Physics Question for you.....

[niki]

The question is a theoretical question and should be answered that way IMO.
The question is asking simply "will takeoff or not" considering only one factor; The conveyer is rotating at the plane speed to the opposite direction.

Actually, if it was B-707 or B-747 the tires would blow before it lifts off. The tire speed limit of both planes is 225 MPH (Goodyear). Usually, a fully loaded 707 or 747 will liftoff around (if my memory is not failing me) 150~160 Knots (Nautical mile/hour) that are around 180 MPH and if the conveyer is rotating at the opposite direction at the same speed......

About speeds, normally, the speeds that are generally known are MPH or Km/H (on the other side of the pond). Those who are in maritime, know another speed; Knot (1 Knot = 1.15 MPH.)

In aviation, all the speeds are represented in Knots (except the tire limit speed) but, there are many "speeds" like:
GS - Ground Speed
TAS - True Air Speed
IAS - Indicated Air Speed
CAS -Calibrated Air Speed
EAS - Equivalent Air Speed
Maybe I forgot a few more

The most important speed for the pilot is the IAS that measure the actual MASS of air acting on the wing for the Lift production (and its different at see level and at 41,000 Feet).

Too low an IAS, and the airplane stalls and turns to a falling stone and the gravity lows of Newton will take care for the rest...
Too high IAS, will break the plane and it will come down in a form of (big) potato chips.

As Russianwolf said, it's a "trick" question just to test us if we know that there is no connection between the engine and the tires, and it should be answered as a "trick" or very theoretical question (at least I don't know an airport that the runway is a conveyer rotating backward).

Mpc
Thanks, I did not know all the facts exept that they had " 2 Engine Sepatation".
The same happend to a B-727 that lost an engine at takeoff at Lapaz airport and turn (steep) to land. (Lapaz airport is at 14,000 feet and sorounded by mountains).

niki

[ejs1097]

I don't think the question can be answered as stated. It all comes down to how "speed" is defined.

Science defines speed as distance traveled over time (such as mph). x=vt (or v=x/t)

One can argue it won't take off because of the conveyer. If you are on a treadmill set at 5 mph you'll have to run to 'stay in place' in relation to your surrounding. But you actually ARE moving your legs at 5 mph because your distance traveled is based upon the length (distance) of conveyour belt passing under your feet.

But if the conveyour is moving at 5mph and you at 10 mph, your moving at 5mph in relations to your surroundings.

So the assumption is made on how the airplane's speed is being measured. By the distance of converyour traveled (or wheel speed)? in which case the plane can't take off. Or by the air speed (amount of air passing around the wings)? in which case it will take off.

Either way the question doesn't state WHAT the airplane's speed is in reference to, conveyour or surroundings. Therefore everyone's right depending upon which assumption they made.

[TheRic]

...This might be a record. At least it is probably a record for the total number of hours of thought and calculation devoted to one thread....

So what is the longest thread on this site?!

[andrew.r.w]

Russian Wolf, I salute you. Never would I have expected this kind of reaction to a simple question. This might be a record. At least it is probably a record for the total number of hours of thought and calculation devoted to one thread.

I read about 8 pages of replies and some of you can come and work for me anytime. Others, nope.

Anyway, I say the plane flies provided (as someone else pointed out) the bearings can handle twice the normal speed and the engines can produce sufficient thrust to overcome the extra drag from the faster-turning wheels.

I don't expect this to be final, but I'd bet a week's pay on it.

[mpc]

Okay, I feel safe replying again. It's amazing how fast this thread grew from when I last logged in...

Yesterday I read a message signed like this:

mpc
Level 5 Engineer - Senior Principal Engineer/Scientist
Aerodynamics, Stability & Control, Flying Qualities, and Simulation
Boeing (formerly McDonnell-Douglas)

I was rather tired (late night sessions in the simulators knocking me totally off schedule) when I posted that; later I realized it looks like I'm trying to say "I'm the world's expert on this so listen to me and shut up" which wasn't my intent. I would have edited it out but others had already quoted it. I apologize to anybody that I offended or annoyed.

Now, today I see at least 20 messages trying to contradict this person, and yet he makes how much to know this stuff, and the rest make how much to make nonesense?

How much do I make? Not enough. haha.

Niki - for that El-Al crash, the circumstances were more than just dual engine failure. One engine actually broke off the wing (due to corroded mounting pins) while the engine was running fine... it tore up part of the wind AND smashed into the engine next to it knocking it off the aircraft too. This happened shortly after takeoff. The flight crew was able turn around and start flying back to the airport; as they turned onto final approach though their airspeed got too low for the torn-up wing and it stalled. That loss of lift caused the loss of control. The airplane did stay airborn though on only 2 working engines for a while.

