Page 40 - Computational Fluid Dynamics for Engineers

1.4 Prediction of Aircraft Performance Degradation Due to Icing 25

economically competitive. Thus, some areas of an aircraft are anti-iced (no ice),
some are de-iced (cyclic ice buildup and removal), and some are left unprotected.
In addition to meeting safety requirements, the aircraft industry must meet
the challenges of rising operating costs and intense economic competition. The
industry, therefore, places heavy emphasis on reducing fuel burn, increasing
range, and improving maintainability and reliability. Aircraft ice protection im-
pacts all four of these economic considerations in surprisingly complex ways.
For example, ice protection devices must be defined accurately so that high
confidence can be placed in the important trade and risk studies. Conserva-
tive assumptions can result in excessive predicted ice protection system weight,
power, and cost.
For both economic and safety reasons, in 1978 NASA established an icing
program at its Lewis Research Center in Cleveland, Ohio. This icing program
is guided by three strategic objectives [22]. One is to develop and validate com-
puter codes that will numerically simulate an aircraft's response to an inflight
icing encounter. This challenging task requires two steps. The first step is to
predict ice accretion on the airframe, which is a very complicated process be-
cause of the numerous parameters involved. For example, both the aerodynamic
and thermodynamic parameters play an important role in the development of
ice accretion at the leading edge of the lifting body. The second step is to pre-
dict the aerodynamic performance of the aircraft and its stability and control
characteristics when there is some ice on the airframe. For instance, in the case
of the flowfield over an iced wing, flow with regions of separation must be com-
puted. The successful development of the desired computer codes offers great
advantages:

1. Validated computer codes will substantially reduce developmental and cer-
tification testing. This results in reduced time and cost of aircraft develop-
ment.
2. Numerical simulation, which is an alternative to extensive flight testing, will
reduce the high risk of flight testing in icing conditions.
3. Accurate numerical simulations will allow earlier assessment of the effect of
ice protection requirements on new aircraft designs.

This section describes the application of a CFD method to predict ice accre-
tion and aircraft performance degradation due to icing, as discussed by Cebeci
and Besnard [23]. The results are presented for the NASA research aircraft
which is a modified DeHavilland DH-6 Twin-Otter. This aircraft is equipped
with electrothermal anti-icers on the propellers, engine inlets, and windshield.
Pneumatic de-icer boots are located on the wing outboard of the engine na-
celles, on both the horizontal and vertical stabilizers, on the wing struts, and
on the rear landing gear struts. The pneumatic de-icers located on the verti-
cal stabilizers, wing struts, and landing gear struts are nonstandard items that