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Macon Aero Modelers Safety News October 21, 2009

Frequency Discipline and Transmitter/Receiver Issues

From:

Safety Coordinator, Macon Aero Modelers

            Within the last two weeks we’ve lost three aircraft. One was flying on 72MHz and two were controlled by 2.4GHz (Spektrum system). Two of these incidences occurred during our Oct 3rd and 4th “Fly-In”. When a commercial or private aircraft goes down, the National Transportation Safety Board (NTSB) conducts a thorough accident investigation to find the cause. As individual flyers, it is important for us to do the same thing. When we’ve invested hundreds or perhaps thousands of dollars we don’t want a repeat. Do we know the exact causes? No we don’t, it could have been the transmitter, or multiple other causes. Let’s examine some possibilities.

            The aircraft flying on 72MHz was an “auto-gyro” controlled by a transmitter with a multi-channel selector module. The owner says he has never had any problems with the transmitter. During the flight the gyro operated perfectly until over the runway when it just fell out of the sky. During the “Fly-in” we had strict frequency control and were using transmitter impound procedures. But could someone with the same type of radio turned their radio on? We don’t know for sure, and probably will never know, what caused the loss of this aircraft but it reinforces the need for frequency control and the use of frequency pins. Never turn on your transmitter if you do not have the frequency pin.

            Now let’s examine the other two losses. Both of these were controlled by a 2.4GHz Spektrum transmitter and were .46 size electric aircraft. At this time the owner does not know if the transmitter was the cause or some other factor. What he does know is that he lost two very nice aircraft.

            I’ve looked at the some of the transmitter websites and some “blog” sites such as RC Universe to see if I could gain any useful information. Most of what I discuss is oriented to Spektrum transmitters and receivers but I believe the comments would also apply to JR, Futaba, and Hitec as well. Assuming one “binds” the receiver to the transmitter all should operate correctly. Right? Not so fast…..read on. While the manufacturers advertise that these new 2.4GHz systems as “glitch-free”, when something happens to your airplane then it isn’t glitch-free. Receiver manufacturers are providing a lot more printed and cautionary information than the instructions printed when these systems first came out several years ago.

            Antenna Polarization: “It’s important that the antennas be mounted in an orientation that allows for the best possible reception when the aircraft is in all possible attitudes and positions……antennas should be oriented perpendicular to each other.”

            Range Testing: “Before each flying session and especially with a new model, it is important to perform a range check.” Note that the manufacturer says to do this before each flight, not just the first time you fly a new aircraft. One “blogger” stated that it is important to have a helper hold the aircraft and change its orientation in respect to the transmitter. This person found a loss of signal when the aircraft was pointed directly at the transmitter and suspected the motor, batteries, etc could be blocking the signal. By repositioning one of the remote antennas he corrected this problem. Some receiver instructions now include a section covering Advanced Range Test procedures using various aircraft orientations and the use of a Flight Log module to detect signal loss.

            Receiver Power Requirements: Most receivers require a minimum of 3.5 volts. “Inadequate power systems that are unable to provide the necessary minimum voltage to the receiver during flight have become the number one cause of in-flight failures.” They advocate using a voltmeter to plug into the receiver, load the servos, and monitor the voltage to make sure it remains above 4.8 volts (4-cell battery). Where weight is not a concern many flyers have gone to 6 volt batteries (5-cell) or use LIPOs with a voltage regulator. Giant scale and IMAA aircraft require redundancy and plenty of power for power-hungry servos.

            Brownout Detection: Some new receivers have a feature where the LED will flash slowly indicating a power interruption (brownout) has occurred. You must look at the receiver after flight and before you turn off the receiver and transmitter.

            Other causative factors causing the loss of an aircraft could be bad connectors; “cold-solder” of connecting plugs; an unconstrained battery that becomes disconnected during aggressive flight; a too small or faulty receiver switch harness.

            Select the right receiver: “Park flyer” receivers (those without a separate remote receiver) won’t have the range or antenna coverage in a large electric, nitro, or gas flyer. Likewise, many receivers are not intended for use in airplanes that have full carbon fuselages (sailplanes), nor significant carbon or conductive structures. Receiver manufacturers now have specific receivers with longer antenna wires that exit the aircraft.

            Summary: Several of our Club’s experienced flyers contributed their many years of flying and wisdom to this article. Greg Doster stated: “It is our responsibility to make sure we understand how the systems operate and ensure that the installation of our equipment is right. That all batteries are charged and connections are backed up with safeties. Also antenna placement and cable routing is paramount. Wiring routed close to an antenna can receive some residual “RF” and therefore in essence change the characteristics of the antenna network. Also, anytime you have a hard landing you should inspect everything about your model. Too many of us treat our airplanes like toys and they are not--by no means. It is all too easy to blame "dumb thumbs" on a radio glitch. Check your 2.4 GHz transmitter antenna and make sure that the antenna is never pointed straight at the model when it is airborne. This creates a lower strength signal path to the model. Keep it perpendicular to the model. Inside the model maintain 90 degrees of antenna separation. I have found that gluing small pieces of “nyrod” (or antenna tube) as antenna guides will hold them in place and maintain the separation required. For those of you that still want to blame your radio system for your crash this will let you know that we all understand that you don’t want to "EAT CROW". Even if it was a radio problem it is more than likely the installation, weak batteries, antenna orientation, or Carbon Fiber interference but chances are it was just "Dumb thumbs".

            ”So for the safety of our sport and all participants, let us be more vigilant and “Fly Safe.”

           

           Bill Duncan

 

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