Prandtl Glauert Singularity

[tigisfat]

The Prandt Gluert singularity is the reason for vapes, shock cones and a few other similar effects on aircraft. It's easily explained as moisture rapidly condensing in a pressure differential. There is a common trend that drives me nuts: TONS of pictures and videos of the singularity around fast moving subsonic aircraft labeled as supersonic. We should be smarter and better than that!! It seems every generation has often told lies passed as scientific fact, and this is fast becoming my generation's rediculous farse.

I distinctly remember a teacher telling us that if a penny were dropped off the Empire State building, it would easily kill someone. Gimme a break!! If you were to follow the same scientifically bankrupt reasoning, an automobile dropped from a thousand feet would easily beam a hole straight through the planet and come flying out of the ground somewhere in India. Then the danger is not as much having it fall on you, it's the hazard of having an object the size of an automobile come flying out of the ground if it was dropped onto the polar opposite of where you're standing. Anyone with any scientific background knows that there must be a terminal velocity for the penny and the car.

Back to the Prandt Glauert Singularity. Do your part and educate people!! Don't let your non-flying neighbors and family members think that moisture condensation means an aircraft must be flying at a million miles an hour.

Here are a few examples:

[YOUTUBE]iRGacDGUGtE[/YOUTUBE]

The worst is when a shock cone is called a "sonic boom:

[YOUTUBE]CF7h3EwBPjc[/YOUTUBE]

[Quixoticish]

I've argued this one so many times with people that I've just given up now. Let people believe what they want to believe, if people are so ignorant as to not even accept that they may be mistaken then I can't be bothered wasting time discussing things with them.

If memory serves there is a cracking picture of B2 sporting a rather fetching singularity that can often be used to prove that it isn't a visual representation of breaking the sound barrier, that sometimes works.

[kilo delta]

If memory serves there is a cracking picture of B2 sporting a rather fetching singularity that can often be used to prove that it isn't a visual representation of breaking the sound barrier, that sometimes works.

I remember seeing a pic of an A-10 (and F-117's!!) in a similar state. :d

[ARDVARK]

Goodpost, I must have been one of those misinformed youngsters who was told it happens when aircrafts hit 'supersonic'...as of this thread I am now a wiser man.

[wombat666]

Hardly supersonic, but another 'Urban Myth'.
It's common (but less dramatic!) when a high performance car race is run in Malaysia or China under high humidity and temperate.

[cheezyflier]

much worse are the peple who believe in chemtrails. i have a neighbor who has been trying to convince me of this for a long time. :angryfir:

[Quixoticish]

much worse are the peple who believe in chemtrails. i have a neighbor who has been trying to convince me of this for a long time. :angryfir:

Argh, this one annoys me so much. I've had otherwise rational people whom I respect greatly try to convince me of mysterious chemtrails in the sky, usually with complete hysteria.

[jhefner]

The wikipedia entry makes it sound like a transonic/supersonic effect.

http://en.wikipedia.org/wiki/Prandtl...rt_singularity
-James

[djscoo]

much worse are the peple who believe in chemtrails. i have a neighbor who has been trying to convince me of this for a long time. :angryfir:

http://www.youtube.com/watch?v=&play...os=pHmn80bk-WU
[YOUTUBE]_c6HsiixFS8[/YOUTUBE]

[Cratermaker]

djscoo, That is too funny. When someone mentioned chem trails, I thought of that video. I guess you did too!

[tigisfat]

much worse are the peple who believe in chemtrails. i have a neighbor who has been trying to convince me of this for a long time. :angryfir:

I wouldn't even bring that up around Boxcar, he believes in them, and very strongly to boot.

[tigisfat]

The wikipedia entry makes it sound like a transonic/supersonic effect.

http://en.wikipedia.org/wiki/Prandtl...rt_singularity
-James

I'd say that it's most easily found and described in transsonic aircraft, but it can and does appear in aircraft moving much slower. It happens on airliners on landing and takeoff if it's really humid. The concorde is a prime example.

