Overspeeding Prop.
Thread Starter
Overspeeding Prop.
'Afternoon Ladies & Gents,
Would someone like to explain to a simple, single-engine, fixed-prop person (me) what 'unfeathering incorrectly resulting in prop overspeed' is all about
I thank you.
Would someone like to explain to a simple, single-engine, fixed-prop person (me) what 'unfeathering incorrectly resulting in prop overspeed' is all about
I thank you.
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
Ian,
To tackle that issue, several points must first be addressed. You indicated that you're familiar fixed pitch propeller installations, so let's build on that.
In normal operations, you're familiar with pushing the power up (increasing, or opening the throttle) to see an RPM increase, and pulling it back to decrease RPM. RPM follows the throttle movement. Additionally, pulling back on the stick causes the airplane to climb, causes airspeed to decrease, and consequently causes engine/propeller RPM to decrease as well. Pushing forward on the stick (decreasing angle of attack) causes the airplane to pitch down, altitude loss, airspeed increase, and RPM increase. So much for the obvious.
A constant speed propeller varies the blade angle to maintain a constant RPM, just as the name implies. If one flies straight and level and increases the throttle setting, RPM doesn't change, but propeller blade angle does. An additional power instrument is added that you don't have with a fixed pitch propeller installation, and that's the manifold pressure gauge. Whereas with a fixed pitch propeller, you can set power by simply setting RPM, you need more information (or rather, have more information) when setting power with a constant speed propeller. This is where the manifold pressure gauge comes in.
Manifold pressure is the air pressure in the induction manifold. When sitting on the ground without the engine running, it's the ambient barometric pressure...at sea level on a standard day, a manifold pressure gauge will read 29.92" of mercury (equal to 1013 mb), sitting on the ramp or apron, at rest. The way it works is much like a vacuum cleaner. The engine acts as an air pump, pushing air out the exhaust, drawing air in through the induction. Just like a vacuum cleaner, if you put your hand over the hose to the vacuum, you'll cause a decrease in air pressure in the induction or the vacuum cleaner hose. The throttle works the same as your hand; close the throttle while the engine is running, and you'll see a drop in manifold pressure...typically about 12" while the engine is running at idle.
Throttle position, then, doesn't affect the engine RPM in much of it's range, just the manifold pressure by varying the opening to the engine induction system (the throttle plate). In a normally-aspirated engine (a non-boosted or non-turbocharged engine), this manifold pressure varies all the way up until one reaches the ambient barometric pressure. This pressure naturally decreases about 1" per thousand feet of altitude increase...which is to say that it's normally about thirty inches at sea level, and about twenty five inches of manifold pressure at five thousand feet. One can't increase manifold pressure more than this value by opening the throttle more, in a normally aspirated engine once one has reached the ambient barometric pressure.
As a side note, in a turbocharged engine, although off-topic, one can keep increasing manifold pressure above barometric...but in that case the RPM remains the same with a constant speed propeller.
A constant speed propeller only maintains RPM when in the "governing range," or an RPM range in which the propeller speed is held constant by the propeller governor (a hydromechanical device that admits engine oil under pressure to the propeller. This oil pressure moves a piston and components in the propeller hub which change blade angle, in order to maintain a constant RPM). The propeller maintains a constant RPM by varying blade pitch...increase pitch (angle of attack), and just like pulling back on the stick where airspeed decreases...propeller RPM decreases as blade angle is increased. This happens simply because when the propeller takes a bigger "bite" of the air with a larger angle or attack, it experiences a drag increase, helping prevent the engine from going any faster.
In order for the propeller governor to increase RPM, it needs to decrease blade angle (similiar to pushing forward on the stick when you fly); this decreases drag, and the engine can turn faster. However, there's only so far that the angle may be decreased in the propeller. When it's at rest on the ramp in a typical piston installation such as a Cessna 182, the propeller pitch is as flat or decreased as it can go...the blades are resting on mechanical stops known as the low pitch stops. What this means is that there will come a point in the engine operation when the propeller governor can no longer decrease blade angle any further, because it's mechanically limited...and as the throttle is retarded then the RPM will fall with the manifold pressure. The point at which the RPM can no longer be maintained and it begins to decay as power is retarded is known as the bottom end of the "governing range," or the effective range in which the propeller governor can maintain a constant RPM.
With these basic principles in mind, let's turn to the propeller's use in flight. Two basic forces drive the propeller. With power to the propeller, the engine drives the propeller, and it varies it's blade angle to maintain a constant RPM. However, when below the governing range, when the propeller blades are as flat as they can go and resting on the low pitch stops, the slipstream in flight may be driving the propeller. This is generally considered a bad thing in a piston engine, for a number of reasons...but here the only consideration will be that the engine is no longer driving the propeller. Any time power is pulled back far enough that positive torque isn't being maintained on the propeller, the fact that it's spinning out there is largely due to the slipstream driving it. This is a big drag rise for the airplane.
To eliminate the drag from a windmilling propeller, one in which the slipstream is driving the propeller, we can feather certain propellers. This means that the propeller blade is rotated nearly parallel with the slipstream, to minimize drag. This may, and usually does, stop the propeller from rotating, and drastically decreases drag. As an illustration, the drag on a windmilling propeller can be, and usually is, higher than if a giant plywood disc were fixed out there in place of the propeller, and the same diameter of the propeller. This may sound sketchy, but it's true. A windmilling propeller can produce a significant amount of drag (I've been unable to maintain altitude in four engine airplanes before when a propeller wouldn't feather during an engine failure...that's with three other engines working hard to keep me flying, but one working against me...it's a LOT of drag).
