Bullroarer

[KG3]

Is anyone familiar with the bullroarer?   https://en.wikipedia.org/wiki/Bullroarer 

It's an ancient instrument thought to date to about 18,000 bc. "A bullroarer consists of a weighted airfoil (a rectangular thin slat of wood about 15 cm (6 in) to 60 cm (24 in) long and about 1.25 cm (0.5 in) to 5 cm (2 in) wide) attached to a long cord"

https://www.youtube.com/watch?annotation_id=annotation_744186113&feature=iv&src_vid=yJN1hDrXtto&v=5PgGDMmBtDg    

I'm curious about the aerodynamics of this.  I know if you simply throw a similar slat of wood like a boomerang (without the cord) it will (sometimes) produce the same roaring sound and travel for a bit parallel to the ground and even sometimes gain a bit of altitude.  It spins around the long axis (not the axis perpendicular to it's flight like the boomerang).  If you look at the demonstration in the video above you can see that the bullroarer produces some lift.  As the bullroarer is swung about it spins twisting up the cord then reverses direction still producing the same sound and lift.  

Is this somehow related to the Magnus effect?  I can't seem to find this described anywhere.  

[StrandedonEarth]

Is that not the same "long distance communication device" used in Crocodile Dundee?

As for generating lift, I believe that's just centrifugal force. Swing any weighted cord around fast enough and it will tend to rise up so that it spins in a circle instead of a cone.

 

[KG3]

You might have seen it in Crocodile Dundee.  It's usually associated with aborigines in Australia but it's been around along time.

"Anthropologist Michael Boyd, a bullroarer expert, documents a number found in Europe, Asia, the Indian sub-continent, Africa, the Americas, and Australia."

It seems to create more lift than it should just through centripetal force.

 

[Gargamel]

I used to make these all the time as a kid.   Just grab a ruler that has a vaguely airfoil cross section, attach a string at one end, and ROuruurruruurrrururuurururrrooooo away. 

The key is to get the foil rotating along it's long axis before spinning it around on the string. 

Here's my guess from playing with these things 30 some years ago. 

(Quick definitions here: Spinning: rotation around the users hand, on the end of the rope.  Rotation: the spinning along the long axis of the foil.   Otherwise the following statements are going to be mighty confusing)

As it's spinning around the rope, it's creating a bit of lift, vectored on one edge of the foil, inline with the direction of axial rotation.  This causes it to continue to rotate in that direction, twisting up the cord some.  The trailing edge of the foil causes a bit of a vacuum, and this cavitation causes the sound, from the collapsing of the vacuum behind the foil.  The minuscule amount of lift it creates, (since there will probably be opposing lift vectors as it rotates, but the one vector will be slightly more than the other, given angles of attack and such)will eventually be overcome by the force of the twisted string, which shows how little lift force there actually is.   The string will eventually cause it stop rotating, leading to a slight pause in the sound, and then start rotating it in the opposite direction.  This will make the previous "trailing edge" into the new "leading edge", and the whole process starts over.  

As long as your initial rotational speed is enough to create enough lift to overcome the most stable neutral position it wants to settle to, it will roar.    If you don't rotate the roarer before spinning it, it will often not produce a noise louder than a soft whoosh.  As it spins, tensions in the rope (as you are holding onto it in a fixed position, not letting the rope rotate in your fingers) and probably a bit of precession will often cause the rope to start the airfoil spinning.   I think that is why, if you use a flat board, you can sometimes get a soft roar out of it too.  The rotation is causing cavitation and a sound.    

I had always used symmetrical airfoils, so the sound was the same in both directions of rotation.  But I would guess that if you used a more asymmetric NACA style airfoil, you could probably get two different tones, maybe even be tunable based on the speed of spin.    Possibly, also using an elastic style string would allow it to build up more rotational energy in the string before being reversed, causing it to rotate faster each time it reversed.  

1 hour ago, KG3 said:

You might have seen it in Crocodile Dundee.  It's usually associated with aborigines in Australia but it's been around along time.

"Anthropologist Michael Boyd, a bullroarer expert, documents a number found in Europe, Asia, the Indian sub-continent, Africa, the Americas, and Australia."

I would guess that since it's so very simple to make one, a stick of the right random dimensions would work, that it was accidentally discovered in many different places over time.  Some dude had a length of vine attached to a stick one day, for some reason, and he got bored, started twirling the board on the string, and whoa, noise.   Hey Goorb, do that again!  Hey look, Ongt made a better one!  And so on. 

 

Random Hypothesis:   The tonal qualities of the bull roarer are due to the spin speed, rotational speed, and the amount of cavitation.  I posit that you could design a multi tone roarer, with a few section sof different sized (width and thickness) airfoil to create different tones.   With a little R&D, these tones could produce harmonics that would greatly increase the volume, and ergo audible range, of the roarer.  Since the spin speed is what gives the roarer it's energy, the actual tones and ergo the harmonic, would be very dependent on the users ability to maintain a fixed spin speed.  Even then, the rotational energy being stored/dissipated in the rope will affect the rotational speed, altering the tones.   So, with a properly harmonic bull roarer, you would get an oscillating wave.  A normal roar with a brief bit of very loud, then a normal roar, a bit of whoosh as it stops and reverses, and then the whole cycle again. 

