F-35 air-to-air - Pro and Con

Discuss the F-35 Lightning II
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basher54321

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Unread post15 Feb 2014, 15:20

mixelflick wrote:I recall a Desert Storm engagement where the F-15 pilot said he took 12g's.

Amazing, though I dunno if I'd try that all these years later!



Yep older jets with no enforced limits - in the heat of battle the pilots gonna pull whatever required even if they risk black out and airframe write off - so there are many accounts from Israeli Mirage III pilots to Mig pilots etc - well the ones that recovered from blackout.

If the airframe is over G'd past its design limits then the damage will vary - for example engines in the F-4 coming off the mounts - but the longer the G is sustained then the damage will likely be more.

F-16 9G limit should be sustained - in the example of the F-15 pulling 12Gs that was probably under 2 seconds - any more and the pilot blacks out and likely more damage will incur.

The Navy have historically set lower G limits on jets - possibly to preserve the airframe life - so the F-35C limit of 7.5G should be computer enforced - like the FA-18EF also 7.5G below defined GW ( FA-18A had an 8G limit. )
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Gums

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Unread post15 Feb 2014, 20:46

Salute!

Still looking for the F-35 flight control laws. With FBW, you can bet that there are limits of all kinds on AoA, gee, pitch and roll rates, and the beat goes on.

OFF TOPIC: In the A-37 early days we routinely went beyond the gee limit and wrinkled the upper surfaces of the wings. Jets required a magnaflux, or whatever, x-ray to ensure the spars were not cracked. The maintenance guys would slice out a groove in the panels and rivet a strip of aluminum on top once the wrinkles settled out. Our problem was we could easily go lots faster than the trainer model and just a slightly hard pull would get us to 7, 8 or more gees. Interestingly, we could pull more gees and reduce stress if we retained the outboard bombs. Think about the wing trying to lift and you have weight out there that is now at 4 or 5 gees. We didn't like this, as we carried the big eggs inboard and used the outboard 250 ladyfingers as "wind bombs", heh heh.
+++++++++++++++++++
The problem in A2A with the high AoA is loss of "e". So I don't think the F-35 will allow 50 degrees willy-nilly. Hence, I would like to see the flight control laws.

I postulate a similar curve to the Viper, where you can max out the gee, then the AoA starts to increase and available gee decreases. I sincerely doubt that the corner velocity has max gee and 50 degrees AoA at the same time. Ours was 15 degrees AoA at 9 gees, then almost a straight line down to 1 gee at 27 degrees AoA. I'll try to digitize the actual function for the Viper and post later.

Gums ...
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neurotech

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Unread post16 Feb 2014, 00:47

Gums wrote:Salute!

Still looking for the F-35 flight control laws. With FBW, you can bet that there are limits of all kinds on AoA, gee, pitch and roll rates, and the beat goes on.

OFF TOPIC: In the A-37 early days we routinely went beyond the gee limit and wrinkled the upper surfaces of the wings. Jets required a magnaflux, or whatever, x-ray to ensure the spars were not cracked. The maintenance guys would slice out a groove in the panels and rivet a strip of aluminum on top once the wrinkles settled out. Our problem was we could easily go lots faster than the trainer model and just a slightly hard pull would get us to 7, 8 or more gees. Interestingly, we could pull more gees and reduce stress if we retained the outboard bombs. Think about the wing trying to lift and you have weight out there that is now at 4 or 5 gees. We didn't like this, as we carried the big eggs inboard and used the outboard 250 ladyfingers as "wind bombs", heh heh.
+++++++++++++++++++
The problem in A2A with the high AoA is loss of "e". So I don't think the F-35 will allow 50 degrees willy-nilly. Hence, I would like to see the flight control laws.

I postulate a similar curve to the Viper, where you can max out the gee, then the AoA starts to increase and available gee decreases. I sincerely doubt that the corner velocity has max gee and 50 degrees AoA at the same time. Ours was 15 degrees AoA at 9 gees, then almost a straight line down to 1 gee at 27 degrees AoA. I'll try to digitize the actual function for the Viper and post later.

