Constant Speed Prop vs Variable Engine Timing

On Tue, 2 Mar 2004 07:32:33 -0700, "Bill Daniels"
wrote:

The gear shifted prop was the last gasp of piston engine development before
the turbine age. Look at the Lycoming XR7755, Napier Nomad or the Rolls
Royce Crecy. These were 5000 HP+ monsters that needed every trick in the
engineers bag. Piston engines produce more HP at high RPM at the cost of
fuel consumption but deliver low fuel consumption at low RPMS. Props
produce more thrust at low RPM and most efficiency with the blades at a
single best AOA. That AOA must be maintained over a wide range of
airspeeds. Just too many variables for a CS prop to deal with alone.

The two speed gearbox isn't perfect but it does buy the engineer a bigger
range of options.

Bill Daniels

Bill, are you talking about a two position propeller, as opposed to a
two position reduction drive transmission for the engine?

Early in WWII, some props used a variable pitch mechanism that allowed
the pilot to adjust the pitch, and therefore the engine rpm to any
setting withing the design limits of the prop's pitch. Normally
they'd select fine pitch for takeoff and coarse pitch for cruise. I
suppose some of the props had just those two settings, but most of
them allowed any setting inbetween.

This wasn't a constant speed prop, just an adjustable prop.

Is this what you are referring to?

Thanks, Corky Scott

PS, it isn't a bygone design, there are some adjustable props on the
market for the homebuilt industry today that allow variable pitch, but
are not constant speed props.

This is kind of a mish mash for a number of posts to this thread.

1. The Merlin (In P-51) had a 2/1 reduction gear. At full throttle
engine turned 3000 rpm and the big prop only turned 1500 rpm.

2. Merlin had a two stage blower. Low blower was set so at 'gate' you
could pull 61 inches at sea level. There was a spring loaded switch
that you could check high blower prior to T/O. The high blower was
controlled by a aneroid and it automatically shifted to high blower
between 12-14K (not a precise altitude). If you were in formation and
the lead bird shifted to high blower and your bird hadn't yet, we had
a fix for that problem. The mech would take a length of safety wire
and loop it under the spring loaded switch and thread it up behind the
safety guard over switch. Then to manually switch to high blower to
stay in formation you would grab the ends of the safety wire and
manually lift the switch to shift to high blower and at the same time
just wrap the safety wire around the safety cover over switch. Of
course after your engine had shifted to high blower automatically, you
could unwind the wire and let switch go back to the automatic position
so you wouldn't inadvertently over boost the engine during descent.

3. I also on a number of missions, flew with the prop pitch pulled
full back (high pitch) and full throttle. (all within the allowable
BMEP). Airspeed was about 140-150 mph under 500 feet where we were
flying. About every 30 minutes Merlin would get rough with that power
setting and we would have to clean the engine out. First pull the
throttle back and then start the prop lever forward (toward flat
pitch). That 'old' Merlin would buck and spit and shake and blow
black balls of smoke out of stacks and cut out and you would have to
stop and let if clean itself out a little at which time you could push
the prop some more forward again. If would take a minute or two to get
the engine to take full throttle at max rpm and you then ran full
throttle for 2-3 minutes to clean things out and then you pulled back
to the low rpm again. At the low rpm the MP was self limiting and full
throttle only gave you 15 or so inches. In this mode the prop was
turning so slow that you could see the blades and count them as they
went by.

All this is just an aside on engine operation with the Merlin in a
time and land far away (sure beats a 65 Cont G).

Big John


On 26 Feb 2004 13:04:46 -0800, (Jay) wrote:

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.

