Can a Plane on a Treadmill Take Off?

"Gary Drescher" wrote:

"Don Tuite" wrote in message
.. .
but what
is the point of the original question in that case? Is it just to
trap a sloppy reader into thinking it's all about a crackpot VTOL
methodology?

I think the more interesting point is to notice the implications of not
transmitting force through the wheels. Even people who know that planes and
cars differ in that way may fail (at least at first) to draw the appropriate
conclusion about what happens to the treadmill plane when it applies takeoff
power.

Totally irrelevant. The answer is the same for a glider being towed to
take-off by a ground vehicle (except that you would want to change the
rear end or wheel size of the vehicle to cut the final drive ratio in
half, so that it could attain the doubled wheel speed necessary to
attain the needed forward speed.)

--Gary


--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

The mistake is in not realizing that the air that the propeller acts upon is
unchanged regardless of the speed of the conveyor belt. Therefore, the
propeller will be able to pull the airframe forward on the conveyor
regardless of how fast the conveyor moves, because it is putting a force on
the airframe relative to the air, not relative to the conveyor belt.

The same thing would happen if you attached a rope to the nose of the
airplane and stood on the ground ahead of the conveyor so that you were not
standing on the moving belt. Start the conveyor and the airplane stands
still while the wheels spin with the conveyor belt. Now pull on the rope
and you can move the airplane forward even though the conveyor is moving
backwards and the wheels are spinning like crazy.

The mistake I made in comparing it to a seaplane on a river is that the
floats are attached to the airframe - where they go, the plane goes. No so
with wheels. Wheel rotational speed has nothing to do with how fast the
airframe moves in this scenario.

BDS

"Helen Woods" wrote in message
...
Nice article explaining this:

http://www.avweb.com/news/columns/191034-1.html


C'mon Helen! .....How can reading that article in 5 minutes possibly
be as much fun as arguing about the problem in a public forum over the
course of 2 days??? ....Huh?!?

Omigod! ....What's happened to me!?! :~(

"alexy" wrote in message
...
"Gary Drescher" wrote:
I think the more interesting point is to notice the implications of not
transmitting force through the wheels. Even people who know that planes
and
cars differ in that way may fail (at least at first) to draw the
appropriate
conclusion about what happens to the treadmill plane when it applies
takeoff
power.

Totally irrelevant. The answer is the same for a glider being towed to
take-off by a ground vehicle (except that you would want to change the
rear end or wheel size of the vehicle to cut the final drive ratio in
half, so that it could attain the doubled wheel speed necessary to
attain the needed forward speed.)

I don't think we're in disagreement. My point is just that (even assuming
frictionless wheels) you have to make some changes (such as the ones you
proposed) to a typical ground vehicle to imagine it running normally at
twice its usual speed (even if the relative wind is merely at the usual
speed). The airplane, in contrast, just takes off normally from the
treadmill without needing to be modified.

--Gary

AES wrote:
If you tie a 100 foot rope to the tail of an airplane (or some other
part of the airframe), attach it to a good strong post, and run the
propellor up to whatever rpm is available, is anyone claiming the
airplane can then lift up say a feet off the ground?

(Assuming the tail doesn't tear off)

(and, a conveyor belt under the airplane is optional)

That completely changes the situation. The plane couldn't take off with a
regular runway if you secured it to a fixed post.

--

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

("Michael Ware" wrote)
You are taking the statement 'a conveyer belt that moves in the opposite
direction at exactly the speed that the airplane is moving forward' to
mean that somehow there is a force being applied to the mass of the
aircraft, equal and opposite the thrust generated by the propellor. The
only place the treadmill can exert any force an the airplane is the only
place the treadmill is touching the airplane: the wheels. Any motion of
the treadmill belt will be translated into rotation of the wheels. This
will not prevent the aircraft from moving forward, through the air and
taking off.


THE AIRPLANE WILL NOT MOVE. (That's my vote)

The rotating wheels + gravity (Thank you Sir Isaac!) ANCHOR the plane to the
treadmill. Plane/prop move forward, treadmill/wheels fall back. The plane is
attached to the wheels. Try it in front of you with a ruler and a magic
marker.

That airplane is doing 150 mph down that runway, only the runway is really a
treadmill which is matching that speed. End result is = to an Olympic
sprinter on the same treadmill - I can stand next to him for his entire 10
second (27 mph) race.


Montblack 83.7
(I thought some of double-digit people needed to be heard from. g)

"cjcampbell" wrote:

"An airplane on a runway sits on a conveyer belt that moves in the
opposite direction at exactly the speed that the airplane is moving
forward. Does the airplane take off?" (Assuming the tires hold out, of
course.)

Now, there are two references to motion in the problem, and the
correct (IMHO) solution is based on both of those motions being from a
consistent frame of reference, i.e., relative to the ground.

The incorrect (IMHO) solution seems to depend on reading these two
motions as related to inconsistent frames of reference, to wit: "An
airplane on a runway sits on a conveyer belt that moves relative to
the surface of the earth in the opposite direction at exactly the
speed that the airplane is moving relative to the surface of the
conveyer [Not sure how those who read it this way fit the word
"forward" into their interpretation.] This reading leads to the
conclusion that the plane is standing still, but flies in the face of
what really would happen if such a device were built, given how a
plane's propulsion is provided -- i.e., this reading of the problem
assumes facts inconsistent with what conceivably could happen were
such a device built. (BTW, many seem to focus on this practical aspect
of propulsion, but that misses the pure logic of the thought
experiment, it seems to me.)

But think about the opposite inconsistent reading of the statement:
"An airplane on a runway sits on a conveyer belt that moves, relative
to the airplane, in the opposite direction at exactly the speed that
the airplane is moving forward relative to the ground." That
inconsistent frame of reference seems just as justifiable as the
other, and is in fact MUCH easier to imagine actually implementing!

I think we should do something to make sure that all future airports
are built with runways that work like this third interpretation of the
stated problem! g
--
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

BDS, aren't you glad you used initials rather than your name?

"BDS" wrote

Nevermind - I finally get it.

DUH!!

BDS

In article ,
"Michael Ware" wrote:

Are you saying 1) the rope is tight, or are you saying 2) you are giving the
plane a 100' running start?


"AES" wrote in message
...
If you tie a 100 foot rope to the tail of an airplane (or some other
part of the airframe), attach it to a good strong post, and run the
propellor up to whatever rpm is available, is anyone claiming the
airplane can then lift up say a feet off the ground?

(Assuming the tail doesn't tear off)

(and, a conveyor belt under the airplane is optional)

Didn't think of that -- and maybe your response is tongue
in cheek -- but I had in mind "rope is tight".

"Flyingmonk" wrote in message
oups.com...
Already rephrased my statement.

Sorry, I missed that. Even having been told it exists, I still can't find
the post with the rephrasing, but I'll assume it essentially retracts the
claim that an airplane can fly from prop wash alone.