Radio Installation, Antennas and Glitches
At the heart of model helicopter survival is the guiding source, the radio. Without a reliable transmitter and receiver system this hobby would have never become as popular as it is today. On board components have shrunk in size and become very robust with the newer surface mount technology. Gone are the days of cold solder joints, chafed internal wiring and frequent electronic realignment.
Still this is not a perfect world and things sometimes can go amiss. Through a better understanding of how things work we can often make wiser choices for a particular situation, both with equipment selection and installation.
There are three methods of radio wave transmission:
Pulse position frequency modulation, pulse position amplitude modulation and last but not least pulse coded modulation on an FM carrier. For helicopters, PPM FM and PCM FM should be the only consideration due to their superior noise rejection as compared to AM. The main practical differences between these two methods are concerning noise rejection and how signal distortion is handled. With PPM FM often momentary glitches will be evidenced with a degraded signal or reception.
With PCM FM these momentary glitches are masked since the servo will maintain the last clean signal position. This is commonly referred to as PCM lockout or hold. When a clean signal is received again things function as usual. The PCM system has a fail safe feature which when enabled will move the servos to a preprogrammed position under lock out conditions… not good for helicopters in numerous attitudinal situations.
Some people disable this feature and some adjust to what they thing is the best servo survival position based upon their preceived safety issues. As with any disease, early symptoms if acted upon soon enough can prevent a major catastrophe. Here lies the advantage of PPM FM.
Concerning possible servo resolution, the current everyday standard for PCM is 1024 steps and PPM FM is theoretically better. In practice, I personally feel your fingers will never notice the difference. For minor glitches and noise rejection the PCM system will be the best performer. The above is true in most cases, but I have seen PCM driven servos “go nuts” and dance all over the place, induced from a worn chattering tail rotor shaft in the bearing inner race generating noise. The servo leads can also pick up RF interference since there is an inputor signal lead here. The choice is yours, so “pick your poison” so to speak.
So now you have a radio system and regardless which version, you should be concerned about optimal long term performance as a safety net. Installation concerns include vibration protection through foam mounting of the battery and receiver. Soft mounting of the gyro will ensure longer life, but too soft will cause anchorage hunting (tail wagging). These items must remain secure under all attitudes and “g” loadings so judge you attachment methods accordingly. I have seen velcro straps, tyraps, double sided foam tape, even rubber bands all used in responsible manners.
The mounting location for the most fragile part, the receiver, is often a compromise based on servo and switch locations due to wire lead length restrictions. While I have seen very professional looking wire routing I have also seen these same nice looking bundles installed under tension and/or laying against the sharp edges of side frames. While this is unavoidable sometimes, one can put a dab of “gorilla snot” over the sharp edge and over the wire to hold it in position. The best looking wiring job may not be the most reliable if common sense is not used.
Some people have used tyraps improperly on wire bundles only to have them chafe into the insulation. Soft Velcro straps are available for this same purpose. They are reusable and can be cut to a shorter length or width if desired. Special attention must be given regarding rocking servos/trays to wire slack which will prevent unnecessary stress at extreme control deflections.
Today there are so many gyros of different sizes and associated supporting boxes (mixers etc) that there is no one best spot for mounting every gyro. It is better to avoid extension cables and additional connections by mounting the gyro where it will fit as is. Since the angular velocity is the same at different points from the mast on the yaw axis one would expect similar gyro performance at reasonable locations.
More wiring means a higher possibility of interference and connector failure incidence. Servo extension leads should be avoided in any case. If a longer servo lead is a necessity, feel free to splice an additional but minimal length using proper soldering methods, remembering to stagger the three individual joints. This makes a neater, thinner, safer and more professional job.
The antenna lead is often left to the mercy of the unknowing. Many plastic machines will tolerate almost any location, while some graphite and metal helicopters will not. Graphite weed eater powered copters should have very special attention paid to this area due to the additional RF noise generated by the ignition system. Running the antenna to the horizontal fin mount parallel to graphite/ aluminum boom supports is a definite thing to avoid on any machine since range will be reduced.
Another consideration with antenna routing is the directional nature of the antenna itself. Polarization eg… maximum reception will be obtained from a horizontally orientated transmitter antenna if the receiver antenna is also horizontal. The same applies for vertical antennas. Maximum energy is radiated perpendicular to the length and the energy is radiated in a doughnut shaped pattern.
As an example VHF radio stations (mobile or fixed) have their antennas pointing up and use the same antenna for transmit/receive. Minimum reception occurs when the antennas are in line length wise with each other….. the hole in the doughnut so to speak. From this we can see that since our machines fly at various attitudes we can sometimes be operating in weaker reception areas. Poor antenna placement can also block radio waves with the helicopter structure in addition to a poor receptive antenna orientation at specific helicopter/transmitter positions.
Fortunately the operator has the option of repositioning the transmitter antenna should a loss of control occur. To be perfectly clear you should not point the doughnut hole (antenna) at the machine when flying. Still, one should always pick a clean (metal/graphite free) area for locating the receiver antenna.
One can often find through trial and error during range checks the best location. Every helicopter is different, even the same model types have slightly different signatures due to varying radio and wiring configurations. Additional range checks should also be preformed with the engine running, the throttle hold on and someone holding the rotor. Remember that the distances where PPM servo chatter occurs will certainly blip the throttle. The distance should not really be any different. If you see a large variance in range it is best to look into matters further.
When preforming a range check if one is not consistent, different results can be had. Personally I like to range check with the antenna collapsed and under the worst antenna orientation. I like to stretch the receiver antenna in a straight line and walk backwards while pointing the collapsed transmitter antenna directly in line at the receiver antenna. Different flying fields will give varying results.
