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Electric Ducted Fan (EDF) Models
Part 2 - Ducting design
Article by Andrew Gibbs
Ducting design
The design of the ducting in an electric ducted fan (EDF) model is a most
important issue to get right if the model is to fly well. Poor duct design
can spoil the performance of an otherwise excellent airframe and EDF unit.
Not all commercially produced EDF models feature good quality ducting,
and in fact there are some really terrible duct designs around, meaning
that the motor and fan are drawing huge amounts of power but generating
relatively little thrust.
Duct design is a complex subject, however adhering to some basic principles
will go a long way towards getting it right. The basics of good ducting
are the following:
Inlet ducting
a) Inlet duct design is in most respects much more important, and more
difficult to get right, than for exhaust ducting. Ideally, inlet ducting
will be designed so that the airflow is smooth, and as far as possible
turbulence-free all the way from the inlet lip to the front face of the
fan. In all cases the duct walls should be as smooth as possible to avoid
promoting turbulent flow. The drag associated with turbulent flow is much
higher than for laminar flow.
b) The area of the ducting should be designed with reference to the fan
swept area (FSA) which means the area swept by the blades minus the center
body. Intakes of 95% of FSA generally work well.
c) It’s important that the inlets have a smooth, rounded intake
lip, preferably with a radius of at least 5mm. A very small radius or
even worse a sharp edge will promote turbulent flow.
d) Changes in the cross sectional area of the ducting should be made
gradually.
e) The air pressure in the inlet duct may become very low under high
power, causing a tendency to collapse. The solution is to reinforce the
duct so it maintains its shape.
f) Don’t try and make the air change direction quickly. Large
radius corners are best.
g) If the inlet ducting diverges, make the angles a shallow as possible
to discourage the airflow from breaking away and becoming turbulent.
h) The fan should be installed so that the air flows smoothly all the
way to the fan blades. Don’t have any sharp edges exposed to airflow.
The rounded lip of this EDF
F86 Sabre inlet promotes a smooth, laminar flow of air all the
way to the front face of the fan (click
image to return to original size).
The rounded lip of this
EDF Sabre inlet promotes a smooth, laminar flow of air all the way
to the front face of the fan (click
small images to enlarge). |
The unpainted right hand inlet
duct of an EDF Vampire designed & built by David and Will
Beavour. The dark colored areas are carbon fibre stiffening hoops,
which prevent the duct from collapsing at high power (click
image to return to original size.)
The unpainted right hand
inlet duct of an EDF Vampire designed and built by David and Will
Beavour (click
small images to enlarge). |
Exhaust ducting
i) Exhaust air flow will always be turbulent, but nevertheless, drag can
be minimised by making changes in cross sectional area as gently as possible.
j) The exhaust ducting will reduce in cross-sectional area from 100%
fan swept area (FSA) to a smaller value at the exhaust nozzle.
k) The nozzle should not be too small and hence restrictive. Nozzle cross-sectional
areas of around 90% FSA generally work well, but the optimum area depends
on the design of the fan unit itself, the design of the inlet ducting
and the flight speed of the model.
The cross sectional area of
the exhaust nozzle will typically be around 90% of Fan Swept Area
(click image
to return to original size).
The cross sectional area
of the exhaust nozzle will typically be around 90% of Fan Swept Area
(click image to
return to original size). |
The exhaust ducting of this
F86 Saber is clearly visible
(click image to return to original size.)
The
exhaust ducting of this F86 Saber is clearly visible (click
image to return to original size.) |
Nick Butler and his superb
EDF SR71. This is a very challenging subject to produce a successful
flying model of (click
image to return to original size).
Nick Butler and his superb
EDF SR71. This is a very challenging subject to produce a successful
flying model of (click
image to return to original size). |
The
design of this sport model requires only very short ducting
(click image to return to original size.)
The
design of this sport model requires only very short ducting
(click
image to return to original size.) |
Bifurcated ducting
Bifurcated ducting, in which the fan is fed from two inlets, one each
side of the model, is common. Bifurcated duting is a little more complex
to design than straight ducting.
Learn
more about EDF ducting here
Click
here to read part 3
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