Constant Speed Prop vs Variable Engine Timing

(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 01 Mar 2004 14:03:35 GMT, (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

Anything that causes the engine to produce more power also causes it
to produce more heat. If the cooling system cannot efficiently rid
itself of that extra heat, you could be looking at detonation, or even
pre-ignition. You don't have to worry about pre-ignition for very
long because just a little of that and the engine is history.

The thing that to me makes the most sense to me would be to have an
engine/prop combination that works well for your application, and pay
attention to gross weight/high density altitude situations.

Corky Scott

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

"AnyBody43" wrote in message
om...
(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.

Not quite. Gears don't have preferred operating conditions, props do.

The engine has its preferred RPM and torque for optimum efficiency and the
prop blades have their optimum angle of attack. If the engine/prop
combination results in the prop operating at a higher (or lower) angle of
attack than optimum to absorb the torque of the engine (Prop governor
increases pitch to hold RPM setting.) then the combination operates below
optimum conditions.

Under some conditions, it would make sense to introduce a third variable
i.e. a gearbox between the engine and prop, to allow both the engine and
prop to operate at peak efficiency. This was the reason that two-speed
grearsets were installed in the nose case of some large radials. This, in
turn, allowed the propeller designer to optimize his prop blades for a
single AOA, thus gaining still more efficiency.

The problem, simply stated was this: How does a heavily loaded, long-range
bomber haul itself off a short runway and climb to cruise altitude and then
shift to highly efficient, long-range cruise. The answer was just emerging
from the labs as the world shifted to turbines. The flight engineer would
shift his engines into a "hole gear" by selecting a cam profile and engine
timing optimized for the low gear that would let the engines scream at high
RPM and pump massive HP into props set for maximum acceleration and climb.
Once in cruise, the engineer would shift his engines back to low RPM, high
efficiency settings.

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

"Bill Daniels" wrote in message news:5_- 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.

Not quite. Gears don't have preferred operating conditions, props do.

The engine has its preferred RPM and torque for optimum efficiency and the
prop blades have their optimum angle of attack. If the engine/prop
combination results in the prop operating at a higher (or lower) angle of
attack than optimum to absorb the torque of the engine (Prop governor
increases pitch to hold RPM setting.) then the combination operates below
optimum conditions.

Under some conditions, it would make sense to introduce a third variable
i.e. a gearbox between the engine and prop, to allow both the engine and
prop to operate at peak efficiency. This was the reason that two-speed
grearsets were installed in the nose case of some large radials. This, in
turn, allowed the propeller designer to optimize his prop blades for a
single AOA, thus gaining still more efficiency.

The problem, simply stated was this: How does a heavily loaded, long-range
bomber haul itself off a short runway and climb to cruise altitude and then
shift to highly efficient, long-range cruise. The answer was just emerging
from the labs as the world shifted to turbines. The flight engineer would
shift his engines into a "hole gear" by selecting a cam profile and engine
timing optimized for the low gear that would let the engines scream at high
RPM and pump massive HP into props set for maximum acceleration and climb.
Once in cruise, the engineer would shift his engines back to low RPM, high
efficiency settings.

First time I've ever heard of gear-shifted props in certified
engines. Which engines were these? I know that many radials (and other
engine layouts) used reduction gearing in the case nose to allow the
engine to run faster and produce more HP while keeping the prop within
safe limits, and that there were two-speed geared superchargers on
many of these engines, but two-speed props?
Jim Bede used a snowmobile-type propshaft drive in the early
BD-5s but abandoned it as unworkable. It still required a relatively
tiny prop to keep the tip speeds subsonic.
As far as the propeller pitch angles go, the constant speed prop
improves takeoff performance by more than just letting engine RPM
reach redline to produce max HP. It reduces the angle of attack so
that more of the prop is unstalled and producing thrust in the static
condition, improving acceleration and shortening takeoff distance. The
inboard sections of a fixed-pitch prop blade have a large angle so
that they still produce thrust in faster forward flight even though
they don't travel the circumferential distance that blade areas near
the tips do, but the large angle means a stalled blade, or at least a
really turbulent flow, at low forward speeds. A gear-shifted
fixed-pitch prop will still have those problems.

Dan

On Mon, 01 Mar 2004 12:38:15 -0800, Dan Thomas wrote:

"Bill Daniels" wrote in message news:5_- 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.

Not quite. Gears don't have preferred operating conditions, props do.

The engine has its preferred RPM and torque for optimum efficiency and
the prop blades have their optimum angle of attack. If the engine/prop
combination results in the prop operating at a higher (or lower) angle
of attack than optimum to absorb the torque of the engine (Prop
governor increases pitch to hold RPM setting.) then the combination
operates below optimum conditions.

