F15C with 14G overload?

[codefox]

Probably because unlike the F-15 FM the Flanker FM is in Beta.

 

This page states the F-15C is still in early access: http://www.digitalcombatsimulator.com/en/shop/modules/f-15c_dcs_world/

[Monkey21]

To be fair, I was able to pull 23.3G in the Eagle in 1.5.4 with no ill effects. Now it was only for a fraction of a second, but I can see why the Flanker dudes are a little upset, even if the Eagle is my baby. Not particularly relevant, but I do understand the reasons for the frustration.

 

[Hummingbird]

Yes I'm able to do the same in the Eagle as well.

 

Also I don't agree with those who say that correcting this will change nothing, of course it will change something, flight limitations always change something - now it might not be a huge deal once people adjust to the new limitations, but the mere fact that they have to adjust IS a change in and by itself :)

 

The most noticable change would be that F-15 drivers suddeny have to be more careful about janking their joysticks around as they then risk ripping their wings off - because atm you can jank that stick around like crazy without a care in the world.

 

Finally this is a sim, so really it makes no sense not to want this corrected.

[pr1malr8ge]

Yes I'm able to do the same in the Eagle as well.

 

Also I don't agree with those who say that correcting this will change nothing, of course it will change something, flight limitations always change something - now it might not be a huge deal once people adjust to the new limitations, but the mere fact that they have to adjust IS a change in and by itself :)

 

The most noticable change would be that F-15 drivers suddeny have to be more careful about janking their joysticks around as they then risk ripping their wings off - because atm you can jank that stick around like crazy without a care in the world.

 

Finally this is a sim, so really it makes no sense not to want this corrected.

 

No one says it should not be corrected. How ever in the grand scheme of things it is not a high priority.

 

The issue is that the flankernutz want the wings to break off. The issue is that is not an option with the f15. The point of why has been said. The hydraulics in the f15 cannot produce enough leverage to generate the gloading needed to do that. At least not with out full fuel and bags. While I guess if the eagle has three bags and it pulls those gs then yes. How ever. If you notice most of the 14g pulls were done from an 700kts dive pull out along with full roll on top of it. Not exactly some one does when going defensive. Tryy achieving those numbers from a standard defensive maneuver. (I could be wrong and it capable haven't loaded up the sim in 4+ months)

 

At any rate the profile I fly I'm never pulling gs until I drop my tank and 75% fuel So wing breakage isn't a concern if they do add a damage model for having three bags.

 

If Ed/bst cave in and add wing breakage for a the f15 instead of fixing the fact the hydros are allowing this then the fact will be as rage puts it fidelity gap did widen all in favor of the su27

Edited July 22, 2016 by pr1malr8ge

[IASGATG]

Okay because everyone seems to just be shouting at this point. IASGATG is here to save the day again.

 

1. The F-15 should have a damage model added like the Su-27 where at the point of extreme over-g (Whatever that is decided to be (Likely around 14g)). This will mean that if the F-15 will explode like the Su-27 once it hits this g force. Nobody is suggesting otherwise, we all agree on this.

 

2. The F-15 should have it's flight model adjusted so that the highest g it can pull is between 12.5 and 13.5 as reflected in observation and documentation. This would effectively make point 1 moot.

 

3. This isn't a priority due to a. The number of massive game breaking bugs that currently exist in the game and b. due to the extremely rare and forced nature of this flight profile. In "live fire" exercises, I can say that my eagle doesn't push past 10g. This isn't through lack of trying. I am never "Gentle" with the stick, and you're right, I never have to be careful about "janking their joysticks around" because I know that my jet wont rip it's wings off. However, at no point am I in this 14g threat terriroty when I am pulling to save my life.

 

4. Because of this, Eagle pilots say that it wont really make that much of a difference. We aren't going to fly any difference even if we get point 1 and not point 2. We don't fly at these extreme profiles. Not in BFM, not in missile jinks, not in BVR. It's never needed. The Eagle wants rate in BFM. Missiles can out-g anything you can pull (AIM-9 and R-73 are 45g rated, you're never out turning an archer). And why you need to suddenly pull 14g at 15nmi I'll never know.

