Re: what is meant by a "runaway" propeller?
Fri Jan 24, 2014 10:50 pm
In simplistic terms........ 
Engine driving prop = good
Prop driving engine = bad
Engine driving prop = good
Prop driving engine = bad
Re: what is meant by a "runaway" propeller?
Fri Jan 24, 2014 11:38 pm
A Hydromatic propeller goes to fine pitch ("increase RPM") when oil pressure is lost. The prop governor ports engine oil pressure to control the pitch. If prop control is lost, the blades go to the fine pitch stops (mechanical stops inside the prop hub).
On takeoff, the prop starts at the fine pitch stops, but rapidly goes into the governing range. You often feel a powerful surge early in the takeoff roll as this happens. That is the prop moving off the pitch stops.
On final approach you normally reduce power and move the prop control all the way forward to the increase RPM position. This once again puts it on the fine pitch stops. At any airspeed much above final approach speed, the airstream would windmill the prop enough to drive an engine at idle power. You aren't developing much power at idle anyhow and it doesn't take much effort for the prop to drive the engine even with the ignition or fuel off, same as with a fixed pitch propeller.
A prop going to the fine pitch stop in cruise would seriously overspeed the engine.
A Counterweight propeller on the other hand, like used on a T-6, goes to coarse pitch ("decrease RPM") when oil pressure is lost so it would not overspeed in that case.
On takeoff, the prop starts at the fine pitch stops, but rapidly goes into the governing range. You often feel a powerful surge early in the takeoff roll as this happens. That is the prop moving off the pitch stops.
On final approach you normally reduce power and move the prop control all the way forward to the increase RPM position. This once again puts it on the fine pitch stops. At any airspeed much above final approach speed, the airstream would windmill the prop enough to drive an engine at idle power. You aren't developing much power at idle anyhow and it doesn't take much effort for the prop to drive the engine even with the ignition or fuel off, same as with a fixed pitch propeller.
A prop going to the fine pitch stop in cruise would seriously overspeed the engine.
A Counterweight propeller on the other hand, like used on a T-6, goes to coarse pitch ("decrease RPM") when oil pressure is lost so it would not overspeed in that case.
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 8:45 am
Jacks got the right answer!
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 10:03 am
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 11:15 am
A constant speed prop varies pitch in order to maintain the selected RPM ... within the mechanical limits of its pitch change; it's not as if it has unlimited ability to maintain an RPM.
When an underspeed is sensed, the governor calls for a lower blade angle. Rather than think of the direction the aircraft is traveling through the air, consider the viewpoint of each blade. A flat blade is more streamlined and will find it easier to slice through the air as it swings through its arc. As a result, less power is required to spin it ... or if the energy put in the prop is constant, the prop speeds up.
Conversely, as blade angle increases, more surface area is presented to the air so that either more power is required to maintain RPM or, if power is constant, the prop slows down. We've all played this game with a hand out the car window and compared the drag of a "karate chop" to a "barn door".
The aerodynamics in play in relation TO the direction the airplane is flying are that a flat prop windmilling generates more drag than one at a high blade angle. If you've never felt this, one way to remember it is that, of course, a feathered prop is the highest blade angle (+/- 90ish degrees), the lowest drag, and the lowest RPM (zero).
Excepting the counterweighted props bdk mentioned, a lack of oil pressure to run the governor plus the physics of centripetal, centrifugal, and blade twist forces will physically drive a prop to flat/fine pitch and high RPM. The blades aren't literally at zero pitch - they stop at the mechanical low pitch stops which are likely the placarded numbers we rivet counters look for stenciled on the blades.
Once the blades are sitting at the low pitch stop, think of it as a fixed pitch prop because the pitch is no longer changing. The common analogy here is the kid's pinwheel on a bicycle. The faster the bike goes, the faster the faster the pinwheel spins - which is the reason the B-17 pilot mentioned earlier slowed to a near stall in an effort to reduce windmilling RPM. (As an aside, multiengine pilots also have to be cognizant of minimum control speeds, since stall speed may allow the airplane wing to fly below the speed at which rudder and aileron can counteract the drag of an asymmetrical prop problem which may lead to a rolling, yawing loss of control before a stall.)
