Aerodynamics

Aerodynamics is generally not considered an airplane system, but it is nevertheless a key design discipline in systems engineering. Airplanes are designed and built to make money for their owners. All aspects of design stem directly or indirectly from this primary goal. The most direct parameters are payload carried, range flown, fuel burned, and flight time. These parameters define much of the equation for profit. The tools used to analyze and optimize the design for these parameters are found in the province of the aerodynamic specialty called airplane performance. Systems engineers need to understand the impact of their design decisions on the economic viability of the airplane being designed.

Knowledge of the economic issues is essential to doing design trade studies. If one accounts only for the costs visible to his or her specialty, one may make the wrong decision. The functional specialties within aerodynamics of interest in this section are design, stability and control, and performance. Some of the many responsibilities include:

• • defining the external shape of the airplane (i.e., the loft surfaces)
• • deciding the size, shape, and location of stabilizing and control surfaces
• • designing the geometry of the high lift system
• • determining the aerodynamic characteristics
• • designing the air data sensing systems
• • calculating performance
• • preparing guarantees

Aerodynamic design defines the external shape (loft surface) of most of the airplane. In theory, the external surface (the skin) of the airplane conforms exactly to this shape. In practice, there will always be reasons for some part of the airplane to deviate from the shape. When a systems or structure design group feels such a deviation is warranted, they should contact the aero design group, perform a trade study, and reach a decision that is best for the company. A design group may deviate from the aero design when intending to solve a problem, but they may not inform aero design and may produce a good solution from their particular discipline’s perspective, but one that is not good for the airplane. Therefore, designers must approach their work as an interdisciplinary endeavor. This applies equally to systems, structure, and aero designers.

Rigging means the positioning of airplane surface components relative to one another during assembly. Aero design works with the mechanical engineering group to establish rigging tolerances. Correct rigging of every airplane is very important, as incorrect rigging can cause significant increase in fuel burn, which tends to make customers unhappy. An outboard aileron misrigged by half an inch (0.5″) can cost as much as 25,000 gal of fuel or $15,000 per aircraft per year.

The Production Flight Procedures Manual (PFPM) is a collection of the tests that an airplane company collectively thinks should be performed on every airplane, to insure that it conforms to design intent. Aero design contributes to determining the set of tests to be done, the conditions under which they should be done, and the quantitative or qualitative results that should be obtained. When an airplane fails a PFPM test, a Flight Work Order (FWO) is generated and must be resolved. If it is decided to change the PFPM test rather than the airplane, an Aircraft Operational Report (AOR) is generated.

The size, shape, and location of the horizontal and vertical tails are chosen to satisfy airplane stability requirements. The elevators, ailerons and spoilers, and rudders are designed to provide control on the pitch, roll, and yaw axes, respectively. Pitch trim is provided (on most but not all jet transports) by movement of the horizontal stabilizer. The stability and control group estimates the hinge moments required to move these surfaces and provides that data to the groups who size the actuators and design the structure. Airplane control considerations are used to estimate the required deflections and rates, which also go into actuator sizing and structural design. Flying qualities are also the province of the stability and control group, and generally refer to those characteristics that describe the response of the airplane to pilot commands and to disturbances. The stability and control group also works with the propulsion group to quantify the effect of the power plant on airplane flight characteristics, from the direct effects of forces and moments to the indirect effects resulting from the air sucked in by the engine and then spit out its exhaust.

The stability and control group is involved, for instance, in the lateral placement of the engine thrust vector to ensure proper flying qualities.

The largest subject area for the certification performance group is the takeoff length data. While the aerodynamic and power plant data are the key contributors to takeoff field length, there are many other factors applicable both to rejected takeoffs and landings—the deceleration contributed by the brakes, the energy-absorbing capacity of the brakes, tire friction, wheel speed limits, etc. The takeoff performance data has almost as much impact on profitability as cruise performance. The limiting takeoff weight out of a given field determines the maximum payload that can be carried. Landing field length is generally a lot simpler and less limiting than takeoff but it is still a concern.

The certification performance group also computes the climb capability of the aircraft following takeoff and for the conditions of an aborted approach or landing. There are FAA requirements for minimum capability, which generally result in maximum permissible weights. There can be cases where climb capability is more limiting than takeoff, such as an airport with nearby obstacles like terrain or building structures. Driftdown refers to the altitude loss when two engines become inoperative while at cruise altitude. This becomes significant when the altitude to which the airplane will drift down is less than the local terrain height. This must be included in the Airplane Flight Manual (AFM).

Since performance is computed for situations where the pilot (or autopilot) has commanded a change in configuration (landing gear up or down, flaps/slats extended or retracted), it is important to know how long the change takes—the actuation and overall system response times. This data is then used in the performance calculation.

The other performance group is called the operational group, and it deals with data that is not required to be approved by the FAA. Operational performance is of great interest to the airline operator, as it translates most directly into expense and revenue. The key data has to do with the amount of payload that can be carried, the distance it can be carried, the time required and the fuel burned. These data appear in quite a few publications:

• • Performance Report, primarily used in selling airplanes;
• • Flight Planning and Cruise Control Manual, primarily used to plan and dispatch flights;
• • Performance Handbook, a reference book of detailed data;
• • Flight Crew Operating Manual, used to train flight crews.

These data are used to define a mission (also called a flight) from one point to another, such as Los Angeles International Airport to Pudong International Airport. A mission consists of a takeoff, climb, cruise, descent, and landing. There is performance data for each of these phases of the flight.

For flight planning purposes, we must include a few other phases: flight to alternate and holding, “contingency,” engine start, taxi out, and taxi in. This data is vitally important for flight planning and dispatch, as the FAA flight rules require that the pilot know the fuel required for his intended flight and that the fuel on board is adequate. While it does not take much arithmetic to determine that a B737 cannot carry enough fuel to go from Los Angeles to Shanghai, most flights require calculation of the fuel required.

This operational performance data have major economic significance to the operator, as distance, time required, payload carried, and fuel burned all affect profit. The operational performance group gets involved in virtually every airplane sale. Given the impact of performance data on their finances, many customers ask that the airplane original equipment manufacturer (OEM) companies guarantee the data. These guarantees lead to significant exposure for airplane OEMs, especially when the airplane has not yet been built and flown.

System design changes after commitments to customers have been made can be problematic. Even more insidious are design features or changes not recognized as affecting performance and/or guarantees and thus not communicated beyond the group making the change. Systems engineers should be aware that the detail specification for the airplane is in effect a guarantee (actually it is a warranty). Failure to meet any of the stated capabilities can result in customers filing warranty claims demanding correction or compensation.