Preventing engine overspeed in a fixed pitch installation is by propeller design; it has to be able to stay within the designated RPM range during a static runup on the ground. In flight, however, it's possible that a fixed pitch propeller might be able to overspeed, or go beyond it's approved RPM limits, especially at higher airspeeds such as in a dive. In a case like that, the only two choices to control RPM are to decrease power, and to decrease airspeed.
Preventing overspeed in a constant speed propeller installation is a function of the propeller governor. It's a mechanical unit which is a valve controlled by a spring. The valve moves open and closed using flyweights that respond to propeller RPM...at higher RPM's the weights pull apart, and mechanically they're linked to the valve (called a pilot valve) that admits or denies engine oil to the propeller dome (which is used to change propeller blade angle). The propeller governor prevents overspeed by increasing blade angle, thus increasing drag on the propeller blade, thus slowing down their rotation, thus controlling RPM. So far so good?
Once an engine is feathered, it must be brought out of feather as part of the engine starting process if one intends to use the engine in flight again. To do this, the engine is rotated and either engine oil or a dedicated supply of pressurized oil from an "accumulator" is used to drive the propeller back out of the feather position.
Now we're finally getting to the point where we answer your question. If the aircraft airspeed is high or the propeller comes out of feather too quickly, it's RPM increase is driven by the slipstream, and the propeller governor may not have time or the ability right away to supply pressurized oil to govern the prop. The engine may not be producing high oil pressure yet, and as the RPM increases, the propeller may not be capable of governing the speed just yet. This may mean that the engine can overspeed. This is a function of airspeed and as a result many times an airspeed range is recommended or prescribed for engine air starts. This may include a minimum speed to provide adequate rotational assistance, and a maximum speed to help prevent overspeed.
An overspeed can be problematic for several reasons. A faster speed can increases stresses on propeller blades, hubs, and shanks. It can affect engine integrity, and put excess stresses on crankshaft counterweights or balances (and can detune a crankshaft), and it can exceed the aerodynamic limitations of the propeller itself. At too high a speed the prop tips experience transonic aerodynamic conditions, which can cause flexing, vibration harmonic stresses, and other problems for not only the propeller, but the engine. Furthermore, as the drag that a windmilling propeller is very high, it becomes particularly detrimental at higher engine RPM's and speeds.
The procedure for unfeathering an engine varies with the propeller system and installation. With some engines, the procedure may be to simply drive the propeller out of feather with respect to the blade angle, and let it windmill up to speed on it's own before introducing fuel or spark. Some engines may require a lower propeller setting until the engine speed is stable. Some may require a higher setting. Using the wrong airspeed or technique may result in an unsatisfactory start, an engine that won't come out of feather, or one that can overspeed when coming out of feather.
To tackle that issue, several points must first be addressed. You indicated that you're familiar fixed pitch propeller installations, so let's build on that.
In normal operations, you're familiar with pushing the power up (increasing, or opening the throttle) to see an RPM increase, and pulling it back to decrease RPM. RPM follows the throttle movement. Additionally, pulling back on the stick causes the airplane to climb, causes airspeed to decrease, and consequently causes engine/propeller RPM to decrease as well. Pushing forward on the stick (decreasing angle of attack) causes the airplane to pitch down, altitude loss, airspeed increase, and RPM increase. So much for the obvious.
A constant speed propeller varies the blade angle to maintain a constant RPM, just as the name implies. If one flies straight and level and increases the throttle setting, RPM doesn't change, but propeller blade angle does. An additional power instrument is added that you don't have with a fixed pitch propeller installation, and that's the manifold pressure gauge. Whereas with a fixed pitch propeller, you can set power by simply setting RPM, you need more information (or rather, have more information) when setting power with a constant speed propeller. This is where the manifold pressure gauge comes in.
Manifold pressure is the air pressure in the induction manifold. When sitting on the ground without the engine running, it's the ambient barometric pressure...at sea level on a standard day, a manifold pressure gauge will read 29.92" of mercury (equal to 1013 mb), sitting on the ramp or apron, at rest. The way it works is much like a vacuum cleaner. The engine acts as an air pump, pushing air out the exhaust, drawing air in through the induction. Just like a vacuum cleaner, if you put your hand over the hose to the vacuum, you'll cause a decrease in air pressure in the induction or the vacuum cleaner hose. The throttle works the same as your hand; close the throttle while the engine is running, and you'll see a drop in manifold pressure...typically about 12" while the engine is running at idle.
Throttle position, then, doesn't affect the engine RPM in much of it's range, just the manifold pressure by varying the opening to the engine induction system (the throttle plate). In a normally-aspirated engine (a non-boosted or non-turbocharged engine), this manifold pressure varies all the way up until one reaches the ambient barometric pressure. This pressure naturally decreases about 1" per thousand feet of altitude increase...which is to say that it's normally about thirty inches at sea level, and about twenty five inches of manifold pressure at five thousand feet. One can't increase manifold pressure more than this value by opening the throttle more, in a normally aspirated engine once one has reached the ambient barometric pressure.
As a side note, in a turbocharged engine, although off-topic, one can keep increasing manifold pressure above barometric...but in that case the RPM remains the same with a constant speed propeller.
A constant speed propeller only maintains RPM when in the "governing range," or an RPM range in which the propeller speed is held constant by the propeller governor (a hydromechanical device that admits engine oil under pressure to the propeller. This oil pressure moves a piston and components in the propeller hub which change blade angle, in order to maintain a constant RPM). The propeller maintains a constant RPM by varying blade pitch...increase pitch (angle of attack), and just like pulling back on the stick where airspeed decreases...propeller RPM decreases as blade angle is increased. This happens simply because when the propeller takes a bigger "bite" of the air with a larger angle or attack, it experiences a drag increase, helping prevent the engine from going any faster.
