Scudrunners

The BMEP of [i]any [/i]super-charged radial on takeoff
is so much higher than an O-470 at 50% power
in slow cruise ...

Hell even the GTSIO-520 in the 421B runs 40 inches
on takeoff.

Technical Interlude for the newbies.

You probably see terms like manifold pressure (MP),
RPM, horsepower, torque and BMEP being casually
tossed around.

Let's start from square one (nothing to do with engines).

There is such a thing as a [b]force[/b], which you could
measure in terms of pounds or anything else.  Units
doesn't matter. 

Let's say you are holding a 20 lb weight.  It wants
to go to the center of the earth, and you are applying
an equal and opposite 20 lbs of pull to hold it motionless
at your waist.

So we have a 20 lb force that you are exterting.  And
even though you will get tired after a while, you aren't
actually doing any [b]work[/b], as an engineer would
think of.  Sorry about that.

[b]Work[/b] is [b]force[/b] applied through a [b]distance[/b].

In our example above, if you raise the 20 lb weight by
one foot, you are now doing [b]work[/b].

However there is no time factor involved in work - you
could take one second or one day, and the same amount
of work could get done.

Time is pretty important to most of us, so there is
something called [b]power[/b] which is [b]work[/b] performed, per
unit [b]time[/b].

Using ancient old British units, an old dead guy called
James Watt measured the power of a horse, and noticed
that it could raise 550 lbs one foot in one second, and
that's a [b]horsepower[/b] - around 750 Watts.

An internal combustion engine doesn't do linear work
like a horse does, though - it actually rotates a crankshaft
and how strong the engine actually is, can be measured
with a virtual torque wrench (actually a dynamometer)
in units of ft-lbs.

RPM is merely Revolutions Per Minute.  You can count
them.

Think of an engine that can put out 100 ft-lbs of torque
at 2000 RPM.  If we can run that engine up to 10,000 RPM
and still get 100 ft-lbs of torque, we get five times as
much horsepower - think of it covering five times the
distance, or doing five times as much work in the same
time.

If you understand the above paragraph, you can understand
that power is torque X RPM.  With British units, horsepower
is actually calculated as torque x RPM / 5272.

Finally, onto airplanes.  This is an aviation website, right?

With a fixed pitch prop, the only control you have over
the engine power (let's ignore mixture and carb heat) is
the throttle.

You probably see the tachometer RPM go up and down
with the throttle, in your fixed pitch trainer.

However, very few fixed pitch trainers have a manifold
pressure gauge installed, because there's nothing you
can do about it anyways - it's "nice to know" information
only.

A manifold pressure gauge is just a barometer downstream
of the throttle, which are flappy butterfly valves which allow
you to control how much air enters the engine.  They are
directly connected to the throttle.  When you push the
throttle forward, the flappy butterfly valves open all the
way, in line with the airflow.  At that point, the MP gauge
will read the ambient air pressure, minus 1 inch of mercury
or so, which is caused by intake restrictions.

Now, if you have a constant speed prop (blue knob) which
is actually a variable-pitch propeller with a governor, you
can manually select an RPM, within the stop limits of the
prop blades.

This means to you, as a pilot, that there is an infinite
number of combinations of MP and RPM that you can
select, to generate the SAME HORSEPOWER.

This causes no end of confusion for pilots, whom generally
aren't too bright, and have problems at the Baskin Robbins
when they try to select an ice cream.

Fact:  for any given horsepower, the lower the RPM, the
higher the MP and BMEP will be, and the more efficient
the engine will be.  To make the same power, it will
burn noticeably less power.

At the other extreme, you can run high RPM and low MP
and make the same horsepower, burning more gas and
making lots of noise.  Not slick.

Now, like a hopeful (and successful) teenaged boy on
prom night, notice I slipped that BMEP thing in.  But
it's really simple:  it's just a measure of the [b]force[/b] that
the combustion pressure is pushing on the piston surface.

Too much pressure in the combustion chamber, the
burning gasoline will detonate and instead of burning
slowly, it explodes prematurely and violently.  The
tetraethyl lead in 100LL breaks up the long hydrocarbon
chains of 94UL into shorter chains, so that if one chain
goes off, it's a smaller, localized event.  The higher
you take off the distilling stack (cracker), the shorter
the hydrocarbon chains.

Detonation can be sudden and catastrophic.  It can
blow chunks out of the piston and will physically
rattle the cylinder heads around.  Auto manufacturers
actually put vibration sensors on the cylinder heads
and back the ignition timing off when they rattle.

However, the margin for detonation with 100LL is
incredible.  In over 40 years of flying, I have yet to
see (or hear) or anyone managing to detonate a
certified normally aspirated aircraft engine.  They
have such low compression and such huge margins.

