Towing Micro RC Gliders, Part 1: The Tow Plane

By Terry Dunn

I've flown lots of micro RC models before. However, this project reflects my first attempt at doing aerotow with tiny airplanes.

During WWII, large gliders laden with troops and supplies were sent into combat zones. There were many instances of this tactic being used throughout the war. One of the more famous examples occurred on D-Day in 1944. American C-47 transport planes departed England while pulling WACO CG-4A gliders on long tow ropes (two gliders per C-47). The British added their array of gliders and tugs as well. This aerial armada flew across the English Channel and over France, where the gliders were released to disgorge their loads of soldiers, guns, and jeeps into the fields of Normandy.

Several years ago, I designed a simple RC model of the CG-4A with a 65" (1651mm) wingspan. I also have a C-47 model of the same scale (about 1/15.5) that I modified to use as a tow plane for the WACO. Of course, each airplane requires a dedicated pilot. So, a lot of my flying buddies have had an opportunity to control one model or the other. Flying this aerotow rig is a little challenging, tons of fun, and always garners a lot of attention from onlookers.

Downsizing

When I saw a press release for Flyzone's new micro-sized C-47, my first thought was that it would be fun to shrink my WACO design to this scale as well (about 1/50). The wingspan of the C-47 model is 23" (584mm). This results in a comparable glider having a wingspan of just over 20" (508mm).

I've flown lots of micro RC models before. However, this project reflects my first attempt at doing aerotow with tiny airplanes. To be honest, I'm still not sure that it's going to work. Micro models tend to have some idiosyncrasies that may render aerotowing impractical. There's really only one way to know for sure. So I'm forging ahead!

The C-47 is a fun little bird. It flies at speeds much faster than scale.

I began work on the WACO model before my C-47 arrived. In the next article, I'll cover my techniques for creating this little green glider. Perhaps I'll also have a favorable flight report to share. For now, I'll focus on the C-47 in its box-stock form and also after being modified for towing.

About the C-47

Flyzone's micro C-47 is a pre-built foam model. It is only available in a Ready-To-Fly package ($160) that includes a radio system. It also comes factory-painted and outfitted with decals. You pretty much just have to take it out of the box, plug in a few wires, and charge the battery to get it airworthy. Flyzone also offers the civilian DC-3 variant in Eastern Airlines livery.

My initial opinion of the C-47 was good. This model definitely has the right look for a C-47. The mold quality of the airframe components impressed me. I also thought that the paint and decals had been applied well.

The included Tactic TTX403 transmitter (left) did not impress me. I much prefer flying the C-47 with my TTX850.

The included transmitter is another matter. The Tactic TTX403 borders on being toy-grade equipment. It is undersized, with plastic gimbals that feel clunky and cheap. Such transmitters are sometimes included with beginner-oriented airplanes. But this micro C-47 is no plane for rookies. It requires an experienced pilot…and I can't imagine that any veteran hobbyist would be happy with the TTX403.

Some good news in all of this is that the onboard receiver can be paired with a different transmitter using the SLT protocol. I used the TTX403 for the maiden flight, just to make sure it actually works (it does). I then tossed the cheesy radio aside and linked my Tactic TTX850…ah, so much better! The bottom line is that the C-47 demands a better transmitter than the included unit. I'd much prefer a Bind-n-Fly option that omits the low-grade radio.

The receiver, ESC, and two of the four servos are integrated into a tiny circuit board.

The airplane side of the radio system is solid. A receiver, electronic speed control, and two servos (for elevator and rudder) are integrated into a small circuit board. Two itsy-bitsy discrete servos are installed in the wing to control the ailerons.

Thrust comes from two super-small brushed motors running through single-stage gearboxes. They are attached to 3-blade propellers with a diameter of 4.25" (108mm). The motors and gearboxes are buried within the airframe. It would require a little bit of invasive surgery if you ever needed to repair or replace one of these units.

The 1-cell 250mAh LiPo battery fits behind the removable nose. Note the thin plywood that provides a secure base for the Velcro patch.

A single-cell 250mAh LiPo battery provides onboard power. Removing the C-47's detachable nose allows access to the battery mounting area. Self-adhesive Velcro is provided to retain the battery. Unfortunately, the adhesive does not stick to the airframe very well. I fixed this by using CA (aka super glue) to lock the Velcro to a small patch of 1/64" plywood. I then adhered the plywood to the battery mounting area with Foam-Tac.

There is a LiPo battery charger integrated into the TTX403. So tossing the radio also means finding a new charger. That's okay, many of the popular chargers for micro-scale models will work. You could actually use the TTX403 as a standalone charger if you wanted to.

Flyzone includes a manual that spells out the few steps required to prepare the C-47. The included info is good and there are helpful photos as well. There is, however, a glaring omission. The proper center of gravity (CG) location is not indicated. My testing has indicated that balancing the airplane at 30mm behind the leading edge of the wing is a good starting point.

Flying the Micro C-47

The C-47 includes removable landing gear. I prefer rolling takeoffs over hand-launching, so I've only flown with the gear in place. Unfortunately, the gear struts are slightly too short. The propellers scrape the ground once the tail lifts during the takeoff roll. Takeoffs from paved surfaces are still possible, but the tips of your propellers will bear scars.

This model has a good amount of power. It will get in the air quickly and has good climb performance. There isn't adequate aileron authority for rolls, but the C-47 does loop nicely. During relaxed cruising around, the controls work well and feel appropriate for this airplane. It flies more like a sport model than a lumbering transport.

This micro model can be a handful in the wind. It isn't for fledgling pilots.

The C-47's cruise speed is rather brisk. It will fly just fine at half-throttle, but that is enough power move at very un-scale speeds. This model obviously needs to maintain a fairly quick pace to keep flying. This isn't really an issue when flying the C-47 alone, but I knew it could be problematic when towing the micro WACO.

Like many micro models, the C-47 is sensitive to gusts of wind. I've been doing a lot of my test flying at field bordered by tall trees. The air here gets turbulent when the wind blows. The C-47 lets you know when it finds an eddy by dropping a wingtip or inserting a bump in its flight path. While this model can be flown in a little bit of wind, it is definitely more enjoyable in calm conditions.

My flights typically last 6-7 minutes per charge. The speed and responsiveness of the airplane make it inappropriate for beginners…especially with the stock transmitter. For intermediate to advanced pilots, the micro C-47 is a fun little airplane and it looks really good.

Tow Plane Modifications

My initial flight tests did not assure me that the micro C-47 would make a suitable tow plane. But I wasn't convinced that it would not work either. So I implemented a few modifications to give it a try. The first order of business was to add an attachment point for the towline. The full scale C-47 hitched the tow rope to the rear of the plane. With RC models, however, I've found that a location closer to the CG works better.

I drilled a 1/8"-diameter (3.2mm) hole in the top of the fuselage using a sharpened brass tube. I then inserted a plastic syringe plunger through the hole from the inside. The flange on the plunger sits flush with the inner wall of the fuselage. Foam-Tac keeps the plunger secured. My next step was to drill a 1.5mm-diameter hole in the exposed part of the syringe. The towline gets attached through this hole. I then trimmed away the excess plastic and rounded the top with a file.

I added a simple post to the top of the fuselage for attaching a tow line.

The short landing gear is mostly just an annoyance with the solo C-47. But I was concerned that it could become a more serious issue with the longer takeoff lengths that will be required when schlepping the WACO. Using the stock gear as a template, I bent new struts out of 1mm music wire. The new struts are about .25" (6mm) longer to give adequate prop clearance on relatively clean paved surfaces.

The last area I addressed was the C-47's flying speed. I wanted to shift the entire flight envelope to the left to match it more closely with the expected performance of the glider. I've done aerotows with fast tow planes before. It was difficult and not much fun. The small size of these models would only exaggerate the challenges.

I made my own landing gear struts from music wire. They are slightly longer than the stock struts (right) to provide more propeller clearance during takeoff.

The first swipe at lowering the stall speed of an airplane should always be to remove any excess weight. In the words of famous aircraft engineer, Ed Heinemann, "Simplicate and add lightness." Unfortunately, this strategy proved to be very challenging with an airplane that weighs less than 3 ounces (85g) in stock form. I scanned the C-47 for any extraneous stuff that could be removed, but found nothing. The few potential things I considered would have required major surgery to extract fractions of a gram…no thanks.

With my dieting hopes dashed, my next tactic for slowing down the airplane was to give its wing more lift. I did this the same way that full-size airplanes increase lift, by deploying flaps. I took a two-pronged approach. The first task was to have the ailerons act as "flaperons". Basically, both ailerons are deflected downward (like a deployed flap) when neutral, rather than being aligned with the rest of the wing.

With models that have each aileron servo on an individual radio channel, implementing flaperons is a simple programming function. The C-47 has both ailerons are on the same channel. So I had to emulate flaps mechanically. I could have shortened the pushrods between the ailerons and their servos. The alternate option I chose was to reposition the servo horns on the servo's output shaft. Now the ailerons still function as ailerons, but they always have a downward deflection like flaps.

The full-scale C-47 has flaps inboard of the ailerons. They are "split flaps", meaning that only the bottom surface of the wing deflects downward. I was able to emulate deployed flaps on my model using pieces of 6mm sheet foam. Each flap measures 1" x 2.75" (25mm x 70mm). I sanded each piece to have a triangular cross section. The trailing edge is 6mm thick while the leading edge comes to a point. It would have been much easier to just use 2mm thick foam.

I deflected both ailerons downward and added static flaps in an effort to make the C-47 capable of flying more slowly.

To be clear, these flaps do not move. They are fixed in the same position all the time. Adding articulating flaps would have needlessly increased the model's complexity and weight.

I wanted the flaps to be easily removable in case they didn't work well. The leading edge of each flap is attached to the wing with a thin strip of Scotch tape. The trailing edge uses foam standoffs that define the amount of flap deflection. The standoffs are glued to the flaps with foam-safe CA. Double-sided tape bond the standoffs to the wing. After some experimentation, I've sized the standoffs to deflect the flaps .5" (13mm) below the bottom wing surface.

My testing of the flaps has been very unscientific. I'm always flying in different conditions and my "measurements" are purely-subjective observations mixed with a little placebo effect. With that disclaimer out of the way, I'll say that I genuinely do think the flaps are providing a positive result. The airplane seems to takeoff, fly, and land at slower speeds. Conversely, it also appears to stall more aggressively. That's something I'll need to watch out for. If nothing else, I feel more confident heading into the first tow flight.

More To Come

As I write this, my micro CG-4A glider is essentially complete and has had several test glides (hand-launched). I'll cover all of that in the next article. As soon as conditions are favorable, I'm ready to see if this crazy micro-aerotow idea is going to work. Stay tuned!

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.