In order for the propeller governor to increase RPM, it needs to decrease blade angle (similiar to pushing forward on the stick when you fly); this decreases drag, and the engine can turn faster. However, there's only so far that the angle may be decreased in the propeller. When it's at rest on the ramp in a typical piston installation such as a Cessna 182, the propeller pitch is as flat or decreased as it can go...the blades are resting on mechanical stops known as the low pitch stops. What this means is that there will come a point in the engine operation when the propeller governor can no longer decrease blade angle any further, because it's mechanically limited...and as the throttle is retarded then the RPM will fall with the manifold pressure. The point at which the RPM can no longer be maintained and it begins to decay as power is retarded is known as the bottom end of the "governing range," or the effective range in which the propeller governor can maintain a constant RPM.
With these basic principles in mind, let's turn to the propeller's use in flight. Two basic forces drive the propeller. With power to the propeller, the engine drives the propeller, and it varies it's blade angle to maintain a constant RPM. However, when below the governing range, when the propeller blades are as flat as they can go and resting on the low pitch stops, the slipstream in flight may be driving the propeller. This is generally considered a bad thing in a piston engine, for a number of reasons...but here the only consideration will be that the engine is no longer driving the propeller. Any time power is pulled back far enough that positive torque isn't being maintained on the propeller, the fact that it's spinning out there is largely due to the slipstream driving it. This is a big drag rise for the airplane.
To eliminate the drag from a windmilling propeller, one in which the slipstream is driving the propeller, we can feather certain propellers. This means that the propeller blade is rotated nearly parallel with the slipstream, to minimize drag. This may, and usually does, stop the propeller from rotating, and drastically decreases drag. As an illustration, the drag on a windmilling propeller can be, and usually is, higher than if a giant plywood disc were fixed out there in place of the propeller, and the same diameter of the propeller. This may sound sketchy, but it's true. A windmilling propeller can produce a significant amount of drag (I've been unable to maintain altitude in four engine airplanes before when a propeller wouldn't feather during an engine failure...that's with three other engines working hard to keep me flying, but one working against me...it's a LOT of drag).
Preventing engine overspeed in a fixed pitch installation is by propeller design; it has to be able to stay within the designated RPM range during a static runup on the ground. In flight, however, it's possible that a fixed pitch propeller might be able to overspeed, or go beyond it's approved RPM limits, especially at higher airspeeds such as in a dive. In a case like that, the only two choices to control RPM are to decrease power, and to decrease airspeed.
Preventing overspeed in a constant speed propeller installation is a function of the propeller governor. It's a mechanical unit which is a valve controlled by a spring. The valve moves open and closed using flyweights that respond to propeller RPM...at higher RPM's the weights pull apart, and mechanically they're linked to the valve (called a pilot valve) that admits or denies engine oil to the propeller dome (which is used to change propeller blade angle). The propeller governor prevents overspeed by increasing blade angle, thus increasing drag on the propeller blade, thus slowing down their rotation, thus controlling RPM. So far so good?
Once an engine is feathered, it must be brought out of feather as part of the engine starting process if one intends to use the engine in flight again. To do this, the engine is rotated and either engine oil or a dedicated supply of pressurized oil from an "accumulator" is used to drive the propeller back out of the feather position.
Now we're finally getting to the point where we answer your question. If the aircraft airspeed is high or the propeller comes out of feather too quickly, it's RPM increase is driven by the slipstream, and the propeller governor may not have time or the ability right away to supply pressurized oil to govern the prop. The engine may not be producing high oil pressure yet, and as the RPM increases, the propeller may not be capable of governing the speed just yet. This may mean that the engine can overspeed. This is a function of airspeed and as a result many times an airspeed range is recommended or prescribed for engine air starts. This may include a minimum speed to provide adequate rotational assistance, and a maximum speed to help prevent overspeed.
An overspeed can be problematic for several reasons. A faster speed can increases stresses on propeller blades, hubs, and shanks. It can affect engine integrity, and put excess stresses on crankshaft counterweights or balances (and can detune a crankshaft), and it can exceed the aerodynamic limitations of the propeller itself. At too high a speed the prop tips experience transonic aerodynamic conditions, which can cause flexing, vibration harmonic stresses, and other problems for not only the propeller, but the engine. Furthermore, as the drag that a windmilling propeller is very high, it becomes particularly detrimental at higher engine RPM's and speeds.
The procedure for unfeathering an engine varies with the propeller system and installation. With some engines, the procedure may be to simply drive the propeller out of feather with respect to the blade angle, and let it windmill up to speed on it's own before introducing fuel or spark. Some engines may require a lower propeller setting until the engine speed is stable. Some may require a higher setting. Using the wrong airspeed or technique may result in an unsatisfactory start, an engine that won't come out of feather, or one that can overspeed when coming out of feather.
Thread Starter
Tech Log the very best in practical TECH discussion on the web
Wow,
That sure is one hell of an answer.
Thank heavens there are folks out there who remember what happened before jets.
So, if you get it wrong in a twin (remember I only know singles) the drag could roll you over?
Which engine is more critical in a British engined twin?
Many thanks for your response
IanBB
That sure is one hell of an answer.
Thank heavens there are folks out there who remember what happened before jets.
So, if you get it wrong in a twin (remember I only know singles) the drag could roll you over?
Which engine is more critical in a British engined twin?
Many thanks for your response
IanBB
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
Ian,
A propeller which doesn't feather won't necessarily "roll you over," but it can certainly be a contributing factor to a loss of control under the right circumstances.
