|Jul 18, 2013, 01:53 PM|
Parkzone Ka8b glider
On the 13th of October 1969 when I got towed the by a Stampe SV4, for unknown reasons the cable snapped at the towship side about 100ft high. I released the remaining cable and landed with full spoilers after a sidestep onto the hard runway, just short of the field end. My logbook shows it was with PL54 and I flew the same airframe a month later during my precision landing tests to obtain my glider pilot license. PL54 also had extra orange colored ailerons so the choice as a model was a no brainer.
Picture of the real Ka8 around the turn of the century at Weelde airfield
The 3,5m span Phoenix model was tempting but maybe too big a step so when Parkzone announced the release of their 2m25 foamy I ordered one at my LHS. It is a very nice ARF kit that will satisfy most flyers as coming out of the box. After analyzing the contents I quickly made up my mind about some changes. Luckily only the nose and tail had been factory painted, the rest of the lines and wingtips were adhesives which could be carefully removed. The z-foam fuselage has been factory coated and care must be taken during the peel off of the adhesives because it is easy to tear complete little foam balls out of their pressings. The wing leading edges needed sanding to get rid of unaerodynamic mold lines, and I didn’t like the mold stars sticking out all over the wings, the many mold hole openings on the bottom where the spars and other reinforcements were, the tape over the too deeply pushed aileron servo channel cover, the lack of cover over the spoiler actuator rod, and the spoiler’s grey color.
I first removed the clear tape holding the aileron servo wire cover into place, tested all servo’s, then hot glued the foam channel cover so they stuck out just a bit. I then preassembled both wings and with only one spoiler connecter, adjusted the throttle travel so movements coincided with the travel without slack nor excess push. I then connected the second spoiler to their single servo, mechanically adjusting the clevis so the spoiler movements match each other perfectly. Just a tad of early deployment of one versus the other already could cause rolling movements with less than full deployment. Only after those final spoiler adjustments was I able to glue scrap foam to fill just the top of the spoiler actuator gutter (wire has to remain free to move). After filling all the intrados holes with lightweight Perfax super filler, sanding the complete wings was easy but I left the factory plastic tape over the aileron pivot joints on the extrados. With the spoilers extended I also took the opportunity to paint the complete assembly white, and also did that to the aileron servo’s and their plastic covers. Why Parkzone hadn’t specified white plastic instead of grey when they had those parts manufactured remains a mystery to me, and aesthetically is anything but pleasing. As per my real Ka8, the spoiler sides will be painted orange, making them very visible when extended.
Whilst all this drying, filling and sanding took place I used the time in between to tackle the fuselage. Pushrods running back to the elevator and rudder were factory installed in their own guides, so the very ugly foam protuberances where they leave the fuselage were cut away flush, and openings filled for continuity. The transparent canopy got separated from its foundation so a more scale looking pilot (almost twice the size and width) got incorporated more backwards (as per real pictures) and a picture of a period dashboard was glued on the ungainly looking Parkzone silver? Interior. Filler also was used on the vertical tail carbon rod holes and to flush the bottom of the stabilizer where a carbon strip caused a rough surface. If you don’t plan on vinyling or painting the surfaces, abide from sanding the factory surfaces, these are glass smooth and even 600 grade sandpaper quickly spoils the surface and causes thin fibers to appear all over.
Covering one complete wing with a single piece of wrinkle free white vinyl proved very difficult but possible. It took me three hours per wing but this is not only for aesthetics, the flight control hinges and overall foam strength were seriously improved in the process. The empty wing weight raised from 195gr to 240gr in the process, but I don’t care. Parkzone made a slow glider optimized for close by thermals. In glider world, the heavier the more range (hence the carriage of water ballast in performance gliders) so I’ll fly a bit faster as other foam Ka8’s but cover more distance per altitude. The additional strength of vinyled wings are a necessity because our club aircraft mainly tow rather large and heavy gliders, and I’m afraid of wing flutter with this ultra-light with all too flexible washout at the wing tips. I then removed and painted all flight control horns and backplates (why did they glue those when they also use screws?), and painted the spoiler plates. Strange they used clear tape reinforcements on the aileron and rudder hinges, but none at the critical elevator. The U wire between left and right elevator helps, but the foam is so light they still twist somewhat apart under load.
