The "Aeroplane on treadmill" conundrum...
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Assuming no friction, the aircraft will remain stationary. Now... start its engine, and it will pull itself forward through the air until it gains sufficient speed to take off.
The whole "system", i.e. the conveyor belt AND the aircraft must be considered to see whether it will take off. As the conveyor belt is not moving through the air, the aircraft will remain on the ground/conveyor belt.
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This ran to 14 pages on jetblast, so to try and save everyone here is the proof that it will fly
http://videos.streetfire.net/player....D-D6BA1A43A06B
http://videos.streetfire.net/player....D-D6BA1A43A06B
Originally Posted by raisin
It will gain sufficient speed to take off relevant to the conveyor belt. The ASI will read zero as the propeller is working to maintain the aircraft's position on the conveyor belt, not to move forward throught the air.
Here is an excellent tongue in cheek analogy from the Straight Dope thread:
Originally Posted by Chronos
OK, how about a different thought experiment? We have a plane on a perfectly normal runway, but next to the runway are a bunch of people whistling. The plane starts its engines, but the more the engines rev up, the louder the people whistle, so the plane can't move at all. If it did move, the people would just whistle even louder, to prevent it. Since the plane can't move, being prevented by all the whistling, there's no airflow over the wings, so the plane doesn't take off.
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Originally Posted by AerocatS2A
If it did move, the people would just whistle even louder, to prevent it.
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Quotes:
Originally Posted by High Wing Drifter
What about the other one I've seen doing the rounds:
If an aircraft is in flight and inside it, a bird is also in flight, does the aircraft feel the weight of the bird?
Sombebody's answer: No
CN- not necessarily, it all depends.
Quote:
Originally Posted by distaff_beancounter
And If the aircraft is flying at, say, 500 kts and the bird has a max speed of 100 kts - will the bird manage to fly from the back to the front of the aircraft?
Sombebody's answer: Yes
CN - again, not necessarily, it all depends. Like the truck driver with a load of birds in the back who had difficulty in climbing a hill, so banged on the back to get the birds to fly and reduce the effective weight. It all depends . . .
Chris N.
Originally Posted by High Wing Drifter
What about the other one I've seen doing the rounds:
If an aircraft is in flight and inside it, a bird is also in flight, does the aircraft feel the weight of the bird?
Sombebody's answer: No
CN- not necessarily, it all depends.
Quote:
Originally Posted by distaff_beancounter
And If the aircraft is flying at, say, 500 kts and the bird has a max speed of 100 kts - will the bird manage to fly from the back to the front of the aircraft?
Sombebody's answer: Yes
CN - again, not necessarily, it all depends. Like the truck driver with a load of birds in the back who had difficulty in climbing a hill, so banged on the back to get the birds to fly and reduce the effective weight. It all depends . . .
Chris N.
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Originally Posted by chrisN
CN- not necessarily, it all depends..
Originally Posted by chrisN
It all depends . . .Chris N.
I'm sure you're right but it would be great if I could see proof of why I am wrong.
Skyhawk.
Why do it if it's not fun?
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It all depends? On what?
First, imagine a totally enclosed cabin. The cabin contains the pilot, the bird, many billions of air molecules, amongst other things. The aircraft's wings have to carry the weight of everything in the cabin. Makes no difference whether the bird is on the floor of the cabin or not, it is still in the cabin, and so still needs to be carried.
Scenario number two: an open cockpit aeroplane, with the bird flying six inches above the top of the cockpit, at the same speed as the aeroplane. The bird is clearly carrying its own weight, and does not require the aeroplane to carry its weight.
The reason for the difference, I think, is that in the first case, the downwash from the bird's wings strikes the floor of the cabin and exerts a force on the floor of the cabin which is equivalent to the weight of the bird. Whereas in the second case, most of the downwash from the bird's wings escapes from the aeroplane. I've never actually seen proof of this explaination, though.
