F-16.net

Senior member Joined: Dec 15, 2006 Posts: 472

johnwill wrote:

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.

I think we are saying the same thing, I just don’t like your wording.

Test pilots often fly uncertified aircraft and weapons don’t they?

The textbook says you can only fly textbook approved load outs, yet test pilots have to fly load outs that are not approved by the conventional book. However the test pilots have their own book that is written by engineers and command that usually have a very strict and specific guidelines to test the new load outs.

Yes it sounds a little arrogant, but then so do I. I feel as long as you’re not excessively arrogant and as long as you're reasonably civilized it would hypocritical to hold a little bit of arrogance against you. I think at least a little bit arrogance is almost required in this field.

No offense meant, no offense taken.

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AOA/G Limiter

In cruise gains, the AOA/g limiter reduces the positive g available as a function of AOA. The negative g available is a function of airspeed. Below 15 degrees AOA, the maximum positive g available is +9g. As AOA increases, the maximum allowable positive g decreases. The positive g limit and maximum AOA depend on the position of the STORES CONFIG switch. In CAT III, maximum AOA varies from approximately 16-18 degrees as a function of GW and g.

That sure is clear as mud. This is the kind of verbage the average line pilot had to work with concerning the limiters. Prepared by an evidently risk averse test team.

I think as you read that “the maximum positive g AVAILABLE, +9g” becomes “a decreasing maximum ALLOWABLE positive g” as AOA increases, you would question whether the limiter did this or the pilot was supposed to control it. Why change the terminology from AVAILABLE to ALLOWABLE when it turns out the limiter does it for you?

I think if you note that in CAT III, max AOA varies as a function of GW and “G”, (or “gee”, the main topic of the discussion) as opposed to max G varying as a function of AOA in CAT I, one might wonder what the heck are they trying to tell me?

They also bluntly state that “THE POSITIVE G LIMIT and maximum AOA depend on the position of the STORES CONFIG switch.” The available guidance doesn't separate the two.

If you can only get 16-18 degrees AOA at a particular speed in one mode and you can get more AOA at that same speed or higher in a different mode, you will be getting more “gee”.

AOA is what is applied or limited from being applied to the wings by the pilot, G is the resulting acceleration, you can’t completely separate them.

johnwill:

Nowhere in any documentation is it stated categorically that at any point in the flight envelope you can get the same "gee" flying in Cat 3 that you can under the same conditions in Cat 1. This statement, if true, would have been a nice confidence builder. I think you can forgive me for violating Section 5 given the ambiguously (I won't call it "poorly" or "incompletely") written guidance available and the tragic occurrence.

I didn't write it.

Any more experts out there?

regards, O.L.

Salute!

I tink the "claw" is right about using actual strain gauges. In any case I'll publish the reference here and maybe John-boy can drive back to the plant and read the thing hisself.

"F-16-060-8 12 August 1975 Technical Description Vol 8 Flight Control System"

These were our only source of data back in 1979, as we only had a Xerox copy of the YF-16 Dash-One and first FSD jet to go by. As I had to teach the crapola, was best I could get my hands on.

It says on page 4-11, sec 4.1.9 "This implementation requires ...continual measurement of its structural deflection" They have a graph two pages later that shows the implementation of wing bending stress as "measured deflection".

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

Now let's get this straight!

The gee available in the Viper is a function of your speed and your AoA, but you can't pull 9 gees above 15 degrees AoA +/- a bit. The FLCS won't let you. By the time you reach 25 deg AoA all you can pull is one steeenkeeng gee! Fer chrissakes, how many graphs do we have to put up!!

You can pull the same gees in Cat I or Cat III at 10, 11, 12 degrees AoA. Go try it!! And we DID show this to the students.

Available gee in the Viper is unlike anything built before it, due to the limiter above 15 deg AoA. Below 15 deg, it's basic math - turn radius a function of speed and gee, not AoA. Granted, AoA is what gets you turning, but different planes will pull more or less gee at a given AoA/speed combo. The Viper's relaxed static stability gave it more gee at less AoA because the tail wasn't fighting the main wing.

