Structural Stress Mechanic

[Belphe]

Hi,

 

I watched Grim Reapers' YouTube video on the "hidden", progressive, additive, G-inflicted damage on the airframe of the Tomcat (and possibly other aircraft?) and found it very unrealistic. Don't get me wrong - I don't mind for the Turkey to fall apart after going over the red line when it comes to G but for the sim to "secretly" add up all the G values and break the plane when they reach critical amount, even when the pilot applies small pressure on the stick in his last ever maneuver, well within the allowed limit is nonsense..

Yes, I know that the potential damage G would inflict in numerous sorties over time is additive but no one would inflict it performing allowed maneuvers in a single flight. Perhaps such long-term structural damage could be simulated along with Heatblur's advertised cockpit deterioration as we keep using the same aircraft in-game? Perhaps it could be totally removed? Personally, I wouldn't mind getting rid of this entirely. No Tomcat has ever fallen apart in mid-air due to a long-term structural stress/failure and if it was nearing such state it would have surely been grounded.

 

TL&DR: If I pull too hard - spank me! But when I'm gentle and taking my time - keep me high, Baby.. ;)

[draconus]

If you over-G the airframe it no longer has the same limits. You really have no idea how bad it can be. Just to add the logic behind it.

 

Or do you say that constatnt pulling 6G will break it anyway even when no over-G happened?

[IronMike]

The Grim Reapers video unfortunately did get a lot of stuff wrong and just assumed stuff, while horribly mishandling the aircraft at the same time. You can't insta pull into 30 AOA and then go "it does not give me any indication at what AOA I am at" - that is not how you fly an aircraft, and even less how you assess it. Sorry if I say this so openly, but a lack of handling ability paired with wrong assumptions leads to many wrong conclusions in said video. Don't get me wrong, the Grim Reapers are awesome guys, I like them a lot, they are especially a great gang to hang out with, and their videos are highly appreciated, but sometimes they do lack a bit of fact checking and assume stuff that is simply wrong.

 

It is not how this works, G does not build up to break your plane, when you just take any G.

 

If you over G it, then you have a very, very slight chance to break it the first time, while this chance increases the more you over G or the more often you over G. And that means pulling well past 12G. If you stay within your limits, nothing will happen to you. Even if you keep pulling Gs for hours on end, if it is within the parameters the aircraft was designed for, it will not matter.

Edited September 2, 2019 by IronMike

[Below Average]

Just a tid-bit on airframe stress from a guy who fly’s military aircraft and went to aviation safety school.

 

Here is the simple part. All aircraft are rated to 150% load limit symmetrically and after that are expected or designed to fail. This is the ultimate structural load limit. Between 125% and 150% we have the range where your causing plastic-deformation where the metal is permanently yielding and becoming less ductile by becoming harder (bad). Between 100%-125% you are yielding abit but the damage is far less severe and the service life is not impacted as much.

 

An overstress that last .1 sec or 3 secs has the same result during that cycle. Less load = more cycles, higher load = less cycles.

 

Here is the thing about over-stresses,they are almost always asymmetrical. Because of this we have bent aircraft. I don’t want to hop in a Jet in DCS and have it stall at different air speeds and AOAs and roll off in different directions or depart unexpectedly. Also consider your going to have to trim more often and also use the roll and yaw trim on a symmetrically loaded jet.

 

Not thanks, to much realism.

Edited September 2, 2019 by Below Average
Grammar

[Ala12Rv-Tundra]

Just a tid-bit on airframe stress from a guy who fly’s military aircraft and went to aviation safety school.

 

Here is the simple part. All aircraft are rated to 150% load limit symmetrically and after that are expected or designed to fail. This is the ultimate structural load limit. Between 125% and 150% we have the range where your causing plastic-deformation where the metal is permanently yielding and becoming less ductile by becoming harder (bad). Between 100%-125% you are yielding abit but the damage is far less severe and the service life is not impacted as much.

 

An overstress that last .1 sec or 3 secs has the same result during that cycle. Less load = more cycles, higher load = less cycles.

 

Here is the thing about over-stresses,they are almost always asymmetrical. Because of this we have bent aircraft. I don’t want to hop in a Jet in DCS and have it stall at different air speeds and AOAs and roll off in different directions or depart unexpectedly. Also consider your going to have to trim more often and also use the roll and yaw trim on a symmetrically loaded jet.

 

Not thanks, to much realism.

 

Those percentage are material related?

The main structural part from the Tomcat was made of titanium

[Below Average]

Just a tid-bit on airframe stress from a guy who fly’s military aircraft and went to aviation safety school.

 

Here is the simple part. All aircraft are rated to 150% load limit symmetrically and after that are expected or designed to fail. This is the ultimate structural load limit. Between 125% and 150% we have the range where your causing plastic-deformation where the metal is permanently yielding and becoming less ductile by becoming harder (bad). Between 100%-125% you are yielding abit but the damage is far less severe and the service life is not impacted as much.

 

An overstress that last .1 sec or 3 secs has the same result during that cycle. Less load = more cycles, higher load = less cycles.

 

Here is the thing about over-stresses,they are almost always asymmetrical. Because of this we have bent aircraft. I don’t want to hop in a Jet in DCS and have it stall at different air speeds and AOAs and roll off in different directions or depart unexpectedly. Also consider your going to have to trim more often and also use the roll and yaw trim on a symmetrically loaded jet.

 

Not thanks, to much realism.

 

Those percentage are material related?

The main structural part from the Tomcat was made of titanium

 

It’s generic for an aircraft with some type of metal frame or wing box. Aluminum, Titanium and composites have different properties from each other which is beyond the scope of the conversation.

[VampireNZ]

It’s generic for an aircraft with some type of metal frame or wing box. Aluminum, Titanium and composites have different properties from each other which is beyond the scope of the conversation.

