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Jan 22, 2013, 06:34 AM
The sky is the limit
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Multiplex Funcub

Multiplex Funcub

Summer 2010 while attending the annual Pampa Model Flyer jet show I stumbled along a German vendor who sold an infrared coax helicopter for a mere 25 euro. Back home flying it around the kitchen and living room it bought back memories from the seventies when I sold all my rc airplanes to invest in a house. After a week I realized the shortcomings of that heli and ordered a Nine Eagles solo pro, with proper tail rotor, more challenging to fly but more rewarding. A month later I discovered an rc club in my town, became member and told them I wanted to pick up my old hobby. The club president sold me his used Graupner electro junior powered glider modified with flaps, but after just a couple of flights I got the hang of it and decided I wanted a real aircraft again to practice takeoff and landings instead of hand launches. My choice felt on the Multiplex Funcub, a design sufficiently benign to pardon my beginners mistakes, but with ailerons and flaps offering me a range of flight maneuvers from basic till aerobatic, with precision landing capabilities as a bonus. I also felt a strong connection with the Cub, having spent many hours instructing in a local aeroblubís aircraft during the seventies. My choice of colors was rapidly made.

At that time the Multiplex Funcub could only be bought as a kit, and I was seriously surprised the basic aircraft price tripled with the purchase of (quality) servoís and a Multiplex power set (consisting in an extremely powerful HimaxC3516-0840kv engine swinging a huge 13x4 prop through a BL30 ESC. The instruction manual was very clear and the box included all the necessary hardware to quickly assemble the model if that is your choice. The fuselage consists of two parts, and I cannot sufficiently stress the importance of gluing the plastic motor mounts very firmly onto the fuselage sides. Forums are full of people encountering deadly resonance in the front fuselage, but most due to use of substitute cheaper (unbalanced) engines or props, and once the nose had been weakened, itís very difficult to correct. You can choose to reinforce this assembly, but beware of CG problems later on if you want an easy battery switch. In that respect I never understood why they elected to mount the rudder and elevator servo in the middle of the plane, with long piano wires in guides all the way back to the tail. The multiplex way of connecting the tailwheel to the rudder requires difficult bending of the rod to glue it in the lower rudder. The main landing gear consists of a pair of large (bush wheels) foam wheels that are very sturdy but cause too much friction on tarmac. The very soft springs will absorb the worst of your landings (after ruining the prop tips) and will require regular bending into shape again. When operating on tarmac it is also very important to have both wheels perfectly parallel to each other, failing to do so will surprise you already during taxi with the friction of the tires causing them to narrow or enlarge the track, with ensuing jumps to relieve the tension of the gear wires.

The servo cutouts in the wing are ok for the ailerons, but unless you bought reversed servoís youíll have a problem with the flaps. Either you install a servo reverser, of after cutting out some foam you can install both servoís exactly the same, and make use of a bit of an angle for the piano wire between the servo arm and the flap horns. Those horns are well designed and allow the flaps to be removed or replaced if necessary. The piano wires are strong enough for pulling the flaps down to incredible angles, but not sufficiently stiff to keep the flaps up when doing negative G maneuvering. If you want to perform inverted flying, you better slide cuffs over those wires to reinforce them. Servo arm angles are important in this aircraft, either to reduce the aerodynamic forces on the servos when flaps are extended, or to already build in some form of mechanical aileron differential because a cub always suffers much from secondary adverse yaw effects. Think about the neutral servo arm angle and test it out before gluing the servo in the wing because it is almost impossible to change it later. I glued the towhook but later never installed the hook release servo. The model being only 1400mm span I glued both wings together for solidity but that was a personal choice.

The engine mount allows easy adjustment of downtrust and sidetrust, but it is my feeling the latter is exaggerated in the manual. The small round opening on the top of the cabin (before mounting the wing onto it), and the heat dissipation hole at the bottom, provide the ideal means for a gynecologist to practice with his fingers, but have to be used both in conjunction (with just feeling) to connect all those servo wires onto the receiver and his eventual satellite. Final part was using the battery to achieve the manualís correct CG position. Instead of ending up in the front below the access cap, it became mostly buried in the hole leading to the receiver compartment. That made it difficult to attach with Velcro on the floor, because you just couldnít get the fingers there to pull it away.

On a cold day in November 2010 I took it to the field, but as the instructor didnít show up, the president suggested I maidened it myself (he seemed more confident as I in my capabilities) . I was terribly nervous because my rc aircraft experience over the last decades was limited to about 10 (uneventful) flight with the Junior mortorglider. After much checking and double checking I lined the Funcub up into the wind, without flaps, and opened up the throttle to full power. Big mistake, the Funcub leaped into the air in less than 3 feet and climbed away with about 60 degree climb angle before I could react. Furthermore, even with left aileron and bank, it still turned right and ended up in the adjacent tall trees about 20 feet off the ground. Luckily one club member climbed into it and shook the branch till the model fell out, being caught by other members before it hit the ground. The model didnít have a scratch, the deepest groove was in my ego but this incident didnít deter me from trying again.

