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

KennyFlys said:
If the conveyor is moving aft at 100 kts and the propeller is providing sufficient thrust to move forward at 100 kts, at what point does the plane develop air over the wings to create lift?
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Wow. Just wow. And you're going for your CFI? Might want to postpone the checkride indefinitely.
 
KennyFlys said:
If the conveyor is moving aft at 100 kts and the propeller is providing sufficient thrust to move forward at 100 kts, at what point does the plane develop air over the wings to create lift?
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What part of "The Propeller does not care what the wheels or the surface they are on are doing" don't you understand.

Geez.

The propeller is pulling on the air. WHAT THE SURFACE IS DOING THAT THE AIRPLANE IS ON IS IRRELEVANT.
 
Answer, Kenny:

Conveyor belt moves rearward at 100knots, the propeller tries to move forward at 100knots, therefor, airspeed (neglecting wind) would be about 99.99 knots.
 
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Look, Ken. The propeller produces thrust by moving air. It does not care what the "runway" is doing. There is nothing the surface can do that changes that.
 
KennyFlys said:
Stop being a smartass and address my question.
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Maybe this would help:
What if you had a 172 and placed it on a conveyor belt moving backward at 500kts (a speed the plane couldn't obtain in S&L flight). Would it take off?
 
Sam Doolittle said:
Maybe this would help:
What if you had a 172 and placed it on a conveyor belt moving backward at 500kts (a speed the plane couldn't obtain in S&L flight). Would it take off?
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The answer is still yes. And the airspeed would be exactly the same if the treadmill were moving at 7000kts.
 
KennyFlys said:
So far, the the propeller is only allowing the airframe to keep pace with the conveyor in the opposite direction. It's not moving forward and PUTTING AIR OVER THE WINGS FOR LIFT!

There's ground speed with relation to the conveyor but there is no airspeed.
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Nope.

The plane moves forward at almost the exact same speed as it would without the conveyor belt.

Groundspeed would be the same too. Wheel speed would be MUCH higher.
 
KennyFlys said:
Stop being a smartass and address my question.
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Do this:

Place table cloth on kitchen table
Find a big Tonka truck with free spinning axles or wheels
Place truck on kitchen table
Pull table cloth off of table
Watch wheels freely spin while "conveyor belt" moves under the truck and the truck stays motionless.

With thrust:
Find 4 class C rocket engines and fasten to tonka truck.
Light engines
pull table cloth
watch truck fly forward off the table and light curtains on fire.

Same concept

WHEELS DON'T MOVE THE DAMN PLANE!!!!
 
Sam Doolittle said:
Maybe this would help:
What if you had a 172 and placed it on a conveyor belt moving backward at 500kts (a speed the plane couldn't obtain in S&L flight). Would it take off?
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That sort of just confuses the issue. If you drop the airplane on the belt that is already going 500 MPH backwards, inertia will keep the airplane in one spot at least for a moment or two. If the airplane was running, and takeoff thrust was applied, the airplane will take off normally. The wheels will probably explode because they are not designed to go that fast.

But if you are saying the airplane is going backwards with the belt, there is no way the prop can overcome that rearward motion. But that is not what the scenario is based on.

Lets not change the rules in the middle of the game.
 
KennyFlys said:
But, in the case of the conveyor theory, the "runway" is moving the opposite direction. There's no forward motion since the propeller is providing only enough thrust to keep pace with the conveyor. You have no air over the wing and no lift. Where could you possibly be getting the lift?
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IF you are only applying enough thrust to counter act the conveyor belt, then No the airplane won't take off. But we have to assume the scenario dictates that the airplane will try to actually take off. That means full throttle, full takeoff thrust.. To assume anything else is just silly. The whole point is to make the airplane fly.
 
Greg Bockelman said:
That sort of just confuses the issue. If you drop the airplane on the belt that is already going 500 MPH backwards, inertia will keep the airplane in one spot at least for a moment or two. If the airplane was running, and takeoff thrust was applied, the airplane will take off normally. The wheels will probably explode because they are not designed to go that fast.

