Physics Question for you.....

[leehljp]

MPC, I am glad you are here.

But you missed some points and added things that were not in the original statement.

Original statement: This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).


No matter what that conveyer belt operator does the aircraft will barely notice the effect. The wheels will spin like gangbusters but the aerodynamics don't care a whit about that.

This is correct.


The engines will easily overcome the extra rolling friction and the whole aircraft will accelerate relative to some fixed point (or that pole)... accumulating real air speed and it will be able to take off.

This is totally incorrect. According to the original statement, if the plane were to accellerate . . .say to 500 MPH on a conveyor going 500 MPH in opposite direction, its resulting airspeed (and relative ground speed would still be ZERO.
Original statement in different format:Conveyor matches the speed of the Airplane. The plane cannot accellerate faster than the conveyor according to the statement. The logical conclusion is that relative to the ground it will not accumiliate any speed, air or ground, only to the conveyor.

I do not understand people's statement that at some point it will accellerate beyond the conveyor when it is stated in a way that it cannot. Without forward air speed (or a typhoon hitting it head on) it will not fly.


IF, per chance, it did accellerate faster than the conveyor to get the Bernouillis rolling over the wing, of course it would, but the original statement is contrary to this.


Even if the belt was moving 100mph backwards (taking the airplane with it due to that rolling drag) BEFORE the pilot advanced the throttles the aircraft would still be able to overcome that and eventually lift off.

AHHH you are going against the original statement here. You and others are assuming that you can get some extra speed up relative to the air or ground but his statement says othewise. Mentally, I think some of you are trying to "help" it into the air. Wish that that could be so.



The REAL ground speed (measured relative to the pole, not to the stupid conveyor belt) and the air speed will be the same; the ground speed experienced by the wheels will be totally different.

Yes, the REAL ground speed and airspeed will be "0" according to the way the original statement is made.

mpc
Level 5 Engineer - Senior Principal Engineer/Scientist
Aerodynamics, Stability & Control, Flying Qualities, and Si

[mpc]

In an aircraft, air speed and ground speed ARE different most of the time, differing by any winds present. For example, a 10 mph headwind will make airspeed 10 knots HIGHER than ground speed. Which is why airplanes generally pick runways aligned with the wind (or have the aircraft carrier aim into the wind) - the headwind is "free" energy. Takeoff distances need to accumulate enough air speed to get enough airflow over the wings to make enough lift to counter the aircraft weight. If the headwind is really strong the ground roll distance can be very low - and the ground speed will be low too. Thus F=ma doesn't need to integrate up long to get sufficent ground speed (velocity). For landing the situation is similar: headwinds combine with ground speed to create airspeed and wing lift. When the brakes are applied they have to overcome only the ground speed of the aircraft... so landing INTO a headwind lowers the ground speed, so stopping distances are shorter. If you watch small private airplanes, that land at 50 to 60mph typically, strong headwinds make them appear to be flying really slowly or almost hovering.

Now on any multi-engined commercial airliner the FAA regulations mandate that it be able to complete a takeoff and climb upwards (at some minimum rate) with an engine completely failed. A failed engine not only doesn't produce thrust... it actually makes a lot of extra drag as it "windmills" in the airflow. Somebody posted a page back wondering if there would be enough engine thrust to overcome all the drag added by the conveyor belt spinning the wheels. Yes, plenty. A two-engined airplane has more than 2 times the thrust it needs to takeoff actually (since it has to fly on just 1 engine); a 3-engined airplane has 1.5 times what it needs. A 4 engined plane can survive TWO engines crapping out and still takeoff. Only private airplanes and some biz jets have wussy engines; commercial airplanes (jet or turboprops) have lots of excess power when all engines are running. Military aircraft are another matter completely; most can barely stay airborn if an engine dies - and they won't be able to take off with a dead engine. Military requirements are totally different than commercial airliner requirements... after all, military pilots can eject and parachute to safety. United Airlines passengers can't.

No matter what that conveyer belt operator does the aircraft will barely notice the effect. The wheels will spin like gangbusters but the aerodynamics don't care a whit about that. The engines will easily overcome the extra rolling friction and the whole aircraft will accelerate relative to some fixed point (or that pole)... accumulating real air speed and it will be able to take off. Even if the belt was moving 100mph backwards (taking the airplane with it due to that rolling drag) BEFORE the pilot advanced the throttles the aircraft would still be able to overcome that and eventually lift off. The REAL ground speed (measured relative to the pole, not to the stupid conveyor belt) and the air speed will be the same; the ground speed experienced by the wheels will be totally different.

