SEMPER VIPER!

Editor’s Note: Those who have been associated with the F-16’s development will know that “Viper” was one name being proposed for the world’s first nine-g fighter. Although it is now officially called the Fighting Falcon, this high-performance airplane will always (semper) be affectionately known as the Viper to many.

 The F-16 is now in the field in real numbers and it is high time that we set to rest some of the rumors concerning just what the F-16 can and can not do. To fully understand the F-16, the Dash One flight manual is the first place to start; but after you’ve thoroughly digested its contents, reading these articles will help fill in some of the blank spaces. It is my intent to try to help clear up some gray areas and point out some things you might not have noticed in the time you have been flying the Fighting Falcon.

If you were to sum up the F-16 in a single word, you would find it nearly impossible to do so. I keep coming up with four words, all beginning with D, that the F-16 is or is not. The F-16 is not difficult, it is not devious, and it is certainly not dangerous. The F-16 is simply DIFFERENT!

If you are a student of aviation history (or perhaps a really crusty old lieutenant colonel), you will recall that there was a lot of nervous hand wringing going on in the late 1940s and early 1950s as jet-propelled aircraft began replacing the props. The feeling was that the tremendous differences between jet and reciprocating engines would be difficult for the pilots to overcome. Well, these "differences" proved to be minor.

Once you've digested what's gone into the F-16, you will see that there's a larger delta between it and the aircraft that immediately preceded it (the F-15, F-14, F-4, F-8, MiG-21, Mirage III, etc.) than ever existed between the jet-powered F-80, F-84, F9F and the prop-powered P-51, P-47, and F4U. Once you can insert this difference in your brain's core memory, you'll begin to see why the F-16 does the things it does so well, and why it does NOT do some of the things you may have been asking it to do.

In this article, I will discuss the F-16 flight control system. Future articles will feature aerodynamics, the cockpit, and engine characteristics. When we're through, you should have a better understanding of and appreciation for this "mysterious" electric airplane.

The flight control system is from whence came the moniker Electric Jet. The flight control system in the F-16 is different (that word again) from anything you've ever flown in an operational fighter. That is worth repeating. "The flight control system in the F-16 is different from anything you've ever flown in an operational fighter.

How different? In the past, any time you moved the stick (or, God forbid, the yoke) you got a corresponding movement of the flight control surface (as long as you didn't exceed the hinge-moment limits of the system). Then, depending on airspeed, center of gravity, or configuration, you got a varying response.

This is NOT the case with the F-16. You only THINK it's you moving the control surface. In fact, the computer positions the control surface to give you the roll rate or g it knows you want, depending on how hard you leaned on the stick. For example, two pounds of pull or push force might move the stabilators two-tenths of an inch at 600 knots air speed. That exact same two pounds of force might move the surface six inches at 180 knots. And in some cases, two pounds of force will drive the surface all the way to the stop! At either airspeed, the two pounds of force gives you an incremental 0.6 g.

The only times you have direct control of the surface are (1) when the weight-on-wheels, or WOW, switch is closed (i.e., you are sitting on the ground), (2) when you have the manual pitch override, or MPO, switch on and push in the nose-down direction, or (3) when the MPO switch is on, the angle of attack is above twenty-nine degrees, and you pull in the nose-up direction. As a result of the computer determining both magnitude AND direction of surface movement, the F-16 gives you a nearly constant response from a constant input across the entire flight envelope. This is only one of the results you'll see with the electronic rate (g) command system.

Since the black box is really flying the F-16, we can instruct it not to exceed a given g, a given angle of attack, or a given roll rate. And it will do this with very few exceptions. These exceptions are why you may have heard some of the horror stories about the F-16. But I'm getting ahead of myself. I've heard the usual reactions: “I don't want any g limiter on my airplane! If I want to pull 10 or even 12 g, I don't want to be limited to ONLY 9 g!” Now, stop and think a minute. While there may be some instances where this is true, they are very rare.

Tell me one other jet in which you're even ALLOWED to routinely attempt 9 g. I'll wait while you think. And while you're thinking, consider this scenario: We're both going straight down at 400 knots. You pull out using 10 g, but I pull only 9 g. You'll be recovered to level flight about 160 feet sooner than I will. Since there are few (if any) of us humans who are blessed with senses keen enough to allow them to delay that additional 160 feet before they start to pull out (about two tenths of a second) the difference between 9 and 10 g’s quickly becomes academic.

