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Elite 1K Joined: May 26, 2005 Posts: 1127

ATFS_Crash:

Well said.

The "easy" answer is meaningless except for the "my plane is better than yours" internet arguments and the RIGHT answer is an awful lot of research.

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johnwill:

There are three flavors of G: (actually 4 if you include G whiz)

1. G available

2. G allowable

3. G attainable

G available is purely a function of calibrated airspeed, the more speed you have, the more G available. In the earlier fighters with mechanical inputs to hydraulically operated flight control systems and without fancy limiters, whatever G was available as a function of airspeed, you could get by pulling on the pole. When G available was in excess of G allowable, it was not wise to use G available unless an unusual situation required it. The pilot was the G limiter. He would certainly write the aircraft up as an over-G if it ever became necessary to use more than the allowable, i.e. avoiding ground impact. I also understand that the entire airframe produces aerodynamic force, not just the wings and that the entire aircraft is subjected to these accelerations. I mentioned available G only to emphasize that there is only one “maximum” G for an aircraft.

The structural limitation or G allowable has to be managed by the pilot when computer controlled limiting is either not available at all, or is limiting only certain functions of the flight control system. I understand this, did read the dash-1, and I understand they do destructive testing on airframes to 150% (inverse of 2/3’s) of the design load factor. My choice of the word fatigue was a poor choice and incorrect but probably describes accurately the state I was in when I wrote that last night. My bad. I still wonder if the destructive testing is done to validate 13.5 G or 11 G for an aircraft with a limiting system for 9G (do they still use 150% for a 9 G limited aircraft?) I understand you can “rate” the limiter, but how much G overshoot you can create I’m not sure.

G attainable is a function of what “HAL” will allow in modern aircraft with computer controlled limiting. It is not always G available, and I understand that. I understand that “HAL” does not provide limiting in all circumstances and the pilot is still responsible for not pulling his aircraft apart. I’m involved with the A330/340 now and limiters can still be frustrating.

With respect to Cat 2, it was just a passing comment on the fact there is no switch position for Cat 2. It’s like having the Master Caution Light come on and when you check the telelight panel for the source of the problem, the “spare” bulb is lit.

Regardless of your view of the Cat 3 position, whether you limit AOA or G, you have potentially prevented the pilot from achieving pitch rates that may be necessary at the time. And if you limit AOA, you also limit G for whatever flight condition you are in. You can’t separate G from AOA if you stop the pilot short of what’s available. It may surprise you, but I don’t know any pilot that I flew with who ever moved the switch out of the Cat 1 position, anywhere, anytime. Of course, everyone I flew with had extensive experience in aircraft which required the pilot to be the G/AOA limiter. I never felt unsafe, maybe you could have saved the taxpayers some money. It was not a big deal and it would be interesting to hear how they do it now.

To answer the question:

F-4 7.33 unless clean (4 AIM-7, 4AIM-9, nothing else) with half internal fuel, then 8.5
Unfortunately half internal fuel in AB to get 8.5 wouldn’t allow you to do it for very long or very far from home.

ATFS Crash,
You are right of course, everyone has a different perspective. Mine happens to be engineering, not military book. I am familiar with the military book of many years ago, having written some of it, so maybe I have two perspectives. My real problem with outlaw162 is why he would intentionally violate handbook limits. I know the limits are conservative, but how conservative? An airplane was lost before the Cat 3 capability was developed, and he risked the same fate by violating them. I also know he was not the only one to do so, as it was common practice. Pilots would try it, find out nothing bad happened, so it must be ok. What they may not have realized was the limiter provided protection for unusual conditions - low airspeed, high AoA rolls with air to ground stores. Do that without the limiter and bad things happen.

The real question is NOT how many aircraft have been lost by test pilots because Cat 3 was not available, but how many aircraft have been saved (or lost) because Cat 3 was made available?

Manufacturers, test pilots & test engineers now tend to weight a compromise between ultimate mission capability and the capability of the weakest link that can be expected to operate the equipment. I think they call it human factors. That is their current charter, both military & civil, and there is no argument with the nobility of that.

In the 50’s & 60’s, the charter was do what it takes to get the capability and expect losses.

I’m not judging it, just commenting on it.