"Corky Scott" wrote in message
...
On Tue, 2 Mar 2004 07:32:33 -0700, "Bill Daniels"
wrote:

The gear shifted prop was the last gasp of piston engine development
before
the turbine age. Look at the Lycoming XR7755, Napier Nomad or the Rolls
Royce Crecy. These were 5000 HP+ monsters that needed every trick in the
engineers bag. Piston engines produce more HP at high RPM at the cost of
fuel consumption but deliver low fuel consumption at low RPMS. Props
produce more thrust at low RPM and most efficiency with the blades at a
single best AOA. That AOA must be maintained over a wide range of
airspeeds. Just too many variables for a CS prop to deal with alone.

The two speed gearbox isn't perfect but it does buy the engineer a
bigger
range of options.

Bill Daniels

Bill, are you talking about a two position propeller, as opposed to a
two position reduction drive transmission for the engine?

Early in WWII, some props used a variable pitch mechanism that allowed
the pilot to adjust the pitch, and therefore the engine rpm to any
setting withing the design limits of the prop's pitch. Normally
they'd select fine pitch for takeoff and coarse pitch for cruise. I
suppose some of the props had just those two settings, but most of
them allowed any setting inbetween.

This wasn't a constant speed prop, just an adjustable prop.

Is this what you are referring to?

Thanks, Corky Scott

PS, it isn't a bygone design, there are some adjustable props on the
market for the homebuilt industry today that allow variable pitch, but
are not constant speed props.


No, I'm really talking about a two-speed gearbox that lets the engine rev
higher for a given prop rpm during takeoff and initial climb. The system,
as I understand it was as follows:

|-----------------|
| Engine ||Two speed planetary gear box||Constant Speed
Prop|
|-----------------|

The engine had cams that could move axially to use two different cam
profiles. One for high-RPM power and another for low-RPM cruise. The
ignition timing was adjustable at the same time.

The gearbox was a planetary not unlike the old Chevy two speed automatics
but without the torque converter. The gear change was accomplished by bands
that braked one part or another of the planetary set. To continue the
analogy of the old Chevy, the prop takes the place of the torque converter.

The prop was designed to absorb the extra torque at takeoff with the engine
screaming in low gear and to turn slowly, about 800 RPM, at cruise. It is
just an attempt to address the issue of the prop being off its design point
in certain flight regimes. Basically you want the prop at its maximum
Thrust/Power at TO and its best Thrust/Drag at cruise.

If the prop is used alone to control engine RPM, then it will not always be
at those best design points. You may know that a Mooney or Bonanza has
"sweet spots" where the range is noticeably better. In the old Mooney, 1900
RPM and 22-25 inches MP gave much better range than running it "square". It
would have been better if the engine turned a bit faster but that would have
put the prop at a lower pitch (think wing AOA) than optimum for the blade
geometry and airfoils.

Bill Daniels
|

The Aerosance FADEC system is currently available for both TCM and
Lycoming engines. It DOES NOT require a "spool-up". In fact the piston
engine deserves more credit than the FADEC with respect to this
characteristic. It has virtually no lag in response to power commands.
The way FADEC is implemented also helps. It responds to throttle valve
movement by sensing air pressure changes. For more info see
www.fadec.com.

Steve

(pacplyer) wrote in message . com...
(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

BllFs6 wrote:
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

Only up to a point. If you put in too much pitch, the prop will start
the takeoff roll in a stalled situation. The prop will be pushing air
around in a circle instead of back. The extreme case is flat paddles
that will push no air backwards at all.

Many people actually do this on purpose as a way to exchange more top
speed when they have enough power to accelerate quickly after wasting
half the runway with a stall prop.

--
http://www.ernest.isa-geek.org/
"Ignorance is mankinds normal state,
alleviated by information and experience."
Veeduber

On Mon, 1 Mar 2004 08:25:38 -0700, "Bill Daniels"
wrote:


A propeller is not a gear box analog. It is more like the torque converter
in an automatic transmission. A torque converter still needs a gearbox
behind it for efficiency.

