Full Time Senior Member
Loc: Roanoke VA USA
02-09-03 11:33 PM - Post#15729
In response to reddragon
Here's the article that was in the March April 2001 Issue OF Rotory Modeler - Enjoy!
As one observes today’s helicopters, each model is somewhat unique when compared to others. One manufacturer might offer carbon frames, yet another offer aluminum. One machine might have an inline flybar, yet another has an underslung flybar. While manufacturers offer an array of hundreds of different features, all current models share a standard set of only three types of control systems.
1) Direct Type - this system has a fixed swashplate and uses a rod or wire connected to
a servo to control collective. This is a single servo collective system.
2) Elevator Control - This system is the most common. Collective control is achieved by linkages
that slide the entire swashplate up and down on the main shaft to achieve collective control.
This is a single servo collective system.
3) CCPM - This system uses two, three or four servos mounted close to the swashplate which
operate simultaneously to move the swashplate up and down to achieve collective and steering
control. CCPM systems offer less complex mechanical linkage.
There are five basic swashplate setups for model helicopters which are supported by mid to high-end radio control systems. They include a “Single Servo” system, and four different types of “CCPM.” A Single Servo system uses one servo for each axis of the swashplate, such as; aileron, elevator, and collective. Single servo type systems may be found in the more popular "Elevator Type" helicopters which slide the swashplate up and down to achieve collective control. A single servo system is also used for the "Direct Type" collective system, where one servo moves the collective thru the use of a pitch change wire or rod. The “Direct Type” is the only system that uses a preferable fixed swashplate.
CCPM type helicopters utilize a combination of servos working together to achieve the 3 axes of control; aileron, elevator, and collective. There are a total of four CCPM types, however the “three servo type” is the most popular. The CCPM system offers far less mechanical complexity to move the swashplate of the helicopter. In addition, multiply servos working together increases the power of the system.
In one of my earlier Mas Technique articles, titled; ”Helicopter Control Systems,” (Jan/Feb ‘94) we examined helicopter control systems in detail. In a relatively short period of time, many helicopter manufacturers have re-tooled their equipment from the traditional “Elevator Type” sliding swashplate, and “Direct Type,” to the now popular - CCPM system.
CCPM which stands for; “Cyclic Collective Pitch Mixing” has recently been promoted to be the greatest thing to happen to model helicopters in a long time. It promises a new control system that will allow you to achieve new levels of expertise in your flying, as well as offer amazing collective and cyclic control. Well . . . this is really not the case. Along with all the glamour and advantages of CCPM mixing, are a few downfalls as well. In this segment of the Mas Technique, we’ll take a close look at CCPM.
One has to ask - why has CCPM almost become standard equipment on model helicopters, and why did most manufacturers switch to it? For the most part, model helicopter mechanical design reached a plateau some 5 - 8 years ago. This was right after we went to, carbon this, and carbon that. There had to be something new with a bit of mystery to attract first time customers and entice the old ones. So along came CCPM.
CCPM evolved a bit differently than past heli evolution’s. Most all changes in helicopter design came from the users and competition - factory pilots who are always striving to improve their helicopter, thus improving their standings and performance. However, there were few competition pilots using CCPM. As a matter of fact, most competition and factory reps. dislike CCPM because of its bad habits. So the moral of this story is; CCPM was mostly introduced by the manufacturers themselves.
Not long ago - a major helicopter manufacturer offered a new model with CCPM as an incentive to market a machine that was unique when compared to other helis. Not surprisingly, the modeling public took interest in the proposed new system and purchased them. In a short period of time other manufactures, not wanting to be left out in the cold, offered the CCPM system as well. Within the last six months a few “diehard” manufacturers, who offer competition helis, reluctantly offered a CCPM machine, even though they vowed to never use CCPM on their equipment.