There's also energy bleed rate to consider. As you continue to increase the angle of attack in search of more g, the increased drag sometimes results in an increased airspeed bleed rate, such that the average g for any given amount of time turns out to be less than if you started with (and maintained) slightly less g at the beginning. If you get deeply into the engineering involved, you'll find that the present g limit is as close to optimum as you're going to get with today's flight control system technology and F-16 aerodynamics. What this limiter ensures, then, is max command in, max performance out.

With the F-16's g limiter, you can snatch symmetrically on the controls without fear of ever overstressing the aircraft. As a result, some of the initial moves you've seen the aircraft perform are astounding. In an amazingly short time, you can have more g than the other guy is even allowed. And then you can add another g or two over the next few seconds. If someone tries to match the resultant pitch rate in any other aircraft, they’ll only succeed in destroying the airplane they’re flying. In addition, the angle-of-attack limiter portion of the electronic flight control system will not allow you to pull the aircraft to an angle of attack where you can get in trouble (more on that later). External stores, however, are no different than you're used to. You must still pay as much attention to the Dash-One and Dash-Thirty-four as you have in the past.  [Ed.: Dash-One is F-16 flight manual; Dash-Thirty-four is the weapons manual.]

Since fighters have historically done some funny things at elevated angles of attack and elevated roll rates, we can also instruct the computer to limit the available roll rate in certain portions of the envelope. The result is an F-16 that achieves 324 degrees per second maximum roll-rate command within the first ninety degrees—and this is then cut back as the angle of attack goes up or the configuration changes (e.g., the CAT Ill switch). [Ed.: CAT III is a position on the stores configuration switch that is generally used when flying with specified weapons and external full tanks.] There's a series of flight parameters that the flight control system looks at in determining just what roll rate it's going to allow, but I don't have time to mention all of them here. (Take my word for it — trying to go completely through the flight control circuit diagrams in their entirety is enough to give anyone religion.)

Now, before someone comes up with what I've heard before (i.e., "My T-38 will roll 720 degrees per second”), let's set a few things straight. First of all, that is a bogus number. The T-38’s actual maximum roll rate is barely half that value — and even then it only occurs during the third consecutive, full-deflection roll under optimum conditions, and is entirely too fast to be of any operational use.

Instead, consider this: the F-16 is as fast to ninety degrees of bank as just about anything you'll run across; and although there are areas of the envelope where the computer limits the F-16 to less than 100 degrees per second, you still have nearly twice the roll rate available, under similar conditions, as any adversary you may meet.

From the obvious amazement of every adversary who views the F-16 across the circle for the first time, it's obvious that this g and AOA "limit" is not a player in any engagement you'll come across in the foreseeable future. But before we go on to other considerations, there are some other points about this "different" flight control system I should make.

Recall that it's a rate command system and not a displacement system, like you've been accustomed to flying. With a displacement system, we've all become accustomed to the different response rates we get as the airspeed changes. Therefore, we've become a nation of samplers — constantly reassuring ourselves that we still have control over the aircraft we're flying. We do this on almost a subliminal level, and are not aware of this habit unless someone points it out.

As the airspeed decreases on final, we tend to sample to ensure that we'll have enough control left to complete the landing. As we try to fly really close formation, we tend to sample to ensure in our minds that we've got enough control not to hit lead. And, because all displacement systems have a small, so called dead band that we must go through, we again tend to sample to continuously reacquaint ourselves with just where the dead band ends and control begins.

With some airplanes, we tend to keep the stick moving in an attempt to reduce the breakout friction to a manageable level. You may not be aware of this sampling phenomenon, but we all do it to some degree.

Now, enter the F-16-with a rate command system that supplies good, constant pitch and roll response so long as the aircraft is physically capable of flying. Further, there is little or no dead band associated with the F-16. Also, since we are not actually moving anything mechanical, there is no friction to consider. So the moral of the story is this: If you don't want the F-16 to move, don't move the stick!

This is NOT to say that the F-16 is too sensitive. Quite the contrary, it is simply a tremendously RESPONSIVE airplane. Resist the temptation to sample or you'll get response in spades. Although this seems like a simple request, old habits die hard. So pay attention to how you're flying the F-16. Become aware of just how you're manipulating the stick and your impression of how hard or easy the F-16 is to fly will improve. Remember, if a correction is necessary, don’t be afraid to move the stick. But if you don’t want the F-16 to move, don’t move the controls (i.e., don’t sample).

It is also very important to realize that this rate command system works both ways. That is, if you move the stick, you get a response. But conversely, if the airplane moves and you haven’t asked it to, the flight control system will try to damp that motion without any help from you. This system is not too different from some of the stability augmentation systems you've seen in previous airplanes, with the exception that this one has more authority than you've ever experienced before.

THIS black box is occupied by an 800-pound gorilla, not by some of the squirrels you have experienced on earlier airplanes. Guys who really know control systems will recognize bits and pieces of this rate command system in other airplanes. The F-111 (General Dynamics’ other airplane) had some aspects of a rate system creeping into the picture. The F-15 and certainly the F-18 share a lot of this design philosophy. However, none go to the lengths the F-16 does in controlling the basic airplane.

The F-16's very good ride at high airspeed and low altitude is one result of this self-checking feature. As soon as the F-16 is disturbed by any type of turbulence, the flight control system has a correction in, almost before you can think about it.

This self-correcting feature is why you see the horizontal stabilizer moving around so much when the airplane is taxiing. The flight control system is not getting any input from you, but is feeling the aircraft move as you taxi across all the bumps on the taxi route. So what you see is the flight control system trying to smooth out the taxiway. This is also why you don’t have to put in any check command to stop the roll rate as you try to do any number of precision point rolls.

One minor drawback of the self-checking feature shows up in what has been described as roll ratcheting. You will recall earlier that I talked about how different the flight control system is, compared to what you’ve been using. The ability to do smooth rolls requires some concentration on your part until you become completely familiar with this different airplane. What’s happening is that you’re putting in some amount of roll command. Since the roll acceleration of the F-16 is so good, you make the subconscious decision that if you’re rolling this fast and this quick then in a couple of seconds you will be rolling at nine million rpm.

The natural tendency is to want to slow the roll rate. With a conventional flight control system, we simply decrease the amount of stick deflection. In order to accomplish this, we relax pressure on the stick and allow the self-centering forces to move the stick closer to center (i.e., less aileron deflection), thus slowing the roll rate to what we want; then apply sufficient pressure to keep the stick at the new position. This relaxing of the pressure will normally go to zero momentarily, and with the F-16 this is sufficient for the self-checking feature to stop the roll rate completely. (Remember — you don’t have direct control over the amount or direction of the control surface deflection.) The roll rate deceleration is ALSO rapid, so your body and hand tend to couple with the aircraft motion and probably make stick inputs that weren’t intended.

The end result is some pretty sloppy rolls until we get used to the system. What you need to do is (1) learn to adjust the roll rate with subtle pressure changes to the stick and (2) get away from the stick position cues you’ve been used to using. Once you can get yourself tuned to using finite pressure changes to control the roll rate, you’ll be able to make smooth roll inputs. This is so despite a force-per-roll-rate slope that isn’t constant. There are two distinct changes in the slope of the curve. This is to make sure that the airplane isn’t too sensitive for small inputs, and that the force required for max inputs is not too high. Those devilish engineers also used two different roll time constants for small and large roll inputs. All this is nice to know, but if you simply pay attention to the amount of force you’re using on the stick, you’ll be able to do very nice rolls with the F-16.

By now I’m sure all of you are asking why it’s necessary to use such a markedly different flight control system. Well this self-checking feature is really one of the main reasons this flight control system is in the F-16. It allows an aircraft design that uses new and different aerodynamic principles.

Joe Bill Dryden was a graduate of the US Air Force Academy, Class of '62. After retiring from the Air Force, he became an experimental test pilot for General Dynamics. His experience with the F-16 began when he was selected to represent Tactical Air Command as one of the original members of the YF-16/YF-17 Lightweight Fighter Joint Test Team. He subsequently flew and helped develop various combinations of F-16 airframe, engine, wing, flight control system, and avionics systems. Joe Bill died on 24 May 1993 when his F-16 crashed in North Central Texas during a company acceptance flight.