We recognize torque as a twisting force. Normally a running engine supplies torque to spin a prop. jtramo made the great analogy of a geared bike. I assume all of us have experienced the pedal force of going up a hill (we drive the wheel) and then the opposite going downhill where you can't pedal fast enough to transmit force to the wheel, the wheel is "driving" the pedal - some prop systems call this "negative torque". You either change gears or accept the situation. The figurative "gears" of a prop change with blade angle; if I can increase blade angle and slow the prop down I have a chance of allowing the engine (just like you pedaling like mad) to catch up and supply positive torque.
In the case of a failed engine, the prop will often overcome the internal friction of the motor and continue to turn - this is full-on negative torque. Most light GA singles with 4-cylinder engines do this too no matter whether the prop is a fixed or constant speed type. (Once variable blades reach their stops, they are essentially fixed anyway.)
Simulating a failed engine by pulling the throttle to idle and gliding, the prop will run to its stops to attempt to maintain RPM. Accepted technique (if restart fails) is to select low RPM ... the prop (absent another malfunction) fixes at its highest blade angle. The pinwheel effect comes into play again and glide TAS determines RPM. The effect in the cockpit is noticeable when low RPM is selected, the airplane feels as if it has surged forward due to the reduction in drag. (Two asides: Flying's Peter Garrison did a piece on throttle position and shared that a wide open throttle on a failed engine will also reduce internal resistance; energy not needed to make the prop spin should then be available to extend glide distance slightly. The other aside is that some pilots say that they will momentarily pitch up, stop prop rotation, and then resume gliding - another writer who tried various experiments in this reports that rarely does this work as well as it sounds.)
And the final thought is the overspeeding prop. Various malfunctions might cause this, and, it may not matter much whether the engine continues to run or is shut down - either way it's in negative torque mode. If one of the elements of the prop problem is that no oil is circulating, then we have a prop (and probably engine depending on which airplane) turning faster than designed and not being lubricated. This can lead to shaft failure, blade failure, or engine seizure - any of which will likely lead to the prop departing the airframe. If that's not bad enough, in some cases, the engine may be torn from the mount, structure damaged, or in the case of a single, the change in center of gravity could cause the plane to be uncontrollable once the prop and/or engine departs. All in all, not good stuff.
Well, I rambled long enough.
Ken
When an underspeed is sensed, the governor calls for a lower blade angle. Rather than think of the direction the aircraft is traveling through the air, consider the viewpoint of each blade. A flat blade is more streamlined and will find it easier to slice through the air as it swings through its arc. As a result, less power is required to spin it ... or if the energy put in the prop is constant, the prop speeds up.
Conversely, as blade angle increases, more surface area is presented to the air so that either more power is required to maintain RPM or, if power is constant, the prop slows down. We've all played this game with a hand out the car window and compared the drag of a "karate chop" to a "barn door".
The aerodynamics in play in relation TO the direction the airplane is flying are that a flat prop windmilling generates more drag than one at a high blade angle. If you've never felt this, one way to remember it is that, of course, a feathered prop is the highest blade angle (+/- 90ish degrees), the lowest drag, and the lowest RPM (zero).
Excepting the counterweighted props bdk mentioned, a lack of oil pressure to run the governor plus the physics of centripetal, centrifugal, and blade twist forces will physically drive a prop to flat/fine pitch and high RPM. The blades aren't literally at zero pitch - they stop at the mechanical low pitch stops which are likely the placarded numbers we rivet counters look for stenciled on the blades.
Once the blades are sitting at the low pitch stop, think of it as a fixed pitch prop because the pitch is no longer changing. The common analogy here is the kid's pinwheel on a bicycle. The faster the bike goes, the faster the faster the pinwheel spins - which is the reason the B-17 pilot mentioned earlier slowed to a near stall in an effort to reduce windmilling RPM. (As an aside, multiengine pilots also have to be cognizant of minimum control speeds, since stall speed may allow the airplane wing to fly below the speed at which rudder and aileron can counteract the drag of an asymmetrical prop problem which may lead to a rolling, yawing loss of control before a stall.)
We recognize torque as a twisting force. Normally a running engine supplies torque to spin a prop. jtramo made the great analogy of a geared bike. I assume all of us have experienced the pedal force of going up a hill (we drive the wheel) and then the opposite going downhill where you can't pedal fast enough to transmit force to the wheel, the wheel is "driving" the pedal - some prop systems call this "negative torque". You either change gears or accept the situation. The figurative "gears" of a prop change with blade angle; if I can increase blade angle and slow the prop down I have a chance of allowing the engine (just like you pedaling like mad) to catch up and supply positive torque.
