Physics Question

[Chode Messiah]

vtol beats conveyor.

 

[91TTZ]

Originally posted by: Chode Messiah
vtol beats conveyor.

The conveyor also has a huge mechanical arm that grabs all vehicles trying to escape its influence.

 

[Chode Messiah]

the harrier or F35 have heavy munitions and ordinance. amraam pwns swing arm IMO.

 

[dawks]

Originally posted by: 91TTZ
Infinite power conveyor > finite power vehicle with rolling resistance


The conveyor will just keep accelerating until the rolling resistance consumes all of the vehicle's power. As long as there is at least some friction between the belt and the vehicle, the vehicle has no chance against the all-powerful conveyor.

Why do you keep saying this? It has already been addressed. The wheels would just blow out. Then of course the plane couldnt take off.

But as long as the belt matches, or even stays just SLIGHTLY below the blowout threashold, the plane will take off without issue.

 

[DeeKnow]

engines produce thrust, not lift.

the speed (groundspeed) in this case is completely irrelevant... it needs a certain airspeed before there is sufficient lift created to carry the weight of the plane... so no take-off.

 

[V00D00]

Originally posted by: JoeFahey1
42

Yeah, souds abot right. I woudl say "yes" THEORETICALLY, but no in reality.

 

[dawks]

Originally posted by: myusername

Originally posted by: Mo0o
No. the "lift" comes from the air passing through the wings and creating a pressure difference . If the plane is not cutting through the air it will NOT lift off. Another question can be, if you're running on a treadmill holding one of thoes 50c glittering foil fans like 6 year old girls like, will that fan spin?

This is based on the assumption the plane is stationary to begin with and any acceleration attempt forward is countered completely by the coveyer belt carrying it back

Really this is a great analogy! I like the glitter fan.

Except for the fact that the Assumption that is made IS NOT VALID IN PRACTICE.

In real life, if you placed an airplane on a conveyor belt, the airplane would accelerate right off the belt.

The problem with this analogy is that the plane does not provide its forward thrust by the wheels (legs in this case). It provides it by turbofan thrust. Put jetpacks on your back while running on that treadmill and see what happens..


Building on that example....

Jump on a treadmill on rollerskates. Grab on to some bars on the side, then crank up the speed of the belt, what happens? You'd stay in place, no matter how fast you sped up the belt, you'd have zero forward velocity. Your wheels would blow apart at a certain speed, but you could hold yourself in place until then. Now, grab a rope or something else, and pull yourself forward. That would work, since your wheels are just free spinning underneath you. If they were spinning at 50km/hr, and you started pulling yourself forward at 2km/hr, the wheels would speed up to 52km/hr, and you could keep moving forward, and even accelerate.

The key is the aircraft moves by pulling air through the engines, not by spining its wheels.

 

[Chode Messiah]

incorrect. vtol aircraft such as the harrier and the F35 have special modifications to all ow complete vertiacl takeoff. the F35 has a ducted lift fan as well as a thrust vectoring nozzle cappable of a 90 degree turn. and both the exhausts on the harrier can bend down to aim at the ground. also there is a yak, i believe the 38 and/or 41 that can take off vertically as well.

 

[91TTZ]

Originally posted by: dawks

Why do you keep saying this? It has already been addressed. The wheels would just blow out. Then of course the plane couldnt take off.

But as long as the belt matches, or even stays just SLIGHTLY below the blowout threashold, the plane will take off without issue.

You're not understanding what I'm saying. Even if the tires can't blow out, the rolling resistance (which increases with wheel speed) combined with a conveyor that can go any speed results in the vehicle lacking the power to overcome the (infinite) resistance. If the plane started to gain ground, the conveyor would simply speed up until the rolling resistance of the wheels stops any progress from being made.

As long as that plane maintained a point of friction with the belt, the belt would always have the advantage.

 

[TheLonelyPhoenix]

Harrier FTW!

 

[DrPizza]

For those in the camp that a conveyor with infinite power could keep it from moving, you're forgetting something: you're going to go from static friction (the wheels rolling on the conveyor) to kinetic friction (the wheels slipping against the conveyor). Thus, you're going to see a lot of smoke coming from the wheels as the plane quite possibly moves down the runway. In your scenario, however, the actual amount of thrust the plane/jet can provide is called into question. If the plane/jet has enough thrust that it can move forward with its wheels locked, then the conveyor cannot stop it. If this helps, think of a high powered car spinning its tires while minimally moving forward. That's kinetic friction. It's not enough to significantly move the car forward; what makes you think the same type of force would be enough to retard the plane from accelerating forward?