The Cat III AoA limiter took a lot of heat when Roy Neize augered in after pressing too far and not aborting his pass. I got the HUD film. Several folks thot he would have made it if he had the switch in Cat I. But he wasn't going fast enough and all he would have done was mush in a little more gradually. Hell, he woulda made it no matter what if he had pulled about 100-150 feet higher.

Cat III feature was intended to have a slower roll rate and lower AoA limit when you had stuff on the wings and rolled the thing when pulling. Little jet, lottsa moments on the wings - you could depart it or bend it. We used it on BFM flights to simulate a Flogger. No knife fights in a telephone booth in Cat III, I assure you - big, fast fights.

Dammit! Where's Roscoe!!

Gums sends ....

Last edited by outlaw162 on Jul 06, 2008 - 02:36 AM; edited 1 time in total

"Several folks thot he would have made it if he had the switch in Cat I. But he wasn't going fast enough and all he would have done was mush in a little more gradually."

Read what you just wrote about "mushing in a little more gradually". Kind of implies there's a difference there in what you're getting. At some point, maybe one or the other quits "mushing" earlier. A 330-350K delivery for computed bombing accuracy is effective, but it does have consequences. We're not talking 9G here but maybe the difference between 4 & 4.5.

Yes, and I know the AOA limit was 25.5, and I don't recall seeing any steeenkeeeng
graphs here. Other than playing Flogger, did you fly in CAT III? Then you can assure me that CAT I or CAT III, in any situation I get the same G?

Too late, I'm too old to fly 'em anymore.

regards, O.L.

Senior member Joined: Dec 14, 2006 Posts: 472

Deleted. Misread/misinterpreted some posts because of careless skimming.

The strain gauge Raptor Claw and Gums mention was installed on every USAF F-16 from Block 1 through Block 15, possibly Block 25. The gauge was mechanical, not electrical, and was never used by the flight control system for any purpose whatsoever. It responded to wing bending moment at the root, recording the load on a small metal disk with scratches from needle, not unlike an old 78 or 45 rpm vinyl recording. The gauge was installed to help keep track of the airplane usage. Usage in this context means how many occurrences of different levels of wing load were experienced compared to design occurrences. Turned out to be not such a great idea, so later blocks contained the Crash Survivable Flight Data Recorder (CSFDR) which recorded 30 different items on tape and were much more successful in obtaining the necessary data.

There is some basis for confusion about those gauges, since one scheme structures and flight control came up with was to install electrical gauges to monitor wing load. The data was to feed into the FCC and be used to limit wing load, not g. Sanity prevailed (for once) and the scheme was dropped. There is a lot more to this story, if anyone is interested.

outlaw162, no question that written descriptions of the various limiters are difficult to understand. They are written by flight control types, so what do you expect? That's why cavemen invented drawing. Take a look at the graphs, and you will see they are very easy to understand. Not sure what you mean by "a risk averse test team". Care to expand on that? No structures test team I was ever on was risk averse. We were prepared, knew what we were doing, and got it done. For example, F-16A No. 2 (75-0746) was the structures test airplane. That airplane went to 9g on flight seven. Any risk aversion was from USAF test management, who were scared silly that there would be an incident on their watch, and their careers would go down the drain.

I'll nit pick just a bit here, g is not acceleration. g is load factor and it is related to acceleration. Here's why. Say you are flying level, erect at a constant speed. What is your g? 1.0 is the answer. What is your acceleration? 0.0 is the answer. Not the same, are they?

I don't think I said you could fly Cat 3 to the same g as Cat 1 in every situation, but maybe I didn't say it very well. Both Cat 1 and Cat 3 limlters are g and AoA limiters. If your airspeed is high anough you will hit the g limiter first. If your airspeed is low enough you will hit the AoA limiter first, No doubt, if you hit the AoA limiter first, g is also limited, but it is the AoA feedback which limits you, not the g feedback. Since the Cat 3 AoA limit is usually lower, at lower airspeeds, you cannot get as much g in Cat 3 as you can in Cat 1. At higher airspeeds, you can get the same g in Cat 3 as in Cat 1.

