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F15C with 14G overload?


flankerted

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All I know is that I can jank pull 14 G's of symmetrical load right now at leasure, and the F-15 a) shouldn't even be capable of that & b) if it ever managed it it would either warp and most likely rip the wings.

 

Should be noted however that the F-14 for example was tested to 13 G's without damage, and its ultimate load limit was the same as the F-15's - so there's a fine line between taking no damage and catastrophic airframe failure.

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All I know is that I can jank pull 14 G's of symmetrical load right now at leasure, and the F-15 a) shouldn't even be capable of that & b) if it ever managed it it would either warp and most likely rip the wings.

 

Should be noted however that the F-14 for example was tested to 13 G's without damage, and its ultimate load limit was the same as the F-15's - so there's a fine line between taking no damage and catastrophic airframe failure.

 

The f14 & F15 are two entirely different air frames manufactured by two different companies/engineers for completely different uses.

Saying that the f14's ultimate load limit is the same as the f15s is wrong.

 

While personally I'd think that the f15 would be stronger then the f14 due to it's pivot point on the wing but that is not always the case in engineering[it was designed for salt & intentional crash landings].

How ever, I could see the f14 being able to tolerate more G's then the f15 when it's wings are swept fully back as it will decrease stress loading due to it having a shorter width 38ft vs 43 along with the fact that it's engines are spaced further apart thus again reducing surface area from the wing root to wing tip or in this case wing root to leading edge at the center of gravity.


Edited by pr1malr8ge

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If your desired effect on the target is making the pilot defecate his pants laughing then you can definitely achieve it with a launch like that.
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All I know is that I can jank pull 14 G's of symmetrical load right now at leasure, and the F-15 a) shouldn't even be capable of that & b) if it ever managed it it would either warp and most likely rip the wings.

 

Should be noted however that the F-14 for example was tested to 13 G's without damage, and its ultimate load limit was the same as the F-15's - so there's a fine line between taking no damage and catastrophic airframe failure.

 

I don't really see how it's relevant but can you provide a source the F–14 test data?

 

The core, if you will, of this argument is one failure modes. AFAIK there is no F-15 airframe strength test data. I may have seen a photograph in a book somewhere but no load factor was given. You can find some data about fatigue life but mostly useless for our purposes. I imagine the F-15 might elongate more than you'd expect subject to a high load.

 

I also know you can "rip the wings off" as a McAir engineer put it. We also know of the crash I've posted. Hence the OWS. That's about it.

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Goose, while what you've learned is correct, it is very generic and unfortunately it can be argued with, even within the scope of what you've learned so far.

 

Without even considering complex issues of airframe structural analysis, there are many factors to consider just about solid mechanics.

 

1. A stress-strain curve of the kind you show is usually generated in lab under very specific, canonical loading conditions. For example, a steel bar put under simple tension. But in many engineering applications, the stress state, becomes far more complicated: you can get up to three distinct normal stresses and another three shear stresses on an element. From the loading conditions they've applied, an inexperienced (or uninformed) engineer might think their element is safe from failure, but there may be characteristic directions in the element that suffer very high principal or shear stresses.

 

Even then there are questions about how you predict for failure. For example, do you use a Tresca condition, or von Mises?

 

2. Even if you understand your stress state very well, the failure mode may not be due to yield, which is a kind of shear failure. (It also depends on what you define as failure, but that's another discussion.) Materials also have other properties that may be critical importance: fracture toughness, directional strengths (such as in composites), corrosion resistance, and in particular (especially in the context of airframe design) fatigue characteristics.

 

There are also geometric considerations such as corners in the airframe (which you may have come across already as 'stress concentration factors').

 

3. Safety factors vary from close to 1 up to values greater than 2. Just 'guessing' 1.5, even because your lecturer told you that aircraft applications feature factors like this, does not give a reliable answer.

 

4. A lot of machine element design, such as for rivets or bolts or linkages or (especially) welding, relies on heaps of empirical data, which has constants here and there which you have no real way of theoretically designing for.

 

5. Heaps of other things I can't even think of right now :(

 

edit: the upshot of the above is that using the yield strength by itself tends to overestimate the strength. There are a whooole lot of things that work together against the designer.


Edited by Scytale
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Obviously, we need a real war between flankers and eagles to settle this dispute!

MS Win7 Pro x64, Intel i7-6700K 4.0Ghz, Corsair RAM 16Gb,EVGA GeForce GTX 1080 FTW GAMING ACX 3.0, w/ Adjustable RGB LED Graphics Card 08G-P4-6286-KR, Creative Labs SB X-FI Titanium Fatal1ty Champ PCIe Sound Card, Corsair Neutron XTI 1TB SSD, TM Warthog Throttle & Stick, TM TPR Pedels, Oculus Rift VR Headset CV1, Klipsch Promedia 4.1 Speakers...

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I've seen an eagle wing off the plane. It's like it has door hinges with big pins that keep it on. They are steel or titanium structures.

MS Win7 Pro x64, Intel i7-6700K 4.0Ghz, Corsair RAM 16Gb,EVGA GeForce GTX 1080 FTW GAMING ACX 3.0, w/ Adjustable RGB LED Graphics Card 08G-P4-6286-KR, Creative Labs SB X-FI Titanium Fatal1ty Champ PCIe Sound Card, Corsair Neutron XTI 1TB SSD, TM Warthog Throttle & Stick, TM TPR Pedels, Oculus Rift VR Headset CV1, Klipsch Promedia 4.1 Speakers...

