# How to calculate pressure frmo force on piston

1. May 2, 2009

### twmc

Hi. I'm new here so apologies if this isn't posted in the correct place.

I'm facing a problem. The situation is this: At a tunnel portal, I am harvesting the energy of the piston effect from trains entering and exiting. I have a number of sails located around the portal edge that. Each sail spans between two lightweight arms that, when hit with the passing air, are forced to rotate around a pivot.
Attached to these arms is a piston. As the arms are forced to rotate outward from the passing air, the piston is exhausted and the air injected into a pneumatic muscle.

I want to work out the rate of pressure increase for the muscle (I have a test rig to determine to rate of contraction from a certain pressure). The average power of each pulse (10s) of the piston effect is 89W. How do I find out the pressure of the system?

My current approach is this:
Let's take one sail as receiving all the power from the piston effect. The sail is pivoted at the top end, with the force hitting the bottom, is 2m long and in a vertical position. The piston is attached 1m from the pivot. Therefore the moment per second will be 89x2=178Nm. As the piston is attached 1m from the pivot, the moment transfered will be 89Nm.

Am I right in thinking I have to calculate the volume of the system (volume of hose = volume of air muscle) then consider the volume or air injected by the piston in order to calculate a pressure increase for each pulse?

I hope someone can help.

Thanks.