Gums, the reduced roll rate in Cat 3 was not needed to protect the structure from "bending". In fact, the entire FSD external stores structural flight test was flown in Cat 1, 'cause there was no Cat 3 at the time. We flew full rate 360 degree rolls with the most severe stores on 3 & 7 - GBU-8 and 3 AGM-65 on LAU-88. Same story for 4 & 6. Worst stores were 370 tanks, 6 MK-82 on MER, and 4 CBU on MER. All flown with Cat 1, without any structural problems.

Hope everyone had a safe Fourth and enjoyed lots of hot dogs, burgers, beer, and the Freedom our military protects for us every day.

Thanks for the strain-gauge history lesson johnwill. I'd either never heard about those, or had just completely forgotten.
Excellent post in general, in fact. I tried really hard to find something to disagree with, but to no avail. Laughing

Maybe next time....

We changed the MSR tape in our block 15's every 50 hours.
Assy was on 325 bulkhead.

Source:

http://www.nlr.nl/id~3040/lang~en.pdf

Salute all!

Nice back-up John-boy. TNX.

We knew that original stores cert was done prior to the Cat III switch.

I didn't know that structural considerations were not part of the Cat III implementation. Most of us understood the aero considerations, and I had flown one plane with serious problems due to "roll coupling" - the Voodoo. The Sluf also had a cosmic departure if you used max roll at a high AoA without gobs of rudder.

To the outlaw:

I flew in Cat I except when "playing around". Pays your money, take your chances.

I could get the same gee at the same speed until 15 degrees AoA.

I posted some plots of the control system on another thread.

Roy's speed was really slow, if you see the tape. So he hit the 17-18 deg AoA limit right off the bat. Problem with the FLCS is/was that gee limiter and AoA are linked above 15 deg AoA. The T-bird crash at Mt Goat is another example of slow speed pullouts. The guy was just not going fast enuf to keep the AoA function from limiting gee, and like Roy, didn't have enuf turning room above the ground.

When working on the "hotline" gunsight, we used simple equations to get a rough idea of the relationships between turn radius and gee and turn rate. One of them was radius = vee^2/gee.

So at 8 gees and 350 knots, you get about a 1400 foot radius. Same speed but at 9 gees it's about 1200 feet. Original Vipers hit the 350 knot "corner", but I suspect the later models need more speed due to their weight/drag.

gotta go

Gums sends ...

VarkVet,
Thanks for the excellent NLR link. Those guys are very good at the fleet management game. You said you changed a tape and I said the the data was recorded on a disk. You are right. An early test version of the MSR used a disk, but it did not have enough capacity, so they developed the metal tape cart, still scratched by a needle.

Back to the Cat 1 & 3 limiter. Here's an oddity. Recall the Cat 1 limiter won't allow 9g above 15 deg AoA, yet the Cat 3 limiter will allow 9g until you reach 16 - 18 deg AoA. So in a narrow airspeed range, Cat 3 will allow more g than Cat 1. All brought to you by our good friends in flight control. All friendly teasing of course. The YF-16 flight control team did some amazing things and continued that through the F-16 program. They were also very easy to work with and tried to do anything we asked of them.

Newbie Joined: Jun 05, 2008 Posts: 6 Status: Offline

F/A-18 started as a 7,5G machine, but was later upgraded to 9G (some legacy Hornets). I have only experience with the 7,5G version.

Mission Computer constantly calculates aircraft weight and FCC sets G-limit anywhere between 5,5 and 7,5G. Weight calculation includes fuel, external stores and some internal devices. In really heavily loaded plane structural G-limit can go below 5,5G. There's a switch on the stick which increases FCC G-limit by 33%. FCC doesn't limit negative G.

FCC limits roll rate depending on external stores config.

FCC doesn't limit AoA. Clean conf doesn't have pilot observed AoA limit. (Aerodynamic limit is around 50 degrees)

Last edited by outlaw162 on Jul 06, 2008 - 09:30 PM; edited 1 time in total

johnwill:

Be careful, the air I flew in was 3D. My physics professor would cringe at you saying G is not acceleration. Feet or meters or furlongs per second squared or minute squared or fortnight squared is acceleration. 1G is 32 feet/second squared which is countering earth's 32 feet/per second squared in the opposite direction. As you say,you do it with lift or load factor.

