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Jens
2017-Jul-03, 08:39 AM
I have a question about the recent incident of a plane that started shaking after (apparently an engine malfunction) while it returned to Perth, where it had taken off for an hour earlier. It's not confirmed that it seems that a blade broke and was contained in the engine. The question I have is (and there will be a report eventually, so it's speculative at this point), why didn't the pilots shut the engine off? In one article it said that they did, but it kept spinning because of the airflow. Is that true? I would think that if you shut it down, it would stop spinning.

swampyankee
2017-Jul-03, 09:46 AM
The bearings in a turbofan are quite good, so unless there is a very severe mechanical failure, everything will keep spinning, or windmilling at any significant airspeed.

trinitree88
2017-Jul-03, 11:03 AM
Swampy. Yep. A neighbor was a US helicopter pilot, and engine failed @ altitude over Atlantic near New Jersey. Emergency procedure involves letting the rotor windmill, while descending, then using the stored angular momentum, while flaring the pitch angle of the rotor blades to land ( hard landing) in water. Skilled pilot...nobody hurt....copter sank in Ocean.
In a commercial jet...the loose piece of prop is contained by a Kevlar bag in the cowling....saves many lives....pete

trinitree88
2017-Jul-03, 11:23 AM
Used to be that rare occurence....the piece of prop would fly through the housing, sometimes rupture a fuel line or tank, and cause a fire.Now the wings are filled with nitrogen, not air, to aid in fire suppression.
Lots of the improvements were made from military needs. One of my favorite planes, the A--10, flying close air support ,low,over battlefields at slow speeds, receives rough ground fire....needs fire suppression, armor, duplicity in all systems, a titanium bulletproof tub for the pilot, with a 3 inch polycarbonate canopy.
When it fires it's gun, the rounds are armor penetrating, incendiary, and antipersonnel/ shrapnel. So receiving ends gets holes and leaking fuel....instant fire, and everybody's hurt. Ground troops on our side love it.

grant hutchison
2017-Jul-03, 11:53 AM
A friend of mine was leading a platoon that was fired on by an A10 in Iraq - so he's one of the ground troops on "our side" that are less inclined to love it.

Grant Hutchison

slang
2017-Jul-03, 12:17 PM
And that's enough of that derail. Please stick to fan blades, not A-10 fans or haters.

Jens
2017-Jul-03, 02:36 PM
So as I understand it, there is no mechanism to slow the turbofan down, so it keeps on spinning. I guess that makes sense, as there isn't really any scenario where you would want to stop the turbine, and having such a mechanism raises the danger of accidental deployment.

swampyankee
2017-Jul-03, 04:39 PM
So as I understand it, there is no mechanism to slow the turbofan down, so it keeps on spinning. I guess that makes sense, as there isn't really any scenario where you would want to stop the turbine, and having such a mechanism raises the danger of accidental deployment.

The number of scenarios that are designed for is large, including failure of the shaft between the power turbine and fan, which figures into the design of the overspeed prevention system, and what happens if all the oil is drained out in flight (which we simulated by taking an engine, windmilling it to cruise rpm, steam heating the engine oil, draining it out, and continuing to windmill the engine), blade failures, ingesting a mechanic's hand tool, shooting birds into them, trying to run them on boiling gasoline, and spraying water into them with a firehose. And keeping all the blades inside.

DaveC426913
2017-Jul-03, 08:51 PM
If the engine were brought to a deliberate stop, it would become a bigger source of drag, affecting flight control. While spinning freely, it has a lower drag effect.

Hornblower
2017-Jul-03, 10:33 PM
With a propeller that has variable pitch, it is straightforward to build it so the blades can be feathered to reduce drag and prevent unwanted windmilling in case of an engine failure. This does not increase the complexity of the propeller hub, but merely extends the range of pitch setting. If I am not mistaken, the big fan on the front end of a turbofan engine is a rigid, fixed pitch device, so there is no way to feather it. Adding a feathering mechanism to those numerous blades would add a lot of complexity. My guess is that windmilling that could be disastrous for a failed reciprocating engine would not be as much of an issue with a turbine. The windmilling does make things dicey if the fan throws a blade and becomes badly unbalanced as a result.

grapes
2017-Jul-04, 12:33 AM
The windmilling does make things dicey if the fan throws a blade and becomes badly unbalanced as a result.
Why it continued to shake, all the way back, right?

billslugg
2017-Jul-04, 01:15 AM
Yes, the windmilling engine fan with a broken blade vibrated all the way back to the airport.

Engine designers would love to be able to control the pitch of turbine blades, it would greatly improve efficiency if different RPM's could have different pitch settings. Unfortunately, the turbine hub is already stressed to the maximum and has no room for additional complexity. It is made from the strongest metal they can justify and has milled slots on its periphery to hold the blades. It is the strength of this single hub that limits the design speed of the entire engine.

swampyankee
2017-Jul-04, 02:03 AM
Yes, the windmilling engine fan with a broken blade vibrated all the way back to the airport.