Buildings, overhead hydro poles, radio towers and metal fences will all affect range. Just a few things to keep in mind. The antenna collapsed range should be more than 100 feet on 72MHz and there will be spots where intermittent control is encountered, which should not be considered as a malfunction. This can occur well before maximum range is encountered.
A bent receiver antenna is not as efficient as a straight one and theoretically in a text book situation some performance will be lost. This might be the case of a loose hanging antenna wire, however since part of it is usually always unobstructed and the fact that helicopters do not fly that far away the weak zones can be avoided or are less noticeable. Range may not suffer and can actually be improved upon in some practical situations. The point here is that relocating the aircraft antenna can improve performance if you are having difficulties with an otherwise serviceable helicopter and radio system.
Under no circumstances should you ever loop your antenna back upon itself. The optional base loaded antenna in the nose has the virtue of being placed away from the metal/graphite helicopter structure, but it is still directionally weaker at certain specific angles. This type designed in a solid wire format and unsupported in a plastic tube has been known to crack at the base attachment area due to vibrations. The best cure is to support the other end with something non conductive.
I have played around with various antenna positions with the gas/glow machines. What I have found to work best through trial and error, is to run the stock antenna down the left (carb side) half way between the skid and the side frames towards the tail. I run it inside a plastic tube (nyrod) and leave enough wire dangling aft so that it will not encounter the rotor when upside down. I have also moved the high tension coil to the other side of the gas X-Cell machine, as a precautionary measure only.
When I had the antenna secured to the horizontal fin (I thought it looked better here) momentary glitches were evident when at certain specific distances and attitudes. I was glad not to have PCM or the antenna might be still attached at this poor location. Try to keep the antenna away from large conductive surfaces and current carrying wires eg: servo/gyro wires, side frames and tail boom.
Experimenting with range checks and antenna locations will deliver dividends. Long servo leads (which can be shortened) may be unplugged from the receiver at the fringes of the range check, to see if the servo chattering stops with a range improvement. You don’t have to be an electronic buff to use these common sense methods.Often machines will glitch after some extended time in use which is unrelated to the radio system and installation.
This is often a gradual thing and if left uncorrected or unnoticed with the PCM system can result in the expensive loss of a machine or worse. Some guys use glitch counters with PCM and others will routinely install a PPM receiver for glitch observations. When these guys crash it is usually do to their “dumb thumbs”. A glitch counter is an electronic device that connects to the on board radio and counts how many times an other wise unseen PCM lock out has occurred. The results can be viewed at the end of a flight. By keeping track of the numbers one can monitor the progress of degrading radio reception.
Newer gas machines are making it to the fields as of late with a larger incidence of glitching. The newer Zenoah G-23 engine has a dual coil high energy ignition system. This coupled with the fact that the cylinder is facing forward on some models with the spark plug very close to the radio compartment is creating grief for some. Kind of like sitting on hot coals! The next paragraph covers asbestos underwear.
The ignition system is supplied from the engine manufacturer unshielded. Although a resistive spark plug has always been used for noise suppression, additional corrective actions are being taken. Here is a brief description of what shielding does. Electronic equipment can be sensitive to stray electrical fields from wires carrying alternating current. Shielded wire in addition to having the insulation around the conductor also has a braided conductive sheath that completely encloses the wire.
This sheath is grounded to the engine so that any extraneous signal that might get into an input wire (antenna or servo wire) or escape from a power lead (spark plug wire) will be carried to ground. In certain cases of sensitive electronic equipment the induced current being fed to ground through the shielding will generate its own electrical field disturbances. This is called a ground loop. To prevent this “ground loop” the shielding is grounded at one end only as an extra measure. Here is how to do it cheaply to a G-23.
One could cut open a piece auto ignition wire and swipe a length of shielding but this material often does not solder, leaving termination to crimping methods. A cheaper method would be to get some RG59 cable (cable TV wire) which has copper braid. Slice the insulation open to get at the braid. The braid is like a Oriental finger trap, push it together to make the center larger.
(The plug connector will have to be removed to slide the braid over the high tension lead.) An “Amp” connector can be soldered on which will keep the stuff from unwinding at the coil end where it will be grounded. The other end can be folded back on itself and a ring of solder used to prevent unravelling.
Use a drill bit the same diameter as the coil wire to avoid melting the insulation or rap some aluminum foil temporarily around the wire when soldering as a heat shield. Only one end should be terminated to ground at the coil. Run the shield up to the plug cap but not under it. Cover it with shrink tubing so it doesn’t look like an ungodly Ergo installation I saw the other day.
This same Ergo gas machine was working completely glitch free using a base loaded antenna up front on an inexpensive JR FM PPM radio system. I do know that the Z230 manual recommends PCM, but rather than waste money on a new radio why not try your servicable FM set first?
Running a radio wire harness be it servo or power next to the magneto can cause it to pick up the magnetic lines of flux which are introduced into the system producing rather nasty glitches. Trust me I have personal experience with that one!
One of the largest causes of glitching is from loose parts vibrating or chattering to create radio noise. Non-lubricated metal parts seem to be the worst. Even a loose washer rattling can cause some problems. The largest offenders are at high rpm, such as a worn tail rotor shaft or a worn bushing equipped chattering retractable start cone. Dried up and ratchet like bearings, cracked paste making sideframes and loose parts are some areas to investigate. Regular maintenance such as routine lubrication and inspection will help.
Hopefully after reading this, solving glitches won’t seem so much like “black magic”. Remember, PCM is no excuse for flying a defective helicopter. As an important ending always remember to never put others at risk by publicly flying a “glitch master”.