This article appeared in the October 2000 issue of Code One Magazine.
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Those few pilots favored with the opportunity to fly the F-22 Raptor quickly become comfortable with the aircrafts unique characteristics. They talk casually about aircraft handling qualities at airspeeds and angles of attack that would make any other fighter spin out of controlled flight. In the same conversation, they describe outstanding handling qualities at supersonic speeds and regularly running chase aircraft out of gas. The F-22 is an impressive aircraft at both ends of the flight envelope. In this first of a series of articles of pilot impressions on the F-22, Paul Metz takes the high speed end of the F-22 flight envelope and Jon Beesley takes the low speed end.
Supercruising the Raptor (cont'd.)
By Paul Metz, F-22 Chief Test Pilot
Paul Metz, chief test pilot for the F-22, piloted the Raptor on its maiden flight in September 1997. He continues to be actively involved in the flight test program for that aircraft. Before joining Lockheed Martin, Metz served in the US Air Force as engineering test pilot. He also served twelve years as chief test pilot for Northrop. To date, Metz has accumulated more than 7,000 hours of flight time in more than seventy types of aircraft, which include the F-86, F-105, F-4, F-5, F-15, F-20, YF-23, and the F-22.
Supercruise, or the ability to travel at high supersonic speeds without afterburner, is one trick in a bag of tricks offered by the F-22. But this trick happens to be exclusive to the Raptor. While fighter aircraft have been flying faster than sound since the Century series of fighters, they almost always required an afterburner for supersonic flight. Because afterburner severely reduced range, supersonic flight was exploited for only relatively short periods of time, for example to avoid ground fire or run down an adversary. Some of todays frontline fighters can maintain slight supersonic flight with non-afterburning thrust without a full weapons load and without external tanks, but the Raptor can sustain much higher speeds for much longer periods with a full load of weapons. This ability to supercruise gives the F-22 huge offensive and defensive tactical advantages. Supercruise performance demonstrated in flight tests with the Raptor is nothing short of eye watering.
The Offensive Advantage
The ability to move against an adversary at high speeds gives fighter pilots advantages they call first look, first shot, first kill. The first pilot to see an adversary is more likely to get off a successful shot and survive the encounter. The kinematic range of an AIM-120 AMRAAM, for example, increases by fifty percent as aircraft speed increases from 0.9 to 1.5 Mach (this assumes an altitude advantage for the shooter). That is, the missile can reach targets fifty percent farther away because its initial speed coming off an F-22 flying 1.5 Mach is much faster. The Raptor easily supercruises in this speed regime. This missile range advantage intensifies the F-22s sensor advantagethe radar on a Raptor can see a bandit long before a bandits radar detects a Raptor.
The first pilot to launch a missile at an adversary is more likely to survive the encounter. The offensive push of supercruise creates shot opportunities earlier and at longer range. The effect of supercruise to subsequently deny the enemys shot is less apparent. Cranking after the shot always reduces the enemys effective missile range, but a supercruise crank places the F-22 way outside an adversarys maximum range, even if it could detect the F-22. Supercruise also plays out in simply getting from point A to point B. Once the Raptor turns on an opponent, the Raptors speed places the intercept point farther from the start point. If the Raptor is protecting a strike force, the adversary fighters will be engaged and downed farther away from the strikers and long before an enemy missile can be launched against the friendlies.
The Defensive Advantage
Supercruise is an added boost to the overall power of the stealth of the F-22. Although not yet a cloaking device, stealth does delay the enemys shot opportunity until late in the engagement. Against a ground-to-air threat, high speed equates to reduced reaction times from detection to launch and reduced kinematic ranges for surface-to-air missiles or antiaircraft artillery.
I liken this advantage against ground threats to skeet shooting against a supersonic skeet, a challenging target made exceedingly challenging by the split second available for tracking, leading, and firing before it flies out of kinematic range. Supercruise has the same effect for the Raptor, and the same dynamics apply to the F-22 in an air-to-air fight. The Raptors higher cruise speed makes intercept more difficult and reduces enemy missile range more significantly. Higher speeds significantly shrink shot opportunities at the beam or front quarter of the F-22 because the missile winds up in a high-speed tail chase or it must make an energy-depleting hard turn because of the high line-of-sight rates.
The Raptors supercruise range is not only good but also sets it apart from previous fighters. The large internal fuel fraction (fuel weight divided by empty weight) allows the aircraft to sustain these high speeds for extended periods. The ingenuity and craftiness of future generations of fighter pilots will determine exactly how this capability will be used. But I can say that this airplane will perform outside the realm of current and projected fighters. Since speedrelates to distance, such things as mutual support and strike escort take on new meanings in terms of positioning and reacting to a threat.
The Raptor is always in a combat configuration, fully loaded and ready for war. The aircraft has no external stores, so drag remains low and Ps stays high. The specific excess power, or Ps, is a measure of the airplanes ability to accelerate or climb at its current flight condition. Wing aerodynamics and overall drag are at a minimum near the design speed of 1.5 Mach at 40,000 feet. This airframe is actually at its best at supersonic speeds, with the best time to climb right off the deck. Conventional fighters have their best time to climb using a Rutowski climb profile. That is, they start with a subsonic climb to the tropopause (about 36,000 feet) and then perform a pushover to supersonic speed and climb supersonically from there. The Raptor can dispense with this complex profile and blast off supersonic from the deck. This machine just likes to go fast.
Level acceleration in military power or less is sprightly at all altitudes but downright astounding in full afterburner. I wish I could state some acceleration times, but they remain classified. Approaching Mach in military power, the acceleration reduces slightly as drag rises, but the aircraft punches on through easily. Accelerating through Mach in military power in the Raptor feels similar to accelerating in full afterburner in an F-15. The Raptor accelerates in full afterburner in one continuous-speed feed. A slight buffet occurs between about Mach 0.97 and 1.08. After that point and to max speed, the aircraft accelerates smoothly and continuously. When testing, we like to get on supercruise conditions quickly so we can get the most out of the relatively small supersonic airspace we have. We use afterburner to boost the aircraft to the point and then pull it out of afterburner to cruise at our test condition. Much of the high-speed testing has been moved to the Pacific Test Ranges off the coast between Vandenburg AFB and NAS Point Mugu. This airspace gives us a longer straight line running room (about 180 nautical miles) and minimizes sonic booms for the locals.
Ive been talking about drag, but the real secret to supercruise is thrust minus drag. The big thrust comes from the incredible F119 engines. We sometimes forget about these beauties as they continue to perform trouble-free at all flight conditionsthe perfect engine for a fighter pilot. They tolerate any throttle motions and pilot demands from ridiculously low speeds to supersonic flight at altitudes above 50,000 feet. Although the F-22 uses a fixed inlet design, the overall engine and airframe are optimized for the high supersonic speeds. Acceleration and Ps are phenomenal at the right hand side of the flight envelope. The Raptor can easily exceed its design speed limits, particularly at low altitude. We have incorporated max speed cues and alerts to remind pilots when approaching the limits.
The best seat in the house for supercruise is from a chase F-16 or F-15. Remember, we fly both these chase jets with just a centerline fuel tank to give them a fighting chance to play with the Raptor. Still, the F-22 usually leaves these aerodynamically slick chase airplanes in the dust. The F100-110, -129, and -229-powered F-16s dont fall very far behind the Raptor in the initial acceleration through Mach. But the race is really no contest at the higher Mach numbers and once on cruise conditions. Nothing can sustain supersonic conditions with the persistence of a Raptor. Load those chase F-16s and F-15s with combat-representative stores and they would not stay with the Raptor during acceleration or sustained cruise.
Invariably, our test mission runs are dictated by the fuel state of the chase aircraft. A curt Bingo forces us to decelerate and take the chase to the tanker for more gas. The Raptor always has lots more supercruising fuel left. I would be a pretty upset taxpayer if this next-generation fighter didnt show clearly superior capabilities over anything flying today. While the Raptor is superior in many areas, the airplane is truly unsurpassed when supercruising.
The Raptor At High Angles Of Attack
By Jon Beesley, F-22 Test Pilot
Jon Beesley, a senior experimental test pilot and project pilot for the F-22, helped develop the handling qualities of the Raptor. His more recent flights have included high angle-of-attack testing.
Previously on the YF-22 program, Beesley performed control law development and flew high AOA tests as well as AIM-9 separation tests. Before joining Lockheed Martin, Beesley served as a primary USAF test pilot for the F-117 program. To date, he has accumulated more than 5,000 hours of flight time in more than forty types of aircraft, which include the F-4, F-117, F-16, YF-22, and the F-22.
Many aircraft define the beginning of high AOA flight at about thirty degrees. On the F-22, stable flight test points are routinely held stable at a positive sixty degrees AOA and a negative forty degrees AOA. Before the Raptor, only experimental or specialized aircraft, such as the thrust-vectoring X-31 or F-16MATV, could claim sixty degrees AOA. For the F-22, high-AOA flight is just one more trick in that bag of tricks Paul Metz talks about. As for the tactical advantages of high-AOA flight, Ill let some of our USAF test pilots explain that. For my part, Ill stick to how the airplane handles at these angles.
Buffeting, a common flight characteristic at higher AOA, begins around twenty degrees AOA in the Raptor and increases slightly up to twenty-six degrees. Buffet is a good cue, with an intensity that compares with the minimal buffet experienced on an F-16 at higher AOA (and much less commanding than the buffet on the F-15). The slight buffet remains constant from twenty-six degrees to about forty degrees AOA, where it decreases. At no AOA is buffeting a problem.
Lateral Characteristics And Control
As we began the flight test program, we were keenly aware that lateral directional stability of most fighter aircraft drops off sharply between twenty-five and thirty-five degrees AOA. Our initial tests of the F-22 showed sideslip excursions around thirty degrees AOA, which were greater than we desired and which indicated a lower stability than predicted. Although this instability was not great enough to stop the testing, we addressed it in the first software update. Through the magic of software and all the smart folks who know how to use it, the airplane now passes through this AOA range with no apparent change in handling qualities.
We also found that the rudder pedals offer the most intuitive lateral directional control at high AOA. Pilots came to similar conclusions in the F-16MATV program. Unlike the production F-16, the rudder pedals in the Raptor are active at all AOA and become the most effective way to guide the aircraft above forty degrees AOA. Rolling maneuvers at high AOA begin to look more like pure yaw inputs. We are exploring easy ways to improve lateral stick characteristics around forty degrees AOA.
Lateral directional control remains good above fifty degrees AOA, but the horizontal tails are working hard to provide that control, moving differentially like the webbed feet of a duck paddling through the water. (Differential tail is the primary yaw control device. This control technique was pioneered back on the YF-22 in 1990 and works very well.) Although these excessive differential tail movements make for great video, they indicate different aerodynamics than predicted. Actually, we have more control power than we originally thoughta nice problem to have. The first control law update tuned the flight control system to the real aerodynamics, so this characteristic no longer occurs. Furthermore, control hasnt suffered. Perhaps high AOA now begins above fifty degrees.
Since negative AOAs are also part of the flight envelope, we found ourselves doing test points down to a negative forty degrees AOA. Other than the oddity and somewhat lunch threatening effect associated with prolonged negative-g flight, these test points went essentially as predicted. We were getting excessive sideslip at negative thirty degrees AOA. The sideslip is now more tightly controlled in the later software updates to the flight control system.
The magic of the flight control system and the wizardry of the thrust vectoring supply ample pitch control in the Raptor. Pitch control is never in question when we move the nose around at high AOA. We have demonstrated pitch rates of greater than forty degrees per second and abrupt pull-ups at 35,000 feet. These Cobra-like maneuvers will be even more impressive at lower altitudes where we will have higher thrust available for thrust vectoring. To date, all of our high-AOA work has been performed at 30,000 feet and above.
We recently completed the flight tests to zero airspeed. (I suppose this makes zero the minimum airspeed.) The Raptor was very honest in all of these maneuvers, always doing the right thing. The nose can still be positioned down to airspeeds of about twenty knots. The aircraft motion in the zero airspeed maneuvers is predictable. The airplane will even recover right side up comfortably and somewhat gently if the stick is released in a vertical climb at zero airspeed.
Maneuvers at or near zero airspeed often involve rapid swings through the vertical as gravity and inertia take control of the airplane. In many modern aircraft, this motion is often the beginning of a pendulum swing to the other side of a vertical arc. In the Raptor, the nose can be stopped easily on the way down, with no dramatic tendency to swing through to the other side.
We have completed throttle snaps from military power to maximum afterburner and back to military power after every condition we have flown, including at zero airspeed. In addition to the normal throttle transients, we add a relatively rapid lateral stick cycling on top of the throttle transients to draw as much horsepower as we can from the engine through the hydraulic pumps. (These test points felt particularly odd going straight up at zero airspeed.) No matter what we have done so far, the engines just keep right on running. They have been flawless, which is important because we rely on them not only for thrust and pitch control through thrust vectoring but also for breathing air. (The onboard oxygen generating system runs off the environmental control system, which is powered by the engine. Without the engines, we breathe from an emergency oxygen bottle.)
Probably the oddest tests we perform at the F-22 CTF are the full-up departure-resistance tests. These tests begin with full stick rolls with full rudder pedal in the direction of roll while simultaneously stomping in full rudder pedal in the opposite direction. These inputs are held for fifteen seconds. The airplane is extremely departure-resistant. These long duration inputs seem to be the most aggravated a pilot might put in. The flight test team decided that, because the departure-resistance tests were going so well, we needed to add a full forward stick push or full aft stick pull into these maneuvers and hold them for fifteen seconds. The pilots then set off to fly these maneuvers. Despite some interesting rides, the Raptor always follows the inputs. When the controls are released, the wild ride stops and the aircraft quickly returns to normal flight.
Fighter Pilot Toolset
Apart from the usual flight test work of identifying and solving problems, we have been refining a set of tools for this new region of the flight envelopetools that fighter pilots have never had before. The next generation of operational pilots will take these tools and develop effective ways to fly and fight at high AOA in the F-22. I look forward to watching their creative efforts.