I have a Saito 72 that I fly with a 14X4 propeller and I love this combonation. My question is that in wide open flight I can hear the propeller resonating like the tips are getting near or at the sound of speed. My question is exactly at what rpm does a propeller start to resonate? Is it at the speed of sound or at a slightly slower speed?

A prop can resonate at diffrent rpms. It depends on the natural resonance of the prop.

A 14" prop will start making the speed of sound noise you hear around 12,000 rpm.

Props will make noise, sometimes without getting even near the speed of sound. I have a Tower 46 two stroke. With a 10-6, it'll run around 14,400 rpm which is not near the speed of sound for a 10" prop but you can hear the prop sound over the sound of the engine.

Whatever it is, I know the four strokes begin making a certain resonant sound at the higher rpms

Props will begin to make a substantually louder noise somewhat before they reach the actual speed of sound.

Often the resonance of the airplane itself enters the picture with the covering acting as a drum. You can hear this sound over the sound of the engine or prop. This seems stronger with the four stroke engine operating at lower rpms during 3D exercises.

Enjoy,

Jim

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Whatever you say Dar.

You must do a lot of reading? Did you never read the book "How to Win Friends and Influence people"?

Enjoy,

Jim

Funny thing about resonance. When you hear it, feel it, or see it something is about to be destroyed. Usually self-destruct is sooner rather than later. It is not unusual for resonance frequencies to drive up the stress loads by two orders of magnitude.

You might want to consider doing something about the resonance noise. Even if the prop is strong enough to withstand the stress tremendous vibration loads are being transmitted to the firewall and to the rest of the airframe. The weaker glue joints will begin to fail first and won’t be noticed. The end result is most likely catastrophic failure at an inopportune time.

It usually ruins the day.

Bill S

Is it, in fact, resonance? Most probably it is cavitation and not resonance as resonance would most likely cause the prop to shatter into little tiny pieces. Cavitation is not necessarily destructive, but results in loss of efficiency of the prop as the air flow over the prop's airfoil becomes turbulent. Resonance occurs when the vibration frequency of the prop is harmonically the same as the wavelength of the prop and will eventually cause catastrophic failure of the prop.

doubledee

Propeller tests have shown, that noise produced sharply increases when the prop tip speed goes beyond 0.6 mach. Extra sonic waves are produced even before the flow reaches mach 1.
If mach 1 equals 18500 rpm, then the best speed for low noise will be below 0.6x18500=11100 rpm. This is lower in rpm, than is customary in the US. In Europe, a 14"prop would be kept below 10500 in locations with noise restrictions.

Explanation for above stated:
1) The airflow around a foil varies quite a lot, and in small foil sections near the leading edge it may approach sonic speed way before the foil speed itself does. This seems to set in at 0.6 mach as indicated by prop noise measurements. Much depends on the local foil thickness and nose radius.
2) The prop rotational speed is not constant, but is maximum at the end of the combustion stroke. In a 4-stroke engine, which is worse than a two stroke, the prop has to do a lot of work before the next combustion stroke, so it slows down a lot. At a mean speed of 18500, the max speed (if maintained) may wel be equal to 20.000 rpm, which is way above sonic.

Summing up 1) and 2), any speed above 11.000 rpm using a 14"prop is prone to produce extra noise, to the extend, that "supersonic slap" can be heard in flight when rpm rise.

Air propellers cannot cavitate. They can stall which is about as close to cavitating as they can get. Unless you are accelorate out of a hover or from a very slow speed your prop is not stalling.

Hi!
Which prop brand do you have?
JK