Constant Speed Prop vs Variable Engine Timing

"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

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

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

"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

Great Stuff Kevin, thanks for your insight. I have a couple of points
I slightly disagree with however further down in your post. :-)

Kevin Horton wrote:

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.

Yeah I mixed those up. Thanks for keeping me honest. Those were both
IMHO over-reliance in airbus automation accidents IIRC. I saw this
frequently with new-to-airbus co-pilots who would stare at the PFD
trying too figure out why the last button push on the FCP didn't do
anything. Instead of disconnecting everything and regaining control.

After you got yourself behind the power curve, however, for whatever
reason, I'm essentially talking about old-school guys like me who were
used to flying non-FADEC machines capable of "overboost." If you got
into trouble, because you were stupid, in say the previous generation
of Boeing products: You could always push up and call for power far
in excess of limiting max GA epr or N1, N2, EGT limits. (but maybe
that's because like you say: old eng's didn't operate so close to the
surge/stall margin.) It's unlikely the engines were going to fail
like a piston or super/turbo charged engine might. Those old buckets
would warp. The blades might creep and stretch and the engines might
have to be scrapped (at say 5 mill a copy.) But you had a better
chance of clearing the trees by going all way the to the mechanical
stops (physical wire to the FCU Hydr/Mech linkage) than you do now
with a Throttle resolver / PFM/MEC/ FADEC arrangement. The airbus
test pilot may think he's called for Jesus power, but FADEC will not
let him have it.
This may have saved me a couple of times in my career flying 60's gen
aircraft overseas. You smash everything to the wall and only slightly
pull back on the engines that are "barking." (compressor stalling.)
ATC would steer you into mountains in those days in some places.
(more war stories.)


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.

I haven't flown any FBW. But we had the predecessor AOA system on
the A310 which had a A/T "alpha floor" mode (Vls) which would not
allow you to command (not select) a speed slower than 1.2 Vso. Check
pilots would scare the **** out of themselves relying on this system,
and come back and rewrite the manual! This also lead to a bunch of
documented (AWST) vertical tailslides at third world airlines where a
little turbulence knocked the A/S below alpha floor for a second.
Throttles (sometimes asymmetrically) would in about six seconds from
(flight) idle reach G/A thrust: locked into what the french call
Thrust Latch mode: meaning if you disconnected A/T's and manually
retarded them, and let go, they would re-engauge themselves (without
your permission) and smoothly place you back up to full pwr again.
(New guys never noticed the uncommanded re-power up. They would
fixate on the airplane departing altitude and start ****ing around
with the pickle switch trim: which was active!) The auto pilot would
respect redline on flaps at all costs. It would pull the airframe up
into a 90 degree body angle and then stall the machine into an airshow
tailslide just like Art Shoal used to. Even if you disc the A/P on
the pull up it's too late. The machine has insuficient down elevator
authority now to arrest the pull up(cuz nugget ran the tailplane down
and "Auhhto" overshot it the back the other direction to get even;
there was no aural stabilizer-in-motion sound in a/p trim so nobody
noticed the comming set-up!) Great French design! Hang on Grandma!
We called these man vs. machine incidents/accidents.

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.

Were the engines in flight idle? Were you guys pulling the ground
sensor breaker? Ground idle takes longer. Older High Bypass designs
eg: the GE CF6-80 series only take about six seconds from flight idle
to reach GA thrust IIRC, but still cannot over boost. But it's more
like twelve seconds in profile mode (slow spool up looking at FMS
parameters.) So I remembered it wrong. I think he tried to change alt
with Level Change and Profile mode engaged first, and when nothing
much happened (norm) he smashed the thrust levers to the wall and ate
wood. But Kevin, I'll concede the acceleration argument to you.
(Older designs were even slower (something like fifteen seconds to
spool up (e.g. GE CF700's aft-fans.) You could bust altitudes
descending, if you didn't lead with the throttles a couple thousand
feet before level off.


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.

Yep, you're right. FADEC by itself didn't put him in the trees. But
most accidents have "a chain" of factors that cause the accident. If
you can break any one of the factorial links the accident would not
happen. I submit the inability to get over-boost power was just one
of those links. Another was a FBW AOA limit that cannot be
temporarily sacrificed to clear obstacles.

