How far a hybrid VTOL drone really flies

A battery-electric hybrid VTOL drone flies from a few tens of kilometres to a few hundred, depending mainly on its size and how much of its weight is battery. The datasheet number is a best case: a defined payload, still air and a planned reserve. The distance you can plan a mission around is lower, and it moves with payload, wind, time on station and the reach of the datalink. This post explains what sets the number and how to derate it.

Range, radius and endurance are three different numbers

Before comparing aircraft, check which quantity a datasheet is quoting.

• Endurance is time in the air, in hours.
• Range is the distance the aircraft can cover through the air before the battery reaches its reserve. In still air it is roughly endurance multiplied by cruise speed.
• Operating radius is how far out the aircraft can go and still return to the launch point. It is less than half the range once any time on station is included.

A fourth number sits beside these: datalink range. An aircraft that can fly 100 km out is only useful at that distance if it can still be commanded and can still send video, or if the mission is designed to run autonomously without a link.

Manufacturers do not always use these words the same way, so ask which definition applies and under what payload and reserve the figure was derived.

The VTOL tax: what a hybrid VTOL drone gives up in cruise

A hybrid VTOL drone carries two propulsion systems. Four lift rotors raise it vertically, a pusher propeller drives it in cruise, and the wing carries the weight between take-off and landing. In cruise the lift rotors are stopped. They still cost something, in three ways.

Mass. Lift motors, speed controllers, propellers, booms and the wiring to feed them are weight that a catapult-launched fixed-wing aircraft of the same size would give to battery or payload.

Drag. Stopped propellers and their booms sit in the airflow for the whole flight. Designers reduce this by aligning the stopped blades with the airflow and fairing the booms, but they cannot remove it.

Hover energy. Holding the full weight of the aircraft on rotor thrust takes several times the power of wing-borne cruise. A vertical climb, the transition and the landing approach last only a few minutes in total, but each minute in the hover costs several minutes of cruise.

How large the total penalty is depends on the airframe, the hover time and the wind at the launch site, so a single percentage would be misleading. What matters in planning is that the penalty is mostly fixed per sortie. It is the same whether the aircraft flies 20 km or 200 km, so it weighs most heavily on short flights and least on long ones. The comparison with a plain fixed-wing airframe is covered in VTOL vs fixed-wing drone: which airframe for which mission.

What actually sets endurance

For an electric aircraft, endurance is usable battery energy divided by the average power drawn. Four factors control it.

Battery fraction

The share of take-off weight that is battery. Structure, motors, avionics and payload all compete for the same weight budget. Larger airframes tend to achieve a better battery fraction because avionics and datalink weigh roughly the same regardless of aircraft size.

Cruise efficiency

The ratio of lift to drag at the cruise speed, combined with propeller and motor efficiency. Every aircraft has one speed at which it stays up longest and a slightly higher speed at which it covers the most ground per unit of energy. Flying faster than that costs range quickly, because drag power rises steeply with airspeed.

Payload power draw

A stabilised EO/IR turret, video encoder, datalink and onboard computer draw power continuously. On a long sortie this is a noticeable share of the energy budget, and it is easy to overlook because it does not depend on how the aircraft is flown.

Reserve

Energy held back for the landing transition, a missed approach, a hold while the landing point is cleared, or a diversion to an alternate. Reserve for a VTOL aircraft has to cover hover power, not just cruise power, so it is larger than for an aircraft that glides to a belly landing.

Published figures as a worked example

The Orbit UAV VTOL platform is offered in three configurations on one airframe family. The published specification is:

Parameter	VT-60	VT-120	VT-300
Range	60 km	120 km	300 km
Endurance	2 h	3.5 h	6 h
Cruise speed	90 km/h	100 km/h	120 km/h
Payload	2 kg	4 kg	8 kg
MTOW	14 kg	24 kg	42 kg
Wingspan	2.6 m	3.4 m	4.2 m
Data link	50 km	100 km	150 km + relay

The product page states that range and endurance follow the payload and battery pack and are confirmed per order, so treat the table as the published configuration figures and not as a guarantee for every payload.

Two things are worth reading out of it.

