| ALFA MODEL EDF 60/15 Mk.2 | 5102 |
| Internal diameter: | 60 mm |
| Design velocity: | 15 m/s |
| Weight (without motor): | 30 g |
The efficiency of the ducted fan is vitally dependent on both the accuracy of the assembly of the device itself, and on its correct installation into the model. Please devote therefore the maximum attention to the following information.
Assembling the power unit
The motor is centred in the fan core by its (motor's) circumference. Inside the core tube there are wide and narrow ribs. The narrow ones are there to support the Speed 300 motor, the wider ones are for the MPJet 25-25/26 motor. To install the MPJ 25-25/26 motor, it is necessary to cut short the narrow ribs, using a knife, a chisel or a sandpaper (sanding block), without damaging the wide ones. If need be to use a motor of even smaller diameter (such as the MM1215/12, Hacker B20 22S), it would be necessary to install taller longerons, gluing them to the inside of the fan core walls. The diameter of the Mega motor 16/17/… requires a marked enlargement of the opening in the motor casing. Therefore it is preferable to use the version made from the outset for the installation of the Mega motor, including the plug.
The duraluminium plug of the fan is either pressed (Speed 300 with the splined shaft end) or glued onto the motor shaft. The correct relative position of the plug and the motor's drive shaft is one of the deciding factors affecting the correct operation of the fan. If the plug would wobble, it would vibrate the fan rotor, too, leading to a pronounced decrease of rpm and therefore to a big loss of thrust. Also the front bearing of the motor would suffer a failure soon, indeed. Therefore ensure that the plug and the shaft would be exactly coaxial when pressing them together, also providing that all the forces would be supported by the shaft itself and the plug, not by the motor body. It is prudent to oil lightly the shaft and the plug before pressing, too. Conversely, when gluing the plug to the shaft, remove any trace of grease from the shaft and the plug; some roughening of the shaft with a very fine sandpaper would not do any harm, either. If the diameter of the shaft of the motor you are going to use is greater than the diameter of the hole in the plug, it is necessary to drill (ream) the plug hole. Using a series of drill bits with diameters growing in 0,1 mm increments, and a fair measure of patience and care, it is possible to do it by hand; without a lathe. Once reamed, the plug could be also glued to the splined shaft of the Speed 300/6V motor. For the eventual disassembly this method is better than pressing the plug on.
The gap between the motor outer shell and the rotor should be about 0,5 to 1 millimetre. If screws are used to attach the motor to the core, it is mandatory to check that the rotor does not bind on their heads. Even if the recessed-head screws would be used, the distance between the plug and the motor front plate must not be less than 3,5 mm. To set the distance (gap) properly, it is advisable to use a scrap of balsa plank or of a polystyrene sheet of suitable thickness (about 1,8 mm, depending on the size of the bearing box, it has to be checked); slide on the motor shaft once the "tight" hole in the backplate is drilled. This "gap gauge" ensures a proper position of the plug on the shaft during the gluing, also preventing glue from eventual getting into the front bearing. Apply a small amount of the glue included in the kit both into the plug hole and to the end of the shaft. Insert the shaft into the plug. Turning it, spread the glue on the whole inside surface of the hole and leave the whole assembly aside in a vertical position to dry for a minimum of 30 minutes. Once the glue dries, remove (gouge out) the "gap gauge" shim between the motor and the plug. At the low motor rpm, you can check both mechanically and (or) optically, whether the plug wobbles or not. During the optical check, observe how much the reflection of the light oscillates on the rotating plug. During the mechanical check, touch the rotating plug very lightly by the ball of finger or a fingernail - do not push! Eventual vibration could be felt. If the vibration seems to be substantial, it is advisable to remove the plug (heat it with a soldering iron and pull it off the shaft), clean thoroughly the shaft and hole in the plug and repeat the procedure again. Little vibration could be either tolerated or removed by careful sanding of the rotating plug with a sanding block; ensure that the motor is well fixed. It is necessary to assess one's own manual abilities squarely, as an indelicate action could worsen the state considerably.
Before inserting the motor into the fan casing, remove all stickers from the outer surface of the motor case, cleaning the motor as necessary. Tighten the motor screws lightly, as they serve only the purpose of preventing the motor from turning or sliding backwards. Secure the screws from loosening by vibrations, using a drop of glue (PU, Epoxy, CA). The motors that could not be secured by screws have to be tack-glued to the ribs inside the fan core, using the PU or Epoxy glue. If using a Mega motor, it is necessary to carefully gouge or rather scrape with a fine file axial (streamwise) grooves beneath the stator vanes. Do not scrape them to the fan casing as it is very thin and some damage might ensue. Once the motor is inserted in the casing, let run a few drops of a thin CA glue into the grooves, cementing the stator vanes directly to the motor outer surface. Solder the power cables and the noise-suppressing capacitors in such position that it would not prevent the motor from sliding inside the fan core.