 

-----    Sometimes the stuff that falls out of my head amazes me.  

Edited August 6, 2018 by Gargamel

[Zeiss Ikon]

The general principle of lift generation of the bullroarer can be demonstrated easily enough.  Take a piece of flat balsa (like one wing from a dime store glider), hold it level, and just release it a little forward.  You'll see it spin rapidly, with the leading edge rising.  This occurs because the center of lift of a flat plate is well ahead of the center of the plate (where the center of mass resides).  The plate builds enough momentum to continue to flip through the stalled regime, until it starts to develop lift again with the other side up -- which supplies more lift ahead of the mass, and continues or increases the rotation (you could also see the plate as flying through a very tight loop, with its air-relative velocity always generally toward the leading edge, so that it never stalls).

I don't understand how a bullroarer can reverse its rotation and still lift upward, however -- by the time the rotation has built up some speed, the Magnus effect contributes substantially to the total lift, and reversing the rotation ought to drive the paddle into the ground.  Perhaps if the second view of the lift (flying through tight loops very rapidly) is correct, the loops can reverse and produce near-zero net lift, so the force of the cord holds the thing up.

The noise comes from pressure excursions as the airflow over the plate changes rapidly -- sometimes the pressure is very high on one side, and very low on the other; other times, it's nearly the same.  This will cyclically alter the pressure (which is the definition of sound), and since this is happening only a few tens of times per second, the pitch of the resulting sound is quite low.

[K^2]

Thin Airfoil Theory 

A bullroarer behaves essentially as a thin airfoil. The important things to note are that if a thin airfoil travels at an angle to the incoming airflow, it will generate lift, and therefore, a pressure difference. That pressure difference depends on the angle to the flow. Secondly, the center of pressure is not at the center of the airfoil. It's one quarter from the leading edge. Since bullroarer is attached near the central axis, the pressure causes an instability. With no other forces present, that instability would cause the airfoil to bob back and forward, but once it is already spinning, Magnus Effect tips the balance, and the instability keeps increasing the rotation speed until the twist in the chord is sufficient to overcome it. At that point, the bullroarer stops very briefly, and twist in the chord starts the spin in the opposite direction.

Since pressure differential changes with the angle to the airflow, and angle constantly changes as bullroarer rotates around its axis, you end up with a source of periodic pressure change, also known as sound.

As @Zeiss Ikon pointed out, this is identical to how a thin rectangular sheet of wood behaves when dropped. But the above is the reason why it works this way.

[KG3]

Ok, thanks everyone!   Been doin a bit of experimentation.  I fished a thick piece of paper out of the recycling and cut a roughly 1"x8 1/2" strip.  I drop this from celling height (roughly) 8 feet and it reliably rotates as it falls and travels forward landing about 8 feet forward from plumb!  So that's a glide ratio of like 1 to 1 right?  Hey, I've made paper airplanes that have flown much worse than that!  Is it possible to improve upon this?      

[kerbiloid]

Now let's make a quaternion-based aerodynamic model of another paleolitic instrument.

Spoiler

 

P.S.
You can combine them by sticking the cap to the bullroarer.

Edited August 7, 2018 by kerbiloid

[Scotius]

You can do something similiar with a bottle partially filled with water (or a small bottle without water). Sound will be lower and softer. Warning for dog owners: My dog doesn't like it and starts howling immediately

[Zeiss Ikon]

On 8/6/2018 at 9:21 PM, KG3 said:

Ok, thanks everyone!   Been doin a bit of experimentation.  I fished a thick piece of paper out of the recycling and cut a roughly 1"x8 1/2" strip.  I drop this from celling height (roughly) 8 feet and it reliably rotates as it falls and travels forward landing about 8 feet forward from plumb!  So that's a glide ratio of like 1 to 1 right?  Hey, I've made paper airplanes that have flown much worse than that!  Is it possible to improve upon this?      

Generally, the lighter your plank relative to its area, the better it will "glide" in this kind of test.  I've seen balsa wing panels from dime store gliders and Sleek Streeks get 4:1 or better.  I'd suggest either the softest, lightest piece of balsa sheet you can find at the local hobby store, or a piece of foam cut from a picnic plate or the lid of an egg carton as having the best strength to weight ratio in this size range.

If you really want to stretch things, you could get some glue, make up a frame (just the outline of a rectangle like your paper scrap, with two or three cross braces), and attach a single layer of holiday wrapping paper or model airplane tissue.  The result will be lighter for its area than any solid material thick enough to be reasonably rigid, and ought to outperform anything heavier.  Look on YouTube for a video on "ten minute squirrel" to see how to build the frame and cover it at the same time.  It's actually possible to build an airplane using wings that work like this -- you'll need stiff rods running through wire loops on one surface of the wing frame, and the rest of a dime store rubber power model (for propeller, rubber, and tail surfaces).  Be prepared to adjust the balance quite far from where it would be with t a fixed wing, and don't expect it to glide like a sailplane, but it can be made to fly.

