FW Ta 183 Huckebein ducted fan comparative measurement of propulsion units     The ducted fan of the FW Ta 183 Huckebein was designed pro Speed 300, 6 V electric motor and for the nominal flight velocity of 15 m/s. Such a velocity is reached in stabilised horizontal flight, as proven by measurements taken over the straight course. The ducted fan as designed is remarkable by its relatively low current drawn and by the thrust characteristics enabling a safe flight of the model in the complete speed envelope - given, of course, the appropriate weight is maintained. In the table the possible alternatives to the Speed 300, 6 Volt motor are listed, either motors with characteristics similar to the Speed 300 (MPJ AC 25/25-26 Mk.1, Hacker B20 22S, providing around 6000 rpm/V), or motors with lower rpm/V performance (Sagami 280BB, MPJ 25/25-26 Mk.2, providing around 4000 rpm/V). With these latter motors it is necessary to use higher number of cells, but, as the currents drawn are even lower than with the first group, the "softer" cells could be utilised. Considering the possible combinations it is mandatory to always assure that the in-flight weight remains within the limits. The static thrust/weight ratio is of a general value only and it can define neither the thrust characteristics of the propulsion unit in flight nor the maximum velocity of the model. This ratio gives the magnitude of acceleration at the launch. It also gives clue to the climb rate of the model at low speeds - i.e. in the flight regime that is generally not desirable with the ducted fan - powered models. The sole way how to get any reliable data on the thrust characteristics is to conduct the aerodynamic (wind) tunnel measurements… The optimum flight characteristics are achieved with the Ta 183 models weighing between 460 and 520 grams. The limiting value to a flight worth its name is 550 grams, at the opposite end of the allowable weight band the models around 420 grams are very sensitive to control inputs - the model loses energy too easily when the control surfaces are deflected and the flight lacks the necessary dynamics. The engine life is another issue to tackle. From the electric point of view the most loaded motor is the Speed 300, yet the practical experience proves that it stands the loads quite well - provided the number of cells is right, of course! It is convenient to run motor for about half hour on 2 V before full loading. From the mechanical point of view the wear is worse, indeed. It has to be borne in mind that none of the motors used has the front bearing designed to take the loads exerted by the fan. There are first the axial forces from the thrust of the fan, second the radial forces caused by the minor imbalances of the rotating parts. Both the motor's rotor (core) and the fan itself are manufactured with certain tolerances. The "meshing" of these tolerances during the assembly may very profoundly influence both the life of the front bearing and the performance of the complete propulsion unit. If the tolerance pluses and minuses, i.e. the vibration caused by them, cancel themselves, you have a very smooth-running, high thrust unit, in the opposite case you have a excessive vibrating unit that draws more amperes, turns less rpm and consequently gives less thrust. Therefore it is necessary to devote much attention to the smooth running of the whole ducted fan. Quite good results may be achieved by changing the relative position of the fan and the motor core and testing the results. It is advisable to mark the starting relative position of the fan and the rotor (core) to avoid "getting lost" soon. If you decide to "tune" the rotor, i.e. to statically balance it, the only allowable method is to sand off carefully and in small increments the bottom (pressure) side of the heaviest blade. Never ever sand the circumference of the blade(s)! As any rotating system also this ducted fan has its so-called critical rpm range, at which it starts to vibrate. Get over this rpm range as quickly as possible, both when spooling up or down.