Manta Ray (oz12550)
About this Plan
Manta Ray. Electric RTP model.
Quote: "Not much is heard of electric powered flight in this country, but its popularity is growing in Great Britain. At the 1970 Model Engineer Exposition in London, four clubs specializing in electric 'round-the-pole' flying demonstrated their craft.
Pylons and power supplies are provided by the clubs, individuals build and fly their own models. This scheme is economical and practical, requiring the young modeler to purchase only the materials for his model, rather than the relatively costly power supply.
While it is expected that many American youngsters will be better able to afford their own 'ground-support equipment,' the club approach is still a good one, providing a superior training method for beginning modelers. A school sponsored club using the talent of an industrial arts teacher, and a gymnasium flying site, seem particularly attractive. Lacking such adult support, an enterprising boy can do the job on his own.
The basic idea is to conduct power up the lines to a model driven by a scot-car racing motor. While regular U-control flying can be done from the hand, the usual method is to have a pylon at the center of a circle, with the pilot sitting relaxed outside the flight path. A rheostat or slot-car racing throttle provides control of the power, while a thin cord or thread serves to operate the elevator.
Lines can be quite short (5 feet), or rather long (40 feet plus). Since no fuel is involved, models are relatively quiet, and very clean - no air pollution indoors. Beginners as young as six have successfully learned to operate these models - yet there is great scope for further development.
Though the power supply is a bit costly, it lasts forever. The models themselves are not expensive to build, and are nearly cost-free to fly. Indoor flying, even in comparatively small roams, is practical. Indeed, it must be admitted that outdoor flight is rarely feasible because of the rather limited power provided by the slot motors.
Models have ranged in complexity from simple sheet balsa crates, similar to that detailed here, to conversions of rubber-powered kit models and fancy scale models with retracting gear, multi-motors or ducted fans. Pylons capable of working up to four models simultaneously have been built. Speeds of over 40 mph have been achieved. Geared props have been used to improve propulsive efficiency. The potential limits have not yet been approached. Who will be the first to hit 50 mph?
Development of really good propeller-gear systems and ducted fan models remains to be done. Can motors giving power to weight ratios as good as gas engines be developed? While loops have been achieved, the ultimate electric stunter has not yet been built. Contest events still need to he worked out, though Payload, Carrier, Speed and Combat have been tried.
Many electrics have been converted from rubber-powered kit models. Elevators are revised to be worked in a manner similar to that shown on the plan, motors and wiring in-stalled, and landing gear beefed up with piano wire and addi-tional structure.
Points to watch are avoidance of large radial type cowlings that block propeller flow, and assurance of cooling air flow to and from the motor. When these points are kept in mind, the results are nearly always successful.
The single most difficult (and most costly) to obtain com-ponent is the power supply. Few train transformers or slot racing power packs have enough output to do a good job of flying a model. MRC's No.642-66 is one that will fly a model powered with a light motor. KF Industries Model CB52 will also operate a light motor. Schauer Manufacturing's 4-amp battery charger handled all sizes of motors. Their 3-amp unit, however, was unsatisfactory. Lionel's large train transformers were not tried, but should do a great job.
Ratings for these units are somewhat deceptive. You would think a nameplate stating '12 volts, 2 amperes' means that you have a current of two amperes at a potential of 12 volts. What it really means, however, is that you get 12 volts when drawing no current, and a 2-amp current when shorted out - no volts.
You can expect to get about half the rated current at half the rated voltage. For the 12V, 2A example, about 6 volts and one ampere. This is only enough to fly a light model with a small motor.
A 12-volt car or motorcycle battery will also do a reasonable job of flying a model. The smooth DC output is much kinder to motors than the ripple from a power supply. On the minus side, you will need a charger, and you will have to care for the battery, keeping it charged and filled to avoid ruining it. Also, battery acid can spill, and is quite hazardous.
The power is controlled by a knob or lever on the power pack, or by using a heavy-duty slot racing throttle. Many different motors have been successfully flown. The ones that do not work out well are the HO car motors, the heavy train motors, and those billed as 'Three Volt: rewind,' 'hopped up,' etc. The HOs simply do not have the power, train motors are too heavy, and the rewinds develop their best power at too high an rpm to operate a decent-sized propeller.
Your best bet is a small 'beercan' type motor. Greatest success has been with the Mabuchi FT series motors. For a small model such as Manta-Ray, the Mabuchi FT-16D-2470 or FT-16D-2475 is suitable, particularly with a light power supply.
