Mythbusters....

timwinters said:
Regardless, this version of the problem (with the treadmill matching the speed of the wheels instead of the airplane) would be virtually impossible to test. And I think this is where madseason is getting confused. He thinks the treadmill is supposed to match the wheel speed...not simply the airplane speed. It would indeed be impossible for the plane to take off in that scenario. But that's not the scenario being contemplated here...matching the AIRPLANE speed is.
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No sir, not confused. In #49 I wrote "My understanding of the myth is that the treadmill could [magically] compensate for the forward movement of the plane."

I appreciate the simple explainations from yall but I'm going a little beyond those with this myth (thanks again to the guy who took the time to explain that the 'wheels weren't attached to the prop'. That was really special :wink2:).

Basically I was thinking that from a standstill the treadmill would need to be accelerated infinately to keep the plane in one place. This is because the plane's weight/friction of wheels on the treadmill would be pulling the plane back but the plane's thrust would be pulling it forward. i.e - plane doesn't move.

But that's not completely accurate. In order for the plane to be pulled back the rolling resistance of the plane would need to increase as the treadmill accelerated. Which probably wouldn't happen unless the wheel bearings got so hot that they expanded and slowed the wheel down. I don't believe the myth is concerned with wheel bearings so I guess I have to concede, I was wrong and I believe the video that I posted was right. Once the plane's thrust is high enough to overcome the rolling resistance, the same as when you pull out of your tiedown spot, then the plane will move forward.
 
MadseasoN said:
No sir, not confused. In #49 I wrote "My understanding of the myth is that the treadmill could [magically] compensate for the forward movement of the plane."

I appreciate the simple explainations from yall but I'm going a little beyond those with this myth (thanks again to the guy who took the time to explain that the 'wheels weren't attached to the prop'. That was really special :wink2:).

Basically I was thinking that from a standstill the treadmill would need to be accelerated infinately to keep the plane in one place. This is because the plane's weight/friction of wheels on the treadmill would be pulling the plane back but the plane's thrust would be pulling it forward. i.e - plane doesn't move.

But that's not completely accurate. In order for the plane to be pulled back the rolling resistance of the plane would need to increase as the treadmill accelerated. Which probably wouldn't happen unless the wheel bearings got so hot that they expanded and slowed the wheel down. I don't believe the myth is concerned with wheel bearings so I guess I have to concede, I was wrong and I believe the video that I posted was right. Once the plane's thrust is high enough to overcome the rolling resistance, the same as when you pull out of your tiedown spot, then the plane will move forward.
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.... and there was much rejoicing.
 
PilotAlan said:
...However, wheel speed only equals treadmill speed at zero airspeed. ...


With any forward speed, the treadmill would instantly have to accelerate to infinity.
Therefore, the hypothetical of treadmill = wheel speed is not possible.
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I always thought that same thing.

Bottom line: In the REAL world, the airplane will accelerate and take off.
 
PilotAlan said:
No.
The wheel movement is based on airspeed. The whole thing of the treadmill matching wheel speed is a physical and mathematical impossibility.

Think of this:
Treadmill speed = wheel speed (your hypothetical)
However, wheel speed only equals treadmill speed at zero airspeed.
If you accelerate the treadmill to 10 knots and hold the plane still, the wheels are spinning at 10 knots.

Now, if I accelerate the plane to 1 knot airspeed, then:
Treadmill speed = 10 knots, and wheel speed now = 11 knots.
If you accelerate the treadmills to 11 knots, the plane is still moving forward at one knot, so wheel speed = 12 knots.
With any forward speed, the treadmill would instantly have to accelerate to infinity.
Therefore, the hypothetical of treadmill = wheel speed is not possible.

The myth is that the treadmill matches the forward speed of the airplane. IOW, if the plane is moving forward at 10 knots, the treadmill is moving backward at 10 knots. Wheel speed equals 20 knots.
Making that assumption, what happens?
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BINGO

Now some planes might not make it off even a non "magic" tread mill because 2Vr > tire speed rating, but you build me the tread mill for that plane and I'll just change the tires;)

It seems that aside from different poor wordings of the myth we all understand it:

Yes a magic tread mill could prevent a plane from taking off but a tread mill simply going backwards at Vr doesn't make the plane "sit there like a brick":rofl:
 
PilotAlan said:
No.
The wheel movement is based on airspeed. The whole thing of the treadmill matching wheel speed is a physical and mathematical impossibility.