My real problem with johnwill is absolutely none. I enjoy reading your posts and respect your expertise.

regards, O.L.

outlaw162,
It seems we are in agreement that limiters are controversial and can cause as much harm as good. Being a contractor engineer, my opinion did not matter, as the F-16 System Program Office decided everything. One way to make limiters more effective and less intrusive is to make them smarter, where they only limit when absolutely necessary. The Cat 3 limiter took a broad brush approach, severely limiting AoA regardless of roll rate or airspeed, when the lower limit was needed only when rolling below 200 kt. The answer of course is to make the limiter a function of roll rate and airspeed, but that takes even more taxpayer dollars to be wasted when pilots don't use it. Recall that these limiters were developed in the days of analog FCC, with their all-hardware computations.

Ground testing to 150% is still done, but is usually not destructive. There are 20 or 30 test conditions, so intentional failures would ruin the test schedule. After all the test conditions are complete, then destructive conditions may be applied.

To say a 9g airplane is tested to 13.5g is not precisely correct. The structural loads for 9g are multipled by 1.5, but that does not mean those are the loads for 13.5g. Why? Because load is usually not linear with g.

Appreciate your comments and your tolerance for my jabs at you.

lamoey,
The g limit on F-16 remains 9, no matter what the FCC allows. The FCC has tolerences, so most F-16s will show you 9.3 or 9.4 under some conditions. Which brings up a point, the g indication system on the F-16 is a mess, to be kind. Since g limit is a structural concern, the g should be measured at the center of gravity. For some reason known only to them, the flight control guys put it 13 feet forward of the cg and above the roll axis. Result - the g reading is affected by pitch acceleration and roll rate. You and I would call that an error. But the pilot doesn't see that g reading on the HUD. It contains its own accelerometer and that's what the pilot sees. Of course it is also affected by pitch acceleration and roll rate, so more errors.

Is it safe to fly at 9.4g in a 9g airplane? Sure is, because the limiter is set to protect the structure at the worst combination of weight, mach, and altitude in a symmetric maneuver. The most critical condition is full fuselage fuel, empty wings, 10,000 ft, 0.95 mach. Faster, slower, higher, lower, lighter, heavier - critical structural loads are reduced at 9g. On top of that, there is always the extra 50% strength built into the structure.

The primitive YF-16 had a variable g limiter, which accounted for reduced load at other than .95/10k. It's limit varied from 7.33 to 9, depending on speed and altitude.

johnwill wrote:

Since g limit is a structural concern, the g should be measured at the center of gravity. For some reason known only to them, the flight control guys put it 13 feet forward of the cg and above the roll axis. Result - the g reading is affected by pitch acceleration and roll rate. You and I would call that an error.

Oh, don't even get me started, jw. It's not just 'some reason' - flight control guys don't do anything without a perfectly good reason. Poke

Sure, g 'limit' is a structural concern, but (unlike certain disciplines) flight control actually cares about the happiness of the human in the seat, which, by the way, happens to be in the general vicinity of '13 feet forward of the cg'. (Apparently you've never had to listen to a pilot complain about G variations during rolls - it's not pretty.)

I'm teasing, of course. But honestly, you can either (a) figure out how to move the cockpit back to the middle of the aircraft, or (b) kick the human out of the aircraft completely. Until one of these thing happens, this 'loads vs FLCS difference of opinion' will continue. And (as it has for the past 30 years) flight control will continue to 'win'...
Laughing

johnwill wrote:

For some reason known only to them, the flight control guys put it 13 feet forward of the cg and above the roll axis. Result - the g reading is affected by pitch acceleration and roll rate. You and I would call that an error. But the pilot doesn't see that g reading on the HUD. It contains its own accelerometer and that's what the pilot sees. Of course it is also affected by pitch acceleration and roll rate, so more errors.

JohnWill: I do enjoy reading your comments and can appreciate your knowledge of the system. You must be talking about the accelerometer pack located right under the ejection seat. Perhaps the idea is to also limit the G's on the pilot. We have heard for a long time that +9G is the generic human limit so perhaps that is the compromise they had to agree on.

The HUD gets its reading from a sensor sitting in front of the cockpit, so even further away, so even more affected by pitch motions relative to center of gravity.

My knowledge of theF-16 is limited to block 15 and older, so if any of this have changed in later blocks none of this may apply.

Rapor Claw,
I know about the accel being at the center of percussion (not the pilot seat), but still don't really understand why, and don't care at this point. "They" thought about computing a g at the cg for structural limiting, but hadn't done so by the time I got out of the business. Some of my best friends are flight control types, but I wouldn't want my daughter to marry one.

Part of the reason pilots complain about g variation in a roll is because the accel is above the roll axis, so gives the FCC a false reading of g. Then the FCC tries to mainatin the commanded g based on a false feedback. I have heard test pilots complain about everything, so surely somewhere in time I've heard the one about g variation in a roll.

