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:

iRGacDGUGtE

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

CF7h3EwBPjc

[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.
:173go1:

[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%E2%80%93Glauert_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=&playnext_from=TL&videos=pHmn80bk-WU
_c6HsiixFS8

[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%E2%80%93Glauert_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=&playnext_from=TL&videos=pHmn80bk-WU
_c6HsiixFS8


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/mb2/content/view/26/27/

[Wing_Z]

http://www.youtube.com/watch?v=&playnext_from=TL&videos=pHmn80bk-WU


There is a remix:
4tL-_ir518E&feature=player_embedded
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/mb2/content/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.
http://i595.photobucket.com/albums/tt32/walkeramerican/forum%20commentary%20pictures/1227953279246.jpg

[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

[tigisfat]

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).


Exactly my point. I refuse to debate anything that doesn't involve whether the aircraft has lift or not.

[tigisfat]

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

Come again? Precisely what am I wrong about? I am 100 percent mostly certainly and positively not wrong in the slightest. Before I get offended and this conversation goes to a wierd place, I'm going to assume that you had to have misunderstood me.

I was saying that an airplane will NOT take off just because the wheels are rolling.

I was saying that the mythbusters episode was obscenely stupid because they claimed to have proven that an airplane will take off from a treadmill, even though the airplane was allowed to move forward through space.

[PRB]

Oh goody! The airplane on a treadmill debate again! This will go on for pages!

Yes it will take off.

My favorite is when a plane that is flying faster than M1.00, and it goes past the observer, who then explaims "wow, it broke the sound barrier just as it passed me!" As if the "Earth Shattering Ka-Boom" is the sound of the sound barrier actually breaking, at that very moment. Boom! It broke! LOLOLOLOL.

[AckAck]

That WAS what the myth was! The myth was that an aircraft could not take off from a treadmill, not that it couldn't take off without forward motion. That's why it was a stupid myth to prove/disprove. It can take off from a treadmill - myth busted. But so far as I can tell, the real myth was that an aircraft would take off from a treadmill with no forward motion, and that was busted too...

Brian

EDIT - PRB - the problem here is that we all are saying the same thing and agreeing - I'm not sure what the argument is about now... I'm right - no, you're right - no, you're right - no, I'm right...

[tigisfat]

Oh goody! The airplane on a treadmil debate again! This will go on for pages!

Yes it will take off.

My favorite is when a plane that is flying faster than M1.00, and it goes past the observer, who then explaims "wow, it broke the sound barrier just as it passed me!" As if the "Earth Shattering Ka-Boom" is the sound of the sound barrier actually breaking, at that moment. Boom! It broke! LOLOLOLOL.

There is no debate. Absolutely none.

A stationary aircraft with no relative wind and no means to produce lift will not take flight just because a treadmill has spun the wheels up.

On the other hand, if you set the parking brake and rig up a treadmill so that it moves the entire aircraft forward, and if the treadmill goes fast enough, the airplane will take off. I call this system the 'tig-o-matic fuel saver 9,000'. You do have to run checklists like a mofo if you took off with the engines out though.

[PRB]

Tig, you're stating the problem incorrectly, which is the source of endless confusion on this.

If you “don't let the plane move”, relative to the planet, then it will not take off. It doesn't matter by what means you succeed in keeping the plane from moving, be it a giant treadmill, or a big rope. If the plane doesn't move, obviously it can't take off.

The question is, can you keep a plane from moving, relative to the planet, by means of this giant treadmill experiment? Answer: no. Therefore, the treadmill plane will take off just like any other.

[Ken Stallings]

Come again? Precisely what am I wrong about? I am 100 percent mostly certainly and positively not wrong in the slightest. Before I get offended and this conversation goes to a wierd place, I'm going to assume that you had to have misunderstood me.

I was saying that an airplane will NOT take off just because the wheels are rolling.

I was saying that the mythbusters episode was obscenely stupid because they claimed to have proven that an airplane will take off from a treadmill, even though the airplane was allowed to move forward through space.

Whether you get offended or not is frankly your choice and I won't concern myself with that.