The Mabuchi FT-26D-2475 will provide superior performance provided that it is fed enough power. The correct propeller to use with either of these is the Cox 3-inch diameter x 1.25-in pitch prop. Full-size drawings of these two motors are shown on the plan to aid your selection. The Mabuchi FT-36D is a larger, heavier motor, suitable for models up to 25-in. spans. It will swing a 4.5D x 4P or 5D x 3P prop, but must be fed 2 amperes or more at 12 to 15 volts to develop its full capability. All of the Mabuchi motors can be ordered from Polk's Hobbies, 314 Fifth Ave, New York, NY 10001, if you cannot find them locally.
It is important to avoid using too much propeller on all of these motors. Props with more diameter or pitch than those mentioned here will result in slowing the motor down, result-ing in more power consumed, but less delivered to the prop. The difference ends up as heat within the motor, causing a burned-out motor.
Almost all slot motors except HO and rewind will handle the Cox 3 x 1.25 prop. When in doubt, start with this prop, and work up to a heftier one if the motor remains cool and shows no excessive arching at the commutator.
Like the power supply, a pylon is a permanent invest-ment. You can go through many models without damaging a well-built pylon. The version shown in the plans is designed for construction from easily obtained material. Speaker cable and wire lugs are available from electronic supply stores. A swivel can be obtained from any store selling fishing gear. The rest of the stuff is from hardware stores, the 5 and 10, lumber yards and our old buddy, the hobby shop. Construction is pretty well covered in the plan and photos, so only special comments will be made here, rather than the 'cut out Part A and cement to part B' routine.
The top end of the inner tube should be filed and sanded very smooth to avoid snagging or fraying the control line. The hole in the hold-down card should also be well smoothed. An eyelet in this hole would be helpful, the control line may be waxed at these points to reduce abrasion. The short bit of tube that holds the pylon head in place should be pushed low enough on the inner tube to insure continuous contact of brush to slip ring, but it should not be so tight that the pylon head can't rotate easily. The slipring should be smooth and clean. Brass is best, but tin can stock will serve. The pylon head should never be oiled. If lubrication is needed, try rubbing the critical spots with the point of a soft pencil.
Manta-Ray was designed to use an FT-16D or FT-26D, or similar small motor. The general appearance is derived from the Davis Manta fighter, proposed early in World War II. It is rather fast, but easy to fly.
The first step in construction is to fit the prop to the motor. If your motor is equipped with a gear, the prop may be forced onto the gear and epoxied in place (enlarge hole in the prop if needed). For gearless motors, the method shown on the plan works fine. Force the sleeve from a Du-Bro Quik-Link onto the shaft. Drill the prop hole to 1/8 in, and force onto the sleeve. Caution! Support the opposite end of the shaft while forcing to avoid shifting the shaft in the motor. A more sophisticated method is to solder a screw with a longi-tudinal hole drilled in it to the shaft. This method requires lathe work.
Bending the wire terminals can be difficult. They should be hooked enough to hold the wire lugs on the ends of the pylon leads; they should also be spread out enough to insure continuous good electric contact with the pylon leads.
Most of the actual construction of the model is covered on the plan, leaving only a 'few points to be covered here. The wheels rotate freely, and must be carefully aligned to prevent the model from turning inward during takeoff and landings.
The elevator roust operate freely. Note that the rudder moves right with the elevator, into a slot in the stabilizer. The stabilizer slot must not bind on the rudder. The rubber band used to return the elevator to neutral must exert only enough force to do the job. When you pull on the elevator line, if the model moves rather than the elevator, the rubber is too strong. Use a weaker or longer rubber band, or a longer front hook.
When flying, make sure the pylon is weighted down. I use three half-gallon bleach bottles filled with water. Also, make sure the model will clear all obstructions. This can be done best by walking the model around the circle. Make sure the elevator Works freely, and that everybody is out of the way - then pour on the coal!
It is recommended that elevator not be used for the first few flights. The nervous tension of trying something new is usually enough to make the pilot apply continuous full up-elevator without realizing what he's doing! If Manta-Ray stubbornly sticks to the ground, add a 1/32 or 1/16 shim under the trailing edge of the elevator.
When plane and pilot are broken in, start using elevator for takeoff, landing and to correct flight path. A quick, light touch and release is all that is normally needed.
When you've had some experience using elevator, try cut-ting back the stop on the leading edge of the rudder to allow more elevator travel. Manta-Ray is fast rather than maneuverable - don't expect to loop it. For lines longer than about six feet, the use of light enameled wire instead of speaker cable is recommended. You will lose some power from the higher resistance, but the weight carried by the model is cut down considerably. "
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Scans from dfritzke, cleanup by Circlip.
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