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F-15C Structual wing surface failure.


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1 hour ago, Cab said:

Maybe, but in my experience I don't think so. It just doesn't seem reasonable that being able to pull up to instantaneous 13g wouldn't be relevant. But, of course, maybe I am not flying well enough to prove the point. 

However, unless you've actually fought someone under those conditions it's just theory. I fight with the Hornet without the paddle whenever I can, but with the paddle up to 11g's when my adversary is flying at those extreme g loads. I can tell you it's like flying two completely different airplanes.

Also, you have to pick your poison (or in this case realism). Personally, if it comes down to maneuvering at 13g's and gloc'ing at 1.5g's above the operational limit, then I pick the former.

But to each his own. No harm.

The ability to spike 13g’s “instantly” would be a control deflection modeling error. That is part of the issue with modeling unrealistic  catastrophic failures. Combined with inaccurate HSTAB deflection rates at high speed, the result is snapping wings off due to unrealistic G onset rates. 

 

 

 

 

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5 hours ago, =475FG= Dawger said:

The ability to spike 13g’s “instantly” would be a control deflection modeling error. That is part of the issue with modeling unrealistic  catastrophic failures. Combined with inaccurate HSTAB deflection rates at high speed, the result is snapping wings off due to unrealistic G onset rates. 

I haven’t seen any validation of the stab deflection rate. Can you elaborate?
 

In the higher transonic it apparently became much easier to rapidly overload the airframe. 
 

This must be a fairly recent change as I didn’t experience it a few months ago. How does this mechanic work? 

I’m curious to test it out now. 

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On 1/29/2022 at 1:45 AM, SinusoidDelta said:

I haven’t seen any validation of the stab deflection rate. Can you elaborate?
 

In the higher transonic it apparently became much easier to rapidly overload the airframe. 
 

This must be a fairly recent change as I didn’t experience it a few months ago. How does this mechanic work? 

I’m curious to test it out now. 


Cant speak for DCS as I havent messed around with the C enough lately, specially in the transonic area. But…with respect to the real thing, it is a real thing. The CAS system is built to maintain a loaded G per stick position. In certain envelope areas, it’s a lot easier to apply the stick you think will give you X, and the result being something different.  The details of how that all happen and the why are well above my personal knowledge…just know that it is for sure an IRL thing. It’s commonly where the IRL over-G occurrences happen. 

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In DCS it is position -> g, IRL I believe it is force -> g which amounts to more or less same minus trimming nuances I believe.

As said above, a transition to transonic (usually from supersonic, but I'm no expert) really does mean a change in airflow type and IIRC you will get more g for that same deflection.  The CAS does its best to protect the aircraft from this but it's neither perfect nor fast enough to deal with it in some cases.  And most importantly, it's there to give you what you commanded it to so if you pull the stick all the way back and you drop into transonic, your 8g might suddenly become 12g  (real numbers unknown, exaggeration mine for effect)

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13 hours ago, GGTharos said:

In DCS it is position -> g, IRL I believe it is force -> g which amounts to more or less same minus trimming nuances I believe.


This is pretty much interchangeable.  Stick travel per G is still a thing in the actual aircraft as well. The feel springs are there so that the stick force increases as positional displacement increases. 

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  • 1 month later...
On 1/26/2022 at 6:17 AM, =475FG= Dawger said:

Thank you for helping me demonstrate that your only concern here is “winning”, not actually modeling realistic behavior with regard to aircraft structural integrity.

It validates my decision to let this issue go.

However, I do have one final request.

You demand from me conclusive proof that the wing does not fail at 1.5 times the design limit load.

I shall do the same. I demand conclusive proof that the wing DOES fail catastrophically the very first time it is subjected to 1.5 times the design limit load.

Video evidence is preferred but official documents will be acceptable. 
 

Cheers!
 

 

The smoking gun for me is this:

 

in the most recent -1 manual we have for the F-15, it is cleared for an emergency pull-out of 10g at a very high weight (~55k lb IIRC). The Eagle should not break catastrophically at 9G at low weight

 

I cannot post it here due to forum rules but it is not hard to find. in fact ED hosts the file "App B Perf Data F100-PW-220"

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On 2/5/2022 at 1:18 AM, GGTharos said:

In DCS it is position -> g, IRL I believe it is force -> g which amounts to more or less same minus trimming nuances I believe.

