Leaving on a jet plane (or, how smart are you again?)
- Thread starter Turok
- Start date
Well, once one startsd thinking about the "wind tunnel" thing, its kinda sticks for some reason
Consider-you have a wheelchair, on a runway length conveyor belt.
The conveyor is moving at 5mph, the wheelchair can just go back with it if you want.
Now, fire up the pratt and whitney turboprop, powering the wheelchair, generating enough thrust to launch a one tonne vehicle along a runway from 0-100 mph in about 10 seconds.
What would happen?
I noticed a mistake in the snippet of mine you posted above. There is a very important "not" missing...
It should read:
*****************
The most entertaining thing is reading the posts that INSIST that the plane will not take off because a plane must have moving air over the wings to fly. -- They are right of course (about the need for airspeed), but they can't get out of the "it can't move" rut.
*****************
Big diffference in the meaning. Sorry.
JB
It should read:
*****************
The most entertaining thing is reading the posts that INSIST that the plane will not take off because a plane must have moving air over the wings to fly. -- They are right of course (about the need for airspeed), but they can't get out of the "it can't move" rut.
*****************
Big diffference in the meaning. Sorry.
JB
Yes, THEY can.....
Thrust, as you say, can be measured in lbs. So what force, in lbs, counteract the thrust? Still apples and oranges?
"IF" someone takes the stand that the plane won't move, then they should be able to solve for X. That is, show what would counteract the engine thrust to make the equation = 0.
But they can't, and that's the point of the Q.
Haroki, your equation makes as much sense as this one ...
Solve for X
X + blue = color of sky + engine thrust
Of course the sky is blue so X = engine thrust. OK, X = engine thrust, so what. All your equation proves is that in fact, you *don't* remember your algebra.
Both sides of an equation must be able to be simplified and in 'here and now' practical terms, "wheel speed" and "engine thrust" can't be simplfied. Mph and Lbs don't mix.
JB
Solve for X
X + blue = color of sky + engine thrust
Of course the sky is blue so X = engine thrust. OK, X = engine thrust, so what. All your equation proves is that in fact, you *don't* remember your algebra.
Both sides of an equation must be able to be simplified and in 'here and now' practical terms, "wheel speed" and "engine thrust" can't be simplfied. Mph and Lbs don't mix.
JB
Then you're not as smart as you think you are.
Mph > conveyor and wheels.
lbs thrust > jets and ?
Think of your excellent wheelchair on the treadmill example. When you push the chair up the ramp, what would be required to prevent you from doing that? An opposing force.
My Q to "them" is what is providing that opposing force?
"They" don't have an answer to that, cuz they're wrong in their assertion that the plane can't move.
Well, ZZzz deleted a post.
In it he states that under the theoretical assumption that a plane couldn't take off if the conveyor belt moved equal to wheel speed.
That's actually true, but the problem with that is it's impossible to achieve in anything else but theoretical circumstances since there is nothing to counteract the force(thrust) provided by the jets since the wheels spin freely and will accelerate to whatever speed is necessary to achieve forward movement.
In it he states that under the theoretical assumption that a plane couldn't take off if the conveyor belt moved equal to wheel speed.
That's actually true, but the problem with that is it's impossible to achieve in anything else but theoretical circumstances since there is nothing to counteract the force(thrust) provided by the jets since the wheels spin freely and will accelerate to whatever speed is necessary to achieve forward movement.
upon further consideration..
Turok is WRONG.. the plane CANNOT take off
why?...
start the treadmill,
place the plane on it engines off.
the plane is moving relative to the air, but not the treadmill, it's wheels are not turning.
there is now a condition of "negative lift", ie air is now flowing in opposite direction moving from posterior wing to anterior. because plane is moving thru space, backward!
start plane's engines. progressively increase thrust.
the plane begins to move "LESS backward", but never forward.
the lift remains "negative".
until sufficient thrust pulls/pushes the plane's movement, and wheels, EQUAL to treadmill's speed.
at THIS point in time, after gradually diminishing "negative lift, there is NO air flow over the wing,
ZERO, NADA, ZILTCH!
and still NO POSITIVE LIFT.
the plane CANNOT take off!
the plane is FIXED IN SPACE!
unfortunately, it remains grounded.
the treadmill is moving CLOCKWISE at a certain speed, the plan'es wheels are moving at a speed EQUAL to treadmill but counterclockwise.
Only FURTHER increase in planes thrust and speed above and beyond that of the treadmill would permit the plane to reach sufficient POSITIVE air FLOW over the wing to liftoff eventually.
at this point the plane would be ADVANCING along the treadmill's belt, ie relative to the treadmill but its wheels FASTER than the treadmill/s backward speed (and the air, which are one and the same for the intent of this problem), but NOT necessarily the PLANE relative to the AIR
only when the plane is advancing relative to the treadmill and air can the plane have any hope to take off.
