How to check frequency, velocity and ounces.

[gary350]

I have 3 pulse jet engines that I designed and built using German V1 flying bomb research data. They starts easy and runs great. I need to determine the number of pulses per second that each engine fires and the exact thrust the engine produces. I have not tried anything yet to test the pulse rate of frequency but it should be very close to 400 pulse per second. I used a spring to test the thrust the amound of movement on the spring gives me a fairly close guess within 90% but it is not accurate. I want to run the engine on different fuels and different fuel mixtures so I need something very accurate to measure thrust in ounces or smaller. I also need to determine the velocity of the1500 deg F exhaust gas this will tell me how fast the engine will fly on a model airplane the speeds should be some where from 100 mph to slightly less than 300 mph. I was thinking a strain guage might work for testing the thrust.

Check out the video.

YouTube - Lowracerman's Channel

YouTube - Lowracerman's Channel

YouTube - Lowracerman's Channel

 

[jpanhalt]

I love the sound of pulse jets too. I had always wanted one since I first saw a Dynajet at a hobby shop in the 1950's and finally fulfilled that dream about 10 years ago. What are you running on? I used nitromethane plus absolute ethanol.

As for your questions: I suspect thrust would be measured in much the same way as with any engine. Model Aviation has has a few devices over the years pictured that did that. I would suggest a pivoted lever and don't see why a spring gauge would not be accurate enough. If it is not, then a modern electronic balance could be pressed upon by the lever.

Exhaust velocity might be a bit harder to measure. I have read somewhere that a substantial proportion of the combustion air actually enters through the tail pipe, so the flow may be turbulent.

I assume you are aware of the pulse-jets.com forum. Quite a few threads were found by searching on exhaust velocity. Many tried to calculate velocity based on temperatures and/or pressure differences, as with a pitot tube and manometer.

John