As said above, a transition to transonic (usually from supersonic, but I'm no expert) really does mean a change in airflow type and IIRC you will get more g for that same deflection.  The CAS does its best to protect the aircraft from this but it's neither perfect nor fast enough to deal with it in some cases.  And most importantly, it's there to give you what you commanded it to so if you pull the stick all the way back and you drop into transonic, your 8g might suddenly become 12g  (real numbers unknown, exaggeration mine for effect)

Is trans-sonic region even modeled in DCS?

 

Cmptohocah=CMPTOHOCAH 😉

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  • 1 month later...

I haven't tested it explicitly, but I have broken the wings at only slightly more than 9g. I think the wings are slightly weaker than they should be. Per an older manual:

 

Dive Recovery - Emergency Pull-Out

Gross Weights of 50,000 to 54,000 Pounds

At airspeeds below 350 KCAS....full aft stick or 10g's to wing rock

At airspeeds between 350 and 500 KCAS....full aft stick of 10g's to wing rock

For a 60,000 pound aircraft...the maximum load factor is 8.7g's below 500kcas

 

"Full aft stick or 10g's to wing rock"


Edited by henshao
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19 hours ago, henshao said:

I haven't tested it explicitly, but I have broken the wings at only slightly more than 9g.

At what weight?  That matters.  The Original F-15A was rated for 7.33G at 37,400lb and that rating remained though the early 90s.  (look up F-15 Standard Aircraft Characteristics) Now, the reason it's only 7.33G is because that was the rating at a critical flight condition.  9G was available for most the envelope and the OWS was programmed to know what the limit was based on the current condition, thus the aural "Over-G".  The weight limit still applies though.  So you really need to think about it in terms of you have 336,600lb of lift available when not at critical condition and 274,142lb of lift at critical condition.  Even with a 150% failure margin if you are 6x2 loaded and full internal fuel (~49,000lb) you should see failure between 8.4 and 10.3G depending on how close you are to critical condition.  5.6 to 6.9G is the rated lift limit for that weight.

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53 minutes ago, Spurts said:

At what weight?  That matters.  The Original F-15A was rated for 7.33G at 37,400lb and that rating remained though the early 90s.  (look up F-15 Standard Aircraft Characteristics) Now, the reason it's only 7.33G is because that was the rating at a critical flight condition.  9G was available for most the envelope and the OWS was programmed to know what the limit was based on the current condition, thus the aural "Over-G".  The weight limit still applies though.  So you really need to think about it in terms of you have 336,600lb of lift available when not at critical condition and 274,142lb of lift at critical condition.  Even with a 150% failure margin if you are 6x2 loaded and full internal fuel (~49,000lb) you should see failure between 8.4 and 10.3G depending on how close you are to critical condition.  5.6 to 6.9G is the rated lift limit for that weight.

Manual page posted says 54000lbs, 10g's to wing rock. Wings were broken at ~9.2g on a clean low-fuel airplane (~33,000LB)


Edited by henshao
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1 minute ago, henshao said:

Manual page posted says 54000lbs, 10g's to wing rock. Wings were broken at ~9.2g on a clean low-fuel airplane

it says 50-54, and notice that at higher speeds it says pull 7G.  My upper lift limit with 150% margin for 54,000lb comes to 9.35G, so 10G is banking on the strength of the structure to survive beyond 150%, which is not unreasonable and could even be because the structural test showed a 160% margin (150% is the minimum and the goal, but the structure going above that is normal).

Now, as to 9.2G on a clean low fuel airplane.  Were you rolling by chance?  Asymmetric load limits are lower.  Just trying to rule out other possible failure modes.

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Well I can appreciate the devil's advocate position but to play the angel's advocate...the manual literally tells pilots to pull 10gs at almost extremely heavy weights to save the airplane. If it would break at 9gs lightly loaded that instruction wouldn't be there. And I always do my best to not roll and pull at the same time for just that reason (are rolling Gs modeled in DCS?) Bear in mind this is McDonnell who built this bird and gave these instructions, the same people who gave us the Phantom over Vietnam. At the point of the F-15 perhaps the industry leader with regard to determining fighter jet maneuvering restrictions


Edited by henshao
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I hear you.  I have broken wings off a Harrier in a loaded roll, so yeah it's modeled.  If they came off in a light plane with no roll at less than 9G is sounds like there was a random number generator bug.  Can you reproduce it?


Edited by Spurts
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