Engine designers would love to be able to control the pitch of turbine blades, it would greatly improve efficiency if different RPM's could have different pitch settings. Unfortunately, the turbine hub is already stressed to the maximum and has no room for additional complexity. It is made from the strongest metal they can justify and has milled slots on its periphery to hold the blades. It is the strength of this single hub that limits the design speed of the entire engine.


Actually, limited production, variable-pitch turbofan engines have been built. Of course, one can get rid of the ductwork and make a propfan.

Speed of rotation is set by the required stage pressure ratio and tip speed; usually turbofans run about 1200 to 1500 ft/s tip speed

profloater
2017-Jul-05, 06:45 PM
I saw an experimental Olympus engine in about 1967 after a simulated bird strike. Unfortunately the chicken was frozen, it was a metal mess. Around that time Rolls Royce 1971 was bankrupted by carbon blades which had huge promise but they never solved the creep problem in time. One problem at that time in turbine blades was flutter, which would lead to fatigue early failure. Another was surge, still a risk at hot take off. The engine can race and then stall the front end compressor blades, effectively blocking flow for a while. I heard a tape from the cockpit of the flying test bed Vulcan with two Olympus strapped underneath. Trubshaw, the test pilot coolly said " left engine surging" and so on, no drama in his voice. The ground engineers had no instrument to detect a surge so they asked how he knew. "I get seven g sideways" was his equally cool reply.

swampyankee
2017-Jul-06, 04:23 PM
I saw an experimental Olympus engine in about 1967 after a simulated bird strike. Unfortunately the chicken was frozen, it was a metal mess. Around that time Rolls Royce 1971 was bankrupted by carbon blades which had huge promise but they never solved the creep problem in time. One problem at that time in turbine blades was flutter, which would lead to fatigue early failure. Another was surge, still a risk at hot take off. The engine can race and then stall the front end compressor blades, effectively blocking flow for a while. I heard a tape from the cockpit of the flying test bed Vulcan with two Olympus strapped underneath. Trubshaw, the test pilot coolly said " left engine surging" and so on, no drama in his voice. The ground engineers had no instrument to detect a surge so they asked how he knew. "I get seven g sideways" was his equally cool reply.

I think that Rolls-Royce was also vilified by the "thaw the chicken" meme. At the time, I believe the birds needed to be fresh-killed, never frozen. Certainly, the bird shots done at Lycoming-Stratford in the 1970s, about the time or after the bird shots were done for the RB211 with Hyfil blades. British airworthiness regulations were probably very close to the US FARs.

publiusr
2017-Jul-08, 04:23 PM
I'm looking forward to distributed lift. Electric motors and propellers on the wings--but actual power-plants maybe in the fuselage--perhaps for in-flight repair--something we haven't had since the days of airships

profloater
2017-Jul-08, 05:10 PM
Oh for room temperature super conductors, you could have a vertical thrust fan with tiny shaft and magnetic tipped blades with a stator of superconducting coils to drive it. Or you charge the air with electron beams and just thrust the charged air through your pulsing magnets. It would look like a flying saucer of myth actually.

publiusr
2017-Jul-08, 05:31 PM
I can see lifter ion wind contraptions around an airship--the whole vehicle being one big air-warp nacelle.

swampyankee
2017-Jul-08, 05:42 PM
I'm looking forward to distributed lift. Electric motors and propellers on the wings--but actual power-plants maybe in the fuselage--perhaps for in-flight repair--something we haven't had since the days of airships

Something we haven't needed, and was not particularly successful even when it was tried. Aircraft engines, especially turbine engines, are incredibly reliable. Besides, you want to be very sure that the passengers can't get to the engines. Putting the engines outside helps that ;)

publiusr
2017-Jul-14, 07:30 PM
They can still throw coins in them--for luck
http://www.cnn.com/2017/06/28/asia/china-coin-planes/index.html

Jens
2017-Jul-14, 11:59 PM
Something we haven't needed, and was not particularly successful even when it was tried. Aircraft engines, especially turbine engines, are incredibly reliable. Besides, you want to be very sure that the passengers can't get to the engines. Putting the engines outside helps that ;)

I think it also helps to reduce noise. And I think it helps to stop fires from spreading. Once you cut off the fuel the engine is basically isolated from the rest of the plane.

swampyankee
2017-Jul-15, 02:22 AM
I saw an experimental Olympus engine in about 1967 after a simulated bird strike. Unfortunately the chicken was frozen, it was a metal mess. Around that time Rolls Royce 1971 was bankrupted by carbon blades which had huge promise but they never solved the creep problem in time. One problem at that time in turbine blades was flutter, which would lead to fatigue early failure. Another was surge, still a risk at hot take off. The engine can race and then stall the front end compressor blades, effectively blocking flow for a while. I heard a tape from the cockpit of the flying test bed Vulcan with two Olympus strapped underneath. Trubshaw, the test pilot coolly said " left engine surging" and so on, no drama in his voice. The ground engineers had no instrument to detect a surge so they asked how he knew. "I get seven g sideways" was his equally cool reply.

