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Rich,

Thanks for the response, You sound like an instructor!.

Attached at the end is your email, please post it for all to read. I did not explain what I wanted to do clearly in my previous email.

What I might be getting confused is what I am reading on the McCauley site about Prop Governors and Feathering Props, ( See following)

From what I can understand that they are saying is that a full feathering prop. will go to full feather / course pitch / low RPM / low resistance with a loss of oil pressure. (In the case of a Twin very important) And if you want to spin the prop to re-start you need an accumulator to apply pressure to the Piston to make it fine pitch and get the Prop. it spinning. (That I clearly understand).

What is confusing is that, I think they are saying a Constant Speed Prop. (Non Feathering) will go to fine pitch / high RPM / high resistance with a loss of oil pressure, which means it works the opposite way. So therefore I figure in the case of a Constant Speed Non Feathering Prop. why not use an accumulator when you have a loss of oil pressure to apply pressure to the Piston to put the Prop. into course pitch / low RPM / low resistance mode? So the question is: is the Hartzell that is made for the Legacy a Full Feathering Prop or a Non Feathering Constant Speed Prop?.

When I flew Lancair's Legacy for training, I found that it came down way too fast for my taste during an engine out exercise (High Key/Low Key) with the prop in fine pitch. Not something I would want to deal with if I can avoid. Yet when the prop was placed in course pitch it came down like a Cherokee, OK maybe a little faster, but very controllable.

Thanks

Kevin Kossi

New York

Legacy 70% IO-550-EXP

http://www.mccauley.textron.com/pro-sup/prosupframeset.html

VARIABLE PITCH PROPELLERS

Full-Feathering vs. Constant Speed

A constant-speed (RPM) system permits the pilot to select the propeller and engine speed for any situation and automatically maintain that RPM under varying conditions of aircraft attitude and engine power. Thereby permitting operation of propeller and engine at most efficient RPMs. RPM is controlled by varying the pitch of the propeller blades - that is, the angle of the blades with relation to the plane of rotation. When the pilot increases power in flight, the blade angle is increased, the torque required to spin the propeller is increased and, for any given RPM setting, aircraft speed and torque on the engine will increase. For economy cruising, the pilot can throttle back to the desired manifold pressure for cruise conditions and decrease the pitch of the propeller, while maintaining the pilot-selected RPM.

A full-feathering propeller system is normally used only on twin-engine aircraft. If one of the engines fails in flight, the propeller on the idle engine can rotate or ³windmill,² causing increased drag. To prevent this, the propeller can be ³feathered² (turned to a very high pitch), with the blades almost parallel to the airstream. This eliminates asymmetric drag forces caused by windmilling when an engine is shut down. A propeller that can be pitched to this position is called a full-feathering propeller.

Changing Pitch

Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.

In a single-acting propeller system, oil pressure supplied by the governor, acting on the piston produces a force that is opposed by the natural centrifugal twisting moment of the blades in constant speed models or counterweights and large springs in full-feathering systems. To increase or decrease the pitch, high pressure oil is directed to the propeller, which moves the piston back. The motion of the piston is transmitted to the blades through actuating pins and links, moving the blades toward either high pitch for constant-speed systems or low pitch for full-feathering systems. (Figs. 1A & 1B)


When the opposing forces are equal, oil flow to the propeller stops and the piston also stops. The piston will remain in this position, maintaining the pitch of the blades until oil flow to or from the propeller is again established by the governor. (Figs. 2A & 2B)

From this position, pitch is decreased for constant-speed systems or increased for full- feathering systems by allowing oil to flow out of the propeller and return to the engine sump. (Figs. 3A & 3B) When the governor initiates this procedure, hydraulic pressure is decreased and the piston moves forward, changing the pitch of the blades. The piston will continue to move forward until the opposing forces are once again equal. Mechanical stops are installed in the propeller to limit travel in both the high and low pitch directions.

Full-Feathering and Constant-Speed Governing Systems

Besides the propeller, the other major component of the system is the governor. Each governor mounts on and is geared to the engine, which drives the governor gear pump and the flyweight assembly. The gear pump boosts engine oil pressure to provide quick and positive response by the propeller. The rotational speed of the flyweight assembly varies directly with engine speed and controls the position of the pilot valve. Depending on its position, the pilot valve will direct oil flow to the propeller, allow oil flow back from the propeller, or assume a neutral position with no oil flow. These oil flow conditions correspond to increasing pitch, decreasing pitch or constant pitch of the propeller blades. (Figs. 4A & 4B)

