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You know when youre an engineer when....

Tomorrow said:
The entire point - the one that people overlook, causing them to think the plane won't take off - is that the plane's mechanics are governed by the propeller and wings. People think of the plane in terms like a car, whose mechanics are governed by wheels rolling without slip on the ground.

Because the wheels of an airplane roll freely, and propulsion is provided by the propeller and not drive to the wheels, it makes no difference whatsoever what's going on underneath the plane; indeed, the problem is exactly the same whether the ground is there or not!

As long as the wheels roll freely and the propeller works, there's no way to arrest its forward acceleration; Mythbusters didn't defeat the purpose or give the plane forward momentum; the plane generated its own velocity with the propeller. It happens whether the plane is on the ground or not. You can't keep the plane stationary, not relative to the ground, not relative to the conveyor, and not relative to the air.
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I think you are missing my point. The point is that I'm assuming that those situations (especially the enclosed building) are dead air situations. So, they only way to generate wind speed (key to lift), is to have wind. If they had executed it perfectly, the plane would be in the same position relative to the ground, and hence, have no airflow over it. Without that, it cannot take off.
 
chrmjenkins said:
I think you are missing my point. The point is that I'm assuming that those situations (especially the enclosed building) are dead air situations. So, they only way to generate wind speed (key to lift), is to have wind. If they had executed it perfectly, the plane would be in the same position relative to the ground, and hence, have no airflow over it. Without that, it cannot take off.
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So if the airplane was light enough that it was able to get off of the ground by the force of the air being pushed over the wings by the propellers alone, it would fly.

Or did I misunderstand?
 
thejadedmonkey said:
So if the airplane was light enough that it was able to get off of the ground by the force of the air being pushed over the wings by the propellers alone, it would fly.

Or did I misunderstand?
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Yes, in theory, it would. I don't know the particulars of airflow due to the disturbance of the propeller alone, however. With a jet, this wouldn't be the case at all, as the engines are for thrust (and indirectly causing air to go over the wings). It is for this reason I suspect that the motionless airplane would have to be near weightless to achieve what you are talking about. That would be some plane :D
 
chrmjenkins said:
Yes, in theory, it would. I don't know the particulars of airflow due to the disturbance of the propeller alone, however. With a jet, this wouldn't be the case at all, as the engines are for thrust (and indirectly causing air to go over the wings). It is for this reason I suspect that the motionless airplane would have to be near weightless to achieve what you are talking about. That would be some plane :D
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Well, in this specific scenario, all that you would require is a thrust to weight ratio greater than 1. I am not an Aero guy but I think there is a MR regular go is.

Back to the subject of the thread, YKYAEW you're commenting on this thread.

Next up, red rubber ball and naked girl on a bicycle jokes.
 
chrmjenkins said:
I think you are missing my point. The point is that I'm assuming that those situations (especially the enclosed building) are dead air situations. So, they only way to generate wind speed (key to lift), is to have wind. If they had executed it perfectly, the plane would be in the same position relative to the ground, and hence, have no airflow over it. Without that, it cannot take off.
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Wind is a relative thing - as long as there is air moving over the wings - which is equivalent to wings moving through still air - the wings will generate lift. The propeller pulls the plane forward through the air, generating lift. Wheels on the ground, or speed relative to the ground, are 100% irrelevant to the problem. The plane's speed relative to the ground is 100% irrelevant to whether the plane takes off. The air's speed relative to the ground is 100% irrelevant to whether the plane takes off.

Unless they physically anchor the plane to the ground, they cannot keep the propeller from pulling the plane forward. They did execute the experiment perfectly.

The second of your statements which I made bold is the fallacy that they were testing. Planes generate lift and thrust based on air speed or movement relative to air; ground speed has absolutely nothing to do with whether the plane gets off the ground, unless you run out of runway first.

People tend to think of this problem the way they would of a car on a conveyor; if the wheel speed is equal to the conveyor speed, the car would appear to be stationary with respect to the ground. This doesn't apply to planes, since they don't generate speed through the wheels; they do so through the air, making wheels irrelevant.

If you think about it, your argument suggests that planes would be able to get off the ground by having an engine turning wheels to generate speed. The reason it won't work is because once you're off the ground, you would have no means of keeping your speed up; the wheels would just spin through the air, burning fuel and doing little else.

thejadedmonkey said:
So if the airplane was light enough that it was able to get off of the ground by the force of the air being pushed over the wings by the propellers alone, it would fly.

