Redline (formerly the Beater thread)

[EDR]

Interesting that many have not defined the origin of the placement of
the "red line" on the tachometer.

I don't know where to look it up, but I suspect it has something to do
with prop tip speed (as one poster mentioned).

Shorten the prop, lengthen the prop, change the pitch and you will
necessisarily change the placement of the redline on the tach.

Change the pistons (bore); change the rods (stroke); change the rings
(compression); change the induction system; change the exhaust (back
pressure) and you change the torque of the engine, again altering the
redline.

[Jay Honeck]

> Shorten the prop, lengthen the prop, change the pitch and you will
> necessisarily change the placement of the redline on the tach.

Doesn't this imply that redline RPM isn't related to engine limitations?

Why would (just for example) shortening the prop allow one to raise the RPM
limit of the engine?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Jay Masino]

Jay Honeck > wrote:
> Doesn't this imply that redline RPM isn't related to engine limitations?

I believe it's both.

> Why would (just for example) shortening the prop allow one to raise the RPM
> limit of the engine?

It's bad to allow the prop tip to go supersonic. I believe that this is
because the shock wave reduces the efficiency of the prop. Others can
chime in on this. Shortening the prop "slows" the linear speed that the
prop tip is experiencing at a given RPM.




--
__!__
Jay and Teresa Masino ___(_)___
http://www2.ari.net/jmasino ! ! !
http://www.oceancityairport.com
http://www.oc-adolfos.com

[C J Campbell]

"EDR" > wrote in message
...
> Interesting that many have not defined the origin of the placement of
> the "red line" on the tachometer.
>
> I don't know where to look it up, but I suspect it has something to do
> with prop tip speed (as one poster mentioned).
>

It can have something to do with it, but the same engine in different
airplanes will have a different redline. The thing that changes is TBO. The
propeller chosen for that installation will be consistent with the engine
service life desired.

[EDR]

In article >, C J Campbell
> wrote:

> "EDR" > wrote in message
> ...
> > Interesting that many have not defined the origin of the placement of
> > the "red line" on the tachometer.
> >
> > I don't know where to look it up, but I suspect it has something to do
> > with prop tip speed (as one poster mentioned).
> >
>
> It can have something to do with it, but the same engine in different
> airplanes will have a different redline. The thing that changes is TBO. The
> propeller chosen for that installation will be consistent with the engine
> service life desired.

That's right. Some airplanes have prohibited operation in the middle of
the RPM range because of harmonics. Certain models of Cessna 210's, for
example.

[Jay Honeck]

> > Why would (just for example) shortening the prop allow one to raise the
RPM
> > limit of the engine?
>
> It's bad to allow the prop tip to go supersonic.

Understood.

However, how is it that you can up the RPM limit of a given engine simply
because you want to keep the prop tips subsonic?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Jay Masino]

Jay Honeck > wrote:
> However, how is it that you can up the RPM limit of a given engine simply
> because you want to keep the prop tips subsonic?

You would decrease the RPM limit, not increase it, to keep the tips
subsonic.


--
__!__
Jay and Teresa Masino ___(_)___
http://www2.ari.net/jmasino ! ! !
http://www.oceancityairport.com
http://www.oc-adolfos.com

[Jay Honeck]

> > However, how is it that you can up the RPM limit of a given engine
simply
> > because you want to keep the prop tips subsonic?
>
> You would decrease the RPM limit, not increase it, to keep the tips
> subsonic.

Understood. If you lengthened the prop, you'd have to decrease the RPM
redline to keep the tips subsonic.

However, the part of the question I was referring to was if you *shortened*
the prop. Part of the original post implied that you would be able to
simply *increase* the engine's redline RPM if you shortened your prop,
regardless of engine limitations. I don't believe this is true.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Richard Russell]

On Mon, 14 Jun 2004 19:39:46 GMT, "Jay Honeck"
> wrote:

>> > However, how is it that you can up the RPM limit of a given engine
>simply
>> > because you want to keep the prop tips subsonic?
>>
>> You would decrease the RPM limit, not increase it, to keep the tips
>> subsonic.
>
>Understood. If you lengthened the prop, you'd have to decrease the RPM
>redline to keep the tips subsonic.
>
>However, the part of the question I was referring to was if you *shortened*
>the prop. Part of the original post implied that you would be able to
>simply *increase* the engine's redline RPM if you shortened your prop,
>regardless of engine limitations. I don't believe this is true.

It isn't true. Theoretically, you could increase the redline rpm
providing you don't exceed the limitations of any other part of the
"system", such as the engine. The redline would have to be based upon
the most restrictive component.
Rich Russell

[EDR]

Found the source, sort of...

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCirc
ular.nsf/0/c2f903127f0f41e4862569ae006d44d7/$FILE/AC20-88A.pdf

ANNOUNCEMENT OF AVAILABILITY OF ADVISORY CIRCULAR 20-88A; LINES ON THE
MARKING OF AIRCRAFT POWERPLANT INSTRUMENTS (DISPLAYS)

g. Tachometer (Reciprocating Engine Aircraft)
Red Radial - At maximum permissible rotational speed (rpm).
Green Arc - From maximum rotational speed for continuous operation to
minimum recommended for continuous operation (except in the restricted
range, if any). For propeller driven small airplanes which must meet
the requirements in Appendix F to Part 36 of the FAR at maximum normal
operating power, from the corrected rotational speed used to establish
the aircraft noise level to the minimum recommended for continuous
operation (except in the restricted ranges, if any).
Yellow Arc - From maximum rotational speed for continuous operation to
maximum permissible rotational speed.
Yellow Arc (Offset) - Conditional operating range for one engine
inoperative flight that exists above the normal takeoff rating limit
(if applicable).
Red Arc - Range(s) in which operation is restricted, except to pass
through, for all operating conditions because of excessive stresses,
etc.

[Jay Masino]

Jay Honeck > wrote:
> However, the part of the question I was referring to was if you *shortened*
> the prop. Part of the original post implied that you would be able to
> simply *increase* the engine's redline RPM if you shortened your prop,
> regardless of engine limitations. I don't believe this is true.

I think at that point, you'd be getting into limitations of the engine
components (with some tolerance).



--
__!__
Jay and Teresa Masino ___(_)___
http://www2.ari.net/jmasino ! ! !
http://www.oceancityairport.com
http://www.oc-adolfos.com

[I'd rather be flying]

> Shorten the prop, lengthen the prop, change the pitch and you will
> necessisarily change the placement of the redline on the tach.
No you won't. Redline is engine speed related only. Redline has a fixed rpm
no matter what you attach or change at the output shaft (in our case a
propeller). Take the prop off and use full throttle... the engine will most
certainly reach redline, the same redline you had before you removed the
prop. Redline speed does not change. The engine has a point where spinning
it faster starts floating valves and causing damage. This is a constant
speed. If you change the pitch and/or increase size of the prop, the engine
won't be able to spin the large prop up to redline. This doesn't mean the
engine won't have huge amount of stress on it trying to do so. It won't
reach redline, but redline remains at the same speed.

Richard

"EDR" > wrote in message
...
> Interesting that many have not defined the origin of the placement of
> the "red line" on the tachometer.
>
> I don't know where to look it up, but I suspect it has something to do
> with prop tip speed (as one poster mentioned).
>
>
> Change the pistons (bore); change the rods (stroke); change the rings
> (compression); change the induction system; change the exhaust (back
> pressure) and you change the torque of the engine, again altering the
> redline.