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OK, even if the eagle can sustain 14G, I think that a common eagle pilot will sustain 10G+ for about 3 seconds staying awake seems to be immpossible, including instantaneous 14G, about 1 senconds 12G, and about 2 seconds 10G. The blackout simulation seems to be unrealistic? To be honest, I think a pilot will be killed in this situation...


Edited by flankerted
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OK, even if the eagle can sustain 14G, I think that a common eagle pilot will sustain 10G+ for about 3 seconds staying awake seems to be immpossible, including instantaneous 14G, about 1 senconds 12G, and about 2 seconds 10G. The blackout simulation seems to be unrealistic? To be honest, I think a pilot will be killed in this situation...

 

THe brain has about 5s of oxygen reserve, so you can virtually sustain any amount of G while staying awake. The question is more whether your eyes will keep on working under such a load, as I've read plently times that pilots can loose sight under crazy amounts of Gs (ie : ejections, or the 70G instantaneous a guy pulled in experiments in the US [this might be hard to read for sensible ones]

[that one straight up

decanted his eyes... ]

) for a short period while staying relatively conscious.

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OK, even if the eagle can sustain 14G, I think that a common eagle pilot will sustain 10G+ for about 3 seconds staying awake seems to be immpossible, including instantaneous 14G, about 1 senconds 12G, and about 2 seconds 10G. The blackout simulation seems to be unrealistic? To be honest, I think a pilot will be killed in this situation...

 

I disagree that a common eagle pilot in DCS will sustain 10+G for 3 seconds. Regardless, if you've read this thread in its entirety I don't know why you think 14G transient will kill you. Like Ktulu said, there is about a 5 second window. Humans can withstand a very high transient G inside that window. John Stapp survived a ~40G deceleration in the forward direction. His colleague Beeding survived a rearward decel measured ~80G for 50ms at the chest accelerometer.

http://youtube.com/watch?v=siau78EFLgc

 

I think if anything, DCS's G-LOC happens a bit too early and far too abruptly if our pilot is wearing an anti-G suit.


Edited by SinusoidDelta
Fixed video link
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Human body cannot handle anything over 11. FOR ANY AMOUNT OF TIME that is not of instantaneous duration. Few, vew few, pilots in prime physical condition may handle 9 perhaps 9.5 instantaneous, then GLOC. Fighter pilots are trained for sustained 7-7.5G ACM fight, 6-7G for SAM/AA missile evasion. At 14G pilot will die. He will have either a catastrophic aneurysm in brain, stroke, or catastrophic myocardial infarction (heart attack). The aircraft itself will suffer an airframe failure. Either in the wingbox (strongest part of airframe) or langeron (a box of airframe bulkheads connection cockpit/nose part to wingbox. Engines will probably keep working due to fuel gravity feed and feed tank pressure. Not sure about fuel pumps. Electronics are hardened to resist damage up to instaneous 100G (to survive shock of detonation and kinetic energy absorbtion, of enemy munitions).

I am fairly certain that should F-15C or any F-15, enter a 12G condition, and return safely, will probably need an airframe rebuild , or be grounded permantly. Hi G and OverG conditions are highly regulated in nearly all Air Forces. G meter can only be reset by maintenance. Crews are trained in authorized G limits during training and have loosened restrictions during combat and emergencies. To overG is below seriousness of violating Rules OF Engagement, but is still very serious issue. Crews have been grounded and lost flight status. From Government's point of view, over-stressing the airframe is equivalent to damaging government property, and decreasing readiness , as a combat asset is decertified. A crew needs to have adequate explanation. Squadron commanders are held responcible (to a point) for performance of their subordinates. So an overG, if a Class A mishap (damage over one million US dollars), expect the crew and squadron CO, to stand before Wing CO, normally O-6 colonel or BG. Careers are not guaranteed to survive. Thats why Air Forces are looking at unmanned combat system, or tele-manned. Aerospace industry can build a combat aircraft for sustained 9-10G combat performance or more. But not with a human inside. Man is not built for this.

 

One of the highest scoring aces in RAF, in WWII, lost his legs but continued to fly. He could outfight the enemy, at nearly 6G, as he had no legs for blood to pool into.

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There are 12g over-g incidents, and the pilots survived. One airframe didn't.

 

Some red bull aircraft are rated for 12g and the pilots get it up there.

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Human body cannot handle anything over 11. FOR ANY AMOUNT OF TIME that is not of instantaneous duration.

 

So how about you look into ejection seats.

he purpose of an ejection seat is pilot survival. The pilot typically experiences an acceleration of about 12–14 g (117–137 m/s2). Western seats usually impose lighter loads on the pilots; 1960s-70s era Soviet technology often goes up to 20–22 g (with SM-1 and KM-1 gunbarrel-type ejection seats). Compression fractures of vertebrae are a recurrent side effect of ejection.

https://en.wikipedia.org/wiki/Ejection_seat#Pilot_safety

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Without comment on the quality of that study itself, that there is an egregious typo in the title is not encouraging

 

Any comments on the actual subject?

 

They might not have English as a native language but they have managed to include full stops after sentences. ;)

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Agreed. Note that it doesn't really describe even what A-LOC is until the results section, when it seems to be defined for methodological reasons more than anything else. One comes away with the impression that there is a thing called A-LOC and that maybe it's worth keeping in mind, but nothing more.

 

The comment about the typo is more a comment on the journal than the authors: one would expect poor grammar to be caught if not in peer-review then by the editors.

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