Based on logged measurements with our very early DS-51-FAN 3-ph during the flight and theoretical background as well as wind tunnel measurements on our DS-51-DIA 3-ph we now would like to describe an approach for the configuration of intake and outlet ducts.
The most outstanding characteristic when designing the ducting is a reasonable distribution of the cross-sectional area. The other parameters such as duct length and cross-sectional shape tend to be given by the original.
The intake should receive most attention here as it is responsible for an adequate supply of air to the ducted fan.
Let’s start with the short airliner nacelle: We want to position the optimum operating point in the take off and ascent phase. High flight velocities do not occur often and are unwanted. In this case the intake area should correspond to the circle area of the ducted fan (e.g. 90mm without subtracting the hub area with a 90mm ducted fan). If required (scale optics) it can be reduced up to 10%. The intake lip radius should amount to at least 6% of the diameter of the ducted fan or more.
If you choose for example a smaller intake diameter of 85mm at a 90mm ducted fan for a slightly faster Business Jet (e.g. Lear-Jet), you should bear in mind that the diffusor has an aperture angle not exceeding 4° per side (otherwise danger of flow detaching).
With longer nacelles for faster jets the intake area should be as big as the FSA of the ducted fan (DS-51-AXI HDS = 80,5mm intake diameter = 51 square cm). Bigger intakes cause an overload of the ducted fan already when flying straight (an overload occurs if the volume flow rate is bigger than the optimum volume flow rate; partial load operation occurs if the volume flow rate is smaller than the optimum volume flow rate).
Smaller intakes shift the optimum operating point towards the speed flight. In the remaining flight phases more or less high frictional losses in the ducts and an impact loss at the blades can be expected. The outlet should also equal the ducted fan’s surface (DS-51-AXI HDS = 51square cm and 80,5mm diameter), if the streamlined fairing is not in the outlet area. Smaller intakes always correspond with choosing a smaller outlet.
Let’s now turn to a more complex scenario with the usual longer ducts for a fast single-engined jet and a speed jet. For such models flying in moderate to high velocity ranges the inlet area should also equal the ducted fan’s surface. For very fast Jets it is possible to minimise the inlet up to 90% FSA in order to avoid overloads at high speed due to congestion at the inlet area. For fast jets it is advisable to also choose an outlet area which equals the ducted fan’s swept area (FSA). With speed Jets the 80-90% reduction of the outlet is possible similar to the inlet as this increases the outer efficiency of the jet propulsion system. As the channels tend to be quite long there is still the remaining question how to design the transition from inlet to the ducted fan and the nozzle.
In order to extend our duct from a small inlet diameter to a larger diameter, which is necessary because of the larger hub area, we have to implement a diffusor into the inlet. This is simply a pipe extended streamwise where the flow velocity is transformed into pressure.
This diffusor should be located directly behind the inlet area with the maximum opening angle amounting to about 4° on each side. Behind the diffusor a longer channel with a larger diameter and a lower flow velocity is attached leading to smaller losses in the channel.
In case of a longer outlet pipe the nozzle is not constricted directly but left at the ducted fan diameter in order to be minimised to the FSA (or 80-90% of the FSA at high speed jets) shortly before the outlet. If the remaining outlet-duct behind the ducted fan is very short, the nozzle should be minimised to the desired size with constant tapering.
Generally speaking one can say about double sided inlets that the curvature progressions should be attached with preferably larger diameters. If at the same time a diffusor is used the wall angles should not be too big due to the danger of detachment. The intake lip diameter should not come below 0,03D even referring to very fast models. If it is required to use sharp-edged inlets for scale reasons the inlet area should be extended by max. 10%.
All statements, of course, also depend on the type of ducted fan. The more blades and the faster you plan to fly the more minimisation of the duct cross-sections is necessary to avoid overloads. However, a high number of blades combined with small channel cross-sections can lead to a severe increase of impeller frictional loss and ducting losses.