Chapter 30. Technical Appendix

Table of Contents

30.1. How helicopters work
30.2. Motor selection guide
30.3. Motor selection table
30.4. Pinion selection guide
30.5. Pinion selection tables
30.6. Servo selection guide
30.7. Electronics modification guide
30.8. Battery info
30.9. Battery care and maintenance
30.10. Maintenance and crash repair
30.11. Useful equations

30.1. How helicopters work

(Please consult Chapter 31, Glossary for any unfamiliar terms)

30.1.1. From the user to the transmitter

The helicopter's control system begins at the joysticks of the transmitter. There are two common configurations for converting the user's joystick movements into helicopter movements:

  • Mode 1 (Asia, Australia, Europe, and some UK)

  • Mode 2 (US, some Europe and some UK)

  • Mode 3

  • Mode 4

Table 30.1. Mode 1 stick assignments

JoystickDirectionUsage
left sticku/dfore/aft cyclic
left stickl/rrudder
right sticku/dthrottle + pitch
right stickl/rleft/right cyclic

Table 30.2. Mode 2 stick assignments

JoystickDirectionUsage
left sticku/dthrottle + pitch
left stickl/rrudder
right sticku/dfore/aft cyclic
right stickl/rleft/right cyclic

Table 30.3. Mode 3 stick assignments

JoystickDirectionUsage
left sticku/dfore/aft cyclic
left stickl/rleft/right cyclic
right sticku/dthrottle + pitch
right stickl/rrudder

Table 30.4. Mode 4 stick assignments

JoystickDirectionUsage
left sticku/dthrottle + pitch
left stickl/rleft/right cyclic
right sticku/dfore/aft cyclic
right stickl/rrudder

30.1.2. Inside the transmitter

30.1.2.1. Throttle stick

The "throttle stick axis" (left stick, vertical motion in mode 2) controls both the throttle and collective pitch channels.

The stick position goes through the throttle curve to generate the throttle channel, and the same stick position also goes through the pitch curve to generate the collective pitch channel.

Note that there are usually two or more flight modes, such as normal, idle-up1, idle-up2, etc and each flight mode usually has a programmable throttle and pitch curve.

30.1.2.2. Rudder stick

The rudder stick goes through the rudder exponential curve to generate the rudder signal. The rudder expo curve can be used to decrease or increase the tail movement near center stick.

30.1.2.3. Fore/aft and left/right cyclic stick

First, the values for both cyclic sticks go through the elevator and aileron exponential curves.

If an eCCPM swashplate mode is enabled, then the output of these curves goes into the swashplate mixer, and the signals for the three swashplate servos are generated.

If an eCCPM swashplate mode is not enabled, then the output of the aileron and elevator exponential curves go directly to the fore/aft and left/right swashplate servo channels.

30.1.3. From the transmitter to the lower swashplate

There are three systems which are widely used to control the lower swashplate from the transmitter:

  • Mechanical mixing (also known as mCCPM)

  • Electronic mixing (also known as eCCPM)

  • non-CCPM

30.1.3.1. Mechanical mixing (mCCPM)

For mechanical mixing, there are three servos that control the lower swashplate indirectly through a mechanical mixer.

So, the transmitter sends separate signals for fore/aft cyclic, left/right cyclic, and blade pitch.

The servos for these functions plug directly into the receiver channels. The control horns for the servos are connected to the mechanical mixer which mixes the movement of the individual servos into the lower swashplate movement.

The fore/aft servo output goes to the mechanical mixer, and its motion is converted into the fore/aft tilt of the swashplate.

The left/right servo output goes to the mechanical mixer, and its motion is converted into the left/right tilt of the swashplate.

The pitch servo output goes through the mechanical mixer, and its motion is converted into the swashplate height.

The most common type of mechanical mixer is the type used on the Raptor and T-rex helicopters. This consists of a "rocker arm" of which one side is connected to the swashplate. The left/right servo is mounted inside the other side of the swashplate, and the fore/aft and pitch servos are mounted under the rocker arm.

The ECO 8 when configured for a mechanical mixer uses a "sliding platform" type of mixer. This mixer performs all the functions of a rocker arm mechanical mixer and also performs revo mixing as well, so there are four servos connected to this type of mechanical mixer.

30.1.3.2. Electronic mixing (eCCPM)

For electronic mixing, there are three or four servos which are directly connected to the lower swashplate which control the height and tilt of the swashplate.

There are usually three servos which are connected to the lower swashplate at 90 or 120 degrees apart, although there are variations which use four servos or have the servos placed 140 degrees apart.

So, the transmitter reads the stick positions and performs calculations internally to determine the positions of the servos to create the correct swashplate height and tilt.

These servo positions are transmitted to the receiver which sends the information to the servos.

The movement of the swashplate is the same as with mechanical mixing. The fore/aft cyclic controls the fore/aft tilt of the swashplate, the left/right cylic controls the left/right tilt of the swashplate, and the pitch controls the height of the swashplate.

The Logo 10, ECO 8 configured for electronic mixing, Hummingbird Elite CP, Hornet CP, and other helicopters use this system.

30.1.3.3. non-CCPM

For a non-CCPM control system, there are two servos which are directly connected to the lower swashplate, and a third servo which directly controls the main blade pitch.

So, the transmitter sends separate signals for fore/aft cyclic, left/right cyclic, and blade pitch.

The servos for these functions plug directly into the receiver.

The fore/aft servo is directly connected to the lower swashplate and controls the fore/aft tilt of the swashplate.

The left/right servo is directly connected to the lower swashplate and controls the left/right tilt of the swashplate.

