So...about that plane on the treadmill...

Looks like airplane treadmill problem, regularly a spark for flame wars
on R.A.P., has made it into the mainstream.

http://pogue.blogs.nytimes.com/

Let the arguing begin!

- Ray

"Ray" wrote in message
...
Looks like airplane treadmill problem, regularly a spark for flame wars on
R.A.P., has made it into the mainstream.

http://pogue.blogs.nytimes.com/

And handled with every bit as much intelligence and consideration as we've
seen here. Which is to say, there's no shortage of people convinced that
the airplane won't take off, even though it will.

Let the arguing begin!

Why? Haven't you had enough by now?

"Peter Duniho" wrote in message
...
"Ray" wrote in message
...
Looks like airplane treadmill problem, regularly a spark for flame wars
on R.A.P., has made it into the mainstream.

http://pogue.blogs.nytimes.com/

And handled with every bit as much intelligence and consideration as we've
seen here. Which is to say, there's no shortage of people convinced that
the airplane won't take off, even though it will.

Let the arguing begin!

Why? Haven't you had enough by now?


It seems that the "non-believers" think that the treadmill is somehow
holding the airplane back.
The way that the problem is posed on the blog states that the treadmill
matches the wheel speed of
the airplane. ("The conveyer belt is designed to exactly match the speed of
the wheels, moving in the
opposite direction.") If friction is taken into consideration then one of
four conditions can exist.
1. no thrust (or not enough thrust to overcome frictional forces) ...
neither the plane nor the conveyor are moving.
2. minimal thrust... the wheels and conveyor are moving but the conveyor
drags the plane backwards.
3. just enough thrust to match friction forces... the airplane remains
motionless relative to the earth but the wheels
and conveyor are moving a little faster.
4. more than enough thrust.... the airplane accelerates until it can take
off. The conveyor also accelerates to match
the wheel's speed until lift off when the wheels rotate to a stop and
the conveyor, somehow sensing this, also
comes to a standstill.
The treadmill's speed is dependent on the wheel's speed, not the other way
around.

"Peter Duniho" wrote in message
...
"Ray" wrote in message
...
Looks like airplane treadmill problem, regularly a spark for flame wars
on R.A.P., has made it into the mainstream.

http://pogue.blogs.nytimes.com/

And handled with every bit as much intelligence and consideration as we've
seen here. Which is to say, there's no shortage of people convinced that
the airplane won't take off, even though it will.


Nope the plane won't take off.

------------------------------------
DW

Darkwing wrote:



Nope the plane won't take off.


You'd better not just be trolling...

Friction generated by wheels is almost negligible. Even for very large aircraft like a
747, wheel friction is nearly constant, regardless of speed (at least at sane speeds). It
only depends on the amount of force pushing down on the wheel.

So let's look at all of the forces acting on this airplane. In the horizontal direction,
we have:
Thrust from the engine (nearly constant at these speeds)
Aerodynamic drag (goes up as the square of speed)
Wheel friction (again, nearly constant)

For any object to accelerate in a given direction, the total force acting on it in that
direction must not be equal to zero. For a normal airplane, on a normal runway takeoff,
thrust must obviously be greater than the other two forces, since we see airplanes take
off every day. Even at the moment of takeoff, aerodynamic drag may have increased, but
the airplane still has a fair bit of excess thrust, and (assuming you don't pull up too
steeply) will continue to accelerate.

So now, let's put the plane on the treadmill. Once again, the ONLY forces acting on it
are thrust (which stays the same), drag (which still increases as speed squared, and wheel
friction (which, again, IS CONSTANT! no matter how fast the treadmill runs, until we start
talking about silly cases like 5000mph treadmills). Sticking to sane, airplane-like
speeds for the treadmill (80-160mph or so) We see that, once again, thrust is greater than
the other two. Therefore, the plane MUST accelerate.

I'd like to also point out that I have tried this on an actual treadmill with a small
model airplane. It takes right off, no problem. Now I just need to go fin

"Bob Martin" wrote in message
...
Darkwing wrote:



Nope the plane won't take off.


You'd better not just be trolling...

Friction generated by wheels is almost negligible. Even for very large
aircraft like a 747, wheel friction is nearly constant, regardless of
speed (at least at sane speeds). It only depends on the amount of force
pushing down on the wheel.

So let's look at all of the forces acting on this airplane. In the
horizontal direction, we have:
Thrust from the engine (nearly constant at these speeds)
Aerodynamic drag (goes up as the square of speed)
Wheel friction (again, nearly constant)

For any object to accelerate in a given direction, the total force acting
on it in that direction must not be equal to zero. For a normal airplane,
on a normal runway takeoff, thrust must obviously be greater than the
other two forces, since we see airplanes take off every day. Even at the
moment of takeoff, aerodynamic drag may have increased, but the airplane
still has a fair bit of excess thrust, and (assuming you don't pull up too
steeply) will continue to accelerate.

