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The fatigue in modern day warbirds


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#1 Skyraider87

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Posted 26 March 2018 - 07:45 AM

Howdy there,

 

I have one question regarding the possible progressive and structural damage of warbirds. If I take f.e. this amazing plane of 1952, an AD-4 Skyraider, which is a veteran of korea and vietnam (where it has been damaged but repaired IIRC), are there certain restricitions for flying/using this plane today?

 

http://www.shermanai...er-plus-spares/

 

The question would go literally for all the warbirds (P-51, Spitfire, etc.), are they restricted to simple take-offs/landings and flybys? What would happen if the warbirds of today would be put into a combat situation on a regular basis (main question here is not about absorbing combat damage, but being exposed to the forces of physics).

 

Since I am neither a pilot nor a mechanic, I can hardly imagine, how things work, forgive me. But to put the question into a contemporary context, what would happen, if this AD-4 would be put into COIN-role, loaded with ordonance and expected to fly ground attack missions, would it literally disintegrate?

 

On the other hand I see old (modified) warbirds like the P-51 being exposed to extreme stress during the Reno Air Races and the last crash of a P-51 there was not caused by the old airframe, but a modification itself.

 

Edit: I know from the F-4 Phantoms here in Germany f.e., which were produced in the 70s, that certain parts had to be replaced after a specific amount of flying hours, which goes for the engine as well, but the frame itself and most parts were making it through the time without trouble, while being exposed to even higher forces then the average warbird possibly could expect.

 

 

Cheerio



#2 Armand

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Posted 26 March 2018 - 05:21 PM

First of all: This is an international forum and it's impossible to include the legislation of the subject for all Nations.
Ex. did UK ban almost all vintage fighter jets as being too complex for private to keep flying. The reason might be an ex pilot who operated some E.E. Lightnings as it caused him to move to South Africa where the legislation was much less radical (however ended crashing due to poor maintanance and died due to a malfunctioning ejectionseat IIRC)!
I'm situated in Denmark, where private jet warbirds became banned after P-80's seemingly became (unexpected) possible to buy, meanwhile a group of privates have brought a F-104 in the air in Norway.

However, I'm sure that there only is one international aproval, hence either it's flyworthy or not, wich likely is prooved in the spectacular displays old warbirds perform regularly, however it's probably not high G's the audience percieve.
In between does classic warbirds actually crash during air displays, but rarely due to structural failure. Lately it was a Hawker Hunter wich looped from too low initial height causing it to hit the ground at the final of the loop and in the mid 90's did a P-38 barrel roll into the ground because the pilot was unaware of this aircrafts characteristics of diving during the second consencutive roll =:-o

A final conclusion is that high G's is hard excercise for the pilot: 2G equals the double weight pressed into the seat - Go figure 5 or 8 G's! Hence a classic warbird pilot have probably not much interrest in exhausting himself by high G maneuvers :-/

Edited by Armand, 26 March 2018 - 05:22 PM.


#3 Skyraider87

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Posted 26 March 2018 - 07:51 PM

Hey Armand,

 

thank you very much for answering, which is much appreciated. I wasnt going so much for the legal restrictions, but still interessting what you have pointed out. My question was regarding more the restrictions by physics :) I remember the crash of the Hawker Hunter in 2011, which resulted in a great tragedy.

 

But how would you asses f.e. the possibilty of a Skyraider of 1952 to become combat operational as a ground attack/COIN-aircraft from the structural point of view?



#4 Armand

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Posted 26 March 2018 - 10:53 PM

But how would you asses f.e. the possibilty of a Skyraider of 1952 to become combat operational as a ground attack/COIN-aircraft from the structural point of view?

Youtube reveal that several Skyraiders actually fly.
Due to my comment of flyworthy or not, hence nothing in between, is flying Skyraiders to expect quite as operational as during the Korean war!

Airforce maintenance might however be beyond the point of servicing the radial engine wich might make the question irrelevant.

Related to the subject i remember to have read that the OV-10 Bronco lately have been reactivated due to it's specific qualities, however it's engined with turboprops.

Edited by Armand, 26 March 2018 - 10:54 PM.


