This certainly seems to be the case subjectively but I wonder if it is physically real.
One would need to record the sound and vibration and analyse it.
I certainly can’t see any mechanical reason why vibration should reduce.
If it’s true then I would expect that older engine would have just slightly larger clearances on bearings so a tiny bit more oil would fill the gaps and provide better sound/vibration dampening. Of course if the clearances become even bigger then the effect will be opposite as the oil is no longer able to dampen vibrations and engine finally falls apart.
Old pilots say that with new engine you have to be more careful when starting cold / warming up as it is tighter. So metal expansion will affect it more than old worn loose engine.
My mechanic has become adept at dynamic prop balancing, so over the next few weeks I’m going to do the Colt/C177B/J3 Cub and see what the difference is. Dynamic balance on the aeroplane is a walk in the park compared to getting a heli balanced laterally and vertically.
A curve ball is an engine with even low compression is also very smooth, don’t ask!
From a medical / human factors point of view: Is there perhaps some degree of familiarity involved in this effect, if it exists?
It is certainly true that a run-out engine vibrates more than a newly overhauled engine, so it makes sense that the newly overhauled engine might run more smoothly after rings have fully seated, variation between cylinders is less and torsional vibration is thereby reduced.
Some motorcycle engine types are well known to smooth out over time, the interesting thing to me is that it often takes 10 to 20,000 miles to occur.
In flight it is not noticeable. But I notice it during taxy. Taxying out, and even just before takeoff, after all has warmed 1000 RPM is never as smooth as 1000 RPM duing taxy-in after a long flight. Particularly in the SR22.
On a single flight I notice my carburated O-320 runs smoother on initial climb out when relatively cold, and it seems to me slightly rougher when doing the same thing when hot. Yesterday after a fuel stop I experimented a bit at DA 4000 ft or so, OAT about 35 C, oil temp about 100 C. Nothing done with the mixture control made a difference.
Four cylinder Lycomings aren’t the smoothest engines in either case.
Silvaire wrote:
Yesterday after a fuel stop I experimented a bit at DA 4000 ft or so, OAT about 35 C, oil temp about 100 C. Nothing done with the mixture control made a difference.
My first response would be to burn off the plugs and see if the symptoms disappear.
I’m pretty fussy, I’m not sure anybody else would notice the hot versus cold issue but it might be fun to try that.
My engine has car-style REM37BY plugs, which are not greatly susceptible to fouling but sometimes little things do make a difference.
We overhauled one of the engines last fall, and initially it was definitely running “square-ish”. Our maintenance guy overwatching the process said one of the cylinder was taking longer to break in than the others for some reason. After the official break-in period was over (oil-consumption stabilized), it would still run smoother if leaned quite agressively. Now 200h later this engine runs super smooth at all rpm’s/powers/mixtures.
