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Rare-earth Brushless Electric Motors

While we most often use IC engines for our UAVs, small, powerful brushless electric motors based on rare-earth magnets are now available in a wide range of sizes so there is a much greater choice of units than for IC engines. Obviously, such motors must be supported by

Table 5.2 Typical IC engine BMEP values taken from various sources.

Engine type

BMEP at peak power (MPa)

Naturally aspirated spark-ignition engines


Boosted spark ignition engines


Naturally aspirated four-stroke diesels


Boosted automotive four-stroke diesels


Very large low speed diesels


Napier Sabre 7 (3055 HP at3850rpm)


Rolls-Royce Merlin 130/131 (2030 HP at 2900rpm)


Pratt& Whitney R-4360 Wasp Major (3600 HP at 2700 rpm)


Typical small aero-modeler-based four-stroke UAV gasoline engines


Typical small aero-modeler-based two-stroke UAV gasoline engines


Note that BMEP values are typically 10-15% higher at the maximum torque speed as compared to maximum power speed and also that by assuming a BMEP of 1.0 MPa, it is possible to estimate the likely achievable peak power of a four-stroke UAV engine at a range of engine speeds.

Hacker brushless electric motor

Figure 5.10 Hacker brushless electric motor.

battery packs, fuel cells, or on-board generators (hybrid systems). In the world of multi-rotor UAVs, they are extremely widely used, and many commercial camera-carrying systems adopt these technologies. They are smooth, powerful, and highly reliable devices since they involve so few moving parts. Their Achilles heel remains the short endurances that are possible with conventional LiPo battery packs, though these continue to improve. We tend to use the motors made by Hacker[1] along with their Master Spin controllers. Brushless motors readily provide powers from a few tens of watts up to 15 kW, see Figure 5.10.

  • [1] - the Motors tab.
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