 

Okay, thread done. Now we wait.

[winchesterdelta1]

I like your comments. Always nice and clear and no Bullshit.

[Fri13]

At least in theory you can exploit the over-g to defeat missiles at close range. The rule of thumb is that a missile has to pull around 3x the g its target is pulling to have a good chance of hitting, so going from the eagle's realistic 12.5g (missile must pull 37.5g) to the less realistic 14g (missile must pull 42g) might make a difference in a dogfight.

 

There was the interesting math for that.

https://defenseissues.wordpress.com/2013/08/17/evading-air-to-air-missile/

 

The missile must pull the amount of g’s the aircraft can pull multiplied by the speed of the missile divided by the speed of the aircraft (in mach).

(Speed of missile in mach / speed of aircraft in mach)^2 * G’s the aircraft is pulling = G’s the missile must pull

 

Aircraft: M 0,5, 9 G

Missile: M 4

 

(M 4 / M 0,5)^2 = 8^2 = 64

64 * 9 = 576 G

 

 

So based to that, missile should pull 576 G to hit you at Mach 4, while you pull 9 G at Mach 0.5.

 

 

And some in the comments about that 1.5-3X

 

https://defenseissues.wordpress.com/2012/12/15/aim-120d-vs-mbda-meteor/

 

[ame]http://www.jhuapl.edu/techdigest/TD/td2901/Palumbo_Homing.pdf[/ame]

Edited July 22, 2016 by Fri13

[GGTharos]

There was the interesting math for that.

https://defenseissues.wordpress.com/2013/08/17/evading-air-to-air-missile/

The missile must pull the amount of g’s the aircraft can pull multiplied by the speed of the missile divided by the speed of the aircraft (in mach).

 

Missiles pull ~3x the amount of G that their targets pull.

 

This is basic PN guidance, and it is what the actual paper that you referenced is telling you rather than the BS that picard578 likes to put out there.

[SinusoidDelta]

Missiles pull ~3x the amount of G that their targets pull.

 

This is basic PN guidance, and it is what the actual paper that you referenced is telling you rather than the BS that picard578 likes to put out there.

[/indent]

 

If you read the paper it says exactly that, page 50.

[Fri13]

Missiles pull ~3x the amount of G that their targets pull.

 

This is basic PN guidance, and it is what the actual paper that you referenced is telling you rather than the BS that picard578 likes to put out there.

[/indent]

 

As I said, I quoted the paper.

[Ktulu2]

Regarding these points, while I don't doubt the F-15 can pull up to 12.5-13.5 on occasion, I'd suspect repeated applications would lower the g tolerance.

 

That cumulative damage is applied to some of the WWII birds and to the Su-27. Probably should be added to the F-15.

 

Seems like the point of the post just went Mach 5 at angel 100 above your head...

I have been saying it wouldn't change much since page 1, now others are saying it.

[Buzzles]

Seems like the point of the post just went Mach 5 at angel 100 above your head...

I have been saying it wouldn't change much since page 1, now others are saying it.

 

No, it didn't go over my head.

 

My actual point was degredation on over g (which is 14+) shouldn't cause the plane to go bang, even if it shouldn't ever be able to pull that much anyway due to hydros, but I've deleted my last post anyway as it wasn't clear.

[GGTharos]

14g should most likely cause it to go bang.

 

The highest known recorded pull in an F-15 is 12.5g, this is with full stick deflection at transsonic speeds. Basically, you probably shouldn't even be able to reach 14g.

[Dr.Goose]

Dear GGTharos

 

No, 14g will most certainly not cause full structural failure.

 

 

Summary:

1.Ultimate strength (max strength (max g))

2.Yield strength (9g*1.5=13.5 this is what the F-15 is designed to withstand you can't argue with this, it's simple engineering)

3.Rupture (full failure)

4.strain hardening region (warping)

5.Necking region

A:Apparent stress (F/A0)

B:Actual stress (F/A)

 

This is because of material science behind metal, it bends to take more stress before it breaks.

