|
|
|
|
Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
|
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.. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
|
Sat Dec 25, 2004 12:45 am |
|
|
|
|
|
|
|
|
|
|