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Rebel S14 wing by Crash Test Hobby
This is a special build, for it is a tribute to my dad who passed in 2015. Within the first couple of years after his death, I restored his two Zagi 400x wings to flying order and find it a treat to channel his flying through them These nearly 20 year old wings endured my antics until summer of 2019, when the patriotic red-white-blue one crashed itself in a near vertical nose dive, after the ESC/BEC shut off during a high speed pass, through mature corn stalks onto the dry clay soil. The right half of the foam wing shattered under the tape finish, the center plastic battery holder fractured and hinged forward, and the twin fins sheared away.
All this damage could be repaired, of course, but not without loosing the original finish work from dad. Which is annoying to me. Instead, his Zagi 400x RWB wing is presently in retirement, and my search for a similar wing yet assembled using modern design techniques is finished. Among the abundance of prefabricated, and kit, wing designs that I researched, the models from Crash Test Hobby stand out. Cut from EPP rubberized foam in one density for the wing, and a heavier density for the control surfaces, I was drawn to their use of string (shock cord) and 3 mil laminate reinforcements. Plus the RC gear is embedded into the foam for protection, streamlining, and simplicity. All new techniques, to me, that peaked my interest. From the abundant choice of wing sizes and shapes from CTH, I selected the Rebel S14 because of its fully symmetrical airfoil (my other flying wings are semi symmetrical), and I estimate that this 14% mid thickness to be thick enough to fully pocket my 2200mah 3-cell flight battery. I do plan to power it a bit over the recommended parts, as that was the direction I was heading with dad's Zagi 400x RWB. When the Zagi crashed it was after a hand full of full throttle passes, with throttle management in the corners. The ESC/BEC appears to have simply over heated and shut off in flight, resulting in an extended brownout and the crash, because after a cool-down time it returned to normal operation. The RC gear going into my Rebel S14 is: ESC/BEC: Castle Creations Phoenix 60A Flight Battery: Turnigy 3-cell, 2200mah, 60C Servos: Tower Pro MG92B high torque RX: Redcon 4 channel full range Motor: Propdrive 35-36-1800KV. 126 gram. Balanced Propeller: Gemfan 7x5 inch. Heavily balanced TX: Spektrum DX7 G2 black Weight RTF: 794 grams, 27-7/8 ounces (added here after build was completed) ---------------------- The main thread for the Rebel by CTH is: https://www.rcgroups.com/forums/show...acers%2A%2A%2A ImagesView all Images in thread |
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Last edited by CorvetteC5; Mar 09, 2021 at 09:49 PM.
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These photos focus on the gluing together of the wing halves, the carbon fiber slat installed for the cross spar, and string (shock cord) routed around the circumference and gluing of its slot/seam. I used an abundance of UHU Por adhesive
ImagesView all Images in thread
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Last edited by CorvetteC5; Mar 04, 2020 at 08:10 AM.
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Next, I installed the Formica sheet to the bottom rear of the wing, drilled holes, and test fit the motor bracket. Another thick application of glue and clamps!
The stainless steel motor bracket was bent along its long leg altering the shape to more closely conform to the airfoil curve of the glued Formica sheet. The motor thrust angle was also adjusted by bending the short leg of the motor bracket out of 90 degrees. Further adjustments to the motor thrust angle are anticipated after test flying. The goal in flight is for there to be no changing in pitch during throttle advancement from cruising to full speeds. ImagesView all Images in thread
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Last edited by CorvetteC5; Mar 04, 2020 at 08:16 AM.
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Next tasks in building of my Rebel S14 wing kit was to add the recommended reinforcement tape onto the bottom and top surfaces. This went quickly for me, working from experience with this same Scotch Extreme Tape that I had first applied to my Muliplex Blizzard medium-liner build years ago. And more recently to my Dancing Wings Rainbow 800 wing build in 2018.
Being cautious of not adding too much tape behind the CG, since my big motor would already likely make it tail heavy, I strictly followed the recommended amount of tape from CTH and did not add extra to it. Up now, I wrapped the 3 mil thick laminate film around the wing. I have been looking forwarding to trying out this stuff on EPP foam to smooth out the bumpy texture for streamlining, to add considerable strength, and to figure out which settings on my Monocoat brand iron would do the job without melting the foam too much! Turns out the laminate is very nice to work with. Temperature range I used was 180 F to 220 F. This required my Monocoat iron to a very low 1.0 to 1.25 on the analog dial with cloth booty on; and measured with an IR temperature gun. As I became more comfortable applying the heat, and trusting that the EPP foam was not melting underneath it, and became less patient with the small iron, I gradually bumped up the heat from the 1.0 to the 1.25 settings. When the laminate was overlapped and stuck to itself, this little higher heat setting also helped smooth out the wrinkles a bit faster too. I found it fantastic feedback from the laminate, how it changed from opaque to transparent in appearance when enough heat was applied to activate the glue and begin the shrinking process. Brilliant! ImagesView all Images in thread
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Last edited by CorvetteC5; Mar 04, 2020 at 04:44 PM.