[wumpus]

Two things about bullroarers.  That sound should include subsonics (or at least subsonics are easiest to produce by bullroarers).  This is sometimes used in religious ceremonies as the subsonics have been known to evoke divine/supernatural feelings in people.

The other is that they were invented by this guy:

Who also invented golf.

[Green Baron]

In Europe they appear from the mid upper paleolithic on (Solutreen), made from bone the ones that lasted, frequently ornamented. It is a very impressive sound, a deep humming vibration, when 5 or more people let them circle. As to the aerodramatics, idk, They do need a starting impulse, as stated, they rotate, coiling up the string, then change direction when force gets too high, resulting in a swelling and ebbing noise.

Apparently they belong to the aerophones, maybe that helps searching more.

Edited August 10, 2018 by Green Baron

[KG3]

On ‎8‎/‎9‎/‎2018 at 6:38 PM, Zeiss Ikon said:

  ...Look on YouTube for a video on "ten minute squirrel" to see how to build the frame and cover it at the same time... 

I looked up "ten minute squirrel" on YouTube and didn't find anything that flew other than flying squirrels.  In fact there seems to be millions of videos of squirrels on YouTube, it's quite a rabbit hole to go down which is funny because squirrels are rodents and rabbits are lagomorphs.  So I abandoned that can of worms before it leapt out the frying pan and into whole nuther kettle of fish.  I think what you are telling me is that if the entire wing is very light with most of it's mass at the edges it will have more momentum to turn and more lift.

 

On ‎8‎/‎7‎/‎2018 at 12:39 AM, kerbiloid said:

Now let's make a quaternion-based aerodynamic model of another paleolitic instrument.

P.S.
You can combine them by sticking the cap to the bullroarer.

Yes, the flute seems to be even older than the bullroarer.  One of the earlier known examples dates back 35,000 years and made out of the wing bone of a vulture.  I'm sure combining the two could only improve the aerodynamic properties of the bullroarer!

 

On ‎8‎/‎6‎/‎2018 at 7:50 PM, K^2 said:

Thin Airfoil Theory 

A bullroarer behaves essentially as a thin airfoil... 

Interesting.  What do you call the movements of a bullroarer, does it spin, pitch or roll?  An aircraft movements are pitch, yaw and roll.  I'm assuming a frisbee and boomerang turn on the yaw axis, a thrown football turns on the roll axis, and a bullroarer turns on the pitch axis?  And an untethered bullroar which turns freely as it falls is called a squirrel?  As it moves forward is it considered to be gliding?  

Ok, I think I got this...

When the squirrel(?) is at the 10 o'clock(ish) position in it's cycle it's acting as a thin air foil so the center of lift is 1/4 of the way from the leading edge.  A fixt wing aircraft needs the center of balance and the center of lift to be the same to remain stable but with the squirrel (really... squirrel?) the 3/4 of the wing behind the center of lift is out of balance and so is going to drop giving the wing momentum to keep turning.  After the wing rotates up to and past 12 o'clock the Magnus Effect kicks in and gives it additional lift up to about 3 o'clock.  

It seems to run so smoothly but if you were to plot the movement of the axis of rotation on a graft would it travel in a straight line or would it dip a bit as the wing traveled through each cycle? 

[K^2]

3 hours ago, KG3 said:

What do you call the movements of a bullroarer, does it spin, pitch or roll?

If we think of it as an airfoil, definitely pitch.

[Green Baron]

It rolls (Edit: seen from the center, for the piece itself that'll be pitch) in the airflow around its longitudinal axis, coiling up the string in alternating directions.

Can a hangglider tuck - loosing its stability around the pitch axis, thus being alternately exposed to the airflow; usually destroys the thing because of alternating g-loads - help explaining the rotation ?

 

7 hours ago, KG3 said:

One of the earlier known examples dates back 35,000 years and made out of the wing bone of a vulture.  I'm sure combining the two could only improve the aerodynamic properties of the bullroarer!

Yep. There are two from southwestern Germany (Geißenklösterle) dated to 40.000bp, and one disputed from Slovenia, dating 42.000. Anyway it is early Aurignacien. One of the Geißenklösterle ones is made from a humerus of a whooper swan, the Slovenian from a femur of a cave bear (that's why it is disputed, it can't really be played). The other ones have been reconstructed and played. Sounds beautiful.

But one should place the bullroarers in a distance from flutes for the palaelolithic symphony orchestra, they are louder ... :-)

Edited August 14, 2018 by Green Baron

[Zeiss Ikon]

The Squirrel I tried to point to is a very simple model airplane, and the video was probably posted by Darcy Whyte.  Let me go look up a link...

Here you go.  My mistake; the title doesn't include the word "squirrel" even though that's the name of the model.  There's another one where a complete Squirrel is assembled (from pre-cut parts) in six minutes flat, but I wasn't mainly pointing to the airplane itself or the speed of assembly, just a simple way to build and tissue cover a frame to demonstrate bullroarer type activity with a very light plank.