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Written by Bob Ormiston Build your own VTOL model As seen in the September 2020 issue of Model Aviation.

Bonus Video

early flight testing in the hover mode
01. Early flight testing in the hover mode with the wings at a 90° tilt. Photo by James Wang.

The OUI Sport Aerobatic RC Vertical Takeoff & Landing (VTOL) aircraft has been under development for several years. "OUI" is taken from the French word for "yes," meaning outstanding, unbelievable, or incredible. Whimsical exaggeration aside, the OUI is a canard, tiltwing tricopter VTOL aircraft designed to fly from hover, through transition, and into airplane mode flight while the pilot maintains full control throughout the entire flight envelope.

I call this "continuous transition, the full VTOL experience." The OUI is designed to perform conventional aerobatics as well as aerobatic maneuvers not possible with other model aircraft. The canard tiltwing configuration is very robust aerodynamically and provides excellent stability, control response, and performance throughout the entire VTOL flight envelope, including hovering, airplane cruise, and transition.

the oui is shown in a stationary hover pitched
02. The OUI is shown in a stationary hover, pitched up 30°, with a 60° wing tilt. The subfins are necessary for directional stability in airplane cruise mode. The trip strips, visible on the canard, cured a slight pitch oscillation in the airplane mode at low speed because of the boundary layer separation. Photo by Bob Parks.

OUI Flight Control

The OUI includes three motors and two servos for the rear wing elevons. In a hover, with both wings tilted up vertically 90°, pitch control is provided by differential rpm and thrust of the front and rear motors, while roll control is provided by differential rpm and thrust of the two rear wing-mounted motors. Yaw control is provided by differential deflections of the elevons.

This arrangement offers powerful control authority about all three axes. In airplane mode flight, with the wings tilted down at 0°, the elevons operate as elevators and ailerons to provide pitch and roll control. Differential rpm of the rear wing motors provides control in yaw.

For transition flight at intermediate wing-tilt angles, the control inputs to the motors and servos are mixed appropriately by the flight controller to provide uncoupled pitch-roll-yaw response of the aircraft in response to a pilot’s transmitter elevator, aileron, and rudder stick inputs.

The front and rear wings of the OUI are linked together mechanically with a single servo to control the tilt angle. The pilot controls transition by operating the back slider control on the left side of the transmitter with his or her left index finger. A small screw is used to extend the knob on the slider and improve the mechanical advantage and ergonomics.

The heart of the OUI and the continuous transition concept is the flight controller that provides feedback stabilization and mixes the pilot controls for the hover and airplane modes, as well as the transitions in between. More specifically, it changes the stabilization and control mixing continuously as a function of the transition—in this case, the tilt angle of the wings.

The open-source software that does this is OpenAeroVTOL (OAV), available through OAV provides extremely versatile functionality that is specifically tailored for VTOL applications and has been widely used by hobbyists, full-scale eVTOL developers, and NASA researchers. Numerous examples of OAV-controlled VTOLs can be found on the RCGroups VTOL thread.

OAV runs on the KK2.1.5 Flight Control Board (it was originally carried by HobbyKing, but it’s now available from other suppliers) that includes rate gyros and accelerometers that are used by OAV for proportional integral derivative controller feedback—typically rate, attitude, and auto-level stabilization inputs to any combination of the up to eight motors or servos that operate the aircraft. Using OAV, the designer specifies two independent sets of stabilization and control mixing gains for the hover and airplane mode flight conditions.

As the transition control is varied by the pilot (a continuous slider on the RC transmitter), OAV interpolates the stabilization and control mixing gains between the hover and airplane modes so that the airplane flies properly throughout the transition flight region.

OUI Rationale and Philosophy

Currently available commercial VTOL models do not seem to have caught on in any significant way. I attribute that to the RC model industry overlooking the fact that continuous transition of a VTOL model is its most appealing virtue. In my view, the industry has missed the potential of the RC VTOL experience.

Manufacturers seem to treat transition from the hover mode to airplane mode as a problem for the pilot, something the pilot will fear and must therefore be protected from. The manufacturers’ solution is to bypass the pilot and automate the transition process. "You have nothing to worry about. Just flip the switch and the airplane will transition automatically for you."

Virtually all commercial VTOLs (the Flex Innovations Cypher is a recent exception) are generally limited to two flight modes and a two-position switch—a clumsy solution that inevitably produces awkward transitions. Most importantly, by ignoring continuous transition, manufacturers are missing their best opportunity to excite pilots who are seeking new challenges and flight experiences and to expand VTOL beyond its tiny niche in the RC flight spectrum.