Gums ...

The F/A-18A-Ds are known for departing controlled flight if yanked and banked at high AoA. Originally, the engineers thought the F/A-18 wouldn't go over 30 AoA due to aerodynamic performance limits but there are parts of the envelope where its possible to do so. After the Super Hornet was cleared to 45 AoA, (and I'd left the program) they upgraded the older F/A-18 FBW to allow flight past 30 AoA without departing controlled flight. I have heard of SHs departing controlled flight, but its extremely unlikely.

It would be my guess the F-35 would handle "slapshot" style high alpha shots better than the F/A-18E/F, but still run the risk of departure in High G, High Alpha conditions if pushed too hard. It could very well be that the G limit occurs before the absolute AoA departure limit in the F-35A. As I understand, even during post-stall high AoA testing they didn't have a full on departure from controlled flight in the F-35A.

I think it was actually Billy Flynn who flew a CF-18 into a (intentional) departure during a test flight, so its not like the test pilots at LM don't know how to fly a F/A-18. Several either flew F/A-18A-Ds, CF-18s or F/A-18E/Fs jets but now fly F-35s for LM.

As for the A-37, the one we flew didn't seem to run out of aerodynamic performance at 6Gs, and the owner said they didn't have major structural issues from high G profiles. The jet was flown with tip-tanks only, no weapons.
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Unread post16 Feb 2014, 01:24

neurotech wrote:The F/A-18A-Ds are known for departing controlled flight if yanked and banked at high AoA.


Do you mean assaulting two different axis limiters at the same time? that will take the F-16 out as well.

In pre limiter jets like the F-4 pilots could intentionally depart the thing to get an advantage in some visual dogfighting situations - but I don't really see any reason for doing it in modern jets?.

Another implication for going out of limits for the 35 is any affect it could have on the LO material I guess.


neurotech wrote:As for the A-37, the one we flew didn't seem to run out of aerodynamic performance at 6Gs, and the owner said they didn't have major structural issues from high G profiles. The jet was flown with tip-tanks only, no weapons.



Did you have a later modified /strengthened airframe that fixed some early faults?
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Unread post16 Feb 2014, 02:03

basher54321 wrote:
neurotech wrote:The F/A-18A-Ds are known for departing controlled flight if yanked and banked at high AoA.


Do you mean assaulting two different axis limiters at the same time? that will take the F-16 out as well.

In pre limiter jets like the F-4 pilots could intentionally depart the thing to get an advantage in some visual dogfighting situations - but I don't really see any reason for doing it in modern jets?.

Another implication for going out of limits for the 35 is any affect it could have on the LO material I guess.

F-16s can depart too, but the F/A-18A-D is known for having a sudden, hard departure if the pilot makes large inputs at high-AoA. An inexperienced F/A-18 pilot in a sudden departure would be a heartbeat away from joining the Martin-Baker fan club.

The reason would be to point the jet at the bandit. The F/A-18 doesn't need to do a full on departure, but the SH has a pirouette mode that comes in handy in a knife fight.

A known mishap with a F-22 being over-G'd didn't cause LO skin damage, but the USAF didn't want to spend the money on a proper laser inspection needed to check for airframe misalignment so the jet was retired. Subsequent inspection revealed slight permanent twisting of the airframe.
basher54321 wrote:
neurotech wrote:As for the A-37, the one we flew didn't seem to run out of aerodynamic performance at 6Gs, and the owner said they didn't have major structural issues from high G profiles. The jet was flown with tip-tanks only, no weapons.



Did you have a later modified /strengthened airframe that fixed some early faults?