However, since the wheels on the aircraft are free floating, and not tied to the propulson drive, the aircraft's forward velocity from prop thrust is not affected by a treadmill since the wheels will freely turn. Because of inertia, the plane may initially roll back on the treadmill due to Newton's law of bodies at rest tending to stay at rest. But, the prop thrust will counter that by providing the necessary force to counter the inertia.

Once the thrust of the prop takes over, the free turning wheels simply increase their rotational velocity commensurate to the linear velocity of the treadmill.

Again, your choice if you choose to be offended by my statements.

Ken

[Ken Stallings]

Tig, you're stating the problem incorrectly, which is the source of endless confusion on this.

If you “don't let the plane move”, relative to the planet, then it will not take off. It doesn't matter by what means you succeed in keeping the plane from moving, be it a giant treadmill, or a big rope. If the plane doesn't move, obviously it can't take off.

The question is, can you keep a plane from moving, relative to the planet, by means of this giant treadmill experiment? Answer: no. Therefore, the treadmill plane will take off just like any other.

Bingo!

And that's precisely what the Mythbusters demonstrated. They concluded that a plane can take off on a treadmill.

Cheers,

Ken

[djscoo]

Tig, you're stating the problem incorrectly, which is the source of endless confusion on this.

If you “don't let the plane move”, relative to the planet, then it will not take off. It doesn't matter by what means you succeed in keeping the plane from moving, be it a giant treadmill, or a big rope. If the plane doesn't move, obviously it can't take off.

The question is, can you keep a plane from moving, relative to the planet, by means of this giant treadmill? Answer: no. Therefore, the treadmill plane will take off just like any other.

Theoretically, would this work?

If the tether was taut from the start, and the engine provided enough airflow over the wing surface, would the aircraft lift off?

7601http://www.sim-outhouse.com/sohforums/images/misc/pencil.png

[Sideshow]

On the other hand, if you set the parking brake and rig up a treadmill so that it moves the entire aircraft forward, and if the treadmill goes fast enough, the airplane will take off.

Actually no it won't. As the wing starts to produce lift the weight on the wheels will decrease. As a result the friction force between the wheels and the treadmill will decrease. You will find that they will slip relative to each other before the plane actually flies.

Don't mind me though. I'm just fanning the flames :redfire:

[Ken Stallings]

Theoretically, would this work?

If the tether was taut from the start, and the engine provided enough airflow over the wing surface, would the aircraft lift off?

7601http://www.sim-outhouse.com/sohforums/images/misc/pencil.png

Assuming the teather's tensile strength exceeded the max thrust of the prop, then of course not because the teather would freeze the aircraft's position relative the wind and therefore unless the wind velocity exceeded lift off speed, the aircraft would remain firmly on the ground.

This isn't even a debate.

The concept is applied by thousands of aircraft every day. The teathers are called tie downs and it is how a pilot keeps his aircraft safely on the ground when parked.

Cheers,

Ken

[AckAck]

Sure. Not very efficient, though, and generating enough wind over the wing with propwash might be problematic - easier just to make the propeller thrust down - but then you have a helicopter.

Can you take off by tying a rope to the wing and anchoring it to the ground, and fly in a circle fast enough to generate lift while being held in a circle by the rope?

Again, yes, but it's probably uncomfortable.

Brian

(EDIT - I type slow, I guess. Ken, as long as the engine is generating the airflow over the wing, then of course the aircraft will take off. There is more than one tie down on an aircraft, and they are in opposite directions, and the force is down to the ground. If you have only one tie down on the nose, and a 120 MPH wind, the plane will fly until the wind speed decreases. (Except we might call it a kite.) Similarly, if the plane is generating lift exclusively from the propwash, then as long as the engine is running, it will fly (nowhere.)

[Ken Stallings]

Actually no it won't. As the wing starts to produce lift the weight on the wheels will decrease. As a result the friction force between the wheels and the treadmill will decrease. You will find that they will slip relative to each other before the plane actually flies.

Don't mind me though. I'm just fanning the flames :redfire:

Not only that, but without prop thrust the flight (even if it happened) would be mighty short because as soon as the wheels broke contact with the treadmill, the thrust would go immediately to zero and the best outcome would be an instant touch down, or worse, a very hard one!