A: the plane CANNOT take off under the initial conditions set in this physical problem!!
ps turok, wrong again, what else is new
psps post edited numerous times to enhance clarity of argument
Turok is WRONG.. the plane CANNOT take off
why?...
start the treadmill,
place the plane on it engines off.
the plane is moving relative to the air, but not the treadmill, it's wheels are not turning.
there is now a condition of "negative lift", ie air is now flowing in opposite direction moving from posterior wing to anterior. because plane is moving thru space, backward!
start plane's engines. progressively increase thrust.
the plane begins to move "LESS backward", but never forward.
the lift remains "negative".
until sufficient thrust pulls/pushes the plane's movement, and wheels, EQUAL to treadmill's speed.
at THIS point in time, after gradually diminishing "negative lift, there is NO air flow over the wing,
ZERO, NADA, ZILTCH!
and still NO POSITIVE LIFT.
the plane CANNOT take off!
the plane is FIXED IN SPACE!
unfortunately, it remains grounded.
the treadmill is moving CLOCKWISE at a certain speed, the plan'es wheels are moving at a speed EQUAL to treadmill but counterclockwise.
Only FURTHER increase in planes thrust and speed above and beyond that of the treadmill would permit the plane to reach sufficient POSITIVE air FLOW over the wing to liftoff eventually.
at this point the plane would be ADVANCING along the treadmill's belt, ie relative to the treadmill but its wheels FASTER than the treadmill/s backward speed (and the air, which are one and the same for the intent of this problem), but NOT necessarily the PLANE relative to the AIR
only when the plane is advancing relative to the treadmill and air can the plane have any hope to take off.
A: the plane CANNOT take off under the initial conditions set in this physical problem!!
ps turok, wrong again, what else is new
psps post edited numerous times to enhance clarity of argument
Haroki:
>Then you're not as smart as you think you are.
I'd be careful in your evaluations of "smart" when you can't generate a valid high school level algebra equation.
LOL
>Mph > conveyor and wheels.
>
>lbs thrust > jets and ?
Are those also supposed to be some sort of "valid" equations?
more LOL
>Think of your excellent wheelchair on the
>treadmill example.
Wasn't mine, twas macal's, but excellent for sure.
>When you push the chair up the ramp, what would
>be required to prevent you from doing that? An
>opposing force.
>My Q to "them" is what is providing that opposing force?
It's an excellent question, and an absolutely invalid, violate the rules of algebra equation.
>"They" don't have an answer to that, cuz
>they're wrong in their assertion that the
>plane can't move.
They are wrong, but the reason they can't solve your equation is because IT ISN'T A VALID EQUATION and violates the simples rules of algebra.
LOL
You ask good question, you just don't write valid equations.
JB
PS: You can have the last word on this -- I'm not going to continue debating a simple high school algebra rule with you.
>Then you're not as smart as you think you are.
I'd be careful in your evaluations of "smart" when you can't generate a valid high school level algebra equation.
LOL
>Mph > conveyor and wheels.
>
>lbs thrust > jets and ?
Are those also supposed to be some sort of "valid" equations?
more LOL
>Think of your excellent wheelchair on the
>treadmill example.
Wasn't mine, twas macal's, but excellent for sure.
>When you push the chair up the ramp, what would
>be required to prevent you from doing that? An
>opposing force.
>My Q to "them" is what is providing that opposing force?
It's an excellent question, and an absolutely invalid, violate the rules of algebra equation.
>"They" don't have an answer to that, cuz
>they're wrong in their assertion that the
>plane can't move.
They are wrong, but the reason they can't solve your equation is because IT ISN'T A VALID EQUATION and violates the simples rules of algebra.
LOL
You ask good question, you just don't write valid equations.
JB
PS: You can have the last word on this -- I'm not going to continue debating a simple high school algebra rule with you.
Ok, let's be more specific.
1-The jets provide , say, 100,000 lbs of thrust.
2- it would take, say 40,000 lbs of thrust to roll the jet along on level ground at 150mph (assumed takeoff velocity).
3- so there's 70,000 lbs of thrust that must be counteracted to negate forward movement.
4- the plane's wheels will accelerate to whatever speed necessary to achieve 150mph airspeed.
So what provides the 60,000 lbs of negative thrust against the plane?
X + (conveyor belt speed*y) = (wheel speed * y) + (100,000 lb thrust - 40,000 lb aerodynamic drag)
y = essentially equals zero since the plan'es wheels will accelerate to whatever speed necessary to achieve 150 mph.
So now the equation will be
X = -60,000lbs of thrust.
What provides the negative thrust?
1-The jets provide , say, 100,000 lbs of thrust.
2- it would take, say 40,000 lbs of thrust to roll the jet along on level ground at 150mph (assumed takeoff velocity).
3- so there's 70,000 lbs of thrust that must be counteracted to negate forward movement.
4- the plane's wheels will accelerate to whatever speed necessary to achieve 150mph airspeed.
So what provides the 60,000 lbs of negative thrust against the plane?
X + (conveyor belt speed*y) = (wheel speed * y) + (100,000 lb thrust - 40,000 lb aerodynamic drag)
y = essentially equals zero since the plan'es wheels will accelerate to whatever speed necessary to achieve 150 mph.
So now the equation will be
X = -60,000lbs of thrust.
What provides the negative thrust?