Adjusting the Power R/C Plane
January 1955 Popular Electronics
If you ever had any doubt whether Bill Winter was one of the model airplane hobby's earliest and most prolific contributors, check out this article that appeared in the January 1955 edition of Popular Electronics. Radio control systems were just beginning to be commercialized and priced at a point where a lot of the public could afford it. Quirks and high unreliability that plagued early systems had become less of an issue so that airplane and power plant design efforts could take priority with aeromodelers. In this article, Bill addresses setting proper wing incidence and engine thrust angles for good, repeatable, stable flight.
January 1955 Popular Electronics
[Table of Contents
old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Popular Electronics was published from October 1954 through April 1985.
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Adjusting the Power - R/C PlaneBy William Winter, Editor, "Model Airplane News"
You can avoid a lot of grief and lost airplanes if you will follow the practical hints for adjusting and flying your R/C plane given here.
Beginners launching their powered airplane models on their first flights make two mistakes. One is to use too much or too little power, and the other, often the fatal one, is to use too much fuel or too long an engine run.
It is possible for the plane to fly erratically and finally, uncontrollably, on an early test flight, and still remain in the air to drift away and be lost. Test flights should be short. The amount of power is most easily controlled by opening up the needle valve for a rich mixture, recognized by a smoky exhaust. This assumes that the engine uses a glow plug. If it is a diesel engine, back off the head to obtain a rich mixture; for ignition engines, retard the spark. It is hard to say exactly what power would be required but, if one had to guess, about two-thirds would be best under average conditions.
Another possibility is to place the propeller on backwards, letting the engine "rev up." This cuts effective thrust by a third or more.
Before attempting to fly the plane, run off a measured amount of fuel and time the engine run. You don't want more than a minute, and remember, if you decide to lean out the needle valve the same amount of fuel may double the engine run. A fuel shut-off device is desirable for first tests; back up the device with a limited amount of fuel.
There are many pros and cons as to whether the radio should be operative on a beginner's first test flight. Chances are the radio will be a help.
We'll assume that you have made the necessary ground checks, flight checks, etc. Holding the plane as described in last month's article, run at a moderate speed (a very slow run if the plane is light and there is any wind) into the wind. When lift is felt, push the plane forward as you release it, with the nose ever so slightly above the horizon. You should follow through and use a smooth technique, the same as you would in golf.
While it is essential to handle glide and power-on adjustments separately, it is not always possible to distinguish between the two in a short first test flight, especially under exciting circumstances. However, if the plane is high enough to glide when the engine stops, try to remember whether it glides too slowly, stalling or appearing to "mush" or falter; or whether it seems to dive ever so slightly. Did it want to turn of its own accord?
Continue to refine the adjustments begun in the hand glide tests, described in last month's article. It is essential to work out glide trim first, if at all possible. If the plane was not in danger of cracking up or getting out of control under power, ignore, for the time being, the power-on adjustments and perfect the glide trim. But if the ship was in danger under power, take the bull by the horns and affect any adjustment that will enable you to get it into the air well enough to observe and evaluate its performance. For example, if the plane turns wildly to the left under power, skimming the ground, adjust the rudder slightly to the right (perhaps 1/16 inch on a small plane, 1/8 inch on a large one). If the plane turns to the right, use left rudder. Naturally, rudder adjustments affect the glide path, but the thrust line (an imaginary line drawn through and parallel to the shaft of the engine) can be offset to one side or the other on subsequent flights, at which time the rudder would be readjusted to normal. Everything is aimed at getting a look at the glide.
Observe very carefully how the plane behaves during its first hand-launched glide. The proper glide path is straight. smooth, and without dips until the plane strikes on wheels. If the plane dives slightly (B), increase the angle that the wing makes with the airstream, or decrease the angle of the stabilizer. In severe cases, move the weights in the plane backwards. If the plane stalls (C) or (D), increase the positive angle of the tail, or move weights forward. Be certain when you launch the plane that you do not throw it nose up, or too fast, especially in high winds, as this will produce a false stall, giving you an incorrect idea of the model's performance.
After the glide is reasonably correct, begin to make thrust line adjustments on subsequent short flights, tilting the engine up or down, left or right, as required to offset the model's power-on tendencies.
If the plane turns under power to the left, the thrust line would have to be inclined to the right. If the plane noses up when the engine runs, but glides perfectly, the thrust line will have to be tilted down, to pull down the nose. The latter is called downthrust. If, under power, the plane noses down, the thrust line should be inclined upward. Downthrust usually is required.
