The figure below shows an exploded view of the final prototype design.
Figure 4.3: Exploded view showing all parts. 1: nozzle outer part, 2: nozzle inner part, 3:
motor cables, 4: motor, 5: stator and (motor)housing, 6: rotor housing, 7: rotor adapter, 8:
rotor, 9: spinner. The bolts and nuts are left out.
Here 1 and 2 denote the inner and outer part of the nozzle and 3 denotes the motor cables, 4 the motor; 5 the stator and (motor)housing; 6 the rotor housing; 7 the rotor adapter; 8 the rotor and finally 9 denotes the spinner.
Stator and motor details The stator and motor housing is an important structural part of the EDF. At each side of the housing sits a bracket to mount the EDF to a plane or test setup. The figure below shows a cross-section of the final prototype design:
Figure 4.4: Cross-section of part of the EDF showing the cooling vents and the mounting of the motor. The dark blue lines denote the edges of the cross-section. The numbering is equal to that of Figure 4.3
Here the numbering is equal to the numbering in Figure 4.3. As can be seen the motor is mounted inside a housing in the hub of the stator. The original plan was to directly glue the motor to the stator blades, as shown by Figure 3.1. In the prototype, this is not possible since the motor diameter is smaller than the stator blade root diameter. The back of the motor is supported by tightly fitting it in the housing. The front of the motor is connected to the housing using screws. Important: the mounting screws can maximum be 5.5mm long.
Otherwise the internal parts of the motor can be damaged.
Because the motor is mounted in a housing, it is not directly cooled by the airflow. To enhance the cooling, the ’LK’ version of the motor is used. This motor actively accelerates air radially through the motor case. The cooling intakes are positioned at the back of the motor; the outlets are positioned at the perimeter of the front of the motor. To allow the motor to breathe, air vents have been made in the nozzle inner part and the motor casing in the stator. The vents and the cooling in- and outlet are marked using blue ovals in Figure 4.4.
Rotor details SLS 3D printing is not accurate enough to create good fit and tolerances on holes. Therefore the rotor is mounted on the motor using an adapter. The figure below shows the rotor adapter and the rotor mounted to it.
(a) (b)
Figure 4.5: The rotor adapter on the motor shaft(a) and the rotor mounted on the rotor adapter(b).
The center hole in the rotor adapter is sized so that it slides onto the shaft. The adapter is fixed using three set screws. In the front of the adapter are 3 threaded holes to which the rotor can be mounted.
The minimum wall thickness that can be made using SLS 3D printing at shapeways is 0.7mm. Especially for the rotor blade trailing edge this causes problems. Therefore for both the rotor as stator blades the section shape is adapted slightly by thickening the trailing edge to 0.7mm. The figure below shows the original and adapted rotor root section.
Figure 4.6: The adapted rotor blade root section to allow production using SLS 3D printing.
The front of the rotor is flat, which would create disturbances in the inlet flow. Therefore a spinner is designed to guide the flow smoothly into the annulus. The Figure below shows a cross section of the rotor with the spinner.
Figure 4.7: A cross section of the rotor with the spinner mounted.
Because it is difficult to create reliable thread in 3D printing, a hexagon shaped cavity is made in the rotor. In this hole, a steel nut is fitted. The spinner is mounted onto the rotor using a single screw in the middle, grasping the nut. The length of the spinner is half its diameter. This dimension has been determined by looking at existing EDFs. The curve of the spinner cross section is arbitrary. The rotor has a small rim on its outer diameter onto which the spinner supports.
Nozzle details The nozzle exits out of an inner and outer part. The outer part is designed in four pieces for easy assembly of the EDF. Since only small forces are acting on the nozzle a wall thickness of 1mm is chosen. Thin 3D printed parts tend to warp. Therefore stringers are made around the perimeter of each part.
The inner nozzle part is connected to the stator hub. To minimize disturbances in the flow, the screws for this connection are places inside the hub as shown below.
Figure 4.8: Cross section of the EDF without motor, showing the mounting of the nozzle inner part.
The screws can be reached with a hexagon key through holes in the front of the housing, as shown in the figure above. To the inner nozzle part a ’tail’ is connected to let the cables
out. The used cables have an outer diameter of 8mm. Fixed cables may not be bend tighter than 3 times their diameter. This has been taken into account in the design of the tail.
Static test inlet In model airplane EDFs, it is very common that no explicit inlet is designed. The inlet is just a sharp edge at the front of the housing. However, when used in static conditions the flow can separate at the inlet. An inlet with a rounded lip is designed to prevent this. The radius of the inlet lip is determined using empirical relations for hydraulic resistance of an inlet with a rounded lip. The figure below shows the prototype with the static test inlet.
Figure 4.9: The prototype EDF with the inlet designed for static tests.