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supersonic blade


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Glen
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Joined: 16 Jun 2004
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supersonic blade

sooooooo, anyone reckon they could do it?
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Post Fri Dec 24, 2004 8:48 pm 
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Knightrous
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yeah, I've already done it. Was the day I connected a 10mm piece of lead onto my over worked P9000 motor with 12v up the leads. Twas a sight and a half listening to the cracking of the sound barrier Cool
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Post Fri Dec 24, 2004 8:53 pm 
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Big AL
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YEAH RIGHT Rolling Eyes are you now deaf from the sound barrier cracking in you shed a few meters away from you head.

Correct me if im wrong but don't they stop fighter jets from doing supersonic speeds over residetial areas cause they can shatter windows and cause burst ear drums

but hey if you can show me proof of this happening i'll stand corrected
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Post Sat Dec 25, 2004 12:08 am 
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Spockie-Tech
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You will need a hell of a lot of power to spin anything not airfoil-shaped into the super-sonic realm.

Some of the direct-drive ultralight guys (who couldnt afford a reduction drive to make their motors spin a bigger propellor at low rpm) would supposedly have the tips of their propellors go supersonic at high-rpm. All it results in is a really loud nasty "blaarrt" sound - like a unmuffled two stroke being played through a distorted amplififer.

You dont get the "crack/boom" that everyone associates with "breaking the sounds barrier" (as if its a piece of glass or something that can be broken). The "crack" most peopl expect is due to our experience of the very tip of a whip going supersonic briefly when cracked by a skilled whipper.. :winK

A rotary blade tracing the same circle over and over again probably makes different shaped wave-fronts and loses that crack sound.

And since wind-resistance goes up rapidly with speed, if your blade isnt a nice low-drag airfoil shape (not a sharp-tip, blunt back typical spinner shape), then you can just forget about it all together. Try sticking your hand out the window of a car moving at 100km/h and watch how much the drag changes as you tilt your hands angle of attack slightly. now imagine that mutiplied by 10.

and now for some maths to back it up..

assume a 1 meter length bar spun from the center. that gives us a diameter of 1 meter, and a circumference of 3.1415Meters (Pi x D). So we cover 3.14 meters every revolution of our bar.

we need to cover 1,225 Kilometers per hour (at sea level) to go supersonic.

Lets get that down to minutes and divide by 60 (since we're used to working in r/pMinute) - meaning our blade tip needs to cover 20.4 Kilometers of distance every minute.

with a circumference (distance around the edge) of 3.1415 meters divided into 20,400 meters, our blade has to reach 6,500 Rpm to *just* crack the supersonic speed.

Good luck finding a motor with enough power to push a 1 meter bar to 6500 rpm (unless its an airfoil), and if you do, all your power gets sucked up in making noise, which is sort of pointless anyway..

So, anyone wanting to build a super-sonic spinner, go ahead, I'll be there with a remote optical tacho to prove it if you think youve done it.. Cool
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Post Sat Dec 25, 2004 12:45 am 
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Knightrous
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8 cylinder 460 chevy motor gover drive gearing to get the 1mm 2kg bar up to 10000rpm. Shame it wouldn't fit in a robot Twisted Evil
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Post Sat Dec 25, 2004 11:35 am 
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Spockie-Tech
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Woo, a 1mm long bar that weighs 2Kg ? Must be made of condensed Neutron star matter...

and to go supersonic with a length of 1mm, lets see, you would need.. 6,493,713 rpm.. nearly 6.5 million rpm. mmm Nice gears..

Sorry, I'm being a pedantic shit stirrer.. I'll go back to christmas day now.. Laughing
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Post Sat Dec 25, 2004 12:00 pm 
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Knightrous
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Was ment to be 1m, but in my posting rush ended up as 1mm Confused Anta Baka!
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Post Sat Dec 25, 2004 2:30 pm 
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Valen
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Joined: 07 Jul 2004
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actuamally for minimum supersonic drag you want a sharp point, the best profile for something going transonic is athe tangent ogive (possibly oogive)
sharp point and blunted curve into the main body.
a supersonic 1m bar would be possible in a lightweight probbly and middle fer sure.
After all nick hits 3krpm in a feather.

would in the end be rather pointless as you would never get a decent hit in on anybody.
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Post Sat Dec 25, 2004 4:43 pm 
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Big AL
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hey if some one pulls it off video it and put it up on the site Very Happy
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Post Sat Dec 25, 2004 6:07 pm 
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Spockie-Tech
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quote:
would in the end be rather pointless as you would never get a decent hit in on anybody.


Very True.. some guy in the states built a Nitro Hobby Engine powered drum and discovered the thing spun so fast, the teeth couldnt get any "bite" and he was basically just "High Speed Sandpapering" the opponent.. Laughing

Which then prompted a big mathematical discussion on optimum Tooth Spacing vs Diameter vs RPM vs Energy Storage, some interesting reading.

Personally I like to see a bit more ingenuity and creativity in designs than "I can build a bigger hand-grenade spinner than you can", but the ways people deal with transferring of those amounts of energy is interesting in its way too..
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Post Sat Dec 25, 2004 6:29 pm 
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Knightrous
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I think the ultimate challenge with a spinner is making the biggest possible spinner while maintaining near perfect relibility. Maybe 1-2 spinners have actually gone close to achieving this.
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Post Sat Dec 25, 2004 9:04 pm 
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