Vinyling the fuselage was not easy and I opted to do the tail first, forward to the point the pushrods come out of the foam, and made a vertical cut there. I then used a shoulder to shoulder joint (no overlap) with the main portion forward of the pushrod exit, and cut off the white slant nose part a 5mm further back from the red nose, in prolongation of the removable cockpit section. The cockpit section already has a questionable engineering, why didn’t they make the clear canopy as large as the foam support? From the box we end up with 3 distinct lines, one between the red and the white (I eliminated that by cheating the paint line), one where the canopy assemble separates, and one between the foam support and the clear part, all within one cm. A bit more thinking between plan and production could have produced a much better aesthetical result without extra cost. The same applies to the choice of pilot, so small in scale he could stand up in his cockpit or fit a passenger next to him. Luckily after no problem removing the hot glued clear canopy, The pilot also came off easily and was replaced by a larger one.
On the internet I also found a picture of wooden old style instrument panel picture, with period instruments in km/h, m/s, altitude in meters and a compass. No radio, no beeper gizmo’s, our Ka8’s had nothing at that time, and we also stayed in the air for more than 5 hours. Together with the tail emblem the instrument panel got properly resized on a word document and printed on a narrow strip of Testors white decal paper. After a while I sprayed the protective coat on it and let it dry overnight. Next morning I cut out the parts of the custom decals I wanted to use and they separated well after about 30 seconds in the water. The pilot got painted in orange coveralls (the early Vietnam type we used well into the seventies) and his arms and legs painted on the flat cockpit section. After assembly with hot glue it again became obvious the white factory canopy paint comes off much too easily so care has to be taken to apply anything over it in one go. I also replaced the loose fitting too large landing wheel with a smaller more scale looking one that fits without play on the axle.
Original wheel was more than twice the size of the too small pilot of the box
On the model, contrary to the wing and fuselage striping, the rudder and nose are just painted plain red. That paint doesn’t adhere too well on the foam but vinyling the rudder over it was acceptable. The nose posed a totally different problem, how hard I tried even with the heat gun over the vinyl, after 3 attempts to stretch the material around the complex variable nose shape, I gave up. In the meantime much of the paint had come off, including the smooth top layer. I had no other choice as to paint the nose orange, with successive coats so the difference between the original painted surfaces and the denuded plaques soaking every paint up became less visible. In the meantime, the back ordered tow-hook release servo arrived but contrary to the instruction notice, no screws were provided. I removed the bottom panel, got rid of the ballast weight and after testing, installed the tow release mechanism. I was used to normal servo’s that either would be open or closed, this thing works differently, just opens temporarily and immediately closed. The tow cable has to make a 90° angle around a rather sharp plastic cutout, something the towpilots don’t like because it prematurely weakens their nylon loops. I therefore hotglued a cut through length of shrink tube around the front of the cutout.
After the last aesthetic touches I started assembling the parts. My initial idea had been to stock and transport the model with both wings attached to each other. Factors driving me to that were the difficulty getting the spoiler links adjusted and connected to the single servo, my choice to connect the ailerons separately to my second hand AR600-X receiver so I can dial in electronic differential, and the possibility to eventually connect all 3 wing servo’s through a single Multiplex connector to the fuselage receiver. The wires of the rudder and elevator servos are very short and much limit the location of the receiver, causing plugging in additional wing servo wires a difficult and delicate job on a daily basis. With the single carbon spar between the wings, and both wings screwed onto the cover plate, I was very surprised how loose that assembly still felt. I toyed with the idea of an additional 6mm wing spar joining both factory carbon tubes running almost the whole wingspan, but had to abandon because the dihedral prevented a straight joint. I’m sure the wing will be sufficiently fastened on the fuselage by the screws, but I would have preferred a stronger unit feel when separated in pairs. The manual is anything but clear concerning the supplied screws and the self-tapping screws for the wing into the cover plate are on the limit concerning length, I used two M3 roundels because I felt they were ready to perforate the plastic at a very visible place. The long one for fixing the horizontal tail works perfectly, and the short ones are for screwing the wing joiner to the fuselage. Spare screws are provide an were put into a small bag labeled Ka8 for possible future use.