Dirty little fact:  If you take 100LL, and before you
add Tetraethyl lead to it, you actually have a very
nice 94UL gas, which will work perfectly in every
certified normally aspirated aircraft engine ever
made - but there is [b]NO PAPER FOR IT[/b] so we cannot
do it, so we spray lead all over the environment
and clog up our exhaust valve guides to keep
TC happy, who know everything about paper
and little about engineering.

You only need the incredible octane of 100LL
for positive-displacement aircraft engines which
see more than ambient (32 inches) of MP and
are capable of creating more BMEP in the cylinder,
by packing in more air and fuel.

Oops - I slipped octane in there.  I actually talked
about it above, re: cracking hydrocarbon chains.

You will notice that I [i]despise[/i] forward references.
I actually was programming in K&R C long before
ANSI came along with it's fancy function prototypes
so you put main() at the end of the C source file. 
However, with ANSI C function prototypes, we could
now have main() at the start of the source file, which
is easier to read I guess.  Not everyone likes RPN or
how Yoda talks.

Anyways, I hope this rambling helps people.  If
you understand the basic physics, everything else
makes sense.

Remember that physics is applied mathematics,
and engineering is applied physics.  If you understand
mathematics, you can really understand everything
except women.

PS  I didn't google any of the above, so it's probably
full of errors - I just did it from memory, like Ben
Stein's wonderful impromptu economics lecture in
[i]Ferris Bueller's Day Off[/i]:

[youtube][/youtube]

As far as ground instruction goes, it's not as good as:

[youtube][/youtube]

but legendary nonetheless.

Pro Tip:  many physics equations can be solved by
merely lining up the units so that they cancel out
and give you what you're looking for.

Sort of like "checklist flying" - you really don't understand
what you are doing, but if the formula is good enough,
and the problem is simple enough, you will arrive at the
correct answer even if you don't understand the reasoning
behind the process.

PS  Just as AvCan is so commercially oriented, I am
contractually obligated by my agreement with my sponsor
to mention that today's technical information is brought
to you by [u]Booker T & the MG's[/u]:

[youtube][/youtube]

Bonus points for noticing R Lee Ermey on the couch.

How did we manage to understand the power being developed in the DC6 that had  BMEP gauges as well as M.P. and RPM gauges for each engine?


It must be because that was way back in the dark ages when we were ignorant and poorly trained as pilots.


Unlike today's pilots who are learning from T.C.'s approved expert trainers.

[quote author=Chuck Ellsworth link=topic=3329.msg9422#msg9422 date=1465773263]
the DC6 that had  BMEP gauges [/quote]

Did the engines have pressure sensor in the cylinders?
If they did, I'm not sure how they could show the average pressure throughout the 720 degree cycle

Jeez, Colonel, I watched that last video and promptly forgot what this thread was about, so I had to start over.
I wanted to chime in here that we are not being explicit enough in separating the apples from the oranges, that being normally-aspirated engines and turbo-supercharged engines. What logic applies in choosing a power setting for the one type does not automatically apply to the other.
An engine that is designed from the outset to be the latter  might well have different Pistons, wrist pins, connecting rods, crankshaft journal sizes and webs and cheeks, etc., from its normally-aspirated brethren, weere such an engine to exist. In effect, it was built from the ground up to withstand running over square.
When a normally-aspirated engine is modded for a turbo supercharged application, (eg: from an I0-520 to a TSI0-520) it doesn't have the strength to withstand the BMEP. The easiest solution is to drop the compression ratio and that is what the engineers do.
In fact, the engine in my Turbo Centurion had such a low compression ratio that it was mentioned in the POH that-with a failure of the turbo supercharger-the engine could still Manage almost full rpm but the power produced would be so low that the aircraft could only manage a gradual descent.
In the aftermarket world of add-on turbo superchargers, the engine must still be handled as if it were normally-aspirated, as the reciprocating parts were not designed to withstand the BMEP.
So my reservation would be to not assume that there is no downside to running at very-high manifold pressures( even if they are still below atmospheric) in combination with low revolutions.
It is interesting to read about Allison V-1710's and "R-" this and that and they were amazing feats of engineering and could be abused with manifold pressures up into the nineties, as experimented with by Reno warbirds, although it was apparently like being pulled around the course by a hand grenade with the pin pulled.
But that, folks, is not our world today.

Wednesday is Hump Day.  Heart, of course:

Admit it, you were worried about Friday.

No worries, Friday is Frank day:

[youtube][/youtube]

I don't listen to any music made in the last quarter
century.  Britney Spears and Hillary Duff just don't
cut it.  I mean, as singers.  I am quite fond of this
picture taken after Hillary Duff got an engagement
ring:

[img width=500 height=324]http://www3.pictures.zimbio.com/bg/Hila ... CIeXtl.jpg[/img]

This stream of consciousness you would never,
ever see on the "other forum" in a million years.

Grim young men are no fun, but it looks like $20
worth of cubic zirconium is all you need to have
a hell of a good time with today's singers.

I agree, the last great full album might be the first Black Crows cd, But... There is still things like that good song by Wolfmother.