Nacelle Design for a Low-

Boom Supersonic Aircraft

Eric Williams

2nd Year Mechanical Engineering Student

Prepared: June 2012

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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 that this new nacelle can reduce drag, and can lower the sonic boom of the

aircraft.

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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

designed several new components for supersonic 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 bypass airway.

Methods

Through experimentation 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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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 in the 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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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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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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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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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]

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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