Or did I misunderstand?
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That's how all planes fly. If you get air moving over the wings (or wings moving through the air) at the proper takeoff speed, they generate enough lift to get the airplane off the ground.

chrmjenkins said:
Yes, in theory, it would. I don't know the particulars of airflow due to the disturbance of the propeller alone, however. With a jet, this wouldn't be the case at all, as the engines are for thrust (and indirectly causing air to go over the wings). It is for this reason I suspect that the motionless airplane would have to be near weightless to achieve what you are talking about. That would be some plane :D
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The plane they used in the episode needs an airspeed of around 25 mph, IIRC. So if you could keep a steady (speed and direction) headwind of 25 mph on the plane, then it would generate enough lift to become airborne.

The problem doesn't change between using a jet or a prop plane, aside from the fact that jets are typically faster and need a longer runway to generate enough speed.
 
Tomorrow said:
Wind is a relative thing - as long as there is air moving over the wings - which is equivalent to wings moving through still air - the wings will generate lift. The propeller pulls the plane forward through the air, generating lift. Wheels on the ground, or speed relative to the ground, are 100% irrelevant to the problem. The plane's speed relative to the ground is 100% irrelevant to whether the plane takes off. The air's speed relative to the ground is 100% irrelevant to whether the plane takes off.

Unless they physically anchor the plane to the ground, they cannot keep the propeller from pulling the plane forward. They did execute the experiment perfectly.
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Not as I see it. The spirit of the myth, it seems to me, is if the plane's engine(s) are working at full capacity, is this enough to achieve flight? That's why there is the assumption there's no motion relative to the ground. The very fault in the myth is that since the wheels are free-spinning, the thrust of the plane will just make them spin the conveyor's speed plus the amount of thrust, giving it relative forward velocity, and hence, wind speed. If you let it have motion with respect to the ground, IMO you're ignoring the fact that the myth inherently misunderstands the mechanics of it. The myth assumes a physical impossibility, so you have to compensate for that somehow. I've acknowledged numerous times it's all about relative wind speed. So, testing the myth was kind of dumb IMO, as they had to change its intent to come up with a test. The closest thing I could think of to equate it to the myth's intent is to tether it with a chain, let the engines get up to full speed, then release it and expect it to instantaneously take off. However, if you took the time to explain that to someone as the same physical situation, they'd probably say, "Oh, ....well that's dumb."
 
Seems to me that everyone here understands the physics. The point of contention appears to be in the wording or intent of the 'myth' itself.
 
P-Worm said:
You can solve multivariable calculus problems, but you can't do long division.
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I can relate. I used to do differential equations and triple integrals when I was in school. Now, I have trouble trying to figure out how to add tax to an amount.

I once was riding on a bus and was getting very annoyed by the noise of the rattling window, so I devised a shim out of a scrap sheet of notebook paper that silenced it. The lady sitting in the seat in front of me said, "You're an engineer, aren't you." And she was right! :D
 
chrmjenkins said:
Not as I see it. The spirit of the myth, it seems to me, is if the plane's engine(s) are working at full capacity, is this enough to achieve flight? That's why there is the assumption there's no motion relative to the ground. The very fault in the myth is that since the wheels are free-spinning, the thrust of the plane will just make them spin the conveyor's speed plus the amount of thrust, giving it relative forward velocity, and hence, wind speed. If you let it have motion with respect to the ground, IMO you're ignoring the fact that the myth inherently misunderstands the mechanics of it. The myth assumes a physical impossibility, so you have to compensate for that somehow. I've acknowledged numerous times it's all about relative wind speed. So, testing the myth was kind of dumb IMO, as they had to change its intent to come up with a test. The closest thing I could think of to equate it to the myth's intent is to tether it with a chain, let the engines get up to full speed, then release it and expect it to instantaneously take off. However, if you took the time to explain that to someone as the same physical situation, they'd probably say, "Oh, ....well that's dumb."
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Does "I'm going to marry him ^^" count as a way to tell? :rolleyes:
 
Signal-11 said:
Says the guy whose avatar is a WWI flying ace who gets shot down by the Red Baron from a stationary doghouse.
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Heh! The imagination is a wonderful thing. And look at Snoopy's scarf - it's obvious that he's really flying! :p
 
So an engineering student is walking through campus when he sees another student from his department with a new red bike. First student says, "Hey, nice bike, where'd you get it?"

Second student says, "It was the darndest thing. This girl rides up to me, drops the bike, takes off all her clothes and throws her arms out and says, 'Take what you want!'"

First student says, "Good choice, the clothes probably wouldn't have fit."
 
You know you're an engineer when...

You make a drawing of a mechanical part that cannot be machined with conventional methods because you don't know any better.

Ask me how much I respect engineers...
 
When your ideal house design is a square box with no windows or doors.
 

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I have that "Tree Swing" picture printed and posted on my cubicle wall at work. (Oh yeah, add that to the list... you know you're an engineer when you spend the majority of your day inside a fabric-lined box...)
 
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