Re: Noise, size, and bypass?

From:         ehahn@wren.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         21 Dec 95 14:01:43 
References:   1
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>In article <airliners.1995.2003@ohare.Chicago.COM> Bob_Crownfield <Bob_crownfield@qc.edu> writes:

<snip>

> Is there a relation between diameter, EPR,thrust, and noise?

> I assume (Yes,I know about ass u me!) that EPR and exhaust velocity are
> directly related to noise. How does bypass relate, if at all? It seems
> that thrust and noise can vary very widely from engine type to engine
> type.

<snip>

To generate thrust, air (mass) has to be accelerated in the opposite
direction to the thrust (Newton's 2nd and 3rd laws of motion).

In general, it is more (fuel) efficient to accerate a large amount of
air a small amount, rather than a small amount of air a large amount,
for a given desired thrust.  (Mass flow, thermodynamics)

Finally, noise is produced as something like the 8th power of exhaust
velocity.

Therefore:  The larger the inlet area, the more air mass can be
accelerated by the engine, thus decreasing the amount of air
acceleration required for a desired thrust.  The decreased amount of
acceleration means that the exhaust velocity is much slower.

Applying this to what you see on the ramp:

The small inlet engines common on 60s/70s aircraft (B727, B737-200,
DC9) accelerate a small amount of air quite a bit, thus leading to
increased exhaust velocity and noise.  The inlet size was limited by
the materials technology available in building the engine.

The larger engine inlets on widebody and recent narrowbody aircraft
(B737-300+, A320, B757, MD80/MD90) accelerate a larger amount of air a
lesser amount, thus leading to a lower exhaust velocity and less noise
(compared with small inlet engines.)  Note that the additional inlet
area more than offsets the increased thrust on these engines, still
leading to a net quieter engine.

As for EPR and thrust, they are related to, but are not as closely
related to noise as the above factors.  Exhaust velocity (& noise)
will increase with increasing EPR and thrust, all else being equal.

Bypass is simply a measure of how much of the air is mixed with fuel
vs. how much bypasses the core via the fan. Bypass to a large extent
is a function of how much inlet area you want to cover, as you don't
need all energy if you tried to burn all of the air passing through
both the fan and core.

Finally, the amount of secondary noise treatment (analagous to a car
muffler) added to the engine varies between models, but the primary
driver of noise is exhaust velocity.

Hope this helps,
ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.