Axial Flux vs Radial Flux Motor: An Engineering Comparison
By Kaan Ozgen · Reviewed
Electric motor selection comes down to two fundamental geometries. In radial flux motors, the design that has dominated the industry for over a century, magnetic flux travels radially outward or inward through a cylindrical air gap. In axial flux motors, flux travels parallel to the rotation axis through a flat disc-shaped gap. Both use permanent magnets and rotating electromagnetic fields to produce torque, but the physical arrangement creates very different performance envelopes across power density, thermal behaviour, form factor, and manufacturing complexity.
For engineers designing weight-critical applications (long-endurance UAVs, eVTOL aircraft, in-wheel EV drives, or compact robotics actuators), these differences carry real design consequences. They determine whether your propulsion system fits the available envelope, meets the power requirement within the weight budget, and sustains high efficiency across the full operating duty cycle. For how an axial flux motor works on its own, with worked examples, see the axial flux motor guide; to compare suppliers, see the axial flux motor manufacturers guide.
The Physics of the Two Geometries
Torque in any brushless motor is produced by the force between the stator's current-carrying conductors and the rotor's magnetic field. That force is proportional to the product of the field strength and the current, and it is maximised when the two are perpendicular. The geometry determines how efficiently the motor arranges this interaction across its active volume.
In a radial flux motor, the stator is a cylindrical iron core with slots cut into it to hold the copper windings. The rotor, which carries permanent magnets, surrounds or sits inside this core. The iron is structurally essential: without it, the magnetic reluctance of the air gap would be too high and torque density would collapse. But iron is dense, and it introduces core losses (hysteresis and eddy current losses) that are present at all speeds regardless of load.
In an axial flux motor, the rotor discs face the stator across an axial gap, and the active magnetic interaction covers the full annular disc area rather than a narrow cylindrical shell. This geometry produces a higher torque constant per unit mass. It also enables a coreless stator design: arranging the rotor magnets in a Halbach array (a specific rotating sequence that concentrates flux on one face) removes the need for an iron backing plate behind the magnets. The stator becomes epoxy-embedded copper windings with no iron, removing all core losses from the motor's loss budget.
This geometric difference has a direct scaling consequence. In a radial flux motor, the active gap is the surface of a cylinder, so its area scales with diameter × length (D · L), and torque, which adds one more factor of radius, scales with D²·L. In an axial flux motor, the gap is an annular disc whose area scales with D², so torque scales with D³ at a fixed inner-to-outer radius ratio. Doubling the diameter of a radial motor of fixed length doubles its gap area and roughly quadruples its torque; doubling the diameter of an axial motor quadruples its gap area and multiplies its torque by about eight. That is why the axial flux advantage grows with diameter, and why in our experience it becomes clear from roughly the 1–2 kW range upward. The axial flux motor guide works through the numbers, including how short a radial motor has to be before an axial disc of the same diameter gives more torque.
Side-by-Side Comparison
The table below summarises the key engineering parameters across both topologies. The Axial Flux column is emphasised where it holds a clear advantage for weight- and efficiency-critical applications. Weight and density figures are compared on an active-part mass basis (magnets, copper, and back-iron where present, excluding housing and bearings), continuous rating.
| Parameter | Axial Flux | Radial Flux |
|---|---|---|
| Form factor | Flat disc | Cylinder |
| Power-to-weight ratio | Higher in short, wide envelopes (from about 1–2 kW) | Moderate |
| Torque density | Excellent at low RPM | Good across RPM range |
| Axial length | Very short | Longer |
| Cogging torque | Very low (coreless variants) | Moderate to high |
| Core losses | None (coreless variants) | Present at all speeds |
| Thermal management | Complex (disc geometry) | Straightforward |
| Air gap sensitivity | High: tight tolerances required | More forgiving |
| Scalability | Stack multiple rotors on one shaft | Extend axial length |
| Manufacturing maturity | Growing rapidly | Highly mature |
| Unit cost at volume | Higher (currently) | Lower |
For conversions across unit systems, use our electric motor unit converters for torque, RPM, power, and temperature.
Why Engineers Choose Axial Flux
The advantages of axial flux are most decisive in applications where system weight, packaging space, or partial-load efficiency are primary design constraints. These are the four areas where the topology consistently outperforms radial flux alternatives.
Shorter axial envelope
The disc geometry fits into tight packaging constraints (inside a wheel rim, behind a propeller hub, within a joint actuator, or flush-mounted in a nacelle) without the protruding shaft length of a cylindrical motor. For an eVTOL rotor arm, where every centimetre of axial depth affects airframe structure, or a satellite reaction wheel, where envelope is fixed by launch vehicle constraints, this form factor advantage is decisive. The active magnetic components sit within a volume that is wide and thin rather than narrow and long, which matches a wider range of mechanical integration scenarios.