After the first tries connecting everything it became obvious my choice proved impractical. Joined wings without fuselage felt too delicate so I opted to transport them separately. On the port wing I cut off the JR plugs and soldered the short wires of the aileron and spoilers into a single 6-pin Multiplex plug. The fuselage side of the plug is taped to the side of the open space towards the wing, together with the extension wire for the starboard aileron servo, making even individual wing connection practicable and error proof. After verifying the correct functioning of the connections I used copious amounts of liquid rubber to make sure the delicate wires and pins would stay out of each other’s harm during the regular plugging and unplugging. All fuselage wires now can remain continuously connected to the receiver ports.
Then came the choice of power source. Having bought the model PNP I had various options, going the original way with a 2s1300 Lipo through a voltage regulator, or a Nimh 4,8 pack straight into the receiver, one my common 2S950 mah Bec batteries, or one of my common 3S1800 batteries with BEC or even ESC from the scrap box. Electronically, they all provide more than enough, the choice depends on personal preference and what the results of the fully assembled weight and balance displays. Due to vinyling of the complete glider, it was obvious the CG moved back and weight had to be added forward. Now was the time to experiment to get adequate power for a day of gliding , whilst balancing the airframe in the process. In the end I choose for a 100gr 2S2100 battery, connected via a Deans plug to an on-off switch under the easily removable canopy, then to a 4A S-BEC reducing to 5 Volt, then into the receiver. That will allow me to charge the battery overnight, leave it connected in the plane all day, just switching it on for flying when it’s my turn to be towed. The thin battery slides nicely into the slot and is blocked sideways by the Deans plug being pushed in between, and it’s length is perfect for the foam canopy bucket to block any possibility of moving backwards in flight, a rather important issue when being towed at very steep angles by powerful aircraft. The S-Bec could be slid between the towhook release servo and foam, so everything was tight and as much forward as possible.
On-off switch near servo’s, receiver forward right, battery middle for 75mm CG.
With my Sig balancer adjusted to 75mm as per instruction manual, I noticed the model to be slightly nose heavy. Adding temporary small foam blocks and fastening them with scotch tape to the battery quickly got everything right. Depositing the flight ready model on the scale I noted an increase from the factory 786gr to my 1020gram. Unfortunately, the wing area is not mentioned anywhere so wing loading cannot be calculated. The second Graupner ASW22 I bought a couple of months ago to get my first experiences being towed, weighs almost twice as much for a similar wingspan so I have no doubts about the flying and gliding abilities of my 12% overweight Ka8b.
My completed model spoilers extended in company of my other weekly models
After a week’s work I drove to the field and first had it held high by a glider pilot running, then he threw it horizontal twice to grossly asses the flying capabilities. I erroneously corrected the sensitive pitch by dialing more elevator expo, but hadn’t caught the fact the sensitivity happened just after “high speed” launch, but after a few seconds was gone and elevator control became excellent at lower speeds. The guys said the Ka8 seemed ready for its first tow. That day it was a very powerful petrol engine towship and I asked the pilot to keep the speed low for my foamies' maiden. We were surrounded by experienced glider pilots and right after take-off I noticed it still overpitched at the slightest stick input. Despite that I was capable of making 2 turns behind the towship and released normally. The pitching continued and I even ended up almost vertical (up and down) for no obvious reasons. Slowing down the plane near stall and applying a copious amount of downtrim made her stable and I brought her in for a textbook landing in the middle of the runway, with just ¼ spoiler.
With the other pilots pointing out at possible tailplane incident angle problems as a cause, I detected no obvious damage anywhere despite some serious wing flex during the tow. To increase stability I removed the foam bits I had taped behind the battery and slid it completely forward in the nose. I hooked up for a second tow, hoping I had taken care of the worst, but the result was the same and I disconnected at altitude when the tow pilot stalled out from flying as slow as he could, pulling me up at probably a 60° angle. With the slight down angle of the elevator after landing we concluded it must be tail heavy and I inserted 25 gram of lead in the nose. This time the tow pilot and I were determined to get to sufficient altitude so I could perform the 45° dive test after being trimmed. As I released at altitude my glider again went hard up and even completely nose down before I could stabilize her at a slow speed. After trimming it hands off (my hands in the air for the other pilots to see), I pushed the nose over and let go. Instead of slowly recovering, the dive just increased till 90° and more. I tried it twice starting from stable gliding flight, and twice got the same result. Landing smoothly again was no problem but I refused another tow before I corrected that problem. I put the Ka8 aside because being a motor instructor I had some teaching to do on that busy day.
