Aircraft designers have long recognized the benefits of an airplane than can take off vertically like a helicopter and then transition to speedy forward flight. The only problem is that Vertical Takeoff and Landing (VTOL) performance is a tough nut to crack. There are lots of engineering challenges and tradeoffs standing in the way. Airplanes like the Convair XFY Pogo and Hawker Harrier helped set the stage for more modern VTOL-capable ships like the V-22 Osprey and F-35B. But VTOL remains a rare and very expensive capability.
The high power-to-weight ratio of most RC models makes achieving VTOL a little easier. Yet, there are still countless challenges to building a practical VTOL machine. For example, I owned a simple foam model of the Pogo about 10 years ago. It was a great performer in forward flight, but a real bear to control during those vertical takeoffs and landings. Crashes were common. Any landing where the model remained upright was cause for celebration.
The X-Vert from E-flite is a new model that provides a unique solution to the challenges of VTOL flight. The airframe is a simple flying wing with twin brushless motors for propulsion. All of the magic comes from the onboard electronics. A single circuit board on the model combines a radio receiver, two brushless motor controllers, and a flight controller with stabilization features. It is the flight controller with its integrated gyros that allows the X-Vert to take off vertically from the ground and then automatically transition to forward flight.
There are two versions of the X-Vert. A Ready-To-Fly (RTF) version ($200) comes with everything you need to get the model in the air. This includes the pre-built model, Spektrum transmitter, 2S-800mAh LiPo battery, and charger. If you already have a 6+-channel DSMX-compatible transmitter and a LiPo charger, you can save a few bucks by going with the Bind-N-Fly variant ($150). It omits the transmitter, battery, and charger. Batteries are available separately for $17. You'll probably want to grab a few spare batteries with either kit option.
I tested the BNF model. I fly it with my Spektrum DX8 transmitter. I have an early version of the DX8, which has slightly different menu options than second-generation versions. The X-Vert assembly manual provides transmitter set-up details for several radios, but mine is not one of them. It wasn't a big deal since I was able to see that the gist of the setup was to link the flight-mode channel to a 3-position switch and the Aux 1 channel (motor arm/disarm) to a momentary switch.
In the DX8 "Switch Select" menu, I set "Aux 1" to be commanded by the "Trainer" switch (a momentary push-button on top of the transmitter). The slightly confusing part is that I then had to set the 3-position "F.Mode" switch to control the "Gear" channel. This was necessary since the X-Vert's receiver does not have a specific "Flight Mode" channel. It uses the "Gear" channel to manipulate the flight modes. I also configured the dual-rate throws per the setup table shown in the manual.
With a wingspan just under 20 inches and weighing less than 7 ounces, the X-Vert is not a large model at all. The entire wing is packaged as a single piece. You only need to screw on plastic propeller shrouds and install clip-on winglets. The winglets actually serve as the landing gear for vertical takeoffs and landings.
Each propeller is held in place with a single screw. You definitely don't want either of these screws to come loose during flight. I removed the screws and reinstalled them with a drop of thread locker…just to be sure.
A large hinged hatch on the belly provides access to the battery compartment and electronics. Tiny 4-gram servos are imbedded in the wing. The molded foam airframe is very clean and smooth. E-flite took a minimalist approach to emulating a cockpit by using a little black paint.
The kit includes a wide selection of self-adhesive decals. I expected the X-Vert to be quite nimble, so I chose a color scheme that I hoped would help me maintain visual orientation during flight. I applied orange panels and some lettering on the top. The bottom side has three parallel red stripes running front to back. This arrangement has proven to work well for me. The airframe surface has a lot of compound curves, but the decals were easy to apply without wrinkling.
My completed X-Vert balanced right where it was supposed to with the suggested battery placed fully forward in the compartment.
There are three flight modes with the X-Vert: Multirotor, Stability, and Acro. Multirotor is the mode used for vertical takeoffs and landings. As the name implies, this mode makes the X-Vert behave like a multi-rotor. The only difference is that unlike traditional multi-rotors, the X-Vert has only two motors and moving aero-surfaces for control. The physics of control are different, but the necessary inputs are the same for the pilot.
I'm going to skip Stability mode for now and describe Acro mode. Acro is meant for forward flight. In short, Acro makes the X-Vert behave like a "normal" airplane. It can do loops, rolls, spins, and other aerobatics.