#5 Armand

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Posted 26 March 2018 - 11:09 PM

BTW: The Skyraider is one of my favourites (too) and IMO one of the big 'what if's' concerning WW2 ;-)
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#6 bearoutwest

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Posted 27 March 2018 - 01:35 AM

Welcome to Aircraft Structures Metal Fatigue 101.

It would be best to consider this as a very broad brush overview of the science behind fatigue damage.
First, a couple of terms:
- metal elastic strength;
- metal plastic strength;
- cyclic loading.
When you gently bend a wire coat-hanger up and down, you are working with the metal in the elastic zone, as it returns to it's original shape.  If you bend it by a large amount, it stays bent.  You have now pushed the wire into the plastic zone - it's strength is compromised beyond it's ability to "bounce back" and it stays deformed.  In the first stage, when the wire was being bent gently up and down; you are applying cyclic loads.  As an example, aircraft wings flex (bend gently) up and down in flight, as the loads increase and decrease momentarily in normal flight.  This is cyclic loading in the elastic zone.

More terms:
- fatigue cracks;
- fatigue damage.
Cyclic loading in the elastic zone causes no damage if there are no microscopic cracks present.  If this was the case, our Skyraider could operate within it's acceptable flight regime indefinitely.  Now the cracks could be present for any number of reasons - the most common include:
- deformation during the metal production stage;
- local deformities or damage during connection of aircraft components;
- minor damage during normal operations (anything from a slightly harder landing, a bullet hole, a stone chip).
With these microscopic cracks, the area in close proximity can propagate the crack during cyclic loads.  The crack can widen or lengthen with repetitive load.  Eventually the crack becomes large enough to be visible (perhaps the size of a very thin hair).  The surrounding area of metal now has to take more share of the load.  Eventually that surrounding metal gets pushed into the plastic zone and loses strength, so a further surrounding area of metal must take a greater share of the load....until it goes plastic... and so on.  If undetected, eventually a sufficient amount of metal around the initial crack is plastically deformed that it cannot carry a share of the load.  In our Skyraider, if the wing could normally take (and I am making up example numbers here) +6G (bend upwards) and -4G (bend downwards), the fatigue damage may have reduced it's capacity to +4G/-3G.  The pilot makes a dive attack with a +5G pull-out, thinking he has the aircraft wing strength to do so.  BANG, wing structure deforms at the crack point, air pressure takes over and rips the wing off at the weak point.

Aircraft before the dH Comet were not designed for comprehensive fatigue loading - as it was not fully understood.  An amount of extra strength capacity was factored in as compensation, but aircraft weren't expected to last more than 10 years, so that was probably sufficient.  Subsequent to the Comet crashes, and the greater understanding of cyclic loading leading to crack propagation and to fatigue damage - aircraft are designed to a greater extent for fatigue analysis techniques, and a much greater emphasis applied to regular maintenance inspections for fatigue cracks (using x-rays, metallic crack detection paint, visual inspection, etc).

So for our 50+ year old Skyraider, it would not have been designed with fatigue as a major concern but most likely has extra safety factors applied in design overall.  In flight displays, the flight loads are limited to well below design limits to specifically prolong the fatigue life of components (wing structure, fuselage, etc) and regular fatigue crack inspections would be performed as part of the maintenance tasks.  Technically, there would be no reason why you couldn't strap 4000-lb worth of disposable ordnance under the wings and go bomb something.  As long as you didn't exceed the strength limits and you regularly inspected for fatigue damage (and you promptly replaced damaged parts), you could keep flying operations indefinitely.  However, as soon as you detect fatigue damage - cracks, deformed metal, etc  - all bets are off, and you better make sure your bang-seat is good to go.

 

Welcome to the Mad-house, Skyraider.  Hope you enjoy your stay with us.

...geoff


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#7 GregP

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Posted 27 March 2018 - 08:01 AM

There are regular inspections of structure, at least annually.

At the Planes of Fame, we had occasion to find some cracked bulkheads in one aircraft. They removed the tail, made new bulkheads, installed them, and re-attached the tail. Problem solved, plane returned to flight status. Most airplanes have life limits for flight hours, but you can change that by replacing major structural components, such as spars, bulkheads, longerons, etc. Still, a life-limited bird cannot be resurrected as an intact airframe. It CAN be if pieces are used to make a new structure.