So now, let's put the plane on the treadmill. Once again, the ONLY forces
acting on it are thrust (which stays the same), drag (which still
increases as speed squared, and wheel friction (which, again, IS CONSTANT!
no matter how fast the treadmill runs, until we start talking about silly
cases like 5000mph treadmills). Sticking to sane, airplane-like speeds
for the treadmill (80-160mph or so) We see that, once again, thrust is
greater than the other two. Therefore, the plane MUST accelerate.

I'd like to also point out that I have tried this on an actual treadmill
with a small model airplane. It takes right off, no problem. Now I just
need to go fin

I am a regular on RAP, not a troll.

Show me video and I will believe it, if the plane is not moving relative to
the wind then the wing isn't making lift. I have ran on treadmills and I
never felt a "wind" blowing in my face.

----------------------------------------
DW

"Darkwing" theducksmailATyahoo.com wrote in message
...
I am a regular on RAP, not a troll.

Then read the extensive thread that we already had on this subject before
you put your two cents in.

Show me video and I will believe it, if the plane is not moving relative
to the wind then the wing isn't making lift.

True.

I have ran on treadmills and I never felt a "wind" blowing in my face.

That's because you're using your feet for propulsion, and the treadmill was
negating your effort. There's no such effect for airplanes, as they don't
use their wheels for propulsion.

Your experience running on treadmills is irrelevant to the question at hand
(except for the intended effect of course, which is to confuse people like
you who haven't thought the whole thing through).

Pete

Darkwing wrote:
Show me video and I will believe it, if the plane is not moving relative to
the wind then the wing isn't making lift. I have ran on treadmills and I


The problem is, that the aeroplane does move forward, it must, there is
nothing to stop it from doing so.

Imagine (or try)...

1. Hold a wheel between your fingers so it can spin.
2. Put wheel on a treadmill which is not moving.
3. Provide thrust (forward motion) to the wheel from your arm.
4. Observe wheel moves forward.
5. Turn on treadmill, and place wheel on the moving treadmill.
6. Apply SAME amount thrust as you did in 3.
7. Observe same forward movement (discounting the very small friction)
is produced, although the wheel spins much faster.

The thrust acts independantly of the treadmill, you need a treadmill at
least as long as a short field takeoff

You propel yourself on the treadmill by the friction interface between your
feet and the conveyor, not by jet propulsion.

mike

"Darkwing" theducksmailATyahoo.com wrote in message
...

I am a regular on RAP, not a troll.

Show me video and I will believe it, if the plane is not moving relative
to the wind then the wing isn't making lift. I have ran on treadmills and
I never felt a "wind" blowing in my face.

----------------------------------------
DW

Peter Duniho wrote:
"Ray" wrote in message
...
Looks like airplane treadmill problem, regularly a spark for flame wars on
R.A.P., has made it into the mainstream.

http://pogue.blogs.nytimes.com/

And handled with every bit as much intelligence and consideration as we've
seen here. Which is to say, there's no shortage of people convinced that
the airplane won't take off, even though it will.


The problem is that as it is stated, the scenario is not one that could
ever be created with a real treadmill subject to normal engineering
constraints.

Let's imagine that the plane gets started in a slow roll down the
runway at a steady 10 mph relative to the calm air & earth. Now the
treadmill has to speed up to 10 mph, but that makes the plane's tires
start spinning at 20 mph. Therefore the treadmill has to speed up to
20 mph which makes the tires spin at 30 mph, which makes the treadmill
speed up to 40 mph, etc.
Even though the plane is only moving slowly relative to the earth, the
tires and treadmill speeds are in an infinite loop to ever higher
speeds.

As soon as the plane starts moving at all relative to the earth, the
tires and treadmill will start their endless positive feedback loop to
try and reach an infinite speed. If the treadmill has a fast enough
response mechanism to keep up with the increasing tire speed the system
will reach some kind of physical limit before the plane can get any
appreciable speed relative to the air. The tires might explode, the
wheel bearings may fail, the treadmill propulsion system may run out of
power, but you can't satisfy the conditions as stated and have the
plane get up to takeoff speed.

Of course the above is based on a particular interpretation of "speed
of the wheels" i.e. that it is measured based on the speed of rotation
and therefore measures show fast they are rolling on the treadmill
surface. If instead the "speed of the wheels" is measured by seeing
how fast the wheel hub is moving forward relative to the earth then the
above infinite feedback loop doesn't arise and the plane can take off
fairly normally although the wheels will be spinning twice as fast as
normal at takeoff.

But that second interpretation of wheel speed doesn't strike me as
consistent with normal usage - i.e. when a bicyclist is on a stationary
trainer we would normally measure the speed of his rear wheel based on
rotation rate, not say that it's zero since the hub is just spinning
but not moving forward.