Think of this:
Treadmill speed = wheel speed (your hypothetical)
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It wasn't my hypothetical. I was inquiring into the hypothetical that Tim Winters posted.

However, wheel speed only equals treadmill speed at zero airspeed.
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The wind velocity has not been specified, so wheel speed cannot be determined from the airspeed.

If you accelerate the treadmill to 10 knots and hold the plane still, the wheels are spinning at 10 knots.
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Since knots are not a unit of rotational speed, it sounds like you must be interpreting "wheel speed" to mean the speed of the surface of the tires (relative to the axles). I was interpreting it to mean the forward or backward speed (relative to the ground) of the centers of the wheels, which are co-located with the axles. (Tim didn't say how he was interpreting the phrase.)

Now, if I accelerate the plane to 1 knot airspeed, then:
Treadmill speed = 10 knots, and wheel speed now = 11 knots.
If you accelerate the treadmills to 11 knots, the plane is still moving forward at one knot, so wheel speed = 12 knots.
With any forward speed, the treadmill would instantly have to accelerate to infinity.
Therefore, the hypothetical of treadmill = wheel speed is not possible.
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I agree that it's impossible if you interpret "wheel speed" as you appear to be. That's why I was using the other interpretation of the phrase.

The myth is that the treadmill matches the forward speed of the airplane. IOW, if the plane is moving forward at 10 knots, the treadmill is moving backward at 10 knots. Wheel speed equals 20 knots.
Making that assumption, what happens?
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The airplane achieves an airspeed of ten knots plus the headwind component, or ten knots minus the tailwind component. What do you think happens?
 
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Skylane81E said:
Yes a magic tread mill could prevent a plane from taking off...
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How can you be sure of that? It seems to me that as soon as you introduce magic into the discussion, the answer becomes arbitrary, and therefore unkown.
 
Cap'n Jack said:
Also- observe this:
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I agree with Greg that the myth is sometimes improperly presented. The words and how they are used are important. Loved the video. It succinctly demonstrates that:

a) the successful achievement of what is generally considered "flight" is a result of a gas moving across a solid lifting surface at sufficient speed to create sufficient lift to overcome the objects weight.

b) mechanical propulsion (airplanes) or gravity (gliders) is needed only to create a thrust vector to guide the "flight" in a direction other than downwind.

c) Sufficient thrust can propel any solid prism through any gas, solid, or vacuum. This is not "flight" in the usual sense; it is "rocketry", "ballistics", "coring", or "falling".

d) ground speed is not relevant to the successful achievement of "flight"; only airspeed is relevant to achieving "flight". Flight at zero ground speed is called "hovering", flight at negative ground speed is called "Hey Yall! Look at this."
 
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jesse said:
You'd never get there. The airplane would stay in one place on the treadmill, but the aircraft would experience a 200 mph wind (headwind or tailwind) and quickly depart the treadmill.
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I don't think that's right. If the airplane stayed in once place, it wouldn't have any wind, and it would stay in place.

You'd add power, and it would pull forward, just like it would off the treadmill. Once it starts moving forward (and it will....) then its just a normal takeoff.
 
SkyHog said:
I don't think that's right. If the airplane stayed in once place, it wouldn't have any wind, and it would stay in place.

You'd add power, and it would pull forward, just like it would off the treadmill. Once it starts moving forward (and it will....) then its just a normal takeoff.
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He didn't say you would add power. He said
Let's do this.... take a 100 mile treadmill. Put a 172 on one end of it, with the brakes set. Start the treadmill moving at 200 MPH (which is much faster than a 172 ever goes).

So your 172 is now moving backwards with the treadmill and will fall off it in 30 min.

Now in the 172, release the brakes. What happens?
Start the engine. What happens?
Increase to takeoff thrust. What happens?


hint: THE EARTH DOES NOT MATTER. Only the treadmill surface and the air matter.
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That would put 200 mph of wind on the tail which would send it tumbling.
 
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And even if we removed Maxwell from the equation, the cat's still dead.

I mean come on, how long ago did that poor creature get locked in that box? It MUST have suffocated by now...
 
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