28.4. Motor selection guide

There are many, many motors available, but only a few are suitable for each helicopter. This is because the rpm/V or the Kv of the motor is very important due to the fixed gearing ratio of the drive system.

There are eleven main criteria to consider when choosing a motor:

28.4.1. Electrical characteristics

28.4.1.1. Motor Kv

For a definition see Kv.

For collective pitch helicopters, the motor Kv and pinion size must be carefully selected so the throttle will be between 90% and 100% when running at the desired headspeed. See the Section 28.1.10, “How ESCs work” section for more information.

For a fixed pitch helicopter, the motor cannot run at a constant speed so some inefficiency is unavoidable. The motor Kv and pinion size should be selected so the hovering headspeed is about 50% of the maximum headspeed possible. (The hovering headspeed for most wide-blade fixed pitch helicopters is about 1200-1300 rpm.)

Given two motors which are identical except for the Kv, the motor with the higher Kv will be more powerful and use more current, and the motor with the lower Kv will be weaker but use less current.

28.4.1.2. Motor Io

For a definition see Io.

The motor's Io is one factor which will help you determine the motor's efficient operating range.

For example, an ECO Piccolo requires about 3 amps to hover with a Hacker B20-36S. This motor has an Io of about 400 ma, so about 2.6 amps is used for hovering (minus other efficiency losses).

A Mega 16/15/4 would fit in the ECO Piccolo frame, but its Io is 1.1 amps. Therefore, the power usage would probably rise to 2.6 amps + 1.1 amps = 3.7 amps. About 30% of the hovering current is consumed by Io, which is a very high value, so therefore this motor is not very efficient when used in the 3 amp range.

A good rule of thumb for micro is to allocate about 10-15% of the hovering current for the motor's Io. For nonmicros, the percentage should be lower - about 7-10% is usually reasonable.

Better quality motors usually have a lower Io because they are more efficient and expend less power just idling the motor.

High-power motors usually have a higher Io because the are designed to run efficiently at high loads and therefore run much less efficiently at low loads.

28.4.1.3. Motor efficiency

Motor efficiency is important because the power not used to fly the helicopter is wasted as heat. For example, you might compare two motors, one with 80% and 90% efficiency. The difference appears to be only 10%.

In terms of heat generation, the 80% efficient motor will generate almost twice as much heat as the 90% efficient motor. If a helicopter requires 150 watts to hover, then the 90% efficient motor will use 166.7 watts of power and waste 16.7 watts as heat.

The 80% efficient motor will use 187.5 watts of power, and waste 27.5 watts as heat. This generates 65% more heat than the 90% efficient motor. So, motor efficiency is very important because efficient motors run much cooler.

28.4.1.4. Motor rated RPM

Some motors are rated for a very low maximum RPM, such as:

  • JETI motors have a maximum rating of 15k-20k RPM

  • Older Kontronik Tangos have a maximum rating of 25k RPM

If you use the motor past its rated RPM, then motor failure will probably occur (most likely the magnets will loosen from the rotor).

28.4.2. Physical characteristics

28.4.2.1. Motor size

Some motors are too large to fit in the some helicopters. For example, the C50-13L is too large to fit in the ECO 8.

28.4.2.2. Motor weight

A quick guide to suitable inrunner motors is to allocate 10-15% of the helicopter's AUW to the motor So, the inrunner motors suitable for an ECO 8 (AUW ~1600 grams) will be between 160-240 grams in weight.

Outrunner motors can be lighter because they are more efficient at dissipating heat. For outrunner motors, it is only necessary to allocate about 7-10% of the helicopter's AUW for the motor.

If you are not experienced with selecting motors, then you should select a motor in the 13-15% of AUW range. Motors in the 10-12% AUW are somewhat small and must be selected carefully to fly the helicopter well and avoid overheating.

28.4.2.3. Motor mounting holes

Some motors are unsuitable for helicopter use because the motor mounting holes are nonstandard.

For example, the "Nippy Black" series of brushless motors cannot be used for micro helis because it is designed for a "firewall mount" and therefore the motor mounting holes are on the rear of the motor. Other examples include the Typhoon Micro series which have a triangle-shaped pattern for the motor mounting holes, which do not fit standard helicopter motor mounts.