It was an A-37B that I'm pretty sure would have been modified/strengthened as needed. It had been fully structurally refurbished when restored, and phase inspections completed. I was suggesting that the rare times the jet exceeded 6Gs didn't result in major cracks in the airframe. This particular jet gave more than a few pilots their first taste of a high performance jet as kids, including the owners' kids, and they wouldn't risk their lives with anything less than full maintenance on that jet.
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Unread post16 Feb 2014, 05:54

Here's the thing about g-limits, they are very conservative. First off, they are set so that no structural load on the airplane exceeds 2/3 of its verified strength (ultimate load). That verification is from ground static test, where no failure is permitted. Second, those static test conditions are based on the worst, worst, worst flight conditions - worst speed, worst altitude. worst gross weight, etc. At any other flight condition, the structural load will be lower for the same g level, and the chance of being at the critical condition is very small. For the F-16, as an example, the critical condition is 0.95 mach, 10,000 ft, full fuselage tanks. Recall Gums' story of wing stores reducing load on the A-37 wing? F-16 wing fuel does the same thing.

So, exceeding a g-limit (pilot-observed or electronic) usually has no bad immediate effect, meaning the wings don't fall off. It is still not a smart thing to do in some cases, depending on airframe age. Static tests are new airframes, and cracks inevitably reduce that strength over thousands of flight hours. At the completion of durability tests (two lifetimes) the airframe is required to withstand 2/3 of ultimate load, called limit load.

g-limiters have two negative aspects. First, a g-limiter could kill you someday. Second, having a limiter encourages pilots to go right to the limiter without hesitation and frequently, while a pilot-observed limit is sometimes approached gingerly for fear of exceeding the limit and owing your crew chief a case of beer for conducting the required (but fruitless) inspection. Frequently hitting the limiter does no short term harm, but will reduce the long term durability of the airframe. Early F-16 experience showed many more 9g events than the airplane was designed for. The design number of events was (thoughtlessly) based on earlier non-limiter airplane experience.

Gums
, I'm afraid you'll never see an F-35 flight control block diagram like you had for the F-16. Flight control engineers have taken advantage of ever more powerful computers to come up with fiendishly complex control laws. The F-16 one page block diagram is laughably primitive compared to later ones. The Korean T-50 trainer, similar to F-16, has a 21 page block diagram for example. I asked my good buddy, a recently retired F-35 flight control engineer, about it today. He said it would take a room full of paper to print it, so it is available only on a computer screen. They don't use simple diagrams for various functions and limiters, but huge tables of numbers.
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Unread post16 Feb 2014, 06:59

Just a short story about going to the G limit constantly in MACCHI 326Hs. I hope this is relevant. The RAN FAA had ten of these training tandem seat aircraft - same model as the RAAF. When fixed wing folded these RAN Macchis were transferred to the RAAF just before in 1982. The RAN did a lot of ACM training in their Macchis from the start and later flew armament sorties with a minigun pod and small practice bomblets only though. So these RAN Macchis were worked hard, with I guess a lot of 'going to the G limits allowed' - if not over sometimes - with subsequent inspections as described above.

Now the point of this story is that one of these former RAN Macchis was flying with the ACM training squadron in the RAAF at Williamtown when one wing broke off to kill the pilot. Bad things do happen. I believe the RAAF had an ongoing wing fatigue and wing replacement program either before or after - not sure about those points. Anyway - beware the overstress.

What you overstress today may not kill you but someone down the line. Always report overstress for inspection.
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And another story from training days in Winjeels with the RAAF at Point Cook. The Basic Flying Course behind the one I was on also had Army Officers learning to fly. One very windy day a RAAF Instructor and an Army Pilot Trainee (1st Lt) went out to do low flying 60 degree AoA turns in the low flying area. Perhaps the forecast was not clear, or the wind was much stronger than anticipated. Anyway during this steep turn training a wing fell off due to overstress from the very turbulent windy conditions at low level, with the added Gs from the steep turn adding to the overstress, in this otherwise very strongly built radial prop trainer of that era. The pilots died, instructor in right hand seat instantly from wing hitting him, with student attempting to escape with no chance before hitting the ground. Again - beware the overstress for whatever reason. One must fly to the conditions and not just a 'limit'.

