The great conveyor belt

KennyFlys said:
Jesse, I see you have this strange fixation.

Ok, the original premise is the belt moved.... ONLY the belt. The aircraft started stationary and was not pulled other than the propeller. And, the belt speeds up in proportion to the forward speed provided by the propeller's thrust. So, the plane never gets any air over the wings. How can it fly???

Now, if you're proposing the propeller is pulling the plane while the wheels are still on the belt... and the wings are being pulled through the air faster than the conveyor is running therefor gaining lift... yes, it could fly.
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Kenny--

The second thing is what would happen. It's impossible for the first to happen. Quit playing in womens clothes and start paying attention.
 
jesse said:
Kenny--

The second thing is what would happen. It's impossible for the first to happen. Quit playing in womens clothes and start paying attention.
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Damn... we agree. Now, I'm scared!

Okay, I'll quit playing in women's clothing if you stop corrupting Tristan!
 
I am not! whatever he says he starts, dont believe it! *points fingers*
 
KennyFlys said:
Jesse, I see you have this strange fixation.

Ok, the original premise is the belt moved.... ONLY the belt. The aircraft started stationary and was not pulled other than the propeller. And, the belt speeds up in proportion to the forward speed provided by the propeller's thrust. So, the plane never gets any air over the wings. How can it fly???

Now, if you're proposing the propeller is pulling the plane while the wheels are still on the belt... and the wings are being pulled through the air faster than the conveyor is running therefor gaining lift... yes, it could fly.
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The only possible way this could happen is with the engine at idle. Anything more than that, and the engine thrust would exceed the wheel bearing friction and acceleration would happen that would move the plane forward on the belt. Not enough to fly at like 800 or 1000 RPM, but enough that the speed of the belt would be exceeded no matter how fast it was going. You need brakes on to keep from moving on a flat surface, when at 1000 RPM don't you? The fact that the belt is turning the wheels at 60 or 70 MPH is irrelevant to the issue. The only forward thrust needed to move the plane down the runway is a little more than the wheel bearing friction. Almost nothing. In fact, at 1000 RPM the forward thrust would produce a speed that would increase until it was offset by the wind resistance produced by that speed and the wheel bearing friction.
 
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Dwight B. Van Zanen said:
The only possible way this could happen is with the engine at idle. Anything more than that, and the engine thrust would exceed the wheel bearing friction and acceleration would happen that would move the plane forward on the belt. Not enough to fly at like 800 or 1000 RPM, but enough that the speed of the belt would be exceeded no matter how fast it was going. You need brakes on to keep from moving on a flat surface, when at 1000 RPM don't you? The fact that the belt is turning the wheels at 60 or 70 MPH is irrelevant to the issue. The only forward thrust needed to move the plane down the runway is a little more than the wheel bearing friction. Almost nothing. In fact, at 1000 RPM the forward thrust would produce a speed that would increase until it was offset by the wind resistance produced by that speed and the wheel bearing friction.
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The thrust is only overcoming the effect of the belt wanting to move the airplane back with it. Even if thrust increases as the belt speed increases... how are you going to acheive flight?
 
KennyFlys said:
The thrust is only overcoming the effect of the belt wanting to move the airplane back with it. Even if thrust increases as the belt speed increases... how are you going to acheive flight?
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You are correct if you are visualizing airplanes such as the Wright Flyer or the early, extremely under powered Fly Babys. They do NOT have enough prop thrust to fly off the conveyor belt even at full power. They must have not only full prop thrust moving air over the wing but MUST also have that thrust combined with the airflow generated by the forward motion of the airplane through the air in order to lift off and continue to fly.

More powerful airplanes need ONLY the full prop thrust to lift off and FLY off the belt at just above their stall speed.

Therefore, neither camp thus far whose answers I've read, (and God help me, I think I've read them all) not the will flys nor the won't flys, has given a completely accurate answer.
 
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SkyHog said:
The reason it is not very accurate is because the conveyor belt (truck) is moving the plane. A conveyor belt in the example problem would not move the plane.
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The truck moves the plane at first because the plane is stationary and being held by the operator, and the plane is not running. If the belt started moving with slow acceleration before the plane fired up in our question, the plane could very well move backwards until it fired up and started breaking the wheel inertia by grabbing air.