The Hacker B20-xxS/L, Feigao 138084xxS/L, Himaxx HA20xx, and Model Motors MiniAC 1215/xx motors have a "bearing bump" which makes installation in helicopters rather difficult. Maxx Products sells the a motor adapter (ACC3900) which eliminates this problem.

28.4.2.4. Shaft diameter and available pinion sizes

Theoretically, an Orbit 15-16 (1067 rpm/V) will fly an ECO 8. However, you would need about a 28-30 tooth 5mm pinion, and the largest available 5mm pinion is 24 tooth. So, the available pinion sizes will limit your motor choices.

28.4.2.5. Cooling options

For inrunner motors, a heatsink or fan is recommended when flying in ambient temperatures over about 70F/21C or when flying for long duration with lithium-ion or lithium-polymer batteries.

The Hacker B20, B40, and B50 heatsinks are probably the most commonly used for helicopters.

GWS style heatsink

Table 28.6. Heatsinks

MotorHeatsink
20mm diameter motors (Hacker B20, HiMaxx 2015/2025 etc)

Hacker B20 heatsink

Sparrow Products 20mm heatsink

Hyperion 20L heatsink

23mm diameter motors:GWS EHS-100 (green color)
24mm diameter motors (Speed 300 motors)GWS EHS-300 (red color)
28mm diameter motors (Mega 16/x/x, Hacker B40, etc)

Hacker B40 heatsink

Hacker B40 heatsink

Kontronik KK480 heatsink

GWS Speed 400c heatsink (GW/EHS-400) (blue color)

Sparrow Products 28mm heatsink

36mm diameter motors (Hacker B50, Kontronik 500/600, etc)

Hacker B50 heatsink

Kontronik KK600 heatsink

The Mega 22/20/3H has the shaft extending out the backside for mounting a small fan using a 5mm prop adapter instead of a heatsink. A fan can be made by modifying a Maxx Products MPIACC328. Cut off the top part of the prop adapter so the prop adapter looks like a cylinder with a disc attached. Next, disassemble a small computer fan to extract the fan portion, and epoxy the fan to the top of the prop adapter.

Be careful of motor amp ratings. These are usually specified for airplanes in a tractor prop configuration where the propeller blows air directly onto the motor. In a helicopter, there is no forced air cooling, so the same motor will only handle about half the max amp rating or less.

Most brushless motors use neodymium magnets. These magnets will lose their magnetization when operated at temperatures over 70C/158F to 120C/248F depending on magnet type. Therefore, it is important to keep the motor temperature below about 150F (by using a heatsink) to avoid demagnetizing the motor magnets.

Some brushless motors use samarium cobalt magnets (Astroflight, etc). These magnets will lose their magnetization above 250C/482F, so overheating is less of a concern with these magnets. The Hacker C40/C50 series is the same as the B40/B50 series except the C40/C50 series has a built-in heatsink so no additional heatsink is necesary. The Hacker B40/B50 series require an additional heatsink available for $15.

The easiest way to install a Hacker B40/B50 heatsink onto a motor is to use a drill bit that is slightly larger than the gap in the heatsink, and press the base (not the spiral side) of the drill bit into the gap to force it open, then slip the motor inside. This technique works extremely well. To install a B40 heatsink onto a Mega 16/15/3, you should use a 4mm drill bit.

Motors are listed from mild to powerful. Please note that I have no experience with most of these motors therefore the ordering should only be considered a rough guide, and not absolute oracle. In general the lower Kv motors with a larger pinion are milder and better for duration flying, and the higher Kv motors with smaller pinions are better for sport/aerobatic flying.

For more info on motors and pinion sizes, I recommend searching the RC Groups electric helicopter and micro helicopter forums for people's opinions on various motor and pinion combinations.

28.4.3. Other characteristics

28.4.3.1. Motor quality

The quality of motors is rather subjective, but here is a list of motors and their general quality:

Table 28.7. Motor quality

QualityManufacturer
BestPlettenberg, Lehner, Actro, Neumotor
BetterHacker, Kontronik, Mega
GoodMultiplex Permax, Astroflight
FairHimaxx, Feigao, JETI, Model Motors AXI

The motors in the best category are relatively efficient over a wide load range.

The Kontroniks are slightly better than Hacker for helicopters due to better part load efficiency.