My goal for this project was to use a micro-sized RC model of the C-47 transport plane to tow a similarly-scaled down replica of the WACO CG-4A combat glider. I've been flying larger models of these iconic WWII airplanes for several years. Shrinking things down to micro-scale introduced several new challenges. I was able to overcome some of these hurdles, but continue to stumble over others.
In part one of this series, I examined Flyzone's new micro C-47. My evaluation covered its performance as a standalone model as well as its potential as a glider tug. I thought that the overall flying qualities of the C-47 were pretty solid. However, I added static flaps, a tow hitch, and longer landing gear before attempting to tow with it.
This time around, I will talk about how I built the tiny CG-4A gliders, explain the wild flight tests, and review the overall successes and failures of this project.
Micro Glider – Take One
My calculations indicated that a CG-4A in the same scale as the Flyzone C-47 (about 1/50) would have a wingspan of just over 20 inches. I scaled down my plans for a 65-inch WACO glider, printed a few templates and started building.
Foam is my preferred material for prototyping because it is so easy to work with. That is especially true when dealing with boxy shapes like the CG-4A. The fuselage is made of .75"-thick blue foam from a home-improvement store. Two layers side-by-side were the perfect width, so I made right and left halves. Using a sanding drum in my Dremel tool, I hollowed out the inner side of each half to create space for the radio gear.
Whenever I crash one of my store-bought micro models, I always make sure to salvage as many parts as I can. I pull the electronics, motor, carbon fiber bracing, control linkages…everything. The resulting stash of tiny parts always comes in handy for projects such as this. My prototype WACO used many of these bits, including a brick (integrated 4-channel receiver, brushed ESC, and two servos) from an old Flyzone Playmate.
I was planning to build a simple flat-plate wing out of thin foam. But then it occurred to me that the span and basic profile of the WACO's wing is very similar to the HobbyZone Champ, a VERY popular micro model. I decided that using a Champ wing would give me a better performing airfoil, look more authentic with its molded-in simulated wing ribs, and save me time as well. All of those benefits made it well worth the meager cost. The only modification I made was to narrow the chord by removing about .5" from the trailing edge.
My favorite aspect of this glider design is the tow release system. It uses a tiny electromagnet weighing only 2 grams. Actually, there is a permanent magnet and an electromagnet. At rest, the permanent magnet holds a small steel pin with plenty of force to tow the glider (the pin is tied to the end of the towline). When the electromagnet is energized, its magnetic field opposes the permanent magnet and the pin is released. The system only draws power for the brief time it takes to ensure separation of the pin.
It just so happens that the operating voltage and current of the electromagnet fall perfectly in line with the capabilities of the brushed-motor ESC on the WACO's brick. I wired the electromagnet to the ESC's output and I was all set. The ESC is normally controlled by the throttle stick on the transmitter (in this case, a Hitec Flash 8). However, I remapped the throttle control to a momentary switch so that I would not accidentally keep the electromagnet energized for prolonged periods and drain the 200mAh LiPo battery.
My initial flights with the WACO began with hand launches. This allowed me to trim the airplane and get a feel for its general handling characteristics. The airplane flew just fine with only rudder and elevator control. I was really surprised by long it would glide from a moderate toss.
I wanted to get a little more flight time on the WACO before hitching it to the C-47. So I added a small metal hook to the bottom of the fuselage and made a "hi-start" launcher by daisy-chaining a few dozen small rubber bands and about 20 feet of fishing line. The hi-start pulls the glider up to altitude in a graceful arc and then automatically falls away from the hook. I was able to consistently launch the CG-4A 50 to 75 feet high, fly a complete circuit and land at my feet. The longer flights and shorter walks were appreciated.
Before long, I felt confident in the WACO's handling and I was ready to hitch it to the C-47. The towline consisted of about 15 feet of 4-pound-test monofilament. I integrated a few fluorescent orange ribbons onto the line so that it would be visible in flight.
I enlisted my buddy Ron Pfender to fly the WACO while I piloted the C-47. Ron is a great pilot, but this was his first taste of aerotowing. I unknowingly handed him a real hot potato. Things started off looking pretty good. The C-47's twin motors had no trouble getting both models off of the ground. But the situation quickly deteriorated. Just a few seconds after becoming airborne, the WACO began to sway right and left. The drag of this motion caused the C-47 to slow down rapidly. Even full power was not enough to muscle out of the situation and gravity took over. Both models were soon reunited with the ground in a most ungraceful manner. Thankfully neither airplane was damaged.
