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

Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...

The Rocket takes a small amount of matter and throws it at great speed
in X direction, getting and equal and opposite reaction in the opposite
direction Y... The prop grabs a large amount of air and throws it in X
direction, getting an equal and opposite reaction in Y... The principal
is the EXACT same.

Its the same principal as aircraft flying - while the common perception
is that aircraft fly because of pressure differential, 'really' they're
flying solely because they're imparting a downward force on the air
that is exactly equal the downward force caused by gravity- that is,
they're forcing exactly enough air downward to accelerate themselves
upward at 1 G...

The simple reality is that these are all various ways of stating the
EXACT same thing (and fundamentally, they all cancel out if you go back
down to the equasions)... Saying that an Aircraft flies via pressure
differential vs. flies via forcing air downward... or saying that a
prop or rocket flies via the same too manners (one might suggest that a
rocket flies with the same pressure differential as an aircraft, just
centered in a MUCH smaller area (inside the bell vs. outside, rather
than in front of the prop vs behind)... in the end, its the same net
result- mass gets forced backwards, I get forced forward... life is
good.


On Dec 13, 8:18 am, Jose wrote:
The thrust of the engine is not against the air. It generates
thrust as a Newtonian reaction to the prop moving air back, not
"pushing on other air." A rocket in space has nothing to push against,
yet it generates the same thrust as it did in the atmosphere.Actually, it's not that way. (but read carefully)

The thrust of a propeller engine is created when the propeller (an
airfoil) creates a high pressure area behind and a low pressure area in
front of the prop, as it pushes air back. The prop is pushing against
the air in order to do this. The air is constantly trying to get out of
the way, but it is not entirely successful, which leads to the pressure
differences. There's nothing funamentally wrong in saying that the
airplane pushes against the air to move forward. The prop (a part of
the airplane) is doing the pushing.

Rockets are different. The tail of fire coming out of the rocket does
push against the air (push the air out of the way to make room for the
fire), but it is =not= part of the rocket. If there were no air to
"push against", the rocket would work just as well, for that reason.
Where the rocket gets its thrust is the tail of fire pushing (the other
way) against the engine bell of the rocket itself. The rocket is
pushing against the fire, in essence. The fire is =not= part of the rocket.

Both cases can be viewed in the newtonian "action/reaction" paradigm,
but something has to push against something else in order to get the
thrust to happen in the first place. In a plane, the propeller pushes
against the air (to make the air go backwards fast and create thrust. In
a rocket, the engine bell pushes against the tail fire (pushing the fire
out, and the rocket forward). I suppose it may be clearer to say that
the expanding gasses of the tail fire push against the engine bell, but
the two are equivalent.

Although the expanding rocket gasses do push the air out of the way,
that doesn't help the rocket in any meaningful way.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

"EridanMan" wrote in message
ps.com...
Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...



At best, you could say that they accomplish the same thing. I don't agree
that they accomplish it in the same manner. Propellers, wings, and sails on
sail boats all work in the same manner. Whether you like to think of them as
redirecting mass or creating a pressure differential, they all do it the
same way. And none of them could do it without moving through a redirectable
mass - like air.

Rockets, on the other hand, carry their own redirectable mass with them, so
to speak.
-------------------------------
Travis
Lake N3094P
PWK

The Rocket takes a small amount of matter and throws it at great speed
in X direction, getting and equal and opposite reaction in the opposite
direction Y... The prop grabs a large amount of air and throws it in X
direction, getting an equal and opposite reaction in Y... The principal
is the EXACT same.

Yes, inasmuch as it is action-reaction by Newton. The nit I was trying
to pick was that a propeller is viewed (correctly) as part of the
airplane, and the exhaust fire is also viewed (albeit incorrectly) as
part of the rocket.

The rocket pushes against the fire (in the nozzle, which =is= part of
the rocket). The airplane pushes against the air (against the
propeller, which is part of the airplane). These forces make the
vehicles go.

Although the rocket exhaust does in fact push against the air, the
exhaust isn't part of the rocket, and that "pushing" doesn't make the
rocket go.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

EridanMan wrote:
Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...

Not exactly the same. Rockets work in a vacuum, but propellors do not.

Matt

wrote in message
ups.com...

Rip wrote:
Yes, the airplane will take off. The thrust of the engine is against the
AIR. NOT the treadmill.

The thrust of the engine is not against the air. It generates
thrust as a Newtonian reaction to the prop moving air back, not
"pushing on other air." A rocket in space has nothing to push against,
yet it generates the same thrust as it did in the atmosphere.

1) Will an airplane on an essentially frictionless surface (say, wet
ice) take off?

Of course, as forward motion creates airflow over the wings.
There is no forward motion on the treadmill.

2) Will a sea plane take off upriver in a current equal to it's take-off
speed (this one is a cheat, since it involves drag not involved in the
original situation, but should be a good "fire starter" for further
discussion).

Yes, it would, but it's waterspeed at takeoff airspeed would
be double the usual takeoff speed. However, this would require
considerable power to overcome the extra drag of the floats on the
water, being a lot more than wheels on pavement.

I have a hard time believing that so many people can't see that it's
airflow over wings, not wheel speed or prop blast, that lifts
airplanes. What do they think wings are for, anyway?

Why don't we discuss something truly valid, like the downwind
turn feared by some (especially a few RC modelers) that they think will
reduce airspeed and cause a stall?

Dan

And, after we've resolved that one, we can move on to the turn to the final
turn for a left crosswind landing from a tight left pattern or a right
crosswind landing from a tight right pattern.

Peter

wrote in message
ups.com...

Rip wrote:
Yes, the airplane will take off. The thrust of the engine is against the
AIR. NOT the treadmill.