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A few steps ago, when the electric motor mount bracket was fitted to the Formica sheet that is secured to the foam wing, the foam control surfaces (elevons) were sized. With the motor and 7 inch propeller installed, one end of each elevon was cut at a pleasing angle and the other end was then cut for length. From experience with noisy pusher-propeller style aircraft, I like a 1 inch gap, or more, around the propeller from all directions. My Rebel elevons are this distance away from the prop tips. These were then set aside.
Note: A bonus for leaving a 1 inch gap between the 7 inch prop tip and elevon edge is room to fit an 8 inch prop, if worthwhile. Moving ahead, with the wing laminated, the control surfaces were also now covered in laminate. The instructions call out for 2 layers. From doing the work I can say that 1 layer of laminate left the springy foam elevons still quite flexible. Two layers stiffened them up significantly more. Like balsa; so writes the instruction manual. Knowing my intend to over power my Rebel S14 wing, and because there was enough laminate with the kit, I covered each elevon in three complete layers of film. Increasing their inflexibility even more so, and adding a bit more tail weight. Pros and cons. They were secured per the instruction using reinforced tape and laminate. Weight of the completed foam parts is 190 grams. ImagesView all Images in thread
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Last edited by CorvetteC5; Mar 04, 2020 at 05:17 PM.
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Finishing Work
My finishing work of the Rebel S14 wing approximated dad's Zagi 400x RWB design when I applied UV resistance tape, made for outdoor signs and displays, in the same patriot stripe pattern. Red-white-blue on top, and contrasting white with a blue border on the bottom. Since the 40 inch Rebel wing is smaller then the 48 inch Zagi RWB, I substituted 1 inch wide colored tape for the 2 inch wide colored tape that dad had on his Zagi 400x. In the Zagi wing design this tape is needed for strength and protection of the foam. With my Rebel the colored tape is strictly decoration. Hopefully it will not peal off at speed! If it does then I will add a some glue to it. Duh.....
On the fins of dad's Zagi, he decorated with stickers of the American flag. Wanting to connect my Rebel to the time period when dad assembled his wing, I found sweet looking stickers in dad's stash from years 1988, 2000, and 2001. These were given out by AMA with dues paying memberships. They look awesome on the fins! ImagesView all Images in thread
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Last edited by CorvetteC5; Mar 05, 2020 at 09:59 AM.
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Electronic Gear Installation
Happy with my effort toward finishing the Rebel S14 wing, I installed the electronic gear.
This was another fun activity for me because of applying the installation method taught by CTH. They position the 2 servos, receiver, and ESC in a single line/slot. The purpose is the minimize the weakening of the wing at the high-stress center section area by maximizing the amount of foam retained. And the single-line/slot greatly simplifies routing of signal wires between these components! The laminate and tapes were cut with a sharp X-Acto blade, and pealed back to expose the EPP foam underneath. The pockets for the servos and battery were sized a little small for a tight fit. The edges of the foam were cut with the knife, and the center routed away using a Dremel tool and grinding stone. The covering materials were then folded back into these pockets, and heated along their edges to conform with the 90 degree bends. The servos are not glued in place. They are held in place by friction and one piece of clear tape across their top. The pockets in the foam for the receiver and ESC were left a bit looser then the servo pockets, yet still provide a friction fit to minimize movement. Between these components the line/slot cut into the foam wing was deepened and widened below the surface enough to smoothly route, coil, and thus hide all signal wiring from sight. I did deviate from the instructions on how I installed the control horns to the elevon control surfaces. The instructions have them glued in place within a slot cut into the elevons. I wanted an easier time, in the future, to reuse these plastic pieces if it came to that, so I simply screwed them in place, as shown, using the backing plate provided and 2mm metric screws from dad's parts stash. Control throw achieved the recommended 3/8 inch up and down at the trailing edge, when setting the servo travel at 100% at my TX and the control rods as shown. It would be very easy to increase the throw with an electrical and/or mechanical change. The pocket I cut for my flight battery, a Turnigy 3-cell 2200 mah, is as far forward and toward the nose as I estimated I could place it and still have the battery fully recessed into the foam wing. I was wrong a bit. See picture of the lipo sticking above the foam about 1 mm, or so, after removing all of the foam, down to the reinforcement tape on the bottom side. Darn! Looks like I should have moved the battery about 1/4 inch further aft from the nose, after all. Bummer, but it will do. Even with the battery that far forward my Rebel S14 came out tail heavy. No surprise because of the over sized motor I installed! To balance the wing, I ended up adding 2 - 1/2 ounces of lead weight into the flight battery pocket along the front edge. See picture. Added strips of reinforcement tape cover the walls and floor of the battery pocket for a bit of extra strength and nose weight. Only thing missing in the photos is the addition of the push-rod guides that I have since been installed. As recommended by CTH. They are simply 2 metal staples that I narrowed slightly to the width of the metal push-rods, and glued through the laminate film into the top of the foam wing with a bit of UHU Por. They do not weight much, so when I measured the ready-to-fly weight at 27 - 7/8 ounces, 794 grams, this should still be accurate enough. ImagesView all Images in thread
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Last edited by CorvetteC5; Mar 05, 2020 at 11:07 AM.
