On Jul 20, 10:48 am, Bertie the Bunyip wrote:
"Mike" wrote innews:qNIgk.158$oU.42@trnddc07:



"Bertie the Bunyip" wrote in message
.. .
"Mike" wrote in
news:WPHgk.144$DS3.119@trnddc01:

"Terence Wilson" wrote in message
...
In the course of trying to understand how a constant speed prop
works I came across the following passage in one of the Jeppesen
books:

"If the throttle is advanced without decreasing the pitch of the
prop blades to increase ___ rpm, the manifold pressure increases as
the prop mechanism attempts to keep ___ rpm constant by increasing
the blade angle. The combination of high manifold pressure and low
___ rpm can cause damage due to high internal manifold pressures."

I found this paragraph to be confusing because it makes several
references to rpm but doesn't clarify whether it is engine or prop
rpm. The blanks were inserted by me. Can someone help me out?

Thanks in advance.

As others have said, unless you have a gearbox (not many planes do),
they are one and the same.

You may also want to ditch your Jepp book as the "theory" they are
describing really doesn't apply to most small piston aircraft. The
old "don't run oversquare" mentality which has been taught for years
originated out of military teachings that applied to very different
pilots doing very different things while flying very different
aircraft.

Actually, they dont, since most military aircraft,even smaller ones,
were supercharged and they ran well oversquare.. A 985, for instance,
is around 37 inches max and a typical cruise MP might be in the
order of 25 inches with a cruise rpm of something like 1850,
depending on how fast you want to go and how much you want to burn.
The geared engines were even less relevant to this argument, since
almost everything larger than about 1500 c.i.d. was geared. The
indicated RPM was usually engine rpm and max for somthing like an
1830 was around 2400 and max MP for takeoff was about 43 IIRC and
cruise was around 30/2,000.
The practice originates from a perceived need to simplify for light
aircraft pilots new to variable pitch props.

And that need is even more of a necessity in military trainers which
have considerably more power and are much more easily red lined.
That's why I always assumed the mentality came primarily from military
instructors giving instruction in training aircraft. At any rate the
myth still persists to this day even with instructors who should know
better.

Well, outside of the T-34 I can't think of anything that would fit the
"square" scenario, and military instructors would not have taken any
sort of soft route with the students in any case. For instance, I happen
to know any Navy student would have to have memorised a very lengthy
series of checklists at the primary student stage for a T-28, for
instance. That's ALL of the checklists. Ever single one, emergencies and
all. And having seen them I know they were very, very complicated
indeed.
They also had to be able to touch every single switch, dial, and lever
in the airplane blindfolded. I can't see them going soft on a little
thing like not having to memorise a given MP RPM combo. Now, during
aerobatics, it would make sense to have a nominal max MP a bit shy of
normal max, as you say, but for operations outside of that, they
certainly would not have done that.
No, the only place I've ever seen he practice touted s by FBOs renting
airplanes or using them for comercial instruction.

Bertie

,

The whole idea of don't run oversquare is not a military technique
taught, but rather a technique taught to radial pilots back in the
day. These pilots then moved over to our flat engines, and decided
running oversquare would still be a bad idea. I agree with the other
guys-and having been through an Advanced Pilot Seminar, I can
certainly say-you will know much more about your engine after going
through the seminar. There is another seminar coming up later this
year, and I'm thinking of attending it again.
Also those pelican perch articles are fantastic, and if Deakin ever
shows up to a seminar, you get to meet the author.