RPM restrictions

[bret]

I have a question for those who know about crankshaft vibrations, is it determined by mechanical factors or dynamic influences like ignition type and ignition timing? So the setup I have came out of a Mooney, IO 360 A1A with C/S Hartzell that had the 2000 - 2250 no continuous RPM restriction. This will now be ran with a full EFII system and the timing will be 30-25 BTDC (depending on map) so does this change the restriction or no? Thanks

[rvbuilder2002]

Originally Posted by bret

I have a question for those who know about crankshaft vibrations, is it determined by mechanical factors or dynamic influences like ignition type and ignition timing? So the setup I have came out of a Mooney, IO 360 A1A with C/S Hartzell that had the 2000 - 2250 no continuous RPM restriction. This will now be ran with a full EFII system and the timing will be 30-25 BTDC (depending on map) so does this change the restriction or no? Thanks

Ign. timing can definitely have an influence.
Harzell has issued warnings regarding operations of their propellers on modified engines.
Other than that, there is no absolute answer to the question regarding your engine/prop combination, because until someone does a vib. analysis of your exact configuration, there is no way to know if there are potential problems or not.

[DanH]

Originally Posted by bret

I have a question for those who know about crankshaft vibrations, is it determined by mechanical factors or dynamic influences like ignition type and ignition timing? So the setup I have came out of a Mooney, IO 360 A1A with C/S Hartzell that had the 2000 - 2250 no continuous RPM restriction. This will now be ran with a full EFII system and the timing will be 30-25 BTDC (depending on map) so does this change the restriction or no? Thanks

+1 what Scott said. You now have an untested, experimental combination, and the usual victim is the propeller. You should not assume anything based on vibration perceived in the cockpit.

You can make some logical assumptions, although you might not like the logical result. Vibration frequency (or frequencies, plural) is based on mass and connecting stiffness. Here the masses and connecting stiffnesses are not being changed, so in general, all natural frequencies will remain the same.

Resonance happens when a natural frequency is matched by an exciting frequency...a driving force. The primary driver is combustion events, and here you're not changing the frequency at which they take place. What you may be changing is the amplitude...the power with which they twist and bend the components. The increased amplitude is magnified by resonant behavior; in resonance, material stress can get very large in a hurry.

The logical result is that the current prohibited RPM range should be observed with real diligence, and you might even want to widen it a bit.

I say "may be changing" because we really don't know the degree to which your new fuel and ignition systems will actually increase the force of each power stroke. If we believed some of the claims, your new prohibited range should extend from 1800 to 2800 Ok, I'm kidding....sorta.

It sounds like you're using a pre-loved propeller from the Mooney, so you should also be aware that some part of each aluminum blade's finite fatigue life has been used up. It's another good reason to avoid the prohibited range.

[bret]

Sorta? you install confidence in me Dan another wrench in this, is the Mooney had the prop bolted to the crank, I now have a 3" extension, so.....I'm not really a kinda save gas money type of Guy, so not looking for how low an RPM I can go as much as what to stay above to keep the stuff in one piece. This also has the new non AD hub. and as I read, the blades can only go 25K hours or something? I'll need to recheck that log book. so if we change the crank to prop distance, does that change the # of orders or maybe cancel some out, didn't sleep at a Holiday Inn Express so I'm a little slow....

[rvbuilder2002]

The back story to Hartzell's warning is the result of some vibe analysis flight testing the were doing at Van's on a new propeller design (I think it was what was later introduced as the blended airfoil prop).

The test results at low RPM conditions at MP's that still allowed for additional timing advance (beyond the normal 25 deg.), were well outside what they expected (and could except) and after some brain storming and some more testing with the engine reconfigured to conventional mags it was verified that the advanced timing from the electronic ignition was the cause.
And this was on an unmodified parallel valve O-360 (carb, standard 8.5:1 comp) on an RV-8

[DanH]

Originally Posted by bret

...the Mooney had the prop bolted to the crank, I now have a 3" extension....so if we change the crank to prop distance, does that change the # of orders or maybe cancel some out

Does not change the number of orders (i.e. the number of times a particular event happens during one crankshaft revolution). It does change some natural frequencies, because the extension is a change in connecting stiffness, and adds a rotational inertia.

No practical change to your overall situation. You still have a combination that has not been subjected to a strain gauge survey.

Scott, not off topic at all. Excellent example.