But if you are saying the airplane is going backwards with the belt, there is no way the prop can overcome that rearward motion. But that is not what the scenario is based on.

Lets not change the rules in the middle of the game.
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AND - the only way this would be possible to begin with is if you had the brakes on, which, last time I checked, was not part of the takeoff sequence.
 
KennyFlys said:
So far, the the propeller is only allowing the airframe to keep pace with the conveyor in the opposite direction. It's not moving forward and PUTTING AIR OVER THE WINGS FOR LIFT!

There's ground speed with relation to the conveyor but there is no airspeed.
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You are assuming that you are only applying enough thrust remain stationary relation to the ground. The whole point is to make the airplane fly.
 
Just for the sake of clarity, we are assuming the conveyor belt is not moving at a constant rate, but is accelerating in the opposite direction at the same rate as the aircraft, right?
 
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/me thinks there is a troll in the thread (at least I hope that's what is going on).
 
Maybe this will help:
 

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If the floatplane is pointed upstream and the river is flowing at 80 knots, and the power is set to maintain 0 relative motion, is it not pulling 80 knots worth of air? But what's the airspeed show? Will the airplane take off? No! No air movement over the wing, therefore no lift.
 
gkainz said:
If the floatplane is pointed upstream and the river is flowing at 80 knots, and the power is set to maintain 0 relative motion, is it not pulling 80 knots worth of air? But what's the airspeed show? Will the airplane take off? No! No air movement over the wing, therefore no lift.
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Bad example, as there's a lot more friction. BUT, to be accurate (and similar):

You're sitting in a skiplane. You add full power. Do you takeoff? And how did you do it if there are no wheels?
 
KennyFlys said:
Now, the terms are changed from the earlier argument. The first argument is identical to Greg's river scenario. Only, you don't have a full river length at your disposal. The conveyor is only as big as the footprint of the airplane.
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That was never the original scenario. Obviously, if you are taking off from a runway (or treadmill) that is 10 feet long, you're not gonna takeoff.
 
Your takeoff speed is 80 knots. You have an 80 knot tailwind as you roll down the runway. Will you take off? 0 airspeed over the wing ...
 
SkyHog said:
Bad example, as there's a lot more friction. BUT, to be accurate (and similar):

You're sitting in a skiplane. You add full power. Do you takeoff? And how did you do it if there are no wheels?
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Is the snow moving underneath my skiis, like trying to take off up an avalanche? :D
 
Holy crap. I do not understand why some people can't open their eyes to this concept. Ed--btw--your post made me laugh for about 5 minutes. I needed that. Thanks :)
 
Ok, guys. You're saying that if the ground moves aft the airplane will want to move aft. So on a normal takeoff where the ground remains stationary...why does the airplane not remain stationary?
 
KennyFlys said:
In order to have lift, you need air over the wing. In order to get air over the wing, you need movement forward through the air.

A conveyor can move aft as fast as it wants and as long as the propeller is not overcoming that aft movement, it's not going to advance along with the wings through the air.
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Hard to believe that someone cannot get this still.

The problem always is that you are starting with a plane that is not moving and trying to explain movement. Start with the plane moving and explain how you STOPPED its forward motion. The question always was " As the plane moves forward the belt moves back wards." Explain what force stops the planes forward movement. Once the plane is moving you have no explanation that can stop that movement EVEN AT IDLE.

Dan
 
Ok, if a plane comes in for a landing on a conveyor moving the same speed opposite the planes path of travel, assuming the brakes aren't applied, will the plane come to an immediate stop and begin traveling backwards?

;)
 
I actually feel dumber for reading this exchange. I concur, time for bed.
 
stingray said:
Hard to believe that someone cannot get this still.
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It has nothing to do with intelligence or knowledge of applied physics or anything else, it is about how one's brain is wired. You know, like those optical paradox illusions where some see the cube one way, some see it the other way.

