Expert: Paul Soderman - 11/13/2004

Question
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Followup To
Question -
I am building an airboat,I'd like to use a ducted fan instead of a propeller.
I've read up enough to know that the air foil for the duct would be different as compared to say a turbo fan jet.
Turbo fan jets are designed to go fast and the airfoil reflects that,here's my question.
How should my airfoil look?  I don't need to go terribly fast.(60kph)Also are there ways to test my airfoil on computer before i build it?
Thanks
Guy Chan
Answer -
Guy

It is difficult to answer your question without knowing anything about your design requirements.  I suggest looking at the many papers on ducted propellers that are in the literature - especially the NASA paper archives.  There have been many studies made by researchers at NASA Glen Research Center (formerly Lewis) and Ken Mort at NASA Ames Research Cener.  I did a Google search on ducted propellers and found several papers on the subject.  As to computer simulation, there are commercial codes that may handle this (do Google on Aerodynamics).  The following website talks about basic aerodynamics and has links to some design tools:  http://www.ae.su.oz.au/aero/contents.html.  The problem is that the propeller has a strong influence on the inflow to the duct, so you really need to analyze the system.  You may be better off using a design that has already been tested and reported.

Good luck.

Paul

Hi Paul
Thanks for your response.
the websites and computer generated test sets are proving invaluable,though it may take me some time to make perfect sense of them.
The more I read the more I understand about the aerodynamics of aircraft. But my problem(?) is about (as I understand) the most efficient way for an
ducted fan input airfoil,to draw in(the design of the mouth or lip) and then best utilize the air flow it gets(fan placement within the duct?/space between the edge of the impeller and the interior of the duct?)
I wonder if my question is too rudimentary or too vague without some more details.
The hull of the boat should be about 250lbs,it's 15'x 7'and the bottom will be slightly crowned.
The engine is a 750cc V4 16 valve motorcycle motor(150lbs.
The prospective impeller is a 32" 7 bladed aluminum fan used in cooling towers,the blade profiles are adjustable.
The boat needs to carry 2 people and some gear (400lbs.)
I'm still looking for the RPM"s available from the engine.
Thanks again for everything your time and expertise is greatly appreciated.
GC  

Answer
GC

You are asking for technical details that are outside my expertise.  I recall a few generalities on fan duct design.  The space between the blade tip and wall should be the minimum possible without rubbing.  Any gap causes aerodynamic losses.  The inlet lip should be elliptical rather than round, but not too thick or you have excess drag and not too thin or the flow will separate from the wall if the inflow wind is not axial.  Unless noise is an issue, the fan duct should be as short as possible, but long enough to get the inflow straight down the duct.  The optimum dimensions require a fan design expert or information from some technical study.  I wrote a paper related to this:  Soderman, Paul T.; Hazen, Nathan L.; and Brune, William H.:  Aerodynamic Design of Gas and Aerosol Samplers for Aircraft.  Airborne Aerosol Workshop, Boulder, CO, Feb 27 - Mar 1, 1991 (also published as NASA TM 103854, Sept 1991).  And in that paper I discuss inlet design and reference other papers that might be helpful.  Looks like you are going to have to do some literature research.

Paul