The interesting things is the in-between situation. For example, the open-cockpit aeroplane with a bird sitting on the seat. During the flight, the bird decides to take off and leave the aeroplane. At what point does the aeroplane stop carrying the bird's weight?
You can apply a very similar argument to the bird moving at 100kt and the aircraft moving at 500kt.
FFF
---------------
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FFF - quite so.
And now for something that really happened, as I read it years ago.
During WWII, some PoW's passed their time building tiny paper gliders from the paper lining of cigarette packets.
Then they progressed to a powered aircraft, by capturing a fly, and gluing it to the tiny model. It worked.
Then they tried aerotowing, but found a 1-fly powered aircraft was too weak.
So they built a twin, one fly each side. There were assymmetry problems, as the flies were reluctant to synchronise their efforts, but eventually they managed to effect an aerotow takeoff successfully.
So the flying creature's efforts, if not totally enclosed, have an effect on drag by overcoming it with thrust, and perhaps on lift too, depending on the circumstances.
Chris N.
And now for something that really happened, as I read it years ago.
During WWII, some PoW's passed their time building tiny paper gliders from the paper lining of cigarette packets.
Then they progressed to a powered aircraft, by capturing a fly, and gluing it to the tiny model. It worked.
Then they tried aerotowing, but found a 1-fly powered aircraft was too weak.
So they built a twin, one fly each side. There were assymmetry problems, as the flies were reluctant to synchronise their efforts, but eventually they managed to effect an aerotow takeoff successfully.
So the flying creature's efforts, if not totally enclosed, have an effect on drag by overcoming it with thrust, and perhaps on lift too, depending on the circumstances.
Chris N.
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HWD, dont, just please for all man kind, dont!
Hmmm. I've gone from one view to the other.
I think the the bird in flight, or a model aeroplane or whatever would not be sensed by the enclosing aeroplane because lift is, as I understand it, the result of pressure differentials in the area immediately around the wing. I think this is regardless of wether you consider Bernoulli, Newton or both responsible for lift. However, if the bird or model aeroplane or whatever were in ground effect, then I think at least some of the weight would be sensed. But then again, if the enclosing aeroplane's cabin is pressurised it must sense the weight of the bird, but that negates the pressure differential lift argument. I'm missing something, I'm sure!
Would I be squidged to a pulp if I lay down on the runway with a 747's wing (attached to 747) flying a few feet above me (assuming I was suitably secured to prevent my otherwise imminent demise in its wake!)?
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Would I be squidged to a pulp if I lay down on the runway with a 747's wing (attached to 747) flying a few feet above me
As yorks.ppl said, this went on for ages on JB, every one decided that it could fly. (well, most people)
You have started summat now HWD, i hope you are proud!
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Coming back to the conveyor belt one.
One thing that we haven't considered is friction.
I accept that under normal circumstances the aircraft should accelerate through that air and take off.
However, lets follow this through a bit.
If the aircraft has its engine turned off, and the conveyor belt is turned off, and we have zero wind, then everything remains stationary.
Now the conveyor belt is turned on, due to the effects of friction the aircraft will stay stationary relative to the conveyor belt, and move back wards relative to the ground (which the conveyor sits on) and back wards relative to the air.
Now the aircraft starts its engine and runs at idle speed. We all know from experience, that at idle power most aircraft don't produce enough trust to over come friction. So the aircraft will still remain stationary relative to the conveyor belt, and move back wards relative to the ground and the air.
Add a little power to the aircraft engine, and it will start to slowly creep forward on the conveyor belt, as it overcomes the friction. However due to the effects of the friction and the small amount of power added, the aircraft will slowly move forward relative to the conveyor belt, but still hasn't over come all the friction, so it will still move slowly back wards relative to ground, and the air, just not as fast back wards as before.
Now add a little more power, and the power of the engine finally matches the effect of the wheel friction. Now the aircraft will move forward relative to the conveyor belt, at the same speed as the conveyor belt moves back wards. Relative to the ground and the air, the aircraft is now stationary.