[tigisfat]

http://www.youtube.com/watch?v=&play...os=pHmn80bk-WU
[YOUTUBE]_c6HsiixFS8[/YOUTUBE]

HAHAHAHAHQA!!! That was awesome. Clearly, she's someone who's respected in scientific communities. There is a new language being spoken by scientists of the highest levels, and they use words like "thrist" and "constituional".

[Naismith]

Arrrgh, someone that stoopid should not be able to operate a camcorder, so I can only assume that some else filmed it whilst she drawled over the top.

[safn1949]

That video is priceless.....I am speechless at the level of ignorance displayed in it.

[Ken Stallings]

What is the terminal velocity of a penny? Is it significantly different when it's dropped edge on versus flat side down?

I'm thinking a penny dropped from a high rise could be lethal. It has a rather high mass to aerodynamic drag ratio.

Flipping a coin several feet into the air and letting it rap you on the top of the head won't feel good, but I strongly suspect a penny doesn't reach terminal velocity in ten feet or less.

Cheers,

Ken

Edit: My favorite reality show (actually the ONLY one I watch) Mythbusters, did their scientific tests. They say a falling penny's terminal velocity is between 35 and 65 mile per hour. That's fast, but it seems not lethally fast. It might break skin and might cause a concussion, but should not be lethal. It can make a mar on concrete and dent asphalt. That's pretty significant. I certainly would not volunteer to put my noggin on the line for science!

http://www.mythbustersfanclub.com/mb...nt/view/26/27/

[Wing_Z]

http://www.youtube.com/watch?v=&play...os=pHmn80bk-WU

There is a remix:
[YOUTUBE]4tL-_ir518E&feature=player_embedded[/YOUTUBE]
You need to light the stick of incense first, perhaps, and darken the room... :d

[tigisfat]

What is the terminal velocity of a penny? Is it significantly different when it's dropped edge on versus flat side down?

I'm thinking a penny dropped from a high rise could be lethal. It has a rather high mass to aerodynamic drag ratio.

Flipping a coin several feet into the air and letting it rap you on the top of the head won't feel good, but I strongly suspect a penny doesn't reach terminal velocity in ten feet or less.

Cheers,

Ken

Edit: My favorite reality show (actually the ONLY one I watch) Mythbusters, did their scientific tests. They say a falling penny's terminal velocity is between 35 and 65 mile per hour. That's fast, but it seems not lethally fast. It might break skin and might cause a concussion, but should not be lethal. It can make a mar on concrete and dent asphalt. That's pretty significant. I certainly would not volunteer to put my noggin on the line for science!

http://www.mythbustersfanclub.com/mb...nt/view/26/27/

Ken, I figured you of all people would think Mythbusters is a crock of crap. They have no idea what they're talking about and their experiments are garbage. I used to love the show until the did the episode with the ultralights. That was it, and it tainted their credibility to me.

There have been other measures of a penny's 'lethality', and suffice it to say that you can throw a penny at someone harder than it would hit them if dropped. Either way, terminal velocity varies depending on how it falls, of which they identified four common ways for a penny to fall. The fastest recorded was not very much at all.

Ken, the penny is not likely to break your skin and give you a concussion.

[djscoo]

Mythbusters frustrates me because the concept is fascinating, but they go for entertainment value over scientific accuracy too often. I would rather watch them work out formulas and try to determine the drag coefficient of a tumbling coin for half an hour than sit through the pseudo-science they sometimes try to pass.

[Ken Stallings]

Mythbusters frustrates me because the concept is fascinating, but they go for entertainment value over scientific accuracy too often. I would rather watch them work out formulas and try to determine the drag coefficient of a tumbling coin for half an hour than sit through the pseudo-science they sometimes try to pass.

Their science is pretty solid.

In the case of the penny, they constructed a variable speed wind tunnel where the speed near the wind source was 65mph and near the entrance where the pennies were dropped was 35 mph. The pennies hovered in the middle.

I think that's a very valid scientific method to judge the average terminal velocity of a dropped penny.