All adjustments should be made minutely and progressively on repeated trials. Downthrust is imparted to a radially mounted engine by inserting washers between the rear crankcase cover, at the top, and the firewall. The washers may be slid onto the mounting bolts. In the case of a beam mounted engine, put the washers between the mounting lugs and the bearers at the rear of the engine and, again, the washers may be slid onto the mounting bolts.
Bit by bit, the plane can be trimmed perfectly. Don't expect miracles. Walt Good, several times winner of the National's radio-control event, once took over 100 flights to trim out his famous "Rudderbug" exactly the way he wanted it.
Only when unquestionable control can be had all during the flight, should you increase the motor run. Two to three minutes is maximum for safety on the first few dozen flights.
The operator can do much to smooth out a rough flight by careful use of the rudder. Stalls, for example, can be alleviated by beginning a turn. The rudder should be applied as soon as it becomes evident that the plane is going to nose up. Control action is more effective at high speeds; it is usually absent at the point of stall, A plane may be stalled due to a hard launch, especially into the teeth of a wind, or after recovery from a spiral or any descending turn, or upon turning sharply into the wind. A straight ahead, nosing-up action, is a dead-end street; if the plane can be turned slightly by applying the rudder at the crucial moment, a zoom or stall will be converted into a climbing turn, dissipating the excess speed. Excess speed simply means excess lift, followed by a stall.
Just as some auto drivers have a heavy foot, and others a light touch, so do some people tend to over-control a radio model by forcing it to do things. A heavy hand on the switch prolongs turn entries until the nose of the plane is screaming earthward. When the control is finally relaxed, a zoom results. This makes for roller coaster flying, with narrow squeaks on every flight.
Try to develop a light touch, stay relaxed, don't get rattled - it's only a model, remember? Begin a turn, when it looks steep allow the plane to recover, then resume the turn. The gentler the flying technique, the fewer out-of-control flights.
For some strange reason it is always easier to maintain good control of a plane that is going away from you, out in front of the transmitter, than it is when the ship is downwind and plodding up behind you. Maintain as much distance upwind as possible, recovering ground after every maneuver. Once the plane is downwind, a skilled hand is required to bring it back with surety. In a strong wind the plane can be flown out a quarter mile or more, with the assurance that it will glide back downwind into control again if anything goes wrong. Or at least, the chase will be shortened. The importance of remaining upwind cannot be overemphasized to the beginner.
Every flier sooner or later gets himself into -hot water. The flier with a little experience may get to stunting too close to the ground, then finds that everything he does magnifies his difficulties. Or the beginner may become excited, turn wildly from side to side. The answer? Relax. The plane is inherently stable and will recover by itself. When it gets right side up and begins to climb, it will slow down and control may be resumed.
Here's another tip: a plane that is trimmed to glide well on a calm day may stall on a windy day. If trimmed on a windy day, it will glide too fast or even dive on a calm day. Rudder control that is adequate in calm weather may be inadequate in a wind. This sometimes fools the flier into thinking he has lost control. In the wind, the same control must be held longer to obtain an equal reaction. It is wise to know how thick a shim it takes to alter a stabilizer angle from windy weather to calm weather trim, or vice versa.
The location of the center of gravity of the plane greatly influences the flying characteristics of the plane. The CG should be located no greater than 40% of the chord (width) of the wing back from the leading edge of the wing. The model can be flown successfully with the CG as much as 50% back on the chord, but the further back the CG is located, the stronger the tendency for the plane to rear up on turning into the wind, or on coming out of fast turns. Forward CG positions are associated with better wind penetration. In fact, some builders place the CG so far forward that a negative angle in the stabilizer becomes necessary to hold up the nose during the glide.
While everything that is known about flying an R/C plane cannot be condensed into two articles, the fundamentals described in this article and last month's will help you get into the air successfully. Once the plane is flying reliably, there is a coordination exercise that will quickly give you the feel of the machine. With plenty of altitude, hold a turn for an eighth to a quarter of a circle, then reverse the direction with opposite rudder, passing by the original heading to a point a similar distance on the other side of the heading. Keep repeating the maneuver "S-ing" upwind. As you gain confidence, hold the turns longer and longer, and you soon will learn how to fly the ship out of any predicament.
Another valuable exercise is to make wide, shallow circles, by repeated applications of the rudder in the same direction. This will teach you how to avoid unwanted rudder positions in the popular self-neutralizing escapements, as well as how to make turns without loss of altitude or picking up excessive airspeed. Repeated short flights is the way to learn safely.
Easy does it.