In a light multi engine airplane with the engines on the wings (as opposed to say, a Cessna Skymaster, where both are on the fuselage), each engine produces thrust along a line which is parallel to, but separate from, the longitudinal axis of the airplane. If only one engine is running, the tendency is to want to push the airplane away from that engine. You can see the same tendency for yourself by standing upright and placing your arms straight out from your sides. Make a fist with each hand, each fist representing one engine of a multi-engine airplane (this example doesn't work so well if you have only one arm, and not at all if you have none, of course).
Have someone push on just one fist from behind (or pull on it from in front of you), and see if it doesn't try to twist you around. Of course it will...and it stands to reason that it won't if you have someone pushing on your fists equally on each side. Same for a multi engine airplane.
To prevent that twisting, or yawing motion, we have a rudder, of course. Unfortunately, the rudder can only do so much. At lower speeds, rudder authority is less, and at higher power settings on the "good" engine, the yawing moment is more. There comes a point at full power when the rudder isn't enough to prevent the airplane from yawing, and the airplane can depart controlled flight. The published speed at which this occurs is called "Vmc, or the point of minimum control with the critical engine inoperative, the operating engine at takeoff power, the aft-most center of gravity, maximum takeoff weight, flaps and cowl flaps set for takeoff, landing gear up, and a maximum of five degrees bank into the "good" engine. That's a lot to remember, but the main thing is that it's the speed at which directional control can no longer be maintained.
How the airplane departs controlled flight varies; it can be rapid, or a slow descending turn to a spin entry, a rollover, or it may simply start drifing in heading until the pilot either decreases angle of attack or most properly reduces power on the operating engine to maintain directional control.
Certain engine-propeller combinations produce a LOT of drag. In those cases, a propeller overspeed or loss of propeller control can indeed be a much greater force to the detrement of performance, than the operative engine. In fact, the operative engine may be the least of one's concerns. Such engines use several forms of overspeed protection, to include dedicated overspeed governors or more than one overspeed governor. These remove fuel from the engine as well as taking steps to control the propeller itself.
The engine that's critical is usually the one with the descending blade on it's propeller disc that's farthest from the fuselage. Tongue in cheek, I recently heard a Navajo pilot opine that his right engine was critical...because that's the one with the air conditioning on his airplane. He considered the loss of that engine much worse than any other calamity which might befall him.
The side of the propeller disc with the descending blade arguably is the side of the disc producing the greatest thrust (I say arguably because this isn't technically correct, but it's a great source for hangar arguments when it's a rainy day, and the topic for another thread). If you have two counter-rotating propellers where the descending blade is on the fuselage-side of theprop disc, then neither engine is critical. In light twins that have all propellers spinning counter-clockwise as seen from the cockpit, then the right engine is critical. The right engine produces a lesser assymetrical thrust situation than the left, so loss of the right puts the airplane in a more precarious condition. The reason for this is that assymetrical thrust yawing moment is greatest when the thrust line is farthest from the fuselage center line (picture the previous example of having someone push on your fists...this time have one person push on your fist, and the other your elbow...the fist is farther from your body, and though your assistants push equally as hard in both places, there's a greater moment on your fist...in this case we could say that your elbow is critical...because it's better to lose the pushing on your fist, than your elbow, and loss of pushing on the elbow puts you in a worse condition than loss of pushing on your fist).
Clear as mud? The opposite is true for clockwise spinning engines. In some cases, airplanes have been designed with the descending blade on both engines to the outside of the prop disc. The P-38 lightening is one example, making both engines "critical," or really neither more disadvantageous than the other...but making the loss of either one pretty darn bad. Certain of the Piper Aerostars were the same way, while others did just the opposite, with the descending blades on each engine on the fuselage-side...making neither engine critical (Piper Seneca is the same way).
A propeller which doesn't feather won't necessarily "roll you over," but it can certainly be a contributing factor to a loss of control under the right circumstances.
In a light multi engine airplane with the engines on the wings (as opposed to say, a Cessna Skymaster, where both are on the fuselage), each engine produces thrust along a line which is parallel to, but separate from, the longitudinal axis of the airplane. If only one engine is running, the tendency is to want to push the airplane away from that engine. You can see the same tendency for yourself by standing upright and placing your arms straight out from your sides. Make a fist with each hand, each fist representing one engine of a multi-engine airplane (this example doesn't work so well if you have only one arm, and not at all if you have none, of course).
Have someone push on just one fist from behind (or pull on it from in front of you), and see if it doesn't try to twist you around. Of course it will...and it stands to reason that it won't if you have someone pushing on your fists equally on each side. Same for a multi engine airplane.
To prevent that twisting, or yawing motion, we have a rudder, of course. Unfortunately, the rudder can only do so much. At lower speeds, rudder authority is less, and at higher power settings on the "good" engine, the yawing moment is more. There comes a point at full power when the rudder isn't enough to prevent the airplane from yawing, and the airplane can depart controlled flight. The published speed at which this occurs is called "Vmc, or the point of minimum control with the critical engine inoperative, the operating engine at takeoff power, the aft-most center of gravity, maximum takeoff weight, flaps and cowl flaps set for takeoff, landing gear up, and a maximum of five degrees bank into the "good" engine. That's a lot to remember, but the main thing is that it's the speed at which directional control can no longer be maintained.
How the airplane departs controlled flight varies; it can be rapid, or a slow descending turn to a spin entry, a rollover, or it may simply start drifing in heading until the pilot either decreases angle of attack or most properly reduces power on the operating engine to maintain directional control.
