be glued on the original bathtub. After much rubbing they were ready for painting but I had to look at my cockpit pictures to find out that again none of the BAC Twin Astirs had the same instrument panels. Some were overall black, others overall gray, and the mushroom type pedestals also differed in sunshade shapes. Luckily I had many detailed pictures of the PL91's front and back instrument pedestals so I could produce a relatively scale reproduction of the real glider.

My early thoughts went into producing a Twin Astir flying solo with only a cadet in the front seat, but I soon realized that the enormous bathtub would look very empty and would necessitate many scale aspects of the seats etc to be built. Expecting the need for weight in the front I thus changed my mind and looked for an appropriate instructor for the backseat. The older looking pilot weighing more, I put him in the front but will have to use paint to have the back guy look older. The busts I found in my stockpile were of the upright type and looked anything but glider pilots laying in a modern glider. I did as on other previous projects and used the bust as a starting point and a foam isolation shapes to form the missing body parts. First step is to draw and cut the foam in one plane, and gluing the torso or busts at the required angle to it.

At that stage I already had fashioned the rubber protections around the instrument glare shield, glued the photo cutouts of the downsized instrument panels, glued the pedestals in place, installed balsa headrests and the Flarm fairings on top of the pedestals. All these jobs were alternated by the spraying of the fuselage (requiring 24hrs between layers of white lacquer) and applying orange Oracover on the rudder and elevator, white Oracover on the lower side of the horizontal tail and Scale White Oracover (with aluminum backing to make it opaque) on the topside of the horizontal stab.

Last edited by BAF23; Jun 28, 2017 at 04:08 PM.
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Jun 15, 2016, 06:06 PM
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Assembly of all bits

Awaiting things to dry I inspected the wings thoroughly. Extensive use of the iron was necessary to eliminate all wrinkles or loose cover material that had been on there since more than a couple of decades. The underside was still white and hardly needed attention. The top part had become yellow due to exposure to the sun but I didn't feel much for a complete strip the wings, I sprayed three layers of white lacquer only over the top. The aileron of the port wing only moved about 10° up but I had no access to the system in the wing where the movement emanates from. Nothing could be adjusted so the only solution was to remove the aileron and shorten its chord by removing 2mm of wood at the upper pivot point. By reinstalling that aileron as close as possible to the wing, the unknown internal system now pushed the lower aileron just a bit further out so as to obtain about 15° of up aileron, still not much for an aerobatic capable glider. Next was the messy part of further shaping the missing pilots' body parts with a knife. This is delicate work because the foam is very brittle and prone to breaking. The shaping is done before the pilots' foam base parts are glued to the bathtub. With the front pilot having arms next to his body, I arranged for the instructor to have a hand around the stick and the other on the trim lever.

To merge the foam with the plastic parts and to further refine the shapes, I used quite a bit of lightweight filler in subsequent layers to obtain a better overall result. Whilst that required much intermediate drying I continued on the wings. The removable tips were solid wood so I just rubbed the shiny red paint away before applying white primer to create a uniform base for the orange Oracover. The 27cm optional wing extensions were red and white, thus forming no good base for applying orange Oracover directly over them. I used white primer all over these surfaces to obtain a nearly uniform base color for a new start. Working on so many sub-assemblies at the same time started to clutter up a good deal of my apartment and many transits were made to the garage for the spray-jobs because that week there were daily showers and thunderstorms. Nothing dried and I often had to quickly close the garage door to avoid that wind gusts would send dust into the still drying surfaces. My car remained parked outside for 3 days, subjected to occasional hail, but luckily without damage. In the meantime I spent my time refining the cockpit interior using detailed pictures of the real PL91 interior.

The pilots were finished in standard air cadets coveralls and sit behind accurate instrument panels with stby compass and FLARM units on top. They have correct sticks, spoiler and trim levers, headrests and side placards in their large greenhouse. While it is not 100% scale, the result is stunning and a major improvement versus the empty shells I started with. I then left for a glider happening in Germany and had the white lacker dry for a week before I touched it any further. Upon return I then taped the nose so I could paint it (and the wooden removable wingtips) with Revel nr30 which is nearly identical to the orange Oracover I used for the wingtips and ailerons. I then applied the Callie graphics and assembled the model completely for the first time. It is a beauty and few people would believe it all originated in a very sorry condition 1980 model.