 

[KillerCharlie]

Allow me to quote myself:

Originally posted by: KillerCharlie

Interestingly, the resistance from the wheels (to a high order of accuracy) depends just on the normal force (weight) of the aircraft, not the speed of the wheels. Given that the aircraft is not constrained in the fixed reference frame, then the conveyor speed actually has 0 effect on the airplane.

This means the ODE governing the aircraft velocity is:

m(dv/dt)=T-D-mu*(W-L)

T=thrust
D=drag
mu=coefficient of rolling friction
W=weight
L=lift

Of course thrust, drag, and lift are functions of velocity. Drag is also a function of lift, but because of ground effect the induced drag changes quite a bit, meaning you'd need a simple numerical ODE solver for this.

The wheel resistance does not depend on the speed, so the conveyor speed is irrelevant (once again, only if the airplane is not constrained in the fixed reference frame). If the aircraft is fixed in the reference frame as per some interpretations, then the thrust needed to cancel out the wheel resistance and keep the plane stationary is constant.

This is exactly how the aerodynamic performance people would analyze an aircraft taking off. Note that the speed of the wheels means nothing. The resistance from the wheels is independent of the the wheel speed to a very high degree of approximation.

 

[DrPizza]

Originally posted by: KillerCharlie
Allow me to quote myself:

Originally posted by: KillerCharlie

Interestingly, the resistance from the wheels (to a high order of accuracy) depends just on the normal force (weight) of the aircraft, not the speed of the wheels. Given that the aircraft is not constrained in the fixed reference frame, then the conveyor speed actually has 0 effect on the airplane.

This means the ODE governing the aircraft velocity is:

m(dv/dt)=T-D-mu*(W-L)

T=thrust
D=drag
mu=coefficient of rolling friction
W=weight
L=lift

Of course thrust, drag, and lift are functions of velocity. Drag is also a function of lift, but because of ground effect the induced drag changes quite a bit, meaning you'd need a simple numerical ODE solver for this.

The wheel resistance does not depend on the speed, so the conveyor speed is irrelevant (once again, only if the airplane is not constrained in the fixed reference frame). If the aircraft is fixed in the reference frame as per some interpretations, then the thrust needed to cancel out the wheel resistance and keep the plane stationary is constant.

This is exactly how the aerodynamic performance people would analyze an aircraft taking off. Note that the speed of the wheels means nothing. The resistance from the wheels is independent of the the wheel speed to a very high degree of approximation.

<--- agrees on this. If you don't believe KillerCharlie or me, why don't you google for formulas to calculate the friction. You're not going to find many (if any) formulas for rolling, kinetic, or static friction that include a term for the velocity.

 

[91TTZ]

Originally posted by: DrPizza
For those in the camp that a conveyor with infinite power could keep it from moving, you're forgetting something: you're going to go from static friction (the wheels rolling on the conveyor) to kinetic friction (the wheels slipping against the conveyor). Thus, you're going to see a lot of smoke coming from the wheels as the plane quite possibly moves down the runway. In your scenario, however, the actual amount of thrust the plane/jet can provide is called into question. If the plane/jet has enough thrust that it can move forward with its wheels locked, then the conveyor cannot stop it. If this helps, think of a high powered car spinning its tires while minimally moving forward. That's kinetic friction. It's not enough to significantly move the car forward; what makes you think the same type of force would be enough to retard the plane from accelerating forward?

That's the problem with these hypothetical problems- we're left having to assume that some major condition doesn't apply and we have to solve the problem around that.

If we wanted to be realistic, we'd ask, "why the hell is a giant conveyor on a runway trying to undermine that aircraft taking off?" But it seems that in this problem, we just have to assume it's there. A realistically thinking person would also ask, "How could the conveyor have limitless power?" or "Wouldn't the tires just slip on the belt after a certain point?". In reality these are all valid questions, but in silly hypothetical problems like this we're left having to assume things.

On this problem, it seems to me that they want us to assume that the belt has limitless power and that the vehicle can't get around the belt's influence in any way. So I take it that it has limitless friction, too.