In the case of a failed engine, the prop will often overcome the internal friction of the motor and continue to turn - this is full-on negative torque. Most light GA singles with 4-cylinder engines do this too no matter whether the prop is a fixed or constant speed type. (Once variable blades reach their stops, they are essentially fixed anyway.)
Simulating a failed engine by pulling the throttle to idle and gliding, the prop will run to its stops to attempt to maintain RPM. Accepted technique (if restart fails) is to select low RPM ... the prop (absent another malfunction) fixes at its highest blade angle. The pinwheel effect comes into play again and glide TAS determines RPM. The effect in the cockpit is noticeable when low RPM is selected, the airplane feels as if it has surged forward due to the reduction in drag. (Two asides: Flying's Peter Garrison did a piece on throttle position and shared that a wide open throttle on a failed engine will also reduce internal resistance; energy not needed to make the prop spin should then be available to extend glide distance slightly. The other aside is that some pilots say that they will momentarily pitch up, stop prop rotation, and then resume gliding - another writer who tried various experiments in this reports that rarely does this work as well as it sounds.)
And the final thought is the overspeeding prop. Various malfunctions might cause this, and, it may not matter much whether the engine continues to run or is shut down - either way it's in negative torque mode. If one of the elements of the prop problem is that no oil is circulating, then we have a prop (and probably engine depending on which airplane) turning faster than designed and not being lubricated. This can lead to shaft failure, blade failure, or engine seizure - any of which will likely lead to the prop departing the airframe. If that's not bad enough, in some cases, the engine may be torn from the mount, structure damaged, or in the case of a single, the change in center of gravity could cause the plane to be uncontrollable once the prop and/or engine departs. All in all, not good stuff.
Well, I rambled long enough.
Ken
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 12:17 pm
During WW-II there were many instances of runaway props, frequently as the result of battle damage. B-17s came home with entire engines wrenched off and major gashes in wing and fuselage structures caused by thrown props. One solution was to equip the aircraft with an auxiliary stand-pipe to contain enough reserve oil to feather the prop of a damaged engine that had lost its internal oil.
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 12:52 pm
dbrown wrote:I suppose the effect would be similar to this?
http://www.youtube.com/watch?v=yLLG2_ErvJs
Amazing video! Kevin has a lot of experience with Maydays at Reno doesn't he?
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 2:28 pm
A good example of the runaway prop from battle damage
http://b-29.org/73BW/500BG/heffner/tokyo-raiders/tokyo.html
There is an accompanying shot that I have seen which has the crew posed in front of the rest of the aircraft. The nose separated on landing.
In this photo posted earlier the hub or barrel of the prop failed and the blades shed themselves off of the spider which is still attached to the prop shaft. If one blade failed then the engine would likely have been torn off from the vibration. It looks like when the hub failed they all departed at close to the same time which was probably quite fortunate for the crew.
You can see a neat slice from the one blade that cut through the nacelle of number three engine and then on to the fuselage. This is what caused number three engine to fail or run pretty rough as the blade probably took out several cylinders on its way through. I would guess that the shrapnel marks on the side of the fuselage were from the parts of number three engine. Double engine failure on one side.
Pilot and the crew certainly earned their pay that day. Love to read the incident report for that flight!
http://b-29.org/73BW/500BG/heffner/tokyo-raiders/tokyo.html
There is an accompanying shot that I have seen which has the crew posed in front of the rest of the aircraft. The nose separated on landing.
In this photo posted earlier the hub or barrel of the prop failed and the blades shed themselves off of the spider which is still attached to the prop shaft. If one blade failed then the engine would likely have been torn off from the vibration. It looks like when the hub failed they all departed at close to the same time which was probably quite fortunate for the crew.
You can see a neat slice from the one blade that cut through the nacelle of number three engine and then on to the fuselage. This is what caused number three engine to fail or run pretty rough as the blade probably took out several cylinders on its way through. I would guess that the shrapnel marks on the side of the fuselage were from the parts of number three engine. Double engine failure on one side.
Pilot and the crew certainly earned their pay that day. Love to read the incident report for that flight!