No pilot I know deliberately overstresses the aircraft unless he/she is about to die.
Last edited by spazsinbad on 16 Feb 2014, 08:10, edited 2 times in total.
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Unread post16 Feb 2014, 07:53

johnwill wrote:Here's the thing about g-limits, they are very conservative.
True, but not nearly as conservative as they used to be (i.e. F22/35 vs F16)

johnwill wrote:g-limiters have two negative aspects. First, a g-limiter could kill you someday. Second, having a limiter encourages pilots to go right to the limiter without hesitation and frequently, while a pilot-observed limit is sometimes approached gingerly for fear of exceeding the limit and owing your crew chief a case of beer for conducting the required (but fruitless) inspection.
I most definitely understand how you see your second point as a negative aspect, but from an FQ point of view, it's actually a positive. That is, if the goal is for the pilot to get max performance out of the jet at any specific flight condition, it's better for him to be able to just pull and/or roll "max stick" and let the control laws figure out and apply the maximum limits, rather than have the pilot have to self-regulate, in which case he is more likely to leave performance "on the table". The life/fatigue issues you allude to are not trivial, but (imo) could be addressed by other means (training, etc)

johnwill wrote:Gums, I'm afraid you'll never see an F-35 flight control block diagram like you had for the F-16...
Very true.

neurotech wrote: As I understand, even during post-stall high AoA testing they didn't have a full on departure from controlled flight in the F-35A.
There's still plenty of testing to be done, so it's premature to declare "success". Also, the most challenging testing doesn't tend to be "post-stall" per se. Rather, the most difficult points tend to start out at higher speeds, lower AOAs (where lots of rate can be generated) and then rapidly transition to high AOA either due to coupling (i.e. the departure actually happens at low AOA and then AOA balloons), or due to a too rapid response to a pilot command. This type of testing is also not complete.
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Unread post16 Feb 2014, 11:33

Raptor_claw wrote:
johnwill wrote:Here's the thing about g-limits, they are very conservative.
True, but not nearly as conservative as they used to be (i.e. F22/35 vs F16)


What do you mean by this? Can you elaborate?
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Unread post17 Feb 2014, 02:30

Raptor_claw wrote:I most definitely understand how you see your second point as a negative aspect, but from an FQ point of view, it's actually a positive. That is, if the goal is for the pilot to get max performance out of the jet at any specific flight condition, it's better for him to be able to just pull and/or roll "max stick" and let the control laws figure out and apply the maximum limits, rather than have the pilot have to self-regulate, in which case he is more likely to leave performance "on the table". The life/fatigue issues you allude to are not trivial, but (imo) could be addressed by other means (training, etc)


I agree completely. It was a problem for F-16, but only because the design g usage spectrum was inadequate. Although I don't know for sure, I suspect the newer airplanes have covered that problem. Obviously being able to go right to the limit easily is a good thing.
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Unread post17 Feb 2014, 03:16

Given modern design principles, materials, and technologies; how many G's do you think the air frame can really take in day to day flying to the limit without any noticeable drop in air frame life expectancy if there was no human factor, let's say purely remote controlled?

From what I can tell, I'm sure most Air Frames are designed to handle far more than what's stated on the brochure, for obvious safety / endurance limits.

Anybody with more insight can help answer this question?
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Unread post17 Feb 2014, 03:41

'KMR' the flight manuals/NATOPS describe the limits. Have a look at any NATOPS for example in the 'LIMITATIONS' section to see what the day to day limits are. Elsewhere in the back section (often missing now from modern PDF manuals) there will be graphs or tables of limits in certain situations. A NavAv Pilot will fly to the limits of NATOPS - no more than that. If going over the limit then explanations and other actions from maintenance required. These limits are not trivial and not to be disregarded. Flying a military jet aircraft is not a trivial matter. Every pilot will know in an instant what any day to day limit is. He will be quizzed constantly (every morning usually) about NATOPS / Emergencies / Limits etc. I read again the other day how a pilot (I think a Marine) said for every one hour flying he will spend about six hours planning/ briefing/ flying for one hour/ debriefing. This seems reasonable to me.
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Unread post17 Feb 2014, 04:04

There are safety margins built into the design of all airplanes. In the static test load case, as I mentioned before, the airplane must withstand 150% of its highest expected load ever encountered. Every part of the airplane is subjected to that rule. Most of the time, limit maneuvers will result in less than 100% limit load - extra safety margin. In the durability loads test, the airplane must withstand two lifetimes of the design usage. The design usage is a spectrum of all the maneuvers the airplane is expected to perform during its design life, low g, high g, rolls, everything.