Ridiculous, I know, and you're right that many real-world variables would prevent the problem from working as it seems to be stated.

 

[mugs]

Originally posted by: 91TTZ

Originally posted by: DrPizza
For those in the camp that a conveyor with infinite power could keep it from moving, you're forgetting something: you're going to go from static friction (the wheels rolling on the conveyor) to kinetic friction (the wheels slipping against the conveyor). Thus, you're going to see a lot of smoke coming from the wheels as the plane quite possibly moves down the runway. In your scenario, however, the actual amount of thrust the plane/jet can provide is called into question. If the plane/jet has enough thrust that it can move forward with its wheels locked, then the conveyor cannot stop it. If this helps, think of a high powered car spinning its tires while minimally moving forward. That's kinetic friction. It's not enough to significantly move the car forward; what makes you think the same type of force would be enough to retard the plane from accelerating forward?

That's the problem with these hypothetical problems- we're left having to assume that some major condition doesn't apply and we have to solve the problem around that.

If we wanted to be realistic, we'd ask, "why the hell is a giant conveyor on a runway trying to undermine that aircraft taking off?" But it seems that in this problem, we just have to assume it's there. A realistically thinking person would also ask, "How could the conveyor have limitless power?" or "Wouldn't the tires just slip on the belt after a certain point?". In reality these are all valid questions, but in silly hypothetical problems like this we're left having to assume things.

On this problem, it seems to me that they want us to assume that the belt has limitless power and that the vehicle can't get around the belt's influence in any way. So I take it that it has limitless friction, too.

Ridiculous, I know, and you're right that many real-world variables would prevent the problem from working as it seems to be stated.

The question doesn't say that the belt will go as fast as it has to go to keep the plane stationary (if that was possible). The question says that the belt matches the plane's speed. It doesn't need limitless power, only enough to keep up with the plane until the plane takes off (which it will).

 

[Savij]

http://www.slweekly.com/article.cfm/theneedforspeed

 

[91TTZ]

Originally posted by: mugs

The question doesn't say that the belt will go as fast as it has to go to keep the plane stationary (if that was possible). The question says that the belt matches the plane's speed. It doesn't need limitless power, only enough to keep up with the plane until the plane takes off (which it will).

You're probably right, and in that case, I'll revert to the answer I gave on page 10.

"If this conveyor thing was in place and moved backwards at the same rate which the plane moved forwards, this would NOT stop the plane from moving forwards, it would just double the tire speed. So the plane would still take off at 100 mph indicated airspeed, but 200 mph tire speed"

 

[Armitage]

Originally posted by: KillerCharlie
Allow me to quote myself:

Originally posted by: KillerCharlie

Interestingly, the resistance from the wheels (to a high order of accuracy) depends just on the normal force (weight) of the aircraft, not the speed of the wheels. Given that the aircraft is not constrained in the fixed reference frame, then the conveyor speed actually has 0 effect on the airplane.

This means the ODE governing the aircraft velocity is:

m(dv/dt)=T-D-mu*(W-L)

T=thrust
D=drag
mu=coefficient of rolling friction
W=weight
L=lift

Of course thrust, drag, and lift are functions of velocity. Drag is also a function of lift, but because of ground effect the induced drag changes quite a bit, meaning you'd need a simple numerical ODE solver for this.

The wheel resistance does not depend on the speed, so the conveyor speed is irrelevant (once again, only if the airplane is not constrained in the fixed reference frame). If the aircraft is fixed in the reference frame as per some interpretations, then the thrust needed to cancel out the wheel resistance and keep the plane stationary is constant.

This is exactly how the aerodynamic performance people would analyze an aircraft taking off. Note that the speed of the wheels means nothing. The resistance from the wheels is independent of the the wheel speed to a very high degree of approximation.

Plenty of other peple have already quoted you - why not address them?

So does it take off? You were vehemetly denying that it could earlier. Now you seem to be waffling. Given what you've said here - that the rolling resistance of the wheels is independent of speed (which I agree with to first order at least), can the conveyer prevent the plane from taking off?

 

[DrPizza]

Originally posted by: 91TTZ

Originally posted by: mugs

The question doesn't say that the belt will go as fast as it has to go to keep the plane stationary (if that was possible). The question says that the belt matches the plane's speed. It doesn't need limitless power, only enough to keep up with the plane until the plane takes off (which it will).

