My Piper Cub

On return from a trip into the south, I stopped at a large model airplane parts supplier in Santiago. In fact I just wanted to see what kinds of planes, engines, and radios were in use nowadays. After all, I had been away from the sport for 10 years!  I started talking to the owner, told him about my plans to eventually return to the hobby. He was a very nice guy, and an even better salesman: Two hours later I left the shop carrying a Carl Goldberg kit for an "Anniversary Cub", two rolls of ColorTex sheeting material, some glue, some paint, an engine, radio set, fuel, and assorted stuff. The owner of the store was smiling over a large check carrying my signature...

Two months later the assembly work had been completed. I put a lot of effort into this plane, building it carefully, and very strong. After all, my piloting skills would be shaky, at best, after 10 years off-duty, so the plane would see some rough landings. While this kit is in fact for an exact scale plane, I did not make all the small details that make the difference between what you see in this photo, and a real scale plane. The wing struts, mock shock absorbers, decals, the black lightning picture, and a lot of other details were not made. I wanted a plane to re-learn flying, but I abhorred those ugly trainers, so I built this semi-scale plane as a trainer...


Here is the front section of my Cub, as I built it originally. The real, full-scale Cub has a boxer engine with the cylinders poking out of both sides of the cowling. Most model Cubs mount the engine sidewards, and use a mockup for the other side, in order to look more original. But this requires a specially shaped silencer, which I didn't have. So I opted for mounting my engine vertically, with the standard silencer at the side.

The engine I bought was a cheap Magnum .40GP. The seller had told me that this was a good and low-cost engine, used by many people. It cost half as much as the well-known O.S. Max of the same size. Only later did I find out that it was bad economy to buy this engine: It lasted for less than 10 hours flying time! It seems that even in model engines you get what you pay for.

The landing gear of this plane is made from steel wire, and for more original appearance some plywood triangles are attached. The cowling, like in most such planes, is made from very thin plastic, and will not resist much mishandling. The rest of the plane is quite sturdy.

I used white glue almost exclusively. It's somewhat slower to use than cyanoacrilate, and slightly heavier, but very much stronger than either cyano or epoxy! And it's less heavy than epoxy.


For months I looked out for someone to ask for help. I felt pretty worried by the idea of flying my new plane, untrimmed, straight away, after not having flown for 10 years. Just by chance I had found a model flying field near La Serena, with a large, weathered sign, but never found any activity there. So, one day I took my heart in my hand, and went out to try my plane and my luck. This photo shows the new bird, seeing the sun for the first time. The engine is running, using very rich mixture. I ran it in this way for around an hour, then slowly approached a more normal mix. This should be enough run-in, and it was working smoothly. Note the plane's tail tethered to a shrub!
I rechecked everything, filled the tank, started the engine, a last check, a deep breath, and off it went! After just 10 meter of runway the plane lifted off easily, and went straight up. I knew I would overcontrol at first, but this was weird! I pushed the elevator, the plane still went up, then at some point suddenly came shooting down. I thought it had stalled, and recovered slowly from the dive, but the plane was like locked into the dive. More and more pull, loud heartbeat, sweat on my back, adrenaline levels at an all-time high, and suddenly the plane broke the dive line and went straight up again!  This was really weird! The only good thing was that the plane was gaining altitude despite the crazy behaviour. But it was far away already.

For several minutes I put up utmost concentration to keep from crashing. Slowly my memories of flying a radio controlled plane came back. Flying RC planes is just like bike riding: If you stay 10 years off, then you will be shaky, but you still can do it. But this plane was not functioning properly. It was absolutely impossible to hold an altitude. It would always go steep up or dive steep down. I tried lower engine power, which helped some, but did not cure the instability. I wondered if I had got the center of gravity wrong, but I was quite sure it was just fine. I had adjusted it quite to the front of the range indicated in the plans, to be on the safe side... so much for that!


I flew the plane in a vertical zigzag pattern for about 15 minutes. By that time I felt as confident as I possibly could, given the crazy behavior of the plane, and started to think about landing. I cut power, but there was no such thing as a glide, just crazy ups and downs. After several aborted attempts, that showed me that I had not the slightest hope of a soft landing, I decided to put the plane down in some shrubs, so at least there was some chance to cut the damage. Now, this is a desert, and there are just a few, hard shrubs. It wasn't easy, and took several attempts, but finally I was successful in putting the plane belly-down onto a larger shrub. The landing gear was ripped off, the prop was shredded, and the plane slid over the shrub onto the gravel ground. The impact broke the plastic servo mount, cracked the firewall, and - worst of all - cracked one of the main wing spars! So the damage is greater than what this photo suggests. But at least it was perfectly repairable. My only problem was how to get this plane stable!