The pitch servo is not connected to the swashplate and controls the main blade pitch through a separate mechanical connection.

From the transmitter's point of view, the mechanical CCPM and non-CCPM control systems are identical, because both have independent servos for collective pitch, fore/aft cyclic, and left/right cyclic.

On the Zoom 400, the main shaft is hollow, and there is a rod which runs through the main rotor shaft and controls the main blade pitch.

On the Piccolo Pro, the there is a hollow tube which goes around the main shaft and inside the swashplate and controls the main blade pitch.

30.1.4. From the upper swashplate to the rotor head

There are three systems which are widely used to control the rotor head from the upper swashplate:

  • Bell control system

  • Hiller control system

  • Bell-Hiller control system

30.1.4.1. The Bell control system

For a Bell control system, the upper swashplate is directly connected to the main blade grips. Usually there are two main rotor blades and these are directly connected to two control balls on the upper swashplate. This is sometimes called a "flybarless" control system.

One advantage of the Bell control system is very quick cyclic response. The control system directly controls the main blade pitch, so the system is very sensitive to swashplate changes.

One disadvantage of the Bell control system is the lack of stability. The system is very sensitive to minor gusts. It also stresses the control linkages because they control the heavy rotor blades directly and therefore strong servos must be used.

The early helicopters using Bell control systems had a "stabilizer bar" which was perpendicular to the main rotor blades, which had heavy weights attached to the ends (similar to a flybar but with weights instead of paddles and no tilting). This added stability to the system.

There are no popular helicopters which implement a pure Bell control system.

30.1.4.2. The Hiller control system

The Hiller control system was invented by Stanley Hiller in the 1940s. This was originally called the "Rotormatic" control system, and was so stable the first prototypes could be hovered hands-off for minutes at a time.

For a Hiller control system, the upper swashplate does not directly control the main blade pitch. Instead, it indirectly controls the main blade pitch by changing the pitch of the flybar paddles. As the flybar seesaws up and down, it changes the main blade pitch.

Two advantages of the Hiller system are:

  • It places less stress on the swashplate servos because they only control the pitch of the flybar paddles.

  • The flybar paddles dampen pitch and roll which improves stability.

One disadvantage of the Hiller system is the lag in control response. The flybar paddles must change their plane of rotation in order to change the main blade pitch.

Two popular helis which implement the Hiller control system are the Piccolo ECO/Fun and the GWS Dragonfly.

30.1.4.3. The Bell-Hiller control system

The Bell-Hiller control system is a hybrid of both the Bell and Hiller control systems. The key component of this system is the Bell-Hiller mixer which mechanically mixes both the flybar tilt and the swashplate tilt.

There are two basic types of Bell-Hiller control systems:

  • Nonmovable flybar systems

  • Movable Flybar systems

In a Bell-Hiller system with a nonmoving flybar, there are usually four control balls on the upper swashplate.

Two of the control balls on the upper swashplate are connected to the washout unit which is a mechanical isolation device which allows the flybar to tilt with the swashplate, but not move up and down.

The other two control balls are connected directly to the Bell-Hiller mixer. The tilt of the flybar also affects the Bell-Hiller mixer. The output of the Bell Hiller mixer controls the main blade pitch.

So, in a Bell-Hiller control system, the swashplate controls the flybar pitch and the flybar tilt affects the main blade pitch. This is the Hiller component of the Bell-Hiller control system.

The swashplate tilt also affects the main blade pitch through the Bell-Hiller mixer. This is the Bell component of the Bell-Hiller control system.

Two popular helis which use a Bell-Hiller system with a nonmoving flybar are the ECO 8 and Logo 10.

The Bell-Hiller system with a moving flybar works exactly the same as the nonmoving flybar system except the washout unit is eliminated and the flybar is allowed to move up and down with the swashplate.

Advantages of the Bell-Hiller system include:

  • Less control force required (as in the Hiller system)

  • More stable than a Bell system (as in the Hiller system)

  • Faster cyclic than the Hiller system due to some direct swashplate input

Disadvantage of the Bell-Hiller system include:

  • Slightly slower cyclic response than a pure Bell system

  • More complexity than either a Bell or Hiller system

The advantage of the moving flybar system is it has fewer parts (no washout unit) therefore has less slop in the control system.

Three popular helis which use a Bell-Hiller system with a moving flybar are the Hensleit 3DNT, Hensleit 3DMP and the Century Hummingbird Elite CP.

30.1.5. The rotorhead

There are two types of rotorhead designs commonly used on R/C helicopters:

  • Flapping head (technically a fully articulated rotor head)

  • Seesaw head (technically a semi-rigid rotor head)

A flapping head allows each rotor blade to flap, lead/lag, and feather independently. This is accomplished by having having three hinges per blade:

  • flapping hinge, which allows each blade to flap (move vertically)

  • lead/lag hinge, which allows the blade to lead and lag (move horizonally)

  • feathering shaft, which allow the blade to feather (rotate along long axis)

This is typically used on helicopters with more than two blades, but some two-bladed helis such as the Corona use this.

A seesaw head typically has the blade grips for two blades connected by a feathering shaft. This feathering is rigid and only allows the two blades to seesaw as a single unit, e.g. when one blade rises, the other is forced to sink. The two blades will still feather and lead/lag independently, however.

Most R/C helicopters use this system, including the ECO 8, Logo 10, Shogun, T-rex, etc.

30.1.6. Basic helicopter aerodynamics

30.1.6.1. Dissymmetry of lift

When a helicopter flies forward, the rotor blades generate unequal lift. This phenomenon is called "dissymmetry of lift".

