OK, I was in SF yesterday, and missed some discussion.

To clarify my attempt at clarification......

Davjosmes is right. Braking on landing was exactly what I was thinking someone would mention.

I'll let those who understand the forces at work better than I (I make maps - what do I know from traction?) debate whether that is traction or not.

(and next time, I'll just keep my mouth shut!)

Thanks for the clarification, killick. And don't shut that mouth. Your input is always welcome.

Apprectiate that!

Actually, I was semi-joking. I just meant that my attemt to clarify was an attempt to avoid just such a debate about "traction", and next time I'd be better off NOT trying to clarify a statment.

Just to, you know, clarify things.

“Perfect clarity would profit the intellect but damage the will”

--Blaise Pascal

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Originally posted by davjosmes:
“Perfect clarity would profit the intellect but damage the will”

--Blaise Pascal

Will probably wouldn't like that.

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Will probably wouldn't like that.

Fire at Will.

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Fire at Will.

Will be shot at the crack of dawn.

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Will be shot at the crack of dawn.

Dawn might not like that.

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Originally posted by davjosmes:

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As a pilot, this was a no-brainer to me. Planes fly because of their motion relative to the air, not relative to the ground. With enough of a head wind, a plane doesn't have to be moving relative to the ground to take off.

And you still have it wrong. The plane moves. No need for a head wind.

I think what this guys means is that a plane can be sitting on the tarmack with a head wind blowing at its takeoff speed and the plane will lift off without moving forward. Thats why normally the plane needs to move forward to build air speed under its wings. But if you already have the airspeed blowing at the plane without it moving it should lift off. Ever watch "Flight of the Phonix"? Thats what is happening at the end when the plane is bouncing without moving forward. The airspeed heading at the plane is the same as its takeoff airspeed...

I thought I should explain alittle more to my previous post and maybe help the explain the myth....

Lets take a plane that has a 70 knot takeoff speed. That speed is airspeed (Speed of air flowing front to back over the wing of the airplane). If you have no wind at all the ground speed will be the same as the airspeed for takeoff.

If you have a wind and its a head wind, its already generating some the airspeed over the wings without the plane moving forward thus generating lift. Lets say that wind is 30 knots. That means the plane only needs to get up to 40 knots of ground speed to make the 70 knots airspeed for takeoff.

Next senerio is a tail wind. (Wind blowing from back to front). If that wind is blowing at 40 knots, the plane needs to now have a 110 knots ground speed (70 knot take off speed 'plus' 40 knot tailwind) to overcome the lost of lift from the wind going the wrong way over the wings.

Only thing ground speed means to an airplane is how fast it needs to go in relation to the ground to generate airspeed under the wings to generate lift. Other than that the ground has no meaning to an airplane....

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Other than that the ground has no meaning to an airplane....

Not technically true. You need a lot less air speed near ground to take off than you would need without the ground. It's called, appropriately enough, ground effect. That's why you don't start the climb as soon as your plane can lift off. You will stall and fall back on the runway. You can, however, use it to help you take off from a poor quality or very short runway.

Also, the takeoff speed for the kind of plane they used in the myth is in the 25-35 knots range. You can have winds faster than that on the windy days, and that's exactly what was talked about earlier. You can take off without moving relative to the ground at all.

I definitely see where all of the confusion comes from with this myth. First time I heard it stated, I actually thought that the treadmill matches the wheel speed. That would keep the plane stationary, and there will be no takeoff. That has nothing to do with whether you propel yourself with wheels or the propeller. With enough treadmill speed, you'll keep even the plane grounded, because there is actually a significant amount of friction in the landing gear.

I agree to a point. The amount of friction from the wheels that you speak of would have to be extreme. It would have to be equal or even 75% less that the thrust produced by the power plant in the airplane. The bearings in the
wheels and the tires on the ground produce very little friction to even be noticeable.

quote:

Also, the takeoff speed for the kind of plane they used in the myth is in the 25-35 knots range. You can have winds faster than that on the windy days, and that's exactly what was talked about earlier. You can take off without moving relative to the ground at all.

Yes, this is true. Thats what I was trying to explain with the plane in "Flight of the Phoenix". If the wind is moving fast enough the plane will lift off. But as soon as it gets in the air it will actually fly backwards unless you have some source of power to move it in the opposite direction of the wind. If your force pushing against the wind is equal, it will hover above the ground like a helicopter because it has lift being generated from the wind moving. If you apply more than equal force, the plane will move forward.

Yes I do know about the 'ground effect' when it comes to an airplane but I was making things simple to whereas most people could understand it (no-flys).

Those are some good reason, especially number 1. They did a successful takeoff, what more could people want?

Heh... Adam's reasoning outdid Jamie's...

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Originally posted by killick123:
I'll let those who understand the forces at work better than I (I make maps - what do I know from traction?) debate whether that is traction or not.

There's nothing to debate, really. Certainly nothing constructive that could come out of it. Traction is traction. The tractor-traction connection should make this obvious.

In order to have traction, there must be adhesive friction (better known as "adhesion") for there to be traction. However that does not mean that they are one and the same. Adhesion comes into play during just about any dynamic automotive situation--acceleration under power, braking, cornering etc. Traction only has to do with the propulsion of the vehicle.

"So what?" you might say. Well, isn't this whole PoaT debacle the direct result of sloppy interpretations of precise language?

Since grade school, a good unabridged dictionary has been a must-have at my desk. When I went to college and studied the sciences, other guides to the jargon of various disciplines took their places alongside my english dictionary. The same holds true for every other technical professional I know.

I believe that over 99% of the needless discord that appears on this forum comes as a direct result of someone failing to learn the meaning of a word, failing to use the correct context, or otherwise not using a word as most others commonly use it.

My question is who wants to strive to communicate, and who wants a Tower of Babel?

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Originally posted by Ishta:

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Originally posted by SnarkFish:

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Originally posted by death_mage:
Read the whole sentence.

i did. i also understand what you meant, just trying to get you to see what you are typing

now if you said
If an RC car moving at 15mph drives onto a treadmill moving whose belt is moving at 15mph in the opposite direction, the car will not move

Yes it would. A car doing 15 is doing 15. A car would only be less effective.
A goat on a conveyor doing the same speed will walk off the belt

if you have ever been on a treadmill you will now that if you go the same speed you stay stationary. but if you accelerate you will walk off the treadmill/conveyor. which this whole myth is about. and thats where most of the people think why they got it wrong.

what if you accelerate the belt as the same as the plane? the plane would remain stationary i believe.(you can correct me if im wrong)

to clarify i believe the plane would still take off

I think that the myth was executed very well. the only thing that would happen at a higher land speed is.. the bearing freeze in the wheels and create loads of friction on the conveyor belt.. resulting in either the conveyor belt tearing or the plane crashing nose first into the conveyor belt.

In order for this to happen the land speed would have to be very very fast.

Plus one more thing to think about, for those of you confused.. think about what would have happened if the plane did not have wheels, but skis or pontoons like an aquatic plane.

the result would be the same, because wheels have nothing to do with how a plane accelerates.

besides wouldnt the motion of the ground help the wheels move the vehicle forward?