ArduPilot Plane / VTOL

ArduPilot Plane firmware covers fixed-wing aircraft, flying wings, gliders and VTOL (QuadPlane) types. They share the same controller and ground station; what differs is the aerodynamic layout and the way they take off. Before choosing, weigh what a fixed wing buys you and what it costs, then see what VTOL adds on top and what it adds in turn.

▲ VTOL aircraft supported by Plane firmware (QuadPlane)

Pros and cons of a fixed wing

Advantages

• Long endurance, long range: the wing makes the lift, so thrust only has to overcome drag. For the same takeoff weight a fixed wing typically flies 3~6 times longer than a multirotor, and the range gap is wider still.
• Fast cruise: typical cruise speed is 15~25 m/s, covering far more ground per hour.
• High payload fraction: the wing carries the lift and structure is a small share of the weight, so the same platform can carry more battery, fuel or payload.
• Good wind tolerance: high wing loading and high inertia mean gusts and turbulence affect the cruise phase less than they affect a multirotor.
• Something left to glide on: with power lost it can still glide down; a multirotor without power simply falls.
• Quiet and low infrared signature in cruise: an electric fixed wing is almost silent while gliding.

Drawbacks

• Takeoff and landing depend on a site or a launcher: it needs a runway, a catapult, a hand launch or a parachute recovery — not any patch of open ground.
• No hovering: no hold-a-point capability, no slow work close to the ground, and no taking off in tight spaces.
• Risk concentrates at takeoff and landing: stalls, wingtip strikes, catapult shocks and hard parachute landings all happen in that phase.
• Poor manoeuvrability: wide turning radius, weak obstacle avoidance and circling in low or cluttered terrain.
• Steep learning curve: the pilot has to master launch and recovery, airspeed management and the stall margin, and training takes longer than for a multirotor; site survey, catapult or runway preparation also needs a dedicated person.
• Many things to get right when tuning: centre of gravity, control-surface direction, airspeed sensor calibration, stall behaviour, pitch trim — any one wrong and the aircraft is destroyed.
• High speed means expensive single accidents: impact energy is large, and airframe and payload are often a total loss.

What VTOL adds

VTOL (QuadPlane in ArduPilot) adds a set of multirotor propulsion to a fixed wing: multirotor mode for takeoff and landing, fixed-wing mode for cruise. That is also why a VTOL fixed wing is sometimes called a compound wing.

• No airfield needed: vertical takeoff and landing, no runway, catapult or parachute recovery — a few square metres of flat ground will do.
• Hovering comes back: it can hold a point over a work site, observe slowly, and take off in confined spaces.
• Better control in the takeoff and landing phase: at low speed the multirotor propulsion controls attitude, so wind tolerance and position accuracy approach those of a multirotor.
• Simpler operations: one site covers both takeoff and transit, which makes mountains, forests and boat decks usable.
• Keeps the fixed-wing cruise efficiency: the wing still makes the lift in cruise, so endurance and range stay far above a pure multirotor.

What VTOL costs

Hovering is not free. The cost shows up in weight, drag, money and reliability.

• Dead structure weight: four (or more) VTOL motors, props, booms and ESCs do nothing during cruise and are pure dead weight.
• More cruise drag: booms and stopped props add drag, so cruise efficiency is below a pure fixed wing of the same size and endurance is typically 15%~30% lower. Top speed is usually lower than an equivalent fixed wing as well.
• Cost close to double: twice the propulsion systems and a more complex frame and connector set make purchase and maintenance more expensive than a pure fixed wing.
• More complex tuning: on top of the fixed-wing parameters there is a whole Q_ set (Q_ENABLE, Q_FRAME_CLASS, Q_TRANSITION_MS, Q_ASSIST_SPEED and so on), and the transition has to be flight-tested separately.
• Transitions are where accidents cluster: losing height on the forward transition, odd attitudes on the back transition — most VTOL accidents happen in those ten-odd seconds.
• Harder to handle a failure: a pure fixed wing can glide without power; a VTOL that loses its lift motors while hovering falls straight down, and both propulsion systems have to be reliable on their own.
• Harder to transport and deploy: booms are usually detachable, there are more connectors, and field preparation takes longer.

Comparing the three layouts

Dimension	Multirotor	Fixed wing	VTOL
Takeoff and landing site	Any flat ground	Runway, catapult or parachute	A few square metres
Hovering	Yes	No	Yes (multirotor mode)
Cruise efficiency	Low	High	Close to fixed wing, slightly lower
Endurance	20~40 min	1~3 h	40 min~2 h
Payload fraction	Low	High	Medium
Wind tolerance in cruise	Medium	Good	Good
Tuning complexity	Low	Medium	High
Cost per aircraft	Low	Medium	High
Failure outcome	Falls	Can glide	Glides in cruise, falls while hovering

Choosing a layout

• Long endurance, large-area mapping and inspection, and you have a site: fixed wing. Highest efficiency, lowest cost.
• No runway but you need endurance: VTOL. The standard choice for mountains, forests, boats and emergency work.
• Fine work at a fixed point, short range, indoors or confined: multirotor.
• Small operating radius but takeoff on demand: multirotor. VTOL dead weight and tuning cost do not pay off here.

Further reading

• Loading Plane firmware: Writing ArduPilot firmware
• CUAV fixed-wing tutorial: https://doc.cuav.net/tutorial/plane/
• CUAV VTOL tutorial: https://doc.cuav.net/tutorial/quadplane/zh-hans/
• Official ArduPilot Plane documentation: https://ardupilot.org/plane/