Higher power-to-weight
The effective torque arm in an axial flux motor is the force-weighted radius of the annular gap, which lies between the mean and the outer radius: about 0.78 Ro for an inner radius of half the outer radius. Because the whole active face sits far from the shaft, a short, wide axial motor reaches more torque per kilogram of active-part mass than a cylindrical motor in the same envelope. How much more depends on the envelope and the power level; in our experience the gain becomes clear from roughly 1–2 kW. On a multirotor every gram counts: saving 100 g on each of four motors frees 400 g for a thermal camera, more battery or more payload. For how printed PCB and DCB stators and housing-free designs push torque per gram further, see lightweight axial flux electric motors, and for how that translates into overall system efficiency, see our efficiency definitions guide.
Direct-drive at low RPM
Because torque in an axial flux motor acts over the full annular disc area rather than a narrow cylindrical shell, the effective torque arm, close to the mean radius of the active annulus, is large relative to the motor's overall size. This produces a high torque constant (Nm/A) per unit mass, enabling direct coupling to propellers, wheels, and joint actuators without intermediate gearboxes. Eliminating the gearbox removes mechanical losses, backlash, lubrication requirements, and a significant source of weight and failure risk. A direct-drive axial flux motor in an EV wheel hub or a drone propeller shaft can reach the same output torque as a geared radial flux motor with less total drivetrain mass and without gear-mesh losses, which weigh most at partial load.
Scalable and stackable architecture
Axial flux motors offer a modular scaling path. Increasing outer diameter raises torque at a given speed; adjusting winding turns changes the voltage and current characteristics to match a target ESC or battery voltage. Additional rotor-stator pairs can also be stacked axially on the same shaft, multiplying power output without increasing the motor's footprint. Two stacks double the power; three stacks triple it. The Turncircles motor platform spans from sub-100 W single-stack units for micro UAVs to multi-kilowatt multi-stack configurations for heavy-lift and VTOL cargo applications, all sharing the same tooling, winding process, and magnet sets. This reduces development cost, simplifies certification, and compresses lead times for custom power ratings.
When Radial Flux Still Wins
Radial flux motors remain the right choice for a significant range of applications. Where continuous duty thermal performance, very high rotational speed, or lowest unit cost at volume are the primary requirements, the century of engineering refinement behind radial flux designs gives them a meaningful edge.
Sustained thermal load
The longer cylindrical casing of a radial flux motor presents a larger external surface area relative to its active volume, making convective and conductive heat paths easier to engineer. Standard finned housings, axial fans, and water-jacket cooling are all well-understood for cylindrical geometries. For continuously-loaded industrial applications (pumps, compressors, conveyor drives, HVAC fans), radial flux motors handle sustained 100% duty cycles more predictably. Axial flux motors in high-duty-cycle applications require more careful thermal design, since heat from the stator disc must be conducted out through the housing in a geometry that does not lend itself to simple finning.
Lower manufacturing cost
Decades of optimised tooling, automated lamination stamping, and precision CNC winding have driven radial flux motor costs to commodity levels in the industrial market. The supply chain for lamination steel, standard motor housings, bearings, and matched motor controllers is mature, globally distributed, and highly competitive. At volumes of thousands of units, radial flux motors remain significantly cheaper per kW than axial flux alternatives. For applications where weight and efficiency are secondary to acquisition cost (general-purpose industrial drives, appliances, low-cycle-count actuators), this cost advantage is often decisive.
High-speed operation
Radial flux designs accommodate very high rotational speeds. The cylindrical rotor structure is mechanically robust under centrifugal loading, and the air gap is defined by a simple radial clearance that remains stable as rotor speed increases. Compressor drives, high-speed spindles, turbocharger assist motors, and centrifugal pump impellers operating above 15,000–20,000 RPM are typically better served by radial flux designs. In axial flux motors, maintaining precise disc spacing becomes more challenging at high speed, as centrifugal deflection and bearing dynamics can vary the axial air gap and affect both performance and noise.
Tolerance to air gap variation
The cylindrical air gap of a radial flux motor is maintained by the radial clearance between rotor and stator, a dimension that is relatively insensitive to thermal expansion, bearing wear, or vibration-induced deflection. Axial flux motors require precise control of the disc spacing: the axial air gap is typically 0.5–1.5 mm, and variations on the order of 0.1 mm can measurably affect torque output and increase axial bearing loads. Manufacturing and assembly processes must be held to tighter tolerances, and the rotor assembly must be engineered to minimise thermal growth differentials between the disc and the shaft. For applications in harsh vibration environments or where maintenance standards do not support precision adjustment, this is a genuine disadvantage.
