F-22 FLIGHT TEST PROGRAM UPDATE
(Note: This technical paper was presented recently at the Society of Experimental Test Pilots' annual convention. The paper remains unedited to retain its original flavor.)Lt Col Steven M. Rainey, USAF (M), F-22 Operations Officer
411 Flight Test Squadron, F-22 Combined Test Force
Edwards AFB, CALt Col (Ret) (M) Allen E. Kohn, Jr. Former Commander
411 Flight Test Squadron, F-22 Combined Test Force
Edwards AFB, CAIntroduction
The F-22 Raptor is well into its EMD flight test program. Last year, Paul Metz, the F-22 Chief Test Pilot presented a paper at the 41st Symposium, "First Flight Preparation and Testing the F-22A Raptor." The purpose of today’s paper is to provide an update of the flight test program since last year. First, to bring everyone up to speed, we will summarize the unique aspects of the F-22 Raptor. We will then review overall progress in the past year and some specific flight test results. Finally, we will preview what lies ahead for the F-22 flight test program in the years to follow.
Background
The F-22 Raptor is a very unique and revolutionary new fighter aircraft. It will be the first production fighter to embody several high technology characteristics in one platform. These include: 1) Low Observables or Stealth, 2) Supercruise, the ability to sustain supersonic speeds without afterburner, 3) High agility and post-stall maneuvering with thrust vectoring, and 4) Highly advanced, integrated, sensor-fused avionics.
Many of these characteristics have been either demonstrated on research or prototype vehicles, or incorporated on other production vehicles. While the F-22 carries all of these key technologies to a new level, the last characteristic - highly advanced, integrated, sensor-fused avionics, has never been done before. This capability is unlike any other avionics system today. It is essentially artificial intelligence. In that light, you will hear us refer to the F-22’s "brain" in two different ways in this paper. It has medulla, or "brain-stem" functions such as landing gear, hydraulics, electrics, flight controls, etc. that are controlled by the Integrated Vehicle Subsystem Controller or IVSC. The "frontal lobe" functions that require reasoning are controlled by Common Integrated Processors or CIPs. This alone makes the F-22 very unique, but combined with stealth, supercruise, and post-stall high agility, the Raptor is truly revolutionary. The synergistic effects of these combined characteristics make the Raptor very lethal, extremely survivable and will ultimately save lives by insuring air dominance for the 21st century.
On September 7, 1997, F-22 4001 or Raptor 01, made its maiden flight at Dobbins ARB, Marietta GA with Paul Metz at the controls. Second flight of the Raptor was much less successful due to an instrumentation failure that forced an early landing with essentially no data. Pilot qualitative evaluations by Mr. Jon Beesley verified first flight findings that the F-22 demonstrated excellent flying qualities in the regime flown. Following the second flight, the F-22 was placed into modification status.
Modifications to F-22 4001
Significant weight reductions were made during initial F-22 design leading up to critical design review to improve performance capability. As a result, the F-22 is significantly lighter than the prototype. Following CDR, subsequent analysis identified areas where the structure did not provide sufficient margin and re-designs were necessary. This led to two structural configurations for the F-22 EMD program. The first two aircraft are Block 1 structures aircraft and have some structural loads limitations associated with them. Aircraft 4003 is the first Block 2 or 100% loads aircraft. Several structural modifications were required to both 4001 and 4002 to insure their readiness and safety to conduct envelope expansion testing. Airframe modifications were completed and the aircraft was disassembled, placed on a C-5 and delivered to Edwards AFB, CA on 6 Feb 98.
Ground Tests Leading Up to First Flight At Edwards AFB
The aircraft re-assembly was completed and first power on the aircraft was 22 Apr 98. The first IDLE power engine run was on April 27th followed by MIL power on April 29th and AB on April 30th. The aircraft was structurally different than it was during first flight in GA and it also had a new software configuration; therefore, sufficient ground testing was required to insure that it was ready for first flight at Edwards AFB. 30, 60, and 110 knot taxi tests were performed to evaluate ground handling characteristics and verify brake operation and functionality.
