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

Ok, how about this one. Everyone knows pilots love a tailwind.

You have a giant fan mounted on a truck positioned behind the airplane.
The fan blows air towards the airplane, helping it to accelerate down
the runway (literally blowing it down the runway) while the truck
follows, keeping up with the airplane. Pretty soon the plane will be
thundering down the runway and the pilot pulls back on the yoke.

Does the pilot have to wait until the airplane's speed down the runway
is twice Vr before he can take off?

Jose
--
There is not enough beer in the world to fuel such a discussion!!!!

Peter

"Jose" wrote in message
. net...
[...]
Does the pilot have to wait until the airplane's speed down the runway is
twice Vr before he can take off?

As long as the fan is blowing the airplane forward, it will be impossible
for the airplane to take off. The only way for the fan to move the airplane
forward would be for the airplane to be moving more slowly than the air,
which means the airplane has a continuous tailwind.

If, on the other hand, you make the assumption that you can remove the fan
at any time, you can arbitrarily decide to remove the fan at the normal Vr
and the airplane can take off normally.

There are a variety of other assumptions one could make, due to the
incredibly ambiguous nature of your question, and the exact outcome depends
on how those assumptions are made. I simply provided the two "most
reasonable" assumptions that came to mind, for the sake of discussion.

Pete

Jose wrote:
Ok, how about this one. Everyone knows pilots love a tailwind.

You have a giant fan mounted on a truck positioned behind the airplane.
The fan blows air towards the airplane, helping it to accelerate down
the runway (literally blowing it down the runway) while the truck
follows, keeping up with the airplane. Pretty soon the plane will be
thundering down the runway and the pilot pulls back on the yoke.

Does the pilot have to wait until the airplane's speed down the runway
is twice Vr before he can take off?


Down wind takeoffs require a higher ground speed.
--
Gene Seibel
Tales of Flight - http://pad39a.com/gene/tales.html
Because I fly, I envy no one.

Ray wrote:
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/

http://www.straightdope.com/columns/060203.html

Ray wrote:
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
Yes, the airplane will take off. The thrust of the engine is against the
AIR. NOT the treadmill. There are two real life situations analogous to
this:
1) Will an airplane on an essentially frictionless surface (say, wet
ice) take off?
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).

Rip

"Rip" wrote in message
. net...
Ray wrote:
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
Yes, the airplane will take off. The thrust of the engine is against the
AIR. NOT the treadmill. There are two real life situations analogous to
this:
1) Will an airplane on an essentially frictionless surface (say, wet ice)
take off?

Of course it will at airspeed X. And it will do so at a power setting that
creates airspeed X.

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 will and it will do so at a power setting that creates airspeed X x2

"Gig 601XL Builder" wrDOTgiaconaATcox.net wrote in message
...

"Rip" wrote in message
. net...
Ray wrote:
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
Yes, the airplane will take off. The thrust of the engine is against the
AIR. NOT the treadmill. There are two real life situations analogous to
this:
1) Will an airplane on an essentially frictionless surface (say, wet ice)
take off?

Of course it will at airspeed X. And it will do so at a power setting that
creates airspeed X.

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 will and it will do so at a power setting that creates airspeed X
x2


Uh. That would not be true. At least not as a general statement. Unless you
somehow believe that the extra friction of water moving at twice the takeoff
speed requires X power to overcome - which seems unlikely.
-------------------------------
Travis
Lake N3094P
PWK

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

wrote:
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."

Oh, I see. Basically, you try to say that moving air back is not pushing
it back?


Leonard

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.