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Power Measurements
From a freshly charged Turnigy 3-cell, 2200mah 60c lipo, I measured motor power input with an Astro Flight Whatt Meter.
It reached 460 watts initial peak, easing to 425 watts steady, on a heavily balanced Gemfan 7 x 5 prop. This is with the Castle Creations ESC set to the slowest 8 khz switching frequency with zero timing advance. I also created a throttle curve at my TX, offering 155 watts input at 50% throttle. Which is 1/3 of maximum. First flights should be fun! I'll initially pay close attention to the temperatures of the CC ESC under load by measuring it after shortened first-flights. And if all is well then after a couple full throttle passes too. Having the ESC hidden in a foam pocket and out of most airflow my cause overheating, of course. I may end of moving into out of the wing pocket and onto the wing surface. |
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Maiden flight video of my Rebel S14 in high winds!
This is a cautious maiden flight with early transmitter removal of excess elevator reflex, and needing a bit of left aileron trim too, where the throttle was managed and the control surface movements moderated. Initial stability was worrisome in the strong gusty winds with dramatic yaw oscillating below half throttle. The two fixed fins are on the smallish side of things! At half throttle the wing smoothed out and kept on tract where I aimed it. It wasn't until midway into the second flight that my Rebel S14 became solid and ignoring of the choppy air, at around 60-70 % throttle. I found a sweet spot for those fins!
With my cautious approach of keeping the flying speed up and the elevator deflections down or risk stalling the wing, per the Crash Test Hobby comments that I have read on RCGroups.com, I still applied full control throws and achieved full throttle at times during this maiden flight. Exploring the flight characteristics, I decided that the recommended aileron roll response is to my liking, yet the elevator pitch control was slow and minimal. My Rebel S14 wing build seems overly nose heavy in a glide and inverted flight, and will be tested with less of the lead weight next to the flight battery. As a temporary tweak, between the maiden and second flights, I dropped the elevator expo from 20% to 15%, and upped the servo throw to 120% from 100%. After this 6 minute timed first flight in low 70 degree Fahrenheit temperatures the electronics were only slightly warm to the touch. The motor and flight battery were each just slight warmer than the ESC. Plus the flight lipo landed with 31% capacity remaining. So there is safety margin room for harder flying and/or more propeller loading. Nice! This maiden flight was flown at the TRaverse Area Model Pilots Society (TRAMPS) RC club flying field on Rennie School Road in Traverse City Michigan on May 24, 2020. I like to think that dad was watching and encouraging me on; as he used to do in person.
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Last edited by CorvetteC5; May 25, 2020 at 01:22 PM.
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Crash Test Hobby Rebel S14 wing 2nd flight in high winds
The 50% throttle at launch was perfect with smooth upright tracking into the winds, but a minute or so later I could no longer hear the propeller noise and thought the ESC had cut off motor power. In the video one sees me gliding the Rebel S14 and aligning it to the grass runway for a dead stick landing. Fortunately, before actual touch-down I gingerly advanced the throttle again and was rewarded with a whirling prop and forward thrust! I resumed with the 2nd flight. Best I can figure is that the wind noise and airplane orientation was such that I couldn't hear the propeller sound briefly and thought it had turned off. At which time I fully lowered the throttle stick on my transmitter. My bad!
Back on the throttle, I soon discovered a sweet spot for those smallish fins! My Rebel S14 became solid and ignoring of the choppy air, at around 60-70 % throttle. Aim and go, with wonderfully tracking turns finished this flight! I'm pleased with my Rebel S14 and look forward to tuning it over its live of years and years (I hope!). Also very happy that the tape decoration hasn't started pealing off.....yet! Sincerely, Curt
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Last edited by CorvetteC5; May 25, 2020 at 01:23 PM.