Lamoey,
The FCC g sensor is still several feet behind the pilot, not at the seat. Just like you, my actual work on the F-16 ended with block 15.

johnwill wrote:

Rapor Claw,
"They" thought about computing a g at the cg for structural limiting, but hadn't done so by the time I got out of the business. Some of my best friends are flight control types, but I wouldn't want my daughter to marry one.

Having a structures guy for a father-in-law? Now that's a scary thought...

Part of the problem with trying to use a computed c.g. accel for limiting is that realistically there's just not a whole lot you (the FCC) can do with it. Classic example is the pushout from max G. At steady-state at limit g (turn or split-s, or whatever), both the pilot and c.g. are at (very nearly) the same load factor (let's say 9.0). If the guy wants to push out of that (for a conventional, i.e. F-16-type, configuration), there's only one way to do it. The horizontal stab moves t.e. down, creating a nose-down aircraft moment by increasing lift at the back end. Since the increase in tail lift 'gets in' before the reduction in AOA reduces wing lift, the net effect is an increase in total lift on the aircraft. More lift, more Nz and the c.g. load increases above 9.0, albeit briefly. (The pilot is also subjected to this 'heave' effect, but is far enough forward that it is more than cancelled by the rotation of the aircraft and his Nz only decreases).
The only way to keep the c.g. from exceeding 9.0 would be to force the aircraft to unload extremely slowly, which clearly would not be acceptable. If you really want to keep the c.g. from exceeding 9.0, the only other option would be to pre-calculate the largest expected c.g. load-up (for the worst case, full push from max Nz at the most-aft c.g.) and bias off the limit by that much. Say you predicted a maximum 0.3g increase - you would have to limit the max G commanded to 8.7. This works, but you are no longer a 9g aircraft.
Getting back to my point, telling the FCC what the load factor at the c.g. is wouldn't help, because it still has only one way to move the aircraft around. (Assuming it doesn't have an active control surface like a canard well in front of the c.g. to use).

Senior member Joined: Dec 14, 2006 Posts: 472

johnwill wrote:

ATFS Crash,
My real problem with outlaw162 is why he would intentionally violate handbook limits.

While in the vast majority of the cases it is best to follow the book.

I consider “books” are essentially laws or rules; and the vast majority of the time they should be followed. However I also think that the intent of the book/rule/law should be considered. I think in some cases violating the book/rule/law is the lesser evil.

Perhaps a pilot has a bogey on his tail and he can see the bogey's tracers walking closer toward him, does the pilot continue to pull textbook Gs, or perhaps an option would be to pull back a little harder on the stick and risk exceeding textbook values slightly?

Perhaps a pilot becomes spatially disoriented and realizes that he’s on a collision course with the Earth and doesn’t seem to have proper parameters for a proper recovery and maybe not even a proper ejection; so what should a pilot do?

F-15 Eagle Spatial (Spacial) Disorientation: World Record G
http://www.youtube.com/watch?v=_-adcRA3u8Q

Also consider that sometimes rules and books are often revised. Often to get an aircraft/armaments or others into service quickly the book will be written encompassing what is known or thought to be the most likely and safe procedures. Often the aircraft and systems will go through additional testing after the initial book was meant, as a result often procedures can change and often values and such can be increased and sometimes regrettably decreased out of necessity.

Like a pilot once said, “I was at the minimum textbook value for ejection, however I elected not to eject at minimum altitude and instead ride it in, because the minimum value was already revised upwardly once because of a fatal failed ejection at textbook minimum altitude values and I don’t want to become a footnote in a memo of another revision revising the minimum altitude upward again.

Also lawmakers/rule makers/book writers/enforcers do not always necessarily have all their rules thoroughly thought out.

I was at a park the other day, a Ranger started to write me a ticket and give me the “letter of law” speech for leaving my dog tied up an unattended while I used the bathroom. I explained to him that I thought that the ticket and rules were unfair. I was using the bathroom, the park rules posted that there was no dogs allowed around the kids playground and the kids playground and encompassed the bathroom, so if I have to go the bathroom while I’m walking the dog there is no legal way I can use the restroom, and if I go to the bathroom in public I would be violating the public urination laws, and that if someone misinterpreted my intentions I might even get charged with indecent exposure. I told him that the park rules were unreasonable and that it wasn’t posted that dogs couldn’t be left tethered and unattended in a dog area for a few minutes while people are using the bathroom, and even if it was the rules altogether are unreasonable. I told him if he gave me a ticket I would not pay out the ticket, instead I would take it to court and publicly point out the ridiculous unreasonable rules and fascist enforcement. I told him I thought I could humiliate him and the park in court. He stopped writing the ticket and let me go.

