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When you start looking at the F-16 from the aerodynamic standpoint, one particular fact immediately stands out: This is the first operational aircraft intentionally designed to have a negative static margin. In subsonic flight, the F-16 is negatively stable (read, unstable) in pitch. I doubt if any of you have had the opportunity to fly an aircraft characterized by true negative stability. But if you did, you’d find that, with a conventional displacement flight control system, you’d spend at least ninety-nine percent of your time just trying to keep the sharp end pointed into the wind. The closest any of you may have come is in the F-4 with three bags and two travel pods, just as you come off the tanker with a full load of gas. In that configuration, if you pull the F-4 to about fourteen units, you’ll find that it wants to keep right on going and that you have to maintain forward pressure to keep the angle of attack, or AOA, from increasing right through the departure boundary. Subsonic, the F-16 is constantly trying to do the same thing, but because the flight control system is constantly monitoring g, AOA, and pitch rate (and comparing these values to what you’re asking for), you’ll never see the same results.

Why design the aircraft in this manner? Because you get several performance benefits from doing so. This negative static margin provides one reason the F-16 turns as well as it does. What you may recall from Aero 101 no longer applies when you try to evaluate the F-16. I’ve seen articles in Air Progress that show how the F-15 will turn so much better than the F-16 because the F-15’s wing loading is lower. But this is where people get in trouble, because you can no longer apply wing loading to come up with a prediction as to how the airplane will turn.

Let me explain this. Since the F-16 is negatively stable, the tail is lifting in order to control the AOA (while you’re subsonic). And while the center of pressure shift is such that the F-16 is positively stable when you’re supersonic, the amount of down force necessary to keep the aircraft trimmed to a given AOA is less than conventional fighters. As a result, the total lift acting on the airplane is more for a given AOA; therefore, the resultant induced or trim drag is reduced. Less drag of any kind means better sustained turn and cruise performance. Also, the F-16 has been designed to take advantage of the vortex lift generated by the strakes. This vortex is what you see trailing back on both sides of the F-16 when you turn it hard in moist conditions. They are not there just for more oooohhs and aaaahhs at air shows.

As a result of this vortex lift, there are areas in the flight envelope where as much as thirty percent of total lift is coming off the fuselage. If you fall into the same trap that Air Progress did and take the gross weight of the aircraft divided by the projected wing area, you’ll come up with a wing loading of about sixty-five pounds per square foot. But (and this is a very big but), when you add in all the contributions of both tail and fuselage lift, you’ll come up with a wing loading of about forty pounds per square foot. Now you’re talking late World War II wing loadings. Can you now understand why you keep hearing, "I never thought you’d be able to make that corner!" Heard that in some of your debriefings? Ah haaaa! Then maybe there is some method in their madness.

So it’s really a combination of these two things that gives the F-16 the different characteristics we have to account for when beginning to fly this multirole fighter. The negatively stable aero and the rate command flight control system both go to make up a fighter that’ll perform like no other!

I mentioned, in passing, some less than desirable features that resulted from new approaches to building a fighter like the F-16. Forewarned is forearmed, so follow me through this discussion of how not to be surprised by the Fighting Falcon.

I’ve heard folks complaining about the lack of cues when flying the F-16. This is true and not true at the same time. The cues are there - it’s just that they’re so suppressed in magnitude (when compared to aircraft you’ve been flying) that they’re often overlooked until you get some experience in the airplane. Without regard to any special order, let’s look at some of the more common ones.

You will have noticed that there’s a decided difference in the amount of time you spend trimming the F-16. This is primarily the result of the flight control system. Since we’re using the flight control system to artificially create a neutrally stable aircraft, trim changes are taken care of automatically as we increase or decrease airspeed. Since the need to re-trim the airplane is removed, we can no longer use this cue to tell ourselves on a subconscious level that we’ve changed airspeed.

Also, the lack of a canopy bow has removed one of the larger sources of wind noise in the cockpit, so we can no longer depend on louder background noise to tell us we’re going faster. The increase in wind noise is really still there; it’s just that the initial noise starts at such a low level (compared to such aircraft as the Rhino) that you really have to be paying attention in order to use this signal as a cue, which is hard to do until you get used to the new feel of the F-16. Both these characteristics are the reason that you find yourself going 450 knots calibrated air speed, or KCAS, when you wanted 250 KCAS, and vice versa. I regret that I have no real clue for you here other than to use the HUD and really listen to the airplane to show you just how fast you’re really going. Rest assured that I don’t want to go back to a positively stable airplane with a canopy bow to recover these cues.