On the other hand, the concept of assisted thrust from a ground source is played out all the time. Imagine a bungee shot for a glider, or a carrier catapault takeoff. It's really a matter of having an additional source of thrust independent of the ground to keep the lift on the wing. For the glider, it is thermal lift exceeding aircraft weight, allowing a bit of forward velocity to translate from lift thrust. For the carrier catapault, it is the aircraft's engine(s) which overcome the drag of air to maintain takeoff speed and accelerate out.

Ken

[Ken Stallings]

Sure. Not very efficient, though, and generating enough wind over the wing with propwash might be problematic - easier just to make the propeller thrust down - but then you have a helicopter.

Can you take off by tying a rope to the wing and anchoring it to the ground, and fly in a circle fast enough to generate lift while being held in a circle by the rope?

Again, yes, but it's probably uncomfortable.

Brian

Well, if it's a long enough rope, and skilled enough pilot, it could be a simple pylon turn and the rope could remain just barely taut.

I would NOT volunteer to test the theory out however! :icon_lol:

Ken

[tigisfat]

Actually no it won't. As the wing starts to produce lift the weight on the wheels will decrease. As a result the friction force between the wheels and the treadmill will decrease. You will find that they will slip relative to each other before the plane actually flies.

Don't mind me though. I'm just fanning the flames :redfire:

Actually, you're wrong. It is quite possible to hold an aircraft down at speeds faster than the speed at which it can begin to fly. If you don't rotate a heavy, it'll likely never get airborne in the runway you have.

[tigisfat]

Not only that, but without prop thrust the flight (even if it happened) would be mighty short because as soon as the wheels broke contact with the treadmill, the thrust would go immediately to zero and the best outcome would be an instant touch down, or worse, a very hard one!Ken

Not only did you fail to outline how I was wrong, but you're agreeing with him (saying he's right) and then telling him he's wrong. Either the plane in his scenario takes off or it doesn't. Going back to my magical machine, it'd be stupid to assume that you can't hold an aircraft on the ground any faster than liftoff speed, and any excess speed would make the flight longer.

Try this on for size, I already know it fits: you're both wrong.

[AckAck]

Yeah, but how about that neat effect when the aircraft exceeds the speed of sound and forms that cone of condensation along the shockwave... :d (or those Formula 1 cars, when THEY exceed the speed of sound and exhibit the same phenomena.)

(tongue firmly in cheek)

Brian

[Ken Stallings]

Not only did you fail to outline how I was wrong, but you're agreeing with him and then telling him he's wrong. Either the plane in his scenario takes off or it doesn't. Going back to my magical machine, it'd be stupid to assume that you can't hold an aircraft on the ground any faster than liftoff speed, and any excess speed would make the flight longer.

Try this on for size, I already know it fits: you're both wrong.

OK, if that makes you feel better, knock yourself out.

But I don't think he implied the pilot had to avoid rotating the aircraft by back elevator forces. On the other hand, while some aircraft have to be rotated to lift off, others do not. You can also simply apply takeoff trim setting to the elevator and have it already set for rotation.

Just curious, did your CFI during your Private Pilot instruction not demo a hands-off-the-yoke takeoff in a Skyhawk? You can do it on a calm day by just using rudder to maintain centerline. Trim the Skyhawk for takeoff, and it will absolutely takeoff without any overt extra elevator inputs. :engel016:

Cheers,

Ken

[Ken Stallings]

Yes, we all know and understand that. Your lack of concern coupled with the fact you know you can get away with it without repercussion here is precisely why you spend your time talking trash, insulting others and loudly offering your opinions on religion, politics and anything else under the sun. It doesn't matter whether you're wrong or right, (such as where you're wrong here and still being arrogant and blunt), you're just here for the game.



No, I'm here for information exchange.

Cheers,

Ken

[Sideshow]

Actually, you're wrong. It is quite possible to hold an aircraft at speeds faster than the speed at which it will fly. If you don't rotate a heavy, it'll likely never get airborne in the runway you have.

Haha sorry Tig, my subtle attempt at humour obviously didn't work.

You are correct in saying that I assumed the wing was already at the optimum angle of attack to produce maximum lift. Obviously this isn't normally the case with most aircraft when they are on the ground and your above statement about rotating is perfectly valid.