After repairing the damage, I started looking for the source of the problem. I finally suspected the elevator pushrod. The kit included a rather short Balsa rod, that had to be joined with rather long pieces of steel wire. The whole affair was quite flexible. I had little doubt now that the pushrod's flexing had caused lack of control over the elevator. I replaced both pushrods by Bowden types, which are very much stiffer. The next flight proved that this had indeed been the problem: The Piper now flew smoothly, without any improper behavior!


The next time I went to the flying field, it was busy! I introduced myself, and became a member of the club. Here you can see my Piper flanked by other planes. Note the aluminum nose: I made it to protect the engine from hard landings, which almost always make the Piper nose over. And hard landings are unavoidable by a shaky pilot on a dirt landing strip!

The club has a very good instructor, Alejandro, the owner of the Extra 300 at the right. When he saw my shaky flying, and heard of my brave try to re-learn flying on my own after 10 years, he volunteered to give me some much needed lessons. He had that cable needed to connect two radios together, and so we had a double-command plane!


But dual command has its problems: One day some confusion happened, and for some seconds neither he nor I was flying the plane. The result was an ugly crash. The wing ripped off the mounts from the fuselage, some windows were broken, a wingtip was shattered... Next weekend I was back on the flying field, but the plane took away some visible scars.
The crash also loosened a patch over a rip in the fabric, which had been caused by the breaking servo mount on my first crash.

Everything was glued back together. New windows were made from a Lindt chocolate container! I would love to know what Lindt people would say about that novel use of their plastic boxes? That material is as good as the chocolates that came inside!

In this photo you can also see the abovementioned Bowden pushrods. The red tubes are the guides.


Here are the scars of a very close encounter! This is the right wingtip of my Cub, greeting the evening sun after caressing a light pole! I was turning in for landing approach, when the instructor started shouting "Pull up, pull up!  The light pole! Pull uuuuuuuup!" I thought I was far away from that pole, but just to quiet his shouting, I pulled up slightly. And then I heard a loud "whack", and the plane jumped and rolled. My reaction was aborting the landing approach, giving full power, straightening the plane and pulling it out there. Then I realized that everything seemed to work perfectly! I flew another circuit, then approached far away from that pole, and landed my plane. Inspection revealed this nice mark at the underside of the wingtip! It had hit the top of the pole! If I had come in just one centimeter lower, my Cub would have been history! Hitting a pole with the wingtip leading edge sends any plane into a flat spin, which from light pole altitude is deadly...

This incident may just show how easy it is to destroy a plane. A model's life is full of such dangers. Just check out my crash page to learn more! But not now, first see how my Cub's life went on!

This photo shows the entire new power plant, ready for mounting in the plane. Note the silencer made from copper and brass. This engine would be mounted horizontally! I crafted the dual-chamber silencer to the exact dimensions required for the space available under the Piper's cowling. The exhaust is centered below the belly of the plane, and a flexible silicone hose keeps oil away from the plane. No need to clean up after flights, from now on!  This silencer is extremely effective! The propeller now makes more noise than the engine! It does steal some power, but then, the .46 engine is more than powerful enough for this plane!

Look also at the carburetor. A piece of panty hose was strung over the air entrance, as a kind of rudimentary air filter. It will not clean away all dust, but at least it will keep the engine from breathing boulders!


Here is the motor assembly seen from the other side. The dual chamber silencer is obvious, and the electric starter can be seen easily. It uses a cobalt-samarium motor, two stages of reduction, a freewheeling mechanism, and a rocking gear to engage the large gear mounted on the crankshaft. The starter was bought from Conrad Electronics, Germany, and modified for airplane use. It was originally designed for model cars.
Mounting that large gear was a real problem! A special, quite complex steel piece had to be custom-made. It fits in cup-fashion over the crankshaft, between the mounting plate and the propeller. It holds the brass gear, and the spinner rear section. Both the gear and the spinner had to be modified for this. It took some engineering, but never has given any problems.

The square black thing with four connections is a rectifier bridge. It was used to pass the starter motor current, causing a 1.5 Volt drop, which is in turn applied to the glow plug. This simple scheme allows using the same 12 Volt starter battery for the plug too.

The engine, starter, bridge and silencer are all bolted to tapped holes in the standard plastic engine mount. So, the entire power plant is mounted by just four screws onto the plane's firewall.


Here is the plane's nose, prepared for accepting the new motorization. In addition to the four screw holes, there are two tube nipples: one for fuel and one for tank pressurisation. Two thick wires bring the electric power for the starter, and two Bowden rods connect to the throttle and to the starter.  By the way, for nonfliers it should be noted that the carburetor control is usually rigged in such a way that the trim on the radio control transmitter can be used to set the idling speed. So, the throttle control puts the engine from idle through max, and the trim control can put the engine below idling speed, thus shutting it off.