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Tuning an aircraft is an ongoing and enjoyable process for me! Over the the last 10, or thereabouts, flights with my Rebel S14 wing, since the maiden flight on May 24th, I have changed the following items:
- My Rebel S14 was plenty stable in flight and the elevator reflex was more than needed per an inverted flight test so I reduced its flying weight from 794 grams to 769 grams (-3%) by removing some nose-lead next to the Turnigy lipo, and by rebuilding the Propdrive motor. I removed the bolt-on prop adapter from the motor, and then rebuild the can assembly by sliding its metal prop-shaft to stick out of the other end of the motor. Onto this shaft I clamped a lighter weight collet-style prop adapter. The motor shaved off about 5.5 grams from the aft end of the aircraft. - While the motor mounting bracket was detached from the wing I bent it about 1 degrees to reduce the down thrust. My Rebel S14 experienced pitching up with throttle. Reducing the down thrust let me reduce the reflex in the elevons (less up-elevator) and thus less pitching up. I did over do the bracket bend slightly, where as my Rebel now dives slightly at full throttle. There is a 2-click trim difference between half throttle and full throttle cruising. Close but no cigar yet! - The propeller was changed from the gray Gemfan 7x5 to a black Master Airscrew 7x6. This MA prop is old stock on hand that was intended for glow engines! Flying with my usual mixed throttle, the energy consumed from the flight lipo is equivalent between the two propellers for the 5:30 minute timed flights. And with the higher pitch of the MA the wing maybe a bit faster. I'm not really sure as it cruises with authority with either prop! - Curt |
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Last edited by CorvetteC5; Jun 08, 2020 at 09:21 AM.
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Motor Reduction due to control surface flexing
The last flight on my Rebel S14 wing showed a new weakness. The control surfaces are flexing now, which is particularly noticeable at full throttle with tight loops at high pitch deflection. The flex is uneven generating spiral loops, or a tip stall. I saw this issue happen with my DW Rainbow 800 wing when I overpowered it by 5 times rated! The fix for my Rainbow was to wrap the elevons in 2 layers of laminate. The same laminate that came with the Rebel S14. Since my Rebel has 3 layers of laminate on the elevons already, I took a different route and dropped some more weight by downsizing the motor.
My Propdrive 35-36-1800KV motor is replaced with an Exceed RC Rocket 3010-1820KV. Still swinging the same Master Airscrew 7x6 propeller. With about half of the added lead nose-weight removed, the ready-to-fly weight is now 698 grams, 24.6 ounces, down from 27 ounces. CG is unchanged. I do expect the control surfaces to continue to flex unwantedly because the power and top speed should be close to what is was, yet hopefully to a lesser extend from the lighter force loads. |
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Last edited by CorvetteC5; Jun 14, 2022 at 02:16 PM.
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Flight report of changes:
(More of a reminder to me on how the above changes impacted the flight characteristics.) I treated myself to 2 new flights with my CTH Rebel S14 wing today, June 18th. The Exceed RC Rocket 3010-1820KV motor is one I haven't flown before, and with the Master Airscrew 7x6 propeller that I had moved over from the previous Propdrive motor, it was lacking in RPM. Appears to be too much prop as the top speed was visibly reduced. I'd estimate by 25% from the Propdrive 1800KV motor. Yet the power consumed nearly matched that of the Propdrive motor with 29% capacity left in the Lipo during my normal 5:30 minute flight time. So I propped down to an APC 7x5E. I also removed another few grams of nose weight because I had missed the CG target! After trimming the first flight at 1/2 throttle the wing climbed with throttle. Quite a bit. Whereas I had it barely changing in pitch with throttle, with the former motor and nose weight. The new lighter weight measures 690 grams, 24.3 ounces. Flight 2 shows much improvement in CG balance and motor balance. RPMs are back up with smooth aggressive acceleration with throttle. Much like with the larger Propdrive motor. Top speed is very satisfying now. Although the 19-21 MPH winds may have skewed my perception. Power consumption dropped with 34% remaining in the Lipo this time. Nice! One negative is that the Rocket motor was warmer to the touch at the end of this flight than the Propdrive ever reached on much hotter days. So the Rocket is less efficient at this load. Something to pay attention too. Overall, I'm pleased with these latest changes and resultant characteristics. |
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Last edited by CorvetteC5; Jun 18, 2022 at 01:26 PM.
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