A coworker was in a 727 landing at LGA when, apparently, the pilot had to do a go 'round (a helicopter was about to cross the runway). He said he noticed that at least one engine was going "pop-pop-pop" as it was going into a mild stall. He was, he reported, getting a bit nervous....

(as an aside, we were able pick up surge pretty quickly by the loud noise; when we did some tunnel testing in Tullahoma it was still obvious even with the engine on the opposite side of several inches of steel)

BigDon
2017-Jul-15, 04:57 PM
Tomcats had a very bad habit of losing an engine at speed. Since "at speed" was often at high altitude and over the ocean, for a long time there were no recoverable wrecks that could pin point a cause. Tomcats dealt poorly with asymmetric thrust and this lead all too often led to a flat spin and the loss of the aircrew. Not shown in movies is the fact that aircrews often died well before the aircraft reaches the ground. Enough energy to drive a manned 25 ton object Mach 2.2 in a linear fashion now has that energy translated into a spinning drop. It's not good for people to be subjected to that. I once entered the ready room on business and several junior aircrew were listening to an incident that happened the cruise before I got there. One of our birds went down in a flatspin and the aircrew died with an open mike. One of those "Don't let this happen to you!" type training lessons.

Mind you a lot of times such lessons can back fire but this was convincing. This loss was during ACM and not a engine failure per se. (Sorry Clev, ACM = Air Combat Maneuvers aka dog fighting practice.)

I bring this up because I was on the flightdeck covering a landing cycle when one of our birds was in the final turn and it started doing uncharacteristic things. Like chugging like a locomotive and blowing enormous black smoke rings, with a distinct shower of ejecta. Stuff that looked important from an aeronautical stand point. This flight had just finished a grudge match with the Airforce so the bird was properly stressed and it was just fortune the aircraft was in a landing configuration when the failure occurred. This brave pilot not only didn't eject, he flew the bird back to then NAS Miramar!

Even worse for the pilot was that fact that he decided the backseater had to go. Didn't know what was going to happen from moment to moment and wasn't going to risk the RIO's life. The RIO really, really didn't want to eject over the ocean so the pilot had to do it for him. :)

The Chief Engine Mech for our squadron, who looked for all the world like a friendly Idi Amin was the first of the ground personnel to the aircraft. As another engine mech I knew said, "I've known the chief for ten years and this was the first time I've seen him bug eyed!"

Why?

When you looked down the intake there was daylight! Waaay too much daylight! And there was a perfect spiral of turbine blades stuck in the interior cowling as something tried to climb its way out of the engine. At this time the engines were encased in three and a half inches of armor grade titanium. This was the first one to come home. And the stator hub was just laying there in the bottom of the intake. The Engine Chief was so amazed he picked it up with both (gloved) hands just to see for himself it wasn't attached to anything.

A highly engineered chrome steel hydraulic access was getting sucked out of its place and into the rotors was the cause. Wasn't highly engineered enough it seems.

By the way, that bit of piloting is on par with the Tomcat pilot who was doing a gun run on a tow target only to discover much to his chagrin that when the ordinancemen replaced the gun the night before they didn't attach the front of the gun, (cannon) properly.

So his six barreled, 20mm autocannon described a small circle inside its housing, while firing at a rate of about 30 rounds a second.

Yep, blew his own nose off. And he did the same thing. Ejected his backseater and flew what was left home. Probably so he could kick his ordinancemen's backsides just that much sooner.

Good thing he wasn't firing warshot or both him and his bird would have become high tech confetti. Simple slugs as opposed to a mix of high explosive armor piercing incendiary rounds. (Amazing how much mayhem you can cram into a 20mm round.)

R.A.F.
2017-Jul-15, 05:12 PM
BigDon's "war stories" :) are always informative and entertaining, and I thought I'd just say so...

Thanks, BigDon...and keep 'em coming.

DaveC426913
2017-Jul-16, 06:59 PM
... a distinct shower of ejecta. Stuff that looked important from an aeronautical stand point.
:rofl:

I have this image of the co-pilot looking out the window and pointing and some piece of something or other receding in the distance and saying "Hey, that's a [X]. Don't we need that?"

otakenji
2017-Jul-30, 03:34 AM
After a jet engine throws a compressor blade, even if the engine is shut down, there will be some windmilling effect as air keeps going into the engine.
(Ex USAF Jet Engine Mechanic)

swampyankee
2017-Jul-30, 12:51 PM
After a jet engine throws a compressor blade, even if the engine is shut down, there will be some windmilling effect as air keeps going into the engine.
(Ex USAF Jet Engine Mechanic)

They can keep running. My manager, back when I was in college, tried to find what would cause a catastrophic shutdown of a T53, the engine (there was only the one) in a Huey. He brought in his M1911 Colt, clamped it in a vice in a test cell, and proceeded, methodically, to shoot up a running engine, which kept running through most of it, excep when the fuel control got holed.