Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Mechanical Engineering


Since the introduction of high bypass ratio turbofan engines, the noise problem from aircraft shifted primarily from jet noise to noise of turbomachine origin. The present dominant noise source is the inlet noise generated by the first compressor stage (or the fan). Acoustic liners to reduce this noise have met with only limited success since the noise reduction to cost ratio is quite low. There are also many technical problems with acoustic liners that are not completely solved. Another method is by means of the choked inlet. This device simply accelerates the air to sonic velocities in the opposite direction of the sound propagation and literally chok8s off the noise. It has been found that choking is acoustically effective and brings about large noise reductions of as much as 30 db* but creates numerous aerodynamic problems including large pressure loss, possible instabilities, etc. However, it has been observed experimentally (see footnote and also from recent tests at NASA-Langley by Dr. Lumsdaine) that some noise reduction can be obtained even when the inlet is operating subsonically. The pressure recovery is high, and instabilities are minimum. It should be noted that large noise reductions are really not necessary since this would be a form of “overkill", because the noise from other sources of the aircraft is usually about 10 to 20 dbs below the maximum noise level which comes from the inlet. Thus, the amount of inlet noise reduction required with the sonic inlet depends on the other noise sources from the aircraft, the. environmental conditions, and the flight conditions (takeoff or landing, gross weight, flap setting, etc.). This means that an automatic control system must be used so that the pilot. can preset the amount of noise reduction desired for the prescribed conditions; then the automatic control would maintain the aerodynamic and acoustic performance of the inlet. The purpose of this thesis is to design, analyze and test an automatic control system to fulfill those requirements.

Library of Congress Subject Headings

Automatic control

Acoustical engineering



Number of Pages



South Dakota State University