Posted for George Braly <gwbraly@gami.com>:

It is my best judgment, based on 10 years of pretty hard data collection on
the subject of cylinder head temps and baffling issues that anything more
than about 380F on a routine basis should be avoid.

If a jug goes to 400 or 410 or even 420 - - briefly,  no big deal.  It
happens.

But, Monday,  I flew to Detroit and back.   4 hours up.  6 hour meeting.   4
hours back (hate big cross winds).

Engine was producing 265 Hp continuous in cruise the entire trip up and
back.  Hottest CHT in cruise or climb never exceeded 380F.  Turbonormalized IO-550.

This is, by Lancair standards,  a "slow" airframe - - a V35.  KTAS ~ 205KTAS
at  12 to 15,500 feet - - is all the cooling air flow that there is
available to the engine. So, how do you keep an engine that cool while producing 265 Hp at airspeeds
that are that slow?    You spend a lot of time learning how and why baffling works and doesn't work
on these cylinders.   Then you have to pay a lot of attention to the details
of the baffling.

We have an engine instrumented up with six CHT probes arranged
circumferentially around each cylinder, so we can find "hot spots".  Would you believe that in measuring the temperature at the 7 o'clock
location and comparing it to  the 2 o'clock location on a cylinder that the
temperature of the cylinder head can vary as much as 150 degrees  F  from
one loctation on a cylinder to another on the SAME cylinder???

What does that do to your engine?  It makes the cylinders go out-of-round at
high temperatures.  Not a good result.

After learning how bad it is, we then instrumented up a cowling with an
array of  standard "tufts" and installed a video camera inside the cowl and
flew it and watched the air flow patterns.  They are completely different
than what you would expect from a study of the theory of the subject.

In short, trying to do this based on "... hey! This looks about right..."
engineering efforts is not likely to get the desired results.

And, no,  I can't describe how to make it right in an email message.  It
takes drawings, and details, and hands on stuff to do that.  There is
nothing mysterious about it,  it is just not particularly easy to do it in
this communication medium.


There are some perfectly "gorgeous"  factory baffling installations - -
engineered by a CAD guy on a computer screen, or an A & P in the shop at the
airframe OEM - - -  that, on 3 minutes of inspection with the cowl off,
will provoke lots of head shaking from me and the mechanics around here who
have worked on this subject with me.

This goes back many many years, when Wichita airframe folks used to take a
Lycoming or TCM engine and slap them in an airframe and add some rubber
strips and go fly them and if the engine didn't hit 450F during a full power
climb, then the installation was good enough to certify - - and that was the
end of it.  They would produce 300 to 1000 of them that year and every year
thereafter.   All equally bad.

Those original baffle layouts and schemes have now become "standard" and are
almost universally copied by almost anybody else trying to put an engine
into a new airframe.... with equally bad results. Regards,  George





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Can we get a more definitive answer as to George Braly's comment on the IO
550N running too hot?