Life-limited items such as rotor blades for helicopters are simply condemned after reaching life hours or sustaining structural damage that cannot be adequately repaired.

All our planes can be flown to structure limit. Most never get flown there in normal operations, even aerobatics. Military fighter specs are such that flight g-limits are usually severe, and not generally encountered unless doing military maneuvers. Nobody pulls 8 g's without meaning to do so. But ours almost NEVER get loaded up to military weights, and as you are aware, lighter means a lower structural maneuvering speed than when heavier. I believe you multiply the stall speed by the square root of the new weight divided by the gross weight to get the new stall speed.

But the structure should be the same strength regardless of weight. The weight simply changes the speed at which structural lift equals the structural strength. In point of fact, I believe the structure DOES change strength with weight, but the relationship is not a simple one to figure, as with stall speed / maneuvering speed changes.


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#8 Skyraider87

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Posted 29 March 2018 - 09:48 PM

BTW: The Skyraider is one of my favourites (too) and IMO one of the big 'what if's' concerning WW2 ;-)

 

nice choice, to be honest beside the amazing performance and capabilities, I think its even an extremely beautiful aircraft. Who in his right mind would choose the Skyraider instead of the beautiful lines of a P-51 or Spitfire, right?! :D But, man this plane has something special, its finest piece of engineering mankind has ever created :) Do you mean with the "big what ifs concerning WW2" her possible role as torpedobomber?

 

 

 

Welcome to Aircraft Structures Metal Fatigue 101.

It would be best to consider this as a very broad brush overview of the science behind fatigue damage.
First, a couple of terms:
- metal elastic strength;
- metal plastic strength;
- cyclic loading.
When you gently bend a wire coat-hanger up and down, you are working with the metal in the elastic zone, as it returns to it's original shape.  If you bend it by a large amount, it stays bent.  You have now pushed the wire into the plastic zone - it's strength is compromised beyond it's ability to "bounce back" and it stays deformed.  In the first stage, when the wire was being bent gently up and down; you are applying cyclic loads.  As an example, aircraft wings flex (bend gently) up and down in flight, as the loads increase and decrease momentarily in normal flight.  This is cyclic loading in the elastic zone.

More terms:
- fatigue cracks;
- fatigue damage.
Cyclic loading in the elastic zone causes no damage if there are no microscopic cracks present.  If this was the case, our Skyraider could operate within it's acceptable flight regime indefinitely.  Now the cracks could be present for any number of reasons - the most common include:
- deformation during the metal production stage;
- local deformities or damage during connection of aircraft components;
- minor damage during normal operations (anything from a slightly harder landing, a bullet hole, a stone chip).
With these microscopic cracks, the area in close proximity can propagate the crack during cyclic loads.  The crack can widen or lengthen with repetitive load.  Eventually the crack becomes large enough to be visible (perhaps the size of a very thin hair).  The surrounding area of metal now has to take more share of the load.  Eventually that surrounding metal gets pushed into the plastic zone and loses strength, so a further surrounding area of metal must take a greater share of the load....until it goes plastic... and so on.  If undetected, eventually a sufficient amount of metal around the initial crack is plastically deformed that it cannot carry a share of the load.  In our Skyraider, if the wing could normally take (and I am making up example numbers here) +6G (bend upwards) and -4G (bend downwards), the fatigue damage may have reduced it's capacity to +4G/-3G.  The pilot makes a dive attack with a +5G pull-out, thinking he has the aircraft wing strength to do so.  BANG, wing structure deforms at the crack point, air pressure takes over and rips the wing off at the weak point.

Aircraft before the dH Comet were not designed for comprehensive fatigue loading - as it was not fully understood.  An amount of extra strength capacity was factored in as compensation, but aircraft weren't expected to last more than 10 years, so that was probably sufficient.  Subsequent to the Comet crashes, and the greater understanding of cyclic loading leading to crack propagation and to fatigue damage - aircraft are designed to a greater extent for fatigue analysis techniques, and a much greater emphasis applied to regular maintenance inspections for fatigue cracks (using x-rays, metallic crack detection paint, visual inspection, etc).