Analyzing the facts I concluded full power was way too much for takeoff, and combined with excessive sidetrust pulled the aircraft to the right. The ailerons had been connected with a Y cable and left application resulted in even more adverse right yaw which I was too late to correct with rudder inputs. I this mechanically offset the rudder to the left before lining up again, this time slowly advancing the throttle till the Funcub gently lifted off and climbed away normally with half throttle. I tried to trim things out in the air but turns looked anything but coordinated (no wonder with excessive engine sidetrust compensated by left rudder). This time I was fully under control and after a few flapless approaches I made a good landing. Back home I adjusted the engine angles and after removing the Y cable, reprogrammed each aileron separately on the DX6 transmitter with 50 percent differential. The second flight was already much better but my limited all or nothing flap command caused wide pitch excursions during safe altitude testing. It became obvious that Spectrum DX6i transmitter was not what I needed, and I soon swapped it for a second hand DX7 with 4 receivers. Now I had a 3 position switch for the flaps and many more programming possibilities. I found out the airplane ballooned a lot when lowering takeoff flaps (about 15į), but had little pitch change between takeoff and full flaps (about 60į down). I dialed in flap elevator mixes to neutralize those effects and it was much easier to fly from there on, especially during go arounds.

My proficiency and confidence rapidly grew and I started doing mild aerobatics. During one of the barrel rolls I must have applied negative Gís when passing inverted, which caused the battery (only held by little Velcro strips due to previously mentioned problems) to come loose, and during the second part of the barrel it slid down. The combination of battery pushing the engine cables against the rotating outrunner (causing a shortcut), and the ensuing extreme forward CG, led to a vertical plunge from 100 feet into soft ground. Amazingly this solid model had little damage and even could have been repaired with little cost, but having the remains of an incomplete spare Funcub box at the attic I decided to take another route. The only damage was one of the nose sides having been crunched. There was no damage whatsoever to the wings or tail. The engine still ran fine but the prop adapter got a whiplash and was replaced.

I didnít want to handpaint the Kiewit aeroclub badge on the tail, nor bend that tailwheel assembly, and decided to mate a new tailless fuselage with all the old parts. I saw both tails off at the exact same place and angles, and used the opportunity to move the rudder and elevator servos into the solid foam parts at the junctions. The extra weight of the servoís and carbon reinforcements along the mating area would allow me to place the battery much more forward and make removal and fastening much more practicable. The only drawback is that inertia from weight concentrations further apart diminishes pitch agility but I never used my Funcub for 3D type of flying so I didnít care.

In the real club both PA18ís existed and sported the same colors so to indicate the change I now flew with the OO-WIK instead of OO-KIW. My modifications worked out very well and I made sure to have the battery blocked not only by Velcro, but also by glued foam blocks that prevented it from moving in any direction. By spring 2011 I started exercising all the maneuvers that had to be flown to obtain the Belgian rc license. In that club in Hasselt nobody had a license so I had no idea how precise that program had to be executed and was very meticulous about the degree of perfection. That summer I made an appointment with an examiner at another club. It was very busy on that field and I wasnít used to that but I passed my test with brio. When I questioned the examiner (European quicky 500 race rc medal winner) about what I had to exercise to obtain the instructor rating (so I could train other own club members), he told me what I had demonstrated already was an overkill and he would submit and sign all the necessary papers for the federation. A few weeks later both licenses were in the postbox, all thanks to the Funcub versatility.

By the end of that year I changed clubs because my tendency to acquire more scale designs necessitated a large hard runway to operate from. Funcubs donít like tarmac runways and it slowly became redundant in my ever growing fleet. I also sold it because I needed more space and those enormous wheels on long legs took lots of storage space. I never disassembled the wings from the fuselage because it fitted nicely on his nose between the front and back seats of my small Mercedes A class car. When most other club members had to remain on the ground during the refurbishment of our runway, I was able to fly from the few flat spots between the rubble of the construction workers, using only partial width (instead of length) of the runway when the wind was cross. I can recommend this airplane to everybody, but accept the price of Multiplex accessories if you want to fully enjoy the aircraft without troubles.

I must have flown about 200 flights with it before I sold it to a fellow club member in December 2012. It had become my most trusted model, being used in any wind conditions to regain currency and feel the weather conditions before flying more demanding models on the field. With takeoff flaps and full power it got airborne in its own length and climbed out almost vertically. Raising the flaps in that situation automatically lowered the nose in a cruise situation. With moderate winds you could just hang there without apparent forward speed, and with stronger winds flying backwards was the real fun. Landing with full flaps could be done precisely and gently in front of your feet, but only with a powered approach till touchdown. Reducing power before that invariably resulted in a loss of elevator control with ensuing hard landing (often damaging the prop). Whatever your experience, you never get bored flying a Funcub, it can be very forgiving and easy for a beginner, but requires knowledge and experience to explore the extreme flight envelope the airplane is capable of handling.
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Jan 29, 2013, 07:32 PM
Registered User
Thanks for all the details, quite enjoyed reading your story. I've got several Multiplex models and quite enjoy them. Might try the funcub one of these days, I've enjoyed using flaps on other models of mine.
Oct 05, 2013, 08:58 PM
Wada ya mean, over engineered?
cubcrafter60's Avatar
Hi BAF23,

Nice write up! I just wish I had found it before I did my 3 part blog post on scale(ish) FunCub's (I would have liked to include some photo's of yours)! Just wanted to say nice job on your FunCub!

Best regards,

Robin van Dorst

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