How to set 55%, 65%, 75% power in a piston
William and Heather Morel
power settings and wants to know how to relate that to RPM settings (I'm
flying Archer II's and III's).
Is throttle fully forward equal to 75%? Help!
John Kunkel
The engine manufacturer determines the rpm/power settings for a given
altitude. There should be a power/altitude chart in your POH, if not, an
operator's manual for your engine will have the numbers.
Airborne full throttle at near sea level on a standard should produce
100% power while full throttle at 5000' MSL might produce only 75%.
John
Jeff Cook
> New inexperienced pilot reads in POH performance charts about 55%, 65%, 75%
> power settings and wants to know how to relate that to RPM settings (I'm
> flying Archer II's and III's).
You have to check the performance charts for altitude, temp, etc, which
will tell you what rpm setting to set to get that percentage of power.
It wasn't (isn't?) completely logical to me either...which I hope
doesn't mean I'm blowing smoke right now!
--
Jeff Cook
je...@cookstudios.com
http://www.cookstudios.com
Washington DC area
Bob Gardner
normally aspirated engine, at density altitudes above 5000 feet the engine
power output is so close to 75 percent (or lower) it is not worth arguing
about.
I don't have an Archer book, but I do have 172 books, and they both use
Lycomings. The Cessna performance charts clearly relate RPM to brake
horsepower. Maybe ten minutes with an instructor or mechanic would help you
interpret the charts.
Bob Gardner
William and Heather Morel wrote:
> New inexperienced pilot reads in POH performance charts about 55%, 65%, 75%
> power settings and wants to know how to relate that to RPM settings (I'm
> flying Archer II's and III's).
>
John Lowry
pseudo-facts to consider.
1. 65% power, e.g., means 65% of rated full-throttle brake horsepower at
MSL.
2. Full-throttle engine torque drops off with sigma approximately as
(sigma - C)/(1 - C) where C is quite close to 0.12.
3. Power is proportional to the product of RPM and torque.
But none of this addresses the problem of only being at partial throttle. To
do that, either get a copy of Performance of Light Aircraft (ISBN
1-56347-330-5) and read Chapter 10, or get hold of the Jul/Aug 1999 issue of
Journal of Propulsion and Power and read the article on pages 497-503.
Either of those will show you how to model engine power, torque, and RPM,
including scaling laws which allow you to get cruise performance tables for
other than maximum gross weight. Also how to get cruise performance for
quite low air speeds like speed for best range and for best endurance.
John.
--
John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: 406-248-2606
Web site: http://www.mcn.net/~jlowry
E-mail: jlo...@mcn.net
William and Heather Morel <wmo...@pobox.com> wrote in message
news:83ofdg$q07$1...@lure.pipex.net...
Michael Richmann
> I'm not sure I'm on the correct wavelength but here are some interesting
> pseudo-facts to consider.
>
> 1. 65% power, e.g., means 65% of rated full-throttle brake horsepower at
> MSL.
>
> 2. Full-throttle engine torque drops off with sigma approximately as
> (sigma - C)/(1 - C) where C is quite close to 0.12.
So what's sigma? Ratio of local air pressure relative to sea level pressure?
>
>
> 3. Power is proportional to the product of RPM and torque.
>
> But none of this addresses the problem of only being at partial throttle. To
> do that, either get a copy of Performance of Light Aircraft (ISBN
> 1-56347-330-5) and read Chapter 10, or get hold of the Jul/Aug 1999 issue of
> Journal of Propulsion and Power and read the article on pages 497-503.
> Either of those will show you how to model engine power, torque, and RPM,
> including scaling laws which allow you to get cruise performance tables for
> other than maximum gross weight. Also how to get cruise performance for
> quite low air speeds like speed for best range and for best endurance.
>
> John.
> --
> John T. Lowry, PhD
> Flight Physics; Box 20919; Billings MT 59104
> Voice: 406-248-2606
> Web site: http://www.mcn.net/~jlowry
> E-mail: jlo...@mcn.net
>
> William and Heather Morel <wmo...@pobox.com> wrote in message
> news:83ofdg$q07$1...@lure.pipex.net...
