No new questions on POA so here is a new old question - prop tip speed and noise

k9medic

k9medic

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ATP-H, CMEL, CSEL, CFI/CFII Airplanes and Helicopters
I recently sold by Cherokee 6/260 and moved into a C310Q. During the past couple of flights, passengers have mentioned how much louder the 310 is compared to the 6/260.

Given this, I decided to do some math to calculate tip speeds and see if there is a true noise increase or if it is just circumstantial.

The 6/260 tip speed at cruise (2400 rpm) calculated to .744 mach.
The 310Q tip speed at cruise (2450 rpm) calculated to .747 mach.
The 310Q tip speed at cruise (2300 rpm) calculated to .701 mach.

Measuring the distance from the cockpit to the blade tip on both aircraft, the distance is similar. The only thing I can think of that would make the perceived difference in noise would be the fact that there are now 2 engines making noise and the relationship of the seats to the tip path plane.

Thoughts?
 
There's more to prop noise in the cabin than prop tip speed. But then there's the fact that the passengers are sitting a lot closer to the engines in the 310 than they are in the single.
 
And harmonics. And wave patterns. And cabin noise insulation. And as mentioned you have two vs one engine.

Solution -
  • Tell them how cool they look in a twin and stop complaining:cool:
  • Spend more of the kids’ inheritance and add more sound insulation.
  • Only use one engine. :)
 
The tip speeds of the props might be the same, but the 310 is producing twice as much power as the other one. In other words, the frequency could be the same, but if you increase the amplitude the noise will go up.
 
Forty five year old insulation, which wasn’t that good from the factory.
 
WDD said:
And harmonics. And wave patterns. And cabin noise insulation. And as mentioned you have two vs one engine.

Solution -
  • Tell them how cool they look in a twin and stop complaining:cool:
  • Spend more of the kids’ inheritance and add more sound insulation.
  • Only use one engine. :)
Click to expand...


I have already mentioned the cool factor. The biggest selling point for my wife was the fact that we can get to our destination 45 minutes quicker.
 
A C310 is a cool SkyKing machine...buy a couple good headsets and grin :)

Cabin insulation might help to some degree, but my friend spent a lot of time insulating his 310 with that black foam and foil system..it is still very loud inside.
 
I can't help but note, that contrary to expectations, the Q-tips are noisier.
 
Have your passengers wear ear plugs under the head sets and boost the head set volume to max. Works especially well with ANR sets.
 
Domenick said:
I can't help but note, that contrary to expectations, the Q-tips are noisier.
Click to expand...
In addition, that’s going to be very painful putting headphones over them. Better options for reducing noise vs cotton swabs on a stick……….
 
I’d guess that it’s the difference between sitting behind the prop vs sitting on the side of the prop (actually between two props). The tip path plane, as you say.
 
The sonic shock wave off the prop seems to travel straight out sideways from the prop. You'll notice that as an airplane flies past you on takeoff. Loudest snarl is when you are close to in line with the prop plane of rotation. Coming at you, or going away, it's quieter. Now, that shock wave in a twin hits the side of the fuselage and creates lots of racket inside.



Jump ahead to 00:48 or so

DUCqoxpbaVo
 
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Dan Thomas said:
The sonic shock wave off the prop seems to travel straight out sideways from the prop. You'll notice that as an airplane flies past you on takeoff. Loudest snarl is when you are right in line with the prop plane of rotation. Coming at you, or going away, it's quieter. Now, that shock wave in a twin hits the side of the fuselage and creates lots of racket inside.

DUCqoxpbaVo
Click to expand...

Strategically placed sound deadoning material glued to the skin of the fuselage interior directly across from the props reduce the noise.
 
I suggest better headsets. Easiest, cheapest solution.
 
261A6188-E992-4A22-8BDC-5EEF8A777647.jpeg Waiting for someone -no one has mentioned it yet —
 
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I got a call yesterday from a buddy of mine asking if I could fly his 310 to pick him up so I figured that it would be a good chance to see if there was a DB change when the prop is pulled back.

Here is what I got -

2300rpm
ee09a5445bb8c6d75ea0ccc1f226326f.jpg


2450rpm
6d215cc9d7741fbe3e5804d574d08f44.jpg
 
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