Tolerance Relaxation at the Wetted Surface
Section 17.3 mentions that a best practice to reduce production costs is tolerance relaxation at the wetted aerodynamic surfaces, which contribute to an increase in parasitic drag. This section describes the important DFM/A consideration of tolerance relaxation, which is a concern of aerodynamicists and structural designers.
Tolerance relaxation during component manufacture could incur problems of the tolerance-chain buildup at the assembly joint. All aspects of tolerance are beyond the scope of this book; only the tolerance allocation at the surface as the aerodynamic smoothness specification is discussed .
In current manufacturing philosophy, the main features contributing to excrescence drag are as follows:
• manufacturing mismatches seen as aerodynamic defects (i. e., discrete roughness; e. g., steps, gaps, and waviness)
• surface contamination with fine particles and dirt adhering to it
• damage, wear, and tear during the life cycle
• fatigue deformation
• attachments of small items on the surface (e. g., blisters, antenna, pitot tubes, gaps/holes, and cooling air intakes/exhausts)
The first and last items are the consequences of design considerations; the remainder happens during operational usage. This chapter addresses only the first item, which gives rise to excrescence drag (i. e., parasitic drag). The nonmanufacturing origin of excrescence drag arising from the last item is treated separately for the Copmin estimation. To keep excrescence drag within limits, aerodynamicists specify aircraft smoothness requirements, which then are translated into tolerance allocations at the subassembly joints on the wetted surfaces. If the finish exceeds the tolerance limits, it must be reworked to bring it within the limits and/or obtain concessions to pass the product to the final line. Tolerance specifications affect aircraft manufacturing costs.
Aircraft wetted surfaces are primarily manufactured from sheet metals and composites. At the subassembly joints, there are some mismatches (e. g., steps, gaps, and waviness) that must be kept under strict control by specifying surface – smoothness requirements. Mismatches result in parasitic drag as an excrescence effect. Aerodynamicists specify aircraft surface-smoothness requirements to keep the drag increase within limits. The stricter is the tolerance, the more is the cost of production on account of rework or rejection. Any tolerance relaxation at the wetted surface reduces manufacturing costs at the expense of an aircraft parasitic drag increase, perceived as a “loss of quality function.” It is assumed that the sheet metal and composites at the surface accommodate a certain degree of tolerance relaxation. In addition, cosmetic appeal is perceived as a customer preference. Loss of some cosmetic quality can save on costs without unduly penalizing the parasitic drag. However, with increases in fuel price, aerodynamicists must be careful in specifying surface-smoothness tolerances.
Figure 17.1. Cost-versus-tolerance relationship. Manufacturing cost reduces as tolerance is relaxed. Savings = amount reduced from the existing level to a lower level due to tolerance relaxation
tight Tolerance relaxed
17.6.1 Sources of Aircraft Surface Degeneration
In aircraft application, degeneration of the wetted surface area results from surface deviations from the specified level. It has many origins; the important ones are as follows:
1. Lifting Surface (e. g., wing, flaps, and empennage)
• control of LE profile and surface-panel profiles (i. e., aerofoil contour)
• rivet and fastener flushness for skin joints
• component geometry and subassembly joint mismatches
• fitment of access panels on the surface
2. Bodies of Revolution (e. g., fuselage and nacelle)
• control of nose profile and profile of the rest of the body joined in sections
• rivet/fastener flushness for skin joints
• component geometry and subassembly joint mismatches
• fitment of doors, windows, and access panels on the surface
17.6.2 Cost-versus-Tolerance Relationship
The relationship for establishing the manufacturing cost, C, at the assembly is derived by summing all costs involved, as shown:
manufacturing cost, C = (basic work time + rework time) x manhour cost + number of concessions x cost of concessions + nonrecurring costs + cost of support/ redeployment/management (17.1)
Changes in tolerance affect the rework time, number of concessions, and the cost of support. Tolerance relaxation reduces manufacturing costs because more components and their assemblies are made right the first time. Tolerance relaxation reaches a limit when any further relaxation has no significant benefit because all components and their assemblies require no rework and/or concessions for acceptance – it is done right the first time. At the limit of relaxation, the cost of manufacturing levels out to what is required for the “basic” work time and the NRC. Figure 17.1 illustrates the nature of the cost-versus-tolerance relationship, a trend that is common to all features.
The X-axis represents the tolerance variation, from the existing level to the level where any further tolerance relaxation has no further benefit in cost reduction.
The Y-axis represents the cost of manufacture, from the existing tolerance level, with current manufacturing costs representing zero savings. Tolerance relaxation results in cost reductions up to the maximum possible level (100%).
Summing tolerance relaxation over an entire aircraft can reduce the manufacturing cost by a small percentage while incurring an increase in excrescence (parasitic) drag. The aircraft DOC reflects the change in cost reduction and drag increase when a trade-off study is conducted. Figure 16.4 shows trends in the tradeoff between cost and tolerance. If the initial tolerance is too strict, the relaxation shows a reduction up to a point at which thereafter the DOC increases as a result of the additional fuel burn due to the drag rise, whereas the aircraft price reduction has leveled out.
Reference  is a study of the trade-offs, describing how a midsized jet aircraft can average about 33% tolerance relaxation with a corresponding net savings in DOC of 0.42%. The conservative estimation given herein is a typical aircraft cost reduction through DFM/A studies (a fuel price of $0.75/U. S. gallon is used):
• an approximate 1.28% DOC savings due to 2% aircraft costs saving through DFM/A studies involving no drag increase
• an approximately 0.42% DOC savings due to 1% aircraft costs saving through tolerance relaxation involving drag increase
This study demonstrates a total of 1.7% DOC savings, which translates into a savings of $530 per sortie for a 150-passenger/3,000-nm range aircraft class. With an annual utilization of 500 sorties, the total is $26,500 per aircraft. For a fleet of 10 aircraft, the savings total $26.5 million in 10 years. For smaller aircraft, the percentage savings is even higher.
This is a good example of how aerodynamic, structure, and manufacturing considerations are needed to conceive designs that result in reduced DOC. Manufacturing cost reductions can be achieved through many other efforts, which is the aim of the DFSS concept. During the trade-off studies of various design parameters, the benefit of cost-estimating activities helps designers investigate and adopt new technologies to advance a product to a competitive edge and generate specification requirements (e. g., tolerance allocations). Designers also analyze the risks involved, balancing the trade-off between cost and performance that eventually leads to affordability for operators as the best buy (i. e., product value), which in turn enables manufacturers to thrive.