Early in the project, I had the dilemma of how I was going to test the engine since all of my previews static tests had been done with the engines upside down (nozzle up), this didn't seem likely to be easy in a hybrid. So I had to design a Test Stand from scratch. The final design called for an upright engine and a hydraulic load cell to measure thrust.

The hydraulic system is based on the $15 Motor Test Stand by Charles Barnett, which by the way is made from PVC.

Illustration 17

There are two ways to calibrate the load cell and both should give the same results:

• Put a known weight on the cell and take note of the reading on the gauge, then find the factor that will make both numbers equal e.g.

Known Weight = (Gauge Reading) * (Factor)

Example. My daughter's weight is 57 pounds, when I put her on the cell the gauge read 35 pounds so the factor is

57 pounds = 35 pounds * 1.628
Force = 1.628 Pressure

• Measure the diameter of the cylinder bore in the hydraulic load cell to get the area of the piston which is then the factor since F = P * A where P = measured hydraulic pressure (psi or N/mm2 ) and A = cross-sectional area of the cylinder bore, A = pi/4 D^2 ( in2 or mm2 ).

Example. The cylinder on the hydraulic load cell from the stand (above image) has a diameter of 1.42 inch, giving a cross-sectional area of 1.58 in2,

thus F = 1.58 P (force in pounds, pressure in psi).

Notice how both results are very similar.

For more on the subject of hydraulic load cells go to Richard Nakka's Hydraulic Load Cell for Thrust Measurement