For a clockwise rotating rotorhead, the blade on the left side of the helicopter is moving into the wind (advancing blade) and the blade on right side of the helicopter is moving with the wind (retreating blade). The advancing blade generates more lift, and the retreating blade generates less lift.

(Technically this is dissymmetry of moment, because the center of lift for the advancing/retreating blades is at different points along the length of the blade, but most most helicopter texts reference this as dissymmetry of lift.)

This dissymmetry of lift is equalized in different ways by the flapping and seesaw rotor heads.

For a flapping head, the upward motion of the advancing blade decreases the relative speed of the wind, and the downward motion of the retreating blade increases the relative speed of the wind. This is similar to holding your hand at the window at 40-50 mph. If your hand is slightly titled up at the leading edge, and you raise your hand, this decreases the apparent lifting pressure, and lowering your hand increases the apparent lifting pressure.

The flapping head may also use a rotor head where the two blades feather together (are rigidly connected on the feathering axis) and use blades where the center of pressure (center of lift) is behind the feathering pivot point. This causes the two blades to naturally equalize lift as the blade generating more lift will push its leading edge downward which also raises the leading edge of the other blade.

For a seesaw head, the advancing blade will rise up due to the extra lift. This rising motion causes the blade to feather and slightly decrease the angle of attack because it pivots around the blade grip's control ball. The amount of feathering is dependent on the angle formed by a line drawn between the blade grip control balls and a line perpendicular to the rotating axis of the blades, and is called the delta-three angle.

The delta-three angle may also be used with the flapping head to induce the blade to feather as it flaps up and down.

Note that this dissymmetry of lift is experienced by both the main and tail rotors on an R/C helicopter. The two rotor heads may use different methods of equalizing lift; for example, the ECO 8 uses a seesaw main rotor but uses a flapping tail rotor relying upon the delta-three angle to equalize lift.

30.1.7. How gyros work

Many people seem to be confused about how gyros work. In order to understand how a gyro works, it is necessary to first understand the relationship between the main rotor and the tail rotor.

Most helicopters have a clockwise main rotor, so for this section, we will assume the main rotor blades are spinning clockwise.

Also, some helicopter use a variable pitch tail rotor and some helicopters use a variable-speed motor for the tail rotor, so this section will use the terminology "increase/decrease tail rotor thrust" to accomodate both cases.

30.1.7.1. The functions of the tail rotor

The three basic function of the tail rotor are:

  • Counter main rotor torque

  • Turn (yaw) the helicopter

  • Stabilize yaw

30.1.7.1.1. Counter main rotor torque

The first function provided by the tail rotor is to counter main rotor torque.

The motor in a helicopter spins the blades clockwise. But in order to spin the blades, the motor needs to push against something. In this case, the motor is pushing against the body of the helicopter. So, when the motor spins the main rotor blades clockwise, the body of the helicopter tends to spin counterclockwise. This is consistent with Newton's Third Law of Motion which states:

"For every action there is an equal and opposite reaction."

In this case, the action is a clockwise rotation of the main rotor blades, and the reaction is the body of the helicopter turning counterclockwise. So, the tail rotor needs to provide the correct amount of clockwise thrust thrust to balance the counterclockwise reaction to the clockwise main rotor. For lack of a better term, we will call this "main rotor counter thrust".

30.1.7.1.2. Turn (Yaw) the helicopter

The second function provided by the tail rotor is to turn (yaw) the helicopter.

If we need to turn left, then we set the tail rotor thrust to slightly less than the main rotor counter thrust. This means the counterclockwise force (reaction of main rotor) will be greater than the clockwise force (tail rotor thrust) so the body of the helicopter will turn counterlcockwise.

If we need to turn right, then we set the tail rotor thrust to slightly more than the main rotor counter thrust. This means the counterclockwise force (reaction of main rotor) will be less than the clockwise force (tail rotor thrust) so the body of the helicopter will turn clockwise.

If we don't need to turn, then the tail rotor thrust is exactly the main rotor counter thrust. For lack of a better term, we will call this the "turning thrust".

30.1.7.1.3. Yaw stabilization

The third function provided by the tail rotor is yaw stabilization.

When airflow hits the side of the helicopter, the helicopter will tend to "weathervane" into the airflow because there is more leverage against the tail of the helicopter than the nose.

This airflow can be either a random gust of wind, or the helicopter may be moving sideways relative in still air.

We can use the tail to correct the orientation of the helicopter by increasing or decreasing the thrust of the tail rotor. For lack of a better term, we will call this the "yaw stabilization thrust".

So the total thrust of the tail rotor should be all three of these variables added together, or:

tail rotor thrust = main rotor counter thrust + turning thrust + yaw stabilization thrust

30.1.7.2. Yaw rate gyros and how they work

For a yaw rate gyro, the functions are controlled by the following devices:

  • Counter main rotor torque - transmitter revo mixing

  • Turn (yaw) the helicopter - rudder stick

  • Stabilize yaw - yaw rate gyro

A yaw rate gyro is a very simple device. It only senses the turn rate (angular acceleration) and it cannot sense the absolute orientation of the helicopter. In technical terms, it "dampens" the tail movement.

Imagine you are blindfolded, and are standing on a frozen lake wearing smooth shoes. A person will try to turn you, and you are only allowed to resist the turning force by digging in your shoes into the smooth slippery ice. Basically, you cannot resist the turning force very much,and once you have been turned, you do not know the original orientation.

This is very much like a yaw rate gyro.

Therefore, a yaw rate gyro can only provide partial yaw stabilization thrust. Usually the amount of yaw stabilization thrust is controlled by the gyro gain. Increasing the gain will make the helicopter more resistant to random turning, but it also decreases the pirouette rate because the gyro will fight against both random and intentional yawing movement.

