Power Setting Tables - Tradeoffs

[O. Sami Saydjari]

I am a little confused about power setting options on my Piper Turbo
Arrow from my power setting table.

Assuming I want 75% power, at Sea Level, I can run it at:
1. 2300 RPM and 34.8 inches manifold pressure
2. 2400 RPM and 33.8 inches
3. 2500 RPM and 32.8 inches
4. 2575 RPM and 31.5 inches

How does one choose between these settings. Are they completely
equivalent? Is one setting easier on the enginer or props than another?
Of course, I could do a whole bunch of experiments, but if someone has
a good rule of thumb and a simple explanation behind it, I sure would
appreciate it.

-Sami
N2057M, Piper Turbo Arrow III

[Tauno Voipio]

O. Sami Saydjari wrote:
> I am a little confused about power setting options on my Piper Turbo
> Arrow from my power setting table.
>
> Assuming I want 75% power, at Sea Level, I can run it at:
> 1. 2300 RPM and 34.8 inches manifold pressure
> 2. 2400 RPM and 33.8 inches
> 3. 2500 RPM and 32.8 inches
> 4. 2575 RPM and 31.5 inches
>
> How does one choose between these settings. Are they completely
> equivalent? Is one setting easier on the enginer or props than another?
> Of course, I could do a whole bunch of experiments, but if someone has
> a good rule of thumb and a simple explanation behind it, I sure would
> appreciate it.
>
> -Sami
> N2057M, Piper Turbo Arrow III

In principle, there are two things to balance:

- the pressure at the engine bearings, and
- the central forces at the prop

High rotational speed means more central forces at the prop.
The forces are proportional to the square of the speed. This
means that 10 % change creates roughly 20 % difference in
forces.

Slow rotational speed means more torque and more pressure
at the engine parts. For the oil film in the bearings there
is a two-fold effect: slower speed pushes less oil into the
contact surface and greater pressure squeezes more oil
out of the contact area.

For normal cruise, I select 2300 rpm and 31 inches, but that
is less than 75 %. I'd use 2400 rpm and 33 inches for 75 %
(although it would be about 31 inches in my Arrow due to
the intercooler).

HTH

--

Tauno Voipio
OH-PYM, Piper Turbo Arrow IV
tauno voipio (at) iki fi

[jmk]

You will find a lot of opinions on this, but generally you want to
climb at high RPM... cruise at the lowest RPM for which you can pull
the desired power. [Allowing, of course, for any limitations on your
RPM/power chart, yellow arcs, etc.] That means 2300 and 34.8 is a good
cruise setting for your turbo Arrow. Lower noise, less vibration, and
a lot less heat. [BTW, check your tachometer some time. Most of the
older ones read anywhere up to 200 RPM low.]

All the above assumes operating ROP, of course. If LOP then the power
settings are determined by fuel flow alone.

[bill hunter]

What do you normally use in cross country flights? Do you ever run LOP.
I just bought a turbo arrow with a TurboPlus intercooler, and have not
figured out all the nuances yet.
email me at billhunter at yahoo dot com if you have any helpful hints.


"Tauno Voipio" > wrote in message
...
> O. Sami Saydjari wrote:
>> I am a little confused about power setting options on my Piper Turbo
>> Arrow from my power setting table.
>>
>> Assuming I want 75% power, at Sea Level, I can run it at:
>> 1. 2300 RPM and 34.8 inches manifold pressure
>> 2. 2400 RPM and 33.8 inches
>> 3. 2500 RPM and 32.8 inches
>> 4. 2575 RPM and 31.5 inches
>>
>> How does one choose between these settings. Are they completely
>> equivalent? Is one setting easier on the enginer or props than another?
>> Of course, I could do a whole bunch of experiments, but if someone has a
>> good rule of thumb and a simple explanation behind it, I sure would
>> appreciate it.
>>
>> -Sami
>> N2057M, Piper Turbo Arrow III
>
> In principle, there are two things to balance:
>
> - the pressure at the engine bearings, and
> - the central forces at the prop
>
> High rotational speed means more central forces at the prop.
> The forces are proportional to the square of the speed. This
> means that 10 % change creates roughly 20 % difference in
> forces.
>
> Slow rotational speed means more torque and more pressure
> at the engine parts. For the oil film in the bearings there
> is a two-fold effect: slower speed pushes less oil into the
> contact surface and greater pressure squeezes more oil
> out of the contact area.
>
> For normal cruise, I select 2300 rpm and 31 inches, but that
> is less than 75 %. I'd use 2400 rpm and 33 inches for 75 %
> (although it would be about 31 inches in my Arrow due to
> the intercooler).
>
> HTH
>
> --
>
> Tauno Voipio
> OH-PYM, Piper Turbo Arrow IV
> tauno voipio (at) iki fi
>