Note some clear signs of accidents... There is epoxy glue filling in some holes. Note also that everything is nicely sealed against oil. While the engine itself has been sealed with silicone rubber, there is always some oil spilling around from the starter system, so good oilproofing of the wood is necessary.


Here you can see how nicely the new powerplant fits in front of the Cub's firewall! Only the starter actuation lever protrudes too much towards the bottom, so this called for the manufacture of a special, not very Cub-like cowling. I didn't find a better way to make it, but with some more tools it would have been possible to make a doubly crooked lever that would fit.

The spinner front is dismounted here, so you can see part of the steel cup that holds the spinner back and the large gear.


And here is a side view. This square-on view gives a clear idea of how it all fits.

The starter battery was made of 10 high-current size AA Nickel-Cadmium cells, making a total of 12 V at 0.6 Ah capacity. This is enough for many dozen starts. As the starter mechanism and the copper silencer add a lot of weight in the plane's nose, the battery had to be pushed into the plane's tail, in order to maintain a correct center of gravity. The battery was mounted by wrapping it in foam, and pushing it in... Anything better would have required removing the fabric from the tail, which I didn't want to do.

The plane gained a lot of weight with the starter and battery, but it still flew very well! It just looks a bit more real now. It doesn't take off after just 10 meter, now it needs 30 or 40. A little weight adds a lot of runway requirements!


After the first tries, several things had to be modified. First of all an expected problem happened: I had soldered the new silencer using standard tin-lead solder, and as soon as I gave full power, the solder melted down and the silencer flew apart. The engine made quite a mess spilling oily exhaust until I brought the plane back for a safe landing. This photo shows Mr. Cub, pilot of the plane, assessing the damage.

The silencer had to be entirely removed, disassembled, cleaned, and resoldered using silver solder. Only then did it hold up. When this photo was made, the silencer was ready for reinstallation. It can be seen under the plane, severely discolored from the very hot soldering.

But two other problems proved harder to solve. One was the proper engaging of the starter's small gear. I had mounted a microswitch in such a way that the motor was energized when the gear was already in position, or at least pressing its teeth against the larger one. But the strong motor started so fast that often the gear did not find time to engage, and kept grinding its teeth against its larger mate's.
The small gear was made from hardened steel, while the large gear was made from brass, so it the softer brass quickly was worn out. The brass gear was replaced by one made from hardened steel too, but still there was the problem of the gears not engaging properly.

This problem was solved by electronics. I built a MOSFET based circuit that after the microswitch being tripped, would wait for 200 milliseconds, and then slowly apply current to the starter. This increased its torque slowly enough to make it turn a little bit at first, enough for the gears to align and engage. Then, after another 300 ms or so, full torque is reached, the engine cranks over and starts. Problem solved.

But the other problem remained: After all, this is a glow plug engine. Combustion will start as soon as compression is high enough for the kind of fuel used and the actual temperature of the glow plug. This means that such engines often kick back badly! It happened in my setup too, and such kickbacks broke gears, bent shafts, disaligned mounts, etc. The solution was again electronics, in the form of a second timer: This one waits until the engine is turning, and only then slowly heats up the glow plug! This assures that the first ignition will occur at the point of maximum compression, so the engine will not kick back! Now the system worked flawlessly!

My self-starting plane was a huge attraction among fellow modellers, wherever it went. Many people wouldn't believe the story until they got a demonstration of me switching on the radio receiver and transmitter, then stepping back, pushing a knob on the radio, starting the engine, taxiing to the runway and taking off...

By the way, I had to add a fifth channel to my radio transmitter, which came from the factory as a 4 channel unit. Fortunately, most modern radio control transmitters are internally 7-channel designs, even if they have control rods for only 4 channels. It's easy enough to add up to three additional channels. The receiver fortunately was ready for 7 channels, so it was a matter of adding a servo to get it controlling the starter.

This cowling was repaired, not replaced!


And this was a rather funny incident. But it hurt...
I had taken the plane to my workplace, to show off in front of my workmates. But I work at 2400 m altitude! I had never flown a model at those levels, where the air is much thinner than at sea level.
The first takeoff attempt was fruitless. The engine ran like mad, the plane ate up a long stretch of runway, but just didn't fly! I changed over to another type of propeller, better suited for the thinner air, and that time the plane just barely got off the ground. Once in the air, things looked brighter, and the plane seemed heavy, but flew rather well. I flew for half an hour, until it was getting dark (this was after working hours!). Then I tried to land. But this proved to be really hard! I was flying from the mountain road that leads to my workplace, and my "runway" was the only straight section of that road. But it had a mountain to one side, and a road barrier plus signs to the other side. So I had to come in nicely centered on the road, but a strong side wind had developed! After all, in the evening cooling air comes down the hills, right?