While CCPM has been promoted as a new and futuristic control system, in reality, it has been around almost as long as helicopters themselves. As modern helicopter radio systems evolved in the early 1980’s, so did CCPM. For the most part, CCPM mixing was developed and used extensively in German manufactured helicopters. My friend, Mr. Ewald Heim was instrumental in the early design and promotion of CCPM. As a matter of fact, CCPM has been standard equipment in past and present European helicopters such as Graupner, Heim, and Vario.
The truth of the matter is, CCPM is a manufacturers dream - gone are the complicated mixing trays, push-pull linkages, numerous bell cranks, ball joints, pushrods, links, etc. It’s a known fact that CCPM reduces 50% of the linkages on a model. Easy calculation, less parts = less cost! However, I have not witnessed this being passed to the customer.
Let’s take a look at some of the advantages of CCPM.
1) CCPM offers short coupled direct control from the servo to the control surface. Servos are mounted just below the swashplate, thereby eliminating the normal two or more pushrods and the use of bell cranks to change the direction of travel.
2) CCPM offers a powerful collective system, since three servos are now used for collective movement.
3) CCPM offers nearly 50% reduction of linkages, reducing the part count. This reduces cyclic/collective freeplay, as well as offer simpler construction when compared to elevator type collective systems.
4) CCPM offers a distinct advantage for scale helicopters since the main frame footprint is reduced. In addition, a forward mounted servo tray is no longer needed.
THE PRICE OF CHANGE
It’s a simple fact that it’s not easy to change model helicopter design. In most cases for every bit of good, there’s a little bad somewhere else. While CCPM offers some distinct advantages, it also has its share of problems. Let’s investigate where CCPM needs to be improved.
With a CCPM system using 3 servos, all servos must move precisely at the same time as we add or subtract collective, otherwise we’ll end up with some unwanted aileron or elevator induced to the control. For example; as we quickly add pitch to perform a straight climb out, the servos tend to not travel at the same speed. This causes the swashplate to tilt slightly during the transition, causing the machine to veer off its straight climb out. This problem is more obvious during a roll. As we move the collective through its full range, from positive to negative, then back to positive, if any unwanted input is introduced during the roll, it results in a less than perfect maneuver.
Regardless of how slow or fast, or how powerful a servo is, most all servos travel at different speeds to reach a given point. This is caused by different tolerances of the servo’s motor, circuitry, and finally the gear train. I’m sure that when you bought your last radio, you wiggled the sticks to see if all the servos were spinning. You might have noticed something else - some servos seem to sound different, even though there is no load on them. This, in most cases, is caused by the differences of the gear train. Some are tight, some are loose. Different tolerances in plastic servo cases, bearings, gears, and their fit, make each servo unique. If you were lucky enough to find three servos alike, there’s another problem, the loads on the swashplate are different as well. This type of problem is only unique to CCPM systems, Elevator and Direct type systems are not affected, since only one servo controls the collective movement.
As CCPM became more popular, modelers moved to better and better servos that would work well with CCPM. The most common is the new digital servos with enormous power and the ability to achieve precise centering. The additional power helps to overcome many of the loads during servo travel, and the precise centering helps to maintain trim regardless of collective position. While digital servos do help - cyclic coupling still remains to a degree. During precision maneuvers such as FAI, where repeatability is crucial, CCPM can knock the edge off a maneuver which could mean winning or losing. This is the reason why few (unsponsored) pilots fly CCPM in competition.
One also has to be aware of the fact that digital servos are expensive. Four digital servos can cost more than the price of an entire sport radio system. Cost factor is important to our industry. Digital servos are not really an option for the new or sport pilot because of the initial cost. I can’t see a new pilot purchasing a $199 helicopter and the sales clerk tells him he now needs a $1000+ radio to make it work. Actually, beginner and sport pilots can still use standard servos in a CCPM machine, since at that level of training the coupling would hardly be noticeable.
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.
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|
"we know around here"
ROTORY ~ THE MOST TRUSTED NAME IN R/C HELICOPTERS
Get Your Free Issue of Rotory