The thrust of the engine is not against the air. It generates
thrust as a Newtonian reaction to the prop moving air back, not
"pushing on other air." A rocket in space has nothing to push against,
yet it generates the same thrust as it did in the atmosphere.

1) Will an airplane on an essentially frictionless surface (say, wet
ice) take off?

Of course, as forward motion creates airflow over the wings.
There is no forward motion on the treadmill.

Why isn't there forward motion on the treadmill? The pressure differential
around the prop or the thrust from a jet will propel the plane forward to
takeoff speed on glare ice (wheels don't have to spin at all) or the
treadmill (wheels spin at twice the speed).



--
-------------------------------
Travis
Lake N3094P
PWK

Travis Marlatte wrote:

Why isn't there forward motion on the treadmill? The pressure differential
around the prop or the thrust from a jet will propel the plane forward to
takeoff speed on glare ice (wheels don't have to spin at all) or the
treadmill (wheels spin at twice the speed).

The Original Question said that the treadmill speed was such as
to counter wheel speed so that the airplane stood still. That's what I
was thinking. Somewhere along this thread the assumption must have
changed and I'm arguing apples against oranges.
No airspeed, no lift. Period. Propeller or jet blast is not going
to lift the airplane. We need forward motion relative to the
atmosphere, or a really strong headwind.

The tires used on light aircraft are Type III and are rated for
120 MPH max. They'd probably explode before 150 MPH, seeing that the
forces increase by the square of the rotational velocity. Or. more
likely, the average lighplane tire being as out-of-round and imbalanced
as it is, the vibration would shake the airplane apart by the time we
got to 120.

Dan

wrote in message
ups.com...


The Original Question said that the treadmill speed was such as
to counter wheel speed so that the airplane stood still. That's what I
was thinking.

That's where you fell into the trap. The statement was that the treadmill
moves backwards at the same speed as the airplane is moving forward - it
didn't say that the airplane stood still (but that's the conclusion it
wanted you to reach). But, since the propeller and not the wheels is
driving the plane forward it doesn't matter how fast the belt on the
treadmill runs backwards, the plane will still move forward.

No airspeed, no lift. Period. Propeller or jet blast is not going
to lift the airplane. We need forward motion relative to the
atmosphere, or a really strong headwind.

wrote in message
ups.com...

Travis Marlatte wrote:

Why isn't there forward motion on the treadmill? The pressure
differential
around the prop or the thrust from a jet will propel the plane forward
to
takeoff speed on glare ice (wheels don't have to spin at all) or the
treadmill (wheels spin at twice the speed).

The Original Question said that the treadmill speed was such as
to counter wheel speed so that the airplane stood still. That's what I
was thinking. Somewhere along this thread the assumption must have
changed and I'm arguing apples against oranges.
No airspeed, no lift. Period. Propeller or jet blast is not going
to lift the airplane. We need forward motion relative to the
atmosphere, or a really strong headwind.

The tires used on light aircraft are Type III and are rated for
120 MPH max. They'd probably explode before 150 MPH, seeing that the
forces increase by the square of the rotational velocity. Or. more
likely, the average lighplane tire being as out-of-round and imbalanced
as it is, the vibration would shake the airplane apart by the time we
got to 120.

Dan

I am not sure what sort of perversity is causing me to continue reading this
insane thread, much less respond to it.

Here is a copy of the text, lifted from the New York Times web site as
linked; except that I have edited out the special characters which appeared
as a result of the text conversion:

-----------begin NY Times excerpt---------

December 11, 2006, 4:37 pm
The Airplane-Treadmill Conundrum
OK, this 's driving me crazy. This brain-teaser is ripping around the
Internet, plus I actually overheard it THREE TIMES in airport conversations
on a recent trip to Canada.'s how I found it presented at
http://boards.straightdope.com/sdmb/...d.php?t=348452 Imagine a plane
is sitting on a massive conveyor belt, as wide and as long as a runway. The
conveyer belt is designed to exactly match the speed of the wheels, moving
in the opposite direction. Can the plane take off? I say no, because the
plane will not move relative the the ground and air, and thus, very little
air will flow over the wings. However, other people are convinced that since
the wheels of a plane are free spinning, and not powered by the engines, and
the engines provide
thrust against the air, that somehow that makes a difference and air will
flow over the wing. The guy behind me at the airport told his buddy that,
in fact, the plane WOULD take off, and his buddy seemed to agree. Do we have
any physicists in the audience?

---------end NY Times excerpt-------

Please note that the embedded link still works.
Please note also that nothing new can be added.

I submit that there are really only two logical presumptions, both of which
have been explored:
1. The aircraft will take off normally.
2. The magic treadmill has no inertia and no limiting speed; and, by some
unexplained means, can sense the rotation of the wheels. Therefore, if the
aircraft is placed very near the departure end of the runway and connected
to an airport tug which is placed on solid ground; then, as the tug begins
to slowly pull the aircraft forward, the magic treadmill will quickly
accelerate until the wheels and tires of the airplane have catastrophically
failed.

I further submit that the second presumption is ridiculous.

Peter

"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/

Let the arguing begin!

- Ray

"The conveyer belt is designed to exactly match the speed of the wheels,
moving in the opposite direction"

Speed of the wheels in relation to what?

The runway under the treadmill, or the surface of the treadmill itself?

If the answer to this question is the surface of the treadmill, the answer
to the original question is undefined (as in division by zero is undefined).

If the answer to this question is the runway under the treadmill, then the
aircraft will take off at the normal airspeed, with the wheels rotating at
twice the airspeed. Should take about the same distance over the runway
(under the treadmill)