I do, however, have some very important facts about what the AOA is, so pay attention. Most of you transitioning out of the F-4 are familiar with the fact that the F-4 would bludgeon you over the head with buffet levels to tell you, in no uncertain terms, that you were increasing the AOA. The buffet cues are still present in the F-16, but the magnitude is probably one-tenth that of the F-4. What happens is this: You’re cruising along at about one or two degrees AOA and you start to turn the aircraft. The first thing you hear or feel is a small increase in the background aerodynamic noise (this usually begins at about six degrees AOA). What you’re most likely hearing is the vortex that’s beginning to be shed by the forebody strakes. This noise increases slowly until you reach fifteen to sixteen degrees AOA where you begin to get flow separation off the main wing. The resultant turbulent airflow impinging on the rest of the F-16 gives you what has always been described as buffet.

This onset of airflow separation follows the rule we are already familiar with, in that it starts at about fifteen degrees at sea level and decreases (as altitude increases) until we see the onset of buffet at about nine to ten degrees alpha at 40,000 feet. The reason for this decrease in AOA for the same airframe reaction has not changed since Icarus, and is unimportant here. The main thing to remember is this: If the F-16 is buffeting (regardless of the power setting), you are slowing down. If you don’t get slow, the F-16 will not depart. If you have enough airspeed, the flight control system will not allow you to do anything that will progress to a departure. It is only when you begin to get below 200 to 250 knots (depending on the configuration) that the F-16 becomes susceptible to departure. But even then, you must still force it to depart. A very important lesson is to be learned here: that is, pay attention to the buffet level. If you don’t want the F-16 to get slow, don’t fly into buffet. The F-16 will fly well beyond buffet very nicely (remember the unstable aero and fuselage lift) so, if necessary, don’t be afraid to do so. Just remember to use the buffet as an important information cue, and you’ll not become famous (or infamous) with your superiors.

As long as the g is low, you can fly the F-16 well at twenty to twenty-five degrees AOA. Just don’t ever forget that, to fly at these angles of attack, you’re getting slow. The flight controls don’t have enough authority at these airspeeds to overcome the bad parts of this unstable aero we’ve been using. If we make rapid pitch or roll inputs at low airspeed, the flight control system will try to honor our request. But then it quickly realizes that there’s not enough energy in the air flowing around the control surfaces to stop the inertia it just started. The unstable aero now gets the upper hand, and the F-16 keeps right on going. If you’ve been paying attention and know you’re slow, you’re still not in any trouble.
Simply put this knowledge to good use and smoothly approach the limits built into the flight control system. If you can do this in the heat of battle, the flight control system can handle the more benign rates that occur and will still keep you out of trouble. (Never fear. Even these benign rates are faster than the guy you’re fighting will be able to generate.)

In addition to using the buffet level as an airspeed clue, another area bears watching - flying the aircraft vertical, or even near vertical. Don’t be led down the primrose path when you read stories about airplanes that supposedly have power-to-weight ratios greater than one-to-one. Writers often don’t have a complete grasp of all the real-world physics involved. For example, despite the stories you may have heard or read, the F-16 will not accelerate straight up for very long. And neither will any other aircraft, for that matter. Although the F100 engine is in the 25,000-pound thrust class, it has never seen 25,000 pounds of thrust in its life. When you (1) deduct installation losses, (2) realize the engine is probably not in perfect trim, and (3) account for the usual thrust-level deterioration from age, you’ll see you don’t have a great big area where the F-16 is really greater than one-to-one. Then, when you superimpose a nominal three percent per thousand feet lapse rate on the remaining thrust, you see that, when you get to 10,000 feet, you only have seventy percent left.

Now remember that, in a vertical climb, thrust must overcome weight and drag. You can see, can’t you, that you need a lot more thrust than you have on hand? This is not an indictment of the F-16. What you’ve just read is true of any fighter aircraft flying in the world today. In a relative sense, the F-16 is still head and shoulders above anybody you’re going to run across in the next few years. A-n-y-b-o-d-y. Again, the real point is in not getting slow. While the F-16 will go straight up farther than anybody you’re going to run across in the near future, you’re still going to be slowing down when the F-16 is pointed straight up. And whether you get slow through pulling a lot of g or through going vertical, you’re now susceptible to a departure if you insist on forcing the issue. Pay attention and you’ll know what energy state you’re in at all times. If you’re in a low energy state (low airspeed), then approach the limiter smoothly and you’ll never get in trouble.