The truck video illustrates quite handily that the airplane can easily overcome the backwards momentum of the surface upon which it sits, and gain forward airspeed to take off.
 
Dave Krall CFII said:
You are correct if you are visualizing airplanes such as the Wright Flyer or the early, extremely under powered Fly Babys. They do NOT have enough prop thrust to fly off the conveyor belt even at full power. They must have not only full prop thrust moving air over the wing but MUST also have that thrust combined with the airflow generated by the forward motion of the airplane through the air in order to lift off and continue to fly.

More powerful airplanes need ONLY the full prop thrust to lift off and FLY off the belt at just above their stall speed.

Therefore, neither camp thus far whose answers I've read, (and God help me, I think I've read them all) not the will flys nor the won't flys, has given a completely accurate answer.
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That makes no sense. We are getting airflow over the wings. The airplane will be moving forward in relation to everything in the outside world. If you are standing there watching it. The airplane will move forward. If you are proposing that the airplane will not move forward and the thrust from the prop is all it has to fly you are wrong.

It will instantly jump forward on the belt not caring how fast the belt is going in the opposite direction. As an observer watching this it would look like the airplane is taking off on a normal runway. You would need a conveyor belt the length of a normal runway. It would gain airspeed and fly away just as it always did.

Stand on a treadmill on roller blades. Have a friend come up and hold onto your hand. If you made this treadmill go really fast could you friend still hold onto your hand holding you in place? Yes. Your wheels would just be spinning like hell but your friend is still holding you in one place. Eventually the friend pulls a little harder and would pull you forward on the belt towards them. The amount of force it would take to do this really isn't that much more than if the belt wasn't moving. As you were pulled forward you would be moving through the air that isn't moving since the belt isn't influencing the air around it.

Your friend grabbing onto your hand and pulling you forward is the same as the prop grabbing onto the air and pulling itself forward.
 
Dave Krall CFII said:
More powerful airplanes need ONLY the full prop thrust to lift off and FLY off the belt at just above their stall speed.
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The propeller does not give enough airflow to generate that lift across the wings. Take the plane to the runup area. Stomp on the brakes and throttle to full. You won't take off. You've got to have airflow across the entire wing at > stall speed.
 
SkyHog said:
The reason it is not very accurate is because the conveyor belt (truck) is moving the plane. A conveyor belt in the example problem would not move the plane.
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But, the fact remains the same, that no matter how fast the truck/conveyor belt is moving, the airplane is going to simply overcome it and take off.

Heck, we can apply it a different way. Consider a truck pulling a long sheet (our treadmill), setup the wind-up plane with a rubber band or stick to hold it as the sheet is pulled underneath it so that the plane doesn't move with the sheet as it did in the video (only preventing backwards motion). Set the plane to flight and see what happens, it'll still take off, and that's about as close as you'll get to the original question without spending large sums of money.
 
jesse said:
Your friend grabbing onto your hand and pulling you forward is the same as the prop grabbing onto the air and pulling itself forward.
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Exactamundo. That spinny thing isn't just there to keep you cool. It pulls you through the air. THE AIR.
 
KennyFlys said:
The thrust is only overcoming the effect of the belt wanting to move the airplane back with it. Even if thrust increases as the belt speed increases... how are you going to acheive flight?
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By increasing thrust to TO power! That is all that is required to get the plane moving to TO speed. The belt is not attached to the plane, so the limit of the backward force it can apply is whatever resistance the wheel bearings apply. The engine has no such problem. It is attached to the plane, and will apply forward force generated by the prop pulling the plane forward. The belt could be moving rearward at 500 MPH and the limit of the rearward force that would apply is STILL just the wheel bearing resistance.

