Mas Technique Continued . . . .

NON-LINEAR MOVEMENT
Even though we now have powerful digital servos with precise centering, they are still not designed for CCPM systems. CCPM requires linear movement which is not achievable with a wheel or arm type output. Cyclic Collective Pitch Mixing or CCPM, is used on full scale helicopters as well. However, on full scale - the swash collective system is operated by hydraulics, which has the same travel and power regardless of position, something our servos do not have.
CCPM uses the same servos for collective and cyclic movement. As the servos change their position from their center, to accommodate positive or negative pitch, the servo wheel now becomes non-linear in its fashion of movement as we add a cyclic command.
Let’s look at a typical example. You’re flying around upstairs at say 8 degrees of pitch, using a typical type 3 CCPM (3 servo) arrangement. The two aileron servos (roll-pitch) are now offset equally (15-20 degrees) to accommodate the positive pitch. At this point everything seems to be fine. The problem occurs when we attempt a roll, one servo which has traveled to 15-20 degrees, now has to move to its full extreme (45 degrees) - however the other servo moves from 15-20 degrees to 0 degrees, center position. Here we have an instant problem. The one servo operating at the end of its arc has less movement and difference in power than the other servo which is in its center of travel. Keep in mind that a servo has more travel in the center of an arc than it does at its extreme. As a servo rotates from center (0 degrees) to + or -45 degrees, the pushrod movement is reduced for each degree of rotation traveled towards the 45 degree position. Five degrees of rotation at the servo’s center of travel (arc) provides more swashplate movement than 5 degrees at the extreme. Not only are the servos non-linear, one servo is fighting the other for position because they are attempting to move the swashplate in pairs. This method of "non-linear" collective movement is contradictive to the set-up that model helicopter control design was built on. In addition, on some CCPM systems, we’re back to single linkage hookup at the servo, exerting considerable side loads on the output arm and bearings.
WE NEED OUR OWN SERVO
Just like in the past, when radio manufacturers finally gave us our own radio system for helicopters - we now need our own "servo" as well. Both standard and digital servos are limited for heli use because they minimally meet our pushrod travel requirements, and are not linear in their movement. We need a servo designed for helicopter use!
Hopefully one of our "Big 4" radio manufacturers will "break the ice" and offer a servo specifically for model helicopters. The "Heli-Servo" will have to be digital, powerful, and offer both rotary and rack driven linear outputs. Pushrod travel distance will have to be increased from our standard 320mm to over 400mm to accommodate needed collective/cyclic movement. This means a larger servo. In order to accommodate new travel requirements, the servo case length will increase from the standard; 1.59" (40.4mm) to approx. 2" in length (50 mm). This will mean retooling each machine to accommodate the new servo size.
These servos will not only conform to CCPM machines, they will make any control system work better. Gone will be the need for push-pull linkages. It would just be a matter of hooking up the pushrod. I really believe it’s just a matter of time - we will someday have our own servo. I have included with this article, a sketch of the Heli-Servo.
POOR SERVO PROTECTION AND ENVIRONMENT
Current CCPM systems offer an inadequate design for the operation and protection of the radio control system. With other typical collective systems, servos are neatly tucked in the canopy (where they belong). With CCPM they’re stuck in the frames, between the frames, and at the end of the frames. One CCPM kit, I recently built, had two servos sandwiched between the frames. If you had a defective servo, you would be required to tear the machine down.
The downside of CCPM installation, besides cosmetics, is the wiring problem. Most machines look like a Christmas tree. The present wiring on our servos and gyros are not designed for exposed use. They are designed to be in a cabin area away from heat, sunlight, fuel, and most of all, turning parts. Another problem occurs when we have a crash. While there is no real safe place on a helicopter for a servo, in the past they were mounted on a flexible (if you will) tray area. In the event of a crash, the servos could dislodge themselves from the tray and hopefully unplug themselves from the receiver. Now servos are exposed and mounted to portions of the frame that will not allow them to break loose. In addition, the servo wiring in now secured to the frame making things even worse in the event of a crash.
RADIO SYSTEM NOISE
With the advent of CCPM we now have more radio interference problems than before. In the past, radio systems (servos/gyro) were housed in the canopy away from the engine and mechanical parts that create RF noise. Now with CCPM, we drape wires over the entire machine to bring the RF noise directly to the receiver.
One of the most troublesome areas is the tail rotor servo. There’s a recent trend to mount the tail rotor servo on the tail tube. While they say it improves the T/R servo’s control of the tail, I find it hard to believe that a servo with a 30" pushrod is going to work any worse by extending it another 10" and put it in the cabin where it belongs. In all reality, it makes little to no substantial difference. What a tail tube mounted servo does accomplish is, the ability to usher in the troublesome RF from the tail rotor system. The highest RF level found on a model helicopter is generated exactly at that point, where the tail tube attaches to the frames. If you’re using a belt drive system, the receiver also has to deal with static electricity from the belt. To give you an idea of how bad it is; when I used to fly at night, during cool and dry weather, it was not uncommon to see a spark shoot from the tail tube to the metal frames every 30 seconds or so.
CURRENT CONSUMPTION
New high powered digital servos draw considerably more current than their older counter parts. There’s also the fact that three servos must move for collective. Even though the load is reduced for each servo, three digital servos moving simultaneously most of the time, is using some excess current. I remember days when you could fly with a 550mh pack. You were the big boy on the block with a 1000mh pack. I wouldn’t even consider flying with a 1000mh pack now.
While digital servos are great for heading lock gyros, to be perfectly honest, I’m not really sure we need all that cyclic speed and power to achieve aerobatics. I never had problems with aerobatics or any centering problems with the older servos such as; Futaba 9201-2 or the JR 4001-31. Just how fast do we need to move our control systems? I’ve had machines with those old servos that would roll so fast on the ground that it sounded like the blades were coming off.
MOVING SWASHPLATES VERSES FIXED
I’m a firm believer that the best control system for model helicopters is one that does not couple collective movement with cyclic movement and of course vise versa. Both Elevator Type, and CCPM control systems require the movement of the cyclic steering system (swashplate) to change collective position. For the same reason the elevator is not coupled to the ailerons on earlier helicopters, neither should they be on modern helicopters. Helicopters such as; TSK, Champion, Kalt, early Graupner, etc. use a "Direct Pitch" control system, where a single servo simply moves a rod or wire up through the main shaft to easily move the collective pitch up or down. This is without-a-doubt the most exact control system for model helicopters because the steering system is unaffected during collective movement, as is the collective unaffected by the steering system. Another advantage of the "Direct Pitch" method is; the swashplate is locked into position to the main shaft, providing the most stable control system available. Gone are the complex sliding trays and numerous levers and bellcranks. It’s unfortunate, but evolution has caused us to move away from the most exact control system for our models. I’m sure, eventually, we’ll go back to the "Direct Pitch" system.
In conclusion - don’t misinterpret this article in thinking that I’m down on CCPM type systems. Like any control system, each has its own place with our models. If you want to test the waters, that’s fine, do it. However, be careful - I’ve witnessed pilots dumping their helicopters just to go out and buy the same machine with CCPM. As I mentioned, for scale CCPM offers distinct advantages for that type of installation and flying style. Does it have a place for sport and competition - I think we need a linear servo designed for that type of use. Remember, CCPM is in its infancy and has a long way to go. The most important point I can leave you with is; make your choice wisely. That’s what model helicopters are about; Changes & Challenges.

Mike Mas