Niki - that belt sander & toy car analogy was terrific.

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

[niki]

I scanned this page from the book.
It shows the forces acting on the airplane during the takeoff roll.

As you can see the only force acting against (or opposing) the engine thrust is the "rolling friction" (axcepr the aerodinamic drag) and it's decreasing as the takeoff speed increases.

[Stytooner]

Actually I was studying speed too much. I should have been thinking about the free wheeling castors on the plane. The guys that said it would fly are correct.

The coveyor belt could be moving forward or backwards and still have no effect on the jet itself because the wheels are freewheeling.

Consider when a plane lands, it goes from the wheels not turning at all to intantly rotating over a couple hundred MPH depending on the plane.
No problem. The plane and its engines operate copletely separate from the landing gear no matter how fast or what direction the gear is traveling. The conveyor is only acting on the gear and not the plane itself.

[leehljp]

The basic way I had the AHAH moment was to think about landing on a moving coveyor belt. Actually my belt sander. You can land on one, so you can do a touch and go or you can take off from a dead start on one.

Hey, you can't do that! it has to have airspeed over the wings or it will fall or stall. Good thing the Wright Brothers didn't know about this on their first plane that had skids instead of wheels. It still took airspeed over the wings for lift.

Once the wheels on a 747 reaches 200 MPH on a conveyor going 200 MPH in the opposite direction, there still is NO lift from the 0 mph wind going over the wings.

Boats, river current and wheel speed are irrelevent to wind speed lift over the wings.

Now as to your belt sander - you can take off and land on it provided the belt sander is also moving (not just the belt) at the speed at which lift takes place.

On the conveyor, everyone is looking at ground speed. They need to be looking a WIND speed. Wind speed is what makes a plane or glider fly, not wheel speed.

[LCHIEN]

The conveyor moves only in relation to the plane's speed, so how fast will the plane be able to move? Once it moves 1 MPH, the conveyor will counter, moving at 1 MPH opposite, bringing the plane's speed down to 0 MPH, which will then make the conveyor 0 MPH. In this instance I don't see how the plane could fly unless it had sufficient acceleration to overcome the time lag between the conveyor sensor reading the plane's speed and the conveyor coming up to that speed. Nonetheless, it would be a very jerky ride!

Once it moves 1 MPH, the conveyor will counter, moving at 1 MPH opposite, bringing the plane's speed down to 0 MPH,

That's where you are wrong, the conveyor will move at 1 MPH but the airplane won't slow, just the wheels will spin faster.

[Stytooner]

The basic way I had the AHAH moment was to think about landing on a moving coveyor belt. Actually my belt sander. You can land on one, so you can do a touch and go or you can take off from a dead start on one.

[niki]

Hi jerry

Lets make an experiment.
Take your car, remove the engine and the gear so the wheels can rotate free forward or back.

Put your car on the conveyer. Put some barrier behind the car just not to let it go backward, the barrier will be anchored to the ground, not to the conveyer.

Turn the conveyer to 500 MPH.
Because the wheels can move free, they will rotate at 500 MPH but the force backward will be only the tire drag + bearing friction, I think that you can stop the car from moving backward only with your hand (lets say that you need 100 lbs to stop the car moving back)

Now, on the roof of you car, install an engine with propeller or jet engine or a rocket than can produce 10,000 lbs of thrust (it will not have any connection to the car wheels that still can move free).

Go back on the conveyer, turn it to 500 MPH, get into the car and start the engine/jet/rocket to maximum power.

To overcome the tire and bearing friction you need only 100 lbs, the rest 9,900 lbs are pushing the car forward off the barrier. Of course the wheels speed will increase but you are pushed and accelerated forward.

niki

[jerrye]

Is this making sense?

The conveyor moves only in relation to the plane's speed, so how fast will the plane be able to move? Once it moves 1 MPH, the conveyor will counter, moving at 1 MPH opposite, bringing the plane's speed down to 0 MPH, which will then make the conveyor 0 MPH. In this instance I don't see how the plane could fly unless it had sufficient acceleration to overcome the time lag between the conveyor sensor reading the plane's speed and the conveyor coming up to that speed. Nonetheless, it would be a very jerky ride!