Stability mode is also for forward flight. There are, however, pitch and bank limits that prevent aerobatics. The X-Vert will also self-level in Stability mode. Simply release the right control stick and the model will automatically bring itself to straight and level flight. Stability mode limits you to little more than circling around, but that's really the point. The automation provided by this mode simplifies the transition from Multirotor to Acro mode…or vice versa. Stability mode also makes your technique much less critical during hand launches.
All three modes utilize AS3X stability. The job of AS3X is to damp out the effects of flying in windy or turbulent conditions. It is especially helpful on small, light models such as the X-Vert since they are more significantly affected by wind conditions than larger models. AS3X doesn't level the wings or fly the model for you. It just makes your flights smoother.
Flying the X-Vert
I've been testing the X-Vert at a local park with a single softball field. It's a perfect location for this bantam model. Most of my flights begin in Multirotor mode with a vertical ascent from the ground. Arming the motors in this configuration brings them to a low idle. The manual suggests gradually increasing power to lift off. Every time I tried that, the X-Vert would get light on its legs and then flop over into the grass. I have much better luck by goosing the throttle to get off the ground quickly.
Flying the X-Vert in Multirotor mode takes a little bit of getting used to. The issue is that the roll and yaw axes are swapped from that of a normal airplane. For example, inputting a left yaw command on your typical fixed-wing airplane causes the nose to swing to the left…regardless of whether the airplane is flying horizontally or pointing straight up. A left yaw command on the X-Vert in Multirotor mode causes the wings to roll to the left. It is 100% correct for a multi-rotor, it's just a little odd when your multi-rotor is an airplane pointing straight up.
The good news in all of this is that the X-Vert is rather docile in Multirotor mode. So even if you make an incorrect control input, the effect will be easy to recover from. I'd say that this model behaves a lot like many of my aerial photography multi-rotors. Control response is immediate, but sedate. On days with anything more than a gentle breeze, I was not able to keep the X-Vert from drifting downwind. It simply didn't have enough control authority to fight the wind.
Transitioning from Multirotor to Stability mode is a hoot. All you have to do is flip the Flight Mode switch. The model automatically powers up and noses over into stable horizontal flight. You then take over and fly the X-Vert as you would any other flying wing. The roll and yaw axes are just where you expect them to be!
After transitioning, you may notuice that the X-Vert is much quieter in forward flight. You'll also find that getting around in Stability mode requires large control inputs. Anyone who is comfortable with fixed-wing models should have no trouble here.
Most flying wing models do not have yaw control. I have to remind myself that the X-Vert has yaw via differential thrust of the motors. Combining yaw commands with pitch and roll really helps to tighten turns in Stability mode.
I've done numerous hand launches using Stability mode. The self-righting feature makes launching really easy. I've tossed it underhand, pushed it from the tail, and flung it sideways from a wing tip. As long as the motors are at about half throttle (or more), the X-Vert immediately whips to straight and level flight from any bad attitude I might give it.
Acro mode is just plain fun. This may be the most aerobatic flying wing that I've ever owned. It has huge control surfaces, light wing-loading, and plenty of power. Rolls with high control rates are really quick and loops can be very tight. It requires an experienced hand, but this little model snaps around and goes where you point it.
The X-Vert has an impressive speed range. You can throttle back for slow knee-high passes. It's not super-fast, but full throttle sure makes the outfield feel small. Thanks to its yaw control, this model can also perform really cool flat spins, stall turns, and other tricks that are not normally possible with a flying wing.
The only funny thing I've noticed about flying in Stability and Acro modes is a slight pitch oscillation. I'm guessing that it has something to do with the gain settings on the AS3X system. It doesn't seem to affect control response. It just looks a little funny to see it porpoising along its flight path.
You have choices when it comes to landing. The most fun way is to switch back to Multirotor mode. The X-Vert will bobble a little and then snap into a vertical orientation. You can then descend vertically and land on the tail. The other option is to belly the airplane in. The propeller shrouds will inevitably snag on the grass and bring the model to a rapid halt. It lands so slowly, however, that damage is not likely. I've had several belly landings and even an actual crash. The X-Vert still shows no sign of damage.
VTOL performance is not a new thing for RC models. But it has never really been simple or easy. The X-Vert introduces both of those elements to the equation. It is a very basic and durable model that relies on modern electronics to make it VTOL-capable. Being able choose specific flight modes makes the X-Vert stable in hovering flight and extremely aerobatic in forward flight, with very few concessions. It is a unique-looking model with equally unique flight capabilities.
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.