Expert: Paul Soderman - 10/14/2005

Question
My question is with regard to the pitch of the lift fan in my hovercraft.

I am using the MultiWing 5ZL fan blades and a Briggs & Stratton engine with an output of 20-hp @ 3,600 rpm. My problem is that I have not been able to achieve more than 2,700 rpm with the fan installed. I guess this must be due to the excessive pitch which has imposed the load to the engine. As a result, the air generated is barely enough to give sufficient lift to the hovercraft. Can you tell me whether I could benefit by re-setting the blades to a lower pitch, as in doing so; the engine would obviously run at a higher rpm. Can I then conclude that the best pitch is when the engine is able to achieve the maximum rpm, considering of course there is always a limit with regard to the lowest pitch I could go for (No load = No air generated). How do I then determine what is the lowest pitch and how do I determine the angle, for record purposes? Finally, when all the above is done, is it correct to say that more air is generated by a given fan and engine when it is set at a lower pitch (resulting in higher rpm) than when it is set at a higher pitch (resulting in lower rpm)?

What is meant by ‘power absorption'?

Note:
Lately I do not seem to get acknowledgement or reply through ALLEXPERTS.COM, possibly due to some fault in the transmission. Could you please extend a copy of the reply to my e-mail address as well? Thank you.


Answer
Sorry Yahaya - My reply was blocked by a KOA campground firewall I believe.  I am on vacation.  Here goes again.

Every fan should have a performance curve that shows mass flow or lift versus fan speed for various pitch angles. It will show the optimum operating point. If you do not have that curve you are operating blind and have to find your own optimum operation, which is not at minimum airflow, but at maximum. I cannot know which way to move without more data, but my first thought is to increase blade pitch to increase thrust.  As long as you don't stall, it should be ok.  Vary rpm and keep track of results and create your own fan map.  Power absorption is the power generated by the fan, which would be equal to engine power if efficiency were 100%.  Since it is not, the motor has to generate extra power.

Good luck.

Paul