Remember that part of a test pilots job is to break the rules and to make new rules. Remember part of the aircraft and weapons certification process is to fly and test aircraft and weapons combinations so that they can attain their certification. Though the test pilot may be violating procedures in one textbook, they usually have their own strict textbook to follow. Remember that the certification process takes a long time and that there may be an initial batch of certified procedures and weapons, though over time the number of procedures and weapons that are certified typically are expanded/revised.

Raptor Claw,
Come to think of it, I'd much rather have a flight control type as son-in-law than the one I have. He'll never read this because I don't think he knows how to turn on a computer.

Your points about the reasons for not calculating nz at the cg are well known and understood by structures guys too. We do abrupt pushovers from high g and measure the nz at the cg, so can precisely tell what that effect is. We don't just calculate stress and strain and hope the wings stay on. In fact, some structures guys have a better overall understanding of how flight control, stability and control, aerodynamics, and structures are inter-dependent than the other disciplines. I can flatly state that we know more about the other disciplines than they know about ours.

ATFS Crash,
You are obviously quite right about violating limits in unusual circumstances. However, outlaw162 and many others routinely violate the Cat 3 switch usage from the -1. That is another matter entirely.

The test pilot's job most assuridly is NOT to break rules and make new rules. That will get you fired in a heartbeat, seen it happen for very minor infractions. During envelope expansion tests, temporary limits may be exceeded, but ALWAYS with approvals up and down the line. We're probably saying the same thing, but please never say it is ok the break a rule routinely. I know what I'm talking about here, as I have been a structures engineer on approximately 1000 test flights spaced over 40 years on everything from B-58 to T-50. I am thoroughly familiar with airframe and weapon certification procedures, including analysis, ground test, and flight test.

Sorry if that sounds arrogant. It's not intended to.

Enthusiast Joined: Apr 28, 2005 Posts: 88 Status: Offline

The F-14 was not a 9g aircraft. It was originally rated for 7.5gs (like the original F-15), but later on it was derated to 6.5gs as the airframes became older.

Test pilots rule ... make the Raptor your Bitch Very Happy

http://www.youtube.com/watch?v=VOro3MDF_sI

Salute!

We need Roscoe here.

1) The GD handouts we used for the first year or so of the production models indicated a strain gauge in the wing root to measure the deflection and limit the gee command. This may not have been implemented, but I can scan the page and post.

2) The Cat I was "unlimited" stick inputs. Trust the FLCS.

3) The Cat II was a "pilot-controlled" limit.

4) The Cat III did not limit gee. The Viper could not pull 9 gees above 15 degrees AoA. The Cat III limited our roll rates and AoA, not gee.

5) A lot of folks fail to realize that pulling lottsa gees with ordnance or tanks is not a big deal. The wing is trying to bend up, and the weight out there keeps the deflection less than a clean wing. The problem is rolling gee. Both aero and mechanical crapola comes into play when rolling and pulling.

The A-37 had a higher gee-limit with bombs out on the wingtips than without them. The cracks we got on our spars were on the top, due to taxiing with 3,000 pounds of bombs and another 1,600 pounds of gas in the wing tanks. The cracks were just inboard of the main gear.

*********************

As far as the original question goes, we have "design" limits, operational limits, and good-judgement limits.

The Viper was the first jet since the F-86 rated at 9 gees for everyday ops. The F-86 had a very high gee-limit, but I believe placard limit was the basic 7.33 gees.

I have seen 10 gees in one plane that was not rated for it. Wrinkled the upper wing skin and got an a$$-chewing from all the maintenance guys and my boss.

I saw various gees on the Viper HUD "history" box ( not the instantaneous display) - somedays 8.7 gees, some days 9.1 or 9.2. So BFD. I didn't have to look at the gee meter and over my shoulder at the same time, and I loved it.

gotta go

Gums sends......

Gums wrote:

1) The GD handouts we used for the first year or so of the production models indicated a strain gauge in the wing root to measure the deflection and limit the gee command. This may not have been implemented, but I can scan the page and post.

No, no such logic is in the production CLAW. If this option ever was used it must have been both (a) pretty short lived and (b) before my time (i.e. before the mid 1980s'). Not actually knowing, and not ever having heard about it, my best guess would be that there may have been a display providing that info to the pilot, to allow for manual limiting. Just really seems unlikely that it would have been part of the very early (analog) CLAW.
Don't know if johnwill was around that early or not, maybe he can help.

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Aircraft G Limits