[Stytooner]

I knew I felt wrong about going against Loring. I should have known better.
All you would need is a flight plan.
I was thinking literally, but it is a physics question.

Sorry to have drug this on so long. I was wrong.
This plane will fly. I suddenly feel so enlightened.

The wording in the original statement is the culprit though.
The conveyor could be running ten times faster than takeoff speed in reverse and the plane would indeed move forward. It's free wheeling casters are what got me.

I actually figured this out on my belt sander.

Thanks for the mind work out guys.

[Stytooner]

There is no mention of tire speed in the original statement. Its tracking the plane's speed period. Ground speed, airspeed all the same. Speed is speed or in the case, the nulifying of forward speed with equal speed in the opposite direction at the same time.

To actually acheive any forward speed on the plane, you would at least have to slow down the conveyor belt. This would at least allow some forward movement of the plane and therefore at least some speed.
No speed, no go. No go, no fly. No fly, no good.

If ya'll want this plane to fly, you will have to adjust the conveyor and therefore the original statement.


I have a question for Russian Wolf. Did you have an answer for this when you popped it to us?

[niki]

I think that all the confusion comes from the wording of the question:

The conveyer DOES NOT track the AIRPLANE SPEED (that is measured relative to the air and the conveyer does not know it), the conveyer is tracking the TIRE SPEED that has nothing to do with the TAS (True AirSpees) or IAS (Indicated Air Speed).

I just Googled "airplane takeoff" and found the same question in the following sites:

http://www8.sbs.com.au/sbsforum/view...3761778c18a3dc

http://community.discovery.com/group...6/m/7451937218

And even a video on YouTube
http://www.youtube.com/watch?v=IZGdUAiMcPs

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

http://digg.com/general_sciences/Phy...lane_take_off_

http://www.airtalk.org/can-a-plane-o...f-vt50373.html


http://forums.anandtech.com/messagev...VIEWTMP=Linear

[jackellis]

There's an analogue to the airplane example, which is a float plane on a river where the current moves from tip to tail of the airplane at exactly the same speed that the airplane moves relative to the current. In this case, the force of the current on the airplane and the airplane on the current exactly counteract one another. Otherwise, the airplane would start moving.

The speed of the treadmill "runway" is probably irrelevant. What is relevant is the force exerted by the treadmill runway against the airplane's wheels in the direction that's perpendicular to gravity (horizontal). A wheel airplane will only move on that treadmill if it's wheels skid. With enough power and a very very long runway, the airplane would fly, but it would be an ugly thing to watch.

[niki]

I would like to explain why I changed my mind.

Take a matchbox car, tie a string at the front and tie a fish-scale to the string.
Put the car on your belt sander, switch it on, and measure the force required to hold the car in place, let's say that you measured 2 lbs, this force is the product of the tire and bearing friction.
now, increase the belt speed, you shall still read 2 lbs (maybe a little more) but the tires are turning faster.

Now, install on top of the car roof some propeller or jet engine that can produce 3 lbs or more. 2 lbs of thrust will be required to overcome the tire/bearing friction (without any connection to the tire speed) and the rest of the thrust will produce forward motion relative to the ground, or more correct, to the space.

The wheels, are only medium between the plane and the belt, they are not tieing the plane to the belt and are free to rotate. They produce some drag force, but ones we overcome this drag force, the plane is like in the free air.

Hope it make sense
niki

[Stytooner]

I agree with Larry on this.
I do see both points now and almost changed my mind as well, but thats only when I don't take the initial statement at face value.

"This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?"


It is the plane's speed, that is being taken away by the conveyor's speed. Not the thrust of air through the engines, but the actual forward progress of the plane itself. Given the information we have and reading as little into it as possible, the plane will not take off.


A fighter jet on an aircraft carrier that is faccing into the wind sitting on deck ready to take off is having its forward progress arrested by the catapult. Even with the after burners on at full throttle, its still just sits there. It's only when they let it go, that it gets enough forward movement to take off.
Will the conveyor counteracting on a planes forward movement, it cannot move forward, therefore it cannot takeoff.

That is my main point. The conveyor is matching the plane's speed which ultimately gives it no speed at all.