Heck, I'm a frickin' genius and I got burned by the treadmill for a while :D (as some of you might remember.) That is why it is fun talking about it, it is not automatically intuitive for everyone.
 
gkainz said:
Your takeoff speed is 80 knots. You have an 80 knot tailwind as you roll down the runway. Will you take off? 0 airspeed over the wing ...
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Dear lord, there's two of them. :eek:

So we're all on the same page... The conveyor moves backward at a speed equal to the airplane's groundspeed.

If the airplane's groundspeed is zero, the conveyor IS NOT MOVING and will thus behave EXACTLY LIKE A NORMAL RUNWAY.

So, you add takeoff power. Let's say the airplane accelerates to 5 knots. The conveyor will be moving backwards at 5 knots, equal to moving forward at -5 knots. The wheels will be turning at 10 knots (5-(-5)=10). If your engine and prop cannot develop enough thrust to overcome 10 knots of rolling resistance, HOW THE HELL DO YOU TAKE OFF FROM A NORMAL RUNWAY???

So then, we've now established that the airplane can move forward. Air is flowing over the wings, albeit slowly. What force is stopping the continued acceleration of the airplane?

There is no such force. The airplane will continue to accelerate. At Vliftoff, the conveyor will be moving at -Vliftoff and the wheels will be turning at 2Vliftoff.

Now, for the "won't take off" the premise of the problem must be changed to "the conveyor will move backward with sufficient speed that the rolling resistance of the wheels matches the forward thrust of the engine and prop at full takeoff power." I have never, ever seen the problem worded that way and that's the only way the plane won't take off.
 
flyingcheesehead said:
Dear lord, there's two of them. :eek:

So we're all on the same page... The conveyor moves backward at a speed equal to the airplane's groundspeed.

If the airplane's groundspeed is zero, the conveyor IS NOT MOVING and will thus behave EXACTLY LIKE A NORMAL RUNWAY.

So, you add takeoff power. Let's say the airplane accelerates to 5 knots. The conveyor will be moving backwards at 5 knots, equal to moving forward at -5 knots. The wheels will be turning at 10 knots (5-(-5)=10). If your engine and prop cannot develop enough thrust to overcome 10 knots of rolling resistance, HOW THE HELL DO YOU TAKE OFF FROM A NORMAL RUNWAY???

So then, we've now established that the airplane can move forward. Air is flowing over the wings, albeit slowly. What force is stopping the continued acceleration of the airplane?

There is no such force. The airplane will continue to accelerate. At Vliftoff, the conveyor will be moving at -Vliftoff and the wheels will be turning at 2Vliftoff.

Now, for the "won't take off" the premise of the problem must be changed to "the conveyor will move backward with sufficient speed that the rolling resistance of the wheels matches the forward thrust of the engine and prop at full takeoff power." I have never, ever seen the problem worded that way and that's the only way the plane won't take off.
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That's the thing people just don't get.

Car Scenario:

Car moves at +55 mph
Conveyor moves at -55 mph

Car exerts forward motion via the wheels, resulting in a net movement of 0. Car remains stationary.

Plane Scenario:

Plane moves at +55 knots
Conveyor moves at -55 knots

Plane exerts forward motion via the prop, acting on the air. Unless the conveyor's motion at -55 knots creates enough drag on the wheels to overcome the forward thrust of the aircraft, the plane will move forward with little hindrance. Thus, unless there is a moment at which the conveyor's friction overcomes the forward motion, the aircraft will take off normally (forward movement = airflow) with the sole exception of the wheels spinning at two times the speed of the aircraft's forward motion.
 
gkainz said:
If the floatplane is pointed upstream and the river is flowing at 80 knots, and the power is set to maintain 0 relative motion, is it not pulling 80 knots worth of air? But what's the airspeed show? Will the airplane take off? No! No air movement over the wing, therefore no lift.
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Floats and skis aren't free moving. Thus any action upon them will exert much greater drag than wheels.

If you stand on skis on a tablecloth and I slide it from under you, you're going to move with it. If you are on rollerblades and I slide it from under you, you're more likely to simply roll off the cloth (if I do it right :D ).
 
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