In what they call level unaccelerated flight, you are countering gravity, which is still acceleration if viewed externally. You may not increase your forward speed and it will not out turn a MiG, but it plots out to almost a straight line (not quite, they told me the earth is not flat).

0 G is zero feet/second squared "acceleration" and you end up in a parabola which eventually intersects the ground regardless of speed a long as you hold G constant. If you are going straight down, like a rock or an apple, you are also accelerating even though at some point you may not be increasing speed (vacuum vs fluid). Load factor is what the wing ('scuse me) airframe feels and it is described as G also because it results in an acceleration in some plane usually selected by the pilot.

Be careful on the narrow airspeed range G thing. You can get a particular non-limited angle of attack at almost any airspeed. My rule was always keep your airspeed equal to or higher than your angle of attack. Example: Zero was my absolute minimum acceptable airspeed (right near the k in knots on the meter) but only at zero angle of attack.

regards, O.L.

Elite 1K Joined: Jul 20, 2005 Posts: 1034 Status: Offline

ATFS_Crash wrote:

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.

Its a shame he let you get to him. Of course there are people like yourself we deal with all the time. He should be let go if he can't do his job. I would have just told you to be quiet and that I'd gladly see you in court.

Senior member Joined: Dec 14, 2006 Posts: 472

checksixx wrote:

ATFS_Crash wrote:

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.

Its a shame he let you get to him. Of course there are people like yourself we deal with all the time. He should be let go if he can't do his job. I would have just told you to be quiet and that I'd gladly see you in court.

So you think that the officer should have enforced the letter of the law and inconvenienced the public in this situation? After all the rule is unreasonable and is violation of the public lands act. Are you telling me that if the park management doesn’t have the foresight to make reasonable and legal rules that encompass common situations that you wouldn’t exercise common sense, instead you would enforce the letter of the law instead of using common sense. So you would inconvenience the taxpayer/public for an unreasonable law that is illegal? Rather than making an exception and contacting your supervisor and suggesting revising the rules to make them more reasonable; or would you prefer to enforce the letter of part of the law? Do you abide by the letter of the law yourself or are you a hypocrite? You don’t think the park and enforcement should follow the laws themselves and exercise good judgment?

Do you know that in many towns and states that it is technically illegal to be out on the road at night without an OIL lamp, because they haven’t bothered to remove the old laws from the books? So do you think it would be reasonable nowadays for enforcers cite everyone that didn’t have an oil lamp while they are on the road? So you think that where the laws forbid firing firearms in city limits; that every time an officer or a member of the public fires in self-defense they should be cited for discharging a firearm inside of city limits? I think that the officers and the public have the right to defend themselves, and that citing an individual for discharging their firearms inside of city limits for defending themselves would be unreasonable. I would consider it a constitutional violation. If you were in enforcement and you were ordered to blockade desegregation in violation of the Constitution, would you follow your unlawful orders? ( would you side with me and the Constitution, or the likes of Democratic leaders of Arkansas Governor Orval Faubus and Alabama Governor George Wallace)

I’m sorry but if you’re a letter of the law enforcement type that doesn’t care about the intent of the law and doesn’t think that the government and enforcement should follow the laws themselves, then I too would like to see you in court.

Outlaw162,

Your physics prof (and mine too) will tell you that the acceleration of gravity at sea level is 32 point something ft/sec^2, agreed. That is sometimes given the name "g" as one gravitational unit. Drop something in a vacuum and it will accelerate at 1g down. That is an entirely different usage of the term "g" compared to the "g" on your g meter in an airplane cockpit. Your cockpit g meter is biased so it reads 1.0 in level erect flight. So I'll say again, a cockpit g meter does not tell you acceleration, it tells you load factor, which is total lift divided by gross weight. A more simplistic case is sitting on the ramp, not moving. What does your g meter read? 1.0. What is your acceleration? 0.0.

Here is an equation relating acceleration and g meter reading:

accel (z axis) = g meter - cosine(pitch angle)

This equation is essentially correct, but there is a small error due to AoA.

I don't understand your last paragraph.

John

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Aircraft G Limits