Its 3m58span is already impressive and it looks very scale that way, but isn't and won't pass close scrutiny by the self-proclaimed scale police. The additional 55cm of the optional wingtip extension will only be tested at a later stage when thermaling will take precedence over scale aspect and roll behavior. Following entry will cover the installation of electronics, the weigh and balance and the first test flights during the summer of 2016 (if we'll have a summer).
Last edited by BAF23; Jun 19, 2016 at 04:06 PM.
Jun 23, 2016, 04:25 PM
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Making it flight ready

There is ample space in the forward fuselage to insert whatever is needed. I knew that even with the heavier canopy, the model would need weight in the nose but I was not prepared for the amount. Instead of stuffing much useless lead, I preferred to use a large useful battery. With this glider I wasn't going the double everything for safety so I decided on a single battery as much forward as possible. Physical restrictions were the aft of the hook assembly, the narrow nose and the front wheel inner arch.

Although a first rough balancing on the fingertips along a quarter of the chord revealed that about half a kilo weight would be needed, 6S batteries could not be used because they are too wide and high to be pushed sufficiently forward, thereby reducing the effect of their heavy weight to create sufficient nose weight. My common 4S4000 batteries fitted just between hook assembly and forward wheel fairing and still allowed the cockpit assembly to be lowered in the nose, the former two blocked the battery fore- and aft, the latter the up and down movements. Four foam blocks were carved and glued inside the fuselage to laterally block the battery.

Whilst that was drying I had time to connect the servo leads to a 6-channel receiver and program the Taranis transmitter for that additional model. A 4amp BEC was then soldered to an EC5 plug (because All my 4S+ batteries use those plugs) and with power to the receiver I was able to adjust all pushrod lengths for neutral positions, and the holes and limits for acceptable throws. The receiver was then Velcroed to the flat spot between the two large servos, and the antenna positioned at 90° angles on the lowest point of the fuselage for minimal inflight signal blocking by the battery or other electronics. A high precision variometer/altimeter was then connected to the receiver, and together with the BEC they were attached with double sided geltape as far forward as practicable.

When all that had dried I measured the wing which came out at 75 square decimeter surface, and the 25 and 33% MAC cg points calculated as 6,5 and 8cm from the leading edge. For the maiden I choose 7cm and prepared my SIG balancer for that value. Although the 4S battery weighed 400gr, I needed another 230gr in the nose for a correct balance. The plateau I had glued on top of the battery in the nose was insufficient to take all that weight so I decided to pour steel blast balls with PU glue to fill the nose cavity around the hook mechanism. With the model vertically on its nose, the glue from the compartment above the plateau found its way around the latter and filled the lower nose much more than planned, resulting in the hook mechanism getting in touch with the PU glue. Having applied that at midnight so it could dry all night, I now was facing staying up for another 4 hours to constantly eliminate excess glue from around the mechanism and activating the rod every 10 minutes so it wouldn't seize.

With the PU glue expanding during the drying this was a cumbersome, messy and annoying task, aggravated by PU glue sliding in the aluminum tube and leaving through the nose standing on a rag. Because all this had to be done with toothpicks and a long screwdriver in the deepest of a nose (from inside and outside), I had my flashlight in my mouth most of the time and off course that battery ran flat at a critical moment, I had to replace it with the battery of the television's remote but the hook moving again before it seized. By 3 in the morning I felt confident that the worst had been adverted and 95% of the excess glue had been removed. The steel balls were already firmly in place and I took the decision to spray some WD40 penetrating oil from within the tube to prevent any remaining glue to grab tight during the night. A typical modeler's evening of self inflicted troubles, not exactly the pleasures you expect from a hobby, but nevertheless very challenging and rewarding.

Next morning I felt relieved that the mechanism operated freely. Having no factory documentation nor other user's figures about the setup or CG, the final adjustments plus weight and balance were made solely on common sense. The battery and steel-balls filled nose helped so much that I only needed to add an 80gram of lead plaque that I Velcroed to the bottom under the battery in between the foam blocks. That is in order to keep it physically in place regardless of the flight movements, but still be removable so it can be adapted to alter the CG according to the results of the flight testing. With the weight settled, I weighted and measured the model, first without and then with the wing extensions. Span changes from 3m58 to 4m11, port wing weight from 1050 to 1085 grams, starboard wing from 1070 to 1110 grams. The fuselage/tail/cockpit/battery is a whopping 3430 grams, bringing flight weight to 5550gr/5590gr. When divided by the respective 75/84 dmē wing areas, this produces a wing loading of 74gr/dmē in standard mode and only 66gr/dmē with wing extensions installed. I still have to find a practical way to keep those wingtip options firmly in place during flight but I figure the standard tips will stay put during the first flights. The method used by many large-glider modelers is to tape the outer wings to the inner ones, but they have those solid white GFK/GRP wings while I have orange colored Oracover on wood and don't want to risk peeling the former from the latter when removing the orange or clear tape. Stay tuned for the imminent flight test results.