 

I would like the source for 12.5g btw, because I read 20+ g all the time and it seams plausible due to the balancing of the stabilator.

 

P.S. I saw a F-104 that had pulled 11.5g (7*1.5=10.5<11.5^=15g for an F-15) it had only marginal but upon further inspection clearly visible warping.

Edited July 24, 2016 by Dr.Goose

[GGTharos]

Dear GGTharos

 

No, 14g will most certainly not cause full structural failure.

 

Yes, it will.

 

2.Yield strength (9g*1.5=13.5 this is what the F-15 is designed to withstand you can't argue with this, it's simple engineering)

I can easily argue with it :) Are we saying 13.5g for an EMPTY F-15?

 

I would like the source for 12.5g btw, because I read 20+ g all the time and it seams plausible due to the balancing of the stabilator.

MA Pilot's lecture referencing his crew chief's readout of the max g recorded by the aircraft. The 20g seems improbable - the hydraulics probably can't generate the necessary deflection at that speed.
Edited July 25, 2016 by GGTharos

[Stuge]

I can easily argue with it :) Are we saying 13.5g for an EMPTY F-15?

 

Uber Pwned :):):):):)

[DarkFire]

Dear GGTharos

 

 

This is because of material science behind metal, it bends to take more stress before it breaks.

 

That's a generic representative graph of strain-induced failure modes in most common metals. Look up similar graphs for high-tensile alloys produced by high-temperature sintered powdered crucible metallurgy, both steel and aluminium - they're not the same as for a 'typical' metal. The proportional & elastic limits tend to be very close together, with a typical offset strain being <0.5%. Necking prior to failure tends to be minimal, the end effect being little or no warning prior to total part failure. The repetitive strain graphs also tend to feature quite a lot of hysteresis, i.e. repetitive strain near to the maximum value will substantially increase failure susceptibility at subsequently lower strain values.

 

As a slight off-topic comment, people were complaining about the observed failure mode in the Su-27 when it got it's G-limits imposed, whereas in actuality the instant failure typically seen above ~1.4x maximum G values from the manufacturer's flight manual is entirely realistic in terms of materials science.

[*Rage*]

Double uber pwned:)

[tusler]

What a testament to the fidelity of a home based computer simulation...People are arguing real aerodynamics using real world charts, math and facts...Great job to the Developers!

[Dr.Goose]

@GGTaros

 

I can easily argue with it Are we saying 13.5g for an EMPTY F-15?

 

The rule of thumb is, if it tells you you can do this then multiply it by the safety factor and you got the designed limitations. And a safety factor of 1.5 is plausible as it is used on other contemporary fighter aircraft.

 

@DarkFire

 

That's a generic representative graph of strain-induced failure modes in most common metals. Look up similar graphs for high-tensile alloys produced by high-temperature sintered powdered crucible metallurgy, both steel and aluminium - they're not the same as for a 'typical' metal. The proportional & elastic limits tend to be very close together, with a typical offset strain being <0.5%. Necking prior to failure tends to be minimal, the end effect being little or no warning prior to total part failure. The repetitive strain graphs also tend to feature quite a lot of hysteresis, i.e. repetitive strain near to the maximum value will substantially increase failure susceptibility at subsequently lower strain values.

 

The graph was supposed to be generic to show the principle behind tensile strength and failure in metals.

And thanks for pointing out the different behavior of high tensile strength metal. I didn't know that (as a 2. semester engineer I haven't had material science yet :music_whistling:).

And I agree that repetitive stress near the maximum will subsequently lower the the strain values (I stated this in my first post

Though the structural strength is eventually compromised by fatigue

) this is the reason for airframe lifetimes.

 

But still 14g is about 4% above the 13.5g (1.5 safety factor and 9g allowed assumed), so it still seams plausible that it wouldn't cause full structural failure at least in a younger airframe.

Loads of repairs and possible warping yes but not full failure. Like the warped 11.5g F-104 I mentioned earlier.