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.

Well I have to disagree with this. We train annually now to fly below
stick shaker to escape microburst wind shear ground contact on t/o.
We will go below stall speed (bugged) momentarily in ground effect
will full power to avoid contact. We don't care about airspeed. We
only look at V/S. If we didn't do this, some dry microbusts would
kill us. Risking a stall is always better than contact with hard
objects. (remember impact g-force energy goes up exponentially with
speed) (besides: most jets don't break fast, they burble and
pre-buffet a bit first. After a positive rate is obtained and we're
still alive, then we fly on intermittent stick shaker (way higher deck
angles than FD/AP AOA limits) until about 1000 ft AGL. AOA FBW
autopilots never fly at speeds this low to escape terrain to my
knowledge. But I'd have to ask an A320 driver to be sure. The other
thing that bugs me about that machine is not being able to bust into a
45 degree bank. (Another thread for that one.)

But keep in mind that if you'd advocated these advanced techniques
twenty years ago, they would've pulled your ticket. :-(

(but you'd still be alive.) :-) Now's its req FAA training. Note:
These techniques vary widely from airline to airline and change from
Chief Kahuna to Chief Kahuna. YMMV. For the record I think FADEC is
great. Do you want it in your GA airplane?

Cheers,

pacplyer

(Veeduber) wrote in message ...
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

It can ruin your whole day. ;-) Enjoyed your sheetmetal "air
drill" stuff last night. Thanks

pac

On Mon, 01 Mar 2004 17:52:00 -0800, pacplyer wrote:

Great Stuff Kevin, thanks for your insight. I have a couple of points I
slightly disagree with however further down in your post. :-)

Kevin Horton wrote:

After you got yourself behind the power curve, however, for whatever
reason, I'm essentially talking about old-school guys like me who were
used to flying non-FADEC machines capable of "overboost." If you got into
trouble, because you were stupid, in say the previous generation of Boeing
products: You could always push up and call for power far in excess of
limiting max GA epr or N1, N2, EGT limits. (but maybe that's because like
you say: old eng's didn't operate so close to the surge/stall margin.)
It's unlikely the engines were going to fail like a piston or super/turbo
charged engine might. Those old buckets would warp. The blades might
creep and stretch and the engines might have to be scrapped (at say 5 mill
a copy.) But you had a better chance of clearing the trees by going all
way the to the mechanical stops (physical wire to the FCU Hydr/Mech
linkage) than you do now with a Throttle resolver / PFM/MEC/ FADEC
arrangement. The airbus test pilot may think he's called for Jesus power,
but FADEC will not let him have it.
This may have saved me a couple of times in my career flying 60's gen
aircraft overseas. You smash everything to the wall and only slightly
pull back on the engines that are "barking." (compressor stalling.) ATC
would steer you into mountains in those days in some places. (more war
stories.)

OK. I misunderstood your gripe against FADECs in the first message. I
too am not happy to have a FADEC limit how much thrust I get out of the
engine. I would much rather have some way to override it if the s*** hits
the fan. But, I don't think this would have made any difference in the
Habsheim accident. All published reports have said either that the
engines didn't respond at all, or that they were still spooling up when he
hit the trees. The only report I can find that actually quotes an N1 says
the engines were at 83% N1, which must be well below TOGA, so the engines
would have still been accelerating, and it wouldn't have mattered what rpm
was commanded.

There was a bit on an internal bun fight at Bombardier when the CRJ-700
was being designed. It has a FADEC engine, and the flight test folks were
not happy about the inability to get more thrust if needed. The engine
does have an Automatic Power Reserve (APR) that commands a thrust bump if
you have an engine failure. The powerplants engineers were persuaded to
add a heavy detent that you can push the thrust levers through to allow
you to get APR thrust with both engines running if you really need it.

The Bombardier Global Express also has a FADEC engine, but there are two
little switches behind the thrust levers that allow the crew to manually
select a back up N1 control mode. If the engine is in N1 control mode,
you are no longer limited except by the overspeed limiter, which allows
you to get much more thrust if needed.




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.