Efficiency Across the Duty Cycle
Motor efficiency varies with speed and load. The peak efficiency on a datasheet is one point on the motor's efficiency map; at the partial-load cruise conditions that define real-world energy consumption, efficiency is often several points lower. For radial flux motors, core losses (hysteresis and eddy current losses in the iron stator) are present at all speeds and increase with RPM regardless of whether the motor is under load. At light-load cruise settings, these fixed iron losses account for an outsized share of total input power.
Coreless axial flux motors have no iron in the stator, so stator core losses are absent. The remaining losses are copper (I²R) loss, which scales with load current, eddy-current loss in the conductors themselves at high electrical frequency, and bearing and windage friction. With no fixed core loss, the efficiency curve stays flatter at light load. The effect on endurance follows directly: at a fixed shaft power and battery, flight time scales with drive efficiency, so a motor running at 91% instead of 80% at the cruise point gives 91 ÷ 80 ≈ 1.14, about 14% more flight time from the motor's share of the energy. Those are illustrative values; the configurator's efficiency map gives the real figures for a specific design.
This broad efficiency plateau also means less heat generated per unit of output power across the entire operating range. Less heat means lighter or simpler thermal management, longer winding insulation life, and lower risk of demagnetisation from rotor temperature spikes. Lower winding temperature also extends insulation life: a widely used rule of thumb is that insulation life roughly halves for every 10 °C of sustained over-temperature.
Application Fit at a Glance
The selection logic is straightforward once the primary constraint is identified. If weight, packaging, or partial-load efficiency is the critical variable, axial flux is the correct starting point. If continuous duty thermal performance, very high RPM, or cost minimisation at volume is the driver, radial flux remains competitive. Most applications where engineers evaluate axial flux are ones where the conventional radial flux solution is already straining against weight or size limits.
| Application | Recommended | Primary reason |
|---|---|---|
| Fixed-wing UAV / long-endurance drone | Axial flux | Power-to-weight and flat efficiency curve extend flight time |
| eVTOL / urban air mobility | Axial flux | Disc form integrates into nacelle or rotor hub |
| EV in-wheel drive | Axial flux | Flat profile fits inside wheel rim without gearbox |
| Collaborative robotics joint | Axial flux | High torque density and near-zero cogging for precision control |
| Satellite / space robotics | Axial flux | Mass savings valued at launch cost; low cogging for pointing; heat leaves only by conduction and radiation, so low losses matter |
| Industrial pump / compressor | Radial flux | Continuous duty thermal performance and lower cost |
| High-speed spindle (>20,000 RPM) | Radial flux | Air gap stability at centrifugal loads |
| Conveyor / general industrial | Radial flux | Cost, supply chain maturity, serviceability |
References
- J. F. Gieras, R.-J. Wang, M. J. Kamper, Axial Flux Permanent Magnet Brushless Machines, 2nd ed., Springer, 2008: axial flux topologies, torque scaling and coreless stators.
- J. R. Hendershot, T. J. E. Miller, Design of Brushless Permanent-Magnet Machines, Motor Design Books, 2010: sizing equations, losses and thermal limits.
- D. Hanselman, Brushless Permanent Magnet Motor Design, 2nd ed., Magna Physics, 2006: the torque equation and core-loss behaviour.
Figures marked "in our experience" come from Turncircles design studies; the 14% flight-time example is arithmetic on illustrative efficiencies, not a measurement.
Frequently Asked Questions
Can I increase the number of stacks to fit the axial flux motor into my envelope?
Yes, if the envelope is limited in diameter but has axial room. Each added rotor-stator stage adds roughly the same torque, so torque grows almost linearly with the number of stacks while the shaft, bearings and housing are shared. Diameter is still the stronger lever where it is available: at a fixed inner-to-outer radius ratio, torque grows with the cube of the outer radius, so a slightly larger disc often beats an extra stack on torque per kilogram. If diameter is severely limited and length is not, a radial flux motor may be the better fit. The Turncircles configurator lets you compare diameter and stack count before committing to a design.
Does an axial flux motor always guarantee the highest power-to-weight ratio?
Not always. Motor performance depends on many parameters working together: winding configuration, magnet grade, air gap tolerance, and operating speed all interact. Our experience shows that axial flux motors deliver a power-to-weight advantage (active-part mass basis) over comparable radial flux motors starting from the 1–2 kW range and above; below that threshold, the advantages narrow. Analytics across different motor configurations, power levels, and stack counts are available in the Turncircles Configurator App, where you can compare simulated performance data for your specific requirements.
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