The minimum airspeed limits given in the handbook are a very good place to start for maximum maneuver limits. Below these limits, we have to use a little skill and cunning. If you inadvertently find yourself slower than the limits set forth in the Dash One, all is not lost. Simply keep your wits about you. Be sm-o-o-o-o-th with your control inputs, and you’re home free.

I know it has happened before (and it will happen again) that someone will not be as smooth as they should be in a low airspeed situation, and a departure is going to happen. Let’s talk about what happens in a departure so you’ll be able to (1) recognize one if it occurs and (2) recover from a deep stall if the departure progresses to that point.

Just what is a departure? First of all, it’s quite different (Heard that before?) from the aircraft you’ve been flying. You’re out of luck if you’re looking for the cues you used in flying the F-4 or A-7. If you’re looking for nose slice to tell you that you’re about to depart, it’s way too late. Why? Because the F-16 usually does not depart directionally (nose slice) but longitudinally (in pitch). By the time you see any left or right nose motion, you’re already well into a departure. What has happened is that you’ve been turning the F-16 hard enough to slow it down or going straight up in an effort to out-zoom the other guy. For whatever reason, you’re slow.

Now suppose you insist on continuing to turn the F-16 hard by snatching on back stick or couple it in pitch by pulling on the pole with a simultaneous rapid roll. What you’ve done is to play right into the hands of all the bad parts of the negatively stable aerodynamics I already noted. You’ve rated or coupled the F-16 into an AOA range where it wants to keep right on going. And at the same time, the slow airspeed means the stabilator doesn’t have sufficient authority to keep the AOA under control. So the F-16 departs. This is never a violent departure. Remember, I told you the F-16 will not depart unless you get slow. If you’re going fast enough to give the airplane enough energy to provide a violent departure, then you’re also going fast enough to give the control system enough authority to prevent that departure - the one you seem so determined to effect. (So perhaps there is a valid reason for the F-16’s flight control system/aero combination.) As a result, departures are very benign. Sometimes, you’ll not even be aware that you’ve departed.

About those previously mentioned exceptions. Most configurations will depart only if you let the F-16 get slow. Nose slices usually do not occur in the F-16; however, as happens in other parts of life, there are no absolutes. Nose slices may occur with a 300-gallon centerline tank, Cat I configuration, under certain circumstances above 35,000 feet, or above about 25,000 feet with wing stores or suspension equipment. These nose slices occur only in the high subsonic speed range (that is, above 0.88 to 0.9 Mach with a moderate KCAS - something well below 300 KCAS) when making roll inputs on or near the AOA limiter. If you ever experience one of these nose slices, your first reaction should be to release the stick and let the jet fly itself out. If the aggravating stick commands are maintained, the nose slice may transition into the more traditional upright pitch departure. This pitch departure will be more dynamic than the low-speed case. The excess energy will quickly bleed off, however, and the two departures will then be very similar.

Back to the discussion about pitch departures. What I’ve described is an AOA excursion to something beyond twenty-five degrees. The control system is trying to maintain a twenty-five degree AOA maximum at one g and as little as fifteen degrees at nine g’s. There is a definite reason for this difference, but it isn’t important here. Even though the flight control system is trying all the time, you can force the F-16 beyond this design AOA limit, either through coupling beyond this in roll by rating it through this limit with an abrupt pull at low airspeed or by aggravating a nose slice. What happens now? Usually nothing of which you’re aware. Once the control system sees an AOA beyond twenty-five degrees, it tries to reduce it to below twenty-five regardless of what you do to the control stick. But if you somehow manage to get the AOA above twenty-nine degrees, then the system, while trying to reduce the AOA to within limits, will also negate any yaw rate. Usually, you’re out of the control loop for a few seconds while the black box lowers the AOA and then gives it back to you. You probably never even knew it.

Why get so excited? Because, occasionally, the F-16 can trim itself into what has been described as a deep stall. If you get yourself into one of these, you do have some problems. Unfortunately, you’ll have to wait until the next thrilling episode to find out just what a deep stall really is and how to get the airplane flying again if you manage to get yourself into one.