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plane question

#121
The trick of the question is the flying part.

The question is whether or not the plane can overcome the conveyor via it's means of propultion. That is what needs to be worked over before anyone worries over aerodynamics and other big more complex words that I can't think of at the moment.

Can a plane fly? sure.

The question is whether or not the plane can gather positive yardage while on the proposed conveyor.
 
#122
Davidoc said:
I don't even get how this is a trick question. No acceleration through the air, no lift. BMC's treadmill comparison is a good one.
Click to expand...

And there's the key to whole thing: acceleration through the air. A stationary airplane WILL fly in a wind tunnel.

Bassic83...yes but not current. Crazy expensive here: club dues of $1000/year. Mandatory insurance of $300-400 a year. Cessna 172 rents for $230/hour. Landing fees everywhere. An IFR approach to Geneva costs $75 each time. To stay safe, you want to log 25+ hours a year. Too expensive.

Plus, language is an issue for going into small fields. I'm fluent in French, but the only words I can say in German are Mercedes and BMW. Italian? Forget it.

Might try gliding once I get the kids through university. Until then, I focus my passion on gigging and try not to think about flying. I'm still in the airline industry and my office is at airport overlooking the widebody private aircraft (B747, B767, etc)


And you?

Brian...aka bmc
 
#123
teej said:
I'm skeptical of all these "no" answers. A planes wheels are free-spinning - they aren't linked to the plane, so there is a possibility that with the massive amount of thrust a jet engine can deliver, that the plane would continue forward, while the wheels spin backwards.
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That's correct.

So what if the conveyor is moving? The plane's wheels aren't driven, and so they're not pushing against the conveyor. The plane's thrust pushes it forward regardless of the conveyor's movement. The conveyor is just making an artificial ground speed, but it's the thrust that pushes the plane through the air. And the air speed, combined with the wing shape and angle of attack that creates lift.

Now, if somehow you could somehow have a giant blower behind the plane generating a large tailwind at the same speed that the plane was moving WRT to ground, then it wouldn't develop airspeed and therefore, no lift.
 
#124
bmc said:
SnoBassMan you are partially right in that the plane pushes air to gain movement, but in this example, the conveyor is matching the speed but pulling it in the opposite direction. If a speedometer was attached to the wheels, it would show 100mph. But the airspeed indicator would show 0 mph.
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The conveyer does not put any force on the airplaine. (well, very, VERY small amounts, created by friction.) It does not pull the plane anywhere. It pulls the wheels, yes. Which causes the wheels to spin. That's it.

No one is saying that the plane will take off from a standstill. The plane will begin rolling forward (with the frame of reference being the ground around the conveyer), it will gain speed, and take off into the air. At the same time, the conveyer will speed up, and increase the speed of the wheels. The wheels, however, dont push on anything.
 
#125
Bob Lee (QSC) said:
That's correct.

So what if the conveyor is moving? The plane's wheels aren't driven, and so they're not pushing against the conveyor. The plane's thrust pushes it forward regardless of the conveyor's movement. The conveyor is just making an artificial ground speed, but it's the thrust that pushes the plane through the air. And the air speed, combined with the wing shape and angle of attack that creates lift.

Now, if somehow you could somehow have a giant blower behind the plane generating a large tailwind at the same speed that the plane was moving WRT to ground, then it wouldn't develop airspeed and therefore, no lift.
Click to expand...


Hi Bob...I remembered you were another airplane junkie.
 
#127
Ray-man said:
If it's stationary, it doesn't matter how fast the wheels are moving. It's all about airspeed. In your scenario, it's airspeed is still zero.

Another example: A plane taxis into position on the runway, and along comes a headwind of, say, 170 mph, or whatever the necessary airspeed is for takeoff, or V1. Assuming the flaps are set properly, all the pilot would have to do is activate the elevator (the rear horizontal control surfaces). The nose would lift and the plane would "take off." Granted, its relative land speed would be zero, but it would be flying.
Click to expand...

However, once it's wheels-up it would have to be tethered like a kite or have some forward thrust in order to maintain that zero ground speed in the 170 mph headwind.
 
#128
bmc said:
So you are saying the airspeed indicator in the cockpit is registering a 200km wind, correct?
Click to expand...

No. The airspeed indicator will read 100 km/h (or MPH as you've now moved to). The conveyor belt isn't creating any wind that I'm aware of.

bmc said:
As a math equation:

100mph (airplane speed) + (-100 conveyor speed) = ?

The only way that airplane attains a takeoff speed of 100mph is when it has 100 mph of wind going over the wings and indicated on the airspeed indicator as 100mph. If the conveyor moves backwords at 100mph, the plane needs to move forward at 200 miles an hour relative to the conveyor and 100 mph relative to the ground.
Click to expand...

Yes. And in the question it was at rest at the start. The conveyor belt just spins the wheels faster.
 
#130
Bob Lee (QSC) said:
That's correct.

So what if the conveyor is moving? The plane's wheels aren't driven, and so they're not pushing against the conveyor. The plane's thrust pushes it forward regardless of the conveyor's movement. The conveyor is just making an artificial ground speed, but it's the thrust that pushes the plane through the air. And the air speed, combined with the wing shape and angle of attack that creates lift.

Now, if somehow you could somehow have a giant blower behind the plane generating a large tailwind at the same speed that the plane was moving WRT to ground, then it wouldn't develop airspeed and therefore, no lift.
Click to expand...

Those principles apply once the plane has shifted it's weight off the ground (eg: taking off), but as long as the weight of the plane is on the wheels, and therefore the ground/conveyor belt, it doesn't matter where the thrust is coming from.

Let's say the plane's sitting on a moving conveyor belt without using it's engines. If what you're saying is true, then the plane would not move at all relative to anything but the conveyor belt. This isn't the case; the plane would move with the conveyor.
 