I think so far we all agree. What happens next is where the discrepancy arises, and is due to the badly worded question.
Scenario 1: At this point the aircraft adds its remaining engine power, and uses this to accelerate forward relative to the airflow, and generates lift, and takes off.
Scenario 2: The conveyor belt starts to move a little faster. This generates extra friction on the wheels. The aircraft still moves at the same speed relative to the conveyor belt, but once again starts to move back wards relative to the ground and airflow.
The aircraft now adds a little extra engine power to over come the extra friction, and now moves forward a little faster relative to the conveyor belt, and once again its engine output matches the friction and its movement relative to the ground and airflow becomes nil.
If the above keeps being repeated (adding friction by increasing the conveyor belt speed, and adding engine power to compensate for the friction only) until the engine is at full power, then the aircraft will eventually be moving incredibly fast relative to the conveyor belt, but will have a nil speed relative to the ground and to the airflow.
Admittedly the conveyor belt would be running at a speed many multiples of the aircrafts normal take off speed, and in practise the tyres are likely to explode, and the the wheels fall off due to the heat from the friction, and the aircraft is likely to be thrown off the conveyor belt by the minor bumps being turned into huge bumps by the speed, but if we are only asking about the theory, then it is possible.
So I guess it all comes down to how we interpret the original question.
The only correct answer is a question back to the questioner. What is the airplane's speed relative to the air?
dp
One thing that we haven't considered is friction.
I accept that under normal circumstances the aircraft should accelerate through that air and take off.
However, lets follow this through a bit.
If the aircraft has its engine turned off, and the conveyor belt is turned off, and we have zero wind, then everything remains stationary.
Now the conveyor belt is turned on, due to the effects of friction the aircraft will stay stationary relative to the conveyor belt, and move back wards relative to the ground (which the conveyor sits on) and back wards relative to the air.
Now the aircraft starts its engine and runs at idle speed. We all know from experience, that at idle power most aircraft don't produce enough trust to over come friction. So the aircraft will still remain stationary relative to the conveyor belt, and move back wards relative to the ground and the air.
Add a little power to the aircraft engine, and it will start to slowly creep forward on the conveyor belt, as it overcomes the friction. However due to the effects of the friction and the small amount of power added, the aircraft will slowly move forward relative to the conveyor belt, but still hasn't over come all the friction, so it will still move slowly back wards relative to ground, and the air, just not as fast back wards as before.
Now add a little more power, and the power of the engine finally matches the effect of the wheel friction. Now the aircraft will move forward relative to the conveyor belt, at the same speed as the conveyor belt moves back wards. Relative to the ground and the air, the aircraft is now stationary.
I think so far we all agree. What happens next is where the discrepancy arises, and is due to the badly worded question.
Scenario 1: At this point the aircraft adds its remaining engine power, and uses this to accelerate forward relative to the airflow, and generates lift, and takes off.
Scenario 2: The conveyor belt starts to move a little faster. This generates extra friction on the wheels. The aircraft still moves at the same speed relative to the conveyor belt, but once again starts to move back wards relative to the ground and airflow.
The aircraft now adds a little extra engine power to over come the extra friction, and now moves forward a little faster relative to the conveyor belt, and once again its engine output matches the friction and its movement relative to the ground and airflow becomes nil.
If the above keeps being repeated (adding friction by increasing the conveyor belt speed, and adding engine power to compensate for the friction only) until the engine is at full power, then the aircraft will eventually be moving incredibly fast relative to the conveyor belt, but will have a nil speed relative to the ground and to the airflow.
Admittedly the conveyor belt would be running at a speed many multiples of the aircrafts normal take off speed, and in practise the tyres are likely to explode, and the the wheels fall off due to the heat from the friction, and the aircraft is likely to be thrown off the conveyor belt by the minor bumps being turned into huge bumps by the speed, but if we are only asking about the theory, then it is possible.
So I guess it all comes down to how we interpret the original question.
