sound barrier

Submitted: Saturday, Jan 15, 2011 at 22:11
ThreadID: 83686 Views:2357 Replies:9 FollowUps:9
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Good evening
I was just wondering how many RPM your fan would have to do to break the sound barrier and what is the equation ?
Murray
Muzbry
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Reply By: Member - Craig F (WA) - Saturday, Jan 15, 2011 at 22:23

Saturday, Jan 15, 2011 at 22:23
As a rough guess I think my fan would need to do 9000 rpm. I dare say my motor would be everywhere before that happened!
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Reply By: dereki - Saturday, Jan 15, 2011 at 22:53

Saturday, Jan 15, 2011 at 22:53
I just tried a rough calculation and got about 15000rpm based on a 44cm diameter fan which is about what my car has.

But I have probably buggered my calcs up somewhere.


Speed of sound at ground level at 20degC is 343 meters/sec (Wikipedia)
Thats 20580 meters per minute.

Pi x Diameter of fan = circumference of the fan. ie distance travelled per revolution.

RPM revs per minute x 3.14 x Fan diameter = meters/minute
RPM x 3.14 x Fan Diameter = 20580m/min
RPM = 6554 / Fan diameter in meters
RPM = 6554 / 0.44
RPM = 14895

That would give a speed at the tip of the blade at the sound barrier.

Please correct me if I have made a mistake.

D
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Follow Up By: Member - MUZBRY(Vic) - Saturday, Jan 15, 2011 at 22:59

Saturday, Jan 15, 2011 at 22:59
Gday Mr D
Thanks , i will do the math when I am at work next. Imagine the racket at 14000 + rpm down the main street at mid night . It would be like a screaming banshee.
Murray
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Reply By: Dave B ( BHQ NSW) - Saturday, Jan 15, 2011 at 23:03

Saturday, Jan 15, 2011 at 23:03
G'day Muzbury, there is a bloke here with me who says "The Washerwoman" would know.

Dave
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Follow Up By: Member - MUZBRY(Vic) - Saturday, Jan 15, 2011 at 23:19

Saturday, Jan 15, 2011 at 23:19
Gday Dave
She is in bed asleep.
Muz
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Reply By: Roughasguts - Saturday, Jan 15, 2011 at 23:55

Saturday, Jan 15, 2011 at 23:55
Heres a link to a Cessna 210 at take off I use to fly these things from time to time there 3 blade prop hits the speed of sound at around 2700 RPM on take off.

Turn the sound up and enjoy, But the prop won't last long at that rate let
alone a fan

C 210 prop hitting the speed of sound.

Cheers
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Reply By: Member - John - Saturday, Jan 15, 2011 at 23:55

Saturday, Jan 15, 2011 at 23:55
Heater fan, air con fan or radiator fan? Just wondering..................
John

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Follow Up By: Member - MUZBRY(Vic) - Sunday, Jan 16, 2011 at 08:27

Sunday, Jan 16, 2011 at 08:27
Gday John
Imagine the noise in the car
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Follow Up By: Member - Doug T (NT) - Sunday, Jan 16, 2011 at 20:29

Sunday, Jan 16, 2011 at 20:29
Muzbry

You want to hear a car, listen to this , turn your volume UP

AWESOME

.
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Reply By: Mick O - Saturday, Jan 15, 2011 at 23:57

Saturday, Jan 15, 2011 at 23:57
WHY?
''We knew from the experience of well-known travelers that the
trip would doubtless be attended with much hardship.''
Richard Maurice - 1903

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Follow Up By: Member - MUZBRY(Vic) - Sunday, Jan 16, 2011 at 08:19

Sunday, Jan 16, 2011 at 08:19
Gday Mick o
Its the ramblings of an old man. I have a new lawn mower and was wondering as I walked around the yard. I thought i would be moderated if i said lawn mower . So far so good.
Murray
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Follow Up By: Hairs & Fysh - Monday, Jan 17, 2011 at 11:02

Monday, Jan 17, 2011 at 11:02
I reckon you woke up yesterday morning and doubled up on the blue pills.
Murray, you need one of them see through lidded boxes that let you know what day your up too.


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Follow Up By: Member - Mark G Gulmarrad - Monday, Jan 17, 2011 at 11:03

Monday, Jan 17, 2011 at 11:03
Jon
i do believe that Cruiser would be able to help him out with that.
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Follow Up By: Member - MUZBRY(Vic) - Monday, Jan 17, 2011 at 11:17

Monday, Jan 17, 2011 at 11:17
Thanks Jon
Thats all I need, pill packets like old grandma.
Muzbry
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Reply By: mikehzz - Sunday, Jan 16, 2011 at 00:14

Sunday, Jan 16, 2011 at 00:14
The velocity= radius X revs. Only catch is you have to make sure all the units are the same. So if the revs are rev per minute and radius is in cm then the velocity will be cm per minute. The radius never changes and velocity of sound is fixed so the revs required will be speed of sound in cm per minute divided by the radius of the fan in cm.....I think....it's been 40 years since I used that equation. My head is now spinning as well...
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Reply By: Cruiser .- Sunday, Jan 16, 2011 at 10:48

Sunday, Jan 16, 2011 at 10:48
Muzbry

At what altitude, as the speed of sounds varies with altitude.

Cruiser
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Follow Up By: Member - MUZBRY(Vic) - Sunday, Jan 16, 2011 at 12:52

Sunday, Jan 16, 2011 at 12:52
Gday Cruiser
The highest hill or mound around here is on the freeway, so i have to get the revs right at 100 kph or they will book me.
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Reply By: Member - Graham H (QLD) - Sunday, Jan 16, 2011 at 12:24

Sunday, Jan 16, 2011 at 12:24
Presumably a lawnmower wont get to any significant altitude.

Except of course if he lives in the Snowy area



LOL
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