Constant Speed Prop vs Variable Engine Timing

(Dan Thomas) wrote:

Horsepower is a function of torque multiplied by RPM. A Lycoming
engine in an older Cessna 172, for example, produces 150 HP at 2700
RPM under standard conditions (sea level atmospheric pressure and
59°F). In the takeoff roll with the fixed-pitch prop, RPM will be
around 2300 RPM, which, according to the POH, would indicate a
horsepower output of about 61% of 150, or about 92 horses. Not very
good, is it?

snip

Dan,

Those numbers can not be correct. The power curves in the Lycoming
operator's manual show that in standard sea level conditions at 2,300
RPM full throttle, a 150 hp Lyc (O-320 A, E) will produce 132 hp or
88% of full rated power. Interestingly, your 92 hp figure closely
matches the propeller load curve at 2,300 RPM. The propeller load
curve, however, is not a full throttle curve. Rather, it is a
variable throttle static run-up curve using a fixed pitch test prop
(or club) chosen to achieve max rated engine RPM at full throttle. If
your C-172 POH says that the 150 hp Lyc produces only 92 hp at 2,300
RPM full throttle in standard sea level conditions, then it is wrong
by a wide margin.

While I'm here, I'd like commend you on your typically spot-on
explanations and your generosity in frequently answering questions
here. Unfortunately, business and other matters keep me from
participating here as much as I'd like. It is folks like you who make
the difference here, not the... (well, I'll let that go). I Hope you
stick around for a long time.

David O -- http://www.AirplaneZone.com

On Fri, 27 Feb 2004 23:43:47 -0500, "Morgans"
wrote:


IMHO, to take advantage of the auto engine's characteristics, you need a CS
prop, even more. Flat pitch for takeoff, then really get the course pitch.......
++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++

COURSE?

Sorry, 'Teach'.
It's your turn in the barrel. g

One need not be gifted or an English major to be educated in the
basics of our native tongue. Just being a teacher should induce one to
become somewhat less of an embarrassment to this noble vocation....
by osmosis or a remedial 'course', if nothing else.


COARSE adj. - Consisting of large particles; not fine in texture


COURSE n.

1. a. Onward movement in a particular direction;
progress: the course of events.

2. a. A complete body of prescribed studies constituting a
curriculum: a four-year course in engineering.

b. A unit of such a curriculum: took an introductory course
in chemistry; passed her calculus course.


Barnyard BOb -

Those numbers can not be correct. The power curves in the Lycoming
operator's manual show that in standard sea level conditions at 2,300
RPM full throttle, a 150 hp Lyc (O-320 A, E) will produce 132 hp or
88% of full rated power. Interestingly, your 92 hp figure closely
matches the propeller load curve at 2,300 RPM. The propeller load
curve, however, is not a full throttle curve. Rather, it is a
variable throttle static run-up curve using a fixed pitch test prop
(or club) chosen to achieve max rated engine RPM at full throttle. If
your C-172 POH says that the 150 hp Lyc produces only 92 hp at 2,300
RPM full throttle in standard sea level conditions, then it is wrong
by a wide margin.

Same thing in my manual.

While I'm here, I'd like commend you on your typically spot-on
explanations and your generosity in frequently answering questions
here. Unfortunately, business and other matters keep me from
participating here as much as I'd like. It is folks like you who make
the difference here, not the... (well, I'll let that go).......

David O -- http://www.AirplaneZone.com
+++++++++++++++++++++++++++++++++++++++++++++++

Bring it up.....and then let it go? g

What the hell....
Is this an attempt to get in touch with your 'feminine side' or what?


Barnyard BOb -

RU ok wrote:

Bring it up.....and then let it go? g

What the hell....
Is this an attempt to get in touch with your 'feminine side' or what?

Just seeking some balance.

David O -- http://www.AirplaneZone.com

David O wrote in message . ..
(Dan Thomas) wrote:

Horsepower is a function of torque multiplied by RPM. A Lycoming
engine in an older Cessna 172, for example, produces 150 HP at 2700
RPM under standard conditions (sea level atmospheric pressure and
59°F). In the takeoff roll with the fixed-pitch prop, RPM will be
around 2300 RPM, which, according to the POH, would indicate a
horsepower output of about 61% of 150, or about 92 horses. Not very
good, is it?

snip

Dan,

Those numbers can not be correct. The power curves in the Lycoming
operator's manual show that in standard sea level conditions at 2,300
RPM full throttle, a 150 hp Lyc (O-320 A, E) will produce 132 hp or
88% of full rated power. Interestingly, your 92 hp figure closely
matches the propeller load curve at 2,300 RPM. The propeller load
curve, however, is not a full throttle curve. Rather, it is a
variable throttle static run-up curve using a fixed pitch test prop
(or club) chosen to achieve max rated engine RPM at full throttle. If
your C-172 POH says that the 150 hp Lyc produces only 92 hp at 2,300
RPM full throttle in standard sea level conditions, then it is wrong
by a wide margin.

Right you are. The 92 HP figure is taken from cruising charts, less
than full throttle. My mistake in assuming that the 2300 RPM would
have a consistent HP.
We once did some physics calcs regarding the acceleration to
takeoff speed for the 172. We found that the energy to accelerate that
mass to that speed came to 24 HP, demonstrating the enormous losses to
prop and airframe drag and wheel rolling friction. Wouldn't it be
great if we could reduce those to a fraction of what they are and make
truly efficient flying machines?

Dan

(Corky Scott snip

That will likely change when auto engines, complete with the
computerized ignition and fuel injection, and all the sensors to make
it work properly get into the air. But then again, the Lycomings and
Continentals would also benefit from such treatment.

Variable timing and fuel injection is coming, it's already running on
several models, it's called FADEC for Fully Automated Digital
Electronic Control.

Corky Scott

I think you are right Corky. FADEC (Full Authority Digital Engine
Control) has been around on jets since the 70's. It is
unquestionably the best way to reach TBO and optimum burn performance
for an individual engine. It however has resulted in unforeseen
accidents (e.g: Airbus 330 in Toulouse, France, where test pilot got
behind power curve, then pushed throttles to the wall, and FADEC
refused due to thermal spool up considerations. Its programming
decided that full power would be available to the crew in something
like five seconds. This saves millions for the fleet every fiscal
year. Problem was: The prototype hit the stand of trees in something
like six seconds… This was caught on video, and the test pilot was
interviewed in the hospital. He stated that nothing happened when he
called for max power. If I had FADEC in a single-engine GA aircraft I
would want a non-software override.

pacplyer

the test pilot was
interviewed in the hospital. He stated that nothing happened when he
called for max power.

--------------------------------------------------------------

I hate it when that happens :-)

-R.S.Hoover

(Dan Thomas) wrote in message . com...
(Jay) wrote in message . com...
Seems to me that some of the benefits of the constant speed prop were
based on the limitiations of timing (ignition and valve) of the
Lyco/Conti engines. If your engine was designed to have a large
dynamic range of efficient operation, you won't need the articulated
prop as much.

.. . . snip . . .
A fixed-pitch prop is a compromise and is like having only second
gear in your car: lousy acceleration, lousy highway speed. Could this
be fixed with fancy engine doodads? Nope. More gears are needed, and
the constant-speed prop is the airplane's transmission.

It seems to me that the gear analogy is spot on. A variable pitch
prop has EXACTLY the same function as the gearbox on a car.

It seems to me that the gear analogy is spot on. A variable pitch
prop has EXACTLY the same function as the gearbox on a car.



Dumb newbie question here...

If you have a prop that is best for cruise....am I right in assuming it has
"too much of a bite" on the air when the aircraft is sitting still...therefore
the engine doesnt have enough torque...and therefore the prop cant spin quite
as fast as it would otherwise...and both these lead to less low speed thrust
than you would like?

And if that is the case...could you not use something like water mist injection
or nitrous oxide to temporarily increase the torque the motor produces?

Probably wount make much sense if you really wanted it for many minutes of
climbing....but it might make sense if all your trying to do is shorten your
takeoff distance.....

take care

Blll

On Sun, 29 Feb 2004 18:52:16 -0800, pacplyer wrote:

(Corky Scott snip

That will likely change when auto engines, complete with the
computerized ignition and fuel injection, and all the sensors to make it
work properly get into the air. But then again, the Lycomings and
Continentals would also benefit from such treatment.

Variable timing and fuel injection is coming, it's already running on
several models, it's called FADEC for Fully Automated Digital Electronic
Control.

Corky Scott

I think you are right Corky. FADEC (Full Authority Digital Engine
Control) has been around on jets since the 70's. It is unquestionably
the best way to reach TBO and optimum burn performance for an individual
engine. It however has resulted in unforeseen accidents (e.g: Airbus 330
in Toulouse, France, where test pilot got behind power curve, then pushed
throttles to the wall, and FADEC refused due to thermal spool up
considerations. Its programming decided that full power would be
available to the crew in something like five seconds. This saves millions
for the fleet every fiscal year. Problem was: The prototype hit the stand
of trees in something like six seconds… This was caught on video, and
the test pilot was interviewed in the hospital. He stated that nothing
happened when he called for max power. If I had FADEC in a single-engine
GA aircraft I would want a non-software override.

pacplyer

Two comments:

You've mixed up two different accidents here. The 330 at Toulouse was a
loss of control due to the aircraft (on autopilot) going way below VMCA
with one engine at idle and the other at full take-off thrust. The sat
and watched until it was too late to recover.

The accident you are referring to was the A320 at Mulhouse-Habsheim. The
pilot did a very low (30 ft AGL) pass with the thrust at idle. The speed
decreased til he was at full aft stick, riding on the AOA limiter just
above the stall. Then he realized that what he had thought were just low
bushes when he was looking down on them as he descended, were actually
trees that were higher than he was. He couldn't raise the nose, as
the fly-by-wire (FBW) was already on the AOA limiter, so the only way to
climb was to get more airspeed. He slammed the thrust levers forward, and
the FADEC accelerated the engine on its normal acceleration schedule.

Turbine engines run more efficiently if they are running close to the
surge line (i.e almost ready to compressor stall). But the engine has to
come closer to the surge line to accelerate. So the closer you run to the
surge line the slower acceleration you'll have.

FAR 25.119(a) requires go-around performance to be calculated using the
thrust that is available 8 seconds after a throttle slam from idle.
Manufacturers want the engine to run as efficiently as possible, but they
don't want to take a hit on the AFM go-around performance. So, they
typically design the fuel controls to allow full go-around thrust to be
reached in just less than 8 seconds from a throttle slam from idle. I've
done tests to check the acceleration on many transport category aircraft,
and the result is usually somewhere between 7 and 8 seconds, and this is
the same no matter whether the engine has a FADEC or an "old fashioned"
hydro-mechanical fuel control unit.

So don't blame the FADEC for the A320 accident at Mulhouse-Habsheim. It
was caused by a pilot who had way too much confidence in the low-speed
protections of the FBW. Fortunately the FBW prevented him from raising
the nose, as then the aircraft would have stalled, any many people would
probably have died. As it was "only" three live were lost.

--
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Ottawa, Canada
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