A yaw rate gyro cannot provide "heading hold" capability because it only pushes against the turning movement but will slip somewhat, and once it's been turned it cannot return the helicopter to the original orientation.

A yaw rate gyro "slips" when trying to countering tail movement, so it cannot effectively counter main rotor torque. Therefore, the main rotor counter thrust is provided by the revo mixing function on the transmitter.

The revo mixing allows you to set the tail rotor thrust for each throttle position so the tail rotor thrust exactly counters the main rotor torque. There is no formula for setting these values; they must be empirically set by trial and error.

The turning thrust is governed by the rudder stick on the transmitter, the same as in a heading hold gyro.

30.1.7.3. Heading hold gyros and how they work

For a Heading-hold gyro, the functions are controlled by the following devices:

  • Counter main rotor torque - heading hold gyro

  • Turn (yaw) the helicopter - rudder stick via heading hold gyro

  • Stabilize yaw - heading hold gyro

A heading hold is more sophisticated than a yaw rate gyro and functions completely differently.

The first big difference between a heading hold gyro and a yaw rate gyro is that the heading hold gyro has a microprocessor on-board and can remember how much the helicopter has turned. Therefore if a random wind gust turns the helicopter, it can always return the helicopter to the original orientation.

Therefore, the heading hold gyro can supply the correct main rotor counter thrust automatically because it doesn't "slip". When you apply throttle and the tail starts to move because there's more main rotor torque, the heading hold gyro can increase the tail rotor thrust to turn the tail back to its original position.

Also, the heading hold gyro can provide the correct yaw stabilization thrust because it doesn't "slip", and therefore it can retain the correct orientation at all times.

The second big difference is the rudder signal from the transmitter does not directly control the tail rotor thrust. Instead, the rudder signal is considered a high-level command to "turn the tail at x degrees per second" and the heading hold gyro will automatically use the correct amount of tail pitch to create this tail motion and compensate for wind and other external factors which affect the tail.

Note that the revo mixing MUST be disabled for the heading hold gyro to work properly. If the revo mixing is enabled, then the heading hold gyro will interpret it as a signal to turn the helicopter.

30.1.7.4. The difference between yaw rate and heading hold gyros

Imagine we have a helicopter with a properly configured yaw rate gyro and the motor is disconnected and it is on the ground where it cannot turn. If we hold left rudder on the transmitter for one second and then center the stick, the servo will move to one extreme servo position for one second and then center.

Now, imagine we have the same helicopter with a properly configured heading hold gyro, and the motor is disconnected and it is on the ground where it cannot turn. Imagine that the setting for this heading hold gyro is full left stick is 180 degreees per second.

If we hold left rudder on the transmitter for one second and then center the stick, the heading hold gyro will know the helicopter should turn counterclockwise 180 degrees.However, since the helicopter is on the ground and cannot turn, the tail servo will stick at one extreme and stay there - the heading hold gyro will keep trying to turn the helicopter. If we manually pick up the helicopter and turn it counterclockwise 180 degrees, the servo will finally center.

Note that both types of gyros only stabilize YAW. Neither type of gyro will stabilize roll or pitch. Technically speaking, a helicopter gyro contains an angular acceleration sensor for only one axis.

30.1.8. GY-series gyro technical info

30.1.8.1. GY401 setup

There are six adjustments on the GY401: direction, DS, delay, limit, gain and pirouette rate. The delay and limit are controlled by trimmers on the gyro itself. The gain adjustment is controlled at the transmitter by setting the value of the gyro channel, and the pirouette rate is set by adjusting the endpoints of the rudder channel.

30.1.8.1.1. GY401 direction switch

For a heli with a tail servo, the GY401 direction switch configures the direction the gyro moves the servo for clockwise and counterclockwise movement.

For a heli with a tail motor ESC, the GY401 direction switch configures whether the gyro moves the throttle up or down to turn the heli clockwise or counterclockwise.

For both heli types, If this switch is set incorrectly, the heli will pirouette wildly and out of control.

30.1.8.1.2. GY401 DS (digital servo) switch

The GY401 DS switch allows the gyro to update a digital servo about 4x faster then a normal servo.

Do not set this switch unless you are using a servo which supports a 270 hz frame rate, such as the S9253, S9254, or Volz Speed-Maxx.

Do not set this switch if you are using a tail motor ESC.

30.1.8.1.3. GY401 delay trimmer

The Delay adjustment is on the gyro itself, and allows you to configure the gyro for the response of the tail rotor system. This is only used in heading hold mode.

The delay setting of 0 is used for very fast servos such as the S9253/S9254. The delay setting of 100 is used for very slow tail servos and for tail motor ESCs. A tail motor ESC requires at least half a second to go from half throttle to full throttle, which is about twice as slow as the slowest tail servos ( 0.25 sec/60 degrees)

For a heli with a tail servo, if the delay setting is too low, then the gyro will assume the servo is faster than it really is. So the gyro will send commands to the servo to move very quickly, and the servo will try to move to the new position but it will be too slow, and it will lag behind the gyro commands.

For a heli with a tail motor ESC, if the delay setting is too low, the gyro will assume the tail motor can change RPM very fast. So the gyro will try to change the speed of the tail motor very quickly but the tail motor RPM will lag behind the gyro commands.

This delay setting seems to affect the end of a turn (yaw). At the end of a turn, the gyro needs to increase the tail thrust to slow down the tail then decrease it to maintain a steady tail position.

For example, imagine you are performing turning the heli (yaw) and you suddenly stop. If the delay setting is too low in this situation, the tail will wag a few times before settling down because the gyro will overshoot the end of the turn and need to correct the heading a few times.

If you increase the delay, this may allow you to increase the gain, although you may not need this as a beginner.

30.1.8.1.4. GY401 limit trimmer

The limit adjustment is on the gyro itself, and allows you to set the endpoints of the servo travel.

For a heli with a tail servo, if the limit setting is too high, then the gyro will attempt to move the tail servo too far in one or both directions. This will cause the servo to bind and emit a buzzing noise, which creates accelerated wear on the servo motor and early servo failure.

If the limit setting is too low, then this will decrease the range of the tail blade pitch. This will reduce your pirouette rate and may cause tail control problems.

For a heli with a tail motor ESC, if the limit setting is too low, then the tail motor ESC may fail to arm when holding the rudder stick full left. Therefore you cannot set the limit too low when using a tail motor ESC, but this is dependent on the tail ESC.

For a heli with a tail motor ESC, if the limit setting is too high, then this causes severe problems. The tail motor ESC will not arm because holding left rudder will cause the gyro to emit a signal that is too low for the tail motor ESC to arm. Also, if you hold extreme right rudder, the tail motor ESC may shut down because the signal is outside the valid range.

Therefore, for a heli with a tail motor ESC, the limit trimmer should be set to 100%.

30.1.8.1.5. GY401 gain setting

The gain adjustment is done at the transmitter via the gyro gain connector on the channel. This MUST be plugged into a receiver channel.

There isn't good documentation for the gain setting available, so the following information is what I have personally deduced from my own observations, which may not be completely correct.

The gain setting seems to control how much the tail is allowed to drift before the gyro will correct the position. It is basically a "fussiness" value. A low gain allows the tail to drift of about 2 or 3 degrees in either direction before the gyro will correct the position. A high gain allows a drift of less than 0.5 degrees.

There are two factors which limit the maximum gain setting:

For a heli with a tail servo, the limiting factors are the tail servo resolution and the amount of slop in the tail rotor pitch control mechanism.

If the tail servo resolution is low, then the gain setting must be fairly low to prevent wag.

For a heli with a tail motor ESC, the limiting factors are the tail motor ESC's throttle resolution and the inertia of the tail motor and tail rotor.

Imagine a tail servo or tail motor ESC with an extremely low resolution; let's say only 9 steps between the low and high endpoints. So, any position between 0 and 9% is truncated to 0%, any position between 10% and 19% is truncated to 10%, etc.

Imagine that hovering requires a tail channel position of 57%. However, the tail servo or tail motor ESC has limited resolution and can only be at 50% or 60%.

If the gyro gain is set very high, then the gyro will be very fussy about the tail position, and will keep changing the tail position. This will cause tail wag.

If the gyro gain is set fairly low, then the gyro will be less fussy about the tail position and will allow some drift before correcting the tail position. This reduces or eliminates the tail wag.

If the tail rotor pitch control mechanism has a lot of slop, then the gyro will need to move the tail servo past the slop in either direction before the tail pitch will change.

If the gyro gain is set to a high value, then the gyro will be fussy and will try to move the tail servo often. This will cause wag.

If the gyro gain is set to a low value, then the gyro will be less fussy about the tail position and won't care about small changes in tail position, and this will decrease or eliminate wag.

Many guides recommend the gain value be set as high as possible without causing wag, but this causes the servo to wear more quickly. I recommend this value be set to a slightly lower value than the maximum possible value to reduce servo or tail motor wear.

If your transmitter o supports the GY mode (Futaba 8U, 9C, 7C, etc) then you can set the mode to either heading hold (AVC) or yaw rate (NOR). If your transmitter does not support the GY mode, then you can set the mode with this formula:

For heading hold mode, take the 50 and add the heading hold gain percentage divided by two, and this is the percentage of the travel you should use for the transmitter's gyro gain channel.

For example, to set heading hold with 60% gain, then this would be (60 / 2) + 50 = 80% of full travel.

For yaw rate mode, take 50 and subtract the yaw rate gain divided by two and this is the percentage of travel you should use for the transmitter's gyro gain channel.

For example, to set yaw rate mode mode with 40% gain, this would be 50 - (40 / 2) = 30% of full travel.

30.1.8.1.6. GY401 pirouette rate

The pirouette rate adjustment is done at the transmitter via the rudder channel EPA. 100% EPA is roughly about 720 degrees/second maximum pirouette rate. Setting this to a lower value decreases the max pirouette rate, and increasing it will increase the max pirouette rate.

30.1.8.2. GY240 setup

There are only three adjustments on the GY240: direction, gain and pirouette rate.

30.1.8.2.1. GY240 delay setting (fixed)

The GY240 has a fixed delay setting and this is not adjustable. However, the GY240 assumes a very slow servo, and the fixed delay setting appears to be greater than 100% delay on the GY401. This is one reason why the GY240 appears to work better with tail motors than the GY401.

See also the GY401 delay setting for more info.

30.1.8.2.2. GY240 limit setting (fixed)

The GY240 limits are not adjustable.

If you are using a heli with a tail servo, you will need to adjust the mechanical linkages so the tail pitch slider does not bind at both extremes of travel. Pragmatically, this means you will need to try different holes in the tail servo horn and/or tail pitch lever until the tail pitch slider can travel to both extremes without binding.

If you are using a heli with a tail motor ESC, then there is no adjustment required.

See also the GY401 limit setting for more info.

30.1.8.2.3. GY240 direction switch

This functions identically to the GY401. See the GY401 direction switch explanation for more details.

30.1.8.2.4. GY240 gain trimmer

The GY240 gain trimmer works similar to the GY401 gain trimmer, however the GY240 gain is less sensitive than the GY401 gain. My rough guess is the GY240 60% gain is about the equivalent of the GY401 30% gain.

30.1.8.2.5. GY240 pirouette rate

This GY240 pirouette rate is configured in the same way as the GY401 pirouette rate. However, the maximum piroutte rate is only about quarter that of the GY401.

30.1.9. Yaw-rate gyro technical info

The gain on a yaw-rate gyro functions differently than on a heading hold gyro. It controls how much the gyro dampens (works against) movement. This is fine if you are not using the rudder. However, if you use the rudder, a yaw-rate gyro will dampen this movement too.

So increasing the gain on a yaw-rate gyro has two effects:

  • It dampens random movement more, so the tail is more stable. This is good.

  • It also dampens intentional tail movement, so this decreases the maximum pirouette rate. This is bad.

A standard single-rate yaw-rate gyro exhibits these bad behaviors. There are more sophisticated yaw-rate gyros which partially fix some of these problems:

Dual-gain gyros allow you to set two gain settings on the gyro and switch between these two gain settings from the transmitter, so you can decrease the gain when you need high pirouette rates.

Remote-gain gyros allow you to set the gain from the transmitter by a knob or switch, so you can decrease the gain when you need high pirouette rates.

30.1.10. How ESCs work

30.1.10.1. Pulse-width modulation

To fully understand how brushed and brushless ESCs work, it is necessary to understand the concept of pulse-width modulation.

First, imagine that you have a water pipe with a valve which can only be fully opened or fully closed. If you open this valve, the water flows through the pipe at 10 gallons per minute. If you close this valve, it stops the water flow.

Now, if you want a water flow of 5 gallons per minute, you can open the valve for 5 seconds, then close it for 5 seconds, and repeat. Since the valve is open for 50% of the time, the average water flow will be half of the max flow rate, or 5 gallons per minute.

If you want a water flow of 2 gallons per minute, you can open the valve for 2 seconds, then close the valve for 8 seconds. The burst (peak) water flow will be 10 gallons per minute, but the average water flow will be 2 gallons per minute.

An ESC works in the exactly the same way. If the throttle signal is 50%, then the ESC will apply full power to the motor for 50% of the time. If the throttle input is 20%, then the ESC will apply full power to the motor for 20% of the time.

This pulse-width modulation technique has several important limitations. If you ignore these limitations, you will overload your power systems, and your heli will likely crash and/or the motor, ESC, or battery may be damaged.

30.1.10.2. Motor startup

An R/C brushless motor controller turns on/off three sets of windings in sequence to rotate the motor. This turning off the windings on/off in sequence is called "commutation".

The brushless motor controller measures the "back EMF" from each set of windings to determine how fast to commutate. Basically, the current draw of the winding drops as the magnet passes by the winding. The problem is this only works properly when the motor is running at a reasonable speed. When the motor is starting from a dead stop, the motor controller must pulse the windings in sequence without back EMF feedback to start the motor spinning to about 100 rpm.

Once the motor is running at 100 rpm, the brushless motor controller can rely on the back EMF to sense the magnet position and can commutate the motor using strictly back EMF sensing.

In an R/C model helicopter, the motor has a high load because it is coupled to the main (and possibly tail) rotor which is fairly heavy. So, with lower torque motors and high motor loads, the motor may not start spinning when the ESC goes through the blind commutation stage. The main symptom for this is the motor will wiggle back and forth instead of starting to spin.

30.1.10.3. Governor mode

A feature of an ESC which will try to keep the motor speed constant despite variable load placed on the motor. This is like the cruise control on a car as it's going up and down hills. Even though the load on the motor is variable as the car goes up and down hills, the cruise control will try to maintain the same speed. The governor mode on an ESC will try to do something similar. Even if the heli is performing wild maneuvers and the load on the main rotor blade is highly variable, it will try to maintain a constant head speed.

If using a governor mode, the throttle curve should not be set to 100%. This is because the governor mode needs a little bit of extra power so it can maintain headspeed. Using the cruise control analogy, if you set the cruise control of a car to its maximum speed the cruise control cannot maintain the maximum speed going up hills. Similarly, if you set the throttle to 100% RPM then the governor mode will not be able to maintain it when the rotor is heavily loaded.

This is why the motor pinion should be selected so the desired headspeed can be achieved at 90 to 95% of the throttle - so the governor mode can work properly.

30.1.10.4. Timing Advance

The timing advance determines how early the ESC applies power to the electromagnets during motor rotation. A higher timing advance will cause the electromagnets to be energized for a longer period, which will increase the motor power but will also increase the current draw. This is appropriate for aerobatic flight. A lower timing advance will cause the electromagnets to be energized for a shorter period, which will decrease the motor power and will decrease the power used. This is appropriate for duration flight.

30.1.10.5. For fixed pitch helicopters

You must select an ESC that can handle the current draw at full throttle, even if you do not plan to fly the helicopter at full throttle.

For example, if a Corona hovers at half throttle and draws 14 amps, then the current draw from the battery and ESC is NOT 14 amps. What actually happens is the battery and ESC are delivering 28 amps to the motor only 50% of the time.

Therefore, if you use a Phoenix 25 for the Corona, it will be overloaded even when hovering, and will probably overheat and shut down when you apply more throttle.

Also, you want to select a motor that will hover your helicopter at no lower than 50% throttle for this reason.

30.1.10.6. For collective pitch helicopters

If you select a motor with the proper Kv and the CP helicopter reaches 1600 rpm of headspeed at 90% throttle and draws an average of 18 amps of current, then the ESC and battery are actually supplying 20 amps of current for 90% of the time.

If you select a motor with an excessively high Kv rating and the CP helicopter reaches 1600 rpm of headspeed at 25% throttle and draws an average of 18 amps of current, then the ESC and battery are actually supplying 72 amps of current for 25% of the time!

Therefore, if you use a motor with a very high Kv rating and you are forced to use a low throttle setting to compensate for the high Kv, this will increase the load on the ESC and battery. In extreme cases, this will shorten the life or destroy the ESC and/or battery.

30.1.11. How batteries work

A battery is basically a electrochemical device which stores electrical power.The closest physical equivalent to a battery is a water tank, which stores water.

Batteries have several important characteristics:

  • Voltage (volts). Voltage is an electrical term which describes the force of the electrical pressure.

    This is similar to the pressure in a water tank. The higher the water pressure, the more water will flow through a pipe connected to the tank. Similarly, the higher the voltage, the more current will flow through wires (with some constraints).

  • Capacity (maH). The capacity of a battery is measured in milliampere-hours, or maH. One thousand milliampere-hours (1000 maH) is equal to one ampere-hour (1 AH).

    One milliampere-hour is the ability to supply one milliamp of current for one hour, or two milliamps for 30 minutes, etc.

  • C rating. The C rating describes the maximum instantaneous (burst) power that a battery can supply. A battery rated for 10C continuous discharge is capable of being discharged at 10 times its rated Capacity, or in other words, it is capable of being fully discharged from a fully charged state in 60/10 or 6 minutes.

    So, a 2100 maH pack rated at 10C continuous can supply approximately 2100 milliamps * 10 = 21,000 milliamps (21 amps) for six minutes.

  • o Pack configuration. This is typically described as 3s1p or 3s2p, etc. The number before the "s" denotes the number of cells in series, and the number before the "p" denotes the number of cells in parallel.

    So, a 4s2p pack of individual 3.7 volt 2100 maH cells would have a total voltage of 11.1 volts, and a total capacity of 4200 maH.

30.1.12. How motors work

A motor is basically a mechanical device which converts electrical power into a rotary mechanical force.

For the motors used in R/C helicopters, there is a permanent magnet and an series of coils of wire (called the motor windings) which act as an electromagnet. The windings are pulsed on and off to sequentially push on the magnets, and either the magnets or the windings are attached to the motor shaft which makes the motor turn.

(Note that there are some motor types which do not have permanent magnets, such as induction motors, but these are not used for R/C helicopters and will not be discussed here).

The part of the motor which is stationary is called the stator. The motor mount is part to the stator, since the R/C model should not turn with the motor shaft. The magnets or windings may be part of the stator, depending on the motor type.

The part of the motor which rotates is called the rotor. If the stator has the windings, then the rotor will have the permanent magnets, and vice versa.

In order for the motor to rotate, the magnet windings must be switched on and off. This process of switching the windings on and off is called commutation. The motor can be commutated either mechanically or electrically.

There are many different arrangements of coils and magnets which will make the motor shaft turn, and therefore many different categories of motors.

The first two categories which we will discuss are: brushed and brushless motors.

30.1.12.1. Brushed motors

For a typical brushed motor, the magnets are attached to the inside of the motor case, and the motor windings are attached to the motor shaft.

The motor windings are commutated by a mechanical device which makes and breaks the electrical connection. There are a few (usually three) copper contacts mounted in a circle around the motor shaft called the commutator. The stator has two brushes which touch thes copper contacts as the motor rotates.

The are two main types of brushes: carbon (graphite) and precious metal.

Carbon brushes are typically used for larger motors designed for higher current because the carbon brushes can handle high currents well.

The carbon brushes in smaller motors are attached to a springy metal which presses the carbon brushes against the commutator. Imagine a tiny toothbrush where the bristles are replaced by a small block of carbon, and the handle is replaced by a springy metal.

The carbon brushes in larger motors are a block of graphite (about 4mm x 4mm) which are pressed against the commutator by a spring. In these types of motors, the brushes can be replaced.

Precious metal brushes are typically used for smaller motors because they are easier to manufacturer for small motors, and they do not handle high current well.

A precious metal brush is usually a strip of springy metal (usually a beryllium alloy) with the end cut into thin fingers. These fingers are pressed against the commutator to make and break the electrical connection.

The are three main reasons for brushed motor failure:

  1. Dirty commutator

    When the motor is new, the commutator will be shiny. As the motor is used, the commutator will become dirty due to arcing and brush material deposition. When the commutator becomes dirty, it will not conduct electricity well, and the motor becomes less efficient and loses power.

  2. Brush wear

    As the motor spins, it will cause wear on the brushes, and eventually the brushes will fail. For carbon brushes, the brush will become so short that it will no longer touch the commutator. For precious metal brushes, the ends of the metal fingers will wear away and the brushes will no longer touch the commutator.

  3. Overheating

    If the magnets of the motor are overheated, they will lose magnetization. This is a vicious cycle - when the magnets are weakened, the motor will run even hotter, which will weaken the magnets further, etc.

The commutator and the brushes are the only parts of a brushed motor which wear significantly, so these are the primary limiting factors for brushed motor life.

There are six main methods of extending brushed motor life:

  1. Proper motor break-in

    Motor break-in is covered in Chapter 10, Helicopter Construction

  2. Proper motor timing.

    Most motors which are used for R/C helicopters are originally designed for R/C car or R/C airplane use. R/C car motors are usually manufactured for counterclockwise rotation, and R/C airplane motors are usually manufactured for neutral timing so it rotates equally well (equally badly) for both clockwise and counterclockwise rotation.

    (The CW/CCW rotation of a motor is the direction of motor shaft rotation when viewing the FRONT of the motor.)

    Most R/C helicopters use a CW rotating rotor with the main shaft driven by a main gear/pinion configuration, and the motor faces downwards. This motor arrangement requires the motor to rotate in a CW direction. The few exceptions to this generalization are: Hornet, Zoom 400, T-rex, and Viper 70/90 which all use upwards-facing motor.

    When a motor which is designed for CCW or neutral rotation is used for CW rotation, then the motor runs very inefficiently because the motor windings are switched on and off at the wrong times. This causes sparking which increases brush wear, dirties the commutator, generates heat and creates RF interference.

    There are some brushed motors which are properly timed for CW rotation, such as the GWS 300H motor and the QRP Hyper S400 (red label).

    On some motors, the timing can be changed. Most Speed 540 brushed motors have an endbell (back face of the motor) which can be loosened by loosening two screws. After the endbell is loosened, it can be rotated to the desired position.

    Some other motors have an endbell which is secured by two tabs of metal which are bent over the endbell. These can be bent out to loosen and rotate the endbell.

  3. Regular motor maintenance

    Regular motor maintenance will maintain the efficiency of the motor and decrease average motor temperature and extend the motor life.

    The commutator should be checked every few flights to ensure it is clean and shiny. If the commutator starts to look dirty, it should be cleaned.

    If the endbell is removable, then the endbell should be removed and the rotor can be removed. The commutator can be cleaned with some extra fine steel wool until it is shiny.

    If the endbell is not removable, then the motor will usually have slots in the motor case near the commutator. A cotton swab (such as a Q-tip) may be used on these types of motors to clean the commutator.

    Motors with metal brushes (such as the Team Orion Elite Micro Modified) do not require the commutator to be cleaned, and the motor should not be disassembled.

    There are some "commutator lathes" designed for cleaning R/C car motor commutators, but these are not recommended, because it is a better investment to buy a good brushless motor and ESC combination.

  4. Use a high frequency ESC.

    A high frequency (about 100 khz) ESC minimizes brush and commutator wear by providing a smoother flow of electricity and minimizing the arcing between the brushes and commutators.

    A high frequency ESC is required for motors with metal brushes because metal brushes will typically disintegrate within 10 flights with conventional ESCs. Motors which use metal brushes include the Team Orion Elite Micro Modified and the grey endbell IPS-style motors.

  5. Keep the motor cool by running the motor in the efficient range

    See Section 30.1.10.1, “Pulse-width modulation” for more info.

  6. Keep the motor cool by using a heatsink

    There are many different heatsinks available which fit R/C helicopter motors.

    See Section 30.2.2.5, “Cooling options” for more info.

30.1.12.2. Brushless motors

For a typical brushless motor, the magnets are attached to the motor shaft (rotor) and the motor windings are attached to the motor case (stator)

The commutation is performed by a brushless motor ESC which pulses the motor windings in sequence to make the motor shaft rotate.

A brushless motor has no commutator nor brushes, so there are no parts which encounter significant wear, so a typical brushless motors should never wear out. In extreme cases, the bearings may fail, but this is rather unusual.

The primary failure mode of brushless motors is motor overheating, which is explained in more detail in the brushed motor section.

Motor overheating can be avoided in two ways:

  1. Keep the motor cool by running the motor in the efficient range

    See Section 30.1.10.1, “Pulse-width modulation” for more info.

  2. Keep the motor cool by using a heatsink

    There are many different heatsinks available which fit R/C helicopter motors.

    See Section 30.2.2.5, “Cooling options” for more info.

  3. Use the correct switching frequency

    Brushless coreless motors require a high switching frequency to run efficiently and avoid overheating. Some motors require an extremely high switching frequency to avoid overheating. For example, the Kontronik Tango documentation states that a brushless ESC with a switching frequency of 32 khz or more is required for this motor.

30.1.12.3. Coreless motors

A brushed or brushless motor can be either a cored or coreless motor.

A coreless motor has no iron core for its motor windings. Therefore the windings are overlapped in a tubular fashion and looks like a woven basket. This woven basket of windings can either be layered around the inside of the motor case for a brushless motor, or wrapped around the motor shaft for a brushed motor.

This type of motor can usually be recognized by spinning the motor shaft while the motor wires are disconnected. A coreless motor will allow the rotor to turn freely because there is no iron core to attract the magnets and therefore has no preferred angular rest position.

Coreless motors have several advantages which are relevant to R/C helicopters:

  1. Size/weight

    Coreless motors can be made smaller and lighter than motors with iron cores of the same size.

  2. Efficiency

    Coreless motors have no eddy current losses, although this can be minimized on a cored motor by using very thin laminations from exotic materials (such as Magnesil).

  3. Lower Io

    Because a coreless motor has no cogging (torque losses), the no-load current is much lower. Some iron core motors eliminate cogging using a skewed armature, but this is complicated.

30.1.12.4. Iron core rotor motors

A motor with an iron core for its windings can usually be recognized by spinning the motor shaft. An iron core motor will exhibit a cogging effect where the motor rotation feels "rough" because the shaft will have certain preferred rest positions. Not all iron core motors will have a cogging effect because this effect can be minimized or eliminated by staggering the rotor.

Cored motors have two main advantages:

  1. Thermal stability

    The motor windings in a cored motor can dissipate heat much better because the iron core acts as a heat sink. This characteristic is important for extended operation at high power levels.

  2. Smaller air gap.

    The clearance between the rotor and iron core can be minimized which increases efficiency.