Last edited by BAF23; Jun 24, 2016 at 02:58 AM.
Jul 13, 2016, 04:50 PM
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Flight tests

The second Saturday of July 2016 I saw many tow-aircraft being readied on the live webcam of my club in Tongeren. An hour later I was on the field and after assembly, pre-flighted the Twin Astir II for its maiden after ?? years of inactivity. I talked to the most qualified tow pilot around and asked him if despite the crosswind he was willing to take me up for the maiden. When they were towing and calling me to join I waved them off, nobody can push me around when I'm performing pre-maiden checks including range and fail-safe checks. During the next tow session I was ready and after a short briefing of the tow pilot regarding the non-standard tow pattern he opened up the throttle and the Twin Astir slowly got in the air, assuming a normal slightly high position on the cable.

The tow went remarkably smooth but visibility in the air was terrible. Our altimeter readouts coincided and after I announced to release, the model quickly stabilized in a stable fast glide. A few club members who stood next to me commented on that speed, but I told them I first wanted to check the behavior and trim before slowing down. Pitch, rudder and aileron authority were more than adequate for normal flying, turn reversals didn't produce much adverse yaw, spoiler operations little pitch change. I then turned upwind and slowly raised the nose to level, let the airspeed bleed off and at the first sight of a nose dip, recovered from the stall. As this went smooth I raised the nose again but this time allowed the stick to come back much more into a full stall. A benign wing-drop occurred at a much slower speed than expected, demonstrating the docile handling of this glider without wing extensions mounted.

I then trimmed the elevator to produce a more typical reasonable gliding speed which again seemed stable so after turning upwind again I pushed the glider into a 45° dive to check the CG. It recovered rather quickly which confirmed the forward CG limit I had calculated and setup for this maiden flight. I was very pleased with the flight progress but as usual, preferred to cut it short for a inspection before exploring the envelope any further. I entered downwind at 60m, flew a rectangular pattern during which I played with the spoilers before stabilizing for a long straight-in with half-spoilers. Final and landing had no surprises, this was a nice addition to my glider collection.

After some slight mechanical adjustments of the flight controls to conform with zero trim settings, I cut off some lead from the front block and inspected all hinges and flight control play around the new neutrals. During the next tow I had a little more trouble staying in position and even lost sight of the bank angle of my Twin being towed straight under a cloudless portion of the sky. When I told Jeroen I was going to disconnect for that, he told me I was doing fine and shortly thereafter I regained visual on the wings and stayed in tow for another turn till we both agreed to disconnect with a cloud as background. This time the glider pitched up rather violently upon release, and pitch control seemed erratic when I had to push the nose down. A simple stall confirmed the CG seemed still within limits, but to my surprise I ran out of down-trim to stabilize the glide, although a new CG dive still resulted in a too-fast recovery.

Something was definitely wrong so I decided to bring her in without delay and chose to fly a much more flat pattern requiring no spoilers, in fact eyeballing a continuous clean glide with minimal control inputs till touchdown. The wind didn't allow that and sometimes pushed the nose up, which couldn't be checked without applying an exaggerated portion of nose down elevator, at which point it developed a too-steep glide angle. I was fighting the controls to stabilize the glide and a lull in the wind came in handy to make a smooth touchdown in clean configuration. Time for a break and analysis of the problem. CG position wasn't the problem, so it must be either the elevator servo or the transmission of the movement to the tail (obvious due to my inability to trim correctly). The culprit was found in the form of the actuator rod rubbing against the opening for it on the top of the tail. Using the knife to cut off a 2mm slice on one side of the hole resolved the problem, that went by unnoticed during the first flight because the more forward CG caused the narrow part of the actuator not to rub the tail close to the neutral elevator position. I moved the lead weight a bit more to the back, gave the model another once-over, and awaited for a towship to prepare.

This time it was another Monsun flown by a pilot I didn't know and he first made a practice flight before I hooked on. He towed much faster and wasn't as smooth, but I managed to stay behind till release at around 200m. The elevator and trim now responded as they should, and after trimming the plane, performing a stall and CG check (still a bit nose heavy), I switched the vario-audio on and started searching for thermals. I caught a few at 100 meters and climbed to 180m again. I never had expected the short-wing configuration to thermal well in such mediocre weather, but to my surprise stayed airborne for 10 minutes before making a nice precision landing after a very steady final glide path. A few beers to celebrate that were next but I first disassembled and stowed my latest glider.