The gap between the rotor and the plug must be minimal - if the gap between the rotor and the plug is too wide, it is necessary to attach to the plug a strip of a thin self-adhesive tape. The fan rotor is manufactured with a minimum clearance (gap) between the blades and the inner surface of the fan casing. Once the rotor is fitted to the plug and the securing screw is screwed in place (the hole in the plug has to be clean, with no glue inside!), check and ensure that the rotor spins freely. If the rotor binds (this could be caused by offset (out of axis) position of the engine, by adverse matching of the manufacturing tolerances during the assembly of the fan, etc.), look down the air duct of the fan and find where the fan blades touch the inside of the fan casing. Mark that spot, and, once you disassemble the fan again, carefully sand away the rib(s) on the opposite side of the core, to move the motor to a position where the blade would not touch the inner wall of the casing. Another cure is to shorten the blades by sanding - place a piece of 200-grit sandpaper inside the fan casing and slowly turn the fan to sand the circumference of the blades. This method is quicker than the previous, but, increasing the gap between blades and casing, it lowers the efficiency of the propulsion unit. Sometimes a removal of the motor from the casing and a reinserting it slightly turned might help.
Check whether the rotor spinner is mass-balanced. Stand its nose on a flat smooth surface such as a glass plate. If it does not stand in upright position, leaning to side, balance it using a piece of self-adhesive tape, attached to the internal surface of the spinner in the position opposite to the "heavy" side. The spinner is snap-fitted to the fan rotor. Especially when using the more powerful motors, it is prudent to secure the spinner in place by tack-gluing it with CA glue or a thin layer of the PU glue (the tack or spot gluing facilitates the eventual disassembly of the joint later). Do that, however, only after balancing the fan. Also the rear (efflux) cone is tack glued to the fan motor core casing. When using a Mega motor, it is necessary to sand away the shoulder (recess) at the rear cone and glue the cone directly to the engine rear face.The streamlined sheath for the power cables is directly downstream of the stator vane.
When checking the operation of the fan, hold it so that its casing would not be deformed - such deformation could cause a potentially damaging contact between the casing and the blades of the running fan. By the "trial and error" method adjust the relative position of rotor and the plug and the position of the spinner and the rotor, so that the fan would run with minimal vibration. It is essential to mark the initial relative position of both the rotor and the plug, as well as of the spinner and the fan - otherwise you would soon "get lost" in the balancing process. This procedure requires a considerable dose of patience, but the thorough balancing pays off substantially by increasing the thrust and life of the power unit. As every rotary system also the fan has its so called "critical rpm" range, that makes itself felt by a sudden increase of vibrations. Therefore pass these rpm ranges swiftly, both when spooling the fan up or down.
The motor in the ducted fan installation is almost without cooling, which does not make any problems when flying at normal regimes up to about 10 minutes. It is necessary to let the motor cool down between flights, this applies especially to the Speed 600/6V. If the motor used has the internal cooling -flow or its small diameter permits airflow to pass between the motor outer surface and the fan casing, it is advisable to drill open the end of the rear cone, and, if necessary, also to drill (open up) auxiliary cooling inlets in the front fan casing. Do not expose the model and its power unit to the sunlight for long periods of time, especially behind a window of a car.
Installation into a model
The ducted fan could be attached to a model by the screws passing through the attachment lugs. Ensure that when installing the ducted fan, its casing does not get deformed - otherwise the rubbing of the rotor blade tips against the duct casing would occur.
The shape and quality of the inner finish of both intake and exhaust ducting and nozzle have a profound influence on the total efficiency of the ducted fan, i.e. its real (or installed) thrust. The intake duct should be as short as possible, of a constant cross-section and no sudden changes of shape. The intake lips have to be rounded, at the best of the semi elliptic or at least of the semi-circular section; in no case a sharp intake lip is allowed!!!
The length of the exhaust tube should not exceed 100 mm and its diameter should gradually decrease from 60 mm to 50 - 52 mm at the nozzle (a non-circular nozzle of the similar area is allowable). The edge of the exhaust nozzle should be as sharp and as thin as reasonably possible. Also the transition between the external surface of the fuselage/nacelle and the nozzle should be very smooth. The whole air tract should be smooth, without any unevenness and protrusions.