> > New inexperienced pilot reads in POH performance charts about 55%, 65%,
> 75%
> > power settings and wants to know how to relate that to RPM settings (I'm
> > flying Archer II's and III's).
> >
> > Is throttle fully forward equal to 75%? Help!
> >
> >
--
Mike
PP-IASEL
My opinions, not Argonne's...
Ron Natalie
John Lowry wrote:
>
> 1. 65% power, e.g., means 65% of rated full-throttle brake horsepower at
> MSL.
Please remove "full-throttle" from the definition. 65% means 65% of
the BHP that the manufacturer decides at the time of certification.
Whether you can exceed or even attain that is dependent on a variety
of conditions.
John Lowry
Sigma is the ratio of ambient air density to standard MSL density. The
similar ratio for pressure is usually called delta.
Also, Ron Natalie objected to my use of "full-throttle" down below. If the
engine is de-rated, or turbo-charged, the "rated" MSL horsepower will be
less than what you can expect at full throttle. So, good point.
John.
--
John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: 406-248-2606
Web site: http://www.mcn.net/~jlowry
E-mail: jlo...@mcn.net
Michael Richmann <rich...@cmt.anl.gov> wrote in message
news:385FDC52...@cmt.anl.gov...
> John Lowry wrote:
>
> > I'm not sure I'm on the correct wavelength but here are some interesting
> > pseudo-facts to consider.
> >
> > 1. 65% power, e.g., means 65% of rated full-throttle brake horsepower at
> > MSL.
> >
John
<83ofdg$q07$1...@lure.pipex.net>...
>New inexperienced pilot reads in POH performance charts about 55%,
65%, 75%
>power settings and wants to know how to relate that to RPM settings
(I'm
>flying Archer II's and III's).
>
Don't make this any more complicated than you need. Just check in the
POH and find the typical power settings to correspond to 55/65/75.
They might vary a bit day to day, and with altitude, but if you check
the numbers for 3000 feet, on any given day your actual power set will
be real close to that. Keep it simple and easy. JG
Marsha Thompson
go to the performance graph to find which RPM will give the chosen power
level at the chosen altitude--this is mainly a speed vs. economy decision.
I usually fly at 65% as a good compromise. Finally, go to the graph which
tells you what TAS your chosen power level will yield at the chosen
altitude. In 1979 Piper changed the prop pitch, so 2 sets of performance
graphs are provided, referenced by serial #--be sure you check your SN so
you use the correct set of graphs. If you do this a few times for X-C trips
you'll see that unless you're really high or the conditions are really queer
(I fly from a sea-level airport) the numbers aren't much different. I
typically use 2450 rpm or so and get about 115 KIAS. at 4500'.
I always run the numbers, just for drill, and generate a nav. log with all
the data--helps keep me out of trouble and up to speed with my pilotage and
D.R.
Good luck
Eric Thompson
Marcelo Pacheco
William,
Most people that have their own aircraft recomends using 65% or less
power for cruising. Judicious use of that plus apropriate leaning
usually will let you get way past TBO (Recommended Time Between
Overhauls) and will also avoid some sorts of possible mechanical causes
for engine failures. 75% power usually requires some not so agressive
leaning where additional fuel helps cool off the engine.
Some aircraft with great induction systems designs can gain manifold
pressure from the aircraft's speed, so you might need to be a little
higher until you can safely use full throttle while being sure you're
getting less than 75% power, also if it's very cool out there, 5000ft
air might be as dense as 4000ft standard temperature air.
I believe most people out there that complains that their engines don't
make it to the TBO are the people that don't pay close attention to
engine performance charts.
I personally only use full throttle for cruise at or above 7000ft,
bellow that I always lookup performance charts, and on long trips, I'll
have to lookup performance charts due to fuel burn issues anyhow.
It helps to understand the concept of manifold pressure, which still
exists and is still the main factor in the available engine power even
to fixed pitch propellor aircraft.
Marcelo
John Lowry wrote:
>
> I'm not sure I'm on the correct wavelength but here are some interesting
> pseudo-facts to consider.
