Some Poop on Props

In preparing for the recent Scale Qualifier meet, I was concerned about the 100 dB noise limitation at the Hemet Model Masters Field.  The Zenoah G62 in my Ercoupe, with a Tatone one-pipe, expansion chamber type muffler, and a  22-10 APC prop, was developing about 7100 rpm and measuring 103 dB as measured by Tom Henning’s dB meter.  Now, only 3 dB over 100 seems very small, but we must remember that the non-linear dB scale is logarithmic, so that only a 3 dB increase equals doubling of the noise.  That’s right!  A 3 dB increase means that 103 dB is twice as noisy as 100dB.

We were able to reduce the noise of the G62 below 100 dB by slowing the engine to 6400 rpm.  Now, I didn’t want to lose all that power, so the solution was to install a prop that would absorb all of  the full throttle engine power at a lower rpm, rather than throttling back with the APC prop.  A Zinger 22-12, which has much more blade area than the APC, in addition to a higher pitch, brought the G62 down to 6100 rpm and solved the noise problem.

But we must remember that the power of an engine is proportional to both rpm and torque, so lower rpm usually means reduced power, even though the engine may be exerting higher torque.  As an example, the following theoretical thrust ratings for 22 inch props are extracted from the Zinger website:

Thrust of a 22 inch Propeller at various RPM and Pitches

RPM               10 Pitch         12 Pitch

6000                 21 lb.               24 lb.

7000                 28 lb.               32 lb.

8000                 38 lb.               45 lb.

Note that the thrust increases at the same RPM as pitch increases, and that the thrust increases at the same pitch as RPM increases.  If I had been running my 22-10 at 7000 RPM, I would have gotten 28 lb. of thrust, while a 22-12 running at 6000 RPM gives 24 lb. of thrust.  Note that the 22-10 at 6000 RPM gives only 21 lb. of thrust, compared to 24 lb. for the 22-12 at the same RPM.  But the noise would presumably be acceptable for both props at 6000 rpm.

Another interesting propeller consideration is that the tip speed of the propeller should not exceed Mach 1, the speed of sound. That is about 760 mph at sea level.  This means that large props must run at lower RPM than small ones.  The Bolly Prop Website On-line Handbook gives some charts showing the propeller tip speed for various prop sizes and RPM.  They say that prop noise increases greatly above 400 mph:

 Tip Speeds of Propellers, in Miles per Hour at Various Diameters and RPM__

          Diameter, inches_______

RPM               20                    22                    24

6000               357                 393                 428

7000               416                 458                 500

8000               476                 524                 571

As we would expect, tip speed increases with RPM and prop diameter.

Note that the tip speed of a 22 inch prop is below 400 mph at 6000 RPM.

In actual practice, the 100dB limit was not strictly applied at the Scale Qualifier meet.  Only a blatantly loud engine was actually measured.  With the 22-12 Zinger prop, my Ercoupe was perceived as not noisy enough to raise any eyebrows, and there was plenty of power for the flight maneuvers.