You seem to think the belt force could equal the engine thrust. It can't. This is not a car where both the engine force and the belt force are applied to the car through the wheels. In that case the belt force could offset the car engine by keeping pace with the tire rotation. The belt force in the case of a plane is applied through the wheels and is limited to bearing resistance unless the pilot applies the brakes. The engine force of a plane is applied by the prop cutting into the air, and is not dependant in any way upon what the wheels are doing. The wheels could be not turning at all if you can get enough headwind to lift off without a ground roll. That is why an aircraft carrier heads into the wind and increases speed for launch of aircraft. The wheels can also be turning at much more than plane airspeed, if you need to TO downwind for some reason. It does not matter what the wheels are doing. The only thing that matters is airspeed, and that is generated by the prop pulling the plane through the air. And a belt speeding up the wheels can not prevent that thrust from causing forward motion through the relative wind. Not necessarilly the same as forward motion relative to the ground, or the belt surface in the case of this hypothetical.

This is not complicated. I don't know what else to say to convice you. How about this. The belt is turning at 70 KTS. The plane is facing in the direction the belt is turning toward and into a headwind of 70 KTS, with power set to move it through the air at 70 KTS. In that case the plane would not move relative to a fixed point next to the runway, even though the wheels are spinning for 70 KTS backwards. But with 70 KTS of wind moving over the wings the plane could fly. It would be intersting to watch a vertical takeoff over a fixed point like that. I understand someting like this is done when a UL plane is launched off a yacht moving into the wind, with no runway or flight deck at all. In this case the wheel bearing friction would ADD slightly to the forward force applied to the plane because that is the direction the belt it going and the wheels are spinning in reverse. Unless the engine and prop were at TO power the plane could not takeoff though, because without the prop turning the plane would not be moving at 70 KTS into the wind - even though that is what the belt is trying to do. Without TO power that headwind would blow the plane backward, and the wheels would spin at more than 70 KTS in reverse until the plane flipped over into a pile of junk.
 
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Dave Krall CFII said:
You are correct if you are visualizing airplanes such as the Wright Flyer or the early, extremely under powered Fly Babys. They do NOT have enough prop thrust to fly off the conveyor belt even at full power. They must have not only full prop thrust moving air over the wing but MUST also have that thrust combined with the airflow generated by the forward motion of the airplane through the air in order to lift off and continue to fly.

More powerful airplanes need ONLY the full prop thrust to lift off and FLY off the belt at just above their stall speed.

Therefore, neither camp thus far whose answers I've read, (and God help me, I think I've read them all) not the will flys nor the won't flys, has given a completely accurate answer.
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If you are saying the prop is producing all the required air flow by its fan effect - sure. That is not what happens. The prop moves the whole plane through the air that it is located in, and all that air is what produces the lift. And the thrust will produce that forward motion regardless of how fast the wheels are spinning and in what direction they are spinning. They are irrelevant unless the brakes are set, except for wheel bearing friction.
 
wbarnhill said:
But, the fact remains the same, that no matter how fast the truck/conveyor belt is moving, the airplane is going to simply overcome it and take off.

Heck, we can apply it a different way. Consider a truck pulling a long sheet (our treadmill), setup the wind-up plane with a rubber band or stick to hold it as the sheet is pulled underneath it so that the plane doesn't move with the sheet as it did in the video (only preventing backwards motion). Set the plane to flight and see what happens, it'll still take off, and that's about as close as you'll get to the original question without spending large sums of money.
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Now if the plane is in a truck going south at TO speed, and the plane is trying to take off going south (in calm air) then all that is needed is for the truck to get up to TO speed, the plane to power up to TO power, and lift off. No ground roll needed since the relative wind is already adequate for TO.

Same situation but the truck is going south and the plane is trying to take off going north. Can't do it. The relative wind is a tailwind and the plane would need a long ground roll to exceed that tailwind by enough to get to TO speed through the relative wind.

The truck situation is nothing like the conveyor belt situation. The truck is actually moving the plane and can apply force to move the plane as fast as the truck tires can turn, assuming the plane is attached in the truck as cargo. If not, the truck could drive out from under the plane as soon as the truck acceleration exceeded the wheel bearing resistance of the plane wheels on the truck bed. The conveyor belt can not apply force to move the plane that is any more than the wheel bearing friction. If more than wheel bearing friction force is applied, the wheels just turn instead of the belt moving the plane. Put a toy plane on a table and pull the tablecloth out from under it. If you do it quick enough the toy plane will not move much at all. All you did was applied more conveyor belt force than the wheel bearing friction of the toy plane wheels. You could do the same thing with a plate. More force than friction, and the object stays while the conveyor belt moves. The amount it moves depends on however much friction has actually transfered force from the conveyor belt (tablecloth) to the object.
 
