This is my first crack at writing a technical report for an assignment in a mechanical engineering course at the University of Alberta. How was the clarity? Any feedback on how I can improve?

1. Nacelle Design for a Low-
Boom Supersonic Aircraft
Eric Williams
2nd Year Mechanical Engineering Student
Prepared: June 2012
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2. Abstract
Although supersonic aircraft is a well-established technology, its use in the public
domain is effectively banned by aviation regulators. The primary concern that prevents
civil supersonic flights is the loudness of the sonic booms these aircrafts
create.Gulfstream, an aerospace company, has done much research into the designing
of supersonic jets that will satisfy regulators’ noise requirements. To achieve this, they
have designed and tested a new type of engine which is enclosed by a high-flow bypass
nacelle. Through heavy-computation tools and wind tunnel experiments, Gulfstream has
shown thatthis new nacelle can reduce drag, and can lower the sonic boom of the
aircraft.
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3. Introduction
The aviation industry is striving to make supersonic transportation available to the
public. This would accommodate the business market that would be willing to pay more
for faster transportation. However, for overland flight to be permitted by regulators, the
noise level and the sonic boom of supersonic aircraft must be reduced. Sonic booms
are fluctuations of pressure caused by aircraft reaching speeds that exceed the speed
of sound. In an attempt to satisfy regulators, Gulfstream Aerospace Corporation has
designedseveral new components forsupersonic aircraft that would produce lower
booms and quieter noise levels.As well, they also insure that their new designs would
not compromise the aircraft’s functionality.[1]
Among the most challenging parts to design was the nacelle: the cover housing for an
engine. Gulfstream’s solution was a high-flow bypass nacelle, in which a portion of the
incoming airflow would be directed around the engine itself in a separate flowpath. (see
Figure 1) Without this type of nacelle, the gearbox attached to the engine would have be
enclosed in a bulge which would increase the boom and drag of the aircraft significantly.
(see Figure 2) The bypass airway of this nacelle also reduces inlet spillage. [1] Inlet
spillage occurs when the intake of air supplied exceeds the intake of air required by the
engine. Excess air is then “spilled over” the edge of the nacelle which increases drag
and sonic boom. However, in a high-flow bypass nacelle, the air is simply redirected to
the bypassairway.
Methods
Throughexperimentation in wind tunnels and through airflow analysis using CFD
(computation fluid dynamics), Gulfstream has made several tests on its nacelle concept.
Initial testing was done in 2009 in the subsonic wind testing facility at the University of
Illinois where they studied the airflow of the bypass airway. [2] They used rapid-
prototyped material to simulate the components of the engine at 1/6th scale.These
components represented the main body of the engine, a gearbox fairing, and two
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4. supporting crane beams. (see Figure 3) A gearbox fairing is an aerodynamic structure
surrounding the gearbox which reduces drag from airflow. In their experiment, they
studied the airflow on each of following configurations: 1) the empty circular wind tunnel;
2) the engine model within the wind tunnel; 3) the engine model with the gearbox fairing;
4) the engine model with the gearbox fairing and supporting crane beams. [2]
In 2010, Gulfstream went to the NASA Glenn Research Center, where they could
conduct tests inthe 8-x6-foot Supersonic Wind Tunnel. [3] There, they tested two
different models. (see Figure 4) One was a dual stream inlet, which would model as the
engine hardware and the surrounding bypass layer. Secondly, they used a single
stream model which allowed room for a camera to visually capture the effects of vortex
generators on the boundary layer. When a boundary is separated from a surface, very
high drag occurs. The vortex generators are micro-ramp structures which were tested to
see if they could prevent a separation of the boundary layer with the surface. Both
models were tested at various angles from the air flow, known as angles of attack.
Results and Discussion
Gulfstream’s wind tunnel experiment at the University of Illinois provided useful data on
the effect of the components on dynamic pressure and velocity of the flow. Most
notably, the fairing caused a major reduction in dynamic pressure and velocity of the
downstream air of the lower side of the model. Conversely, the fairing increased the
dynamic pressure of the velocity of the downstream air on the upper side of the model.
(see Figure 5) This indicated that a better flowpath channels would need to be
considered in future designs. In addition, the data accumulated in this experiment was
used to develop more accurate CFD (computational fluid dynamics) models. [2]
The wind tunnel experiment at the NASA Glenn research centre yielded very positive
results. The models performed well and a maximum pressure recovery of 96%, even at
high angles of attack.Also, it was shown that a near-zero inlet spillage is possible.
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5. Figure 6 shows the relationship between the pressure recovery and the mass flow ratio.
The mass flow ratio is the ratio between the air intake of the inlet and the total air
supplied. In addition, the shock on the lip of the cowl, the casing of the nacelle, was also
very low at supersonic wind speeds. The vortex generators were observed to have very
little effect on overall inlet performance. [3]
Conclusion
Gulfstream’s goal was to design a turbofan engine that would produce a relatively low
boom. They decided the best way to do this was to design a streamlined nacelle with a
bypass flowpath. After creating some prototypes and models, they tested the concept at
wind tunnel research facilities. After gathering data and using CDF analysis they
concluded that the high-flow bypass nacelle design for had good pressure recovery,
good performance, low shocks and reduced drag. All these factors indicate that the
concept is functional and will produce a lower sonic boom. In conclusion, the concept of
a low boom aircraft is feasible and merits more research.
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6. Figure 1 – Comparison of a traditional
nacelle with a high-flow bypass nacelle. [2]
Figure 2 – Comparison of engines with
and without a nacelle bypass [2]
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7. Figure 3 – Computer-modeled rendering of the components
used in the wind tunnel test at the University of Illinois [2]
Figure 4 – Single stream and dual stream
models used in the Glenn Research Centre [3]
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8. Figure 5 – Normalized dynamic pressure mapping of engine model
with gearbox fairing. (Downstream) [2]
Figure 6 – Pressure Recovery vs. Mass Flow Ratio at various angles
of attack (AOA) and at Mach 1.7 wind speed. [3]
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9. References
[1] http://www.aviationweek.com/Article.aspx?id=/article-xml/AW_06_04_2012_p50-
461842.xml&p=1 (Last accessed July 2, 2012)
[2]http://www.ae.illinois.edu/icing/papers/09/AIAA-2009-4207-202.pdf (Last accessed
July 2, 2012)
[3] http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110014468_2011015053.pdf
(Last accessed July 2, 2012)
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