[hpv-boats] CFD & Swing Arms

[Rick Willoughby]

Bruno
You can get good results for foil modelling using JavaFoil or Xfoil.   
I prefer JavaFoil because it has friendlier front end:
http://www.mh-aerotools.de/airfoils/jf_applet.htm
It is a 2-D analysis but it has corrections in the options page for  
the foil aspect ratio.  It gives good results for a wide range of  
sections that can be selected and modified if required.  You can  
simply add the drag components of each element.  You might get  
different results with full 3D CFD analysis but you could not rely on  
them anyhow.

For a foil mounted of a large flattish surface double the selected  
aspect to allow for the end plane.

The high velocity body element with pedalling is the feet.  If they  
are moving in a high density fluid such as water then there is a lot  
of wasted energy in stirring the water with the motion of the feet.  
With the swing arm system the power stroke is more effective.  The  
force is in line with the movement of the power stroke of each foot.   
For example if the thrust is applied in a horizontal direction then  
the feet mostly move back and forth horizontally with a small  
vertical lift at either end.  With a crank the feet travel both  
vertically and horizontally at the same velocity.  In a recumbent  
position the vertical component of the motion does not delivery much  
force to the cranks - per your observation for the elliptical  
cranks.  Hence the feet travel at high velocity for 4 quadrants of  
the cycle with roughly only 1 quadrant being effective power delivery  
or 75% of feet spinning energy just to make the next power stroke.   
With a swing arm system it is closer to 50% of feet moving energy to  
make the next power stroke.

In air there is the benefit of the harmonic motion if the swing arms  
are set up right so there is negligible cost in the reversal.  That  
may be more evident in this clip courtesy of now departed colleague  
Warren Loomis:
http://www.rickwill.bigpondhosting.com/Warren_HC.wmv
The reason I played around with this drive system on my V12 boat was  
to lower the centre of gravity of the engine such that it was  
compatible with unstabilised slender hull.  I have also contemplated  
its use on a faired trike for endurance record setting because it  
requires less volume than cranks and chainrings commonly used - hence  
lower wind resistance.  It is a compact arrangement for delivering  
power that still permits the HUFF configuration.   I have seen other  
configurations that try to reduce frontal area but they look  
incredibly awkward to operate.

Rick


Re: CFD

Hello Rick,

Thank you for your recommandations and advices. Actually my CFD quest  
is not
only for the submarine drag reduction and estimation aims but for more
general questions such as sizing of the tail and interaction with  
hull and
transcient analysis when pitching, yawling and rolling.

Your point about efficiency with an arm swinging system for  
propulsion is
interesting but looking at the video on your weblink it seems that  
you are
pedaling (with high speed results!) or did I miss something? Could  
you be
more specific about this system please?

Thank you.
Bruno

Rick Willoughby
rickwill at bigpond.net.au
03 9796 2415
0419 104 821