12/12/2007 - the Conveyor Belt myth will be busted

Dave Siciliano said:
I was very disappointed in the entire 'xperiment. I thought the conveyor belt was to be going the same speed as the plane in the opposite direction; in that case, if the plane flew, it would rise straight up over a fixed spot on the conveyor belt. That just flat didn't happen. In each case, the plane moved forward and accelerated off the conveyor belt. They said they matched speeds, but the plane didn't match that speed in either case.

I would have liked to see the plane remain fixed over one spot, but that just didn't happen.

"Course, they really aren't trying to settle anything; just get their ratings up <g>

Best,

Dave
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Dave, Dave, Dave!

You're still missing the physics.

The only way the plane would stay fixed in RELATIVE (in relation to spot on the ground outside of the conveyor) space is if it had a headwind matching the airspeed. NOTHING you do to the wheels would make that happen!

*sigh* *sigh* *sigh*

I'm beginning to see that the inability of people to get the model clear in their brains is fascinating in itself. It's gotta be something along the lines of the misdirection that magicians use. There are other commonly held illusions, but I can't think of them at the moment.
 
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oops! Yea, I see what you're saying now! Didn't think 'bout that part! I guess it just would have been clearer to me that the lift was being created if it lifted over a fixed spot, but, now I see your point 'bout the lack of a head wind. It wasn't the wheels part that was getting me, it was the relative forward speed of the plane over the conveyor, but I'm with ya now!

Thanks.

Best,

Dave
 
The thing is, if the airspeed needs to be 25kias, as in the show, obviously the aircraft will accelerate to 25kts by the force of the propeller, regardless of what is happening to the wheels.

Since the treadmill is moving the opposite direction at 25kts, the wheels on the aircraft must attain 50kts (in a no wind situation). The speed of the tires is really irrelavent unless in order to get to the required Vr, you exceed to the max tire speed, in which case the tires fail, the aircraft doesn't fly AND you have a real mess.

Sorry to keep this thread going...I was a naysayer myself, but in this scenario it makes sense. Of course, if the conveyer belt speed were to increase as the aircraft attempted to accelerate (tire speed + acceleration), then the aircraft would remain stationary, and there would be no relative wind over the wings.
 
Dave Siciliano said:
I was very disappointed in the entire 'xperiment. I thought the conveyor belt was to be going the same speed as the plane in the opposite direction; in that case, if the plane flew, it would rise straight up over a fixed spot on the conveyor belt. That just flat didn't happen. In each case, the plane moved forward and accelerated off the conveyor belt. They said they matched speeds, but the plane didn't match that speed in either case.
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It took a while but I've seen the light. Airplanes move through AIR. That big fan thing pulls the plane through the AIR. Once the airplane is moving in the air the ground speed, or how fast the wheels are turning are irrelevant and the amount of drag / friction is removed from the airframe by the wheel bearings. Ground roll and take off difference are virtually unaffected by the ground moving backwards at a speed = take off speed.

for instance if the belt would not rip start the truck towing the plane first with the brakes lock or a friction coeffecient of 1.

Result is a ground speed -25 and air speed -25

In the airplane release the brakes.

Belts still moves at -25, airplance slows down the wheels are still tunring at 25 mph. IAS < 25 and > 0

Apply full power and the airplane stop and takes of in the other direction normally through the air. The speed at which the wheels turn does nto effect the take off.

The plane can never rise straight up, unless the relative airspeed = take off speed. In the ultralight example wind > 25 MPH down the runway. Apply power and the plane flies with no ground roll.
 
The only way the plane would stay fixed in RELATIVE (in relation to spot on the ground outside of the conveyor) space is if it had a headwind matching the airspeed. NOTHING you do to the wheels would make that happen!
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A lot of people seem to interpret the question as "the treadmill matches the wheel speed".

The problem is that it's pretty trivial to show that if the wheels aren't skidding, then the wheel speed is the sum of the treadmill speed and the airframe speed, i.e.:
wheel speed = treadmill speed + airframe speed

It's pretty easy to solve the above equation when "treadmill speed" equals "wheel speed", i.e. the airframe speed is necessarily 0.