 

While being 'beyond the scope' - here are a few basic comparison figures for Ultimate Tensile Strength between a standard 2" plate of the Titanium used in the F-14 Wingbox (Ti-6Al-4V), and a generic aluminium used for machined critical aircraft components (7075-T651) - Ref MMPDS-03, B-Basis. Just for the interested lay-person.

 

Ti-6Al-4V Ftu = 137,000 psi

 

7075-T651 Ftu = 80,000 psi

 

Also for comparison - the same 2" sheet of AISI 1025 standard Carbon Steel (not used in aircraft) Ftu = 55,000 psi

 

An interesting document detailing the design of the Wing Box section - but I am probably getting off on a tangent now....

[Jansgi]

Hi,

No Tomcat has ever fallen apart in mid-air due to a long-term structural stress/failure and if it was nearing such state it would have surely been grounded.

 

Starting @ 0:15

 

The ship passed behind us, and I initiated an aggressive right hand climbing turn that would carry us back up to our holding altitude. As I pulled the stick back and nudged it slightly right I set a little more than six “Gs” for the climb.

 

As soon as I loaded the aircraft up with “G” there was a troublesome “bang” and the jet rolled dramatically and uncontrollably left. Instinctively I countered the left roll by moving the stick right, but despite my best attempts to control the aircraft we kept rolling harder left. In an instant it felt like the nose snapped downward in full left yaw, and I was certain we were on a vector headed downward toward the water. My head banged hard off the right side of the canopy, and all of a sudden time stood still.

 

In a span that was perhaps a few hundredths of a second the comfortable air-conditioned cockpit of our Navy fighter became foreign and hostile. I was confused about what had happened, and I was desperate to sort our situation out. I stared at the engine instruments and flight instruments, but the gauges held no usable information. In short, the instrument panel was a blank slate staring back at me while telling me nothing about our dire predicament.

[Ala12Rv-Tundra]

While being 'beyond the scope' - here are a few basic comparison figures for Ultimate Tensile Strength between a standard 2" plate of the Titanium used in the F-14 Wingbox (Ti-6Al-4V), and a generic aluminium used for machined critical aircraft components (7075-T651) - Ref MMPDS-03, B-Basis. Just for the interested lay-person.

 

Ti-6Al-4V Ftu = 137,000 psi

 

7075-T651 Ftu = 80,000 psi

 

Also for comparison - the same 2" sheet of AISI 1025 standard Carbon Steel (not used in aircraft) Ftu = 55,000 psi

 

An interesting document detailing the design of the Wing Box section - but I am probably getting off on a tangent now....

Thanks, checking that link too.

Starting @ 0:15

That was a compressor/engine issue.

[StandingCow]

You really have to take GR videos with a pillar of salt. They are fun videos to watch but they often get a lot of things wrong.

[Jansgi]

 

That was a compressor/engine issue.

 

That does not close the fact that a F-14 has fallen apart from a failure in mid-air. Official story said a faulty component on the oil system caused the engine to explode.

 

In the case our Tomcat crash there was no definitive explanation as to what caused our jet to explode. The board blamed a faulty component in the engine oil system, but with the wreckage buried in 17,000 feet of water, they never knew for certain. Personally I am at a loss to explain the event, but I am thankful that we made it back.

[draconus]

That does not close the fact that a F-14 has fallen apart from a failure in mid-air.

But you answered against the quote "No Tomcat has ever fallen apart in mid-air due to a long-term structural stress/failure."

[Belphe]

Thank you all for your input. I stand corrected and maybe even learnt a thing or two.. ;)

[toilet2000]

While being 'beyond the scope' - here are a few basic comparison figures for Ultimate Tensile Strength between a standard 2" plate of the Titanium used in the F-14 Wingbox (Ti-6Al-4V), and a generic aluminium used for machined critical aircraft components (7075-T651) - Ref MMPDS-03, B-Basis. Just for the interested lay-person.

 

Ti-6Al-4V Ftu = 137,000 psi

 

7075-T651 Ftu = 80,000 psi

 

Also for comparison - the same 2" sheet of AISI 1025 standard Carbon Steel (not used in aircraft) Ftu = 55,000 psi

 

An interesting document detailing the design of the Wing Box section - but I am probably getting off on a tangent now....

 

The reason why this is "beyond the scope" is that those percentages are exactly that: percentages, not units. Doesn't matter what you're working with, as long as it acts like most metals: a first mostly elastic deformation (up until yield strength) and then a plastic deformation part (up until ultimate strength).

 

If say you're at 150% load limit with both an aluminum structure and a titanium structure, the actual strain (in units) might be at 70 kpsi for the alu structure and 128 kpsi for the titanium structure.

 

Below Average's comment was just a general rule of thumb on strain and deformations caused to airframes over-g'ing.

[fl0w]

The Grim Reapers are good for entertaining content, but it is best to not use them for actual factual data as IronMike mentioned.

 

Structural stress is going to be depending on areas of the airframe and how they're built, like the truss design in the wings and the block sections of the Tomcat in the manufacturing process, where the thickness of the body is the thinnest, and etc.

[bear.is.flying]

Also the reason they add a 1.5x margin of safety in the design is not so pilots can take it up to 12 Gs if needed in a combat environment. It’s because the aeronautical engineers are using mathematical models that assume perfect pieces of metal without any cracks, corrosion, inclusions, etc. Add in 15 years of salt water exposure, thousands of carrier traps, and who knows what the actual limit of any individual airplane is. Maybe it will take 11 G’s just fine, or maybe there’s a tiny micro-crack in just the wrong place and the wings come off at 7.1 G’s.

[Jester2138]

I watched Grim Reapers' YouTube video

 

That was your first mistake...