Tach Vs. Hobbs Time

Dylan Smith wrote:

In article , John Roncallo
wrote:

Why? Do the Archer's engine components wear less at high RPM than an
engine on an Arrow?

Yes they probably do wear less. They are under less load for the given
circumstance. Cylinder pressures are much higher in an engine at 65%
power 2300 RPM than they are at 65% 2480 RPM.


But a far greater effect will be the additional wear from friction of
running at 2480 RPM instead of 2300 RPM - a significant increase in the
number of friction cycles per hour. I'd say the Archer's engine
will wear more quickly.


Wear is a function of pressure and velocity. Mechanical dynamic friction
is a function of mostly pressure. High cylinder pressures cause high
mechanical pressures everywhere , journal bearings, piston side forces
etc. With both cases of high and Low RPM at equal power output the wear
should for the most part be relatively equal.

The absolute which case is better argument and where is the exact equal
wear points as far as RPM goes will probably never be much more than an
educated guess but it should be clear that the lower loaded Archer
engine should be allowed more RPM than the higher loaded Arrow engine
even if they were the same exact engine, which they are not.

John Roncallo

Ron Natalie wrote:

"Dylan Smith" wrote in message ...


But a far greater effect will be the additional wear from friction of
running at 2480 RPM instead of 2300 RPM - a significant increase in the
number of friction cycles per hour. I'd say the Archer's engine
will wear more quickly.


The number of revolutions acquired isn't the driving factor in engine
life. Flight school aircraft that are flown frequently and full throttle
tend to far exceed TBO where as babied, infrequently flown single
owner birds tend not to get anywhere near TBO.


This is true but the tach doesn't know any more than RPM. The rest of
the story is all assumed. What I'm wondering is did we replace our clubs
perfectly good running engine last year at 2000 hr or was it more like
(2300RPM/2566RPM)* 2000hr = 1793 hr. My most recent flying and some of
the posts in this group are telling me we may have replaced it at 1793 hr.

Thanks to all who posted

J. Roncallo

In article , John
Roncallo wrote:

This is true but the tach doesn't know any more than RPM. The rest of
the story is all assumed. What I'm wondering is did we replace our clubs
perfectly good running engine last year at 2000 hr or was it more like
(2300RPM/2566RPM)* 2000hr = 1793 hr. My most recent flying and some of
the posts in this group are telling me we may have replaced it at 1793 hr.

It doesn't matter how many hours are on the engine, it's why you had to
replace it?
Was it making metal?
Were parts worn out of spec that they could not be replaced?
Many things affect TBO.
Some engines go significantly beyond TBO, some significantly less.
Operation and environment make a difference.

EDR wrote:
In article , John
Roncallo wrote:


This is true but the tach doesn't know any more than RPM. The rest of
the story is all assumed. What I'm wondering is did we replace our clubs
perfectly good running engine last year at 2000 hr or was it more like
(2300RPM/2566RPM)* 2000hr = 1793 hr. My most recent flying and some of
the posts in this group are telling me we may have replaced it at 1793 hr.


It doesn't matter how many hours are on the engine, it's why you had to
replace it?
Was it making metal?
Were parts worn out of spec that they could not be replaced?
Many things affect TBO.
Some engines go significantly beyond TBO, some significantly less.
Operation and environment make a difference.

No it was running just fine. But our club feels that Lawyers will eat us
alive if an engine fails and we let it go beyond TBO. I tend to agree,
but I would like to see our rules changed to as required or first annual
past TBO which ever comes first.

John Roncallo