Aircraft Design: Horizons Expansion (Part II)

In this article we are about to discuss about the flow of a modern aircraft design cycle and use an example of how to use the services of a prototype manufacturer for the prototype testing.

An excess of computational power use at this stage is deemed useless and time waster by most contemporary engineering scientists. It is much more efficient to have approximated techniques involved, which could also help to decrease the time required for parametric modeling executions. Such techniques include Reduced Order Modeling that helps reduce the mathematical complexity of the system, and meanwhile ensures that the physics of the governing differential equations keeps its integrity.

After the implementation of the initial analysis, an iterative procedure starts to take effect where the results command changes for optimization in the design. This procedure is considered the bond between the Conceptual and Preliminary design phase.

Let us have a detailed look at of how the famous Howe model for Project Synthesis Process works through a sum-up.

It is deemed as an extension of the feasibility analysis, but with greater detail and complexity involved.
At the first stage of this process is to select one or more configurations.
The second stage is the Flight Regime and the selection of Powerplant.

At this phase two, for a given set of operating conditions i.e. Mach number,etc. And turbo-prop, turbofan, low bypass turbofan, piston-prop, turbojet, ramjet,etc, are selected to be the types of Powerplant.

The third stage is considered the fuselage layout selection. The details of the payload are often considered the driving factor behind the scene at this stage, because this would have a better first prediction on the aircraft mass.

Then the next comes with Wing Configuration. This is a complex procedure for the aerodynamics lab, for a massive number of parameters are referred. This is an essential phase during the preliminary design process. It takes great part in examining the mass of an aircraft and an initial estimation of lift, drag. Meanwhile, it helps in achieving wing loading estimation calculations after the successive analysis is finished. Theoretical equations tuned according to empirical data for various flight conditions are what Wing loading estimations could apply to. It also assists to implement a crude estimate of the thrust to weight.

Lastly, the parametric analysis phase starts playing its role. Wing and fuselage dimensions are combined at the first stage to produce a series of results for each flight phase and resulted in forming a design space.

For the second stage of the parametric analysis, proper sets of Wing loading and Thrust to Weight ratios are to be selected. At this stage, it comprises the selected sets of data to calculate the aircraft mass overall. The sets that provide the optimal mass values are used for creating a referee design to be used for in-depth analysis and evaluation later.

After the referee design is evaluated, as in return it provides with more details:

Approximated sizes for control surfaces.
Greater estimation of lift, drag and mass values.
Assistance in completing landing gear layout.
Modified calculations for performance characteristics based on
Adjusted input data and complex estimation methods.
Procedure repetition carries out till mass convergence criteria is met.

At the end of the referee design phase, sensitivity design studies are implemented in order to recognize critical design areas by using either graphical or mathematical techniques. Additionally, other acts including the design of hydraulic, fire suppression, ice protection, electric, and pneumatic systems are undergoing synchronously.

Then we came to the most intriguing part –i.e.The Detailed Design Phase. After the design is completely defined, scaled models for testing are ordered from a prototype manufacturer, final drawings based on Design for assembly and design for manufacturing are laid out with actual geometries, tolerances, topologies, dimensions and material specifications.

Detailed Design

At this stage, getting verification for the design procedures that outlined in the previous phases is the main focus, which is also the most extensive phase of the whole design process. It allows to focus on each part’s ultimate design, prototyping, and testing. The use of Computer- Aided Design and Computer-Aided Manufacturing packages are involved in this phase, in order to support design acts based on the data acquired from the preliminary design phase.

Performances, time-costs, manufacturing costs and operational deficiencies are the four key factors to consider. In order to get an integrated result, two types of testing procedures are involved which are Ground Testing and In-Flight testing. Below are the greater detailed references of the two types.

Ground Testing:It comprises wind tunnel testing to sustain results from CFD packages, structural tests, aeroelectronic evaluation, and system check. To cut off unnecessary costs and time wasted, prototyping scaled parts play its role well in initial testing. According to the required material specifications from your end, an eligible prototyping service provider will use adequate expertise to craft the structure. Meanwhile, a professional prototype is applied to be more accurate to analyze rigidity, flutter, strength and elastic stability, etc. There are four key testings to be carried out are: static loading, dynamic loading, vibrational modal analysis, and flutter analysis. The required accuracy for synthetical evaluation between outlined design and experimental results can also be provided by the stereo-lithography 3D printing techniques when applying to scaled aircraft parts.

In-Flight Testing: The certificated agencies, which are known as airworthiness authorities are involved to verify the actual aircraft’s performance and flight characteristics. In accordance with Federal Aviation Regulations Airworthiness Standards will they evaluate the design of an aircraft based on the preset design and safety requirements. The following chart comprehensively outlines all airworthiness standards and their each usage.

Let us pay a close attention to FAR Part 23 among all of the standards outlined above. Part 23 is applicable for utility, normal, and acrobatic with a capacity of less than 12,500 pounds and 9 or less passenger in Maximum Takeoff Weight(MTOW).

For commercial transport category airplanes, i.e. Airbus A320, or Boeing 737, FAR Part 25 dictates several standard requirements. FAR Part 25 includes sub parts which are A, B, C, D, E, and F, all dictate standards for various systems and subsystems to a commercial transport aircraft. Moreover,for rotorcrafts, which it is known as helicopters commonly,  FAR Part 27 and 29 dictate the standards for normal and transport category accordingly. At this phase after having the airworthiness certifications achieved will the design cycle practically ends with 95% of the life-cycle cost incurred. Then followed by the stage of large scale manufacturing is on put.

However complex it may seem on such in-depth review of the design cycle in an aircraft. Still having the design cycle in an aircraft well achieved is approachable.

In the field of aviation, rendering the service of the right prototype manufacturer is of great importance for the accuracy of the prototypes matters the most especially we are in the era where stakes are too high in terms of time and cost. Taking step by step, digging on critical thinking and making mature decisions will be a plus on the design cycle of aircraft.