Like I said, don't mind me. Infact your whole argument till this point has been solid and I actually agree with you.

[tigisfat]

Haha sorry Tig, my subtle attempt at humour obviously didn't work.

You are correct in saying that I assumed the wing was already at the optimum angle of attack to produce maximum lift. Obviously this isn't normally the case with most aircraft when they are on the ground and your above statement about rotating is perfectly valid.

Like I said, don't mind me. Infact your whole argument till this point has been solid and I actually agree with you.

And I apologize if I was rude. If I did in fact offend you, I wish to offer you a dancing ninja as a form of apology for my bitterness. This Ninja is valued at +1 internets. You can have the receipt in case you wwant to return it and get something more practical.

http://i595.photobucket.com/albums/tt32/walkeramerican/forum%20commentary%20pictures/dancingkarate.gif

[tigisfat]

No, I'm here for information exchange.

Cheers,

Ken



And no, I never had that demo'd to me. It may have had something to do with the winds in Abilene.

[Sideshow]

No offence taken. And thanks for the Ninja :wavey:

[tigisfat]

No offence taken. And thanks for the Ninja :wavey:

You're welcome, it was the least I could do.

[Wing_Z]

On a really fast, sticky, long, treadmill, an aeroplane would fly off the back :gameon: and crash... :gameoff:.

Does that count?
:d

[AckAck]

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.

I think it is a completely different scientifically bankrupt reasoning (from the penny dropped from the Empire State Building and killing someone) that would have a car dropped from 1000 ft go completely through the planet and kill someone on the opposite side. That has absolutely nothing to do with the terminal velocity - unless you assume that a penny and a car dropped from the same height (Emipire State Building is around 1000ft, no?) accelerate at different rates, and the car, by virtue of its larger mass, is going a heck of a lot faster than a penny - somewhere near the speed of light, I would guess, to go completely through the planet.

I would, however, stipulate that if you dropped a car from the top of the Empire State Building, you could kill someone on the ground...

Brian

[tigisfat]

On a really fast, sticky, long, treadmill, an aeroplane would fly off the back :gameon: and crash... :gameoff:.

Does that count?
:d


Yes, I believe it does. Especially if the pilot has the parking brake set and he's not paying attention to the airplane being placed on a giant treadmill.

[Wing_Z]

Tch...all the things a pilot has to remember...
Now:
If a plane's takeoff speed is 94kts
and there is a train going at 110kts with a treadmill on its roof running backward at 116kts...
pointing at the locomotive end, will the plane take off from the treadmill?

[tigisfat]

Tch...all the things a pilot has to remember...
Now:
If a plane's takeoff speed is 94kts
and there is a train going at 110kts with a treadmill on its roof running backward at 116kts...
pointing at the locomotive end, will the plane take off from the treadmill?

Yes, and if the train is long enough, it will take off backwards too. Both are assuming the airplane has the excess thrust required for flight and isn't held to any position on the train. This is also assuming that the plane won't fall off the train and the joints between cars are somehow seemless. I do suspect that you could do an immediate liftoff though.

It'd work just like this:

EIV1ZcIzhLQ

[robcap]

Beaver launching, cool:cool:

[tigisfat]

now if you put a non-amphibious floatplane on a treadmill...

[OBIO]

How come flatulence, which every one knows is faster than the speed of smell, never has those vapor cone thingies? Sure would make it easier to know who floated the air biscuit.

[tigisfat]

Theoretically, would this work?

If the tether was taut from the start, and the engine provided enough airflow over the wing surface, would the aircraft lift off?

7601http://www.sim-outhouse.com/sohforums/images/misc/pencil.png

umm,

If that were going to work, it'd have to be on a prop plane with enough lift produced from the prop's accelerated slipstream to make it hover. I may be wrong, but there hasn't ever been one of those. There are a few prop planes that can hover in the vertical, but none that can do what you're offering. That's assuming a zero wind condition.

[Quixoticish]

My gripe with the "plane on a treadmill" Mythbusters episode was that testing it goes against the spirit of the problem, their awful grasp of science aside.

It's one of those theoretical physics questions that relies on a lot of assumptions that you can't actually re-create in real life.

