Multi-rotors aren't the only RC vehicles being used for FPV racing. Lots of pilots are racing flying wings as well. It's a different kind of flying but the thrill is equally infectious. And just like quads, wings are fun to fly off the racetrack too. Today, I'll show you how I assembled a simple, inexpensive flying wing and set it up for FPV.
The Flite Test Mini Arrow
The wing that I built is the FT Mini Arrow from Flite Test. If you're not familiar with Flite Test, they have a very popular YouTube channel where they promote scratch-building and other unique aspects of the RC hobby. When it comes to building a Flite Test model, you have two options. You can download the plans for free and do it all yourself, or you can buy a pre-cut kit to get a head start.
The beauty of downloading the plans is that it provides a deeper sense of satisfaction when your creation takes flight. Transforming simple raw materials into a well-behaved flying machine is definitely rewarding. If you're like me, however, and already have a number of scratch-builds on your resume, the $20 kit is a no-brainer.
Like all Flite Test airplanes, the Mini Arrow is constructed of foamboard that can be purchased at many dollar stores and craft stores. Foamboard is really nothing more than a thin sheet of foam with coarse paper laminated to both sides. It is a unique material for constructing RC airplanes and requires a few specialized techniques. Don't let that scare you. It's all easy stuff.
I'm a fan of the Flite Test YouTube channel, and my fingerprints are on a couple of their online articles, but this is the first of their models that I've had on my workbench. Yet, the Mini Arrow is hardly my first flying wing model. I've had tons of them in my hangar over the years. Some were built with traditional balsa construction. Others were cut from solid foam with a hot-wire. I've even built flying wings using sheet foam--just not foamboard.
I've always been drawn to flying wings. Their simplicity often makes them ideal platforms for trying out off-the wall ideas. That characteristic really appeals to me because I love to experiment and push the envelope with my RC stuff. Wings also tend to be tough and easily repaired when things go wrong.
Building the Mini Arrow
With a 30.5" wingspan, the Mini Arrow is not a big model. The kit parts are contained on just two sheets of foamboard-- with lots of area left over. Also in the kit is music wire for control pushrods, a pair of zip ties, three laser-cut plywood parts and a couple of bamboo skewers. "Skewers?" you ask. Yes, skewers. They are primarily used as tools, but could actually become a part of the airplane as well.
Flite Test does not provide written instructions for the Mini Arrow. The web site has a few captioned photos of the build process, but what you really want is the detailed build video. Flite Test front man, Josh Bixler, walks you through assembly from start to finish. The process is geared towards modelers who are new to building (but not necessarily flying). If you've done some building before and have preferred ways of doing things, just substitute as you see fit. I pretty much followed Josh's example to the letter. I subsequently made some modifications, but more about that in a bit.
I don't think there is any point in detailing the build process here. It's all covered in their video. Since this is such a simple model, it went together very quickly. I opened the kit before lunch and had the Mini Arrow ready for flight later the same afternoon. Much of that time was spent scrutinizing sections of the video to make sure I was doing things correctly. Now that I understand the techniques better, the next build should be even faster.
One of the reasons that the Mini Arrow comes together so quickly is that it is assembled with hot glue.
One of the reasons that the Mini Arrow comes together so quickly is that it is assembled with hot glue. While I've used hot glue for repairs on other airplanes, I've never used it as the primary adhesive. The short working time forces you to plan out your steps carefully, but the super-fast cure time is awesome.
I was slow getting the parts together on one of the wing panels and allowed the glue to cool too much. The resulting bond was loose, but I didn't think I could pull apart the compromised pieces without causing damage. So I spread a thin layer of carpenter's glue along the seam and clamped the joint. I later went straight to carpenter's glue when attaching the center pod. All of the bonds using carpenter's glue are holding strong.
Powering the Mini Arrow
There are numerous ways to equip the Mini Arrow. I took the simple route and used Flite Test's "Power Pack F". This set includes an E-Max 2204-2300Kv brushless motor, two 6x4.5 props, a BL-Heli 12-amp ESC, four E-Max 5-gram servos, and a few other small items. The Power Pack is made for several different airplanes, so there are 2 extra servos and other parts that aren't needed with the Mini Arrow. I'll use them in a future project.
The only thing I changed about the electronics was to swap the JST battery connector on the ESC for a Hobbico Star Plug. I don't trust JSTs in any system that pulls more than 5 amps, and this one peaks a little over 12 amps at full throttle. I soon found that the tradeoff for this swap is that the Star plug required more of the precious little space within the battery compartment. It gets cramped in there, but everything fits with a little finesse and TLC.
To control the Mini Arrow, I installed a Futaba R617FS receiver just ahead of the motor pod. I routed the antennas into opposite sides of the wing spar. My Futaba 7C transmitter has all of the features needed for this model, and then some. It's the same transmitter I use for my racing quad, so that should cut down on the amount of gear that I have to haul for my FPV outings.
I use an ElectriFly 3S-850mAh LiPo to power this model. It is a snug fit in the battery compartment, but I wouldn't trust that alone to keep the battery in place. I ran a strip of self-adhesive Velcro down the length of the center pod. This keeps the receiver and battery where they should be.
Completing the Mini Arrow
My plan for the Mini Arrow was to first make it airworthy without FPV gear. That would give me a chance to trim it out and get used to the plane's handling before adding the extra weight and drag of the FPV bits. I had to improvise just a little. My model was a tad tail heavy with the battery as far forward as it would go. Rather than add ballast to the nose to get the correct CG, I went ahead and added my RunCam 2 that would serve as my FPV video source and HD camera. Another strip of Velcro atop the battery access door was all it took to get the camera in place and the CG squared away.
