Why does airspeed change when I adjust the prop?

Tony,

Ah, "oversquare" is obvious BS, we can agree on that (said the man coming
from the land of SI units, where the whole idea of oversquare goes away
when you use Pascal to measure MP ;-)).

I cannot think of a cirumstance
when following the commonly taught and used procedure of first reducing
throttle then rpm, or first advancing rpm then throttle, is not the
prudent technique.


Again, I mentioned two examples, one common, one maybe not so:

1. Take-off. Leave throttle at full and reduce RPM to 2500 is better for
the hardware in most cases than reducing to 25/2500.

2. Dogma. If you're reducing RPM by, say, 200 but leaving the MP at the
set value, it makes no sense to reduce MP, then reduce RPM, then increase
MP again. Yet some CFIs will insist you do it that way.

Here's a third: People flying the "full LOP regime" will often leave the
throttle at full for the whole flight, controlling engine power with RPM
and mixture alone. If you really know what you're doing, that's perfectly
safe and good on the equipment.

--
Thomas Borchert (EDDH)

We are not disagreing on any point here. The 'dogma' about which
adjustment to make FIRST is if both throttle and prop are being
adjusted. The cases you mentioned are circumstances where the PIC has
decided to pull or push on ONLY one knob!

It's also worth mentioning that cowl flaps and CHTs are among a pilot's
best friends when stretching things.




On Jan 17, 8:44 am, Thomas Borchert
wrote:
Tony,

Ah, "oversquare" is obvious BS, we can agree on that (said the man coming
from the land of SI units, where the whole idea of oversquare goes away
when you use Pascal to measure MP ;-)).

I cannot think of a cirumstance
when following the commonly taught and used procedure of first reducing
throttle then rpm, or first advancing rpm then throttle, is not the
prudent technique.Again, I mentioned two examples, one common, one maybe not so:

1. Take-off. Leave throttle at full and reduce RPM to 2500 is better for
the hardware in most cases than reducing to 25/2500.

2. Dogma. If you're reducing RPM by, say, 200 but leaving the MP at the
set value, it makes no sense to reduce MP, then reduce RPM, then increase
MP again. Yet some CFIs will insist you do it that way.

Here's a third: People flying the "full LOP regime" will often leave the
throttle at full for the whole flight, controlling engine power with RPM
and mixture alone. If you really know what you're doing, that's perfectly
safe and good on the equipment.

--
Thomas Borchert (EDDH)

"Thomas Borchert" wrote

Again, I mentioned two examples, one common, one maybe not so:

1. Take-off. Leave throttle at full and reduce RPM to 2500 is better for
the hardware in most cases than reducing to 25/2500.

I'm not so sure about that - but I also don't really know. As you said,
these engines are fuel/air cooled, but as you ask for more power you also
produce more heat, and I suspect that the engine is actually less efficient
thermodynamically at full power than it is at reduced power. In other
words, as you approach full power you get increasingly more heat and less
actual BHP for that heat.

I wonder if the oversquare-phobia is a carry-over from adjustable-pitch
props where it was much easier to "lug" the engine (like driving in 4th gear
at low speed and heavy on the gas).

BDS

Tony wrote:

The habit of backing off throttle first, then reducing prop RPM, and
the habit of increasing RPM first, then advancing the throttle, is one
of those things that can save wear and tear, and maybe an engine. Can
anyone offer a good logical reason to do it any other way?



Can you offer a good reason TO do it that way?

Take a look the POH or engine manual for the plane. Often you will see
oversquare settings and their corresponding fuel flow and percent
power. Obviously, if this is the case it is OK to run there. I actually
think that sticking to the published rpm/manifold is the best policy.
You KNOW that running the engine there is OK.

Note that if there are two or more settings of a certain percent power,
the setting with the lowest rpm gets the best fuel economy.

Some airplanes have a "redzone" on the tachometer indicating no
prolonged running the engine at that rpm.

Someone once posted that he called Lycoming to ask about vastly
oversquare running (like 1800 rpm and 25" mainifold) and if it was ok.
The engineer hemmed and hawed and said that Lycoming actually had never
tested the engine for long periods of time at those settings, but that
since it wasn't explicity restricted....well who knows? To me running
that low an rpm with that high a manifold pressure seems wrong. But I
can't actually proove it.

On a closing note, Lycoming recommends 1000 rpm immediately after
startup so you have enough oil pressure to cycle oil. Lycoming also
recommends no cooling faster than 50 degrees per minute (as I recall).
Best to stick with those guidelines also.

Tony writes:

Old wives are often right.

Not nearly as often as engineers are right.

--
Transpose mxsmanic and gmail to reach me by e-mail.

BDS writes:

I'm not so sure about that - but I also don't really know. As you said,
these engines are fuel/air cooled, but as you ask for more power you also
produce more heat, and I suspect that the engine is actually less efficient
thermodynamically at full power than it is at reduced power. In other
words, as you approach full power you get increasingly more heat and less
actual BHP for that heat.

In general, the greater the temperature gradients in a heat engine,
the more efficient the engine becomes.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Thomas Borchert writes:

So read it.

I have. But it doesn't go into much detail on the fine adjustments of
mixture, throttle, and prop, and their interactions. I think one
reason for this may be that many of the possible adjustments just
aren't that important. This would also explain why so many pilots can
depend on voodoo or tea leaves to determine how they make the
adjustments, and yet they never have any problems.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic wrote:
Thomas Borchert writes:

So read it.

I have. But it doesn't go into much detail on the fine adjustments of
mixture, throttle, and prop, and their interactions. I think one
reason for this may be that many of the possible adjustments just
aren't that important. This would also explain why so many pilots can
depend on voodoo or tea leaves to determine how they make the
adjustments, and yet they never have any problems.

--


Now that sort of statement is only going to **** off the people who
have kindly answered your question!!!

For my money it is nice to know the in-depth stuff but it simply isn't
necessary to be an aeronautical expert just to fly a light aircraft!

Thomas Borchert wrote:
Chris,

OK, I stand corrected!! As I indicated in my first post I am a fixed
pitch pilot so I am quite short of knowledge in this area, and I am
always keen to learn more ...


Go for the avweb.com columns of John Deakin on engine management.

--


Hi, thanks for that - I didn't know about those columns, and the first
one I came across - the one I am reading right now is fascinating, so I
will keep on reading them..

Thanks heaps..

Chris