When you're answering a physics exam paper there are always certain rules laid out before a question. Ice that is completely frictionless. The force of gravity is always exactly 9.8 ms-2. Air resistance is negligible. Energy lost as thermal energy is negligible. Friction is a constant force. Acceleration is constant and does not fluctuate.

There are a million and one of them and anyone with any qualifications in physics will be well aware of them. And in the case of the plane on the moving runway it relies on one of these assumptions, in this case a conveyor belt that can instantly and precisely match the aircrafts speed. We're not taking into account the effects of friction, wind resistance, pretty much anything. The only fact we are given is that the treadmill instantly matches the aircrafts speed. Of course, I'm sure that since it was released into the wild the question has now become bastardised and shortened, but in its original form that is how it would have been phrased.

Of course in real life we can't manufacture something like that, it is simply impossible, and this leads to two answers. In real life the aircraft will take off, the aircraft will be able to accelerate quicker than the conveyor belt can in the opposite direction. Obviously the net result is a take off.

But on paper it can't, due to these little physics assumptions that we make, and the correct answer is obviously no, it wouldn't take off. This is why it's against the spirit of the question trying to test it. It is a question designed to hone your analytical skills. Carrying out experiments is easy, but picturing obtuse and often illogical concepts and actions in your head can prove exceedingly difficult, and that is exactly what this question is designed to test.

[Wing_Z]

Interesting...
What would be the conditions you impose for the plane not to take off, then?

[JohnC]

Considering the phrases "Normal Shock", "Boundary Layer", and "Ground effect" have been not been mentioned once in this conversation, I would suggest the following.

Read: http://www.amazon.com/Fundamentals-Fluid-Mechanics-Bruce-Munson/dp/0470262842/ref=sr_1_1?ie=UTF8&s=books&qid=1273678454&sr=8-1

then Read: http://www.amazon.com/Mechanics-Flight-Warren-F-Phillips/dp/0470539755/ref=sr_1_1?ie=UTF8&s=books&qid=1273678490&sr=8-1-spell

Then come back and talk until you're blue in the face. I've read them both cover to cover and would vouch for their quality of content.

[Ken Stallings]

Another good book is Aerodynamics for Naval Aviators.

I have learned quite a bit from that book.

Cheers,

Ken

[GT182]

Well, if an automobile was to be dropped and travel "through" the earth and come out the other side...... we probably wouldn't live long enough to even know about it, much less see it.

[Ken Stallings]

An automobile achieves thrust by the friction between the tires and the road as the tires achieve rotational velocity through a drive system.

An airplane achieves thrust by a propeller or jet engine propelling the aircraft forward on free turning wheels.

The basic difference is summed up by a straightforward examination:

1. Take a car and put it on a treadmill and tie a chain to the frame and the other side of the chain to a steel pyling. Turn the treadmill on and something on the car will break, or the chain will break, or the treadmill's surface will tear. This is because the drive wheels on the car will resist the velocity of the treadmill surface.

2. Take an airplane and similarly chain it to a steel pyling. Turn the treadmill on and the aircraft at rest will initially move aft until the force on the chain counters the friction of the tires at rest on the treadmill surface. At that point, the airplane's wheels will match the velocity of the treadmill's surface. There will be no additional force on the chain.

Now, with regard to the airplane, leave the chain attached. Fire up the engine and add power. How many vote that the aircraft will not move forward and relax pressure on the chain?

The point is that all the thrust that is needed is what is required to break the initial at rest inertia of the wheels on the surface.

One final point. When aircraft are engine run, you have to put chocks down to keep the aircraft from rolling forward.

When cars are engine run, you simply place them on a dynamometer, which is a free rotating cylinder on the drive wheels. The faster you go, the faster the cylinders are rotated to match the velocity. This how race cars are engine tested.

The critical difference is that cars again get thrust from the frictional effect of tires on the road as the wheels they are attached to rotate. The car only moves if it encounters resistance on a stationary surface. Another example of the point.

I am sure everyone has sat in a car at idle on sandy soil and tried to leave quickly. The wheels spin, right? You go nowhere while the wheels spin and they keep spinning until the tires make contact with ground surface firm enough to provide resistance to the spinning drive wheels.