Different club members later came up with different solutions for my problem, but later at home I analyzed it as a pure CG problem which I could have detected from the first hand launch, but failed to do so because I believed in the 75mm CG figures of the Parkzone instruction sheet. I put the airplane on the balancer again and found out the last flight had been performed with 65mm CG. That was already 1cm forward with little result, so I decided to approach the problem from a different side. Moving the CG to 55mm (for a 21cm wing root and 10cm tip chord with a straight leading edge) still seemed more than acceptable and if too much forward, at least the Ka8 would be more stable during the tow. Experimenting I found out with the battery fully forward I needed 75 gram of lead to get to the 55mm CG, and removed the tow servo mechanism, carved out the nose portion just forward of it and added the compacted 25 gram of lead there, for the other 50 gram I used lead plates slid under the forward part of the battery. Nothing being glued, it can be easily adapted later but in the meantime is held firmly in position. Because we fly off a tarmac surface I added a wooden skid plate (as per original) because the plastic on the forward nose got worn too rapidly during takeoff and landing.
The third flight went much better and the tow was finally flown with normal stability and pitch response. The stall check demonstrated with that forward CG I still had sufficient up elevator available for the landing flare. The 45° CG dive test showed neutral stability, it just kept going at the same angle so I’ll put an additional 25 grams in the nose before the next flight with about 50mm CG position. For the rest the flying capabilities were much more stable and a precision landing with half spoilers finished this flight. There were no thermals that day and the long dive check substantially shortened the possible flight duration. After retrimming in the air I noticed the elevator is now better streamlined with the horizontal stabilizer (instead of showing down after the first two flights. I presently fly with the linkages as per suggested servo and horn holes in the Parkzone manual, and use a 35% aileron differential in conjunction with 25% expo, the elevator at 35% expo and the rudder at 20%. I still have to figure out the perfect spoiler to elevator mix and when I finally tweaked out all the settings I will amend this message to reflect those values. In the meantime just enjoy the comparison between my model and the original
After about 20 towed flights I'm getting the hang of it, but it still requires uttermost concentration and quick but smooth control inputs to remain in position behind the powerful tow aircraft in our club. The Parkzone Ka8b being much more at ease at the lower end of the speedrange, tow pilots are reminded before each tow to keep their speed as slow as practicable. Hooking on the cable is highly impractical if using the recommended Parkzone system. The cable loop has to be inserted vertically (bottom to top) through a square plastic cutout on the bottom of the skid area. With the pin only retracting for about a second when commanded, there is no way to get the connection done lifting the plane up, activating the switch on the transmitter and inserting the nylon loop within that one second. Two people (at least three hands) can do it, or you have to turn your Ka8 upside down and let it rest inverted on the grass or even worse, tarmac, using one hand to position the nylon loop against the towpin pushing slightly, whilst using the other hand to first arm, then open the pin mechanism before it retracts by itself after only one second. If the connection checks good and free from nylon jamming, the glider can be turned back on its wheel and the cable stretched for takeoff. The nylon loop then exists vertically through the bottom and makes a 90° bend around a sharp plastic edge. I did protect that edge with a glued portion of rubber because I was afraid rubbing could destroy the nylon prematurely, such things mostly happen at the worst moment of the tow. Although there is a small slit that allows you to manually retract the pin for connection, you'll need another extra hand and a very small screwdriver to operate it.
The position of the hook is acceptable but tends to augment the AOA of the wing, leading to the glider to assume a much too high tow position if not checked by down elevator during the climb. With the club towships just above their stallspeed, and the Parkzone Ka8 close to overspeed, it is not the easiest combination to get to altitude. Even with the CG moved forward from the Parkzone recommended 75mm to a stable 50mm position, the Ka8 remains very sensitive in all axis, even using expo. This is mainly because of the speed and the lack of rigidity of the foam. The wings can take a lot of flapping, but the towhook pin mechanism cannot absorb the beatings of strething towcable after some inevitable slack sometimes occur. The mechanism was designed for dropping light bombs etc, but the forces of a one kilo glider being pulled is beyond the (expensive but recommended) Parkzone mechanism.
Even worse is the design philosophy, if you want to release, the cable better be stretched because chances are if the pin is pulled back only one second, it will not fall out of the box by weight alone. On the transmitter, making a second attempt requires two flips o,n the switch to rearm the hook mechanism, unacceptable if having to disconnect in anger, for example when the towship stalls. During one of my last flights, we were close to release point when I my Ka8 started some wild uncommanded gyrations in roll and yaw. I told the other guy I was disconnecting, but we saw no proof of that. While I was battling the controls I tried to rearm the release button, but before I succeeded the towplane stalled, dropped a wing and the pilot wisely disconnected his side of the line.
