Solution
2.4 Introductory Flight Test Concepts
2.4.3 Flight Test Techniques
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planning process are covered in a later section. After the test plan has been written, reviewed, and approved, test cards are written that specify the step-by-step procedures to be used to set up and perform the test maneuvers. Test cards are discussed in more detail in a later section. Prior to the test, there is usually a technical and safety review of the readiness to proceed to flight, often called a flight readiness review. This final, formal review is usually presented to technical, safety, and management personnel who are not associated with the flight test, so as to provide an independent, objective assessment and approval as to whether the flight test team is ready to proceed to flight.
Once this readiness review is passed, the flight test team is complete with the preparation phase of the test and is ready to move on to test execution.
On, or very near to, the day of the flight, the test team meets to brief the planned flight to ensure that everyone understands the test objectives, the flight test techniques that will be flown, test data that is required, and any flight restrictions or limits. The test cards for the day’s flight are talked through, as they will be flown. Finally, it is time to go flying, and the test flight is performed, following the briefed plan. The well-known adage for this process is to “plan the flight and fly the plan”. After the flight is completed, the test team holds a post-flight briefing to review the flight, discuss what went well and what did not go as planned, and to identify any issues or discrepancies.
The flight test data is analyzed by the engineering analysts to ultimately determine if the objectives and requirements have been met.
k k Table 2.12 Ground and flight test techniques discussed in text.
Discipline
Ground or flight test
technique (GTT or FTT) Aircraft used in description
Fundamentals Familiarization flight McDonnell Douglas F/A-18B Hornet
“Trim shot” Extra 300
Aerodynamics In-flight flow visualization NASA F/A-18 HARV
Drag cleanup GTT
Wind tunnel testing GTT
Computation fluid dynamics GTT
Lift and drag in steady, gliding flight North American XP-51B Mustang Aerodynamic modeling Boeing F/A-18E Super Hornet Visualizing shock waves in flight Various
Stall, departure, and spin Christen Eagle II Hypersonic flight testing North American X-15
Propulsion Engine test cell and test stand GTT
Flying engine testbed Various
In-flight thrust measurement Convair F-106B Delta Dart Performance Altitude and airspeed calibration Northrop T-38A Talon
Cruise performance Ryan NYP Spirit of St. Louis
Climb performance Cessna 172 Cutlass
Energy Lockheed F-104G Starfighter
Turn performance Lockheed F-16 Fighting Falcon
Takeoff performance North American XB-70 Valkyrie Stability & Control Longitudinal static stability Piper PA32 Saratoga
Lateral-directional static stability NASA M2-F1 lifting body Longitudinal dynamic stability Piper PA31 Navajo Variable-stability aircraft Various
First flight New or modified vehicle
Time
Land H (KFT) Level accels at H = 43K
(MIL & MAX power)
Aeromodeling 0.9 M, H = 25K POPU, WUT, Split-S Normal climb to H = 43K
Aerobatics, stall
Normal descent Tower flyby 300 KIAS Max power takeoff
45 40 35
25
15 10 5 0 20 30
Figure 2.45 Example flight profile.
2.4.3.2 Flight Test Cards
The details of each test point and FTT are usually written down on a set of flight test cards. The test cards provide step-by-step procedures for each test point. Details about setting up the test
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point, the maneuvers to be flown, and the post-maneuver prompt (i.e. what test point is coming next) are usually included. In addition to the individual test point cards, there are usually some overview cards that describe the big picture of the test, often including a card showing the flight profile, as described in the previous section. The overview cards usually include other items that are applicable to all the test points, such as aircrew assignments, communications frequencies, airspace information, aircraft and test limits, aircraft weight and balance information, performance charts, or other pertinent information.
In constructing test cards, the format selected depends on the test situation and on personal preferences. For example, textual directions, tables, pictures, or diagrams are different formats that may be appropriate. The physical size of a test card can be small or large, depending on the size of the cockpit or cabin where it is utilized. Typical elements of a test card include the test point name and number, aircraft type, and card number. Items specific to the test point might include the aircraft configuration, limits, data band and tolerance, setup of instrumentation or data system requirements, and setup or trim shot flight conditions. There is usually space on the card to record data or write comments. The order in which data is recorded should be prioritized on the card:
the most important data that is required should be written down first. The test card should specify data responsibilities, such as specifying that the pilot make a verbal call at given time intervals or the flight test engineer starts a data system and records certain data. An example flight test card is shown in Figure 2.46. The test cards are a record of the events and data from a test flight. They should not be altered or rewritten after a flight, as this might taint the original information or data on the card.
2.4.3.3 Flight Test Data Collection
Almost always, a flight test technique involves the collection of data. The data may be quantitative, such as obtaining performance or stability data about an aircraft. Alternatively, the data may be qualitative, such as a subjective pilot opinion of how capable an aircraft is at performing a mission task, such as aerial refueling or formation flying.
Typically, data bands and tolerances are specified for the data collection. For many FTTs, espe-cially those that require maneuvering, it is not possible to maintain the aircraft at perfectly constant flight conditions. A data band of ±1000 ft (300 m), above and below a specific altitude, may be used for a constant altitude test point, since the atmospheric properties are essentially constant over this altitude range. Tolerances for holding flight conditions, such as airspeeds or load factors, are set by the accuracy required in the data. Tighter tolerances usually translate into more pilot effort in flying the aircraft precisely.
The first step in the flight data collection process is the definition of what data needs to be col-lected. This definition flows down from the test requirements in the form of the data that is needed to perform the data analyses. In addition to this data – the direct inputs to analyze a specific aircraft characteristic – other data is usually required to validate the test conditions. For example, in an aircraft climb performance test, primary data such as airspeed, altitude, engine power, and time are required for the performance analysis. In addition, data such as the angle-of-sideslip may be desired to ensure that the climb was performed with zero sideslip. The sideslip data may not feed directly into the climb performance analysis, but it validates the quality of the data.
Of course, the desired number of data parameters may exceed the number available. This con-straint is usually due to the number of sensors on the vehicle and the architecture and size of the data acquisition system. Another factor to be considered is the data sampling rate, the data samples per second that are collected. The required data sampling rate is a function of the frequency of the physics that is being measured and other data acquisition related requirements.
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F-16B S/N: F-16 PERF FLT
LIMITS: 600 KCAS, 1.6 M
TRIM SHOT @ 41 K, 0.8 M (15 sec) DAS: On (REC LT ON)
A B C
D E
5B 0.85 M → MMAX
ALT ± 100 FT, VVI±100 FPM Mi
0.85 0.90 0.95 1.0 1.05 1.1
ΔHi VVI FUEL TIME
LVL ACCEL -- MAX PWR
DAS: OFF (REC LT OFF) COMMENTS:
NEXT: DESCENT TO 27.5K
CONFIG: S/B IN, MAX PWR DATE:
MAX PWR LEVEL ACCEL 43 K ±500 FT / 0.85 M → MMAX
Figure 2.46 Example of a flight test data card for a maximum power level acceleration in an F-16.
Flight test data may be collected in a variety of ways, ranging from simple hand recording of information and data in flight to the use of a sophisticated data acquisition system (DAS). There are many types of data acquisition systems available, from simple inexpensive systems to com-plex costly systems. A DAS can be commercial off-the-shelf (COTS) equipment or it can be a custom-made system. Modern data acquisition systems are capable of measuring and recording thousands of parameters. DAS data may be recorded onboard the aircraft or may be telemetered to a ground station, or both. Telemetry systems require a transmitter and antenna on the aircraft, as well as receiving equipment and an antenna on the ground. A benefit of data telemetry is that personnel in a ground control station or control room can monitor the flight data in real time.
At the other end of the data collection spectrum is the use of “hand-held” data, where personnel in the aircraft manually record flight data by hand. Hand-held data is the simplest source of flight test data, requiring only a pencil and paper in its most basic form (of course, an electronic tablet,
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laptop computer, or other electronic device could be used for hand recording). For testing where budget or schedule precludes use of a data acquisition system, hand-held data may be the only option. However, even when data is electronically recorded, hand-held data is useful in providing a real-time record of the test and as a convenient way to reconstruct the test events after the flight.
The quantitative, hand-held data can serve as a “backup” to the DAS or telemetered data. There may be instances when the electronically recorded data is unavailable or incomplete, making the hand-held data extremely valuable. Hand-held data may be quantitative or qualitative. Numerical readings may be taken from cockpit gauges and instruments, providing quantitative data collection.
Qualitative data may consist of observations, comments, or descriptions which provide valuable insights of the test.