Certain engine-propeller combinations produce a LOT of drag. In those cases, a propeller overspeed or loss of propeller control can indeed be a much greater force to the detrement of performance, than the operative engine. In fact, the operative engine may be the least of one's concerns. Such engines use several forms of overspeed protection, to include dedicated overspeed governors or more than one overspeed governor. These remove fuel from the engine as well as taking steps to control the propeller itself.
The engine that's critical is usually the one with the descending blade on it's propeller disc that's farthest from the fuselage. Tongue in cheek, I recently heard a Navajo pilot opine that his right engine was critical...because that's the one with the air conditioning on his airplane. He considered the loss of that engine much worse than any other calamity which might befall him.
The side of the propeller disc with the descending blade arguably is the side of the disc producing the greatest thrust (I say arguably because this isn't technically correct, but it's a great source for hangar arguments when it's a rainy day, and the topic for another thread). If you have two counter-rotating propellers where the descending blade is on the fuselage-side of theprop disc, then neither engine is critical. In light twins that have all propellers spinning counter-clockwise as seen from the cockpit, then the right engine is critical. The right engine produces a lesser assymetrical thrust situation than the left, so loss of the right puts the airplane in a more precarious condition. The reason for this is that assymetrical thrust yawing moment is greatest when the thrust line is farthest from the fuselage center line (picture the previous example of having someone push on your fists...this time have one person push on your fist, and the other your elbow...the fist is farther from your body, and though your assistants push equally as hard in both places, there's a greater moment on your fist...in this case we could say that your elbow is critical...because it's better to lose the pushing on your fist, than your elbow, and loss of pushing on the elbow puts you in a worse condition than loss of pushing on your fist).
Clear as mud? The opposite is true for clockwise spinning engines. In some cases, airplanes have been designed with the descending blade on both engines to the outside of the prop disc. The P-38 lightening is one example, making both engines "critical," or really neither more disadvantageous than the other...but making the loss of either one pretty darn bad. Certain of the Piper Aerostars were the same way, while others did just the opposite, with the descending blades on each engine on the fuselage-side...making neither engine critical (Piper Seneca is the same way).
Thread Starter
That Damn Overspeed
One last question please:
Is operator error the only possible cause of overspeed in the unfeathering process or can mechanical misfunction do it?
-Am going to lie down now as my little ole' head is hurting
Thanks for your time - i'm still struggling with your report that 3 good engines find it hard to fight 1 flat blade disc
Is operator error the only possible cause of overspeed in the unfeathering process or can mechanical misfunction do it?
-Am going to lie down now as my little ole' head is hurting
Thanks for your time - i'm still struggling with your report that 3 good engines find it hard to fight 1 flat blade disc
Join Date: Apr 2008
Location: Nearer home than before!
Posts: 524
Likes: 0
Received 0 Likes
on
0 Posts
It depends on how much thrust the engines produce.
Most twins will not keep flying if one engine fails and you can't feather it.
Commercial turboprops use an autofeather unit to protect life and limb on take-offs.
The only commercial twin I can think of that will fly on one with the other unfeathered is the Dash8Q400, so overpowered it's almost silly- but damn god fun as well, and only then with luck on your side.
It's just about the worst situation you can train for in any propellor aircraft and most simulators cannot replicate it fully simply because it's such a dangerous situation, flight testing would be crazy to try it in a real aircraft......
Most twins will not keep flying if one engine fails and you can't feather it.
Commercial turboprops use an autofeather unit to protect life and limb on take-offs.
The only commercial twin I can think of that will fly on one with the other unfeathered is the Dash8Q400, so overpowered it's almost silly- but damn god fun as well, and only then with luck on your side.
It's just about the worst situation you can train for in any propellor aircraft and most simulators cannot replicate it fully simply because it's such a dangerous situation, flight testing would be crazy to try it in a real aircraft......
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
Is operator error the only possible cause of overspeed in the unfeathering process or can mechanical misfunction do it?
The most common scenario for an overspeed isn't during an unfeathering process, but a governor failure in cruise flight. If you're flying the local club's Cessna 182, and notice that the RPM is increasing, check to see that the friction lock or vernier control is secure, and that the propeller control isn't creeping forward. If the propeller is approaching the redline and appears to be moving on it's own, you can try to control it with the propeller control, but you may have no success. In this case, your only choice is to retard power using the throttle...and begin to treat the propeller like a fixed pitch prop until you can get somewhere to land and have the engine looked-after by a qualified mechanic.
You seem very concerned about an overspeed when unfeathering. Can you share a little of what might be driving this concern?
Truthfully, overspeed is seldom an issue if procedures and limitations are followed, and the equipment is properly maintained. In most cases when an overspeed does occur, it's easily controlled. If it isn't, then in most cases a shutdown and slow-down can be performed, and the aircraft then reverts to basic engine-out procedures...simple stuff.
Thread Starter
You seem very concerned about an overspeed when unfeathering.
SNS3Guppy (or by now may I call you SN?)
Not only are you a fine teacher of technical mysteries to those of us who only just understand the '3Fs' - (fixed wing, fixed gear, fixed prop) but you are clearly sensitive and perceptive as well.
Yes, as you suspect, I have a specific incident in mind.
From a book about post-war RAF crashes
"The pilot feathered the port engine but because he used the incorrect procedure to unfeather it, the propeller overspeeded and so the propeller was feathered again. An emergency landing was attempted but on final approach, the port wing struck a tree and the aircraft crashed. Although a passenger; AC1 xxxxxxxxxxx was injured, the pilot F/O Geoffrey Burgess was killed."
The pilot in question was my father, and the information above was researched, I understand, from official RAF records. While I knew that the
purpose of the flight was a feathering exercise, the above information had never been given to me. I only found this out a few days ago.
