Testing for Resonant Harmonics

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Daleandee

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Keep in mind that I'm the uneducated one here so please be gentle. I read the quote below on another forum and found it interesting:

"They started it up at night and put a standard automotive timing strobe light pointed at the prop and running off of one spark plug. The prop was appearing stationary and solid until they got up to the normal full power climb RPM range of 2500-2700. Then the prop started hopping around with each strobe pulse. This was entirely due to resonant harmonics being amplified."

This quote is from a post on the Jabiru Groups io site: Re: Low RPM

Don't know if you have to be a member to see the entire post but wanted to give credit to the source.

Comments?
 

TFF

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My first question, did they try each different cylinder with the gun? Next is airflow cavitating the prop making it slightly surge the RPM? Is there an advance curve on a Jabaru? Might it be a connection pickup problem.
Strobe tracking helicopter blades is common but a little old school. Strobe goes off with the ignition, so if the prop is not in the same place something is retarding it or advancing it. Cavitation would be a resonance from
varying loads. Another question is does it do it in flight? Set it up for a night flight and see what you get.
 

wsimpso1

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As a powertrain vibe guy, I gotta tell you it was an unusual event for an ignition strobe to show it. If you do not have a resonance at low rpm, but as the rpm goes up the resonance comes in, then one of several forcing functions is becoming more lines up with a natural frequency. These forcing functions are usually related to firing, and so is the resonance. Yeah, sometimes you will get erratic responses, which showed up on the Jabiru case cited. But do not count on it.
 

BBerson

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All engines shake more at full power under load. Each firing impulse shakes some like a gun recoil. These pulses are not noticed much at lower power.
 

Daleandee

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Great info gentleman. Seemed to me that if this method worked well that it would be more widely used. I appreciate the input.
 
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