An aircraft doesn't have this problem. Every time you apply full and immediate power, whether on grass, dirt, or asphalt, the aircraft never spins its wheels, does it? Of course not! Instead, the wheels don't need any surface resistance for the aircraft to move forward. Instead, for a prop plane it merely needs the prop to cut through the air and produce a lift force in the horizontal direction. A jet engine simply produces more thrust out the back and relies upon Newtonial physics to provide the matching thrust in the opposite direction.

And this is why, folks, a treadmill cannot prevent an airplane from taking off.

Cheers,

Ken

[Wing_Z]

Well, you've left out stuff that will make the anal ones among us squirm a bit...and of course I include myself in that grouping ;)
But of course the aircraft will take off, we've established this already.

I'm waiting to hear about what conditions you could theoretically apply to make this a hard problem!

...In real life the aircraft will take off, the aircraft will be able to accelerate quicker than the conveyor belt
...But on paper it can't, due to these little physics assumptions that we make, and the correct answer is obviously no, it wouldn't take off...

[Ken Stallings]

Well, you've left out stuff that will make the anal ones among us squirm a bit...and of course I include myself in that grouping ;)
But of course the aircraft will take off, we've established this already.

I'm waiting to hear about what conditions you could theoretically apply to make this a hard problem!

I agree. This doesn't appear to me to be a "hard" physics problem. The additional component is that if you allow the treadmill a full head start, then in addition to the inertial force, the thrust of the aircraft would have to also overcome the horizontal momentum in the aft direction. But, momentum is not thrust, and therein lies the essential point.

Yes, it will take more time for the aircraft's forward thrust to counter the aft velocity the treadmill was allowed to generate -- again due entirely to the frictional force and inertia Newtonian laws.

But, that thrust will overcome that aft momentum and soon as those aft forces are broken, the aircraft will experience forward velocity relative the stationary ground and provided the treadmill is long enough, that velocity will increase and once rotate velocity is achieved, it becomes a normal takeoff with the free rolling tires going twice as fast in rotational velocity as they normally would be. The only practical difference once airborne is that the wheels on the aircraft will take longer to spool down.

Ken

[Wing_Z]

Actually, as JohnC has pointed out, ground effect might be quite significant.
It's the reason cars are put in wind tunnels with rolling roads to tweak aerodynamics.

[Ken Stallings]

Actually, as JohnC has pointed out, ground effect might be quite significant.
It's the reason cars are put in wind tunnels with rolling roads to tweak aerodynamics.

The effect of ground effect is simply to reduce the airspeed at which an aircraft can remain aloft. It has the effect of lowering the stall speed. So, if anything, same as for a soft field takeoff, it should assist the takeoff, not harm it.

For those who do not know, a soft field takeoff has the pilot rotate and lift off at a lower airspeed and then remain in ground effect while the aircraft builds up speed to climb out at Vx (best angle of climb).

Cheers,

Ken

[OBIO]

And this is why, folks, a treadmill cannot prevent an airplane from taking off.

Cheers,

Ken

If that said treadmill were thrown through the windscreen, striking the pilot across the bridge of the nose...I bet that air plane will not take off until said pilot gets out of the plane and whoops the hind end of the joker who threw the treadmill through his wind screen.

[N2056]

What is the airspeed velocity of a swallow on a treadmill?

[Ken Stallings]

If that said treadmill were thrown through the windscreen, striking the pilot across the bridge of the nose...I bet that air plane will not take off until said pilot gets out of the plane and whoops the hind end of the joker who threw the treadmill through his wind screen.

Not to mention I'd kick his butt fierce for breaking my airplane! :icon_lol:

Ken

[Ken Stallings]

What is the airspeed velocity of a swallow on a treadmill?

African or European?

And stay away from that gorge .... it looks awfully deep! :icon_lol:

Ken

[Wing_Z]

... it should assist the takeoff, not harm it...
Correct from a pilot's perspective, yes, on a stationary runway.
Because the treadmill moves however, there will be more rotation of air (tumbling anticlockwise seen from the port side) under the aircraft, due to boundary layer drag.
Now I'm trying to imagine whether this would drop the pressure under the plane.
I think at higher speeds: yes
So that would suck the plane back down!
The faster plane and treadmill go, the more it would suck!
Is there a moral here??

