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Introduction to CCPM and Heli Swashplate by Dave Day
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More and more recently introduced machines are using some form of moving swashplate system incorporating Cyclic/Collective Pitch Mixing (CCPM). They can take many forms and the essential mixing can be accomplished mechanically, electronically, or by a mixture of both.
In order to understand just what is involved in a moving swashplate system, let's first explain how a fixed swashplate works. In its simplest form, only two inputs are required (Fig.1). One link tilts the swashplate in a for/aft direction and the other in a lateral direction. The centre of the swashplate is a large ball joint which is fixed in position on the main shaft. On the recently introduced 'Hornet' electric helicopter, the swashplate is fixed in its position by the links going up to the flybar. Some systems use more than two inputs which serves to steady the swashplate and give more precise control (Fig.2). This arrangement is normally only found on the higher priced competition machine.
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The essential point here is that a fixed swashplate system uses some other way of changing the collective pitch of the rotor blades. A moving swashplate moves up and down to control the collective. For all practical purposes, CCPM means 'moving swashplate'.
3 and 4 input systems
If a swashplate is to move up and down with pitch variation and still give satisfactory control of the cyclic inputs to the blades, it must be held firmly in any given position. This requires a minimum of three inputs, or operating links. These can be spaced at 90 degrees to each other (Fig.3) or at 120 degrees (Fig.4).
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90 degree spaced inputs are normally arranged so that there is one on each side and one at the front (or rear). The inputs at each side move in opposition to each other for lateral cyclic control (aileron), and the front input moves to give fore/aft cyclic (elevator) control (Fig.5). All three links move together for collective control.
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There is, of course, no reason why there should not be two linkages at front and rear, moving in opposition, for elevator control and a single linkage at one side for aileron control. However, the writer is not aware of any model which has used this system.
When the inputs are spaced at 120 degrees, there are two arrangements which can be used. One (Fig.6) has one link at the front and links at either side, similar to a 90 degree system, but the side links are further to the rear. The actual control inputs for this system are rather more complicated, since the cyclic inputs cannot be totally separated.
For example, when the front link is moved up and down for elevator control, the whole swashplate will move up and down unless the two side links are moved a smaller amount in the opposite direction to compensate (Fig.7). The two side links will still operate the aileron input as before.
Once again, the same linkage can be used with the single link at the back and the side links fitted a little to the front of the side location (Fig.8).
Another variation on this particular set-up is as shown in Fig.9. Here the single side link will give lateral control, with some opposite compensation from the other two, while the fore and aft linkages will move in opposition to each other for elevator control. However, this is not a common arrangement.
The final variation, which gives even more support to the swashplate, has four links at 90 degrees to each other (Fig.10). Here both axis are controlled by a pair of links moving in opposition, with all four moving together for collective control
The manner in which these swashplate movements are communicated to the blades varies from model to model and does not concern us here. The basic points to remember are:
1). The fixed swashplate tilts in the direction that the helicopter is required to move. Collective pitch control is effected by some other means.
2). The moving swashplate has two functions. It tilts in the direction that the helicopter is required to move, and it moves up and down to control the helicopters height.
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Mechanical or electronic?
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There are many ways of operating a moving swashplate via the three radio functions involved - aileron, elevator and pitch. Until recently all of these were entirely mechanical, with any mixing being performed by means of some form of mechanical mixer.
The advent of increasingly complex radio equipment with various mixing circuits incorporated in the transmitter has made some of the existing systems simpler and also paved the way for more complex systems. For example, the 120 degree swashplate systems described above would be virtually impossible to achieve mechanically. It has also made the adjustment of such systems much simpler.
Fig.11 shows the mechanical system used on many of the Morley machines. Note that each input is controlled by a completely separate servo, which makes adjustment fairly straightforward. While some interaction is inevitable, careful design, as here, can reduce this to a minimum. This is a 90 degree system.
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Another mechanical system is found in the various versions of the Heim mechanics. While this is a four input 90 degree system, only the two lateral inputs are used to raise and lower the swashplate for pitch control (Fig.12). This means that only these two inputs need to be mechanically mixed. This is normally achieved by means of a 'rocking servo' mixer (Fig.13).
Note that there is some interaction between collective and elevator due to the swinging arm carrying the elevator bellcrank. This was slight and never caused any problems. It could be mixed out with a modern system
The Heim method of actuation is very easily converted to a hybrid electronic system by substituting the mechanical mixer for an electronic type commonly used for 'V' tail models. Here the aileron and pitch inputs (say A and B) from the receiver are fed into an electronic circuit which mixes them together and gives two outputs. One of these outputs is the sum of the two inputs (A+B) and the other is the difference (A-B) (Fig.14). When this system was popular, some radio manufacturers incorporated it into their transmitters and referred to this as '180 degree CCPM'. Few modern systems have this.
One problem with this method is that electronic mixers are usually arranged to reduce the final servo movement to avoid over driving the servo when the two inputs are added together. The only solution here is to use long servo arms.
The latest, entirely electronic, systems have all of the circuitry in the transmitter and each of the swashplate inputs (3 or 4) is controlled by a separate servo. For optimum results, these should be mounted below the swashplate and connected by a short, direct, link (Fig.15).
There are many advantages to this layout. Many of the connections of previous set-ups are eliminated, as are many bellcranks or levers. It is, therefore, lighter and more compact. However, it will almost certainly be necessary to use long servo arms to obtain sufficient movement and this does involve some exaggeration of any servo deficiencies.
There are certain limitations involved in the use of electronic mixing systems. Early types suffered from the fact that increasing the throw of one of the mixed inputs would automatically reduce the other. For example, if the pitch and aileron channels were being mixed, increasing the aileron throw would reduce the pitch throw.
The latest equipment does not suffer from this problem, but care must still be exercised when two channels are mixed. In the example above, full aileron throw and full pitch throw together could drive the servo well beyond its normal limit. This can produce a very non-linear output or even interaction of the controls.
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In this situation, the mixed servos must have their throw reduced via the travel adjustment facility and then, if necessary, longer servo arms will be needed. Remember that you cannot get something for nothing, there has to be a snag somewhere. There is a definite limit to how much travel any servo can give. If you exceed this there will be problems somewhere.
The advantage of electronic mixing with a modern radio is that everything will be adjustable and, with patience, you can get everything set to its optimum.
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Micro Heli Collective Pitch Setup source Dave Ganzer
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So now that you understand CCPM, you will need to decide which servo configuration / CCPM you would like to physically install.
Of course, your setup will have to comply with the type of Swashplate you own (90 / 120 / Dual Armed / Quad Armed ) and the compatible configuration which your Tx is capable of supporting. As a side note, it is possible to configure "CCPM" using Y connector cables and mechanical arms, but we highly recommend again this kind of a setup due to the complex nature of CP and the limited (if any) mixing capabilities of such configurations with regards to Micro Helicopters.
In our case we will explain the most widely used Swashplate / CCPM setup, the 120 degrees 3 servo setup.
First disconnect the motor, either by disconnecting the wires or moving the main motor a few millimeters back so it does not engage with the main gear. This way you will not damage the Heli or cause injury while playing with the many setting and features of CP. The last thing you want is to have the rotor spinning at 1500+ RPM while you are trying to figure out "What does this button do".
Next you will continue by configuring the Tx (A.K.A Transmitter or Radio).
The Helicopter transmitter is equipped with Swashplate electronic mixing. The term you will be looking for in your Tx manual is CCPM ( Cyclic / Collective Pitch Mixing ).
For the 120 degree setup, we chose the SR-3 setup using the 8UHF Tx menu system. Your Tx should have a diagram similar to the one below which represents the servo configuration used. The Swashplate should behave such as:
With Aileron inputs, the aileron and pitch servos tilt the swashplate left and right; With Elevator inputs, the three servos tilt the swashplate fore and aft; With Pitch inputs, all three servos raise the swashplate up and down.
Most Tx will allow to reverse individual servos to achieve proper directional motion of the servo arms. In some cases you will simply control such behavior using the reverse menu, while in other Tx you can change a positive servo value to a negative, depending on the Tx model you own.
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Make sure that your electronic setup is as follows per Futaba (JR and other systems may vary ):
channel 1 is the aileron (roll)
channel 2 is the elevator (fwd/aft)
channel 3 is ESC
channel 4 is Rudder ( connected to Gyro and then to Servo )
channel 5 ( Not used, usually dedicated to Gyro gain )
channel 6 is Main blades Pitch control
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At this point we are not yet concerned with any servo travel adjustments or any trim settings. We just want to make sure that the basic functionality of the swash with relation to the Tx is correct. If you completed this step, you are in a very good shape.
You have completed the installation of the Collective Pitch kit and the Swashplate is properly moving in the correct directions in relation to the Tx stick controls.
Now reset all of the trims (the clicks that adjust small servo movement) as well as the sub-trims (Internal trims per servo) so that everything is center and zeroed out.
Now that the servos are reset (trims and sub-trims are at zero level) we will setup the CP mechanical setup.
First turn on the Tx, this will insure that all Servos are at 0 offset while we adjust linkage and horns.
Its no fun to spend time setting up the perfect adjusted Swashplate configuration just to find out that when you turn on the Tx your servos are not calibrated at the zero offset and you have to re-adjust the servo horns; thus, we keep the Tx on.
Remove all servo screws and adjust the horns so they are parallel (or 90 degrees) to the ground. Continue by adjusting the ball links turning a single turn at a time until the Swashplate is completely parallel to the ground and all three links are equal in length.
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Now that the Swashplate mechanical configuration is complete, we turn to the flybar and rotor head. It is crucial that each paddle is of equals length from the center of the flybar. Verify that the flybar paddles and main blades are even and centered. This completes the mechanical setup and all things should seem completely even and balanced.
We now turn to the fun part, the Tx setup. At this point the Swashplate and all servos should be moving in the correct direction. It is now a matter of pitch adjustments, mixing ratios and travel setup to get our little guy in the air.
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ATV and Swash Travel:
First, move the cyclic Tx stick Fwd/Aft and verify that the Swashplate does not bind or rub against the main shaft and that the Servos do not produce a buzzing sound; an indication that the travel of the servo arm exceeds the free limit. Repeat this step for left and right tilts of the Swashplate. Also, make sure that enough travel is allowed for the Swashplate.
The degrees of tilt depend on your construction and setup but should be within 25 to 40 degrees respectively.
To adjust these settings you will first enter the ATV (Adjustable Travel Volume) section of your Tx. Increase or decrease the ATV as needed per servo to achieve just the right amount of travel per servo. Make sure that the applied adjustments are identical to each mixed servo or you will end up with an uneven cyclic control. Micro Helis are not easy to fly and applying human mixing due to uneven swash configuration will make it that much more difficult.
When you are satisfied with the Swash movement limitations, you should read through your Tx manual and find out if your Tx supports Swash travel adjustments. This feature will allow for even further ATV adjustments per the CCPM system in relation to all servos.
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Throttle Curve:
As briefly explained before, as the throttle is increased the Swashplate should evenly lower itself (main blades pitch changes).
Using your Tx menu system, enter the throttle curve settings. The throttle curve controls the speed of the motor so as you increase throttle (raise the stick), the pitch of the main blades changes (due to the pitch curve setup) and the pitch of the tail changes (to counter react the increased drag of the spinning rotor blades ).
In our case we are using an ESC with no Governor mode, so we must determine the speed of the motor at each point (stick level).
If you are using a Governor mode ESC, then you can set the Tx to 100% throttle output regardless of the position of the stick.
The reason is that Governor mode will slowly increase the speed of the motor / rotor and then keep it at that speed.
If the RPM is dropped (Like when the pitch is increased) the smart Governor ESC will automatically apply more power to counter react. If on the other hand the motor is off loaded the work (for example when you decreasing the blade’s pitch) the ESC will reduce power to the motor.
However, since in this article we are using a None Governor mode ESC, we suggest you start by setting the throttle curve points to: 0-30-50-70-100 (given that your radio supports a 5 point curve). You can then go and optimize point positions to better suit your flying style later on when you are ready to do so. The goal is to have an RPM around 1750 RPM when flying indoor and you will need to adjust your 100% throttle accordingly. If your Tx supports Idle up (a fancy word for other saved settings that you can enable or disable with a click of a button).
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Rotor Blades Pitch Curves:
Pitch curves control the angle of attack of the main blades as they turn. As the angle increases (positive shift), so does the lift power of the blade. Of course the blade will further resist the air flow and more power is needed (to make a long story short).
Most of the CP kits are supplied with a pitch gauge. Install the Pitch Gauge. The pitch gauge is sort of a tape measure that will display the current pitch of the blades.
