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electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are
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on aircraft modeling.
Bill Winter is one of the best-known names in the aeromodeling realm since he has been around writing columns on modeling events, construction, flying, and product features, serving as editors of modeling magazines, and participating in modeling events throughout the country since the middle of the last century. He went above and beyond the call of duty in his attempt to introduce people to the model aircraft and model rocketry hobbies. This particular article is one of a handful Bill wrote for Popular Electronics magazine in the 1950s and 1960s. An amazing transformation has occurred in the radio-control aspect in that when this article was published, participation required knowledge of electronics, a larger hobby budget than your average modeler, and a willingness to be continually battling problems.
R/C Reliability Escapements and Batteries
By William Winter
Editor, "Model Airplane News"
Concluding last month's discussion of reliability, that taken-for-granted device, the escapement, had our attention. In the author's log, covering 15 radio control airplanes and thousands of flights, the escapement was found to be second only to the relay as a cause of erratic control. The point was made that the escapement should be considered as a relay, since it has pull-in and drop-out currents which, aside from the mechanical features, require observation, occasional adjustment, and an accessible and removable installation that enables convenient maintenance. Do not bury the escapement in a "blind" installation.
How the escapement works has been described in earlier articles of this series, as well as in Mr. Safford's articles in this magazine. Our concern now is how to keep one working. Properly installed and regularly checked, the escapement is reliable. Any new escapement should be examined and bench-tested before installation in the plane. Howard Bonner, whose SN and compound escapement types are familiar to all R/C modelers, states that the overlap of the revolving arm or claw, on the pawl, should be 0.015 to 0.020, armature pulled in. With the armature released, the claw should barely clear the pawl. Also, when armature is pulled in, the claw should barely clear the neutral pawl position. These values apply approximately to the other familiar makes of escapements.
If the escapement functions suitably when hooked up on the bench, leave it alone; the above figures are given as a rough guide in the event that the item skips or sticks, requiring adjustment that is unlikely when new. After operating the escapement perhaps 100 times on the work bench, examine it closely for burrs that might develop where claw and pawl meet. The tiniest burr can prevent the escapement's working in the air. Usually the escapement so afflicted functions while the engine is running, but when the motor stops and vibration disappears, response to a transmitted signal does not occur. Sometimes new escapements develop burrs quickly, but once smoothed off will function properly for long periods of time.
Pushrod and bellcrank type of linkage and escapement drive mechanism.
Torsion bar type of linkage and escapement drive.
Set up batteries similar to those in the plane, also the same size rubber strand wound to 20 per-cent excess of a single row of knots (not counting the first row of turns). It is vital to check the spring tension. Does the escapement always release when the rubber is fully wound? Does the escapement always pull in and work easily under simulated service conditions? One excellent way of checking is to hook up a set of batteries with a potentiometer in order to vary the voltage available to the escapement magnet, Increase voltage gradually until the escapement armature pulls in. Just as important, note the voltage at which the escapement releases. This was the most important lesson learned by the writer in the 1954 flying season.
The escapement should never require more than 2 1/2 volts to pull in and should not require a cutting off of current in order to release. First, the pull in. Under load, two new 1 1/2 volt pen cells in series drop off to 2 3/4, volts. As the no-load voltage decreases with use, the voltage then available under load may be less than the voltage required to pull in the escapement. Once during a demonstration, the writer installed a new, unchecked escapement and immediately lost control of the plane during the glide. A check revealed that the escapement required 2 7/8 volts to pull! It is best, therefore, to allow an adequate margin for falling voltage, especially under load, by adjusting the escapement (its spring tension is increased or decreased) to pull in at 2 to 2 1/4 volts.
Why is drop out so important? If spring tension is too low, the flow of current '\has to be cut off to allow release of the escapement armature. This is a timely tip that in the air the escapement may not release. Many a crash has been attributed to interference, sticking relays, etc., when the escapement was out of adjustment. The difficulty is that if the condition is marginal, the escapement may appear to function properly after the accident, so that cause of the accident may be undetermined. Eventually, the spin-in will be repeated. Sometimes, the plane has a mysterious tendency to come out of a turn very slowly after the rudder is released or to continue overbanking momentarily after the rudder goes back to neutral. This can be caused by a sticking relay, but also suspect that escapement.
It has been found that if the escapement will release with current caused by 1/4 volt flowing through the coil, it should release reliably by spring tension in the air. The current is a measure of spring tension.
Mounting affects an escapement. Do not screw an escapement base tightly to a slightly warped piece of plywood. The frame bends, throwing adjustments out. Then the modeler may file the end of the revolving arm to make it shorter. Loosen the escapement and the frame springs back into alignment. Now the gaps are too big and the escapement is needlessly junked. So the mount must be firm (if the escapement does not incorporate its own mount), never less than 1/8 inch thick plywood with the long edges reinforced with balsa strips, and it should also be warp free.
It is important to use the proper rubber strand for the escapement drive power and the correct voltages. The type of linkage affects the required size of rubber. There are two types of familiar linkages. First, the push rod connected between the rudder horn and the bell crank, converts the rotary motion of the escapement drive pin into linear motion. The second is the torsion bar, which is rocked back and forth by the rotation of the escapement drive pin. The push rod arrangement increases the load on the escapement, especially during maneuvers (centrifugal force multiplies the weight of the rod) when the entire weight of the linkage may have to be lifted by the actuator. The torsion bar is easier to move, does not overload the escapement and, therefore, favorably affects the size of rubber which is required, in addition to escapement adjustment and operating currents.