Were the engines in flight idle? Were you guys pulling the ground sensor
breaker? Ground idle takes longer. Older High Bypass designs eg: the GE
CF6-80 series only take about six seconds from flight idle to reach GA
thrust IIRC, but still cannot over boost. But it's more like twelve
seconds in profile mode (slow spool up looking at FMS parameters.) So I
remembered it wrong. I think he tried to change alt with Level Change and
Profile mode engaged first, and when nothing much happened (norm) he
smashed the thrust levers to the wall and ate wood. But Kevin, I'll
concede the acceleration argument to you. (Older designs were even slower
(something like fifteen seconds to spool up (e.g. GE CF700's aft-fans.)
You could bust altitudes descending, if you didn't lead with the throttles
a couple thousand feet before level off.

The 8 second requirement is for an acceleration from flight idle. There
is typically some worst case condition (specific bleed configuration,
altitude and temperature) where the engine will be close to 8 seconds, and
it will be a bit better at other conditions. So I am not surprised if you
saw about 6 seconds in many cases.



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.

Yep, you're right. FADEC by itself didn't put him in the trees. But most
accidents have "a chain" of factors that cause the accident. If you can
break any one of the factorial links the accident would not happen. I
submit the inability to get over-boost power was just one of those links.
Another was a FBW AOA limit that cannot be temporarily sacrificed to clear
obstacles.

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.

Well I have to disagree with this. We train annually now to fly below
stick shaker to escape microburst wind shear ground contact on t/o. We
will go below stall speed (bugged) momentarily in ground effect will full
power to avoid contact. We don't care about airspeed. We only look at
V/S. If we didn't do this, some dry microbusts would kill us. Risking a
stall is always better than contact with hard objects. (remember impact
g-force energy goes up exponentially with speed) (besides: most jets
don't break fast, they burble and pre-buffet a bit first. After a
positive rate is obtained and we're still alive, then we fly on
intermittent stick shaker (way higher deck angles than FD/AP AOA limits)
until about 1000 ft AGL. AOA FBW autopilots never fly at speeds this low
to escape terrain to my knowledge. But I'd have to ask an A320 driver to
be sure. The other thing that bugs me about that machine is not being
able to bust into a 45 degree bank. (Another thread for that one.)

Well, the AOA limiter an the Airbus's is set very close to the stall. It
is well beyond where a stick shaker would be. The curve of lift vs AOA
tends to have a fairly flat top with modern swept wing jets, so once you
get up on top of that curve there isn't any benefit to pulling more AOA,
as you don't get any more lift. I wish there was some way to get in a FBW
Airbus sim with you. We could do two windshear recoveries - one using
full aft stick riding on the AOA limiter, and one in Direct Law, with no
AOA limiter. I'm convinced you would do better just using the AOA limiter.

For the record I think FADEC is great. Do you want it in your GA airplane?

Well, I'm a suspicious type, and I want to see some more service history
first to assure myself that they've sorted all the bugs out. So not on
my RV-8 project, but maybe on the next one.

--
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

Kevin Horton wrote in message ...
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.


Yes, I knew that about the supercharger gearing to allow different
settings at altitude, but the poster I was questioning had discussed
(or seemed to hint at) a two-speed propeller drive; in other words, a
transmission. I had never heard of it, outside of Jim Bede's
belt-driven variable-ratio system in the early BD-5.
The only propeller gearing I've ever seen is a fixed reduction as
used in many larger radials, all the V-12s except very early ones, and
many opposed engines such as the Continental GO-300 (Cessna 175),
Lyc's GO-480 (Helio), and GTSIO-540 (Cessna 414 or 421?), and
Continental's Tiara engine that never reached significant production.
And, of course, all turboprop, turbofan and turboshaft engines. All
with a fixed ratio, single reduction. And all to allow the engine to
develop high RPM and therefore higher HP, while allowing a larger,
slower prop to operate in an efficient range.
It seems to me that turning a prop faster in cruise flight is
self-defeating, since drag rises as tip speeds rise with forward speed
factored into the operation. It's why airplanes with big props like
the Dash 8 turn at around 1300 for takeoff and 850 or so in cruise,
and why variable pitch of the blade is absolutely necessary.
Dan