I made several landing attempts, aborted several times, until it was getting really dark and I just had to land, or I would no longer be able to see my plane! So, I came in low, put my best concentration into compensating the side wind, working rudder, ailerons and elevator. There, a wind gust! Almost against that road sign! Ailerons, rudder, elevator, some power! Saved! Ooops! Now it goes against the hill, catches the downdraft! POWERRRRRRR! Some of my workmates run for their life, others lie on the ground... Why do these guys stand right there, after all...   Plane saved, so far... reduce power, next approach. Now it's fast. Need to burn off some speed before touching down. Still some 200 meter available. Engine to idle, altitude of 2 m above the road, compensating side wind... Now lower, closer...  Another colleague running for shelter... Now I HAVE to land! But what's that? No rudder control!  The rudder, so small in this plane, has stalled! Well, now it's all on the ailerons, at just 1 m altitude! Plane veers sidewards, another fellow engineer, affectionately known as "the goose", jumps over the road barrier and disappears downslope, frantically beating his arms... Wingtip almost touches ground, HAVE to level off now! Plane coming straight to me, one meter altitude... REALLY straight! No rudder control, I can't run, I have to land this thing! But not against me! Power? No, too late! I shut the engine off, and OUCH!!!!!!!  You may guess just WHERE the plane came in! Good that I shut off the engine, or the prop would have made some scrambled eggs! The spinner found home just THERE! It still hurts just thinking about it!

Oh, yes, the landing gear broke when the plane fell to the ground, from the altitude of my balls.

But the plane came out in a dive, and I had real trouble pulling it out. It barely cleared the planet! I flew it a bit higher, and noticed that I had to hold the elevator almost fully back to maintain level flight! But it was very stable. A clear indication that the center of gravity had shifted way forward!
It was out of the question to try landing that way. I needed extreme speed to keep the plane in the air! So I decided to rely on gravity, and on my engine's pull: I climbed to great altitude, then dived down, pulled up and got the plane looking vertically into the sky. There I shook rudder and elevator, returned to a normal flight position, and... my plane was back to normal!  In the crash the starter battery had come loose from its cushion, and my maneuvers had returned it to its rightful position in the tail!


I then landed close to Miguel's wreckage. His plane really looked bad. It had dived straight into the ground, at full power. Miguel didn't even try to shut off the engine! He had spent the last second of his plane's life trying to pull it out of the dive, a hopeless affair, since my prop had cut through his elevator hinges! A full reconstruction of the event was made:
His plane had come in front of mine, from above. My prop cut his right elevator hinge (he had only two hinges there!). My next prop blade caught his plane's tail below the fuselage. That spot was sturdy enough to stop my engine, while his tail was thrown up, making the plane face down. Then his plane slid to the left, blocking my left wing and sending my plane into the flat spin. His plane eventually slid over my left wing, leaving paint marks there, and then went straight down, engine still running.


I felt a strange mix of guilt, shock and proudness. Guilt for engaging in such dangerous formation flying, but I'm sure that he did the hickup, not me... Shock for thinking that it could have costed my plane instead. And proudness, not for shooting a friend down, but for having built my plane so strong that it could tear another one up, without suffering any damage!
In any case, I couldn't miss the chance to make this photo. My plane, in one piece, barely scratched, saved only thanks to a combination of having the electric starter and reacting quickly, while Miguel had to get helping hands to pick up all the many small pieces remaining from his plane!


And here is a closeup of my left wing, showing the paint marks left by Miguel's plane. It's noteworthy that absolutely no damage was done to the wood, and not even to the fabric covering! This ColorTex material is really good! If I had used Monokote or similiar plastic foils instead, they would have ripped. And a large hole in a wing makes a plane uncontrollable.
You see, many things played their part in order to save my plane. Mr. Murphy really must have been sleeping that day!

Maybe someday I add an alternator and a larger transmitter, but since I now am flying myself, for real, there is less incentive in TV-equipped model planes. For the moment, I have retired my Cub from active service, after it has given me many hundred hours of enjoyment. Its elevator needs strengthening, as it is suffering heavily from fatigue. It has become unsafe to fly the plane that way. So, maybe someday I fix that elevator, fit new batteries to replace the present ones, which are 6 years old and no longer reliable, wash the rockyfied oil out of the engine, and fly it again. In the meantime, the Cub is living a peaceful life, hanging from the ceiling in my appartment. I'm building a new plane, an aerobatic Chipmunk, and flying myself too (see the volatrix page!).