The great conveyor belt

smigaldi said:
What happens if there is a tailwind of equal speed as the thrust?
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Richard Collins says it won't take off...or you need to add the tailwind to the airspeed needed for rotation.

(He has said repeatedly you need to add airspeed when you land with a tailwind.)
 
mikea said:
Richard Collins says it won't take off...or you need to add the tailwind to the airspeed needed for rotation.

(He has said repeatedly you need to add airspeed when you land with a tailwind.)
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Cause the tailwind won't make you land long enough already?
 
mikea said:
(He has said repeatedly you need to add airspeed when you land with a tailwind.)
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Where does he say this? I think I'd need to see some really solid logic before I'd ever do that.
 
Greebo said:
Explain why.
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I thought the entire concept was to see a plane fly while only the conveyor belt moved. If the plane is moving in relation to the fixed point upon which the conveyor rest then it will gain airspeed. Otherwise, not a chance.
 
KennyFlys said:
I thought the entire concept was to see a plane fly while only the conveyor belt moved. If the plane is moving in relation to the fixed point upon which the conveyor rest then it will gain airspeed. Otherwise, not a chance.
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The concept is to understand airplanes move because of thrust. Not because of the wheels spinning. It doesn't matter what you do with the conveyor belt. You can make it go as fast or as slow as you want. The airplane will grab onto the air and move forward. As it pulls itself forward through the air (and forward in relation to a fixed point outside of the conveyor belt) it will gain airspeed and takeoff.
 
OK, I think I get it. If the entire runway consists of a conveyer/treadmill, the direction and speed of the treadmill is relevant only to the tires since the airplane MUST move forward relative to the runway threshhold because it is being dragged through the air by the propeller.

If the treadmill is going the opposite way from the plane, the wheels must go faster than the plane to keep up. But, you could turn the treadmill the other way and the wheels don't rotate at all while the airplane accelerates down the runway (still pulled by the propeller).

If, you turn off the propeller and just let the treadmill move the airplane until it flies, though, the plane would stall as soon as it lifts off.
 
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[HOMER]Doh! Stupid conveyer belt! Good thing I got suspenders![/HOMER]
 
AuntPeggy said:
If, you turn off the propeller and just let the treadmill move the airplane until it flies, though, the plane would stall as soon as it lifts off.
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Sort of. The belt would have a hard time moving the airplane to a fast enough speed in a controlled manner unless you held the brakes. You could actually *HOLD* the airplane on the conveyor belt until you built sufficient airspeed to pull off and climb some but you would be coming back down.


You nailed it though.
 
mikea said:
Richard Collins says it won't take off...or you need to add the tailwind to the airspeed needed for rotation.

(He has said repeatedly you need to add airspeed when you land with a tailwind.)
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Wow is he way off on that. Why in the G-d of flight would you add airspeed with a tailwind for landing. You already are going at a faster GS than a normal landing. Sounds like pretty well reasoned insanity if you ask me. It also sounds as if Mr. Collins may not understand how planes move through the air. You got a reference for him saying that??
 
jesse said:
Where does he say this? I think I'd need to see some really solid logic before I'd ever do that.
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I read it twice in his column and he mentions it on his Sporty's DVD series. If I was inclined I'd try to extract it from the DVD but I don't have the time.

He says something like you want to add half of the tailwind to your airspeed. In the meantime you read Peter Garrison in the same magazine writing (correctly) how stupid of an idea that is.
 
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KennyFlys said:
I thought the entire concept was to see a plane fly while only the conveyor belt moved. If the plane is moving in relation to the fixed point upon which the conveyor rest then it will gain airspeed. Otherwise, not a chance.
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Ok - explain how the conveyer belt can keep the plane in place since the airplane's wheels are free spinning. :)
 
Greebo said:
Ok - explain how the conveyer belt can keep the plane in place since the airplane's wheels are free spinning. :)
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If the conveyor moves at a speed equal to the inverse of the frictional coefficient of the bearings in the wheels times the forward velocity of the plane the plane will not move.
 
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EdFred said:
If the conveyor moves at a speed equal to the inverse of the frictional coefficient of the bearings in the wheels the plane will not move.
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The bearings would fail and fire would be imminent long before that.
 
jesse said:
The bearings would fail and fire would be imminent long before that.
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Not if you have super-awesome bearings. I mean, we are after all talking about a conveyor belt the length of a runway - and those don't exist, we might as well allow for super bearings that won't fail.
 
EdFred said:
But can it spin at speeds of up to 120kts?
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With some improvements we can make it happen.

Or we can just stick a real light rubber band powered plane on it to demonstrate the theory. But we can do that with a treadmill just as easily. In fact I saw a video where someone did.
 
mikea said:
He says something like you want to add half of the tailwind to your airspeed. In the meantime you read Peter Garrison in the same magazine writing (correctly) how stupid of an idea that is.
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I don't know who this guy is, but I'd love to hear his reasoning on that one. Actually, there could be an argument for decreasing your airspeed in a tailwind because as you fly down through the wind gradient, the tailspeed will drop off. However, we're all approaching at the minimum safe airspeed already, so that doesn't seem like a good idea. I can't begin to fathom why an increased airspeed would make sense.