[Matt Barrow]

"bill hunter" > wrote in message
...
> What do you normally use in cross country flights? Do you ever run LOP.

I _always_ run LOP.

> I just bought a turbo arrow with a TurboPlus intercooler, and have not
> figured out all the nuances yet.

> email me at billhunter at yahoo dot com if you have any helpful hints.

GAMIjectors.

[john smith]

Just remember to follow "Deakin's Rules"...
- Do not run Lean of Peak above 65% power.

[Matt Barrow]

"john smith" > wrote in message
...
> Just remember to follow "Deakin's Rules"...
> - Do not run Lean of Peak above 65% power.

For a normally aspirated engine.

[Tauno Voipio]

> "Tauno Voipio" > wrote in message
> ...
>
>>O. Sami Saydjari wrote:
>>
>>>I am a little confused about power setting options on my Piper Turbo
>>>Arrow from my power setting table.
>>>
>>>Assuming I want 75% power, at Sea Level, I can run it at:
>>>1. 2300 RPM and 34.8 inches manifold pressure
>>>2. 2400 RPM and 33.8 inches
>>>3. 2500 RPM and 32.8 inches
>>>4. 2575 RPM and 31.5 inches
>>>
>>>How does one choose between these settings. Are they completely
>>>equivalent? Is one setting easier on the enginer or props than another?
>>>Of course, I could do a whole bunch of experiments, but if someone has a
>>>good rule of thumb and a simple explanation behind it, I sure would
>>>appreciate it.
>>>
>>>-Sami
>>>N2057M, Piper Turbo Arrow III
>>
>>In principle, there are two things to balance:
>>
>> - the pressure at the engine bearings, and
>> - the central forces at the prop
>>
>>High rotational speed means more central forces at the prop.
>>The forces are proportional to the square of the speed. This
>>means that 10 % change creates roughly 20 % difference in
>>forces.
>>
>>Slow rotational speed means more torque and more pressure
>>at the engine parts. For the oil film in the bearings there
>>is a two-fold effect: slower speed pushes less oil into the
>>contact surface and greater pressure squeezes more oil
>>out of the contact area.
>>
>>For normal cruise, I select 2300 rpm and 31 inches, but that
>>is less than 75 %. I'd use 2400 rpm and 33 inches for 75 %
>>(although it would be about 31 inches in my Arrow due to
>>the intercooler).
>>
>>HTH
>>
>>--
>>
>>Tauno Voipio
>>OH-PYM, Piper Turbo Arrow IV
>>tauno voipio (at) iki fi
>>
bill hunter wrote:
> What do you normally use in cross country flights? Do you ever run LOP.
> I just bought a turbo arrow with a TurboPlus intercooler, and have not
> figured out all the nuances yet.
> email me at billhunter at yahoo dot com if you have any helpful hints.
>
>

That's what I have, with a power upgrade to 220 hp for 5 min at start.

Leisurely cruise with 28 inches (including the intercooler 3 inches)
and 2300 rpm. Fuel flow a little less than 10 gallons/hour.

--

Tauno Voipio
tauno voipio (at) iki fi