The flyweights change the position of the pilot valve by utilizing centrifugal force. The L-shaped flyweights are installed with their lower legs projecting under a bearing on the pilot valve. When engine RPM is slower than the propeller control setting, the speeder spring holds the pilot valve down and oil flows to the propeller in a full-feathering system and from the propeller in a constant-speed system. (Fig. 5) As engine RPM increases, the tops of the weights are thrown outward by centrifugal force. The lower legs then pivot up, raising the pilot valve against the force of the speeder spring so no oil can flow to or from the propeller. (Fig. 6) The faster the flyweights spin, the further out they are thrown, causing the pilot valve to be raised and allowing more oil to flow from the propeller in a full-feathering system and to the propeller in a constant-speed system. (Fig. 7)

The cockpit control lever is connected to the governor control lever which in turn is attached to a threaded shaft. As the lever is moved, the threaded shaft turns and moves up or down to increase or decrease compression on the speeder spring. (Fig. 8) For example, when the cockpit control is moved forward, the governor control shaft is screwed down, increasing compression on the spring. This increases the speed necessary for the flyweights to move the pilot valve and produces a higher RPM setting. The cockpit control lever allows the aircraft pilot to shift the range of governor operation from high RPM to low RPM or any area in between.

This system results in constant speed by producing what is known as an ON SPEED condition, which exists when the RPM is constant. Movement of the cockpit controls have set the speeder springs at the desired RPM. The flyweights have positioned the pilot valves to direct oil to or from the propellers. This, in turn, has positioned the propeller blades at a pitch that absorbs the engine power or RPM selected. When the moment of RPM balance occurs, the force of the flyweights equals the speeder spring load. This positions the pilot valves in the constant RPM position with no oil flowing to or from the propellers. (Figs. 9A & 9B)

At constant-speed, an OVERSPEED condition results and airspeed increases when the airplane begins a descent or engine power is increased. Since the pitch of the propeller blades is too low to absorb engine power, the engine RPM begins to increase. At the instant this happens, however, the flyweights move out and raise the pilot valves, causing oil to flow from the propellers in a full-feathering system (Fig. 10A) and to the propeller in a constant-speed system (Fig. 10B), increasing the pitch of the blades in both cases. Engine speed then slows to the original RPM setting.

If the airplane begins to climb or engine power is decreased, an UNDERSPEED condition results. Airspeed is reduced and, since the pitch of the propeller blades is too high, the engines begin to slow down. At the instant this happens, the flyweights will droop, causing the pilot valves to move down. Simultaneously, oil flows to the propellers in a full-feathering system (Fig. 11A) and from the propeller in a constant-speed system (Fig. 11B), reducing the pitch of the blades in both cases. This automatically increases the speed of the engines to maintain the original RPM setting.

Feathering

Feathering is achieved through a mechanical linkage that overrides the flyweights and speeder spring. When the cockpit control is moved to ³feather,² the governor lever and shaft are turned beyond normal low-RPM operating limits. As the threaded shaft backs out, the shaft lift rod engages the pilot valve spindle and lifts the pilot valve. This causes oil to flow out of the propeller, and it moves to feather pitch position. (Fig. 12)

Unless the airplane is equipped with the unfeathering accumulator option, the pilot can ³unfeather² the propeller by moving the propeller control to high RPM (low pitch) and engaging the engine starter. When the engine is turning over fast enough to develop sufficient oil pressure, the propeller blades will be forced out of feather.

The unfeathering accumulator option permits a feathered propeller to be unfeathered in flight for air-starting the engine. With this option, the governor is modified to provide an external high-pressure oil outlet through a check valve, as well as a device for unseating the check valve. The external outlet is connected to an accumulator. One side of the accumulator is filled with compressed nitrogen and the other side with oil. This allows the oil to be stored under high pressure, as it is during normal flight. (Fig. 13) When the propeller is feathered, the check valve maintains oil pressure in the accumulator. (Fig. 14) When the propeller control is moved from feather to low pitch, the check valve is unseated, permitting the high-pressure oil in the accumulator to flow to the governor pilot valve. With the governor control lever and shaft in low pitch, the speeder spring forces the pilot valve down so that the oil flows to the propeller and moves the blades to low pitch. (Fig. 15)

On Oct 26, 2006, at 5:05 PM, rtitsworth wrote:

Kevin,

From reading your message, I believe you may have some things regarding
feathering props backwards/confused. I sent this off-list - feel free to
circulate it if you want. Or reply directly if you think I missed something
or have other questions. Hope this helps...

On light twins, (and other normal feathering props), the prop "rests" in the
feathered position - i.e. if there is no oil pressure, the prop will
automatically feather. The oil pressure is used to bring the out of feather
(oil is used to move from high pitch (feather) to low pitch) - which is
opposite a normal (non-feathering) prop.

The high pressure accumulator (reservoir) is used to momentarily bring the
prop "out of feather" if no engine oil pressure is available, which is used
for re-starts in the air after an engine has been shutdown and the prop
feathered. Note: completely shutting down one engine and feathering the
prop is a commonly practiced maneuver in a twin. Without the accumulator,
there would be no way to get the prop out of feather after a engine has shut
down and thus a wind-milling re-start would be impossible.

If you look at most light twins sitting on the ramp, you might notice that
the props are not in the full feather position. You might think to
yourself: "if the prop rests in the feathered position with no oil pressure,
then why isn't it feathered while sitting on the ramp?" The reason is,
because that would make it hard to start on the ground (a feathered prop
"beats" at the air). So, to prevent the prop from moving to the feathered
position during normal engine shutdown (on the ground), the prop has small
centrifugal "pins" which automatically drop into position during low rpm's
(i.e. ground shutdown) and prevent the prop from feathering (unless it's
already feathered). Note: this is important to know during an engine out
scenario on a twin, because you have to move the prop lever to the feathered
position before the prop slows down. Otherwise, the lock-out pins will
engage and prevent the prop from falling into the feathered position (even
though that is the normal resting (i.e. no oil pressure) state.

Interestingly, if you look as most dual stage Twin Gas Turbine planes (i.e.
a King Air), while sitting on the ramp, you'll notice the props are
feathered. This is because they do not have the feather lock-out pins.
They don't need them because the prop is not directly/mechanically connected
to the engine's primary compressor/turbine shaft. Rather, the prop is only
"air coupled" via a 2nd'ary prop turbine (in the engine). Thus, during a
start operation the prop still spins freely (from the primary turbine shaft)
and does not create appreciable back pressure even though it is in the
feathered state (beating the air).

"Counterweighted" props are a bit of as hybrid. They are "limited" so as to
not move (in pitch) all the way to the feathered state (otherwise they would
be a feathering prop). However, they "act" like a feathering prop in that
the prop "rests" in the high pitch position (with no oil pressure), which is
opposite from a normal variable-pitch prop. They typically do not have
accumulators since the prop does not go all the way to the feathering
position - i.e. there is not need to get them back out of feather. However,
without oil pressure the prop will always be at the high pitch position.
They typically also do not have feather lock-out pins, since they do not go
all the way to feather. You can see this by looking at a counterweighted
prop sitting on the ramp. It will be at high pitch, whereas all others will
be at low pitch (i.e. flat). Thus, (in theory) a counterweighted prop will
be harder to start since the starter has to push more air (via the high
pitch prop) during the starting operation. After the engine starts and oil
pressure is available the prop will move to low pitch (if the controls are
positioned that way).

The upside of a counterweighted prop over a feathering prop is that it is
simpler and thus usually lighter and cheaper. The high pitch position
offers only slightly more drag than feathered (especially if you can slow
the plane enough to get the prop to stop windmilling). In a twin the
asymmetric drag can be a killer and thus they go for full feather. In a
single, it's just "incremental" glide distance, which is not really "free"
considering the weight and cost.

The upside of a counterweighted prop over a normal variable pitch-prop is
that if the engine or governor fails (no oil pressure) the prop will
automatically move to the high pitch position (less wind-milling drag and
thus better glide). However, the downsides are 1. theoretically harder to
start (minimal), 2. typically a bit heavier, 3. If the governor fails (or
the engine losses oil pressure) on take-off the prop will go to high pitch -
which is typically not what you want and depending on the engine may give
very poor takeoff/flight performance.

This last emergency scenario (downside #3) does not occur with a normal
variable pitch prop - since it would go the other way (to low pitch) which
is what you normally want at takeoff and is easy/safe to fly the pattern and
return to the runway. Note: if the engine completely fails the advantage
switches back to the counterweighted prop - better glide.

Tradeoffs, Tradeoffs :-)

Rick Titsworth
ES/TSIO550 - building (with an MT counterweighted prop)

-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Kevin
Kossi
Sent: Wednesday, October 25, 2006 8:05 PM
To: Lancair Mailing List
Subject: [LML] Re: Prop Feathering

Does anyone have any experience with the use of pressure reservoirs
to place a Prop. in full feathering mode when you have an engine
failure or loss of oil pressure?

I am thinking about purchasing a Hartzell for my Legacy but that
model is not offered with counter weights or a full feathering option.

I heard that one can install a pressure tank to store oil pressure,
kind of like a well tank. The tanks has a bladder or piston and
spring to store the energy that you can use to feather the prop when
all else fails by opening a valve.

Thanks

Kevin Kossi
New York
Legacy 70%

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