Bill Daniels

Sounds like a CVT to me. Of course, I don't know nothing about props.
--
dillon

Life is always short, but only you can make it sweet

On Tue, 02 Mar 2004 07:32:33 -0700, Bill Daniels wrote:


The gear shifted prop was the last gasp of piston engine development
before the turbine age. Look at the Lycoming XR7755, Napier Nomad or the
Rolls Royce Crecy. These were 5000 HP+ monsters that needed every trick
in the engineers bag. Piston engines produce more HP at high RPM at the
cost of fuel consumption but deliver low fuel consumption at low RPMS.
Props produce more thrust at low RPM and most efficiency with the blades
at a single best AOA. That AOA must be maintained over a wide range of
airspeeds. Just too many variables for a CS prop to deal with alone.

The two speed gearbox isn't perfect but it does buy the engineer a bigger
range of options.

Bill Daniels


Bill,

Thanks for pointing out these fascinating engines. I had heard of all of
them, but had never really looked into the details before.

The Lycoming XR-7755 certainly was a huge, complicated monster.

http://www.aviation-history.com/engines/xr-7755.html
http://www.people.virginia.edu/~rjr/engines/

The Napier Nomad was a bizarre combination of two-stroke diesel and gas
turbine. It managed a very impressive specific fuel consumption of 0.345
lb/ehp/hr. The only reference I can find to a gear box was a variable
ratio gearbox between the gas turbine and the piston crankshaft of the
Nomad 2. Not exactly what the original poster was referring too, but
interesting non-the-less.

http://www.home.aone.net.au/shack_one/nomad.htm
http://www.yourencyclopedia.net/Napier_Nomad

The Rolls-Royce Crecy was a highly supercharged diesel, that supposedly
produced about 5,000 on the test stand. I can't find any reference to a
two-speed gear box between the engine and the prop either on-line, or in
Aero Engines, Bill Gunston, but neither have I found a detailed technical
description of the engine. So perhaps that detail was left out.

http://www.stobbe.dk/technical_literature/combustion_engines/rolls-royce/Rolls-aircraft.html

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

Bill Daniels wrote:

"Corky Scott" wrote in message
...
On Tue, 2 Mar 2004 07:32:33 -0700, "Bill Daniels"
wrote:

The gear shifted prop was the last gasp of piston engine development
before
the turbine age. Look at the Lycoming XR7755, Napier Nomad or the Rolls
Royce Crecy. These were 5000 HP+ monsters that needed every trick in the
engineers bag. Piston engines produce more HP at high RPM at the cost of
fuel consumption but deliver low fuel consumption at low RPMS. Props
produce more thrust at low RPM and most efficiency with the blades at a
single best AOA. That AOA must be maintained over a wide range of
airspeeds. Just too many variables for a CS prop to deal with alone.

The two speed gearbox isn't perfect but it does buy the engineer a
bigger
range of options.

Bill Daniels

Bill, are you talking about a two position propeller, as opposed to a
two position reduction drive transmission for the engine?

Early in WWII, some props used a variable pitch mechanism that allowed
the pilot to adjust the pitch, and therefore the engine rpm to any
setting withing the design limits of the prop's pitch. Normally
they'd select fine pitch for takeoff and coarse pitch for cruise. I
suppose some of the props had just those two settings, but most of
them allowed any setting inbetween.

This wasn't a constant speed prop, just an adjustable prop.

Is this what you are referring to?

Thanks, Corky Scott

PS, it isn't a bygone design, there are some adjustable props on the
market for the homebuilt industry today that allow variable pitch, but
are not constant speed props.


No, I'm really talking about a two-speed gearbox that lets the engine rev
higher for a given prop rpm during takeoff and initial climb. The system,
as I understand it was as follows:

|-----------------|
| Engine ||Two speed planetary gear box||Constant Speed
Prop|
|-----------------|

The engine had cams that could move axially to use two different cam
profiles. One for high-RPM power and another for low-RPM cruise. The
ignition timing was adjustable at the same time.