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 4:10 pm
k5083 wrote:Yeah, same thing happened to Hinton when he spent the first 8 of his lives that day in the RB-51. I get how the flat pitch causes huge drag. I don't get how it turns the engine fast if the engine is not making power.
Is it just that the engine may have gone to very high RPM when still making power and there is not enough pitch to slow it back down even with power off? Especially with the airflow giving it at least a little push.
August
The RB-51 and Hinton had an engine failure and no way to get the prop to feather, therefore his ensuing flat pitch and drag caused him to not make the runway and crash. This was not a prop runaway event.
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 5:11 pm
dbrown wrote:I suppose the effect would be similar to this?
http://www.youtube.com/watch?v=yLLG2_ErvJs
Yep. And it couldn't have happened to a better pilot. That's his second engine that's blown up in his face during a race.
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 8:50 pm
Having that huge Hamilton prop do that on a B-29 would have been something to hear (or not).
Are there any instances where pilots of bombers described the sound? Would it have been similar to the split second high wind-up sound as on the video?
Makes one want to keep a few feet in front of or behind the center line of the prop.
DB
Are there any instances where pilots of bombers described the sound? Would it have been similar to the split second high wind-up sound as on the video?
Makes one want to keep a few feet in front of or behind the center line of the prop.
DB
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 9:25 pm
Having had one in a C-54 (R-2000) on takeoff, I don't what another thanks. Shut it down and stayed in the pattern and landed, it feathered as I turned off onto the taxiway. Saw 3300 rpm, engine change.
Re: what is meant by a "runaway" propeller?
Sat Jan 25, 2014 10:39 pm
K5DH wrote:Look at the rear fuselage shape on that plane in Mark's photo. Izzat an X-plane dropper?
I bet you're right. Looks like they're on a lakebed surface and the background looks like Edwards, er um, I mean Muroc
Re: what is meant by a "runaway" propeller?
Sun Jan 26, 2014 6:55 pm
Dave Hadfield wrote:This happened to a friend of mine in B-17 CF-HBP in about 1967 while on high-altitude photo survey for Kenting.
Apparently the Wright-1820 did not have an oil reserve to allow feathering the prop if the engine failed due to an oil leak. The crew had an oil quantity total loss at 33,000, with the result that the prop couldn't be feathered, and the thing "ran-away".
I don't understand this?
The Wright 1820 itself does not hold an oil reserve, infact I don't think any aero engine of these types (warbird engines) actually contain any quantity of oil other than that which is in the pressure system, as it is almost then immediately scavenged back to the oil tank, normally via an oil cooler.
B-17 oil tanks have two oil pick off points, one for the normal running conditions and a second lower pick off point that only supplies the feathering pump.
The only situation where an engine/prop arrangement had no oil left to enable the prop to be feathered, is if the tank is completley compromised right to the very bottom and so drains to empty, or the oil line from the fearhering pick off point has become disconnected at some point, again leaving the oil tp drain completley away.
Re: what is meant by a "runaway" propeller?
Sun Jan 26, 2014 7:19 pm
Dave Hadfield wrote:A friend of mine started his career as a Flt Engineer on DC-6s. It was an El-Cheapo outfit. If they had a problem with a starter, or couldn't get an engine going, one technique they had was to roll fast down a long runway and try to get the airflow to start rotating the propeller.
This is indeed a very viable way of starting an angine and having been on board 'heavy metal' with the same 'starter' problem, I can say this method of starting works very sucessfully, so long as the Pilot(s) at the helm of the aeroplane knows what he is(they are) doing!
I'm not sure I remember it being started in the way you describe here. I seem to remember it the other way around, because in feather, it can take possibly too long for the feather pump to start moving the blades back to where you want them before the runway runs out having taken up more runway than usual, in order to get up enough speed in the first place. Obviously for a very long runway, this might not be an issue.Dave Hadfield wrote:As you say August, it wouldn't budge in fine pitch. They had to start in coarse, working back from "feather". That provided more leverage on the blades to overcome the initial stiffness and reluctance to rotate. But once turning, going to full fine pitch, as in the case of the runaway, would cause it to accelerate, not stop.
Dave
The other things of course are that a taildragger is more difficult to start this way than a tricycle and that an outboard engine is a little more tricky to start this way, than an inboard.
If both of these elements apply, then there is a nice multiplier for you, but I can tell you it can be done and I was both surprised and impressed the first time I saw it.