Under some conditions, it would make sense to introduce a third
variable i.e. a gearbox between the engine and prop, to allow both the
engine and prop to operate at peak efficiency. This was the reason
that two-speed grearsets were installed in the nose case of some large
radials. This, in turn, allowed the propeller designer to optimize his
prop blades for a single AOA, thus gaining still more efficiency.

The problem, simply stated was this: How does a heavily loaded,
long-range bomber haul itself off a short runway and climb to cruise
altitude and then shift to highly efficient, long-range cruise. The
answer was just emerging from the labs as the world shifted to
turbines.
The flight engineer would shift his engines into a "hole gear" by
selecting a cam profile and engine timing optimized for the low gear
that would let the engines scream at high RPM and pump massive HP into
props set for maximum acceleration and climb. Once in cruise, the
engineer would shift his engines back to low RPM, high efficiency
settings.

First time I've ever heard of gear-shifted props in certified
engines. Which engines were these? I know that many radials (and other
engine layouts) used reduction gearing in the case nose to allow the
engine to run faster and produce more HP while keeping the prop within
safe limits, and that there were two-speed geared superchargers on many
of these engines, but two-speed props?
Jim Bede used a snowmobile-type propshaft drive in the early
BD-5s but abandoned it as unworkable. It still required a relatively
tiny prop to keep the tip speeds subsonic.
As far as the propeller pitch angles go, the constant speed prop
improves takeoff performance by more than just letting engine RPM reach
redline to produce max HP. It reduces the angle of attack so that more
of the prop is unstalled and producing thrust in the static condition,
improving acceleration and shortening takeoff distance. The inboard
sections of a fixed-pitch prop blade have a large angle so that they
still produce thrust in faster forward flight even though they don't
travel the circumferential distance that blade areas near the tips do,
but the large angle means a stalled blade, or at least a really
turbulent flow, at low forward speeds. A gear-shifted fixed-pitch prop
will still have those problems.

Dan

Some of the supercharged recips had a gear box with two different gear
ratios to drive the supercharger. They needed to spin the supercharger at
high rpm at high altitude in order to get enough manifold pressure. But
if they used the same supercharger gear ratio at low altitude it would
produce more manifold pressure than the engine could handle at full
throttle. The engine would then have to be run very throttled, and there
would be a lot of wasted power used to spin that supercharger at a
needlessly high rpm. So, they used a different gear ratio to spin the
supercharger at a lower rpm for take-off and low altitude flight.

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

"Dan Thomas" wrote in message
om...
"Bill Daniels" wrote in message news:5_- 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.

Not quite. Gears don't have preferred operating conditions, props do.

The engine has its preferred RPM and torque for optimum efficiency and
the
prop blades have their optimum angle of attack. If the engine/prop
combination results in the prop operating at a higher (or lower) angle
of
attack than optimum to absorb the torque of the engine (Prop governor
increases pitch to hold RPM setting.) then the combination operates
below
optimum conditions.

Under some conditions, it would make sense to introduce a third variable
i.e. a gearbox between the engine and prop, to allow both the engine and
prop to operate at peak efficiency. This was the reason that two-speed
grearsets were installed in the nose case of some large radials. This,
in
turn, allowed the propeller designer to optimize his prop blades for a
single AOA, thus gaining still more efficiency.

The problem, simply stated was this: How does a heavily loaded,
long-range
bomber haul itself off a short runway and climb to cruise altitude and
then
shift to highly efficient, long-range cruise. The answer was just
emerging
from the labs as the world shifted to turbines. The flight engineer
would
shift his engines into a "hole gear" by selecting a cam profile and
engine
timing optimized for the low gear that would let the engines scream at
high
RPM and pump massive HP into props set for maximum acceleration and
climb.
Once in cruise, the engineer would shift his engines back to low RPM,
high
efficiency settings.

First time I've ever heard of gear-shifted props in certified
engines. Which engines were these? I know that many radials (and other
engine layouts) used reduction gearing in the case nose to allow the
engine to run faster and produce more HP while keeping the prop within
safe limits, and that there were two-speed geared superchargers on
many of these engines, but two-speed props?
Jim Bede used a snowmobile-type propshaft drive in the early
BD-5s but abandoned it as unworkable. It still required a relatively
tiny prop to keep the tip speeds subsonic.
As far as the propeller pitch angles go, the constant speed prop
improves takeoff performance by more than just letting engine RPM
reach redline to produce max HP. It reduces the angle of attack so
that more of the prop is unstalled and producing thrust in the static
condition, improving acceleration and shortening takeoff distance. The
inboard sections of a fixed-pitch prop blade have a large angle so
that they still produce thrust in faster forward flight even though
they don't travel the circumferential distance that blade areas near
the tips do, but the large angle means a stalled blade, or at least a
really turbulent flow, at low forward speeds. A gear-shifted
fixed-pitch prop will still have those problems.

Dan

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

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

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.