#131
Davidoc said:
That would make sense only if the plane was stationary on the conveyor belt, and the belt was moving in the direction the plane was facing. Then the pieces would fit with that explanation; the plane would have no net acceleration with the belt, but would be accelerating relative to the air, causing it to take off.

If the belt is moving in the opposite direction as the plane at the same velocity, then that explanation does not fit. The plane would have double acceleration relative to the belt. (not none at all), and would have no acceleration relative to the air.
Click to expand...

I'm sorry but you're wrong here. The only thing to undestand is that the conveyor belt can impart almost zero force to the aircraft. Unless you can come up with a way that the conveyor can impart a force that is significant compared to the thrust of the aircrafts engines then there's no way that the conveyor could stop it from taking off.

Regarding the direction of the conveyor, it would make almost no difference what direction it is going in. The only way for the conveyor to impart a force to the aircraft is through the wheels through friction (unless the brakes are on).
 
#132
Geoff St. Germaine said:
I'm sorry but you're wrong here. The only thing to undestand is that the conveyor belt can impart almost zero force to the aircraft. Unless you can come up with a way that the conveyor can impart a force that is significant compared to the thrust of the aircrafts engines then there's no way that the conveyor could stop it from taking off.

Regarding the direction of the conveyor, it would make almost no difference what direction it is going in. The only way for the conveyor to impart a force to the aircraft is through the wheels through friction (unless the brakes are on).
Click to expand...

Again, think of a stationary wheeled object on a conveyor belt (could be anything, a plane, car etc.) Does the object move with the belt? Yep. Is the object moving with the belt? You betcha.
 
#133
I've been converted, i now side with geoff.

I mean, instead of the engine or whatever what if the conveyor belt was matching the hand of god which had reached down to move this plane? Provided that god didnt move any faster/slower than the conveyor belt, wouldnt the wheels just spin doubly fast? now that ive typed that im not so sure about its merit as an analogy, but heres another. take an axle with a wheel, attach it to a stick and put the axle on a conveyor. make sure it is a free spinning axle with very little friction.push it forward at 5 mph and set the conveyor to 5 mph. wouldnt it move anyway? god, now that ive typed that im not sure about its merit as an analogy...someone with a brain could discount/backup my theories please? (Geoff?)

in the time it took me to type this, six more people posted! wow.
 
#135
Davidoc said:
Those principles apply once the plane has shifted it's weight off the ground (eg: taking off), but as long as the weight of the plane is on the wheels, and therefore the ground/conveyor belt, it doesn't matter where the thrust is coming from.
Click to expand...

It does matter. The thrust isn't coming from the wheels.

Let's say the plane's sitting on a moving conveyor belt without using it's engines. If what you're saying is true, then the plane would not move at all relative to anything but the conveyor belt. This isn't the case; the plane would move with the conveyor.
Click to expand...

Yes, if it's brakes were engaged. If the wheels are free rolling, the conveyor's motion will be transferred to the plane's mass only through the friction of the wheels, which means it will take some time to get it moving at a velocity close to the conveyor's.
 
#136
Davidoc said:
Again, think of a stationary wheeled object on a conveyor belt (could be anything, a plane, car etc.) Does the object move with the belt? Yep. Is the object moving with the belt? You betcha.
Click to expand...
Only due to the VERY slight force of friction. You apply any force at all in the other direction, and it will stop moving in that direction, and start accelerating in the other direction.
 
#137
Bob Lee (QSC) said:
Yes, if it's brakes were engaged. If the wheels are free rolling, the conveyor's motion will be transferred to the plane's mass only through the friction of the wheels, which means it will take some time to get it moving at a velocity close to the conveyor's.
Click to expand...

Dan1099 said:
Only due to the VERY slight force of friction. You apply any force at all in the other direction, and it will stop moving in that direction, and start accelerating in the other direction.
Click to expand...

This sounds accurate when I think about it again. I am beginning to think that I am wrong. My stationary wheeled object comparison was accurate, but the way I said it would respond is not.
 
#138
Davidoc said:
Again, think of a stationary wheeled object on a conveyor belt (could be anything, a plane, car etc.) Does the object move with the belt? Yep. Is the object moving with the belt? You betcha.
Click to expand...

um, wouldnt any object ever (wheeled or not) when placed on a conveyor belt that is moving, move with the belt?

and does the weight of the plane have anything to do with this? like, put a pin on a 30 mph treadmill and itll fly off, but put a billion pound anvil on (super-strong) wheels, the wheels will spin like mad but that anvil is gonna take a little longer to get goin.

my take on it is that according to the no theories, if you placed the plane on an already moving conveyor than it would just shoot off instantly because the wheels and the plane are connected. i dont think so...but i dont know much on the subject and am definetely missing something. please, someone enlighten me!
 
#139
As I understand it the prop/jet pulls the plane forward through the air. The wheels are free spinning, they aren't connected to the plane's method of forward thrust, they're just there so the plane doesn't scrape along the conveyor belt.

When the plane engine is off and the conveyor is on the plane would move with the conveyor (i assume?) but as soon as the engines begin generating thrust and pulling the aircraft through the air the wheels will spin at the speed necessary to keep up with the engine, moving in the opposite direction to the conveyor belt and at double the speed of the conveyor.
 
#140
Bob Lee (QSC) said:
Yes, if it's brakes were engaged. If the wheels are free rolling, the conveyor's motion will be transferred to the plane's mass only through the friction of the wheels, which means it will take some time to get it moving at a velocity close to the conveyor's.
Click to expand...

THATS what i was trying to say in my last post! ah, you educated persons with yer big words and yer, yer small, difficult words.
 

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