So for our 50+ year old Skyraider, it would not have been designed with fatigue as a major concern but most likely has extra safety factors applied in design overall.  In flight displays, the flight loads are limited to well below design limits to specifically prolong the fatigue life of components (wing structure, fuselage, etc) and regular fatigue crack inspections would be performed as part of the maintenance tasks.  Technically, there would be no reason why you couldn't strap 4000-lb worth of disposable ordnance under the wings and go bomb something.  As long as you didn't exceed the strength limits and you regularly inspected for fatigue damage (and you promptly replaced damaged parts), you could keep flying operations indefinitely.  However, as soon as you detect fatigue damage - cracks, deformed metal, etc  - all bets are off, and you better make sure your bang-seat is good to go.

 

Welcome to the Mad-house, Skyraider.  Hope you enjoy your stay with us.

...geoff

 

Hey Geoff, thank you for taking your time to write this and your nice welcome. It helps me a lot for developing an understanding for the fatigue. If you would have just one more minute please :) and check out the flight maneuvers of the Skyraider in this short film. They start at 3:43 until 4:50.

 

 

So regarding the old airframe of 65 years, if there are no cracks, then this plane should be able to do this (as Cyclic loading in the elastic zone causes no damage if there are no microscopic cracks present.  If this was the case, our Skyraider could operate within it's acceptable flight regime indefinitely) ? What is really interessting in this case is the aspect of maintenance. If the crack (after a hard landing f.e.) is microscopic and becomes just visible when it reaches a dangerous level, then were these planes regulary checked after every possible cause occured? And furthermore how to spot a microscopic crack with something else then an electron micrsoscope, wouldnt that be extremely labour intensive?

 

 

There are regular inspections of structure, at least annually.

 

At the Planes of Fame, we had occasion to find some cracked bulkheads in one aircraft. They removed the tail, made new bulkheads, installed them, and re-attached the tail. Problem solved, plane returned to flight status. Most airplanes have life limits for flight hours, but you can change that by replacing major structural components, such as spars, bulkheads, longerons, etc. Stil, a life-limited bird cannot be resurrected as an intact airframe. It CAN be if pieces are used to make a new structure.

 

Life-limited items such as rotor blades for helicopters are simply condemned after reaching life hours or sustaining structural damage that cannot be adequately repaired.

 

All our planes can be flown to structure limit. Most never get flown there in normal operations, even aerobatics. Military fighter specs are such that flight g-limits are usually severe, and not generally encountered unless doing military maneuvers. Nobody pulls 8 g's without meaning to do so. But ours almost NEVER get loaded up to military weights, and as you are aware, lighter means a lower structural maneuvering speed than when heavier. I believe you multiply the stall speed by the square root of the new weight divided by the gross weight to get the new stall speed.

 

But the structure should be the same strength regardless of weight. The weight simply changes the speed at which structural lift equals the structural strength. In point of fact, I believe the structure DOES change strength with weight, but the relationship is not a simple one to figure, as with stall speed / maneuvering speed changes.

 

Hey Greg, thank you for sharing your experiences. Were these cracked bulkheads found during the annually inspection (by chance) or did something occur before, like what Geoff wrote, did the plane have a rough landing or something similar? Are you scanning during the annually inspection every single square centimeter of the plane to measure if there are any microscopic cracks or are you going for the areas which are exposed to the most stress?

 

Regards,
Che



#9 bearoutwest

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Posted 30 March 2018 - 01:13 AM

Hello Che,
I remember your @tag from the old days - should have said "Welcome Back" instead.

OK, you seem to understand my babble, so let's go a bit more in-depth with fatigue-science.  It's a good video (of course it's good - it has aeroplanes in it!) - but the maneouvres, although spectacular looking, weren't particularly load-intensive on the airframe.  At a guess, I'd say of the order of 1-2G load only.  You see that sort of loading during display flying.  The prototype Boeing 707 was rolled 360 degrees during a filmed flight test.  The test pilot knew what he was doing and intended to demonstrate the control responsiveness of the 707 flight systems.  (Well, that's what he said in the interview....he was probably just having fun too.)  The entire roll was conducted smoothly with 1G load all the way around, and minimum side load.  Looked GREAT on film and probably caused Boeing management a few heart flutters!