This is why I designed the OUI Sport Aerobatic VTOL. Unlike commercial VTOLs, the OUI is aimed at exploiting the VTOL opportunity and letting a pilot embrace continuous transition to fly with full control throughout the entire flight envelope.

The OUI is actually many different models in one. Instead of two airplanes—one for hovering and one for conventional flight—the OUI becomes a different airplane for each wing-tilt angle. The aim is to put a pilot in charge and let him or her embrace the full VTOL experience.

the fuselage side pieces were cut from 6
03. The fuselage side pieces were cut from 6 mm Depron foam plastic with separate pieces for the tilting nose.
the initial buildup of the fuselage structure shows
04. The initial buildup of the fuselage structure shows the balsa reinforcement. The flat-bottom airfoil simplifies construction. Six mm Depron is used for the lower surface with 3 mm Depron for the ribs and upper surface.
the rear fuselage structure shows the balsa and carbonfiber
05. The rear fuselage structure shows the balsa and carbonfiber reinforcement for the rear wing mounting, with balsa anchors for nylon hold-down bolts.

The OUI Flight Experience

What is it like to fly the OUI? The wing-tilt slider that controls the transition is the key to the OUI experience. It’s also used for maneuvering. During slow or incremental transitions, the OUI responds smoothly to the four primary pilot stick controls at each wing-tilt angle.

The OUI is stable and maintains trim for all slider positions. Additionally, the tilt slider gives the pilot another control degree of freedom. When the wing-tilt control is combined with the four stick controls, a new realm of VTOL flight opens up a new way to fly using throttle, rudder, elevator, aileron, and wing tilt—all together.

The VTOL transition can be blended into the OUI’s flight maneuvers to choreograph a variety of aerial aerobatic routines and open new potential for VTOL aerobatics.

The tiltwing VTOL offers some unique benefits over other VTOLs such as tiltrotors, multicopters, and helicopters. In hovering flight, the wing provides powerful translational dampening that makes the aircraft feel more stable and controllable. Maneuvers are smoother and it’s easier to accurately control the hover position.

The tiltwing is more sensitive than a tiltrotor to steady wind gusts during hovering, but with the OUI tilt control at a pilot’s fingertip, these disturbances are easily dealt with. A pilot simply moves the slider until the wing tilt cancels out the wind gusts.

The tiltwing affords excellent translation and maneuvering control near hover. In fact, with auto-level stabilization, the wing-tilt angle slider is the easiest way to fly because the translation velocity is proportional to the wing-tilt angle, and control of vertical motion with the throttle is largely decoupled from the wing tilt. Alternatively, the elevator stick can be used to pitch the aircraft to translate but throttle might be needed to coordinate height control at the same time.

The tiltwing is an excellent flier within the transition regime. For large tilt angles near a hover, the propellers provide most of the lift, and the OUI is able to maneuver smoothly and quickly as though it were a stall-proof airplane. Imagine flying slowly, as though you were on a landing approach, without worrying about stalling.

With continuous transition control, a pilot can vary the speed smoothly with the slider while maneuvering with rudder and aileron. The OUI can be flown like an airplane in a small area by simply increasing the wing-tilt angle to lower the flight speed as needed. For these conditions, the propeller’s slipstream minimizes wing stall and flow separation.

the nearly completed rear wing shows the fixed
06. The nearly completed rear wing shows the fixed inner panel and tilting outer panels mounted on a 10 mm carbon-fiber tube tilt spindle. Elevons are attached with simple, flexible Mylar hinges.
an early depron and blue foam concept mockup
07. An early Depron and blue foam concept mockup shows the KK2 flight control board located in the forward fuselage.

At smaller wing-tilt angles, the flight speed increases and the flight area enlarges. At the increased flight speeds, typical sport aerobatic maneuvers can be performed.

When fully transitioned, the OUI flies much like a conventional RC sport airplane, but because it carries the extra motors and mechanisms for VTOL, the wing loading is slightly higher, and it flies faster. By slightly increasing the wing-tilt angle, the speed can be reduced to match a conventional airplane.

The beauty of continuous transition is that the OUI can be instantly tailored to suit a pilot’s mood and the size of the flying area. Fully transitioned into airplane mode, control system feedback is limited to rate damping stabilization, and the OUI is fully capable of sport aerobatics.

A particularly interesting feature of the tiltwing aerodynamics is the OUI’s ability to perform stationary transitions. As long as the propellers are pointed straight up, the OUI is able to hover in a fixed position. This can occur for any tilt angle, as long as the sum of the fuselage pitch attitude, plus the wing-tilt angle, equal 90°.