This all happened over 60 years ago now and is of little relevance to anyone, except perhaps me - but I'm sure you can understand why, in my dark ignorance of complex aircraft, I turned to the esteemed Tech Log in order to try and learn more about the procedure.
See PPRuNe Forums 'Where are they now' thread 'Flying Officer Geoffrey Burgess'.
Thanks & Regards Ian BB
PS
WingoWango - thanks for the interesting read.....Sweden.
Not only are you a fine teacher of technical mysteries to those of us who only just understand the '3Fs' - (fixed wing, fixed gear, fixed prop) but you are clearly sensitive and perceptive as well.
Yes, as you suspect, I have a specific incident in mind.
From a book about post-war RAF crashes
"The pilot feathered the port engine but because he used the incorrect procedure to unfeather it, the propeller overspeeded and so the propeller was feathered again. An emergency landing was attempted but on final approach, the port wing struck a tree and the aircraft crashed. Although a passenger; AC1 xxxxxxxxxxx was injured, the pilot F/O Geoffrey Burgess was killed."
The pilot in question was my father, and the information above was researched, I understand, from official RAF records. While I knew that the
purpose of the flight was a feathering exercise, the above information had never been given to me. I only found this out a few days ago.
This all happened over 60 years ago now and is of little relevance to anyone, except perhaps me - but I'm sure you can understand why, in my dark ignorance of complex aircraft, I turned to the esteemed Tech Log in order to try and learn more about the procedure.
See PPRuNe Forums 'Where are they now' thread 'Flying Officer Geoffrey Burgess'.
Thanks & Regards Ian BB
PS
WingoWango - thanks for the interesting read.....Sweden.
Last edited by Ian Burgess-Barber; 9th Jul 2009 at 19:21. Reason: missing word
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
Ian,
I suspect that the term "overspeeded" may be misleading in this case. I believe the intent of the term "overspeed" isn't mean to imply that the pilot in question actually caused a propeller overspeed, but rather brought the airplane inadvertantly back out of feather.
A typical propeller at that time was a hamilton standard hydromatic prop. This required a dedicated oil pump called a "feather pump," and this pump had to be energized to drive the propeller into feather. The pump was supposed to shutoff when the propeller was feathered. To drive it into the feather position, a button was pushed, and the button was held in by a solenoid. When the feather position was reached, the button was supposed to pop out by itself, interrupting the pump process, and leaving the propeller in feather.
A common problem with the hydromatic prop was that the button wouldn't pop out. The solenoid would hold the button in, and the propeller would feather, then with the pump running, continue to drive itself out of feather again. The solution was to guard against the button, watching the propeller, and popping the feather button out by hand once the prop had feathered.
An integral part of training in multi engine airplanes is performing engine-out work, just like it is in single-engine airplanes. As complexity grows, so does weight, and performance on reduced power or less engines tends to suffer, accordingly. Procedures tend to become lengthier and more exacting, and the requirements or demands on the pilot also tend to increase.
I don't know all the circumstances regarding the loss of your father, but it's important to realize that one can do everything right in an airplane, and still pay the ultimate price. In this case, the actions with the propeller don't appear to have been the cause of the mishap, but rather striking the tree. How the two events may be related, one can't say with such little information. However, when an emergency develops, one can only contribute toward improving the situation such as one is able; there are never any guarantees that one will be successful, and accordingly there is no ringing condemnation when one is not. One can only do ones best with what one has available.
I'm sorry to learn of your father's passing long ago, yet would urge you to remember that this incident does not speak ill of your father. It's quite conceivable that your father did the right things, and was the victim of a situation which simply deteriorated beyond his, or anyone else's control. Simply put, we can all fly into places from which we cannot return, and in my opinion, some of us are destined to do so...no matter how much we might wish to escape the fact. We must all go at some time, without regard to our particular skill, circumstance, age, caste, or form of employment.
I suspect that the term "overspeeded" may be misleading in this case. I believe the intent of the term "overspeed" isn't mean to imply that the pilot in question actually caused a propeller overspeed, but rather brought the airplane inadvertantly back out of feather.
A typical propeller at that time was a hamilton standard hydromatic prop. This required a dedicated oil pump called a "feather pump," and this pump had to be energized to drive the propeller into feather. The pump was supposed to shutoff when the propeller was feathered. To drive it into the feather position, a button was pushed, and the button was held in by a solenoid. When the feather position was reached, the button was supposed to pop out by itself, interrupting the pump process, and leaving the propeller in feather.
A common problem with the hydromatic prop was that the button wouldn't pop out. The solenoid would hold the button in, and the propeller would feather, then with the pump running, continue to drive itself out of feather again. The solution was to guard against the button, watching the propeller, and popping the feather button out by hand once the prop had feathered.
An integral part of training in multi engine airplanes is performing engine-out work, just like it is in single-engine airplanes. As complexity grows, so does weight, and performance on reduced power or less engines tends to suffer, accordingly. Procedures tend to become lengthier and more exacting, and the requirements or demands on the pilot also tend to increase.
I don't know all the circumstances regarding the loss of your father, but it's important to realize that one can do everything right in an airplane, and still pay the ultimate price. In this case, the actions with the propeller don't appear to have been the cause of the mishap, but rather striking the tree. How the two events may be related, one can't say with such little information. However, when an emergency develops, one can only contribute toward improving the situation such as one is able; there are never any guarantees that one will be successful, and accordingly there is no ringing condemnation when one is not. One can only do ones best with what one has available.
I'm sorry to learn of your father's passing long ago, yet would urge you to remember that this incident does not speak ill of your father. It's quite conceivable that your father did the right things, and was the victim of a situation which simply deteriorated beyond his, or anyone else's control. Simply put, we can all fly into places from which we cannot return, and in my opinion, some of us are destined to do so...no matter how much we might wish to escape the fact. We must all go at some time, without regard to our particular skill, circumstance, age, caste, or form of employment.
