Page 38 - Computational Fluid Dynamics for Engineers

1.4 Prediction of Aircraft Performance Degradation Due to Icing 23

The test covered a range of Mach number from 0.6 to 0.97 and the results were
reduced to a nominal chord Reynolds number of 3.6 million.
The test yielded the following results:

- Large drag improvements were obtained with the re-contoured fuselages at
all Mach numbers above Mach 0.70. The largest drag reduction (3% to 5% of
total aircraft drag) was obtained with the shaped fuselage B172. The shaped
pylon P73 contributed substantially to the weakening of the shock wave in
the channel.
- All drag reductions were associated with a lowering of the peak Mach number
in the channel between the fuselage and the nacelle, a reduction of the pylon
download, and a better control of the diffusion in the aft end of the channel.
- The optimum orientation of the nacelle for drag was found to be precisely
the one predicted by using the MBTEC multi-block Euler code for optimal
nacelle pressures.

Figure 1.24 shows a comparison of MBTEC predictions with pressures
measured on the fuselage above and below the nacelle pylon on the initial
(B165/P72) and final (B170/P173) configurations. This comparison shows that
the inviscid Euler results (Chapter 10) on the fuselage were a good indicator of
the flow field generated on this part of the aircraft.

1.4 Prediction of Aircraft Performance Degradation
Due to Icing

Aircraft icing presents a serious hazard for flight at subsonic speeds in visible
moisture and at temperatures near or below freezing. Many aircraft have been
lost due to ice accumulation. Some twenty accidents where icing was a con-
tributing factor are listed in Fig. 1.25. In the absence of thermal ice protection,
ice on wings, control surfaces, and engine intakes can reduce the aerodynamic
performance of the aircraft. Therefore, the Federal Aviation Administration
(FAA) requires an airplane manufacturer to demonstrate that its aircraft can
fly safely in icing conditions as defined by the so-called icing envelopes in the
FAA's Federal Airworthiness Regulations (FAR) Part 25, Appendix C [21].
Ideally one would like to prevent ice from accreting anywhere on the air-
frame, which is unfortunately not always possible. Thus, the analysis of an
aircraft's response to an inflight icing encounter plays a key role during the
development and certification phase of an aircraft. All icing testing is relatively
expensive, however. In today's competitive environment, cost-effective calcula-
tion methods must be developed so that the aircraft manufacturer can evaluate
the performance of a system for a range of icing conditions and consequently
reduce development and certification time and cost. Full-scale icing experiments
over a wide range of conditions would be very expensive.