There's a rather informative website, with a somewhat morbid title, that collects this accident type of stuff. I actually make new folks in my group read some of this to remind them of how serious their jobs can be:
http://www.airdisaster.com/cgi-bin/v...ne=El+Al+Cargo
It does have pop-up adds though, fair warning. Many of the pics are of maintenance goof-ups... guys forgetting to set the parking brake and finding the airplane rolled into a ditch the next morning; etc. It's not all sick.

mpc

[bigfoot]

OK

enough is WAY too much

Yesterday I read a message signed like this:

mpc
Level 5 Engineer - Senior Principal Engineer/Scientist
Aerodynamics, Stability & Control, Flying Qualities, and Simulation
Boeing (formerly McDonnell-Douglas)


Now, today I see at least 20 messages trying to contradict this person, and yet he makes how much to know this stuff, and the rest make how much to make nonesense?

I freekin give up... I was POSITIVE the plane would not move, I am a stubborn swamp yankee by birth, and even I saw the light.

The plane will fly.. period...

OK folks try this answer for size:

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

or

http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/

or Google

airplane + "conveyor belt" and read to your hearts content.....

[backpacker85]

My head hurts just reading all of this, but I STILL stand by my last decision.....it depends on the assumptions made in the original statement.

As stated in response #55, ..if the plane isn't moving, neither is the conveyor!!

What am I missing here????

[25]

and if we dropped a bullet on the conveyor, what impact would it have? and how fast would it be traveling afterwards?

Is the bullet perpendicular or parrell to the motion of the coneyor?

[Stytooner]

I can't beleive we went through this whole thread and no one mentioned the gription rule.

[Russianwolf]

Taking into account the planes air speed, and the speed of the conveyor belt, we know the wheels will be humming along. So this should give us plenty of time to beat it for awhile. Now if the conveyor belt is limited in length, then we would have finite time (and numbers) to beat the horse.

I'm going to go with the conveyor belt is one large loop, since most are, therefore we would have infinite amount of time and numbers to beat the horse. As the speed of the beatings approach the speed of light, and taking the mass of an average sized horse, we can...........

I think the real question is what is the maximum page length allowed?!

and if we dropped a bullet on the conveyor, what impact would it have? and how fast would it be traveling afterwards?

[TheRic]

.....how many times can we beat a dead horse?

Taking into account the planes air speed, and the speed of the conveyor belt, we know the wheels will be humming along. So this should give us plenty of time to beat it for awhile. Now if the conveyor belt is limited in length, then we would have finite time (and numbers) to beat the horse.

I'm going to go with the conveyor belt is one large loop, since most are, therefore we would have infinite amount of time and numbers to beat the horse. As the speed of the beatings approach the speed of light, and taking the mass of an average sized horse, we can...........

I think the real question is what is the maximum page length allowed?!

[scorrpio]

I will reiterate - in reference to what is speed being tracked?
1. In reference to a fixed point outside of the conveyor:
As object moves over the conveyor, the system tracks its absolute position, and adjusts conveyor speed till it matches speed of the object. If this was a car showing 100mph on its speedometer, the system would take the conveyor to 50mph. Relative to the fixed reference point, car is moving at 50 mph, and so does the conveyor.
In case of a plane, whetever speed plane's engines give it, is matched by the conveyor, causing the wheels to spin twice faster than they would on a stationary runway, but not affecting the overall plane's ability to take off.

2. If the system tracks speed at which object moves relative to the conveyor itself, however, the picture changes drastically. In case of a car, no matter how fast the car goes, conveyor matches the speed at which the car passes over its surface, keeping the car motionless for an outside observer.
In case of a plane, the system becomes recursive. No matter how fast the conveyor goes, the tracking system perceives plane body moving at a greater speed, and continues to accelerate the belt in its effort to match.

[Russianwolf]

I was just thinking about it. The original question doesn't say HOW the conveyor "senses" the speed. So lets say for a moment that there is a radar gun tracking the plane and that tells the conveyor how fast to go (so we are reading the speed of the body of the plane in relation to the radar station).

Even if the plane were powered by it's wheels it would eventually lift off (assuming it could go any speed).

Reason: Lets say you needed 100MPH of ground/air speed to acheive lift and flight. Plane starts at 100MPH radar reads and conveyor goes 100MPH. Conveyor would slow down as the body speed of the plane slowed until they both reached 50MPH (plane is trying to go 100MPH forward but 50 MPH backward yeilding a body movement of 50 MPH forward). If the plane increased speed to 200MPH the conveyor would counteract half of this and yeild 100MPH which is needed for lift and flight.

how many times can we beat a dead horse?

[LarryG]

If it were a car on the conveyor what would happen?.

But has been pointed out many times, it's not a car. It's an airplane. And as everyone has now agreed, the conveyor will not stop the plane from moving. So no one who is thinking objectively would argue that "plane speed" can possibly mean anything but its speed relative to some fixed point on the ground. The conveyor speed is dependent on the speed of the airplane, not the other way around.

This will be my last post to this thread, since all this recent discussion amounts only to an attempt to alter the original problem as it was stated.