If you're disappointed that they didn't do that experiment, well, don't be, because they did. Right before anything started moving, the treadmill speed was matching the wheel speed, as they were both 0.

If this isn't satisfying, and you'd prefer to see this "experiment" done at a variety of different speeds, that can be done, too, but it's inherently a "low throttle" experiment. For any speed you like, you can get the treadmill going that speed, and the throttle set to whatever power is necessary to overcome rolling resistance at that speed, this is necessarily the power needed to keep the plane in the same spot.

Hopefully you don't expect to see the plane begin to levitate in such an experiment.
-harry
 
Soo, what is the relevance of the conveyor belt? Are folks thinking it accelerates the aircraft?

The whole point of the experiment is to show the disconnect between the speed of the belt and the aircraft. They are not connected so therefore there is no influence between the two.
 
121Dispatch said:
Sorry to keep this thread going...I was a naysayer myself, but in this scenario it makes sense.
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Argh! You should be!
121Dispatch said:
Of course, if the conveyer belt speed were to increase as the aircraft attempted to accelerate (tire speed + acceleration), then the aircraft would remain stationary, and there would be no relative wind over the wings.
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NO!!!! :no: The speed of the conveyor belt does not affect the takeoff!
 
121Dispatch said:
The thing is, if the airspeed needs to be 25kias, as in the show, obviously the aircraft will accelerate to 25kts by the force of the propeller, regardless of what is happening to the wheels.

Since the treadmill is moving the opposite direction at 25kts, the wheels on the aircraft must attain 50kts (in a no wind situation). The speed of the tires is really irrelavent unless in order to get to the required Vr, you exceed to the max tire speed, in which case the tires fail, the aircraft doesn't fly AND you have a real mess.

Sorry to keep this thread going...I was a naysayer myself, but in this scenario it makes sense. Of course, if the conveyer belt speed were to increase as the aircraft attempted to accelerate (tire speed + acceleration), then the aircraft would remain stationary, and there would be no relative wind over the wings.
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You're still wrong. If you ignore friction and in the real world you do, the conveyor belt speed can't do anything to make the plane not move forward, ...never, ever, ever, ever, to infinity and light speed because the wheels are free spinning.

One more time. If you had a roller skater standing on the belt, would you say, "Sure he can stand still at 25 MPH, but if the belt went to like 50 MPH then he'd be thrown on his backside!"

I've really, really had enough of this. I will continue to be amused.
 
Checkout_my_Six said:
Soo, what is the relevance of the conveyor belt? Are folks thinking it accelerates the aircraft?

The whole point of the experiment is to show the disconnect between the speed of the belt and the aircraft. They are not connected so therefore there is no influence between the two.
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If they don't get it after that they never will. It was explained very well on the program. As far as friction goes did you notice that the tarp did not tear with the plane but did when he ran on it. No friction! The truck could have been going 100 mph against the planes 25 and it made no difference. Even though the pilot was rather uninformed (Being kind here) he did say that he did not notice any difference from the belt to the runway takeoffs.

It is intersting to see that the people that thought it would not move thought they did something wrong because it did. Just shows how strong the mind really is, even when it's wrong. The only way to prove to some people would be to have them fly the plane. After all that's what it took for Mark Johnson.

Dan
 
The belt didn't match speeds with the plane. The belt went FASTER than the plane. Jamie said he gunned it as soon as he saw the plane moving - so he went well beyond the 'matching' speed of the plane and it *still* had no effect on the plane.

The only way that plane wasn't gonna move forward no matter HOW fast that belt moved was if the wheels were locked and unable to spin.
 
smigaldi said:
No I was just wondering about conveyors, do they work in Texas like everywhere else on the planet except for Georgia? :D:D:D:D
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No clue... I haven't met any pilots here who dwell on such issues. There are too many planes to fly and too many Indians to avoid! :)
 
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