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The only way the truck could be comparable to the conveyor belt is if you were to use thrust to hold it in place with the rest of the world and accelerate with the truck finally adding more power to overcome the wheel bearing friction caused by the truck and taking off.
 
Or the truck bed could be long enough to allow for the roll distance needed to overcome the backwards momentum.

About 3 miles long maybe?
 
If everyone here would pool up to build the conveyor belt and buy the airplane I'd fly it. Of course if I live it's my plane.
 
jesse said:
The only way the truck could be comparable to the conveyor belt is if you were to use thrust to hold it in place with the rest of the world and accelerate with the truck finally adding more power to overcome the wheel bearing friction caused by the truck and taking off.
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I am not understanding this one. Use plane engine thrust to hold the truck in one place on the runway? If the plane is on a truck and thrust is applied it will move through the relative air. If the truck is going the same direction at the same speed the plane will stay on top of the truck. Unless the plane is attached to the truck by some strong cargo straps, the plane thrust will not have much effect on the truck. If the plane moves slower through the air than the truck it will fall off the back. Faster and it will fall off the front (or run into the truck cab, and at that point it would help push the truck along). If they match speed until TO airspeed the plane can lift off, but in all that time the wheels would not have turned at all.
 
Dwight B. Van Zanen said:
I am not understanding this one. Use plane engine thrust to hold the truck in one place on the runway? If the plane is on a truck and thrust is applied it will move through the relative air. If the truck is going the same direction at the same speed the plane will stay on top of the truck. Unless the plane is attached to the truck by some strong cargo straps, the plane thrust will not have much effect on the truck. If the plane moves slower through the air than the truck it will fall off the back. Faster and it will fall off the front (or run into the truck cab, and at that point it would help push the truck along). If they match speed until TO airspeed the plane can lift off, but in all that time the wheels would not have turned at all.
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No Dwight. Drive forward with the truck (using trucks engine) and use the thrust of the airplane to hold the airplane in one place in relation to the rest of the world. If you think of it this way the problem would be very similar to the conveyor belt. The trucks moving underneath it and the airplane is staying stationary due to the thrust. The airplane could then use more thrust to move forward through the air and takeoff. You'd need a damn long truck.
 
jesse said:
No Dwight. Drive forward with the truck (using trucks engine) and use the thrust of the airplane to hold the airplane in one place in relation to the rest of the world. If you think of it this way the problem would be very similar to the conveyor belt. The trucks moving underneath it and the airplane is staying stationary due to the thrust. The airplane could then use more thrust to move forward through the air and takeoff. You'd need a damn long truck.
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Ok. Got it. With a 3 mile long truck bed and the plane thrust moving the plane opposite the truck, right? But again, it would only take idle power. If the truck is moving through the relative air, and the plane were facing the rear of the truck, that relative air would already move the plane toward the rear of the truck bed. The only additional power needed to hold the plane in place is the little bit necessary to roll those wheel bearings and wheels. Inertia holds the plane in place. The force of the relative wind holds the plane in place. Just that would cause the plane to roll off the rear of a normal truck bed, and on a longer bed it would keep rolling.

Idle thrust would keep it in one place relative to the world no matter how fast the truck was going, and of course at idle and no relative wind over the wings it could not fly. However, if you increase thrust to TO power the plane would accelerate toward the rear of the 3 mile truck bed and take off when it got to TO speed. I see how this is similar to the conveyor belt situation, but as you said it would take one very long truck bed.
 
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Dwight B. Van Zanen said:
Ok. Got it. With a 3 mile long truck bed and the plane thrust moving the plane opposite the truck, right? But again, it would only take idle power. If the truck is moving through the relative air, and the plane were facing the rear of the truck, that relative air would already move the plane toward the rear of the truck bed. The only additional power needed to hold the plane in place is the little bit necessary to roll those wheel bearings and wheels. Inertia holds the plane in place. The force of the relative wind holds the plane in place. Just that would cause the plane to roll off the rear of a normal truck bed, and on a longer bed it would keep rolling.

