Sound & Model Aeronautics
Howard Crispin, Jr.
LARGE ENGINES
Many modelers have considerable interest in large engines and large models because a high percentage of models being constructed and flown today fall into this power category. Typical power ranges begin at about .12 cubic‑inch two‑ and four‑stroke engines and upward. Engines in the .12–.18 cubic‑inch range are not especially difficult to operate quietly, though some engineering is required to "hide" or otherwise treat the muffler. When larger gasoline engines are used, however, a different set of problems arises.
A recurring question is propeller tip speed and its contribution to radiated noise. Two primary contributors to high sound levels are:
- lack of an effective muffler, and
- propeller noise (including harmonics).
For engines in the "big‑banger" range (about 5,500–7,000 rpm) the observed values are approximately:
- exhaust frequency: 92–117 Hz
- blade‑passage frequency: 184–234 Hz
On an A‑weighted sound level meter these components are relatively reduced (exhaust about −18 to −16 dB(A); blade passage about −10 dB(A)), but the levels would be much higher on a flat (C) scale. Using a spectrum analyzer often reveals high harmonics caused by an inefficient muffler and the propeller. A single dB(A) reading does not disclose the distribution of sound energy across harmonics, so a perceived muffler improvement may be masked by propeller‑generated harmonics.
This masking effect is often responsible for reports that mufflers did not produce the expected dB reduction; frequently the difference is a change in propeller. The problem can be aggravated in flight when engine RPM increases due to unloading. An airplane that checks reasonably on the ground may sound much worse in the air if RPM rises. Conversely, an efficient, well‑loaded propeller may sound quieter in flight because of reduced turbulent flow and different lift vectors at airspeed. A dramatic increase in airborne noise is often a sign of the propeller approaching cavitation conditions.
There must be compromise and careful testing to define operating limits that permit acceptable sound levels without lowering standards. For example, a 26‑inch propeller turning at 4,500 rpm has a tip speed of about 510 feet per second — a regime where propeller noise becomes significant. Engines that could develop full torque at low rpm would require very large‑pitch propellers; in many cases the engine will not supply the power needed to turn such a propeller at useful airspeeds, so design compromises are necessary.
The objective is not to lower standards but to determine practical limits and methods for achieving reduced sound levels through engineering and testing.
THE STUDY
The purpose of the study was to clarify the relationships among factors contributing to audible sound levels from model aircraft. One critical aspect is the directional characteristic of the sound source, quantified by the Directivity Index (DI), which describes the change in sound pressure level at different geometric locations around the source.
Key findings (piston‑engine cases, various exhaust/muffler configurations):
- At frequencies above 1,000 Hz, DI values were within about 5 dB of the space‑average sound pressure level, indicating nearly uniform radiation.
- At frequencies below 1,000 Hz, large DI variations were observed.
- At frequencies corresponding to engine firing and blade passage, trial exhaust cases showed DI variations of about +6 to −18 dB; muffled cases showed variations of about +6 to −12 dB.
- For the muffled engine, DI at the blade‑passage frequency was nearly constant with engine rotation except near angles where the exhaust was at approximately ±90°, where large variation in DI was noted. This was attributed to interference between discrete acoustic sources (blade passage and the second firing frequency).
- Pointing the exhaust away from the observer produced about a 7 dB drop with the unmuffled engine, but only about a 3 dB drop with the muffled engine.
- Overall, muffling reduced sound pressure levels at most frequencies; for piston engines the reduction from an unmuffled state was on the order of 20–30 dB in sound power for the cases studied.
These results align with model‑scale experience and one‑third‑octave band analyses: a good muffler will reduce many harmonics, but once exhaust noise is reduced the propeller becomes the dominant noise source. Therefore, propeller design and lowering propeller tip speed where feasible are crucial to further reductions in overall radiated sound.
Further parts of the study will address the effects of propellers on radiated levels and will provide practical conclusions and tables for model aircraft design and operation.
Transcribed from original scans by AI. Minor OCR errors may remain.