With self-neutralizing escapements, 1/8 inch rubber has been used with both push rod and torsion bar, although, in the author's opinion, it is decidedly marginal in the case of the push rod, particularly on a cold day when rubber loses much of its natural vitality. Therefore, it is better to use 3/16 inch rubber with push rod deals. When using 3/16 inch rubber, allow 20 per-cent extra length over and above the distance between hooks. This prevents too much tension being placed on the escapement thrust bearing which could cause a jamming action. The compound escapement should never be used with 1/8 inch rubber with a push-rod linkage. The reason is that the compound incorporates a rattle wheel to slow down the action of the escapement and allow the man on the ground enough time to get off the required number of pulses necessary for its operation. So the compound requires more drive power than the self-neutralizing escapements.
In this self-neutralizing escapement the mechanism is also used to cover and uncover air bleeds in the fuel line to control the motor.
Some builders claim that the flier must have a sense of timing to pulse the compound (one, two, or three signals, depending on the desired control). Therefore, they claim it is better to use the weaker, 1/8 inch rubber to slow down the escapement so that the flier can keep up with it. That is poor advice. The compound can hang up when powered by 1/8 inch rubber, if only due to the drag of the electrical contacts in the third control position. Actually, it is better and easier to operate the compound with 3/16 rubber, when the unit works faster instead of more slowly. With the 1/8 inch rubber, timing is important, because it is possible to pulse too fast and pick up the wrong control. With the 3/16 inch rubber it becomes impossible to pulse too fast with a Microswitch held in the hand. At the same time it is not hard to pulse fast enough, especially after a few practice dry runs on the bench. Even a mechanical ground control unit will not time properly the 1/8 rubber driven compound. As to reliability, the compound is often criticized, but one of the author's compounds has given trouble-free operation equivalent to three self neutralizing units.
It is frequently argued that the compound does not have the ability to hold the rudder over when air speed picks up or to hold the plane in a prolonged spiral, especially in the direction of the control (usually left rudder) that requires two pulses. Supposedly, the compound is not effective on big, heavy, fast machines. The truth is that the compound is suited for all installations, provided an aerodynamic surface (see drawing) is used along with the3/16 inch rubber drive.
Blaming escapement ills on batteries is both commonplace and groundless. People are forever putting 4 1/2 and even 6 volts on a 3-volt escapement. Not only is this unnecessary under any circumstances, but it leads to further complications. To begin with, battery drain is increased greatly with higher voltages when the resistance of the coil remains the same. This is a basic law of electricity. Therefore, the batteries run down faster, not more slowly, when voltage is stepped up. A 5 ohm escapement which might function for several flying sessions on 3 volts, may make only one long flight on 6 volts! Battery life is increased by hooking batteries in parallel, not series. Most planes can carry four pen cells, instead of the standard two, for escapements. Two pen cells may give a dozen good flights, depending on how many times the control is applied and how long it is held on. Planes with slow response are rough on batteries all down the line, even in the transmitter. Excessive voltage on many escapements builds up a residual magnetism which can cause the armature to stick in the control position. Higher actuator voltages accelerate damage and dirt on the relay contacts.
With some builders, batteries may be the second or even first source of trouble. In the writer's log they happen to be third, mostly due to odd and unexpected failures, such as one abrupt failure resulting from the battery having been dropped by a clerk. A connection between cells gave way. This suggests care in the handling of "B" type batteries.
Choose battery sizes that provide adequate life and reserve, unless, of course, the plane is a midget. For example, two pen cells on filament will give an afternoon's flying on a single (gas) tube receiver. Such receivers have even been flown on one pen cell, but if the plane will carry one or two medium flashlight cells, it is an unwise risk. A two (gas) tuber will operate on two pen cells for a busy half-day flying session, but two mediums would last for weeks. Similarly, why fly on 22 1/2 or 30 volt hearing aid "B" batteries (in series for 45, 67, 60 volts, etc.)? A single Burgess XX-30 or K-45 or the equivalent in other brands will last for weeks, if not months. The typical transmitter will operate for at least a season on Burgess M-30's or larger (or the equivalents). Hearing aid "B" batteries certainly are not desirable for long term results with hard tube receivers that idle at 3 or 4 mils. Two mediums on an escapement may last a summer.
Possibly the gravest error made by the beginner is to measure voltages without placing a load on the batteries. The transmitter should be checked with filament turned on, Microswitch closed for "B's." It will be noted that "B" batteries may drop several volts under load, but this is normal. On the other hand, a drop of 10 volts or more from the initial reading (not new voltage necessarily) under observation means that the batteries are weak. Hold the meter probes in place for 5 to 10 seconds and watch for a slight, steady falling off in voltage. The battery is no good. Do not operate anywhere near the minimums specified by the radio manufacturer. The writer discards flight "A" batteries that read 1.4 volts or less under load, when 1.5 volts is the normal filament voltage. The voltage can drop further in the air and 1.3 volts is the safe minimum.
For 3 volt escapements, a bitter-end 2 1/2 volt minimum under load is desirable, unless the escapement happens to be one that works on 1.5 volts, as does the Macnabb Citizenship. After a 67 volt "B" battery drops 5 volts to about 62 volts, there is no percentage in continuing it in service. Battery costs are low compared with the total cost of plane, radio, engine.
In cold weather, allowance must be made for a falling off of voltage due to temperature. Some modelers keep batteries in a warm place, as in the pocket or on a car heater. Obviously, this is inconvenient, but batteries should be allowed to recuperate between long flights. It is a good rule to allow the batteries to rest for a period twice as long as the last flight. Between flying sessions, batteries recuperate so that they almost regain the normal new voltage. After that, they should be checked after every few flights.
Make it a rule to check batteries before going out to the flying field. If they are down to a serious degree, install new ones and enjoy an outing free of concern.
Posted March 14, 2015