To answer your question, a 9g airplane could be expected to survive at least 13.5g (1.5 x 9). For reduced fuel, non-critical speed/altitude, probably even more. But there would be an inevitable loss of life expectancy, no way around it.

But spazsinbad is correct, limits should never be intentionally exceeded in day to day flying.
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Unread post17 Feb 2014, 07:05

johnwill wrote:There are safety margins built into the design of all airplanes. In the static test load case, as I mentioned before, the airplane must withstand 150% of its highest expected load ever encountered. Every part of the airplane is subjected to that rule. Most of the time, limit maneuvers will result in less than 100% limit load - extra safety margin. In the durability loads test, the airplane must withstand two lifetimes of the design usage. The design usage is a spectrum of all the maneuvers the airplane is expected to perform during its design life, low g, high g, rolls, everything.

To answer your question, a 9g airplane could be expected to survive at least 13.5g (1.5 x 9). For reduced fuel, non-critical speed/altitude, probably even more. But there would be an inevitable loss of life expectancy, no way around it.

But spazsinbad is correct, limits should never be intentionally exceeded in day to day flying.

As I understood, the F-16Ns were retired early because they didn't allocate funds to fully inspect and refurbish the jets. Those jets were flown hard, but with minimal external stores. It would have made economic sense to keep them flying, if budgeted.

The thing I have trouble with is that a comprehensive structural SLEP for a jet fighter is a fraction the cost of replacement, but the Navy/Air Force seem to want to do low cost "inspections" and minor structural refurbishment instead. Pulling 7Gs+ or slamming onto the deck will definitely shorten service life.

@raptor_claw: The Super Hornet that departed was a high speed transonic (~M0.95) departure at lower AoA. The claim was that it was a glitch in the FBW control laws that caused it but the pilot definitely used large, sudden control inputs. The jet was recovered but not before rapidly exceeding the 7.5G limit.
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Unread post17 Feb 2014, 08:03

KamenRiderBlade wrote:Given modern design principles, materials, and technologies; how many G's do you think the air frame can really take in day to day flying to the limit without any noticeable drop in air frame life expectancy if there was no human factor, let's say purely remote controlled?
The essence of this question gets back to the "design g usage spectrum" that johnwill referred to just above. When you design a structure you have to have some estimate as to how it's gonna be used. To really oversimplify: let's say the customer lays out the expected usage (say, based on every 10 flights): max G once , 80% G three times, to 50% six times, and the rest of the time it's just tooling around at "low" G. So, you design the structure based on that usage. If you have that requirement, but you actually design it to go to max G every flight, then you would have over-designed, and the extra weight is hurting something else (performance, most likely). On the flip side, if the customer requirements call out a more modest usage spectrum and then they wind up flying much more aggressively, there's gonna be problems.

spazsinbad wrote:'KMR' the flight manuals/NATOPS describe the limits. Have a look at any NATOPS for example in the 'LIMITATIONS' section to see what the day to day limits are. Elsewhere in the back section (often missing now from modern PDF manuals) there will be graphs or tables of limits in certain situations. A NavAv Pilot will fly to the limits of NATOPS - no more than that.
Kinda gets back to my earlier point. Modern control laws now are able to be much more complex (due to computing power, largely) so all those graphs and tables are now built-in to the system. Point being, the pilot of a sufficiently-modern jet doesn't have to know all that stuff (not that he shouldn't...), the control law knows which stores are loaded and where, how much fuel there is (and where it is) - basically everything it needs to know to arrive at the proper limits. It will never be perfect, obviously, but the stuff the pilot has to remember to keep from breaking the jet is dramatically reduced. For instance, he may still have to know that he's not supposed to slam in full pedal while rolling and pulling at certain Machs. But the more basic limits (how many G's he can pull at a certain speed, or how fast he can roll with certain stores) should all be automatic.
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