Abstract
Mounting engines on the wing causes complex wing
flutter characteristics. The location of the engine mass
and the stiffness of the pylon relative to the wing are
important in preventing hazardous wing flutter. In
addition, if the nacelles are installed over the wing,
aerodynamic interference between the wing and the
nacelle may cause unfavorable flutter characteristics, in
particular, at transonic speeds. The flutter
characteristics of an over-the-wing engine mount
configuration obtained from theoretical analyses and
low speed and transonic wind tunnel tests, are
presented.
Introduction
Small business jets are becoming very popular among
business people. Market surveys show that demand for
comfort, in particular, a large cabin, is critical to the
success of business-jet development. Mounting the
engines on the wing instead of the fuselage is one way
to maximize cabin size by removing the engine support
structure from the fuselage.
Recent research by Honda R&D shows that an optimum
over-the-wing engine mount configuration minimizes
aerodynamic interference at transonic speeds and
reduces wave drag such that the range parameter for the
over-the-wing engine mount configuration is about five
percent higher than that of the conventional rear-fuselage engine mount configuration (ref. 1).
Mounting the engine on the wing, however,
significantly changes the vibration characteristics of the
original wing and, as a result, influences the aeroelastic
characteristics (e.g. ref 2, 3). In addition, the nacelle
aerodynamic load and interference may affect the flutter
characteristics (e.g., ref.4). Positioning the engine ahead
of the elastic axis of the wing to increase the flutter
speed is a well-known design rule, which has a marked
effect on the configuration of modern transport aircraft.
For the present over-the-wing engine mount
configuration, however, the engine is positioned aft of
the elastic axis of the wing and the aeroelastic
characteristics are, therefore, considered to be critical.
Also the aerodynamic effect on the flutter
characteristics induced by having the engine nacelle
positioned over the wing must be carefully evaluated,
especially in the transonic flight regime. It is necessary
to validate these characteristics for the present
over-the-wing engine nacelle configuration.
In the present study, the engine location relative to the
wing was first systematically varied and the effect on
the flutter speed was studied theoretically and in the
low speed wind tunnel tests. (The engine pylon is rigid
in this study.) The general tendencies were evaluated
using a cantilever-wing flutter model. The study
determined the effect of the chordwise and spanwise
location of the engine on the flutter speed.
The pylon stiffness was then varied to alter the
side-bending frequency, yawing frequency, and pitching
frequency of the engine-pylon mode. The effects on the
flutter characteristics of the over-the-wing engine
mount configuration were thus quantitatively evaluated.
The results show that the flutter characteristics change
at a certain frequency ratio.
To investigate the flutter characteristics under the
aerodynamic influence of the nacelle for the
over-the-wing engine mount configuration, transonic
flutter tests were also conducted. These tests show that
there is no large flutter speed reduction at the transonic
dip or undesirable flutter characteristics for the
over-the-wing engine mount configuration.

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