The only correct answer is a question back to the questioner. What is the airplane's speed relative to the air?
dp
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dublinpilot
You have made an incorrect assumption that invalidates your case.
Friction of movement between two solid objects does not have a simple relationship with speed. Friction is greatest when they are stationary in relation to each other, but drops immediately when they start to move and thereafter remains low - it doesn't rise quickly with speed. Bearing friction is a small fraction of total drag for an aircraft taking off; parasite drag and induced drag make up a much greater force.
The case you suggest cannot happen - until the aircraft moves (through the air) the conveyor would have no speed, so the conveyor cannot match the speed - it must always lag the aircraft, and the aircraft would have an airspeed.
You have made an incorrect assumption that invalidates your case.
Friction of movement between two solid objects does not have a simple relationship with speed. Friction is greatest when they are stationary in relation to each other, but drops immediately when they start to move and thereafter remains low - it doesn't rise quickly with speed. Bearing friction is a small fraction of total drag for an aircraft taking off; parasite drag and induced drag make up a much greater force.
The case you suggest cannot happen - until the aircraft moves (through the air) the conveyor would have no speed, so the conveyor cannot match the speed - it must always lag the aircraft, and the aircraft would have an airspeed.
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Has anyone got ?
1. a remote controlled airplane.
2. a electrical running machine that goes faster than the remote controlled aircraft can fly striaght and level at with max power.
The experiment would be simple.
1 tie the aircraft with 2 bits of string in a V out the front secured by 2 hooks which would unhook when the aircraft goes past.
2. Start the aircraft
3. Start running machine and get it going faster than the aircraft flys at.
4. Open throttle and take a video as it farks off the end of the running machine.
Then we can put this ****e to bed.
1. a remote controlled airplane.
2. a electrical running machine that goes faster than the remote controlled aircraft can fly striaght and level at with max power.
The experiment would be simple.
1 tie the aircraft with 2 bits of string in a V out the front secured by 2 hooks which would unhook when the aircraft goes past.
2. Start the aircraft
3. Start running machine and get it going faster than the aircraft flys at.
4. Open throttle and take a video as it farks off the end of the running machine.
Then we can put this ****e to bed.
Can I drift onto something that i've wondered about since A-level physics? Let's say you're in space standing on the inside wall of a cylinder that rotates. The cylinder rotates at such a speed that when you are standing on the wall, you experience 1G. If you now jump up, what happens? 
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After you have left the cylinder you will still have the side velocity at the point of jumping. Then add what ever downwards compent by using the velocity triangle to work out the resultant.
The point were you will hit the wall again will be determined by that.
For very small jumps it will appear that you just go in and out relative to the wall. For larger jumps you will see the slight angle.
You used to be able to do it on fair ground rides. They spin you up and put it vertical and you stick to the wall. If you stand horizontal and skip you can feel the slight sideways force on your ankle when you land.
You also feel a great sideways force round your ear afterwards when your mum gets her hands on you after watching said physics experiment.
The point were you will hit the wall again will be determined by that.
For very small jumps it will appear that you just go in and out relative to the wall. For larger jumps you will see the slight angle.
You used to be able to do it on fair ground rides. They spin you up and put it vertical and you stick to the wall. If you stand horizontal and skip you can feel the slight sideways force on your ankle when you land.
You also feel a great sideways force round your ear afterwards when your mum gets her hands on you after watching said physics experiment.
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Mind you, if the treadmill were running in the other direction - ie, in the direction of take off - that would be a simply _marvellous_ asset for short field ops. Especially if you had an enormous hairdryer at the end of the runway. 
Don't try the above at the gym though. On second thoughts, if they have CCTV there...
Don't try the above at the gym though. On second thoughts, if they have CCTV there...
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Originally Posted by FlyingForFun
the downwash from the bird's wings strikes the floor of the cabin and exerts a force on the floor of the cabin which is equivalent to the weight of the bird---------------
This is real interesting! can't think of anything I'd rather be doing on a still, cloudless morning......hang on a minute!!!
Skyhawk.