[N2056]

Yes! There is...
Don't be associated with things that suck. Get off that treadmill! :jump:

[Bjoern]

Anyone with any scientific background knows that there must be a terminal velocity for the penny and the car.

It's a lot more than just terminal verlocity.

Energy dissipation on impact, surface properties, etc...

A car is rather weak compared to your average ground.

[tigisfat]

It's a lot more than just terminal verlocity.

Energy dissipation on impact, surface properties, etc...

A car is rather weak compared to your average ground.

My comment was only to say that gravity has it's limits. Whatever you drop from any height will not continue to accelerate to the speed of light. Eventually, and in most cases, there will largely be an equilibrium.

Of course gravity and atmosphere changes with distance, but we're talking about stuff being dropped 1,300 feet, or however high the empire state building is.

[Ken Stallings]

Correct from a pilot's perspective, yes, on a stationary runway.
Because the treadmill moves however, there will be more rotation of air (tumbling anticlockwise seen from the port side) under the aircraft, due to boundary layer drag.
Now I'm trying to imagine whether this would drop the pressure under the plane.
I think at higher speeds: yes
So that would suck the plane back down!
The faster plane and treadmill go, the more it would suck!
Is there a moral here??

OK, I see your point. It would be an interesting measurement to take. I suspect it wouldn't suck enough.

Although I confess on that point, I have no empirical data to back up my assertion. It's just a hunch based upon my own flying.

Cheers,

Ken

[boxcar]

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


Wow... I come here to check in for the first time in weeks to find you speaking for me. Never being be so audacious & reckless to do so again. You've done this elsewhere & I'll speak for myself. I do not speak for you, Memphis man, so stop speaking for me.

If anyone cares to read the full-meal-deal on how this assertion of "tigis" came about then you are most welcome to read the thread: http://forums.flightsim.com/vbfs/showthread.php?t=209947 (http://forums.flightsim.com/vbfs/showthread.php?t=209947) He subsequently accused me in another forum of saying that "we were being gassed" by our government which is completely untrue.

Like I said, read the whole thread. You'll see me speaking for myself as well as standing up to a trouble maker.
..

[tigisfat]

Wow... I come here to check in for the first time in weeks to find you speaking for me. Never being be so audacious & reckless to do so again. You've done this elsewhere & I'll speak for myself. I do not speak for you, Memphis man, so stop speaking for me.

If anyone cares to read the full-meal-deal on how this assertion of "tigis" came about then you are most welcome to read the thread: http://forums.flightsim.com/vbfs/showthread.php?t=209947 (http://forums.flightsim.com/vbfs/showthread.php?t=209947) He subsequently accused me in another forum of saying that "we were being gassed" by our government which is completely untrue.

Like I said, read the whole thread. You'll see me speaking for myself as well as standing up to a trouble maker.
..

How on earth do you consider the comments of mine you quoted offensive OR speaking for you? It's clearly neither. I shouldn't even be concerning myself with this....


Mods, not only is a lock called for but a delete is as well. Please do me a favor. No good can come of this (way) off topic public display. This can only go REALLY downhill from here.

[boxcar]

.
You .just ... don't ... get it. Someone mentions "chemtrails" & you bring up my name in connection, saying not to bring them up around me, how greatly I believe in them, how strongly. That certainly is speaking for me. Was going to edit out your city of residence though, but you've already quoted me.

Am seeing no need for any lock on this interesting thread, as I'm done, though what will be will be.
.

[tigisfat]

.
You.just ... don't ... get it. Someone mentions "chemtrails" & you bring up my name in connection, saying not to bring them up around me, how greatly I believe in them, how strongly. That certainly is speaking for me. Was going to edit out your city of residence though, but you've already quoted me.

Am seeing no need for any lock on this interesting thread, as I'm done, though what will be will be.
.


I was wondering why you called me 'Memphis Man'. I've never lived in Memphis....

[N2056]

Sorry...gotta do it :salute:

[Wing_Z]

...Don't be associated with things that suck. Get off that treadmill! :jump:
:running::running::running: