Constant Speed Prop vs Variable Engine Timing

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.

"Jay" wrote in message
om...
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.

Prop blades are just rotating wings. The goal is to run the blades at their
most efficient angle of attack for the RPM and aircraft airspeed. The
performance of the prop is best at low RPM but the piston engine driving it
is likely to be most efficient at a higher RPM. That is the reason that
high performance piston aircraft have both PRSU's and constant speed props.

Some experimental powerplant/prop systems included a two speed gearbox in
addition to the CS prop to run the engine at high RPM at takeoff and low RPM
for cruise. These experimental engines also shifted the cam and ignition
timing for the two PRSU ratios. This helped the prop blades stay at the
best AOA to maximize thrust and optimized the engine at two set points, high
RPM for takeoff and low RPM for long range cruise. This was at the very end
of large piston engine development and an attempt to wring the last bit of
performance out of these monsters.

Having an engine with a wide "dynamic range" is nice for a car but less
useful for an airplane where it is best to optimize the engine for one RPM
and let the CS prop and PRSU operate the prop in the most efficient way.

Bill Daniels

Thanks for your insight, which raise a few questions:

Apart from the geared Cessna (which isn't all that hi performance)
which aircraft have a PSRU AND a CS prop?

Which experimental aircraft has 2 speed gear boxes? I heard of a guy
flying a WW1 replical biplane on a Honda motocycle engine. He just
kept the original gear box and said he actually shifts gears depending
on if he's climbing or cruising.

My point about using an engine that can operate efficiently over a
large range of RPMs (like a modern automobile engine) is that the CS
prop is NOT as necessary although it certainly does help, no doubt
about it. Certainly you will get you peak horsepower at high revs,
but the moderm engine has a fatter torque curve due to being able to
change valve AND ignition timing in a manner optimum for the
particular revs it is at. The Lyco/Conti design takes a double hit
for operating at low revs, its off the peak HP point, and its timing
was peaked for a specific RPM.


"Bill Daniels" wrote in message ...
"Jay" wrote in message
om...
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.

Prop blades are just rotating wings. The goal is to run the blades at their
most efficient angle of attack for the RPM and aircraft airspeed. The
performance of the prop is best at low RPM but the piston engine driving it
is likely to be most efficient at a higher RPM. That is the reason that
high performance piston aircraft have both PRSU's and constant speed props.

Some experimental powerplant/prop systems included a two speed gearbox in
addition to the CS prop to run the engine at high RPM at takeoff and low RPM
for cruise. These experimental engines also shifted the cam and ignition
timing for the two PRSU ratios. This helped the prop blades stay at the
best AOA to maximize thrust and optimized the engine at two set points, high
RPM for takeoff and low RPM for long range cruise. This was at the very end
of large piston engine development and an attempt to wring the last bit of
performance out of these monsters.

Having an engine with a wide "dynamic range" is nice for a car but less
useful for an airplane where it is best to optimize the engine for one RPM
and let the CS prop and PRSU operate the prop in the most efficient way.

Bill Daniels

"Jay" wrote in message om...

Apart from the geared Cessna (which isn't all that hi performance)
which aircraft have a PSRU AND a CS prop?

Navions, Helio Couriers, Twin Commanders, Beech Twin Bonanzas,
Republic SeaBee, Beech Queen Air all have models that use variants
of the Geared Lycomings (435 and 480). They almost always drive
a variable pitch (some cases not constant speed ) prop.

All moderate to high power geared piston engines and all turboprops have
gear reduction and CS props. It is not just about engine efficiency, it is
about prop efficiency over a range of airspeeds and altitudes. As airspeed
increases, AOA of the prop decreases so you can not have a prop which works
efficiently for takeoff and cruise unless the airplane is very slow. My
airplane advances 19 feet for each revolution of the prop at cruise vs. 0
feet at the beginning of the takeoff roll. Similiarly, the prop needs more
pitch when the air is less dense to be efficient..

Mike
MU-2

"Jay" wrote in message
om...
Thanks for your insight, which raise a few questions:

Apart from the geared Cessna (which isn't all that hi performance)
which aircraft have a PSRU AND a CS prop?

Which experimental aircraft has 2 speed gear boxes? I heard of a guy
flying a WW1 replical biplane on a Honda motocycle engine. He just
kept the original gear box and said he actually shifts gears depending
on if he's climbing or cruising.

My point about using an engine that can operate efficiently over a
large range of RPMs (like a modern automobile engine) is that the CS
prop is NOT as necessary although it certainly does help, no doubt
about it. Certainly you will get you peak horsepower at high revs,
but the moderm engine has a fatter torque curve due to being able to
change valve AND ignition timing in a manner optimum for the
particular revs it is at. The Lyco/Conti design takes a double hit
for operating at low revs, its off the peak HP point, and its timing
was peaked for a specific RPM.


"Bill Daniels" wrote in message
...
"Jay" wrote in message
om...
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.

Prop blades are just rotating wings. The goal is to run the blades at
their
most efficient angle of attack for the RPM and aircraft airspeed. The
performance of the prop is best at low RPM but the piston engine driving
it
is likely to be most efficient at a higher RPM. That is the reason that
high performance piston aircraft have both PRSU's and constant speed
props.

Some experimental powerplant/prop systems included a two speed gearbox
in
addition to the CS prop to run the engine at high RPM at takeoff and low
RPM
for cruise. These experimental engines also shifted the cam and
ignition
timing for the two PRSU ratios. This helped the prop blades stay at the
best AOA to maximize thrust and optimized the engine at two set points,
high
RPM for takeoff and low RPM for long range cruise. This was at the very
end
of large piston engine development and an attempt to wring the last bit
of
performance out of these monsters.

Having an engine with a wide "dynamic range" is nice for a car but less
useful for an airplane where it is best to optimize the engine for one
RPM
and let the CS prop and PRSU operate the prop in the most efficient way.

Bill Daniels

On 27 Feb 2004 09:28:04 -0800, (Jay) wrote:


My point about using an engine that can operate efficiently over a
large range of RPMs (like a modern automobile engine) is that the CS
prop is NOT as necessary although it certainly does help, no doubt
about it. Certainly you will get you peak horsepower at high revs,
but the moderm engine has a fatter torque curve due to being able to
change valve AND ignition timing in a manner optimum for the
particular revs it is at. The Lyco/Conti design takes a double hit
for operating at low revs, its off the peak HP point, and its timing
was peaked for a specific RPM.

Aircraft engines, even the direct drive ones, don't take as much a hit
on efficiency as you might think. They're designed to run at
relatively slow steady speed from the outset, unlike auto engines
which are designed for many rpm ranges. Just because aircraft engines
don't run like auto engines doesn't mean it's bad, or inefficient.

When you get to cruise power, and lean it way back, the aircraft
engine generally gets a better brake specific fuel consumption (BSFC)
than does the auto engine when configured to run as an aircraft
engine.

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

(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

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.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

Either that or a WEP setting (break a wire at full throttle) that
basically says to the microcontroller "Its now or never." Something
that indicates that there is a real possibility of loss of vehicle and
also disconnects the field current for the alternator.

(pacplyer) wrote in message . com...
If I had FADEC in a single-engine GA aircraft I
would want a non-software override.

pacplyer