Last year, Paul Metz detailed his Mission Control Room (MCR) training plan that was instrumental to insuring a safe first flight. Eight months had transpired since first flight; therefore, four additional Mission Control Room training sessions were conducted in the Vehicle System Simulator at LMTAS. A dress rehearsal was flown with F-15s and an F-16.
First Flight at Edwards AFB (Video)
On 17 May 98, the F-22 Raptor made it’s first flight at Edwards AFB, CA with Lt Col Steve Rainey at the controls. Paul Metz was the primary safety chase and Lt Col Al Kohn was the backup safety chase and primary photo chase. The flight was very successful and included flying qualities at 15,000 ft, 10,000 ft , and 5000 ft. Speed brake transients and flying qualities were evaluated. Formation flying qualities were evaluated in the CRUISE configuration and in the POWER APPROACH configuration.
Envelope Expansion Testing
The initial goal of envelope expansion testing was to clear the E-0/E-1 zones of the F-22 flight envelope. These were the first two zones to be cleared and were required to ferry aircraft 4002 from Marietta, GA to Edwards AFB, CA. (Show slide of envelope) (Discuss number and type of test points required to clear the envelope as well as flights and hours). Approximately 7 flights were planned to complete the E-0 and E-1 envelope clearance, but 12 sorties were actually flown. Envelope expansion testing proceeded first with flying qualities, and engine transients. Simulated single engine flying qualities were evaluated on May 29th. The first aileron rolls were conducted on June 5th. On June 6th, Lt Col Al Kohn made his first flight in the Raptor. This was a picture perfect flight and simulated short field maximum braking was demonstrated. On June 19th, the first inflight APU start and first engine shutdown and airstart were conducted with outstanding results. Roll acceleration tailoring was evaluated by using pilot selectable flight test control law options.
Meanwhile in Marietta, GA, Raptor 02 flew it’s maiden flight on June 29th (11 days early), with Paul Metz at the controls. It flew a second flight on July the 1st – again flawless.
On July 14th (flight 1-14), the Flutter Excitation System was first used inflight on aircraft 4001. Envelope expansion typically included a Leading Edge Integrated Test Block (LEITB) that consisted of doublets and sideslips to insure control response, followed by a series of FES bursts and/or sweeps at predetermined conditions to identify critical modes. A full Integrated Test Block (ITB) followed flutter testing. This consisted of doublets, maximum sideslips, abrupt release, maximum abrupt rudder input, 45-45 bank-to-bank rolls with a release, left 180 degree full stick roll with a release, left 360 degree roll with a release, right 360 degree roll with a full check, and a wind up turn. Flying qualities continued to be excellent as the maximum speed of the Raptor was increased. The highest speed to date in the Raptor is 423 KCAS attained on Aug 29th.
The Raptor also began envelope expansion in altitude on July 14th (flight 1-14). Altitude capabilities were expanded and engine performance verified on seven flights to attain the current maximum altitude of 40,000 ft. This capability was achieved in a rather short time and in comparison with other fighters, the F-22 powered by Pratt & Whitney F-119 engines, demonstrates excellent performance and flying qualities at altitudes where other aircraft tend to become unresponsive and sluggish. In fact, as altitude and speed increase, it becomes very obvious to the pilot that the F-22 "wants" to go much faster. The aircraft accelerates faster, the faster it goes. This may in fact be a problem for the operational Raptor pilot. There is an "AIRSPEED ICAW" to alert the pilot that he is about to over-speed the aircraft - a real possibility in the F-22. To compare, during a MIL power or less climb in the Raptor, the F-15 and F-16 are in AB in an attempt to keep up.
In addition to flying qualities evaluations to increase the speed and altitude of the F-22, testing has included flying qualities with the landing gear retracted at all speeds and altitudes (flights 1-22, 1-24, 2-2), landing gear down (flight 1-13, 2-2) and with the speed brakes extended. The F-22 has no dedicated speed brake surfaces, but uses the standard flight control surfaces for the speed brake function. In the CRUISE configuration, the ailerons deflect up, flaperons down, and rudders deflect out or the "barn door" mode. This is very effective in slowing the aircraft. In comparison, the F-22 and F-15 in side-by-side formation both deployed their speed brakes and the F-15 wound up in front. In the Power Approach or AR configurations, only the rudders are utilized for the speed brake function. (flights 1-13 (PA and landing), 1-15, 1-23 (CR), 1-29). Testing always includes formation maneuvering (flights 1-12, 1-20, 1-22) which continues to be a pleasant task with the Raptor.
Simulated single engine handling qualities have been evaluated in the CRUISE and PA configurations with speed brakes in and out. For these tests, the failed engine is simulated by placing one engine in IDLE for the handling qualities tests. Pilot selectable control law options are available to select single engine control laws. In this mode, the F-22 flight control system reconfigures itself for single engine. The flaps reconfigure from 35 degrees down to 20 degrees. Roll rates are reduced and the on-speed indication is reset from 12 to 11.5 degrees AOA. (flights 1-14, 1-17 (landing), 1-22, 1-23 (landing), 1-24,m 2-2).
Landings have been increased in complexity since the first few flights, which were flown to a straight-in full stop landing. Offset approaches with abrupt corrections on short final have been flown in the normal configuration (flight 1-14, 1-20, 1-23, 1-25, 1-28), with a simulated engine failure (flight 1-17, 1-24, 1-27) and with the speed brakes extended (flights 1-27. Touch and go landings have also been conducted (flights 1-18, 1-14). Finally, precision landing touch down tasks have been flown with consistent performance within desired criteria (flight 2-11).
To increase test efficiency and in preparation for ferry of aircraft 4002, initial air refueling qualification was completed on aircraft 4001 on July 30th (flight 1-21) at 20,000 ft and 300 KCAS. The initial air refueling qualification included boom tracking tasks in pre-contact position (HQDT), followed by boom control authority tasks throughout the AR/boom envelope without a contact. Next, aircraft/boom compatibility was verified with a contact and tanker initiated disconnect followed by another contact and receiver initiated disconnect. Contacts and disconnects were then flown to map the entire boom envelope for the medium, long, and short boom positions. Station keeping was conducted as a closed loop flying qualities evaluation during actual transfer of fuel. Since I don’t believe in a perfect HQR-1, I gave the Raptor an HQR 1.5. It was an exceptionally stable aircraft for the air refueling task and the best handling aircraft on the boom that I have flown. On Aug 6th (flight 1-23), AR qualification at 30,000 ft and 300 KCAS opened the AR envelope from 18,000 to 31,000 ft at 300 KCAS +/- 10 KCAS.
After a short down time for scheduled modification work, Raptor 02 resumed flight test on July 30th. This was a rather monumental day for the program. We flew three F-22 sorties at two separate operating locations for the first time. While aircraft 4001 was busy clearing the air refueling envelope and the right side of the E-0/E-1 envelope for flutter, aircraft 4002 was conducting airworthiness and delivery testing to include some flying qualities envelope expansion work. After Raptor 02 had six sorties to it’s credit, sufficient confidence in the aircraft and it’s systems was gained to conduct a long duration sortie, a prerequisite for the ferry mission to Edwards AFB, CA. On Aug 23rd, Raptor 02 flew for 4.6 hours, the longest sortie to date, and air refueled two times. During this duration sortie, flying qualities, engine transients, speed power points, air refueling flying qualities, and a rapid descent to test the MAX Defog capability of the aircraft were all tested. The duration sortie was highly successful and served as a dress rehearsal for the ferry flight.
On August 26th 1998, Raptor 02 was flown non-stop from Dobbins ARB, GA to Edwards AFB, CA. Rick 50, an Edwards AFB KC-135E, departed Dobbins ARB, GA at approximately 1030 EST. At 1033, the safety chase (Paul Metz in an F-15) and the photo chase (Jon Beesley in an F-16) conducted an airborne pickup departure. Raptor 02, piloted by Lt Col Steve Rainey, released brakes at 1035 EST for a MIL power takeoff with airborne pickup and visual rejoin with Rick 50. Rick 49, a backup KC-135R from Edwards, then made a trail departure on Raptor 19 flight.
This armada provided tremendous contingency options for the ferry of aircraft 4002. Real time telemetry was only available within approximately 100 nm of Marietta, GA. The Lockheed Martin Mission Control Room was manned and verified the operation and health of all major F-22 systems. No safety of flight parameters remained at the time of the ferry mission. Control of the mission was handed off to the pilot and the Mission Control Room team aboard the KC-135 prior to losing TM coverage with the Lockheed Martin Mission Control Room. Ferry Critical parameters were available to the pilot from the on-board instrumentation system. Each discipline generated a "ferry critical" parameter page for presentation on the pilots Flight Test Instrumentation Display. At pre-determined intervals between air refuelings, the pilot read these parameters to the on-board mission control room team. In the event of communication failure, the pilot’s test cards included normal, minimum, and maximum allowable values. In the event of an emergency, the primary plan was for the mission control room engineers aboard the tanker to piece together the situation based on the Integrated Caution and Warning System (ICAWS), parameters from the instrumentation system read by the pilot, and system design documentation carried aboard the tanker. Either chase was sufficient to continue the mission. If the primary tanker had to divert, the spare tanker had sufficient (although limited) mission control room personnel to safely continue the flight under most circumstances.
Overall, the ferry of Raptor 02 was very successful and marked a milestone for the F-22 program. With the addition of a second aircraft, the F-22 CTF will be able to expand the envelope in two different directions simultaneously - high and fast with 4001 and slow or high AOA for 4002.
Since arriving at Edwards AFB, Raptor 02 has conducted XX test missions, continuing to clear the flying qualities envelope, conducting speed power performance testing, pacer and tower fly-by performance tests, and engine transient tests. Additional instrumentation was also added to support continued envelope expansion testing. Currently, full Ground Vibration Testing (GVT), weapon bay door calibrations, and friction and freeplay tests are being performed on aircraft 4001.
Referencing figure XX, it can be seen that we have cleared the E-0/E-1envelope zones. Testing has been initiated in E-2 as shown. To date, Raptor 01 has flown 29 sorties and 40.2 hours and Raptor 02 has flown 11 sorties and 21.2 hours for a grand total of 40 sorties and 61.4 hours. The F-22 1998 flight test requirements are shown in the next slide. The exit criteria for long lead funding of our first production lot include several specific test events as well as a flight hour requirement. The specific events include initiation of testing on aircraft 4002 which was accomplished on Jun 29th. Clearance of the E-0/E-1 envelopes was completed on Aug 18th. Flights in the E-2/E-2A envelopes were conducted on July 16th and Aug 6th. The first air refueling was accomplished on July 30th, and the first sortie above 30,000 ft was flown on July 7th. In addition, there are three other events that we have yet to complete. The full Ground Vibration Test (GVT) on aircraft 4001 is currently underway and will be complete by the end of September. We have already conducted flight test at 18 degrees AOA in Raptor 02. The first flight above 20 degrees AOA will also be complete by the end of September. The first supersonic flight will be in October. We are also required to achieve 183 flight hours by the end of the year; our team goal is to achieve this early.
We hope that this brings everyone up to date on the progress of the F-22 test program. Now, we would like to present some specific results from our testing to date. In other words, it’s time to air our dirty laundry a little. In general, the results discussed in this paper represent both lessons learned as well as successes for the F-22 flight test program.
FLIGHT TEST RESULTS
One of my all time favorite axioms in life is "keep it simple stupid." In our attempt to integrate virtually every system on the F-22, we have violated this axiom numerous times. For instance, simply lowering the landing gear requires the "global manager" software in the Integrated Vehicle Subsystem Controller (IVSC) or the "brain stem." Before going much further, let me explain the IVSC or "brain stem" functions in a little more detail.
The Medulla, Brain – a.k.a. Integrated Vehicle Subsystem Controller (IVSC)
The function of the IVSC is to coordinate operations which involve several subsystem functions and act as a controller for subsystems without dedicated controllers. For aircraft sizing and increased survivability, the IVSC is divided into six assemblies. The IVSC assemblies and their functions are depicted in figure XX. You can see from the diagram that the physical locations provide some margin of survivability for these functions. In addition, the assemblies were located closer to the functions that they control. This additionally reduces line length and ultimately reduces weight. The bottom line is that this architecture provides significant capability and redundancy for the F-22. Unfortunately, a by-product is that we require a computer for something as simple as lowering the landing gear. This has proven to be more of a challenge than anticipated, but in the end will ultimately result in an incredibly resilient and superior combat aircraft. We will now review some specific examples of problems encountered with our "brain stem" functions and the systems that they control.
Environmental Control System (ECS)
The F-22 ECS is very unique. From a design perspective, we are asking for more from this system than any other fighter ECS in history. It is designed to meet the MIL 210C 1% criteria, rather than the less demanding 10% criteria specified for most current systems.
The heat generated by aircraft systems is carried away by aircraft fuel, refrigerated air, or liquid coolant. Heat absorbed by the liquid coolant (PAO) and also heat generated by the air cycle machine is transferred to the fuel system. Fuel temperatures are then controlled by either burning the fuel in the engines or transferring the heat into the ambient air using the ram air heat exchangers. The simplest, most reliable method of cooling is with refrigerated air; therefore, critical aircraft systems such as the flight control converter-regulators, and 28 VDC converters are air cooled. To reduce the bleed air requirement from the engines, other systems are liquid cooled (PAO). In the event that refrigerated air is no longer available, RAM air cooling is available and it’s effectiveness varies with ambient conditions. The ECS has and continues to be one of the systems that concerns us the most. If the air cycle machine fails and cooling air is lost to the flight control converter-regulators and the 28 VDC converters, limited flight time is available. For this reason, we have been very concerned about the AIR COOLING ICAW.
During first flight, Paul got an AIR COOLING ICAW. This can indicate the air cycle machine failure discussed earlier which we were very concerned about. It can also indicate other less critical malfunctions. The pilot’s only indication that it is the air cycle machine failure will be his inability to breath because OBOGS fails. Paul could, in fact breath, and the control room determined that the ACM had not failed. What really happened was that at this test condition, 12 degrees AOA, insufficient airflow was available through the inlet diverter duct (shown) for cooling the primary ram air heat exchangers. This results in hotter than normal air to the air cycle machine. As a result of this problem, one of the modifications made to the aircraft during the down-time was a modified inlet diverter duct (shown). Subsequent testing has shown that this modification was successful. The figure shows the air flow on first flight and the airflow with the diverter mod at the same condition. Mass flow is improved with a subsequent increased cooling effect and no AIR COOLING ICAW - at least not for this reason.
In preparation for the third flight of Raptor 01 (first flight at Edwards AFB), a new OFP was loaded in the aircraft. During the initial ground runs, we discovered that the Raptor is very much like my five year old and has a very difficult time "waking up." With battery power, not all six IVSC assemblies are powered. ECS is on the backup controller, IVSC A3. During APU start, the ECS self tests itself and hands off control to IVSC A4, the primary controller. Unfortunately, the timing and "handshake" was not well coordinated in this version of the software. Our work around is for the pilot to initially start the APU with the ECS in OFF and then select NORM after the system has had an opportunity to "wake up" (approximately 10 seconds). This will be fixed in a later OFP update, but serves to illustrate how convoluted things can get when virtually every system is integrated and the aircraft "brain" or "brain stem" in this case, is allowed to control each and every aircraft subsystem function with multiple redundancy.
Another valuable lesson learned is that going for the quick, cheap, light weight solution is usually not the right answer. Rainey’s corollary to this axiom is " if it seems too good to be true, it probably is." Unfortunately, we have had four inflight occurrences of the AIR COOLING ICAW, not associated with the inlet diverter duct. These are a direct result of some decisions made early in the program. Referencing the ECS diagram again, you can see that there are essentially four legs that receive conditioned air for cooling. These legs are not controlled by temperature sensors within the components cooled on that leg, or by measuring flow rate. Temperature control would have required sensors for each and every component requiring more lines and ultimately more weight. Measuring delta pressures within the leg was determined to be the cheaper, and easier method for ECS air cooling control. The cockpit pressurization leg and the 19A leg (DC converters, and other critical electronics) were determined to be the most critical and essentially have direct control over the air cycle machine. In other words, if they are not receiving sufficient cooling air as determined by their leg’s associated delta pressures, the branch demands more air from the air cycle machine. If the cooling requirements (DPs ) for these legs are satisfied, the ECS goes into a conserve mode and reduces airflow. Unfortunately, if the DPs for the other legs without control of the ACM (i.e. 19B and 19C) are not adequate, they have no way of demanding more air. The result is issuance of the AIR COOLING ICAW. While the DPs in the 19A leg were predicted to be the worst case, they are not. Insufficient system modeling and simulation was completed to insure that the delta pressure command system would work in the actual aircraft.
The type of tool required to correlate the delta pressure command system with actual cooling effects is an ECS Flow Balance Stand. Again, to save money, an ECS flow balance test stand was not used. For the four flights mentioned, the "work around" is to trick the "brain stem" into thinking that one of the command legs (cockpit or 19A) is not being satisfied so that the ACM will again be commanded to a higher flow state. By partially closing the cockpit manual flow diverter valve or by selecting maximum cold or hot on the cockpit temperature selector, the ACM generates more flow and satisfies the starving leg that can not ask for more air by itself.
Our solution will be some system re-designs to optimize the delta pressure command system. We are currently re-evaluating the potential use of an ECS Flow Balance Stand. The lesson learned for everyone else is to develop and use appropriate modeling and simulation facilities early in the design process to avoid design problems and flight test delays later on.
One other lesson learned here is that configuration control between the simulation facility and the aircraft is essential. Other subsystems on the F-22 used the Vehicle System Simulator in Ft. Worth, TX; however, configuration control between the aircraft and the SIM also generated problems during initial integration into the aircraft.
Gimme a Brake!
The F-22 braking system is also very unique. The Brake Control System (BCS) is a redundant, brake-by-wire, deceleration command system. Last year, Paul Metz showed a video of the first taxi tests where a simple software logic error resulted in Paul receiving a pedal gain 40 times greater on one side than the other. We obviously fixed this problem prior to first flight. Well, we have discovered more unique features of the Raptor’s brakes.
On flight 1-11, I landed the Raptor with a slightly heavier fuel load than normal; however, not what would be considered a heavy weight landing. The nose wheel was started down from the aerobrake at 120 KCAS and on the ground by 118 KCAS which was approximately 120 KGS. Brake pedal commands were initiated at 118 KGS. As can be seen on the HUD video, braking appeared to be normal. There was a slight crab during landing and the right brake was favored initially for directional considerations, but the delta pedal command as perceived by the pilot was very small. Rather than maximum brake, I elected to release brake pressure and continue to the end of runway 22 at Edwards. This decision was made well before taxiway B which left at least 8000 ft of runway for stopping. The pilot considered this to be a conservative decision based on a slightly higher gross weight. Approximately one minute later, braking was again initiated and the aircraft brought to taxi speed with exit at the end. Both the pilot and the control room identified that the brakes were hot. While the HUD video and pilot perception indicated normal braking technique, erring on the conservative side, the result was a hot brake condition. Looking at the data reveals another software logic problem that will be changed with the next F-22 OFP update.
During normal braking, the pilot is commanding a deceleration rate. The BCS uses wheel speed and Inertial Reference System (IRS) velocity feedback to determine if the desired deceleration is being generated and actively controls hydraulic pressure to the brakes to achieve the commanded deceleration rate. This sounds well and good; however, when a pilot steps on a brake, he wants some deceleration. If he steps on the pedal a lot, then he wants a lot of deceleration. This logic that did not make it into our BCS software.
For flight 1-11, the chart shows that the aircraft had a normal aerodynamic deceleration rate of 3.1 ft/sec2. The pilot pedal commands for the first 15 seconds of braking increased from 35% to 75% on the right pedal and 15% to 70% on the left pedal. For the first 6 seconds of braking, the left pedal command was less than 35%. The next chart shows that to command a deceleration rate in excess of 3.1 ft/sec2 (the aircraft’s aerodynamic deceleration rate at the time), the required pedal deflection was 38%. Therefore, even though the pilot was commanding some deceleration, and from his perspective wanted the aircraft to respond, the command was less than that required for the BCS to generate actual brake pressure. All of the high speed energy was absorbed by the right brake. The result was an apparent normal landing roll to the pilot, with hot brakes at the end of a 14,000 ft runway. This is obviously counter to normal pilot sense and expectation. Our next software update will include a "delta" deceleration command BCS so that any pilot commanded pedal deflection will result in some deceleration and provide normal pilot sense.
In addition, the brakes appear to respond differently depending on pilot technique. The next chart illustrates the technique requested by the Landing Gear team. The residual idle thrust in the Raptor varies with elevation, but is significant at Edwards AFB. IDLE thrust is sufficient to taxi and the aircraft will continue to accelerate without pilot action. Specifically, the groundspeed is allowed to build to 35 KGS and then the pilot slows the aircraft to approximately 10 knots. Using this technique, brake response is very predictable and data analysis is possible. During the time with pedal pressure released, some brake cooling is actually occurring and is depicted on the next chart. Conversely, less predictable, but perhaps normal pilot technique on the brakes results in noisy and unpredictable brake response that makes analysis almost impossible because the conditions are changing so rapidly and the BCS is attempting to react to these changes. Brake pressure is never allowed to completely release and the result is significantly higher brake temperatures for rather normal pilot action. This condition will improve with the new software and with the new hardware configuration for the brakes scheduled to be incorporated on Raptor 03. The hardware changes are designed to improve thermal heat protection, reduce brake vibration, and increase durability. More to follow on this in another paper.
Overall, we have identified some deficiencies with the brakes; however, they stop the aircraft very well and we are normally able to stop the aircraft well before taxiway B at Edwards AFB. The preferred technique is a symmetric, 2 second rate, full application of the brakes initiated at 100 KGS to approximately 30 KGS and then release prior to the 20 KGS transition to manual braking, then reapplication of manual brakes to bring the aircraft to a stop. As we continue testing, we will strive for a BCS that accommodates various pilot techniques yet still provides the specified braking capability.
"Let Me Out!" – Caught in the Cockpit
The normal sequence during aircraft shutdown is:
- Throttles – OFF
- APU – OFF
- BATT – OFF
- OBOGS – OFF
- Canopy – Unlock
- Canopy Open
During an engine run in aircraft 4001 at Edwards AFB, the pilot followed the checklist steps and discovered that the canopy would not open. The canopy actuator had just been replaced and this anomaly was attributed to the new actuator at first. Another pilot almost 2000 miles away in Marietta, GA was also trapped inside Raptor 02 in a very similar situation. While there are much worse places to spend time than trapped in the cockpit of the world’s most advanced fighter, when it’s 100+ degrees out and getting hotter under the canopy every second, this is not a desirable condition. After further investigation, we discovered that again, the system handshakes are very time dependent. We now know that if this occurs, wait 10 seconds for the handshake to occur and then attempt to reopen the canopy. The command from the canopy switch waits 5 seconds after all power is off the aircraft. Even if the pilot selects the battery switch off, there are fans that remain powered for several seconds. An attempt to open the canopy during this time results in the command being ignored.
While there were numerous other small subsystem design deficiencies identified during our flight test thus far, these were the most interesting and perhaps most clearly provide the message that the test pilot needs to be involved in every step of the design process. And of course, "keep it simple stupid!"
FLYING QUALITIES
The Raptor’s flying qualities are excellent for the conditions flown to date. Very little closed loop evaluations have been conducted. Formation flying tasks, air refueling (to include boom tracking tasks), offset landing tasks, and precision landing tasks have been evaluated at this point in the test program. It was felt that a larger G and AOA envelope was really needed for tracking tasks, BFM exercises, and initial BFM evaluations. These limited military utility tasks will be evaluated by the end of the year.
Nose Kick, Roll Jerk, or Nose Wander
Every Raptor test pilot has observed what has been termed a roll jerk, nose kick, or nose wander during rolling maneuvers. At higher speeds this is generally observed as a rapid jerkiness upon roll initiation. At higher angles of attack and lower speeds this characteristic is observed as a nose kick or sideforce upon roll initiation and a beta kick from proverse to adverse on fully checked rolls. On approach for landing in the PA configuration, there is a yawing nose wander with roll inputs.
This anomaly can be seen in the following video. The HUD video shows proverse yaw on roll initiation that increases and then kicks to adverse yaw on release or check. While it is subtle in the HUD video, it is very noticeable and objectionable at the pilot station. This is verified by the flight test data as shown on the strip charts. The solid lines depict the predicted response and the dashed line shows actual flight test results. For full stick roll initiation, proverse yaw is shown. The beta increases with increasing roll angle, but is controlled tightly. While the sideslip angles are relatively small, the fully checked roll results in a nose kick to adverse yaw (or proverse to the check input) of up to 4-6 degrees. This is obviously objectionable and would make fine tracking tasks very difficult.
After reviewing the predicted data, it has been determined that the directional stability and rudder control power is greater than predicted. This is actually good news. During DEM/VAL, the prototype flight test data did not match flight test results. Correction factors were applied to the ATLAS models for the differences between the wind tunnel and flight test data. An engineering assumption was made and corrections based on prototype flight test data were applied to the F-22 wind tunnel results. F-22 flight test results show close correlation with F-22 wind tunnel data. As a result, the corrections will be deleted from the ATLAS models and the flight control laws.
Pilot selectable flight control law options were available to investigate aileron rudder interconnect (ARI) reductions to account for the variations. Available options included 20% and 40% ARI reductions. The following data is for a 40% ARI reduction. The response to a full stick roll is much more normal and shows a slight adverse yaw that is controlled tightly. During fully checked rolls, the beta transients are much smaller. This data verifies that directional stability is higher than planned and that without the corrections, the F-22 wind tunnel data matches flight test results.
Ultimately, these findings will be incorporated as part of the baseline control laws in a F-22 OFP update.
Closed Loop Flying Qualities Evaluations
Closed loop flying qualities evaluations have been rather limited to date. As the envelope expands, more closed loop tasks will be conducted. To date, evaluations of the air refueling station-keeping task and precision landing tasks have been conducted, as well as subjective evaluations of formation flying qualities.
AR - Desired criteria was essentially to maintain the optimum or "captain’s bars" position with only momentary deviations to a single green up or down arrow. Multiple deviations or deviations beyond a single green arrow constituted adequate performance, while red arrows with momentary limit positions was considered unsatisfactory. The flying qualities were outstanding with essentially no pilot workload to maintain desired criteria. The more relaxed the pilot was on the controls, the better the performance. The aircraft was very stable, required no trim changes, and responded very well to small pilot corrections. It was very difficult to identify the AR switch by blind feel and the HUD AR indications were not in the instantaneous FOV, requiring head movement to see. Perhaps due to these minor inconveniences and perhaps because I don’t believe an HQR 1 exists, I rated the Raptor HQR 1.5 for AR at 300 KCAS and 20,000 ft.
PRECISION LANDING - Precision landings were evaluated by aiming at a precision landing square painted on the runway surface. Existing runway remaining markings on the runway were used to define the desired and adequate performance. The aircraft has a tendency to float requiring the pilot to put the airplane on the ground instead of flying it onto the ground (HQR-3).
FORMATION FLYING - In general, formation flying has been extremely easy, requiring very little compensation by the pilot. Handling qualities ratings have not been assigned, but all Raptor test pilots have very favorable subjective comments about the formation task.
THE FUTURE FOR F-22
The future of the F-22 flight test program is bright. The remainder of 1998 will be focused on achieving the test objectives required to meet the DAB exit criteria for long lead funding. In addition, the flight test team has established goals for extending flight test into zone E-3 this year. Finally, the F-22 Flying Test Bed (FTB) is a Boeing 757 modified with the F-22 radome, sensor wing, radar function, F-22 avionics suite as well as an F-22 simulated cockpit and multiple engineering work stations. The FTB will be conducting F-22 avionics test to reduce risk prior to installing the avionics on the Raptor.
This is the F-22 flight test planning schedule. The next F-22 will be complete in approximately 14 months. Aircraft 4004 through 4009 are our avionics test aircraft and will have the Common Integrated Processors (CIPs) or "frontal lobes" that we mentioned earlier. The development test and evaluation is scheduled to end in 2002 with the initiation of dedicated IOT&E. Initial Operational Capability (IOC) is slated for Nov 2004.
Overall, the F-22 test program has been very successful to date. Many challenges lie ahead and the team is ready to face them. We are trying to learn from our mistakes and we hope that you too can benefit from our lessons learned throughout the next few years. This concludes our paper; are there any questions?
09 Apr 1999