Idle thrust would keep it in one place relative to the world no matter how fast the truck was going, and of course at idle and no relative wind over the wings it could not fly. However, if you increase thrust to TO power the plane would accelerate toward the rear of the 3 mile truck bed and take off when it got to TO speed. I see how this is similar to the conveyor belt situation, but as you said it would take one very long truck bed.
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You got it
 
Greebo said:
The propeller does not give enough airflow to generate that lift across the wings. Take the plane to the runup area. Stomp on the brakes and throttle to full. You won't take off. You've got to have airflow across the entire wing at > stall speed.
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Not true for all planes, that's why I differentiated above.
You've seen planes hanging on their props at air shows haven't you?
 
Dave Krall CFII said:
Not true for all planes, that's why I differentiated above.
You've seen planes hanging on their props at air shows haven't you?
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Still, the only aircraft I know of that can lift off without the help of airflow over the wings would be rotorcraft including the Osprey or jet-thrust lift like the Harrier. Unless you also count the "lifting body" aircraft that resulted in the Space Shuttle, that essentailly have no wings or are all wing - depending on how you look at it.
 
I can't take it any more!!!!

CB053452.jpg
 
Dave Krall CFII said:
Not true for all planes, that's why I differentiated above.
You've seen planes hanging on their props at air shows haven't you?
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Yes but two things:
1) You said "more powerful" in relation to the Wright Flyer. A C150 is more powerful ... and can't hang off the prop.

2) A plane that can hang off the prop isn't doing so because of prop wash over the wings. Its doing so on pure thrust. Park it on the ground, lock the wheels so they can't move, and go to full power. It will not take off on prop wash. (It may, however, pull the plane anyway destroying the tires...but it won't get enough speed to take off doing that...) :)
 
Hard to believe that this is still going on. I would think that every pilot already has got this. The problem with most of the analogies on the other boards was that everyone always stopped the plane and put it in a station mode or on a train.

When you took your pilot test one of the questions was or rather the answer was "An object in motion stays in motion unless...". The question: "The plane moves in one direction..., So the plane moves first, now stop it and prove how you stopped it. You will not be able to stop it, and it will continue to pick up speed and take off (Even the Wright Brothers plane). To get this question the plane has to move or be moving. Then the doubters have to stop it. Rather than the knowledgeable ones trying to prove it will move.

Dan
 
lol - the treadmill experiment is happening at Gastons. Dave Gaisky said he'd bring a few of his RC planes, and I'm planning on constructing some sort of treadmill (might need donations, depending on what is required).

This question will be answered, and as such, there will never be another topic about it again.
 
SkyHog said:
lol - the treadmill experiment is happening at Gastons. Dave Gaisky said he'd bring a few of his RC planes, and I'm planning on constructing some sort of treadmill (might need donations, depending on what is required).

This question will be answered, and as such, there will never be another topic about it again.
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:rofl: yes it will
 
Greebo said:
Yes but two things:
1) You said "more powerful" in relation to the Wright Flyer. A C150 is more powerful ... and can't hang off the prop.

2) A plane that can hang off the prop isn't doing so because of prop wash over the wings. Its doing so on pure thrust. Park it on the ground, lock the wheels so they can't move, and go to full power. It will not take off on prop wash. (It may, however, pull the plane anyway destroying the tires...but it won't get enough speed to take off doing that...) :)
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1) Yeah, I should have been more specific.

2) That same thrust is also generating lift over the airfoil.
Haven't you seen the scale models take off like that in videos?
 
Dave Krall CFII said:
1) Yeah, I should have been more specific.

2) That same thrust is also generating lift over the airfoil.
Haven't you seen the scale models take off like that in videos?
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In the cases where an airplane can take off and hang on the prop climbing straight up. This isn't due to the lift from the air thrown over the wing. This is pure thrust from the prop pulling the airplane up. More thrust than weight and up you go. You could remove the wings and it'd do it just the same.
 
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