Of course a penny can be thrown faster than terminal velocity, and it then slows down until either gravity brings it to the ground, or wind resistance reduces the speed to terminal velocity.

Ken

[TeaSea]

I can remember in my CFS3 days arguing with someone about the "smoke" coming off bullets being fired from an aircrafts guns in gun camera videos.....

I dared to point out that it was water vapor....and was soundly shouted down.

[djscoo]

I just rewatched the penny segment on youtube. They determined with the wind tunnel that the terminal velocity of a penny was somewhere between 13.5 and 30 m/s. Adam then says "It's going out on television that we did the experiment that showed exactly how fast the penny goes. No math, no ideas, no timing off a building, we got it right here". For some reason they decided to go with the value that was highest rather than estimate or measure the velocity of the area where the penny was hovering. From then on they referred to the velocity the penny was being fired at as the terminal velocity, when in all actuality the value was totally contrived.

The penny myth is actually not their worst work, the episode where they tested gun barrels getting chopped off with samurai swords was painful to watch.

[tigisfat]

The penny myth is actually not their worst work, the episode where they tested gun barrels getting chopped off with samurai swords was painful to watch.

I beg to differ. The ultralight episode was the worst. They asked a bunch of people if a stationary airplane could take off from a treadmill. They said no. They laid a tarp onto the ground and placed the ultralight at point x. They moved the tarp backwards at say, 40mph and then accelerated the airplane foward at 40mph, so that the wheels were doing 80. The plane took off, as it had not remained stationary but had accelerated forward from X; they declared all the pilots and others wrong on the spot. I wanted to climb through my TV and strangle them.

[Ken Stallings]

I beg to differ. The ultralight episode was the worst. They asked a bunch of people if a stationary airplane could take off from a treadmill. They said no. They laid a tarp onto the ground and placed the ultralight at point x. They moved the tarp backwards at say, 40mph and then accelerated the airplane foward at 40mph, so that the wheels were doing 80. The plane took off, as it had not remained stationary but had accelerated forward from X; they declared all the pilots and others wrong on the spot. I wanted to climb through my TV and strangle them.

You'd be wrong then.

Aircraft are not powered with the wheels. They are powered with the prop. That makes all the difference in the world as the free turning wheel would not be rolled back equal to the velocity of the treadmill. The source of velocity has to be countered for takeoff to be affected. The threadmill works for a car, but not an airplane.

This discussion was held at the AOPA forums over a year before Mythbusters did their work. In fact, they did the work because of all the letters they got from AOPA members who wanted to see it tested out. Most pilots on the AOPA forums concluded that an aircraft would takeoff on a treadmill set at rotation speed and the Mythbusters experiments validated that conclusion.

In addition to the actual aircraft, they also replicated the results with a RC aircraft on an actual treadmill. That helped dissipate views that their version of a treadmill for the actual aircraft wasn't valid.

Cheers,

Ken

[AckAck]

Am I reading you guys wrong? The episode I saw was that they COULDN'T take off a stationary aircraft (scale model from a treadmill, or full size on the ground), but they did take off an aircraft (scale model from a treadmill, and full size on that tarp) once the aircraft reached flying speed (regardless of the wheels).

Ken in his first paragraph says that an aircraft's propulsion has nothing to do with the wheels (correct), and in the next paragraph says that an aircraft takes off when placed on a treadmill at rotation speed. (Um, no, that's not what I remember them finding at all). Two contradictory statements...

(Most pilots on the AOPA forums concluded that an aircraft would takeoff on a treadmill set at rotation speed and the Mythbusters experiments validated that conclusion.)

I'll have to find the episode, but I think they busted that myth, not confirmed it... If they did confirm, they were horribly wrong, just like the people on the AOPA forums...

Brian

EDIT - apparently I AM reading you wrong - the myth was that it wouldn't take off at all, not that it would take off with 0 ground speed. Nevermind... I apologize - I am misrepresented the myth, and can't figure out what the fuss is about