 

But you're free to disagree and show my why it isn't plausible or even possible.

Edited July 26, 2016 by Dr.Goose

[Bushmanni]

How are the load carrying structures inside modern fighters wings built? If they are hollow sheet metal structures they would behave different from solid metal bars.

[Dr.Goose]

How are the load carrying structures inside modern fighters wings built? If they are hollow sheet metal structures they would behave different from solid metal bars.

 

Well they use both. The wings of fighter-aircraft are built, like most other Wings or for that matter most aircraft in general, as a semi-monocoque structure in which spares (structure) and the skin carry the load of the aircraft.

 

(A notable modern exception is the A-10, it mostly uses the structure to carry the load, this gives it its damage resistance, because the skin can be torn off without causing structural damage.)

 

The benefit of a semi-monocoque is it's relatively low weight and high strength (because you can downsize the structure by putting load on the skin).

Edited September 27, 2017 by Dr.Goose

[Bushmanni]

My experience with hollow structures in general is that they tend to completely fail soon after they start to give in instead of permanently warping. You can have only very little warp in a small stretch of structure and if you want to have a large bend you need to distribute it along a long distance (like you do in pipes). I would think semi-monocoque wings behave somewhat similarly?

[SinusoidDelta]

How are the load carrying structures inside modern fighters wings built? If they are hollow sheet metal structures they would behave different from solid metal bars.

 

Something resembling this:

 

Edit to add:

Stress strain curves won't tell us much about ultimate strength of a structure. Quote from this thread:

 

http://www.pprune.org/archive/index.php/t-509868.html

 

"What is written in a Flight Manual (FM) may not be a "true" (again, from my point of view) limit g-factor. I did see that in F-15 data. The baseline usage spectrum for F-15 assumes 2 exceedances of 100% limit load per 1000 FH. That is 8 exceedances per original service life of 4000 FH. An absolutely clear indication that this 100% limit load is not a maximum load encountered in a normal operation is the presence of a certain number of exceedances at around 110% of limit load. From the prospective of the FAA-type definition of the limit load, this is just a nonsense.

In a test, the stress corresponding to the 100% limit load was 30 ksi gross. Net stress would be around 32 - 35 ksi. Therefore, the design ultimate stress is

(32 - 35) x 1.5 = 48 - 52 ksi. For 7075-T73 aluminum alloy, the ultimate tensile strength is 71 ksi. Yield limit is 60 ksi.

 

This means that when you pull F-15 to +9g you are nowhere close to the material strength or even yield limit (the design combat weight is assumed).

This is partially what I called the distorted (can't hide there was some dramatization in my words) understanding of the structural issues among pilots."

Edited July 26, 2016 by SinusoidDelta

[DarkFire]

Something resembling this:

 

Edit to add:

Stress strain curves won't tell us much about ultimate strength of a structure. Quote from this thread:

 

"What is written in a Flight Manual (FM) may not be a "true" (again, from my point of view) limit g-factor. I did see that in F-15 data. The baseline usage spectrum for F-15 assumes 2 exceedances of 100% limit load per 1000 FH. That is 8 exceedances per original service life of 4000 FH. An absolutely clear indication that this 100% limit load is not a maximum load encountered in a normal operation is the presence of a certain number of exceedances at around 110% of limit load. From the prospective of the FAA-type definition of the limit load, this is just a nonsense.

In a test, the stress corresponding to the 100% limit load was 30 ksi gross. Net stress would be around 32 - 35 ksi. Therefore, the design ultimate stress is

(32 - 35) x 1.5 = 48 - 52 ksi. For 7075-T73 aluminum alloy, the ultimate tensile strength is 71 ksi. Yield limit is 60 ksi.

 

This means that when you pull F-15 to +9g you are nowhere close to the material strength or even yield limit (the design combat weight is assumed).

This is partially what I called the distorted (can't hide there was some dramatization in my words) understanding of the structural issues among pilots."

 

Very interesting indeed, thanks for posting that :thumbup: