Code One Magazine: Report From The Future

F-22 I have flown the future.

Last fall I began flying an aircraft designed for the twenty-first century, the Lockheed/Boeing/ General Dynamics Advanced Tactical Fighter. The YF-22 was supposed to have low observability (stealth), be able to cruise supersonically without afterburner, and maneuver with unequaled agility. It has, it is, and it does.

The two YF-22s are powered by new engines, General Electric YF120s in one and Pratt & Whitney YF119s in the other. The YF-22s let us demonstrate both our ability to predict aircraft performance and handling qualities, as well as let us work through the initial agonies of many technologies to reduce the risk of the F-22 program.

We did that in ninety days. This was indeed a remarkable demonstration/validation flight test program, since most test programs require the first several months just to work out the bugs and to get the machines to function as designed. As it happens, dem/val is not the only remarkable thing about the ATF.

The cockpit or "office" has no round dials, gauges, or engine instruments. There are five liquid-crystal displays and a small multifunction display for the flight test boom displays and, of course, the head-up display. There are very few switches in the cockpit, although the number grew slightly because some computers were a bit too smart. (Remember HAL from 2001? He's still alive.) A few extra reset switches helped these computers get their minds straight. Prior to the test program, I was very skeptical about the cockpit, because my experience on other programs was that it took more than three months to get less-advanced avionics to even turn on repeatedly. I am a believer now. The throttles are similar to F-15/F-18, and the aircraft has a sidestick with slightly larger motions than an F-16, which worked very well.

We did the flight control system development and simulation here in Fort Worth, spending what seemed like an obscene amount of time in the simulator doing both development work and hardware checkout. It turned out to be time well spent.

The handling qualities and the performance of our digital flight control computers were excellent. We initially flew with a twenty-degree angle-of-attack limiter because - let's face it - we're a conservative bunch of guys. Later we flew at much higher angles of attack. Thrust vectoring was designed into the aircraft from the outset. Although the first flights were flown without thrust vectoring, we checked it out very early and used it throughout the remainder of the program. From the very first flight by Dave Ferguson on 29 September to the last flight on 28 December 1990, we made only two small changes to the flight control system.

The YF-22 is one of the easiest aircraft to fly in the landing pattern of any airplane any of us have ever flown. Roll rate is very responsive and pitch control is very precise. Throughout the program, we attempted increasingly aggressive handling qualities tasks in the landing pattern to ferret out any problems. The airplane was always very predictable and easy to land precisely. Turbulence and crosswinds posed no problems; we were able easily to clear a twenty-knot crosswind limit within about a month of first flight. We had two unscheduled opportunities to fly single-engine landings: Dave Ferguson on the third flight, and me on my first flight. They were as easy as a normal landing, with no special compensation, thanks to the flight control folks and to many an agonizing hour "crashing" the simulator doing these same tasks.

With the landing gear up, the aircraft has many F-16-like qualities — precise nose control, responsive roll control, and impressive roll rates. One of the two small deficiencies we discovered early on was a roll "twitch" at higher subsonic speeds. Basically, we were asking the aircraft to start rolling too fast at these speeds. By smoothing the roll input, we could compensate and minimize this effect. But it was still irritating.

This discovery was most noticeable during air refueling when the YF-22 would roll-twitch occasionally with rapid corrections. A modest change to a roll stick filter solved the problem and eliminated all the irritation and complaints. The only other change in the flight control system was a reduction in flap setting during air refueling. Incidentally, a nice feature about our advanced cockpit was an ability to evaluate three flap settings with two roll stick filters on the same flight. This made it easy to make side-by-side comparisons and to pick the best configuration.

The F-15 STOL demo is the only other fighter I know of that has thrust vectoring as part of its flight controls. This modified F-15 went through an initial flight test program before thrust vectoring was implemented with a proven engine. While several other programs (F-18 HARV, X-29, and X-31) are planning to use thrust vectoring on proven engines, we successfully integrated thrust vectoring into two new engine/nozzles and had the confidence to flight-test them shortly after first flight of a new aircraft design. In our cockpit, there are no "side effects" when thrust vectoring is on. Throttle transients, altitude changes, afterburner lights, etc., have no effect. The only indications occur at slow speed where the already excellent pitch response becomes even more precise, and the flight controls allow faster roll rates than with vectoring off. These are the kinds of indications that we found easy to accept.

I'm a little reluctant to talk about agility, because there are a variety of rock-hard opinions about just what that means in a fighter aircraft. They range from formal-symposium, video-aided extravaganzas to stag bar hand motions that sometimes result in the simulated destruction of a wrist watch. Actually, hand motions come closer to describing agility. It's simply the ability to put the airplane any place that I'd like to, whenever I'd like to, and have the aircraft never do anything that I didn't ask it to. (Very few pilots have severe wrist buffet, wing rock, or lose control of their wrists while talking with their hands.)

So that's the definition I'll use here. Agility was one of the things we hoped to achieve with the YF-22. Slightly more than two months after first flight, Mark Shackelford and I were able to evaluate the high-angle-of-attack behavior of the YF-22. The angle-of-attack limiter was "turned off' and we proceeded to methodically evaluate the aircraft's characteristics. We evaluated not only pitch characteristics (the favorite barroom wrist maneuver) but also roll capabilities (a more difficult barroom maneuver because of the lack of a ready velocity vector reference and a free-swiveling wrist). To point the aircraft wherever you want, you have to be able to do it in more than one axis, e.g., point the nose in pitch and then change heading while not sacrificing nose position (by unloading, rolling, then loading up again). Now that's an impressive capability.

Test maneuvers were all conducted from one-g slowdowns. Minimum speed during the program was approximately eighty knots, and the maximum angle of attack was sixty degrees. The pitch control was very precise and positive nose positioning could be done at all angles of attack up to sixty degrees. Light buffet, no wing rock, and positive and correct response to all pitch and roll inputs made the YF-22 an absolute joy to fly at these angles of attack. It was very impressive to be able to stabilize on these points for long periods of time, get slightly off the point during a pitch maneuver, and easily correct back to the exact condition.

Rolling maneuvers are often neglected in discussions about agility because they become hard to understand at higher angles of attack and they become harder for the engineers to make happen, even with their magic. I consider the F-16 to be the current standard for roll capability at high angle of attack. As a comparison, the YF-22 roll rate is equal to, if not slightly faster than, the F-16 roll rate, within the F-16 angle of attack envelope.

The important advance is that the roll capability continues to sixty degrees angle of attack. As angle of attack increases, the maximum rate decreases, but the roll capability at sixty degrees still allows the aircraft to change direction very well. All rolling maneuvers can be done 'feet on the floor," with none of the dancing on the rudder pedals required in some other fighters. The aircraft rolls around the velocity vector. Most fighters have reasonable rolling characteristics up to twenty degrees angle of attack, so this seems quite natural. The rolls from twenty- to forty-degree angles of attack begin to take on a "barrel roll" appearance. Above forty degrees, the yaw part of the roll begins to predominate the pilot's impression and rolls become virtually heading changes. At sixty degrees, they appear very much like pure yaw (video certainly makes them look like a spin). At sixty degrees angle of attack, a thirty-degree bank angle change results in a heading change of approximately ninety degrees. It's important to remember that most of these maneuvers were accomplished at speeds closer to the average speed on a California freeway than takeoff speed for the average fighter.

So you begin to understand that we were able to fly the aircraft to angles of attack beyond the capability of most aircraft. A few aircraft have reached these extreme angles of attack, but never with the precise pitch, yaw, and roll control we demonstrated. I am happy to report that the aircraft always did exactly what I wanted it to do. The high-angle-of-attack handling qualities are just outstanding. Our objective in this phase was to reach sixty-degree angle of attack and perform pitch and roll maneuvers. We did this in one week.

We hope to continue this testing to higher angles of attack and many different maneuvers. In the maneuvers flown, I believe it is superior to any fighter ever built. With further testing, we could prove that it is the most agile aircraft ever flown, by anybody's definition of that term.

Because the YF-22 is a full-spectrum aircraft, it is agile at both low and supersonic speeds. Since a great deal of the fighter's flight time will be supersonic, however, we especially felt that it had to handle very well at these speeds. At 1.5 Mach, the YF-22 feels as crisp and responsive as an F-16 at 0.8 Mach, and we were able to demonstrate supersonic roll rate and instantaneous turn capabilities. And we were able to demonstrate our predicted super-cruise performance.

One last important thing. Internal weapons carriage is fundamental to the YF-22 design. We felt that it was important to demonstrate actual missile firings of both the Sidewinder and AMRAAM missiles. I had the opportunity to fire the Sidewinder and was very pleased as I watched the missile leave from the aircraft. (There had been many what-if warnings, which increased exponentially as we got closer to the mission.) Tom Morgenfeld fired the AMRAAM and reported that it also went flawlessly. I believe this is the first AMRAAM firing from an internal carriage on any aircraft.

That's about all I can say now. There's a lot of other good stuff I may be able to go into later, but I do want to mention just one more important thing: I am immensely proud to have shared perhaps the most aggressive flight test program in history with the hardest-working and brightest people I know. Because of them, I got to fly the future.