Chris
 
gibbons said:
Can we create a new forum for this thread so I can ignore it?
:D
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Like you would, Chip, like you would!

It is compelling, irresistable, like looking at a car wreck as you drive by...
 
jesse said:
You admit defeat?

Or you think you can't convince us you're right and we're wrong?
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This is only temporary. The only thing I should allow to drive me to insanity at this point is preparing for the checkride on Sunday.

Right now, I'm convinced of only two things... I'm right and y'all are nuts! :D
 
Are you guys serious about this? Unless the conveyer belt is moving the air over the runway, the plane thrust will move it relative to the AIR, and not relative to the runway surface. It is impossible for the conveyor belt to offset the forward motion through the AIR by increasing the rotation speed of the wheels. The only backward force that would create is the minimal resistance friction in the wheel bearings, or if the pilot applies the brakes. The conveyor belt becomes irrelevant as soon as the prop or jet thrust begins moving the plane through the air over the runway. The only way it could be relevant is if the conveyor generated its own wind by friction, causing a headwind for the plane - and in that case you are talking about a wind tunnel type situation. Think of it as a momentum problem. Ordinarilly, the plane engine is just overcoming inertia and wind resistance and wheel bearing friction to get a plane moving forward. If the plane is on a conveyor belt that is trying to move it backward, it also has to overcome inertia and wind resistance. And it can only apply wheel bearing friction to move it backward. And unless the brakes are set the plane will not move at the same speed as the conveyor initially because inertia and wind resistance exceeds wheel bearing friction. The wheels transfer very little force from the plane to the conveyor or from the conveyer to the plane, based entirely on the effectiveness of the bearings. So, when the engine is turning the prop at max RPM, where is all the force going? A very little bit is lost if the conveyor is turning the wheels at high speed. The rest MUST overcome inertia and wind resistance and accelerate the plane forward - regardless of the wheel RPM. The conveyor belt can't apply force to the plane in excess of the wheel bearing resistance. The engine and prop (or jet thrust) can.
 
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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.
 
Dwight B. Van Zanen said:
Are you guys serious about this? Unless the conveyer belt is moving the air over the runway, the plane thrust will move it relative to the AIR, and not relative to the runway surface. It is impossible for the conveyor belt to offset the forward motion through the AIR by increasing the rotation speed of the wheels. The only backward force that would create is the minimal resistance friction in the wheel bearings, or if the pilot applies the brakes. The conveyor belt becomes irrelevant as soon as the prop or jet thrust begins moving the plane through the air over the runway. The only way it could be relevant is if the conveyor generated its own wind by friction, causing a headwind for the plane - and in that case you are talking about a wind tunnel type situation. Think of it as a momentum problem. Ordinarilly, the plane engine is just overcoming inertia and wind resistance and wheel bearing friction to get a plane moving forward. If the plane is on a conveyor belt that is trying to move it backward, it also has to overcome inertia and wind resistance and wheel bearing friction to move it backward. And unless the brakes are set the plane will not move at the same speed as the conveyor initially because inertia and wind resistance exceeds wheel bearing friction. The wheels transfer very little force from the plane to the conveyor or from the conveyer to the plane, based entirely on the effectiveness of the bearings. So, when the engine is turning the prop at max RPM, where is all the force going? A very little bit is lost if the conveyor is turning the wheels at high speed. The rest MUST overcome inertia and wind resistance and accelerate the plane forward - regardless of the wheel RPM. The conveyor belt can't apply force to the plane in excess of the wheel bearing resistance. The engine and prop (or jet thrust) can.
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What he said! :)
 
Let me make this simple for you...
conveyor2.jpg


Kenny is holding the rope which is tied onto the airplane (let's say he actually strong enough). They conveyor belt starts to move 70 mph. Do you think the airplane could still be held in place? Yes. You would hold it and the wheels would just spin. Now if you started to pull the rope towards you what would happen? It would move forward over the belt, it would move forward through the air.

Kenny pulling on the rope is the same as the prop pulling on the air. The airplane grabs onto an outside source not effected by the conveyor belt, grabs onto it, and pulls itself forward.

Those of you that simply do not understand how it could takeoff...What do you think is happening to the prop when it's spinning and grabbing air? Are you saying the airplane wouldn't move forward, along the belt, and through the air? *THINK* about this. Draw it on paper. Do whatever you gotta do.
 
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jesse said:
Let me make this simple for you...


Kenny is holding the rope which is tied onto the airplane (let's say he actually strong enough). They conveyor belt starts to move 70 mph. Do you think the airplane could still be held in place? Yes. You would hold it and the wheels would just spin. Now if you started to pull the rope towards you what would happen? It would move forward over the belt, it would move forward through the air.
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Great point. If the conveyor belt started slowly I think I could hold my Cherokee with one hand. Easier than getting the plane moving to drag it out of the hangar, since in this case inertia would be on my side.
 
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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