what is meant by a "runaway" propeller?

[hbtcoveralls]

Reading a book about B-17s in WWII and I've come across the term runaway propeller. Can somebody explain what it is and, I'm certain it's bad, what causes it and what can happen due to one. Thanks
Tom Bowers

[Dave Hadfield]

If the prop pitch goes to full-fine, and the control mechanism fails, and the aircraft is at flying speed, the airflow is sufficient to cause the prop to run up to a very high RPM, possibly beyond it's structural limits. In that case, shutting off the engine via the mags or mixture would not help -- the prop will spin the engine regardless.

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". They were somewhere over northeast BC I believe, or may the NW part of the Territories -- not the high Arctic.

So my friend Jerry F. hauled the nose up to the edge of the stall, to keep the rpm as low as possible, and kept the airplane "mushing" forward and down, and fluttered like that all the way down to where he was able to plop the airplane onto a short gravel strip. The whole way....

The worry of course was that the prop would tear off and a blade would come through the fuselage and slice them up.

Dave

[mike furline]

In simple terms it means the prop governor is no longer controlling the blade angle of the prop. You're moving the prop control and the prop aint doing what it should.

Causes - too much or lack of oil pressure to the prop, prop governor, prop control linkage, planetary gear/nose case failure, any type of battle damage to these components.

What can be done - shut down the engine, but the prop may still spin uncontrollably based on blade angle and problem.

Last edited by mike furline on Fri Jan 24, 2014 3:20 pm, edited 1 time in total.

[sandiego89]

Yes a bad day, A good, easy to understand, overview is here (scroll to the very bottom):

http://www.pilotfriend.com/training/fli ... /props.htm

[jtramo]

It's like riding a bike downhill with it in the slow gear and your legs spinning the crank until your legs (the engine) burn out.

Not sure how the Wrights, with a complete lack of oil, would keep spinning at high rpm. I'd assume the real issue was (with no dedicated feather oil supply) that the blades would be flat (high rpm fine pitch) with an oil loss engine failure. Aka drag.

[Mark Allen M]

Allegedly the cause here ...

[K5DH]

Look at the rear fuselage shape on that plane in Mark's photo. Izzat an X-plane dropper?

[k5083]

If the prop pitch goes to full-fine, and the control mechanism fails, and the aircraft is at flying speed, the airflow is sufficient to cause the prop to run up to a very high RPM, possibly beyond it's structural limits. In that case, shutting off the engine via the mags or mixture would not help -- the prop will spin the engine regardless.

That surprises me. I'm not arguing with it, I just don't understand it.

if the engine is still running, I can understand how the prop in full fine presents almost no air resistance and the engine can overspeed, just like if it has no load (prop) on it.

If the engine is stopped, it's hard for me to see how airflow coming nearly perpendicular to the blade angle makes it spin fast. Seems to me it would windmill faster with about 45-degree pitch than at full fine.

August

[Mark Allen M]

A fine pitch propeller has a low blade angle (more "vertical") and will take a 'small' bite of the air, each rotation will try to move forward a small distance through the air. It requires relatively low power to rotate, allowing high propeller speed to be quickly developed, but achieving only limited airspeed. This is like having a low gear in your car.

[Badman]

If the prop pitch goes to full-fine, and the control mechanism fails, and the aircraft is at flying speed, the airflow is sufficient to cause the prop to run up to a very high RPM, possibly beyond it's structural limits. In that case, shutting off the engine via the mags or mixture would not help -- the prop will spin the engine regardless.

That surprises me. I'm not arguing with it, I just don't understand it.

if the engine is still running, I can understand how the prop in full fine presents almost no air resistance and the engine can overspeed, just like if it has no load (prop) on it.

If the engine is stopped, it's hard for me to see how airflow coming nearly perpendicular to the blade angle makes it spin fast. Seems to me it would windmill faster with about 45-degree pitch than at full fine.

August

It's my understanding this term really applies to the situation where the prop governor system as described above fails with the engine on and making power. In that situation the runaway prop can quickly overspeed the engine and result in engine damage/failure and/or prop damage/failure as the photo of the B-29 shows. Pulling the throttle back to idle, pulling the nose up, and even shutting the engine down can all mitigate the overspeed situation and permit a safe recovery of the aircraft. The flat pitch of the prop in this situation also creates a lot of drag whether the engine is on or off. In 1982(?) at Reno Skip Holm had this happen in Jeannie at about 400mph and pulled the nose up and only gained a few thousand feet before bleeding off all that speed and then landed safely, albeit unconventionally.

[k5083]

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

[hbtcoveralls]

Thank you all, it makes a lot of sense now and I do appreciate the time you took to reply
Tom Bowers

[jtramo]

I don't get how it turns the engine fast if the engine is not making power.



August

Centrifugal force from the spinning prop drive the engine. The centrifugal forces from the prop and engine spinning out of control....if not shut down immediately....end up frying bearings and snapping the prop shaft. Best case, it fries the bearings and seizes the engine as opposed to it breaking the shaft and throwing the prop into the fuselage or the other engine.

When an engineer/pilot is watching gauges, identification of the type of failure/issue could take a few seconds. The runaway gone unchecked for a few seconds may be too late.

[CoastieJohn]

Thank you all, it makes a lot of sense now and I do appreciate the time you took to reply
Tom Bowers

If you want to muddy up the water then look at "runaways" vs "overspeeds" and the causes/theories. Also compare recip vs turboshaft. There is alot of discussion to be had with those topics.

[Dave Hadfield]

It's critical. A runaway prop, as stated, can detach, and cut apart the airplane and the people in it.

August, I know what you're saying -- at fine pitch, you wouldn't think the airflow could turn the engine over. And, if you were accelerating from a dead start, it probably wouldn't. But in this case you're already flying, the prop is already turning, and if the prop goes to the limit-setting (to the fine-pitch "stop"), it accelerates the prop to far beyond it's specified limit RPM.

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. 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.

Since they are already turning, the blades are not stalled. If they were stalled, they'd probably stop, but they aren't, so they don't!

Dave