Forward flight and VTOL cruise

Hover numbers tell you how long a build can hold position. Forward flight is where range comes from, and the Builder models it two ways depending on the Airframe type you pick. Multirotor types (Quad, Hexa, Octo, X8, and so on) carry their weight on the props and tilt into the wind; the model for them is driven by one input, the flat-plate drag area. The VTOL transition type is a tailsitter-style craft whose airfoil arms become a wing in cruise, so the wing carries the weight and the props become pure forward pushers.

The hover analysis is identical for both: pure momentum theory, unchanged by anything on this page. Only the cruise regime differs.

Multirotor forward flight#

Open the Forward flight group in the console and set Flat-plate drag area f in m². This is the equivalent flat-plate area, Cd times frontal area. When f is greater than zero, cruise power, endurance, range, and top speed all come from a full power-required curve across airspeed instead of a flat scaling of hover power.

The curve is the sum of three terms:

• Induced power, from momentum theory in forward flight. It starts at the hover value and falls as the aircraft speeds up.
• Profile power, the non-ideal part of hover power, held roughly constant across speed.
• Parasite power, 0.5 * rho * V^3 * f, which rises as the cube of speed.

Required thrust grows with speed because the disc tilts to beat body drag: T = sqrt(W^2 + D^2) with D = 0.5 * rho * V^2 * f. At zero airspeed the curve equals hover power exactly, so the cruise figures are continuous with the hover figures.

From the curve the tool reports:

• Best-endurance speed: the minimum of the power curve. Fly here to stay up longest.
• Best-range speed: the speed with the most distance per unit of energy. This is the default cruise speed when Cruise speed (in the Advanced model factors group) is 0; set a cruise speed yourself and the model uses that instead (the readout tags the source as set or best-range).
• Cruise endurance and range at the cruise speed, from the usable energy on the motor bus.
• Max speed, the smaller of the thrust-limited and power-limited speeds, tagged in the readout as thrust-limited or power-limited.

The speed sweep is adaptive: its ceiling is derived from your build's own thrust and power limits rather than a fixed constant, so it covers a slow heavy lifter and a fast low-drag racer equally. If neither thrust nor power binds before the model's own sanity ceiling of 180 m/s, the top speed is tagged model ceiling, meaning the aircraft is genuinely faster than this first-order model should claim.

The curve is plotted as Prop power required vs airspeed in the web readout and in the HTML report, with the cruise speed marked.

Leaving f at zero#

Set Flat-plate drag area f to 0 and the forward-flight model switches off. Cruise power falls back to the legacy flat cruise factor (Cruise power factor in the Advanced model factors group, a fixed fraction of hover power) and top speed falls back to the Pitch slip estimate, a fraction of the geometric pitch speed. Profiles saved before the forward-flight model existed load with f = 0 automatically, so their numbers do not change; see Build library, import and export.

Estimating f#

f is Cd times frontal area in m². The default of 0.020 m² suits a compact multirotor bullet body. Estimate your own from the frame and payload frontal area, or back it out from a logged cruise: adjust f until the predicted cruise power matches what your logs show at a known speed. f is the single input that turns range from a comparison number into a planning number, so it is worth calibrating.

VTOL transition: wing-borne cruise#

Pick VTOL transition in the Airframe type dropdown and the wing inputs appear. The arms are the wing: total wing area equals the number of arms times the arm length times the Airfoil arm chord (in mm). On a coaxial layout the arm count is half the motor count. Setting the chord to 0 disables the cruise wing; picking the type seeds a 70 mm chord for you.

The VTOL transition (wing) group holds the wing inputs:

Flat-plate drag area f still matters here: it is the parasite drag of the bullet body, added on top of the wing drag.

How the cruise model works#

In cruise the wing carries the weight, so the props fight only drag. Total drag is the wing profile drag (Cd0), plus induced drag from the wing's aspect ratio and span efficiency, plus the body parasite (f). Power required is drag times speed divided by the cruise prop efficiency, the classic fixed-wing form, which is why this regime reaches much higher speeds than a weight-carrying multirotor.

The wing can only carry the weight at or above the stall speed, sqrt(2W / (rho * area * CLmax)), so cruise is only defined from stall upward. The stall speed is the transition gate: the craft must exceed it before the wing flies.

The tool reports:

• Wing area and wing loading (N/m²).
• Stall / transition speed, shown as "wing flies above" this speed.
• Best-endurance speed (minimum power) and best-range speed (minimum drag).
• Cruise endurance and range at the cruise speed (best-range by default; a cruise speed you set is clamped to at least stall).
• Max speed, thrust-limited or power-limited, with the same adaptive sweep and a 200 m/s model sanity ceiling.

The transition-feasibility check#

Before any of the cruise numbers matter, the thrust-borne phase has to accelerate the craft past its stall speed. The tool runs the multirotor forward-flight model on the same airframe and compares its top speed against the wing's stall speed. The result appears in the readout and in the checks list: "Transition feasible: wing flies above N km/h stall", or "Cannot reach transition" with the thrust-borne top speed. When f is 0 (so the thrust-borne curve is inactive), the check falls back to requiring a thrust-to-weight ratio of at least 1.5.

A very small wing gives very high wing loading and a high stall speed, so the arm chord and CL max matter more than any other wing input.

Related#

• Airframe types
• The Builder
• Mission planner
• Physics reference