In other words, the OUI can hover with the fuselage at any angle from 0° to 90°, from a level fuselage in a conventional hover all the way to a 90° nose-up hover like a 3D airplane. As long as the pilot properly coordinates the wing-tilt slider control with the elevator stick, a full transition can be performed while the airplane hovers in a stationary (fixed) position.

The stationary transition is easier to describe than to perform, and it offers an interesting challenge for flying aerobatics and can be incorporated into other aerobatic maneuvers.

The OUI is designed to take off and land in a variety of ways in addition to natural, level-fuselage VTOL. When flying from a runway and with the wing tilted down, the OUI performs conventional takeoffs and landings using its tricycle landing gear, but the fun factor really goes up for Short Takeoff & Landing (STOL) flight, with the wing at intermediate tilt angles. Touch-and-gos in STOL flight become a new experience, especially with the flexibility afforded by continuous transition. Combined with the stationary transition capability, an endless variety of slow and vertical takeoffs and landings are possible in different wind conditions.

The excellent high angle of attack (AOA) harrier flight characteristics in the airplane mode enable the OUI to fly landing approaches and landings at low speeds, limited only by the tail clearance and landing gear height. The OUI is even capable of "tailsitter" VTOL operation in the airplane mode.

The OUI canard tiltwing excels in transitions between a hover and cruising airplane flight. Again, the slider control enables many ways to do this. The easy way is to take off vertically then perform an outbound transition by slowly moving the slider to tilt the wings down and accelerate to the airplane cruise mode. The OUI remains in trim and fully controllable at all points in the transition.

The OAV can be programmed to adjust the trim in transition, but the OUI requires virtually no trim adjustments. Inbound transitions from the airplane mode back to a hover are similarly easy because the slider is simply moved slowly back to the hover position.

Alternatively, the pilot can transition rapidly (the OUI’s tilt servo travel time is typically set at 5 seconds) by flipping the slider quickly from one extreme to the other. The OUI also transitions smoothly when climbing, diving, and while turning in flight. The OUI can perform both outbound and inbound stationary transitions.

The true potential of the OUI is realized when all of these unique capabilities are combined for aerobatic flight. Many of these are beyond the realm of conventional aerobatic model airplanes. In fact, the OUI’s full capabilities are still being explored. An example of VTOL aerobatics includes tailsitter hovering with the wings tilted down in the airplane mode. This is the same as hovering a 3D aerobatic airplane.

the wings and landing gear are attached
08. The wings and landing gear are attached to the fuselage. The wingtilt servo is located in the forward fuselage and moves a carbonfiber rod connecting both wings to ensure that they maintain the same tilt angle.
the oui is shown during initial flight testing
09. The OUI is shown during initial flight testing without the fuselage and nacelle covers attached. The receiver is visible behind the KK2.1.5 flight controller and the battery squeezes into the remaining space.
the completed oui is shown in airplane cruise
10. The completed OUI is shown in airplane cruise mode before the final trim markings were applied.

Although stabilization in the airplane mode uses only rate feedback, the OUI canard configuration is nearly inherently stable in a tailsitter hover, and control is positive so that torque rolls in either direction are easily accomplished.

Similarly, harrier flight in the OUI’s airplane mode works well. The propeller slipstream reduces flow separation at high AOA, and the OUI flies smoothly and is easily controlled without the characteristic "wing walk" of typical 3D airplanes.

Torque rolls in a tailsitter hover and pirouettes in a conventional hover are pleasing maneuvers, but a "pitch-up pirouette" presents a challenge. The OUI starts in a stationary, 45° nose-up hover. A pilot’s task is to rotate the airplane smoothly on a vertical axis using rudder and aileron (crossed) controls while holding the aircraft in a fixed position. This is much easier said than done. In fact, a pitch-up pirouette can be performed in any pitch attitude.

Other VTOL maneuvers include aerobatic transitions such as inbound from airplane mode to a hover during rolls or loops, a loop-to-hover, and a roll-to-hover.

Another unique feature of some canard configurations is the ability to descend vertically in a stable stall, with the wings at a 90° AOA. In the airplane cruise mode, with power off and full up-elevator, the OUI will produce dramatic "parachute descents." Simply adding power returns the OUI to normal flight.

Plans for the OUI are not currently available, however, the OUI thread on includes additional photos, drawings, and information that should enable an advanced modeler to build the OUI.

For modelers who are interested in the unique experience of VTOL flying, several suitable build projects are accessable on, including Ran D. St. Clair’s Bixler VTOL Trainer, Multiplex Fun Cub Quad Copter, and FPV VTOL Explorer.


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