I quickly recovered but was surprised the rather long and heavy cable (they mostly tow gliders of 4 to 6 meter span) seriously pulled my nose down, requiring a delicate mix of sufficient speed to keep minimal pitch control, but dragged down by the weight and resistance of the stabilizing cloth at the ends of the rope. I tried hard but couldn't release the cable electrically, and my dive angle ended up around 45°. I had insufficient glide ratio for the cable to clear surrounding trees and when it caught, it pulled the glider's nose into one of the last trees before our landing area. Thanks to helpful clubmembers, a rope, a saw and long pole, we got it out of the top of the trees without much damage.
Subsequent investigation revealed a broken spoiler servo because when I instinctively pulled my throttle back prior to impact, this caused the spoilers to extend against tree branches, leading to nylon gear failure. The noise of the release servo sounded normal, but only detailed inspection after the inability to insert the nylon loop revealed erratic inconsistent operation op the pin, which seemed anything but steadily aligned. That system marketed as "Servoless payload release" is barely suitable as a tow release mechanism and better be replaced by either homemade or dedicated glider hook release mechanisms. After dismantling that system I found the culprit: the metal pin is inserted in a nylon pullback mechanism, but not held in place by anything but own friction. Not only are the chances the cable doesn't release when there is no pull significant, but there is also an additional risk if the cable is pulled too tight (as when the glider is uncontrollable or the hook is opened the moment the cable tightens with a shock), the nylon is pulled back but the pin slides out of the guide, making it impossible to make further releases even if the servo mechanism works perfectly. I strongly recommend anybody using that system to firmly glue that pin into the nylon guide (with CA) before it eventually will slide out in time. It only takes 5 minutes and might save your glider in the long term.
After a normal release, get rid of the (tow)speed, retrim for lower speeds and enjoy mild aerobatics or thermals. I made many half hour flights, the same time expensive large GFK gliders with telemetry were back on the ground after 15 minutes. The Ka8 is very good in thermals, but has less capabilities moving from one bell to another. Thanks to the spoilers, landing it in front of you is easy but beware of strong headwinds. To give you an idea of Parkzone Ka8 towed flights, watch this movie taken on a day without thermals. I made 6 flights that day, and I used shots from 3 of them top compose this sequence. Click on following link to watch the video shots.
Over winter my Ka8 will remain in the rack, and although I already replaced the spoiler servo (with a metal gear one), and repaired the towhook mechanism, I will modify the nose to cope with a normal towhook inclined 45° down through the nose (weight is no problem with the ballast required to achieve a 50mm CG). I will post pictures of that modification when available, but in the meantime will fly with my old Robbe ASW22 and prepare a recently acquired second hand, decades old Multiplex Ka6E glider with a wingspan of 3m88.
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|Dec 03, 2013, 06:51 PM|
Partial winter rebuilt
During autumn I encountered another mishap during a tow. The wind at altitude was very strong that day and the sun low, But we decided to try it anyway. Immediately after takeoff the towship turned 180° (away from the sun as agreed before takeoff), but the winds aloft really blew us away and by the time we initiated another 180° turn to come back to the field, we already were almost half a kilometer downwind. Being that far didn't make it easy to watch and steer for a correct tow position, and the inevitable happened without anybody even noticing the cable got slack at the end of the tow.
Three quarters into the turn things didn’t look right anymore but it was too far away to see exactly what was happening. Whatever input I did, the glider went from one wild gyration into another, and I was unable to disconnect. With more than sufficient altitude, my tow pilot with his powerful aircraft gave me ample time to rearm my hook release, in the meantime dragging me back closer to the field, waiting to disconnect me till absolutely unavoidable. My glider finally disconnected from the rope, but was still uncontrollable. I noted inputs in pitch and rudder gave some result, but roll could not make it recover from inconsistent gyrations, not really a spin or so. Being such a light glider, it just slowly came down as a falling leaf, while being blown further away. By the time he disappeared, I feared it had come down on a 4 lane busy road. I immediately jumped into my camper and headed for that road.
