Re: Low RPM

Jay Scheevel
 

This kind of bugs me. The resonant harmonics that are most damaging are torsional resonances. If the power pulse frequency overlaps with one of the natural resonance frequencies in the crankshaft then they become additive and the stresses can really build up fast, especially if the engine is at higher power output.

 

Here’s what bugs me: The resonant frequencies are dominated by the geometry and stiffness of the crank. Whatever those resonance frequencies are, determines the power pulse frequency that is critical. The 4 and 6 cylinder models vary in both respects, so the unless there is a huge coincidence/accident in the resonant frequency spectrum of both cranks and how that relates to power pulse frequency, it is very UNLIKELY that both engines share the same critical RPM range. On top of that the whole thing is modulated by the prop’s moment of inertia and even by the stiffness of the engine mount. Jabiru’s “boilerplate” warning is suspect right out of the gate. I am sure it is based on specific testing for a specific engine in a specific mount with a specific prop/counterweight, but application of these results across the board is not likely.

 

I have a friend in town who has designed a nice little airplane (you will see it one day at OSH, if they ever reopen), and he was using a certain unnamed manufacturer’s ground-power engine as his powerplant. This engine was modified by the manufacturer to suit his specs (my friend is an engineer who has been in the aviation manufacturing business for some time). Unfortunately, this engine had two crankshaft failures on two consecutive brand new engines requiring emergency landings. Each failure occurred at almost exactly 75 hours of total time on the engine. The original test engine ran fine through all phases of a 500 hour test in the engine test cell, with crank loads similar to those in the air, was torn down and showed no abnormal wear. The test cell runs were before the first engine was mounted on the plane for test flying.  

 

The prototype plane was not harmed in either emergency landing, but on the third engine, they decided to run a test with it on the plane. 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. Stresses were apparently sufficient to induce metal fatigue in the crank after a number of takeoff and climb cycles reached  at about 75 hours .

 

My friend told me about this auto stobe method, and it sounded dead simple. One of these nights I will go out to the airport, tie my plane to my car’s bumper hitch, and get a buddy to run the strobe. Thankfully, I have water cooled heads so do not run the danger of overheating while ground running at high power, so I can run through my whole RPM range up to and including 2800 (which is my static RPM).  Once I have done this, I will report back about my results. I think this would be easy enough for most of you to do with your plane. The great thing is that it is specific to your entire running configuration.

 

It would be nice to compare our results to see if they line up with Jabiru’s recommendations.

 

Be safe when you try this. A lot of weird stuff can happen with strobes in the dark and powerful machinery!

 

Jay

 

From: main@JabCamit.groups.io <main@JabCamit.groups.io> On Behalf Of Rob Turk
Sent: Wednesday, May 13, 2020 10:51 AM
To: main@JabCamit.groups.io
Subject: Re: [JabCamit] Low RPM

 

It's actually in the 2200 installation manual JEM2202-6, page 35:
"Do not cruise or climb in the range 2100rpm – 2400rpm"

The same warning is also in the 3300 Installation manuals, so it's not limited to 2200's.

At the time there was some information that resonance caused through bolts to break. The through bolts have since been radically changed and I doubt the tests that justify this warning were repeated. Rather than retest, they just left the warning in. This is even more clear as the same warning is also still present Gen 4 manuals, where the through bolts are entirely different.


On 5/13/2020 6:05 PM, dean l via groups.io wrote:

I remember reading a report some years ago which was carried out by Jabiru. It was produced following detailed and complete 2200 engine vibration and resonant frequencies analysis at different RPM. It contained a quite a few interesting graphs and a lot of information on how methodology.

In a nutshell, It found that running the engine low RPM (2000-2200) caused pretty noticeable spikes in resonant vibrations. The recommendation that came out of report, was correct static and dynamic balancing  of prop was essential as well as to run engine outside of the "high resonance" RPM range. I did a search for document on the internet but no luck. Does anybody have it ?
 

 

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