A few days later I swapped the rudder and elevator servos because I had a problem with the servo arm orientation and lengths. The lead was again moved a bit back in the nose, small rings were glued over the wing aft carbon rods because they tended to go deeper in the wing instead of penetrating till halfway the fuselage diameter during field assembly. I also designed a quick way to swap short wing stubs for the extensions, without using tools but ensuring they would remain in position during flight. Using small flat metal strips I found in the scrap-box, I bolted one end in the underside of the wing, the other end was bent down 100° and slits were cut in the correct positions of the tips and extensions for the bent-down tips to grab into. The metal is light and can be lifted by nail to allow the outer items to slide out. Here is a picture of this home-made system before it was painted orange to make it less obvious.

The model is now ready to be taken along to an annual international large-glider gathering along the French border at Pottes, where I will continue exploring the flight envelope using the wing extensions.

Although the wing extensions promised a lowering of the wingload from 72 to 66gr/dmē, a couple of flight in moderate thermal conditions revealed little practical improvements (I had made a 10min flight on short wings just before). As expected the ailerons were even less efficient and barely adequate for the job in anything but smooth air conditions. Next day I switched to the short configuration and explored the aerobatic capabilities. Elevator power was perfect, rudder efficiency not that good for stall turns, but ailerons totally unsuitable, barely allowing to pass an aileron roll or barrel roll even at high speed.

The problem of the ailerons is that as already mentioned earlier, the up deflection is (mechanically) minimal and cannot be corrected without major surgery within the wings. The only thing I can try is to eliminate electronic aileron differential and substitute it by rudder mix. This could very well increase the roll rate substantially without affecting the gliding characteristics, but care will have to be taken not to let the speed drop of too much in the pattern or flare because the higher deflection down will quickly bring that wing into a stalled condition.

Final flight configuration

In August during a glider meet in the North of Holland I first flew the Twin Astir with eliminated aileron differential and rudder to aileron mix. It now requires a bit more manual rudder during turn reversals but the roll-rate became such that normal glider aerobatics can be easily flown with the short wings, whist still benefitting adequate thermal qualities and landing speed. Under the same conditions I added the long wings but found out the roll-rate to be disappointing, directional stability reduced, and little gain in thermalling abilities. The wing extentions were quickly removed again and now lay deep in the shelves, probably never to be used again.

That 35 year-old model will never be my favorite but looks very attractive, flies very well with the short wings and doesn't take much space to be carried along to events as a spare or sport glider. Be patient for inflight pictures and videos after I find a photographer for that job.
Last edited by BAF23; Aug 25, 2016 at 07:40 AM.
May 04, 2017, 10:54 AM
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Follow on report and airborne pictures

The elimination of electronic differential aileron and substitution of an aileron to rudder mix greatly enhanced the model's flight characteristics. The roll rate is now sufficient to perform scale-like aerobatics, including aileron roll and split-S in short wing configuration. Even with these changes, flight characteristics with long wings still were marginal, mostly due to poor directional stability. The wing extensions were therefore permanently redirected to the stock bin. With the short wings, the model now flies and thermals so well that the Wik Twin Astir replaced my Flair Ka8b as the weather-check/new field explorer. I can compete rather well against larger and much more expensive modern plastic gliders, and on the ground it looks much different from the omnipresent ASW and Nimbus type gliders. September 2017 I had flights up to half an hour during the Bastogne meet

The Twin Astir is very stable under tow but difficult to follow at altitude due to its predominently white wings and slim body, 300 meters under tow and 400m in thermals is the practical limit for my eyesight. Spoilers are effective but aerobatics are also a way to come down if the thermals are too strong.

The scale-like aerobatics are a delight now but its looks in the air are just soo real that you have to look twice if this is the model or the full size Air Cadet Twin Astir II. Here is a picture of it during a landing at the BiGGS gathering of Odoorn in 2017

The end

After another year of many successful flights during which I was able to alternate between thermaling and aerobatics, I started using it as a proving model if I had to regain proficiency or tryout a new airfield or towship (pilot). During one such flights in May 2018 at Enghien the towship lost altitude after the second (too far away) turn in mediocre visibility (sun haze), picked up speed and as I saw the Twin Astir catch up I disconnected. Somewhere the cable must have caught a wing because as he started climbing again I saw my glider making a violent 180° snap turn around its vertical axle, after which it dove straight into the ground from 200m without me having any form of control anymore. As I approached the debris in the nearby field it just confirmed this was a total loss. It might not be visible on the picture but not a single surface nor structure was kept free of serious cracks. A sorry end for this fine model.
Last edited by BAF23; Oct 13, 2019 at 09:43 AM.

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