You're probably right, and in that case, I'll revert to the answer I gave on page 10.

"If this conveyor thing was in place and moved backwards at the same rate which the plane moved forwards, this would NOT stop the plane from moving forwards, it would just double the tire speed. So the plane would still take off at 100 mph indicated airspeed, but 200 mph tire speed"

Page 10?? This thead is only on the 3rd page.
Page references are irrelevant as the number of posts per page is something you can change in the settings. It's pretty pointless to keep the posts per page set low. Even on dial-up, 100 posts per page doesn't slow me down enough to care.

 

[91TTZ]

Originally posted by: DrPizza

Page 10?? This thead is only on the 3rd page.
Page references are irrelevant as the number of posts per page is something you can change in the settings. It's pretty pointless to keep the posts per page set low. Even on dial-up, 100 posts per page doesn't slow me down enough to care.

Good point. I changed the settings. Now I'm on page 3.

 

[HardcoreRobot]

afaik airplane wheels are free rolling, the conveyor will just roll them resulting in no net movement for the airplane

unlike a car, an airplanes thrust is not applied through the wheels

 

[mobobuff]

Yes. Obviously it can.

Can I nominate this thread for Ownage of the Year 2006?

 

[g8wayrebel]

Being that it was a hypothetical question , albeit an impossible one in the first place , you could also assume the infinite speed of the conveyor.
If the conveyor moves as fast as the thrust of the engine is propelling the plane , it never moves. It would sit in place at full speed just rolling on the coneyor. It's forward propulsion would be matched by it's rearward progress on the conveyor, thereby never developing lift in relation to the earth or the surrounding environment , hence: the inability to defeat gravity.
The conveyor is simply a different form of defeat. It negates the propulsion by motion rather than fixed support(ie. a test stand).
How do I look stupid?
Perhaps you could explain how it would take off if it never develops any forward speed.
I would certainly like to hear from someone who is so clearly more intelligent than I.
If I'm pwnd , show me.

EDIT:
This opinion is based on the fact that this is a conveyor. It moves independently. It will move at the same time as the plane attempts to , preventing the plane from making any forward progress. If you think it will still take off if the conveyor matches it's speed and prevents it from moving forward , explain yourself. If not , we aren't seeing the same issue and this is moot anyway.

 

[dawks]

Originally posted by: g8wayrebel
Being that it was a hypothetical question , albeit an impossible one in the first place , you could also assume the infinite speed of the conveyor.
If the conveyor moves as fast as the thrust of the engine is propelling the plane , it never moves. It would sit in place at full speed just rolling on the coneyor. It's forward propulsion would be matched by it's rearward progress on the conveyor, thereby never developing lift in relation to the earth or the surrounding environment , hence: the inability to defeat gravity.
The conveyor is simply a different form of defeat. It negates the propulsion by motion rather than fixed support(ie. a test stand).
How do I look stupid?
Perhaps you could explain how it would take off if it never develops any forward speed.
I would certainly like to hear from someone who is so clearly more intelligent than I.
If I'm pwnd , show me.

EDIT:
This opinion is based on the fact that this is a conveyor. It moves independently. It will move at the same time as the plane attempts to , preventing the plane from making any forward progress. If you think it will still take off if the conveyor matches it's speed and prevents it from moving forward , explain yourself. If not , we aren't seeing the same issue and this is moot anyway.

The key is the wheels are free spinning, and the plane is not driven forward by the wheels. If the plane just sat there on a conveyor, with the jet engines off, assuming you could overcome the initial rolling resistance, the plane would remain stationary. No matter how fast you cranked up that conveyor, the wheels would just accelerate to match that. Now, throw in an additional factor, in that an airplanes jet engine literally pulls it forward, the plane will begin to move forward. The faster the plane goes, the faster the wheels turn. The faster the conveyor spins, the faster the wheels turn. No matter how fast the belt goes, the fact that the wheels are free spinning, negates its existance. The only way this would prevent a plane from taking off is if the wheels blew up due to heat/friction caused by highspeeds.

 

[giantpinkbunnyhead]

Holy thread necromancy Batman!

How is this still being debated?!? Whatever, it's still funny to read all this.