How to Get Into Hobby RC: Building Foam Airplanes

Electric airplanes made of molded foam are very popular in the RC world right now. While this class of airplanes used to be limited to small models with modest power, there is seemingly no limit to the size and power handling of modern “foamies”. Perhaps the largest contributor to their popularity is the marginal effort that’s required to assemble an attractive and nice-flying foam model. There are, however, some things to be aware of, and habits you should develop to court success with these aircraft. I recently assembled and flew a newly-released foamy to illustrate what I’m talking about.

THE COMPLETED FLITEWORK STEARMAN IS AN ATTRACTIVE AND AEROBATIC MODEL. UNFORTUNATELY, I ALSO TESTED ITS TOUGHNESS.

The Flitework Stearman

The model that I used for this article is the Flitework PT-17 Stearman. It is a 1:8 scale model of the 1942 Boeing PT-17 that is owned and flown by The Flying Bulls in Austria. Most of the model is constructed of molded Expanded PolyOlefin (EPO) foam, a popular material for RC planes. This is a Receiver-Ready (RR) model, meaning that all of the control servos and power system components are included and installed. The user must provide a radio receiver and transmitter, as well as an appropriate battery to power the airplane.

This was my first experience with a Flitework model. Overall, I would consider it a little above average among the current crop of RR foamies that I’ve seen. The mold quality of the foam components was excellent and the finish applied to the airplane was well executed. There is nothing worse than factory-applied trim schemes with sloppy paint overspray or crooked decals. I was happy that neither sin was displayed here.

THIS SHOT WAS CAPTURED WITH A MOBIUS CAMERA MOUNTED IN THE FRONT COCKPIT OF THE STEARMAN…WEEE!

Despite my positive first impressions with this model, my unboxing inspection also revealed a few common shortcomings that I would need to address. The positive side of this is that the corrections were easily implemented and didn’t incur any extra cost. As I outline the basic assembly steps, I will cover those changes, as well as some tips and tricks that may not be intuitive.

Strategery

I suggest that you go through the assembly steps mentally and envision any hang-ups that you could encounter.

Before I actually begin assembly of a model, I like to read through the entire manual first. This lets me inventory the box contents and also tells me what tools and supplies that I will need. I may even decide that I want to rearrange some of the assembly steps. For instance, the Stearman’s manual has you glue on the wings prior to installing the radio receiver. The top wing partially obscures the hatch for the radio compartment, so I decided to install the receiver first. Little tweaks like that can prevent frustration down the road.

I suggest that you go through the assembly steps mentally and envision any hang-ups that you could encounter. In this case, I realized how short the nose of the Stearman is. That factor alone had me concerned that the model would be tail-heavy and I would have trouble getting the proper Center of Gravity (CG). I couldn’t know for sure until it was all put together. But I hedged my bets during assembly by being very diligent about avoiding any unnecessary weight in the tail.

Paint Care

The durability of EPO foam makes it a good choice for airplanes. The downside, however, is that not many paints will adhere well to this foam…especially the latex paints used by many model manufacturers. While I really liked the paint job on the Stearman, I knew that its beauty was truly only skin deep. To preserve the finish for as long as possible I have been very careful with it. Incidental contact with tape (even low-tack masking tape) is often all it takes to pull off a section of paint. Forget about removing any of the decals that you don’t like. You will peel back the decal and paint.

MOST PAINTS DO NOT STICK WELL TO THE EPO MATERIAL COMMONLY USED FOR MOLDED FOAM AIRPLANES. IT IS IMPORTANT TO STRIP PAINT FROM GLUED AREAS TO ENSURE A GOOD BOND.

The glues required for EPO models are often different than those used on other types of foam. Cyanoacrylate (CA) adhesives (aka “super glue”) will dissolve most Styrofoam materials, but they work very well on EPO. Even solvent based glues such as GOOP will bond well to EPO without causing damage. Some modelers use 2-part epoxy with EPO, but I typically avoid it. I just don’t think that epoxy creates a very strong bond with EPO. The Stearman manual instructs you to use epoxy to attach the tail feathers and the wings. I used medium CA and GOOP respectively.

Whatever type of glue you choose, you have to remove the paint from the mating areas. Otherwise, your glue joint will only be as strong as the paint’s lackluster bond to the foam. I found that if I scored the perimeter of the area that I wanted to strip with a sharp X-Acto, I could usually peel off the paint in one stretchy sheet. On areas where I had to glue plastic parts to the foam, I roughed up the mating surface of the plastic with a few swipes of 100-grit sand paper. I also cleaned the area with denatured alcohol to remove any oils or mold release that could compromise the glue joint.

Power

Many models already have the motor bolted to the airframe and mated to the Electronic Speed Control (ESC). The Stearman had a brushless outrunner motor on an aluminum firewall. The motor on the Stearman was already connected to a 40-amp ESC, which also siphons power from the flight battery to the radio system. While I appreciate the convenience of having those tasks completed for me, I always double check the factory’s workmanship. I have found models with loosely mounted motors and poor connections between the motor and ESC. I have also seen motors that had become fouled because loose items such as screws and washers had found their way to the motor and attached themselves to its internal magnets.

YOU SHOULD VERIFY THE WORKMANSHIP OF FACTORY-ASSEMBLED COMPONENTS BEFORE FLIGHT. MOTORS MAY BE LOOSELY MOUNTED OR FOULED WITH FOREIGN OBJECTS.

I removed the plastic cowling that covered the Stearman’s motor and found that everything was in good shape. With that complete, it was ready for a test run-up. Before running a pre-installed motor for the first time, I declaw it by removing the propeller. An improper setting could cause the motor to spool up unexpectedly, so you don’t want the prop suddenly whipping around inside your workspace. I’ve had that happen before and it wasn’t much fun.

The radio system that I used on the Stearman consisted of a Futaba 7C transmitter and R617FS receiver. I connected the ESC (but not the servos) to my receiver. After attaching the flight battery, a 4-cell 2450mAh Flight Power Lipo, I verified that the ESC armed correctly. I then cycled the throttle stick and verified that the motor responded appropriately. Everything worked well, with no errors to troubleshoot.

Controls

As is common with EPO models, the control surface hinges for the Stearman are molded into the parts. They are just thin sections of foam that allow adequate flexing. It’s a good idea to loosen up these hinges by manually articulating the control surface through its full range of motion several times. If the servos and pushrods are installed at the factory, you should detach the pushrods for this break-in step. Keep them detached for the next step as well.

I have seen two types of control horns on EPO models. Some are glue directly to the foam. Others, like those on the Stearman, are screwed into place with a backing plate. Either type is fine. Just be sure to remove any paint if using the glue-in type. With the screw-in type, you must avoid over-tightening the screws. You want the foam to be just slightly compressed. There is little resistance, so it is easy to keep turning the screws until you have gone too far and crushed the foam.

SCREW-IN TYPE CONTROL HORNS CAN EASILY CRUSH THE FOAM IF YOU OVERTIGHTEN THE FASTENERS.

Whenever you have a model with preinstalled servos, you can never be sure that they were centered correctly before the servo horns were installed. If things were skewed, you could damage the servo or the control surface the first time that you power up the radio system. To mitigate this risk, I make sure that the servo is isolated from the control surface before connecting it to my receiver. You can do this by detaching either end of the respective pushrod, as I mentioned previously.

I connected the servos to the receiver and verified that all of the trims and sub-trims on the transmitter were centered. I again connected the flight battery to power up the system. I cycled the servos to make sure that they were all operating correctly. I then reattached the pushrods to the servos. This was also a good time to make sure that the servos were moving the surfaces in the correct direction. The servo reversing options on the transmitter provide an easy way to correct any of those issues.

The plastic clevises on the ends on the pushrods did not have a very positive locking feature. I was concerned that flight loads could cause them to pry open and detach from the control horns. If that were to happen, a crash would soon follow. To mitigate this risk, I secured every clevis with a small piece of silicon fuel tubing. Small zip ties could also be used.

I ADDED BANDS OF STRETCHY SILICON TUBING (YELLOW) TO SECURE THE PLASTIC CLEVISES ON EVERY PUSHROD.

Most assembly manuals provide recommended control surface throws. A few even provide recommended high-rate and low-rate throws, as well as exponential settings. I use the End Point Adjustment and Dual-Rate menus on the transmitter to dial in the throws. Even if exponential throws are not suggested, I typically input about 35% as a starting point. After I have flown the model, I will refine these settings to suit my preferences.

Balance

Perhaps the most important element controlling the flying characteristics of a model is the Center of Gravity (CG). CG is the fore/aft balance location of the model. The manual will provide a CG location specific to the model. Do not assume that the CG will be correct just because you used the recommended components. Always verify the CG location of your model and do whatever you must to get it correct.

Perhaps the most important element controlling the flying characteristics of a model is the Center of Gravity (CG).

As predicted, the Stearman was slightly tail-heavy. Before adding useless lead weight in the nose to balance things out, I first looked for opportunities to lighten the tail. Some models have heavy tailwheels or other hardware that can be swapped for lighter units. The back end of the Stearman was already optimized with lightweight components, so I looked for ways to add useful weight to the nose.

I noticed that there was a significant open area in front of the forward wall of the wooden battery box. I removed the wall, which allowed me to slide the flight battery further forward. I also had to cut away a portion of a foam bulkhead to allow the battery to move forward. Some modelers are very reluctant to remove any foam from their airplanes. But removing small amounts such as I did here will rarely produce any negative effects. Keep in mind that the final shape of the foam reflects the aerodynamic wishes of the designer in addition to any concessions that had to be made for ease of manufacturing. So there is usually plenty of wiggle room.

Moving the flight battery forward was not sufficient to fully correct the CG. My next option was to try a larger battery. In these cases, you want to use a battery with the same voltage, but higher capacity. I found that a Flight Power 4S-3350 would fit in the battery box and also add nearly three ounces of ballast up front. I don’t mind a little extra weight when it provides ancillary benefits. In this case, the higher capacity battery delivers increased flight time.

Even with the repositioned and larger battery, the airplane was still slightly tail-heavy. I resigned myself to the prospect of adding lead to the nose. It is always better to be heavy rather than tail-heavy. Whenever you must add nose weight, place it as far forward as you can to get the most benefit. I added 1.5 oz. of lead near the front edge on the bottom of the motor cowling. I attached the weights with Goop.

More Power

I can’t overstress the importance of using a balanced propeller. Unbalanced props cause vibrations that can create all kinds of problems….not the least of which is reduced power output from the motor. On the dramatic end, I have even seen airplanes literally shake themselves apart in flight. I use a simple fingertip balancer to check the props and add small pieces of clear tape to the light blade until it balances.

ALWAYS BALANCE YOUR PROPELLER TO MINIMIZE VIBRATION AND GET THE MOST OUT OF YOUR POWER SYSTEM. NOTE THE TAPE USED AS A COUNTERBALANCE ON THE LIGHTER BLADE.

Once I had the Stearman’s prop balanced, I reinstalled it on the airplane. The first time I ran the motor with the prop, I had a Whattmeter attached. A Whattmeter is a RC specialty tool that measures the real-time voltage, current, power and amp-hours. It is an essential tool for electric flyers. With a preconfigured power system such as this, the Whattmeter is handy for verifying that all components of the power system are operating within their design specifications. With the 4S-3350 battery, the system pulled 28 amps and produced 403 watts of power…comfortably within spec and pretty good power for a sport airplane weighing 4 pounds.

Flying

There is always a little bit of uncertainty and nervous energy when flying a new model for the first time. Sometimes there are nasty surprises, but it’s rare. If you’ve been diligent about the CG, control throws, and power system performance, things usually end well. Even so, it’s a good idea to have a co-pilot who can help make trim inputs and talk you through the flight.

My only real goal for a model’s first flight is to get it trimmed. I accomplished that in the first few circuits with the Stearman, so I began testing its aerobatic capabilities. I was performing loops, rolls, knife edge, hammerheads, and other stunts with it. I was impressed with how well it flew and had fun with it.

After landing, I verified that the battery, ESC, and motor were not excessively hot. They were barely above ambient temperature…a positive sign that they were not being overworked. I also checked over the airframe for any signs of damage or fatigue. I noticed that the top wing had become loose in its mounts, so I tightened all of the screws on the wing struts. They have held firmly since then.

Wrap Up

I think that my experience building and flying the Flitework Stearman is typical of modern foamies. The few simple techniques demonstrated here helped me to verify that the model’s great paint job would last a while and also gave me confidence that all of the model’s systems would perform reliably. Beyond looking good and flying well, there isn’t much more that you can ask of a molded foamy.

I suppose there is one more desirable quality for a molded foam airplane: durability. That trait was inadvertently tested on the model’s tenth flight. I had become quite comfortable flying the Stearman by then, so I proceeded to execute low altitude rolling circles. Somewhere along the way, my fingers and my brain got out of synch. I crashed the airplane (hard) into the adjacent sorghum field. After hearing a loud, resonant thud from the impact, I expected to find total carnage. I was amazed to find only that the top wing had cleanly separated and there were cracks in the bottom wings and motor cowling…all very repairable. The Stearman will return…the rolling circles may not!

Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Follow Terry on Twitter: @weirdflight

SOURCE:http://www.tested.com/art/makers/478282-how-get-hobby-rc-building-foam-airplanes/

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