Thread Starter
SN
Thanks again for your informed and considerate assessment (noting of course that we don't have all the facts before us). I suppose now that I'm over the shock of these revelations so long after the event, I can contact the RAF Museum at Hendon - they have a Dept. of Research & Information Service and will have the RAF Form 1180 Accident Card for the A/C plus the findings of the court of enquiry. I am grateful also to the PPRuNe member who contacted me by private message and directed me to other forums who have steered me towards the relevent archives. However, whatever is revealed by my inquiries, I will be fortified by the content of your last two paragraphs.
As I said, I just wanted to learn something about the procedure, before engaging with the guardians of the records of the event.
Does your PPRuNe handle mean you knew Statocruisers? I remember an article by Ernie Gann in which he described the propellers as "four villains dancing in a row"- so evocative of those different times - & different airplanes.
Thanks again and Good Night
Thanks again for your informed and considerate assessment (noting of course that we don't have all the facts before us). I suppose now that I'm over the shock of these revelations so long after the event, I can contact the RAF Museum at Hendon - they have a Dept. of Research & Information Service and will have the RAF Form 1180 Accident Card for the A/C plus the findings of the court of enquiry. I am grateful also to the PPRuNe member who contacted me by private message and directed me to other forums who have steered me towards the relevent archives. However, whatever is revealed by my inquiries, I will be fortified by the content of your last two paragraphs.
As I said, I just wanted to learn something about the procedure, before engaging with the guardians of the records of the event.
Does your PPRuNe handle mean you knew Statocruisers? I remember an article by Ernie Gann in which he described the propellers as "four villains dancing in a row"- so evocative of those different times - & different airplanes.
Thanks again and Good Night
Join Date: Apr 2008
Location: nowhere
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
Ian,
A typical propeller at that time was a hamilton standard hydromatic prop. This required a dedicated oil pump called a "feather pump," and this pump had to be energized to drive the propeller into feather. The pump was supposed to shutoff when the propeller was feathered. To drive it into the feather position, a button was pushed, and the button was held in by a solenoid. When the feather position was reached, the button was supposed to pop out by itself, interrupting the pump process, and leaving the propeller in feather.
A common problem with the hydromatic prop was that the button wouldn't pop out. The solenoid would hold the button in, and the propeller would feather, then with the pump running, continue to drive itself out of feather again. The solution was to guard against the button, watching the propeller, and popping the feather button out by hand once the prop had feathered.
A typical propeller at that time was a hamilton standard hydromatic prop. This required a dedicated oil pump called a "feather pump," and this pump had to be energized to drive the propeller into feather. The pump was supposed to shutoff when the propeller was feathered. To drive it into the feather position, a button was pushed, and the button was held in by a solenoid. When the feather position was reached, the button was supposed to pop out by itself, interrupting the pump process, and leaving the propeller in feather.
A common problem with the hydromatic prop was that the button wouldn't pop out. The solenoid would hold the button in, and the propeller would feather, then with the pump running, continue to drive itself out of feather again. The solution was to guard against the button, watching the propeller, and popping the feather button out by hand once the prop had feathered.
In all honesty, I was never 100% comfortable with my knowledge of the inner workings of that complicated prop and it seemed difficult to get answers sometimes, but going from memory, it seems to me that a propeller blade angle switch and a pressure switch in combination would stop the solenoid operating the motor on the pump.
Perhaps our system was different because there was no way to push in that button on the Herc. It went in automatically when the condition lever was placed to feather and then popped out or if necessary was pulled out.
P.S. We had a maximum airspeed of 180 knots for an airstart.
Last edited by JammedStab; 10th Jul 2009 at 04:08.
Join Date: Jul 2009
Location: N 06/W 75
Posts: 81
Likes: 0
Received 0 Likes
on
0 Posts
@ SNS3Guppy
Hi everybody; first post, woo-hoo!
This thread draw my attention.
You said that, with increasing aircraft pitch, there's an altitude gain, and an airspeed reduction, so the propeller governor would increase blade angle to maintain an specific RPM; the opposite for an aircraft decreasing pitch. So, my question is this: Do you have in the cockpit any way to check for the blade's angle of attack? Something like an AoA sensor gauge, or do the prop (or condition) levers move (not likely, me thinks, since the governor works on oil pressure, and the lever tells the governor which speed it wants via the weights, am I right?)?
Thanks in advance!
This thread draw my attention.
You said that, with increasing aircraft pitch, there's an altitude gain, and an airspeed reduction, so the propeller governor would increase blade angle to maintain an specific RPM; the opposite for an aircraft decreasing pitch. So, my question is this: Do you have in the cockpit any way to check for the blade's angle of attack? Something like an AoA sensor gauge, or do the prop (or condition) levers move (not likely, me thinks, since the governor works on oil pressure, and the lever tells the governor which speed it wants via the weights, am I right?)?
Thanks in advance!
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
I was under the impression that if the button did not pop out that it would cause the motor for the aux feathering pump to burn out as it was continuing to try to operate the now feathered prop. I had not heard about the prop coming out of feather by itself in this scenario if the motor continued to run.
Perhaps our system was different because there was no way to push in that button on the Herc.
In all honesty, I was never 100% comfortable with my knowledge of the inner workings of that complicated prop and it seemed difficult to get answers sometimes, but going from memory, it seems to me that a propeller blade angle switch and a pressure switch in combination would stop the solenoid operating the motor on the pump.
