Ducted Fan Design
This is a Mathcad document converted into HTML. I hope the
formatting works in your browser, as it is rather a bear to
convert. I probably won't be updating this page very often.
I will instead try to keep the source MathCad file up to date. There
are some obvious shortcomings to some of these
calculations. The thrust required is based on what I could
find on the web, and is only based on parasitic drag.
Clearly this should have both parasitic and induced drag,
but that would require lift/drag information that I don't
have. If anyone has information that could help me fine
tune these numbers, I would appreciate it.
Values in blue are inputs, values in yellow are interesting
outputs.
Establish the atmospheric conditions:
Fan parameters:


< Fan and disk combined
efficiency estimate

Targeted performance:
Value typically listed
for Cozy MkIV, which includes induced drag

Estimated engine parameters:
Calculate the available torque
Estimate cruise power needed assuming infinite mass flow
Calculate mass flow given additional power available
Calculate the "capture area" given relative airspeed and
mass flow
Calculate the air pressure at the front face of the fan
Estimate the amount of swirl power:
Determine the required exit velocity the attain the desired
thrust
Calculate the air pressure on the aft fan face
"Stinger" radius at exit
Alternate ways to estimate pressure at aft face of fan.
Calculate the power required at the fan to pump the
pressure up from p2 to p3
If necessary, try fine tuning mass flow to reduce the power
requirements
Recalculate capture area:
Determine the pressure at the front face of the fan:
Calculate the swirl power
Determine the exit velocity to create the required thrust:
Calculate the pressure at the aft face of the fan:
Calculate the power required at the fan to pump the
pressure up from p2 to p3
Check the fan blade tip speeds to ensure the airfoil
critical Mach is not exceeded
Find the effective blade pitch
Estimate the fuel efficiency:
Evaluate the performance envelope of a particular fan
geometry