>
> 1. 65% power, e.g., means 65% of rated full-throttle brake horsepower at
> MSL.
>
> 2. Full-throttle engine torque drops off with sigma approximately as
> (sigma - C)/(1 - C) where C is quite close to 0.12.
>
> 3. Power is proportional to the product of RPM and torque.
>
> But none of this addresses the problem of only being at partial throttle. To
> do that, either get a copy of Performance of Light Aircraft (ISBN
> 1-56347-330-5) and read Chapter 10, or get hold of the Jul/Aug 1999 issue of
> Journal of Propulsion and Power and read the article on pages 497-503.
> Either of those will show you how to model engine power, torque, and RPM,
> including scaling laws which allow you to get cruise performance tables for
> other than maximum gross weight. Also how to get cruise performance for
> quite low air speeds like speed for best range and for best endurance.
>
> John.
> --
> John T. Lowry, PhD
> Flight Physics; Box 20919; Billings MT 59104
> Voice: 406-248-2606
> Web site: http://www.mcn.net/~jlowry
> E-mail: jlo...@mcn.net
>
> William and Heather Morel <wmo...@pobox.com> wrote in message
> news:83ofdg$q07$1...@lure.pipex.net...
> > New inexperienced pilot reads in POH performance charts about 55%, 65%,
> 75%
> > power settings and wants to know how to relate that to RPM settings (I'm
> > flying Archer II's and III's).
> >
Michael Kowalsky
rpm's less engine wear.
over leaning will definitly cut your engine life short,but pull back
them rpms why run at the same speed when you can jog.
Newps
Depends on the engine. The worst thing you can do to an O 470 is run it
at slow RPM's.
Ron Rosenfeld
life?
--ron
Michael Kowalsky <mik...@webtv.net> wrote in message
news:17283-38...@storefull-235.iap.bryant.webtv.net...
Marcelo Pacheco
engine.
Very low RPM's at high MP's is also bad for the engine because you could
be overheating the engine (too much power being generated and less oil
cooling).
For a plane with RPM control, whatever lowest RPM is allowed for cruise
*should* be the best. For a 182 it looks like 2100 RPM is great. That's
why both the tachometer and MP gauge has a green arc. Lower than that
might cause overheating.
What really caught me is that we might not be hearing the right advice
(I don't mean mine is right, just to question's everybody's advice
validity) is that my CFI told me not to oversquare a Piper Arrow without
any such recommendation in the POH, just because he was taught that way.
Now we know that if you look at www.avweb.com, this is an old wife
tail's come from old supercharged/geared large radial engines where
oversquare was bad (imagine 3000 RPM with 35 inches, while the prop is
running at something between 1500 and 2000 RPM), for example in old
Super Constelation, ... . It just stroke me that most wisdom out there
was learned just because it was passed mouth to mouth, without much
technical thinking behind it. Before you take such engine advice from
your instructor or whatever, ask where he learned this from. If it just
came from his former instructor, question it. If you question it nicely,
who knows both you and your instructor might agree there's a better way.
Same thinking about going full rich when starting a descent from
altitude. My personal belief is if you were already at 65% power or
less, a little power reduction will get you at 55%. At this power
output, unless the engine is running rough, it shouldn't be overheating,
detonating or whatever, so all you should do is to enrich the mixture as
the engine get's rough. However, every instructor I had told me to go
full rich before starting to descend, which might even work at 4000ft,
but could kill the engine due to an overly rich mixture at 12000ft.
Marcelo
HLAviation
>(I don't mean mine is right, just to question's everybody's advice
>validity) is that my CFI told me not to oversquare a Piper Arrow without
>any such recommendation in the POH, just because he was taught that way.
And it is pure unadulterated BS. There is absolutely nothing wrong with
running "oversquare", it's all BS. There are engines that never get down to
square. If this was followed during WWII our bombers could have not flown
their deep penetration missions.
During the war (any of you WWII pilots correct me if I'm wrong) allied pilots
flew by the Lindberg Doctrine which boiled down to Max MP with Min RPM = Max
Range. This is also the technique that got him across the Atlantic.