Now, I refer to G-loading as a convenient point of reference.  G, of course is an acceleration.  What causes damage is the actual force applied to the airframe - Force (F) = mass (M) x acceleration (A).  The more mass being carried, means more force being applied.  So our Skyraider(1), 10,000kg carrying 5,000kg under-wing is experiencing more load than Skyraider(2) carrying 1,000kg (again made-up numbers for the example).  G as an acceleration is normally 9.81m/s/s, but I'll use 10m/s/s to simplify the calculation.
Skyraider(1): F = (10,000 + 5,000)kg x 10 m/s/s = 150,000 Newtons (unit of force = kg.m/s/s)
Skyraider(2): F = (10,000 + 1,000)kg x 10 m/s/s = 110,000 N
- both during normal 1G flight.
Now in a 6G pull-out from a dive - so 6 x 10 m/s/s acceleration:
Sky(1): (15,000) x 10 x 6 = 900,000 N
Sky(2): (11,000) x 10 x 6 = 600,000 N
- Skyraider(1) is experiencing 50% more airframe load than Skyraider(2).
We tend to use G-load as an easier pilot work-load reference point - simply because most modern aircraft have a G-meter in the instrument panel.  A pilot experienced in operating a certain type of aircraft should be familiar with the maneuvre G-load based on aircraft weight or load (eg. 2,000 kg fuel and load = 6G limit; 4,000 kg = 4G limit; 6,000 kg = 2.5G limit, etc).

More fatigue terminology:
- zero timing.
What this means is to bring the fatigue life of the aircraft back to "almost" as new off the production line stage.  Several ways to do this:
- replace main components (as Greg indicated with the Planes of Fame display aircraft), if you replace the main spar of the aircraft, you put in a component with zero effective fatigue flight hours on it;
- inspect and reanalyse - if you have a $400,000,000 FA-18 or F-111, etc, you will probably have a comprehensive fatigue analysis report which gives you "expected" fatigue lives (in terms of flight hours or landing cycles).  It should also highlight the critical items for inspection at which period of maintenance.  It you do a regular appropriate-level inspection on these items and find no fatigue-induced cracks (or any other damage), you can effectively "zero-time" that component - i.e. declare it safe, and start the flight-time/landing cycle clock on that item.  For our 65+ y.o. Skyraider - probably no fatigue analysis report - so regular inspection is the key.

Now to your question - yes, x-ray techniques are expensive and time consuming, so you may only schedule that in for the major overhaul (e.g. once every 5 years, rather than annually).  You need to expect there to be microscopic cracks - just can't avoid it due to the manufacturing process with metals.  The microscopic crack should become visible well before it becomes dangerous. Hence the regular visual inspection - every pre-flight, and more closely inspected 6-monthly or every 100 hours.  Magnetic particle inspection, and/or crack-revealing paint (this stuff we use in offshore oil and gas, not sure how regularly the aviation indistry makes use of it) - dust or paint it on, then use ultra-violet light to reveal crack lines.  Some areas you can't inspect easily, so if it's a reasonably available part, just replace it every 200 hours.  (Again, all the time periods, etc are made up numbers for comparative example.)  ....and you don't need an electron microscope that's housed in a lab and need 32 technicians to operate, there are simpler field use kits for application.  The tricky bit is exposing the aeroplane part for inspection.

Again, all the above stuff is what some engineers spend a life-time studying (not me!), this is meant as a very rough explanation in (hopefully) easy to understand terms.

...geoff


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#10 GregP

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Posted 30 March 2018 - 02:47 AM

The cracks were found when doing an inspection. We inspect our warbirds annually plus hours requirements ... or whenever they are apart enough to afford the opportunity. In this case, I do NOT know when the cracks were found, only the result of taking a flying warbird down for some months while replacement bulkheads were fabricated and installed.
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