Expert: Paul Soderman - 10/11/2005

Question
I learnt a lot by reading your various answers to questions posted by people who wants to know more about propellers and thrusts.

My question is with regard to the air volume generated by the lift fan and the thrust propeller in a my hovercraft.

The lift fan is 34" diameter and pitched at 23 degrees. The problem I am facing is that with a 20-hp Briggs & Stratton engine, the maximun engine speed achieved under load is only 2,700 rpm although the manufacturer specifies 3,600 rpm as the maximum engine speed. At 2,700 rpm the air volume generated is barely sufficient to provide the lift capability of the hovercraft.

I know that if I re-pitch the fan blades to a lesser degree, I may be able to achieve a higher rpm, say 3,100 rpm due to a now much lighter load.

Would this change to lower pitch (but the resultant increase in rpm) generate more air volume?

If it does, then the principle would work as well for thrust, wouldn't it?

The thrust prop is belt driven by a 100-hp automotive engine via a 1.65:1 reduction pulleys. I understand this is to reduce the prop speed to avoid excessive wear and noise when the engine is operating at 5,000 rpm maximum speed. Would it make a difference running the prop via 1:1 pulleys, thus running the engine and the fan at the same speed; just lower the engine speed to lower the prop speed accordingly.

Hope to hear a reply from you. Thank you

Answer
Hi Yahaya
I apologize for being late with my reply as I am on vacation and just found your message.  I am also away from my engineering books so an unable to give you a detailed answer.  Generally speaking, your fan must have a performance curve that shows mass flow and thrust versus speed for various pitch angles.  If you don't have that, you are operating blind.  So, you will have to try different speeds and pitch angles to find the optimum.  I can't tell you which way to move the pitch angle without data. My first thought would be to increase pitch angle to increase thrust.  Clearly, you don't want stall, but beyond that, it won't hurt to try various angles.

Good luck.
Paul