First, the range figure is much smaller than endurance multiplied by cruise speed. For the VT-300 configuration, 6 h at 120 km/h is about 720 km of flight through still air, against a published figure of 300 km, which the same page also describes as the operating radius. Read that way, a 300 km outbound leg and a 300 km return use about five of the six hours, and the remainder is what is left for time over the target and reserve. The same arithmetic on the VT-60 gives roughly 180 km of still-air flight against a 60 km figure, which leaves more than half of the endurance for work on station. This is simple arithmetic on published numbers, not a measured mission profile, but it shows the trade: distance out and time on station come from the same budget.

Second, the datalink figure is shorter than the range figure in each column. The aircraft can fly further than the direct link reaches. Beyond that distance the mission needs a relay, which the VT-300 line indicates, or it has to be flown autonomously with the link re-established on the way back.

Derating the number for a real mission

Headwind and crosswind

Wind does not cancel out over an out-and-back route. The aircraft spends longer on the upwind leg than it saves on the downwind leg. Take a cruise speed of 100 km/h and a 30 km/h wind along the route: ground speed is 70 km/h one way and 130 km/h the other. A 100 km radius then takes about 2.2 h for the round trip instead of 2.0 h, an increase of roughly 10 %. At 50 km/h of wind the same trip takes about a third longer. The effect grows quickly as wind speed approaches cruise speed, so slower aircraft are hit hardest.

Payload versus battery

At a fixed take-off weight, a heavier payload means less battery, and a heavier aircraft needs more power to stay in the air. Both push in the same direction. If the mission needs a heavier sensor, expect less time; if it needs more time, look at whether a lighter sensor will do the job.

Hover time

Plan the vertical phases to be short. A direct climb to transition height and a direct descent cost little. A long hover while the crew decides where to land, or a slow descent in gusty conditions, costs a lot. The Orbit UAV page states wind tolerance of 15 m/s at launch; near any aircraft’s limit the hover phase uses more power, because the rotors are working against the wind on the wing as well as holding the weight.

The datalink

Line of sight limits the link before transmitter power does. Terrain between the ground station and the aircraft, or an aircraft flying low at long distance, will break the link earlier than the datasheet figure. Options are a higher antenna, a relay aircraft or ground relay, or a mission flown autonomously. On aircraft with GNSS-denied navigation and onboard computer vision, as listed for the Orbit UAV platform, the aircraft can continue the route and return on its own when the link drops.

A planning checklist

1. Fix the payload first. It sets the configuration and the usable battery.
2. Measure the route as flown, including turns and the climb, not as a straight line on the map.
3. Decide the time needed on station and convert it to distance at cruise speed.
4. Apply the forecast wind to the outbound and return legs separately.
5. Add the hover phases and the reserve required by your procedures.
6. Check the datalink path for terrain and decide whether a relay is needed.
7. If the total exceeds the aircraft’s endurance, shorten the time on station or move the launch point closer. Vertical launch makes the second option easier than it would be for an aircraft that needs a strip.

If the launch site has open ground and the payload is light, a hand-launched fixed-wing drone avoids the lift-system penalty altogether. Crews who will plan these sorties routinely should practise the calculation, which is part of UAS operator training.

FAQ

How far can a hybrid VTOL drone fly?

It depends on size. Small battery-electric hybrid VTOL aircraft are typically specified in tens of kilometres and larger ones in hundreds. As an example, the three published Orbit UAV configurations are listed at 60 km, 120 km and 300 km. The plannable distance for a specific mission is lower once wind, time on station and reserve are included.

How long can a hybrid VTOL drone stay in the air?

Endurance for electric hybrid VTOL aircraft ranges from around two hours for a light mapping configuration to several hours for a large long-range one. The published Orbit UAV figures are 2 h, 3.5 h and 6 h for the three configurations. Hover time, payload power and cold weather all reduce it.

Why is the range lower than endurance multiplied by speed?

Because a useful range figure allows for the return leg, time over the target and a landing reserve. Endurance multiplied by cruise speed gives the total distance flown through still air with nothing held back.

Can a hybrid VTOL drone fly beyond its datalink range?

It can, if the autopilot is able to fly the route without commands and the mission accepts a gap in video. Otherwise the link has to be extended with a relay. Check both the aircraft range and the datalink range when comparing platforms.