The gearbox was a planetary not unlike the old Chevy two speed automatics
but without the torque converter. The gear change was accomplished by bands
that braked one part or another of the planetary set. To continue the
analogy of the old Chevy, the prop takes the place of the torque converter.

The prop was designed to absorb the extra torque at takeoff with the engine
screaming in low gear and to turn slowly, about 800 RPM, at cruise. It is
just an attempt to address the issue of the prop being off its design point
in certain flight regimes. Basically you want the prop at its maximum
Thrust/Power at TO and its best Thrust/Drag at cruise.

If the prop is used alone to control engine RPM, then it will not always be
at those best design points. You may know that a Mooney or Bonanza has
"sweet spots" where the range is noticeably better. In the old Mooney, 1900
RPM and 22-25 inches MP gave much better range than running it "square". It
would have been better if the engine turned a bit faster but that would have
put the prop at a lower pitch (think wing AOA) than optimum for the blade
geometry and airfoils.

Bill Daniels
|

I vaguely misremember some thing like that cam shifting and stuff.

But I don't think anything ever came of it.

I'm holding out for software controlled valves.

Look Ma, No Cams!

So what did you do if your plane shifted to high blower but the lead didn't? Pull it back real quick I suppose...

I like that 15" and (maybe) 500 rpm - really lugging it...

--
Dan D.



..
"Big John" wrote in message ...
This is kind of a mish mash for a number of posts to this thread.

1. The Merlin (In P-51) had a 2/1 reduction gear. At full throttle
engine turned 3000 rpm and the big prop only turned 1500 rpm.

2. Merlin had a two stage blower. Low blower was set so at 'gate' you
could pull 61 inches at sea level. There was a spring loaded switch
that you could check high blower prior to T/O. The high blower was
controlled by a aneroid and it automatically shifted to high blower
between 12-14K (not a precise altitude). If you were in formation and
the lead bird shifted to high blower and your bird hadn't yet, we had
a fix for that problem. The mech would take a length of safety wire
and loop it under the spring loaded switch and thread it up behind the
safety guard over switch. Then to manually switch to high blower to
stay in formation you would grab the ends of the safety wire and
manually lift the switch to shift to high blower and at the same time
just wrap the safety wire around the safety cover over switch. Of
course after your engine had shifted to high blower automatically, you
could unwind the wire and let switch go back to the automatic position
so you wouldn't inadvertently over boost the engine during descent.

3. I also on a number of missions, flew with the prop pitch pulled
full back (high pitch) and full throttle. (all within the allowable
BMEP). Airspeed was about 140-150 mph under 500 feet where we were
flying. About every 30 minutes Merlin would get rough with that power
setting and we would have to clean the engine out. First pull the
throttle back and then start the prop lever forward (toward flat
pitch). That 'old' Merlin would buck and spit and shake and blow
black balls of smoke out of stacks and cut out and you would have to
stop and let if clean itself out a little at which time you could push
the prop some more forward again. If would take a minute or two to get
the engine to take full throttle at max rpm and you then ran full
throttle for 2-3 minutes to clean things out and then you pulled back
to the low rpm again. At the low rpm the MP was self limiting and full
throttle only gave you 15 or so inches. In this mode the prop was
turning so slow that you could see the blades and count them as they
went by.

All this is just an aside on engine operation with the Merlin in a
time and land far away (sure beats a 65 Cont G).

Big John


On 26 Feb 2004 13:04:46 -0800, (Jay) wrote:

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.

Dan

Yep. Not a problem it you shifted first to high blower. You just
adjusted your power to stay in formation. You had more power than lead
and so just tucked it in and went on with mission.

Engine did not lug. We ran the figures on the BMEP and well within
acceptable range. Was not stressing the engine at all.

Big John

On Wed, 03 Mar 2004 01:17:54 GMT, "Blueskies" wrote:

So what did you do if your plane shifted to high blower but the lead didn't? Pull it back real quick I suppose...

I like that 15" and (maybe) 500 rpm - really lugging it...