Now adjust the ball links and arms (again), but this time look at the pitch gauge while you are shortening or lengthening the connecting arms, one turn at a time. The goal is to have a zero pitch while the Swashplate is in its default position ( almost all the way up ).
Remember that the servo arms and servo rods below the Swashplate are not to be touched (since we already calibrated them); we are now dealing with the upper portion of the Swashplate only.
Now that the Swashplate is in its default upper location and pitch is equal to zero degrees on the main blades and the Swashplate is completely parallel to the ground and so are the servo arms, we can finally deal with the positive pitch.
Enter the pitch setup menu in you Tx and setup the initial 5 points to 65-70-75-85-90. Now slowly increase the throttle and watch how the lowering Swashplate increases the pitch of the blades. If the behavior is the opposite, you may need to reverse the pitch servo from + to - or use the reverse menu option (again, depending which Tx you are using).
Now set the throttle stick to 25% of the allowed travel. Watch the pitch gauge and using your pitch adjustment Tx button increase or decrease the pitch for this stick level (25%) until the pitch gauge reports +3 degrees. Now move the Tx stick half way and adjust the Tx until the pitch gauge reports around +6 degrees. Continue to the next point and setup for a pitch of around +9 and for the last point finish it off with +11 degrees. Now that was easy.
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Tail Pitch:
Now that we are done with the rotor mechanism, we can turn our attention to the Tail.
We highly recommend you install some sort of an adjustable tail system (such the one posted on the site). Also, it is a common mistake for beginners to think that plenty of pitch is necessary to control the tail thus they setup the tail pitch control rod on the furthest servo hole allowing for the maximum servo travel, where in fact in most cases the lowest servo hole will produce the most accurate tail control.
In our case we will be using a Gyro with a Heading Hold (since it more difficult to setup and we wanted to cover it all).
HH Gyros will apply pitch adjustments to the tail pitch to counter react unmanly yawing of the tail (like a regular gyro) plus they will return the tail back to its original position from where the wag originated (i.e.: holds the head). First, make sure you turn off the HH feature in your Gyro. This way we can start the hard way using mechanical mixing and complete the setup with the HH to reach a solid tail, always a challenging feature in Micros.
First, assuming your servo is calibrated and the trims are reset (from the prior steps) confirm that the tail pitch is at zero pitch when the servo horn is at 90 degrees and has plenty of pitch upon servo movement. Remember that when the tail blades pitch are at zero pitch your Heli will yaw since no counter reaction is applied to the fuselage.
Now that the mechanical setup is done, we can setup the Tx pitch. Like the rotor pitch, we want to setup each of the 5 curve points to -50 -25 0 +25 +50 to start with where at point 0 we have no tail pitch. Continue and increase the throttle stick at 25% increments while adjusting the tail pitch values (just like the rotor pitch curves).
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Test flight:
The moment of truth, if you followed the above instructions the you will be at least at a point where you should be able to hover and with some simple troubleshooting and adjusting you Micro Heli should produce consistent and controllable flight characteristics.
Set the Heli in an open area with no wind. Connect the battery pack and stand behind the Heli. Now slowly increase the throttle and closely look for any out of the normal movements. Don't worry if the Heli is a little unstable and seems to be skidding a bit on the ground; this is called GE and the only way to fix it is to get out of it, lifting the helicopter above GE.
However, the Micro Heli should remain roughly in the same are and no sudden spiraling of the tail should take place.
If all seems well then continue and lift the Helicopter while slightly adjusting the sticks to control the Micro Heli in a stable hover. Try and pay attention to your fingers and feel what is wrong. In other words, if you keep on applying too much left rudder, then now it is time to land and adjust your Tail pitch. The goal is simple, you should be able to hover for 3 - 5 seconds hands free.
Hands free does not mean you can take your hands off the sticks and grab a sandwich, it just means that you can relax for 3-5 full seconds without fighting the sticks. When you achieve such flight control then you are ready to re-enable the Heading Hold module on the Gyro.
Now that you completed the basic setup we would highly recommend two things:
(1) Setup all switches to same settings so if you mistakenly hit a switch, you will not crash the Heli
(2) Backup (copy) the model's memory to another free Tx model location for backup purposes or write down the settings on paper for future reference.
Congratulations, you achieved CP hover, the rest is now easy... kind of..
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