Ron and I attempted several more flights with similar results. In one instance, Ron was able to release from the towline before the oscillations got out of hand and we both landed safely. He even managed a nice long glide. But it didn't take long before we got ourselves into another dramatic pileup. This time, a broken prop on the C-47 ended any further attempts.
As I drove home that day, I contemplated potential causes of the failures. Was the WACO unstable under tow for some reason? Was the C-47 too underpowered for the job? Was the towline too short? Rather than isolate and solve each concern individually, I chose the nuclear option.
Better Flying Through Technology
I built a second glider model that incorporated a number of changes from the prototype. I increased the size of the fuselage templates by 10% so that I could use an unmodified Champ wing without it looking too badly out of proportion. The extra boost in size also provided a little more space for mounting the radio gear.
My second fuselage was built with 2mm-thick Depron foam (similar to meat trays) rather than .75" insulating foam. I basically built a hollow box using the top, bottom, and side profiles. It was a fairly quick process and saved some weight over the previous method.
The most significant change for this glider involved the electronics. I utilized the brick from a HobbyZone T-28 Trojan because it has two built-in stabilization systems: AS3X and SAFE. AS3X detects small perturbations in the flight path and sends corrective commands to the control surfaces. This makes the model fly more smoothly in rough air. SAFE is more of an auto-pilot system. In the absence of inputs from the pilot, SAFE automatically controls the pitch and roll of the model to keep it in level flight.
To make the best use of the gyro-stabilized brick, I incorporated ailerons into the wings with tiny 2.4-gram servos. This model also received a hook for hi-start launching and an electromagnet tow release. Despite being larger and having two more servos, this model came in at the same 41-gram flying weight as the prototype.
I addressed my other concerns my lengthening the towline by approximately 10 feet. I also swapped the C-47's stock 250mAh LiPo with a 260mAh high-voltage LiPo (LiHV). The fully-charged voltage of the LiHV battery is .15 volts higher than the LiPo. Equally important is that this battery uses a larger connector with less resistance than the original. Consequently, I had to change out the mating connector on the airplane side. A quick test suggested that the props spin 1000rpm faster with the LiHV…significantly more than the voltage increase alone would account for.
More Failure and Some Success
I'll cut to the chase here. My second round of flight testing was much like the first. Only this time, I was able to single out the most significant problem. The bottom line is that the C-47 just doesn't have enough power to pull the CG-4A effectively. Having SAFE enabled on the glider completely eliminated the swaying that plagued my prototype. Yet, even with more power on hand and a stable glider behind, any maneuvering eventually bogged down the C-47 until it stopped flying. I am now convinced that my original goal of the flying the C-47 and CG-4A together is impractical unless the C-47 gets a serious power upgrade.
There's no question that I have to concede defeat with this project. But I'm not upset (or deterred) by that fact. Some very bright rays of hope emerged. First of all, I'm blown away by how well SAFE worked to stabilize the WACO when being towed. I am not typically a huge fan of SAFE (and similar stabilization systems) because it can very easily become a crutch for rookie flyers. This application has forced me to change my tune. In fact, the glider flew best when the pilot was completely hands-off during the tow! I'm serous. There were flights where the glider pilot did nothing but release the towline. SAFE did all of the flying, including some landings.
Once I determined that the C-47 lacked sufficient power as a tow plane, my clubmates and I experimented with some non-scale tugs. We found that brushless-powered micro planes such as the E-flite UMX Cessna 182 and E-flite UMX Timber make superb tow planes! The Timber was especially good. With these tugs, we were able to take off, fly circuits of the field and release at any altitude we wanted, whenever we wanted. There were no unplanned emergency releases! That's how it is supposed to be!
And speaking of releases, the electromagnet system has worked flawlessly. I love it. I am now planning to create an upsized version than can be used with my larger gliders. Those gliders are currently equipped with different flavors of servo-actuated release mechanisms, which require maintenance and are sometimes prone to binding. With no moving parts, the electromagnetic rig seems like the right way to go.
There's no doubt that I will continue flying both of my micro gliders with the hi-start and various tugs. I may even upgrade the C-47 one day. We'll see. But I am especially excited to explore some new variations on the "micro aerotow" theme. Judging from the reaction of my clubmates, I think that a few of them are jazzed about the small-scale aerotowing as well. Fun can be very contagious!
Terry is a freelance writer living in Buffalo, NY. Visit his website at TerryDunn.org and follow him on Twitter and Facebook. You can also hear Terry talk about RC hobbies as one of the hosts of the RC Roundtable podcast.