It didn’t take me long to spot my Ka8 in the wood on the other side of the road. Wood entry was hindered by a 4 barber wire fence but luckily I found a hole I could crawl through further down. Second luck, the glider seemed intact and with a dead branch of about 5 meters I found in the vicinity, I was able to dislodge him and bring him slowly down to earth with minimal additional damage. I immediately noted the outside section of the starboard wing had no torsional integrity anymore with the inner part, almost free to rotate around the main forward wingspar tube. I had my glider already in my camper when the first guys arrived from the airfield to help me. The thorough inspection at the club revealed the towrope became entangled between the aileron inner split and the main wing (after the Ka8 apparently had turned inside the towship and caught up with the latter). When the rope tightened again, it snatched forward through the foam all the way till the first obstacle, the forward wing rod stiffener. No wonder I couldn’t disconnect with the fuselage hook, no wonder the glider being dragged by the back or its right wing behaved wildly and couldn’t be controlled.
What finally caused the disconnect from the cable I still don’t know, but any input to the starboard aileron just resulted in the complete half of the wing just torsioning the opposite way, now wonder I had no control over the dwindling glider. My initial thought was to just repair the wing by gluing it back together and reinforcing it with 2 carbon strips at the bottom and one on the top. This went well and could be done with minimal disruption of the vinyl covering. When dry, I started bending the wing to test my repair, that seemed strong enough, but I wondered about the structural integrity of the rest of the wing and removed the top vinyl to investigate under the fold traces.
Those wings had been overstressed during the first flights with the erroneous Parkzone c.g. and also had suffered (mainly aesthetically) from ending up twice in the high trees and the subsequent actions to get them back to terra firma. A mail to the local hobby shop confirmed he could deliver a set of new wings within a week for 40 euro so I ordered new ones for the sake of confidence.
When they were delivered I was surprised the package included the spoilers and its actuators, the aileron servo trays with caps and full linkages and control horns. New wing skids were also in position. First thing I did was reinforce the wing intrados with a 6mm carbon strip glued solid to the aileron cable gutter over its full length, end a 4mm carbon strip over the full width of the wing , in a custom cutout just forward of the ailerons. This last one not only would decrease notorious wing flex tendencies, but in case a towcable got caught again between aileron and wing, it wouldn’t be allowed to cut through more than 20% of the wing depth, thereby keeping wing torsion solid over its complete wingspan. Even with the carbon glued in wing flex remains but will be hopefully a bit less under tow. After filling all gutters and mold holes, and sanding the wings smooth I applied new vinyl, but this time with half wing panels shoulder to shoulder. This method with 8 panels worked much easier and seams are almost invisible.
With wood from branches of the last tree still perforated in the fuselage, a pincer got all of them out and elastic filler used to smooth the surface out again along the cockpit. I don’t know how much the Parkzone tow release system played a role in the inability to disconnect quickly during last mishap, but I got tired of continuously rearming the servo before trying again, and decided to remove that payload release servo from the bottom of the fuselage, and replace it with a multiplex system in the nose, actuated by a 2,5kgcm servo mounted in the void. I “finger drilled” the 10mm tunnel from the nose into the lower void under the landing skid plate. Having the hook in the middle of the nose instead of under the fuselage hopefully will reduce the nose up pitch each time the cable comes into tension, thereby diminishing this light thick wing slow glider’s tendency to crawl up in the tow behind faster planes.
Anyway, the nose hook will now open and stay open in a single command, and the nylon towrope bit will not scrape in that 90° bend under the landing skid plate anymore bit have a clean inline separation. The foam nose might not be very solid to glue this Multiplex system in, but in case under extreme stress the tube should get pulled out, the servo further back will probably remain in place with the pushrod, and the release will thus happen automatically. This system weighing less as the bulky Parkzone payload release servo, I glued the original supplied 20gr weight as far as possible just above the pin into the multiplex nose system, another 25gr on the plastic skid plate before adjusting the pin length and servo throw to operate the hook and closing down the nose room with the original screws.
