17.6 Tolerance Relaxation at the Wetted Surface

design rules,” and “I refuse to use the tools.” DFSS demands a culture change – not easy to achieve but possible.

17.6 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 [5].

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 CDpmin 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 surfacesmoothness 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

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) × manhour cost

+number of concessions × cost of concessions

+nonrecurring costs + cost of support/

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 manufac- turing 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.