I used standard Rust-Oleum spray paints on my model. I first tested each of the colors on scrap pieces of foamboard. I made sure to apply the paint in several light coats and I couldn't see any adverse effects to the paper laminate or foam. I prefer asymmetric paint schemes for in-flight orientation. So, I applied an orange/grey/black fade on the left wing. Low-tack masking tape worked well and didn't cause the paper to tear or delaminate.
Mini Arrow First Flights
The maiden flight of any hand-launched model can be a little stressful since you know the model isn't trimmed. I practiced the launch technique a couple of times so that I would be prepared to quickly release the plane and get both hands on the control sticks right away. As it turned out, I didn't need to worry with this model. I powered up to just over half-throttle and gave it a gentle side-arm launch. The Mini Arrow began flying away with the nose above the horizon and the wings level. No sweat.
The Mini Arrow flies…well, like a flying wing. It is not difficult to fly, but you should be comfortable with aileron-equipped models before trying this one. This model is capable of all the usual flying wing aerobatics: rolls, loops, inverted flight, and the assorted variations on those themes.
I found that I had to hold quite a bit of down elevator to sustain inverted flight. That is likely a product of the design's flat-bottom airfoil. I could probably correct that somewhat by moving the CG aft, but I like how the plane feels in other maneuvers. So I'll leave the CG alone.
Another trait that the Mini Arrow shares with other flying wings is prop noise. Pusher props tend to be noisy in general, but especially so in wings. Some models are downright obnoxious in both the volume and harshness of the prop noise. This plane doesn't approach that level, but it makes its presence known.
I think this motor/prop combination is a good match for the airframe. It provides a nice mixture of climbing power and velocity. Top end speed is fast for a park flyer, but probably not beyond the skills of an average pilot.
Landing is nearly as easy as the launch. Just chop the throttle and feed in up-elevator as the plane nears the ground. The Mini Arrow will touch down nose-high and quite slow. Time will tell how long the paper skin holds up to repeated belly landings…especially if there is any moisture on the ground.
Tweaking the Mini Arrow
I had three goals after logging a few initiation flights: finish the paint job, install FPV gear, and attempt to muffle the prop noise. The only remaining part of the paint job was to add a Tested logo. I used the same spray paints, but utilized frisket paper instead of masking tape. The frisket paper was easy to work with and didn't damage the model's paper skin.
Since I already had the RunCam 2 in place, I only needed to install a video transmitter (VTX) to complete the FPV system. I used a Lumenier TX5G6R 5.8GHz 600mW VTX with a standard whip antenna. There was no room left in the center pod, but the hollow wing panels provide alternate mounting locations. The kit even has access doors to facilitate this option.
I placed the VTX in the right wing panel. A sharpened brass tube worked great for drilling clean holes in the foam for the antenna and wires to poke through. I added a few extra holes to promote air flow through the compartment. This should prevent the VTX from getting too hot.
I added a tap to the battery plug so that the VTX could pull power from the flight battery. Next, I mated the video input and power output connections from the VTX to their partners from the RunCam 2. Having just one camera on board really simplifies things.
What lets me use the RunCam 2 for both FPV and recording is its low latency on the video out signal. I've been using the 720p/120fps setting and I can't detect any delay between my control inputs and the visible response in my goggles. Whatever latency there may be in this FPV setup, I'm not calibrated well enough to detect it when flying the Mini Arrow.
While the prop noise of the Mini Arrow isn't too bad, I felt like there was some low-hanging fruit to improve it. I often fly at a large field that is adjacent to my neighborhood. So I try to be very conscientious and courteous to avoid upsetting anyone with my toys.
A few simple tests in my workshop using a sound meter app on my phone indicated that I would see the best results by simply moving the motor further aft. This is easy to do since the motor is mounted on a detachable pod. I moved the pod back 1" and secured it in place with hot glue. I shifted the RunCam slightly forward to retain the proper CG.
I also experimented with different size propellers. However, the only significant reductions in noise were accompanied by unacceptable declines in motor power. My next move was to clean up the air around the cutout. The cutout's tall, blunt trailing edge doesn't do much to promote smooth airflow to the prop. I figured that mitigating some of this turbulence could only improve the noise level as well as the model's overall efficiency.
I made two relief cuts in the top sheeting up to the rear of the spar. The cuts allowed me to fold down the top sheeting on either side of the center pod and glue it to the bottom sheeting. This got rid of the blunt trailing edge.
Next, I used scrap foam to fill in the open sides of the cutout. I also applied lightweight drywall spackle to create smooth fillets along all of the seams in that area. When all was said and done, I measured a 3db decrease in sound pressure level. That may not seem like much, but it tells me that whatever power was previously being hijacked to generate the sound had been cut in half.
All of my workshop testing and tweaking was a fun sidebar (I told you flying wings are good testbeds), but what really matters is how those changes affect the model's noise when flying. I don't have a way to accurately measure such things, so my observations are mostly subjective. Audio tracks from onboard video clips taken before and after the mods point to a reduction in prop noise, but it's difficult to say with certainty due to the overpowering wind noise that is also present. Listening from the ground, the Mini Arrow is still far from stealthy, but it sure seems to whistle a softer and somewhat mellower tune. Or maybe it's just placebo effect. Either way, I think the airplane looks better with the restyled cutout.
I think we'll see a surge in flying wing activity as FPV racing continues to grow in popularity. These aircraft seem to be tailor-made for the demands of head-to-head competition. They pack plenty of speed and excitement. Yet, with just one motor, two servos, and no flight controller, they are much simpler and less expensive than their multi-rotor brethren. I don't see me giving up my racing quad anytime soon, but I'll definitely be keeping a flying wing in my FPV fleet from now on.