Next I assembled the complete glider so I could rig the spoilers and ailerons to center on the receiver’s zero point, then place the machine on the balancer to adapt the nose weight in the battery compartment so the glider balances out at 50mm (instead of the Parkzone suggested 75mm) behind the leading edge. Weighing the flight condition Ka8 resulted in 1000gr dry, and 1140gr with battery, which is a 40% increase compared to the model as intended by Parkzone. Most of it comes from the wing reinforcement, complete vinyl covering, and additional nose weights to correct the c.g. Additional weight is not always a penalty in gliders, high performance gliders carry additional water ballast to improve gliding distance, something our slow speed Ka8 can use when battering against strong winds (even on final to get to the field). I’m glad my Ka8 is in the rack again before Christmas, completely refurbished and ready to fly another season when temperatures raise a bit after winter. Best of all, in the future I won't have to flip the glider on its back anymore to connect the towrope, it will now be done standing in takeoff position on it's wheel, just in the nose. No more rearming the release system, open is open and close is close, on the transmitter as well as on the towhook.
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|Dec 19, 2013, 06:40 PM|
United States, CA, Bishop
Joined Aug 2006
This entry has given me more food for thought (before flying the model for the first time) than any other entries; I feel especially aggrieved at having shelled out for the HZ 'towrelease' which is clearly inadequate. I will also make some of your other mods, esp. GG position. Thanks very much for taking the trouble to record your trials.
|Mar 31, 2014, 03:42 AM|
Flight test after wing and towhook modifications
With the first spring weather I took my completely refurbished Ka8b out to the field and got in tow even if it was rather windy that day. I was apprehensive because due to cold winter weather hadn't been towed for half a year, but that wasn't necessary.
The glider tracked behind the towship as if on rails. The replacement of the awkward recommended release mechanism by a nose mounted hook really is a major improvement. The glider now remains nicely behind and slightly above the towship, irrespective of speed or eventual snaps after the cable got slack. It also was a reassurance once I opened the hook, it stayed open without having to rearm.
The additional carbon stiffeners also greatly reduces wing flex, and with the hook pulling straight at the frontal center of the aircraft, the wings don't flex anymore during the tow. Loopings and rolls pass very well, and the increased overall weight produced a much better all-around glider on the 50mm CG position. It's penetration against strong winds became better, it caters well for higher towspeeds, and is much stabler and preciser to land in a dime. This is the Ka8b Parkzone should have marketed in the first place, it wouldn't have cost much more, but is a much more versatile and solid glider.
It still is a bit of a toy, but is useful to get back in the picture after a lull in flying, and for new towship pilots getting a hang of towing light ones before they start the serious stuff. Liking the Ka8b so much, I just made a purchase of a secondhand 3m75 span Flair traditional wooden true 1/4 scale example. Watch these pages for an update when I flighttest it before restoring it further.
|Nov 27, 2014, 01:17 PM|
Thank you very much for your detailed account of your experiences with the Parkzone Ka8b glider. As I intend to acquire it, the info will help me assemble it in a proper way and now I can avoid mistakes concerning the CG point. Your other posts are very interesting too and I will take the time to read more of them.
I am a member of "RMZC De Rechte heide" in Goirle, the Netherlands. I have visited Weelde a few times and once took a ride as a passenger in one of the (modern) gliders.
It was awesome!
Thanks again and wish you lots of fun flying those wonderful planes you have..
|Dec 01, 2014, 04:07 AM|
Thanks for the positive comments. If you intend to buy one, you better be fast because the model has been discontinued. Luckily some importers are getting rid of their stock at discount prices. That is why I took the opportunity to order yet another Ka8 down-priced from 139,99 to 89,99 euro from Schweighofer last weekend.
During the summer of 2014 I started slope flying, and my Parkzone Ka8 was the ideal trainer for that. In the meantime I acquired and restored a more specific slope glider (see Breitling/woody page on my blog) that can be flown in stronger winds (3 beaufort or more). The Ka8 was a good performer on the slope with force 2 winds, but due to my (weighty) modifications to make it suitable for towed flights behind our strong club aircraft, it became less than suitable for slope gliding in very light wind conditions.
My latest Parkzone Ka8b will therefore remain completely stock, no reinforcements, no paint, no towhook, and just sufficient weight in the nose to bring the CG to slope acceptable limits. At that weight, it will remain in the air with the slightest breeze.
The discount price was just too good to pass for such a good glider. The one glider fits-all approach isn't optimal, I like to adapt my models for better performance in each specific bracket I'm operating them in. I'm already looking forward being the last one holding on to the slope with a pure glider, while fellow pilots already got their electro-gliders out to cope for the diminished wind conditions as the day progresses and thermals have vanished with the lowering sun.
Have fun with your Ka8b's
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