You said that, with increasing aircraft pitch, there's an altitude gain, and an airspeed reduction, so the propeller governor would increase blade angle to maintain an specific RPM; the opposite for an aircraft decreasing pitch.
The opposite is true in a dive; the propeller tends to spin faster in a dive with a fixed pitch installation. In a constant speed propeller, in order to prevent the RPM from increasing, propeller blade angle is increased in the dive.
So, my question is this: Do you have in the cockpit any way to check for the blade's angle of attack? Something like an AoA sensor gauge, or do the prop (or condition) levers move (not likely, me thinks, since the governor works on oil pressure, and the lever tells the governor which speed it wants via the weights, am I right?)?
In a piston airplane, we can determine our power from manifold pressure and RPM, and in turbopropeller airplanes, we can determine our power from RPM and either torque or horsepower/shaft horsepower (a few airplanes use horsepower gauges, rather than torque). What the blade angle is doing at any given time isn't really meaningful, as it's constantly changing with flight conditions to maintain a constant RPM. The constant RPM simplifies the use of the propeller from the cockpit, and simplifies power management, as well as serves to make the propeller more efficient and useful.
You are correct regarding the cockpit control of propeller RPM. When the pilot moves the propeller lever toward increase or decrease, he or she is controlling a spring in piston airplanes. That's it (except for feather and reverse operations, which vary with the propeller type, and may involve mechanical connections between the propeller control and other engine functions). Not actually moving anything else, the pilot is simply compressing or releasing compression on a "speeder spring." This spring acts against the flyweights in the propeller governor. The flyweights want to open a pilot valve, and the speeder spring wants to close it. At a given RPM for the flyweights in the governor assembly, the ability of the centripetal force on the flyweights to open the pilot valve is tempered by the resistance to moving the valve...and that's provided by the speeder spring, and the amount it's compressed, in turn, is determined by the pilot's use of the propeller control.
Does your PPRuNe handle mean you knew Statocruisers?
I worked on C-97's, and was in line to fly them, but separated from that operation before I had the chance. I believe we flew the last operational working aircraft of that type...so I'll never get that chance again. It was an impressive airplane.
Join Date: Mar 2000
Location: Arizona USA
Posts: 8,571
Likes: 0
Received 0 Likes
on
0 Posts
It was an impressive airplane.
I personally flew the type for awhile (not the C97, the B377, and yes there is a difference in many areas) and most of these airplanes that were delivered with CurtisElectric propellors, were later converted to HamiltonStandard Hydromatic types.
If those that operated the Hydromatic prop think it's complicated and unreliable, you should have seen the complexity of the CurtisElectric design.
Not only that, but the steel blades tended to corrode under the de-icing boot...with the resultant loss of a portion of blade, most times the entire engine...and usually the airplane.
The CurtisElectric prop system (installed on some Stratocruisers) had a couple of advantages however....it did not use engine oil for prop control/feathering and...to unfeather, the feather button was pulled out.
So, feather button pressed, prop feathered electrically, button released automatically.
Feather button did not release automatically?
Prop stayed feathered, anyway.
When Hamilton Standard Hydromatic propellors were later fitted, this feather button arrangement was retained, for fleet commonality.
For most light aircraft the prop will remain on the fine pitch (high RPM) stops
untill it reaches 70 to 100 knots on the takeoff run. At that time the prop will start to unload and speed up, so the govenor will increase the pitch of the prop blades to stop the prop from speeding up and exceeding the seelcted RPM (Redline in the case of the takeoff)
If there was a failure which caused the prop togo to the fine pitch stops during cruise flight the prop will very significantly exceed the redline RPM. If this occurs an immediate sharp pull up should be initiated in order to reduce airspeed and thus RPM. Prop RPM will now be controlled by airspeed as well as throttle position.
Since light aircraft use engine oil pressure to control the prop a hunting prop (ie uncommanded RPM excursions above and/or below the selected RPM) can be a sign of dropping oil pressure and therefore the engine oil pressure should be immediately checked if the prop is not operating normally.
It is also important to know which prop type, Mcaulley, or Hartzell is on your aircraft as they work in opposite ways. A decrease in oil pressure will cause a
Mcaulley prop to go to course or low pitch. The Hatzell will go to fine pitch or high RPM with the loss of oil pressure.
In Multi engine aircraft an overspeeding prop should be feathered immediately (except for an overspeed right after takeoff) .
untill it reaches 70 to 100 knots on the takeoff run. At that time the prop will start to unload and speed up, so the govenor will increase the pitch of the prop blades to stop the prop from speeding up and exceeding the seelcted RPM (Redline in the case of the takeoff)
If there was a failure which caused the prop togo to the fine pitch stops during cruise flight the prop will very significantly exceed the redline RPM. If this occurs an immediate sharp pull up should be initiated in order to reduce airspeed and thus RPM. Prop RPM will now be controlled by airspeed as well as throttle position.
Since light aircraft use engine oil pressure to control the prop a hunting prop (ie uncommanded RPM excursions above and/or below the selected RPM) can be a sign of dropping oil pressure and therefore the engine oil pressure should be immediately checked if the prop is not operating normally.
It is also important to know which prop type, Mcaulley, or Hartzell is on your aircraft as they work in opposite ways. A decrease in oil pressure will cause a
Mcaulley prop to go to course or low pitch. The Hatzell will go to fine pitch or high RPM with the loss of oil pressure.
In Multi engine aircraft an overspeeding prop should be feathered immediately (except for an overspeed right after takeoff) .
Join Date: Jul 2009
Location: N 06/W 75
Posts: 81
Likes: 0
Received 0 Likes
on
0 Posts
In Multi engine aircraft an overspeeding prop should be feathered immediately (except for an overspeed right after takeoff) .