A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Home Built
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Constant Speed Prop vs Variable Engine Timing



 
 
Thread Tools Display Modes
  #1  
Old February 26th 04, 10:04 PM
Jay
external usenet poster
 
Posts: n/a
Default 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.
  #2  
Old February 26th 04, 10:47 PM
Bill Daniels
external usenet poster
 
Posts: n/a
Default


"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

  #3  
Old February 27th 04, 06:28 PM
Jay
external usenet poster
 
Posts: n/a
Default

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

  #4  
Old February 27th 04, 06:46 PM
Ron Natalie
external usenet poster
 
Posts: n/a
Default


"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.

  #5  
Old February 27th 04, 08:25 PM
Mike Rapoport
external usenet poster
 
Posts: n/a
Default

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



  #6  
Old February 27th 04, 10:00 PM
Corky Scott
external usenet poster
 
Posts: n/a
Default

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
  #7  
Old February 27th 04, 11:45 PM
Dan Thomas
external usenet poster
 
Posts: n/a
Default

(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.



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?
So, we only have about 60 percent of the engine's power in the
takeoff. Worse yet, this diminished power is going into a propeller
that is largely in a stalled condition at the beginning of the takeoff
roll (because of high blade pitch angle and low forward speed) and is
producing much less than its max thrust as a result, and acceleration
is pretty dismal.
What can we gain by fooling with valve or ignition timing in a
situation like this? Not much. We add weight and failure points,
neither of which are welcome here, and gain very little in
performance.
So the constant-speed prop was invented. It is controlled by a
governor so that the engine is allowed to reach full rated RPM, which
produces full rated HP (if at sea level and standard temp), and
because the propeller's pitch is much lower in this mode, much more of
it is producing thrust instead of useless stall turbulence. In
cruising flight, the pitch increases to keep the engine RPM within
limits while still producing more thrust and a higher cruise speed
than a fixed-pitch prop can.
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.

Dan
  #8  
Old February 28th 04, 01:35 AM
Dale Alexander
external usenet poster
 
Posts: n/a
Default

This is directed more at the original poster,

Latest Kitplanes, March 2004 top of page 25 under heading of "Performance".
Comparisons of 6 constant speed props and one fixed pitch prop on a RV-8.
Speeds within 4% fastest to slowest, Constant speed props are very useful
for take-off and speed reduction to pattern, but counter-intuitively, not
much difference in top speed. Given that the piece that takes the bite out
of the air would be difficult to optimize more than it is, it's hard to
figure how current engine tweaking could do any better and I see all the
bells and whistles in New Model Training all the time for cars. There are
practical limits with what you can do with add-on doo-dads.

Dale Alexander
Velocity Stealth RG Gullwing
Toyota Master Tech
Mazda Master Tech
Been working on cars WAY too long...

"Dan Thomas" wrote in message
om...
(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.



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?
So, we only have about 60 percent of the engine's power in the
takeoff. Worse yet, this diminished power is going into a propeller
that is largely in a stalled condition at the beginning of the takeoff
roll (because of high blade pitch angle and low forward speed) and is
producing much less than its max thrust as a result, and acceleration
is pretty dismal.
What can we gain by fooling with valve or ignition timing in a
situation like this? Not much. We add weight and failure points,
neither of which are welcome here, and gain very little in
performance.
So the constant-speed prop was invented. It is controlled by a
governor so that the engine is allowed to reach full rated RPM, which
produces full rated HP (if at sea level and standard temp), and
because the propeller's pitch is much lower in this mode, much more of
it is producing thrust instead of useless stall turbulence. In
cruising flight, the pitch increases to keep the engine RPM within
limits while still producing more thrust and a higher cruise speed
than a fixed-pitch prop can.
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.

Dan



  #9  
Old February 28th 04, 04:00 AM
UltraJohn
external usenet poster
 
Posts: n/a
Default

Great expanation Thanks
John
PS my first post to a newsgroup (I'm a Virgin no more;-) )


Dan Thomas wrote:

(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.



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?
So, we only have about 60 percent of the engine's power in the
takeoff. Worse yet, this diminished power is going into a propeller
that is largely in a stalled condition at the beginning of the takeoff
roll (because of high blade pitch angle and low forward speed) and is
producing much less than its max thrust as a result, and acceleration
is pretty dismal.
What can we gain by fooling with valve or ignition timing in a
situation like this? Not much. We add weight and failure points,
neither of which are welcome here, and gain very little in
performance.
So the constant-speed prop was invented. It is controlled by a
governor so that the engine is allowed to reach full rated RPM, which
produces full rated HP (if at sea level and standard temp), and
because the propeller's pitch is much lower in this mode, much more of
it is producing thrust instead of useless stall turbulence. In
cruising flight, the pitch increases to keep the engine RPM within
limits while still producing more thrust and a higher cruise speed
than a fixed-pitch prop can.
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.

Dan


  #10  
Old February 28th 04, 05:43 AM
Morgans
external usenet poster
 
Posts: n/a
Default


"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.


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
at high speed and high altitude, so the engine can loaf along at really slow
and low HP output, to keep the thrust up, while at the low engine RPM'S.

Most of the successful auto conversions tend to keep it simple, and variable
valve timeing is not in that spirit. YMMV.
--
Jim in NC


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.594 / Virus Database: 377 - Release Date: 2/24/2004


 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
V-8 powered Seabee Corky Scott Home Built 212 October 2nd 04 11:45 PM
IVO props... comments.. Dave S Home Built 16 December 7th 03 12:43 AM
want variable pitch prop Ray Toews Home Built 5 October 7th 03 09:59 PM
Corky's engine choice Corky Scott Home Built 39 August 8th 03 04:29 AM
Gasflow of VW engine Veeduber Home Built 4 July 14th 03 08:06 AM


All times are GMT +1. The time now is 12:14 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AviationBanter.
The comments are property of their posters.